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| United States Patent |
6,674,785
|
|
Sato
, et al.
|
January 6, 2004
|
Vertical-cavity, surface-emission type laser diode and fabrication process
thereof
Abstract
A vertical-cavity, surface-emission-type laser diode includes an optical
cavity formed of an active region sandwiched by upper and lower
reflectors, wherein the lower reflector is formed of a distributed Bragg
reflector and a non-optical recombination elimination layer is provided
between an active layer in the active region and the lower reflector.
| Inventors:
|
Sato; Shunichi (Miyagi, JP);
Takahashi; Takashi (Miyagi, JP);
Jikutani; Naoto (Kanagawa, JP);
Kaminishi; Morimasa (Miyagi, JP);
Itoh; Akihiro (Miyagi, JP)
|
| Assignee:
|
Ricoh Company, Ltd. (Tokyo, JP)
|
| Appl. No.:
|
957507 |
| Filed:
|
September 20, 2001 |
Foreign Application Priority Data
| Sep 21, 2000[JP] | 2000-286477 |
| Mar 12, 2001[JP] | 2001-068588 |
| Jul 16, 2001[JP] | 2001-214930 |
| Current U.S. Class: |
372/96; 372/45; 372/46 |
| Intern'l Class: |
H01S 005/183 |
| Field of Search: |
372/96,45,46
|
References Cited [Referenced By]
U.S. Patent Documents
| 5544193 | Aug., 1996 | Devaud-Pledran et al. | 372/96.
|
| 5903586 | May., 1999 | Ramdani et al. | 372/45.
|
| 6026108 | Feb., 2000 | Lim et al. | 372/50.
|
| 6061380 | May., 2000 | Jiang et al. | 372/96.
|
| 6072196 | Jun., 2000 | Sato | 257/87.
|
| 6207973 | Mar., 2001 | Sato et al. | 257/98.
|
| Foreign Patent Documents |
| 6-37355 | Feb., 1994 | JP.
| |
| 7240506 | Sep., 1995 | JP.
| |
| 7307525 | Nov., 1995 | JP.
| |
| 8340146 | Dec., 1996 | JP.
| |
| 10126004 | May., 1998 | JP.
| |
| 10303515 | Nov., 1998 | JP.
| |
| 11-4040 | Jan., 1999 | JP.
| |
| 11145560 | May., 1999 | JP.
| |
| 20004068 | Jan., 2000 | JP.
| |
Other References
Masahiko Kondow et al., "GaInNAs: A Novel Material for Wavelength-Range
Laser Diodes with Excellent High-Temperature Performance", Jpn. J. Appl.
Phys, part 1, No. 28, Feb., 1996, pp. 1273-1275.
M. Kawaguchi et al., "Low Threshold Current Density Operation of GaInNAs
Quantum Well Lasers Grown By Metalorganic Chemical Vapour Deposition",
Electronics Letter, Oct. 12, 2000, vol. 36, No. 21, pp. 1776-1777.
|
Primary Examiner: Davie; James
Attorney, Agent or Firm: Cooper & Dunham LLP
Claims
What is claimed is:
1. A vertical-cavity, surface-emission-type laser diode comprising:
a semiconductor substrate; and
an optical cavity structure provided on or above said semiconductor
substrate,
said optical-cavity structure comprising an active region containing at
least one active layer that produces a laser beam, and upper and lower
reflectors sandwiching said active region to form said optical cavity,
said lower reflector including a semiconductor distributed Bragg reflector
having a refractive index that changes periodically, said lower reflector
reflecting an optical beam incident thereto by diffraction,
said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1),
wherein a non-optical recombination elimination layer is provided between
said active layer and said lower reflector including an interface between
said lower reflector and said active region.
2. A vertical-cavity, surface-emission-type laser diode as claimed in claim
1, further comprising another non-optical recombination elimination layer
provided between said active layer and said upper reflector including an
interface between said active region and said upper reflector.
3. A vertical-cavity, surface-emission-type laser diode as claimed in claim
1, wherein said non-optical recombination elimination layer forms a part
of said lower reflector.
4. A vertical-cavity, surface-emission-type laser diode as claimed in claim
3, wherein said non-optical recombination elimination layer forms a
low-refractive index layer in said lower reflector.
5. A vertical-cavity, surface-emission-type laser diode as claimed in claim
1, wherein said semiconductor substrate comprises GaAs and wherein said
non-optical recombination elimination layer comprises a Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer.
6. A vertical-cavity, surface-emission-type laser diode as claimed in claim
5, wherein said Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer has a lattice constant smaller than a lattice
constant of said GaAs substrate and has a tensile strain therein.
7. A vertical-cavity, surface-emission-type laser diode as claimed in claim
5, wherein said Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer has a lattice constant larger than a lattice
constant of said GaAs substrate and has a compressive strain therein.
8. A vertical-cavity, surface-emission-type laser diode as claimed in claim
5, further comprising a current confinement layer on or above said
Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1)
layer, said current Confinement layer containing Al and As as primary
constituent element and is formed by selective oxidation of a
selective-oxidized layer.
9. A vertical-cavity, surface-emission-type laser diode as claimed in claim
1, wherein said active layer comprises any of GaInNAs and GaInAs.
10. A vertical-cavity, surface-emission-type laser diode as claimed in
claim 1, wherein said active layer accumulates a compressive strain of
2.0% or more.
11. A vertical-cavity, surface-emission-type laser-diode array comprising,
a plurality of vertical-cavity, surface-emission-type laser diodes, each of
said vertical-cavity, surface-emission-type laser diodes comprising a GaAs
semiconductor substrate and an optical cavity structure provided on or
above said semiconductor substrate, said optical-cavity structure
comprising an active region containing at least one active layer that
produces a laser beam, and upper and lower reflectors sandwiching said
active region to form said optical cavity, said lower reflector including
a semiconductor distributed Bragg reflector having a refractive index that
changes periodically and reflecting an optical beam incident thereto by
diffraction, said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1), a non-optical recombination elimination layer
of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) being provided between said active layer and said lower
reflector.
12. An optical transmission module having a vertical-cavity,
surface-emission-type laser diode as an optical source, said
vertical-cavity, surface-emission-type laser diode comprising:
a semiconductor substrate; and
an optical cavity structure on or above said semiconductor substrate,
said optical-cavity structure comprising an active region containing at
least one active layer that produces a laser beam, and upper and lower
reflectors sandwiching said active region to form said optical cavity,
said lower reflector including a semiconductor distributed Bragg reflector
having a refractive index that changes periodically, said lower reflector
reflecting an optical beam incident thereto by diffraction,
said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1),
wherein a non-optical recombination elimination layer is provided between
said active layer and said lower reflector.
13. An optical transmission module having a vertical-cavity,
surface-emission-type laser-diode array as an optical source, said
vertical-cavity, surface-emission-type laser-diode array comprising:
a plurality of vertical-cavity, surface-emission-type laser diodes,
each of said vertical-cavity, surface-emission-type laser diodes comprising
a GaAs semiconductor substrate and an optical cavity structure provided on
or above said semiconductor substrate, said optical-cavity structure
comprising an active region containing at least one active layer that
produces a laser-beam, and upper and lower reflectors sandwiching said
active region to form said optical cavity, said lower reflector including
a, semiconductor distributed Bragg reflector having a refractive index
that changes periodically and reflecting an optical beam incident thereto
by diffraction, said semiconductor distributed Bragg reflector comprising
a low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1)
and a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1), a non-optical recombination elimination layer
of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) being provided between said active layer and said lower
reflector.
14. An optical transceiver module having a vertical-cavity,
surface-emission-type laser diode as an optical source, said
vertical-cavity, surface-emission-type laser diode comprising:
a semiconductor substrate; and
an optical cavity structure on or above said semiconductor substrate,
said optical-cavity structure comprising an active region containing at
least one active layer that produces a laser beam, and upper and lower
reflectors sandwiching said active region to form said optical cavity,
said lower reflector including a semiconductor distributed Bragg reflector
having a refractive index that changes periodically, said lower reflector
reflecting an optical beam incident thereto by diffraction,
said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1),
wherein a non-optical recombination elimination layer is provided between
said active layer and said lower reflector.
15. An optical transceiver module having a vertical-cavity,
surface-emission-type laser-diode array as an optical source, said
vertical-cavity, surface-emission-type laser-diode array comprising:
a plurality of vertical-cavity, surface-emission-type laser diodes,
each of said vertical-cavity, surface-emission-type laser diodes comprising
a GaAs semiconductor substrate and an optical cavity structure provided on
or above said semiconductor substrate, said optical-cavity structure
comprising an active region containing at least one active layer that
produces a laser beam, and upper and lower reflectors sandwiching said
active region to form said optical cavity, said lower reflector including
a semiconductor distributed Bragg reflector having a refractive index that
changes periodically and reflecting an optical beam incident thereto by
diffraction, said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1), a non-optical recombination elimination layer
of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) being provided between said active layer and said lower
reflector.
16. An optical telecommunication system having a vertical-cavity,
surface-emission-type laser diode as an optical source, said
vertical-cavity, surface-emission-type laser diode comprising:
a semiconductor substrate; and
an optical cavity structure on or above said semiconductor substrate,
said optical-cavity structure comprising an active region containing at
least one active layer that produces a laser beam, and upper and lower
reflectors sandwiching said active region to form said optical cavity,
said lower reflector including a semiconductor distributed Bragg reflector
having a refractive index that changes periodically, said lower reflector
reflecting an optical beam incident thereto by diffraction,
said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1),
wherein a non-optical recombination elimination layer is provided between
said active layer and said lower reflector.
17. An optical telecommunication system having a vertical-cavity,
surface-emission-type laser-diode array as an optical source, said
vertical-cavity, surface-emission-type laser-diode array comprising:
a plurality of vertical-cavity, surface-emission-type laser diodes,
each of said vertical-cavity, surface-emission-type laser diodes comprising
a GaAs semiconductor substrate and an optical cavity structure provided on
or above said semiconductor substrate, said optical-cavity structure
comprising an active region containing at least one active layer that
produces a laser beam, and upper and lower reflectors sandwiching said
active region to form said optical cavity, said lower reflector including
a semiconductor distributed Bragg reflector having a refractive index that
changes periodically and reflecting an optical beam incident thereto by
diffraction, said semiconductor distributed Bragg reflector comprising a
low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and
a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1), a non-optical recombination elimination layer
of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) being provided between said active layer and said lower
reflector.
Description
BACKGROUND OF THE INVENTION
The present invention is related to a vertical-cavity,
surface-emission-type laser diode and the process of making the same.
Further, the present invention relates to a vertical-cavity,
surface-emission-type laser-diode array, an optical transmission module,
an optical transceiver module and also an optical telecommunication
system.
Vertical-cavity, surface-emission-type laser diode is a laser diode that
emits an optical beam in a vertical direction to a substrate. It is used
for a light source of optical interconnection systems and optical pickup
devices, and the like.
A vertical-cavity, surface-emission-type laser diode has an active region
including an active layer that produces a laser beam. The active region is
sandwiched with a pair of reflectors, wherein a semiconductor distributed
Bragg reflector, in which a low-refractive index layer and a high
refractive index layer are laminated alternately, is used widely for the
reflectors. Materials having a wider bandgap than the active layer and not
causing absorption of the optical beam coming from an active layer are
used for the semiconductor distributed Bragg reflector. Particularly, the
materials that achieve a lattice matching with the substrate are used so
as to avoid lattice relaxation.
Meanwhile, the reflector has to have a high reflectance of 99% or more.
Generally, the reflectance of the reflector becomes higher by increasing
the number of stacking. However, production of the vertical-cavity,
surface-emission-type laser diode becomes difficult when the number of
stacking in the reflectors is increased excessively. Because of this, it
is preferable that there exists a large refractive index difference
between the low-refractive index layer and the high refractive index layer
constituting the semiconductor distributed Bragg reflectors. AlAs and GaAs
are end-member compositions of the system AlGaAs having a lattice constant
almost the same as that of GaAs. Further, the materials of this system can
provide a large refractive index difference therebetween. Because of this
reason, it is possible to achieve a high reflectance with fewer number of
stacking by using the material of the AlGaAs system. Thus, the material of
the AlGaAs system is used widely.
However, the material containing Al is very reactive, and crystal defects,
originating from Al, are formed easily. For example, oxygen molecules or
water molecules contained in the source material or growth atmosphere are
easily incorporated into the crystal as a result of reaction with Al. Once
they are thus incorporated, they form a crystal defect acting as
non-optical recombination center, resulting in degradation of efficacy of
optical emission. Further, there is a concern that the reliability of the
device may be degraded due to the existence of such crystal defects.
Even when the active region is formed by a layer not containing Al, the
problem of non-optical recombination still occurs when Al is contained in
the low-refractive index layer (formed of a widegap layer) of the
reflector that makes a contact with the active region. More specifically,
such a non-optical recombination may occur at the interface between the
active region and the reflector when carriers are injected for
recombination. Thereby, the efficacy of optical emission falls off
inevitably. In order to avoid this adversary influence, it is necessary to
carry out rigorous process control, material purity control, optimization
of growth condition, and the like. Still, it is not easy to produce a
device with high quality.
Meanwhile, there are proposals in Japanese Laid-Open Patent Applications
08-340146 and 07-307525 to form a semiconductor distributed Bragg
reflector by using GaInP and GaAs, which are free from Al. However, the
difference of refractive index between GaInP and GaAs is only one-half the
refractive index difference between AlAs and GaAs. Because of this, the
number of stacking in the reflector has to be increased significantly, and
the production of the laser diode becomes difficult. Associated with this
there arise various problems such as degradation of yield, increased
device resistance, increased time needed for producing a laser diode.
Further, because of the increase of total thickness, there appears a
difficulty in providing electric interconnection in such a laser diode.
Meanwhile, it is practiced to use a current confinement structure in the
art of laser diode for reducing the threshold of laser oscillation.
Japanese Laid-Open Patent Application 7-240506 discloses a structure that
uses a current confinement structure including a high resistance layer
formed by an ion implantation process in combination with a semiconductor
distributed Bragg reflector that consists of AlAs/GaAs. Further, Japanese
Patent 2,917,971 proposes a vertical-cavity, surface-emission-type laser
diode that uses, in addition to an optical cavity formed by the
semiconductor distributed Bragg reflectors of the AlGaAs/GaAs stacked
structure, a current confinement structure that includes an oxide film
formed by selective oxidization of a part of the Al(Ga)As optical cavity
structure. In this proposal, the oxidation is conducted by supplying water
vapor of high temperature. It should be noted that the oxidation process
using water vapor of high temperature is capable of forming a true
insulator of Al.sub.x O.sub.y. According to such an approach, the distance
between the active layer and the current confinement layer can controlled
exactly by controlling the process of crystal growth. Further, it is
possible narrow the current path significantly. In view of these, the
foregoing construction is suited for reducing reactive current and for
reducing the active region. Because of this, it is also suited to for
reducing electric power consumption. Thus, the construction is used widely
recently.
It should be noted that the foregoing Japanese Patent 2,917,971 uses the
phenomenon that the oxidation rate starts to increase sharply as the Al
content in the AlGaAs layer is increased. Thus, in order to ensure that
only the part to be oxidized is oxidized, the foregoing process increases
the Al content of the layer in which the oxidation is to be caused. In
this way, it is possible to obtain a current confinement structure by a
selective oxidation process. In view of this, the Al content of the AlGaAs
layer forming the low-refractive index layer of the semiconductor
distributed Bragg reflector is set smaller (Ga content is increased) than
the Al content of the Al(Ga)As/GaAs oxidation layer. The composition of
Al.sub.x Ga.sub.1-x As (x=0.97) is used for the selectively oxidized layer
in the foregoing Japanese Patent 2,917,971, while a composition of
Al.sub.x Ga.sub.1-x As (x=0.92) is used for the low-refractive index layer
of the semiconductor distributed Bragg reflector.
In the art of forming a current confinement structure by such a selective
oxidation process, an approach is adopted to oxidize an AlAs layer from a
sidewall surface thereof. In order that such a process is to be conducted,
it is necessary to remove unnecessary layers by means of a mesa etching
process such that the sidewall surface of the AlAs layer to be oxidized is
exposed. However, in view of variation in the etching rate, there may be
caused variation of mesa height within a lot. Further, there may be caused
a lot-to-lot variation of mesa height. When such a variation has been
caused, the device characteristic may be scattered correspondingly.
Current optical-fiber telecommunication technology uses a laser diode of
long wavelength band of 1.3 .mu.m or 1.55 .mu.m for utilizing the
wavelength slot of quartz optical fibers in which the optical loss is
minimum. The optical fiber telecommunication system is spreading rapidly
and it is expected that it may reach a subscriber terminal (Fiber To The
Home; FTTH) in a near future. Furthermore, the technology of information
transmission by way of optical signals is going to be introduced even to a
device-to-device interconnection system inside an apparatus or even to an
interconnection system inside a device. Like this, the technology of
information transmission will become important still more. In order to
realize such an optical interconnection system, it is essential to realize
an optical telecommunication module of unprecedented low-cost. Thus, there
is a keen demand for a small, long wavelength-band laser diode of low
electric power consumption, with excellent temperature characteristics,
capable of eliminating the need of a cooling system.
Currently, the material of GaInPAs system formed on an InP substrate, which
is a group III-V semiconductor material, monopolizes the market. It should
be noted that the material of the GaInPAs system can be tuned to the
foregoing wavelength band. However, the material of the InP system has a
drawback, because of the small discontinuity in the conduction band
between the cladding layer (spacer layer) and the active layer, in that
the electrons injected into the active layer are poorly confined,
particularly when temperature of the device is increased. This results in
a decrease of efficiency. Further, the materials that achieve lattice
matching with an InP substrate cannot provide large refractive index
difference suitable for realizing a semiconductor distributed Bragg
reflector. As a result, the vertical-cavity, surface-emission-type laser
diode of the long wavelength having a performance suitable for practical
use has not been obtained.
The material of the GaInNAs system formed on a GaAs substrate is proposed
in the Japanese Laid-Open Patent Application No. 6-37355, as the material
that can settle the foregoing problems. It should be noted that GaInNAs is
a group III-V mixed crystal containing N in addition to other group V
element. In the system of GaInNAs, it is possible to achieve lattice
matching with a GaAs substrate by adding N to GaInAs having a lattice
constant larger than that of GaAs. By doing so, the bandgap energy is
reduced also. Thus, it becomes possible to realize optical emission at the
wavelength band of 1.3 .mu.m or 1.5 .mu.m. Kondou, et al., calculated the
band lineup of this system in the article, Jpn. J. Appl. Phys. Vol. 35
(1996), pp. 1273-1275. As this is a material that can achieve lattice
matching with GaAs, a large band discontinuity can be realized by using
AlGaAs for the cladding layer. Because of this, there is an expectation
that a laser diode having a high characteristic temperature may be
realized by using such a material. Further, it should be noted that the
material of GaInNAs can be formed on a GaAs substrate. Thus, it becomes
possible to construct a/the semiconductor multilayer reflector by using an
Al(Ga)As/GaAs material system. Thereby, it becomes possible to reduce the
number of stacking in the multilayer reflector significantly as compared
with the case of forming the multilayer reflector on the InP substrate.
Further, it becomes possible to form an AlAs selective-oxidation layer as
the current confinement structure, and the operating current is reduced
effectively.
However, the problem noted above arises in the case the material system of
Al(Ga)As/GaAs is used for the semiconductor multilayer reflector, as
proposed in the Japanese Laid-Open Patent Application 10-303515 or
Japanese Laid-Open Patent Application 11-145560. Further, the problem
similar to above arises also in the case an AlAs selective-oxidation layer
is used for the current confinement structure.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a
novel and useful vertical-cavity, surface-emission-type laser diode and
the process of making the same wherein the foregoing problems are
eliminated.
Another and specific object of the present invention is to provide a
vertical-cavity, surface-emission-type laser diode having excellent
reliability and easily fabricated, without increasing the total thickness
thereof.
Another object of the present invention is to provide a vertical-cavity,
surface-emission-type laser-diode array, an optical transmission module,
an optical transceiver module, and an optical telecommunication system.
Another object of the present invention is to provide a vertical-cavity,
surface-emission-type laser diode having an optical cavity structure on or
above a semiconductor substrate, the optical-cavity structure comprising
an active region containing at least one active layer that produces a
laser beam, and upper and lower reflectors sandwiching the active region
to form the optical cavity, the lower reflector including a semiconductor
distributed Bragg reflector having a refractive index that changes
periodically, the lower reflector reflecting an optical beam incident
thereto by diffraction, the semiconductor distributed Bragg reflector
comprising a low-refractive-index layer of Al.sub.x Ga.sub.1-x As
(O<x.ltoreq.1) and a high-refractive-index layer of Al.sub.y Ga.sub.1-y
As (0.ltoreq.y<x.ltoreq.1), wherein a non-optical recombination
elimination layer is provided between the active layer and the lower
reflector.
According to the present invention, a non-optical recombination elimination
layer is provided between the active layer and the lower reflector in the
construction in which the active region (an active layer is included), in
which injection of carriers is made, is sandwiched by the upper and lower
reflectors. Thus, the phenomenon that the crystal defects that originate
from Al crawl up to the active layer at the time of crystal growth is
effectively restrained, even in the case the lower reflector is formed of
a semiconductor distributed Bragg reflector including a semiconductor
layer that contains Al. Thereby, the adversary effect caused by the
defects is suppressed, and the active layer can be formed with high
crystal quality. Accordingly, non-optical recombination caused by the
crystal defects that originate from Al is reduced, and the efficiency of
optical emission and the reliability of the laser diode are improved. As
compared with the case in which the low-refractive index layers of the
semiconductor distributed Bragg reflector are all formed of Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1), the
semiconductor distributed Bragg reflector of the present invention can
maintain a large refractive index difference. It should be noted that the
reflector of the present invention is formed mostly of the material of the
AlGaAs system. Thus, a reflectance is achieved for the reflectors with
fewer number of stacking. Because of this, it is possible to obtain the
high reflectance without increasing the number of stacking of in the
reflector or increasing the total thickness of the device. In the laser
diode of the present invention, the total thickness of the
vertical-cavity, surface-emission-type laser diode does not increase, and
the operating current is small. Further, the laser diode has excellent
reliability. As such, the vertical-cavity, surface-emission-type laser
diode can be produced easily.
Another object of the present invention is to provide a vertical-cavity,
surface-emission-type laser diode having an optical cavity on or above a
semiconductor substrate, the optical cavity comprising an active region
containing at least one active layer that produces a laser beam, and upper
and lower reflectors sandwiching the active region to form the optical
cavity, each of the upper and lower reflectors including a semiconductor
distributed Bragg reflector in which a refractive index is changed
periodically, the upper and lower reflectors reflecting an optical beam
incident thereto, the semiconductor distributed Bragg reflector comprising
a low-refractive-index layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1)
and a high-refractive-index layer of Al.sub.y Ga.sub.1-y As
(0.ltoreq.y<x.ltoreq.1), wherein a non-optical recombination
elimination layer is provided between the active layer and the lower
reflector and a non-optical recombination elimination layer is provided
between the active layer and the upper reflector.
According to the present invention, a non-optical recombination elimination
layer is provided between the active layer and each of the lower and upper
reflectors in the construction in which the active region (an active layer
is included), in which injection of carriers is made, is sandwiched by the
upper and lower reflectors. Thus, the phenomenon that the crystal defects
that originate from Al crawl up to the active layer at the time of crystal
growth is effectively restrained, even in the case the lower reflector is
formed of a semiconductor distributed Bragg reflector including a
semiconductor layer that contains Al. Particularly, the active region, in
which carrier injection occurs, is sandwiched by the non-optical
recombination at both top part and bottom part thereof. Thereby,
non-optical recombination caused by the crystal defects that originate
from Al is reduced particularly effectively, and the efficiency of optical
emission and the reliability of the laser diode are improved easily. While
the effect of the non-optical recombination elimination layer is obtained
when it is inserted to one of the reflectors, the construction in which
the non-optical recombination elimination layer is provided to each of the
reflectors is extremely effective for eliminating the influence of the Al
defects. As compared with the case in which the low-refractive index
layers of the semiconductor distributed Bragg reflector are all formed of
Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1),
the semiconductor distributed Bragg reflector of the present invention can
maintain a large refractive index difference. It should be noted that the
reflector of the present invention is formed mostly of the material of the
AlGaAs system. Thus, a reflectance is achieved for the reflectors with
fewer number of stacking. Because of this, it is possible to obtain the
high reflectance without increasing the number of stacking of in the
reflector or increasing the total thickness of the device. In the laser
diode of the present invention, the total thickness of the
vertical-cavity, surface-emission-type laser diode does not increase, and
the operating current is small. Further, the laser diode has excellent
reliability. As such, the vertical-cavity, surface-emission-type laser
diode can be produced easily.
By using Ga.sub.x In.sub.1-x PyAs.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) for the non-optical recombination elimination layer in
combination with a GaAs substrate, the carriers that cause a leak to the
layer containing Al through the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) is eliminated substantially
particularly in the case that the bandgap of the material used for the
active layer is smaller than that of GaAs, in view of the fact that the
bandgap of the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer, which is substantially free from Al (Al content is
1% or less with regard to the group III elements), is larger than the
bandgap of GaAs. Because of this, non-optical recombination can be
prevented. Accordingly, a vertical-cavity, surface-emission-type laser
diode, operating with small current and having excellent reliability is
realized.
In the case the lattice constant of the Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (0<x.ltoreq.1,0<y.ltoreq.1) layer is smaller than the
lattice constant of the GaAs substrate, the Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (0<x.ltoreq.1,0<y.ltoreq.1) layer accumulates a tensile
strain therein. Thus, crawling-up of defects from the substrate to a
growth layer during a growth process is effectively suppressed. As a
result, the efficacy of optical emission is improved. Further, it becomes
possible to grow a layer accumulating a compressive strain of 2% or more,
for example, as the active layer. Furthermore, it becomes possible to grow
a strained layer with a thickness exceeding the critical film thickness.
In view of the fact that the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layer contacts with the active region,
and in view of the fact that the bandgap energy of the Ga.sub.x In.sub.1-x
P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer become larger
with decreasing lattice constant, the hetero-barrier height between the
active region and the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) is increased. Thus, the efficiency of
carrier confinement is improved. Thereby, improvement with regard to
temperature characteristics and threshold current are achieved.
In the case the lattice constant of the Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer is larger than the
lattice constant of the GaAs semiconductor substrate, the Ga.sub.x
In.sub.1-x PyAs.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) accumulates a
the compressive strain. Thus, crawling up of defects formed during the
growth process or existing in the substrate to the growth layer is
suppressed, and the efficiency of optical emission is improved. Further,
it becomes possible to grow a layer accumulating a compressive strain of
2% or more. Further, it becomes possible to grow a strained layer with a
thickness exceeding the critical thickness.
In the case the sense of strain of the Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer is the same as the
sense of the strain of the active layer, there is an effect, in addition
to the above-noted effect of insertion of the strained layer, in that the
compressive strain that the active layer senses is reduced substantially.
Thus, the adversary effect of the defects existing on the surface of a
foundation layer, on which the growth is made, in the state immediately
before the start of the growth process is reduced substantially. As a
result, the crystal quality of the active layer improved and the
characteristics of the laser diode are improved. Especially, this
improvement is effective in vertical-cavity, surface-emission-type laser
diode of long wavelength band in which growth of thick film is necessary.
The non-optical recombination elimination layer of the GaInPAs system
containing P functions as an etching stopper with respect to the layer of
the AlGaAs system that contains Al as a principal component. Because of
this, the height of the mesa structure provided by a wet etching process
for selective oxidation process is controlled exactly. By using the mesa
structure, it becomes possible to form a current confinement layer by
selectively oxidizing the layer that contains Al and As at the location
above the non-optical recombination elimination layer. In this way, the
accuracy of process control is improved. Further, the homogeneity and
reproducibility is improved with regard to the device characteristics.
Furthermore, the yield is improved, and the fabrication cost it reduced.
Further, according to the present invention, it becomes possible to form a
vertical-cavity, surface-emission-type laser diode for use in long
wavelength band of 0.9.mu.m or more on a GaAs substrate by using any of
GaInNAs or GaInAs.
By providing a compressive strain of 2.0% or more to the active layer in
the present invention, it becomes possible to realize a vertical-cavity,
surface-emission-type laser diode operable at the wavelength hitherto not
possible. For example, by using GaInAs for the active layer, it becomes
possible to provide a vertical-cavity, surface-emission-type laser diode
operable at the wavelength of 1.1 .mu.m or longer. By using GaInNAs for
the active layer, the crystal quality of the active layer is improved, and
the threshold current density is reduced. Thereby, it becomes possible to
provide a vertical-cavity, surface-emission-type laser diode having
excellent reliability and still operable at the wavelength band of 1.3
.mu.m or longer.
By arranging such a vertical-cavity, surface-emission-type laser diode in
the form of one-dimensional or two-dimensional array, it is possible to
provide a vertical-cavity, surface-emission-type laser-diode array with
excellent homogeneity and reproducibility. In the case of forming an
array, in-plane homogeneity influences the element-to-element variation of
characteristics. As noted before, it is possible to use the crystal layer
GaInPAs system as an etching stopper with respect to the crystal layer of
AlGaAs system. Because of this, the height of the mesa structure used for
the selective oxidation process is controlled exactly over the elements
included in the array. Because of this, not only the precision of process
control at the time of device fabrication is improved, but also the
homogeneity of characteristics between the elements in the array and
reproducibility of the vertical-cavity, surface-emission-type laser-diode
array are improved also.
By using the vertical-cavity, surface-emission-type laser diode or the
laser-diode array of the present invention as an optical source, in other
words by using the vertical-cavity, surface-emission-type laser diode
low-cost, high-quality and excellent reliability for the optical source, a
low cost, highly efficient and reliable optical transmission module is
realized.
By using the vertical-cavity, surface-emission-type laser diode or
laser-diode array of the present invention as an optical source, in other
words by using the vertical-cavity, surface-emission-type laser diode
low-cost, high-quality and excellent reliability for the optical source, a
low cost, highly efficient and reliable optical transceiver module is
realized.
By using the vertical-cavity, surface-emission-type laser diode or
laser-diode array of the present invention as an optical source, in other
words by using the vertical-cavity, surface-emission-type laser diode
low-cost, high-quality and excellent reliability for the optical source, a
low cost, highly efficient and reliable optical telecommunication,
including an optical-fiber telecommunication system and an optical
interconnection system, is realized.
By providing a process for removing residual Al source material, residual
Al product, residual Al compound or residual Al from a location such as
the gas supply line or growth chamber, in which contact with a nitrogen
compound source material or impurity included therein tends to occur, in
the interval after the growth of the semiconductor layer containing Al but
before the start of growth of the active layer that contains nitrogen in
the fabrication process of the vertical-cavity, surface-emission-type
laser diode, it becomes possible in the present invention to decrease the
amount of oxygen taken into the active layer that contains nitrogen during
the growth process of the active layer. Thereby, it becomes possible to
grow the semiconductor light-emitting device without decreasing the
efficiency of optical emission even in the case the active layer
containing nitrogen is formed on the upper part of the semiconductor layer
containing Al in the semiconductor light-emitting device.
By providing a process for removing residual Al source material, residual
Al product, residual Al compound or residual Al from a location such as
the gas supply line or growth chamber, in which contact with a nitrogen
compound source material or impurity included therein tends to occur, in
the interval after the growth of the semiconductor layer containing Al and
before the end of growth of the non-optical recombination elimination
layer in the fabrication process of the vertical-cavity,
surface-emission-type laser diode, it becomes possible in the present
invention to decrease the amount of oxygen taken into the active layer
that contains nitrogen during the growth process of the active layer.
Further, the adversary effect of non-optical recombination originating
caused by oxygen taken into the growth interrupt interface at the time
electric current is injected for device operation is successfully
eliminated. Thereby, it becomes possible to obtain the semiconductor
light-emitting device having a high efficiency of optical emission even in
the case the active layer containing nitrogen is formed on the upper part
of the semiconductor layer containing Al in the semiconductor
light-emitting device.
By providing a process for removing residual Al source material, residual
Al product, residual Al compound or residual Al from a location such as
the gas supply line or growth chamber, in which contact with a nitrogen
compound source material or impurity included therein tends to occur, in
the fabrication process of the vertical-cavity, surface-emission-type
laser diode by an MOCVD process that uses source materials of at least a
metal organic Al source and a nitrogen compound source, it becomes
possible to improve the efficiency of optical emission of the
semiconductor light-emitting device as compared with the case in which no
such a removal is made.
Other objects and further features of the present invention will become
apparent from the following detailed description when read in conjunction
with the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are diagrams showing the construction of a vertical-cavity,
surface-emission-type laser diode according to a first embodiment of the
present invention;
FIGS. 2A and 2B are diagrams showing the construction of a vertical-cavity,
surface-emission-type laser diode according to a second embodiment of the
present invention;
FIGS. 3A and 3B are diagrams showing a first constitutional example of the
vertical-cavity, surface-emission-type laser diode of a sixth embodiment
of the present invention;
FIGS. 4A and 4B are diagrams showing a second constitutional example of the
vertical-cavity, surface-emission-type laser diode of the sixth embodiment
of the present invention;
FIG. 5 is a diagram showing theoretical value and experimental value of
critical thickness for a system of GaInAs layer formed on a GaAs
substrate;
FIG. 6 is a diagram showing the relationship between PL central wavelength
and PL intensity for the PL emission occurring in a GaInAs single quantum
well layer;
FIG. 7 is a diagram showing an example of relationship between the
threshold current density and nitrogen contents for a GaInNAs laser diode
(edge-emission type) having an In content of 10%;
FIG. 8 is a diagram showing the relationship between oscillation wavelength
and threshold current density of a GaInAs/GaAs-DQW laser diode of the
present invention having a high compressive strain;
FIGS. 9A and 9B are diagrams showing the construction of a vertical-cavity,
surface-emission-type laser diode of Example 1;
FIGS. 10A and 10B are diagrams showing the construction of a
vertical-cavity, surface-emission-type laser diode of Example 2;
FIG. 11 is a diagram showing the overall construction of an optical
transmission module that combines a 1.3 .mu.m band GaInNAs
vertical-cavity, surface-emission-type laser diode of Example 2 and a
quartz optical fiber;
FIG. 12 is a diagram showing the overall construction of an optical
transceiver module that combines a 1.3 .mu.m band GaInNAs vertical-cavity,
surface-emission-type laser diode of Example 2 and a receiver photodiode
with an optical fiber;
FIG. 13 is a diagram showing a room temperature photoluminescence spectrum
from an active layer formed of GaInNAs/GaAs double quantum well structure;
FIG. 14 is a diagram showing a basic structure of the sample used in a
thirteenth embodiment of the present invention;
FIG. 15 is a diagram showing the depth distribution profile of nitrogen (N)
and oxygen (O) in a laser diode of FIG. 14 for the case the laser diode
has an active layer of GaInNAs/GaAs double quantum well structure, a GaAs
intermediate layer and an AlGaAs cladding layer, for the case the laser
diode is formed by using a single epitaxial growth (MOCVD) apparatus;
FIG. 16 is a diagram showing a depth distribution profile of Al in the same
sample of FIG. 15;
FIG. 17 is a diagram showing an example of the laser diode device according
to a fourteenth embodiment of the present invention;
FIG. 18 is a diagram showing a depth distribution profile of Al in depth
direction of the laser diode of FIG. 17 for case the growth crystal layers
is interrupted between the a first lower intermediate layer and a second
lower intermediate layer and conducted a purging process for 60 minutes;
FIG. 19 is a diagram showing a depth distribution profile of nitrogen (N)
and oxygen (O) in the same device as FIG. 18;
FIGS. 20A and 20B are diagrams showing the construction of vertical-cavity,
surface-emission-type laser diode of Example 7; and
FIGS. 21A and 21B are diagrams showing the construction of a
vertical-cavity, surface-emission-type laser diode of Example 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
FIGS. 1A and 1B are diagrams showing a constitutional example of a
vertical-cavity, surface-emission-type laser diode according to a first
embodiment of the present invention, wherein FIG. 1B is an enlarged view
of the active region of FIG. 1A.
Referring to FIGS. 1A and 1B, the vertical-cavity, surface-emission-type
laser diode of the first embodiment includes an active region 3 on a
semiconductor substrate 1, wherein the active region 3 includes at least
one an active layer 2 that produces a laser beam therein. Further, it
includes a cavity structure formed of upper reflector 4 and a lower
reflector 5 that sandwich the active layer from upward direction and
downward direction for obtaining the laser beam. Each of the upper
reflector 4 and the lower reflector 5 is formed of a semiconductor
distributed Bragg reflector having a periodically changing refractive
index, wherein the semiconductor distributed Bragg reflector reflects an
incident optical beam incident thereto by optical interference. The
semiconductor distributed Bragg reflector includes a low-refractive index
layer of Al.sub.x Ga.sub.1-x As (0<x.ltoreq.1) and a high-refractive
index layer of Al.sub.y Ga.sub.1-y As (.sub.0.ltoreq.y<x.ltoreq.1).
Further, a non-optical recombination elimination layer 6 is provided
between the active layer 2 and the lower reflector 5.
In the first embodiment, it is noted that the semiconductor distributed
Bragg reflector forming the lower reflector 5 includes a semiconductor
layer containing Al. On the other hand, it is also noted that the
non-optical recombination elimination layer 6 is provided between the
active layer 2 and the lower reflector 5. Because of this, crawling up of
crystal defects, originating from Al and tend to be caused at the time of
crystal growth of the active layer 2, to the active layer 2 is suppressed
in the active region 3 (includes the active layer 2) sandwiched by the
upper reflector 4 and the lower reflector 5. Thereby, the adversary
influence associated with the Al defects is suppressed, and the active
layer 2 can be formed with high crystal quality. In this active region 3,
carrier injection is made. Thereby, non-optical recombination originating
from crystal defects, which in turn are caused by Al, is effectively
reduced, and the vertical-cavity, surface-emission-type laser diode of the
present invention can be operated with reliability. Further, efficiency of
optical emission and reliability are improved. As compared with the case
in which all the low-refractive index layers of the semiconductor
distributed Bragg reflector forming the lower reflector 5 are formed of
Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1)
a large refractive index difference is achieved in the present invention.
Because of this, a high reflectance is achieved with fewer number of
stacking. In other words, it is possible to obtain the above-mentioned
effect without increasing the number of stacking of the reflector, or
without increasing the total film thickness of the device.
It is possible to construct the reflector 4 by a dielectric multilayer
mirror. In this case, the cladding layer used for confining the carriers
may be provided between the upper reflector 4 and the active layer 6. In
the construction of FIG. 1A, the optical output is taken out from the
upper reflector 4. However, it is also possible to take out the optical
output from the substrate 1.
Second Embodiment
FIGS. 2A and 2B show the constitutional example of a vertical-cavity,
surface-emission-type laser diode according to a second embodiment of the
present invention, wherein FIG. 2B shows an enlarged view of the active
region of FIG. 2A.
Referring to FIGS. 2A and 2B, the vertical-cavity, surface-emission type
laser diode of the second embodiment of the present invention has an
optical cavity structure including an upper reflector 14 and a lower
reflector 15 that sandwich an active region 13 therebetween respectively
from upward direction and from downward direction on a semiconductor
substrate 11 for obtaining a laser beam, wherein the active region 13
includes at least one active layer 12 that produces the laser beam. Each
of the upper reflector 14 and the lower reflector 15 has a refractive
index profile that changes periodically and forms a semiconductor
distributed Bragg reflector that reflects an incident optical beam
incident thereto by optical interference. The semiconductor distributed
Bragg reflector includes a low- refractive index layer of Al.sub.x
Ga.sub.1-x As (0<x.ltoreq.1) and a high-refractive index layer of
Al.sub.y Ga.sub.1-y As (0.ltoreq.y<x.ltoreq.1). Further, non-optical
recombination elimination layers 16 and 17 are provided respectively
between the active layer 12 and the lower reflector 15 and between the
active layer 12 and the upper reflector 14.
By providing the non-optical recombination elimination layers 16 and 17
respectively between the active layer 12 and the reflector 14 and also
between the active layer 12 and the reflector 15 in this second
embodiment, crawling up of the crystal defects, originating from Al and
tends to occur at the time of crystal growth of the active layer 12, to
the active layer is suppressed effectively, even in the case the upper and
lower reflectors 14 and 15 are formed of a semiconductor distributed Bragg
reflector that includes a semiconductor layer containing Al. Thereby,
adversary influence associated with this is also suppressed, and crystal
defects caused by Al are reduced. Thus, the active layer 12 can be grown
with high crystal quality. Furthermore, in view of the fact that the
non-optical recombination elimination layers 16 and 17 are formed at the
top and bottom of the active region 13 in which injection of carriers is
conducted, the non-optical recombination caused by the crystal defects
originating from Al is also reduced. Thereby, the efficiency of optical
emission is improved, and the vertical-cavity, surface-emission-type laser
diode can be operated with reliability. Also, the reliability of the
device is not failed by the crystal defects. It is true that insertion the
non-optical recombination elimination layer to only one side of the
reflectors, as in the case of the first embodiment, is effective. When the
non-optical recombination elimination layer is provided on the reflectors
both sides, as in the case of the second embodiment, the effect is
enhanced. Also, a large refractive index difference is realized in the
upper and lower reflectors 14 and 15 as compared with the case in which
all of the low-refractive index layers of in the semiconductor distributed
Bragg reflector are formed of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1). Because of this, a high reflectance is
obtained with fewer number of stacking. Thus, it is possible to obtain the
above-mentioned effect without increasing the number of stacking in the
reflectors and without increasing the total film thickness of the device.
In FIG. 2A, the optical output is taken out from the upper reflector 14.
However, it is possible to take out the optical output from the side of
the substrate 11.
Third Embodiment
In a third embodiment of the present invention, GaAs is used for the
semiconductor substrate 1 or 11 in the vertical-cavity,
surface-emission-type laser diode of the above first or second embodiment.
Further, the non-optical recombination elimination layer 6, 16 or 17 is
formed of a Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer.
It should be noted that the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1,0<y<1) layer is free from Al (Al content with
regard to group III elements is 1% or less) in the present embodiment and
has a bandgap that can be larger than that of GaAs. Thus, by using the
non-optical recombination elimination layers 6, 16 or 17 of Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) in
combination with an active layer having a bandgap smaller than that of
GaAs, it become possible to eliminate the leakage of carriers to the layer
containing Al through the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layer. In this way, non-optical
recombination is effectively and positively eliminated.
Fourth Embodiment
In a fourth embodiment of the present invention, the lattice constant of
the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer used for the non-optical recombination elimination
layer in the vertical-cavity, surface-emission-type laser diode the third
embodiment is set smaller than the lattice constant of the semiconductor
substrate of GaAs. Because of this, the non-optical recombination
elimination layer of the present embodiment accumulates a tensile strain
therein.
In an epitaxial growth process, a crystal layer is grown while reflecting
the information of a foundation on which the epitaxial grow is made. Thus,
when there is a defect on the substrate surface, the defect crawls up to
the layer grown on the substrate. On the other hand, it is known that such
a crawling up of the defect can be suppressed when there is provided a
strained layer. In the event the defect has reached the active layer, the
efficiency of optical emission is inevitably reduced. When the active
layer has accumulated a strain therein, there occurs a decrease of
critical thickness, and there arise problems such as growth of a layer
with necessary thickness is not possible. Especially, the problem of
failing to grow a layer due to the existence of defects arises in the case
the compressive strain in the active layer is 2% or more, or in the case
of growing a strained layer beyond the critical film thickness. In such
cases, the growth is not possible even when a low-temperature growth
process or other non-equilibrium growth process is employed. In the
present invention, the lattice constant of the Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer used for the
non-optical recombination elimination layer is set smaller than the
lattice constant of the semiconductor substrate formed of GaAs, and thus,
the non-optical recombination elimination layer accumulates a tensile
strain, and the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer forming the non-optical recombination elimination
layer becomes a strained layer. As a result, the problem of crawling up of
defects is effectively suppressed. Thereby, the efficiency of optical
emission is improved and it becomes possible to grow the active layer even
in the case the active layer accumulates a compressive strain of 2% or
more. Further, it becomes possible to grow a strained layer with a
thickness exceeding the critical thickness.
In the present embodiment, the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layer used for the non-optical
recombination elimination layer makes contact with the active region and
functions also to confine the carriers in the active region. In the case
of a Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer, it should be noted that the bandgap energy
increases with decreasing lattice constant. In the case of Ga.sub.x
In.sub.1-x P (y=1), for example, the composition approaches to GaP with
increasing compositional parameter x. Associated therewith, there occurs
an increase of lattice constant and also the bandgap energy Eg. It should
be noted that the bandgap Eg for direct transition is given as Eg(.left
brkt-top.)=1.351+0.634x+0.786x.sup.2, while the bandgap energy for the
case of indirect transition is given by Eg(X)=2.24+0.02x. Therefore, the
hetero-barrier height formed between the active region and the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer is
increased and the carrier confinement is improved. As a result of this,
effects such as decrease of threshold current or improvement of
temperature characteristics are obtained.
Fifth Embodiment
In a fifth embodiment of the present invention, the lattice constant of the
Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1)
layer, used for the non-optical recombination elimination layer in the
vertical-cavity surface-emission type laser diode of the third embodiment,
is set to be larger than that of the semiconductor substrate of GaAs.
Because of this, the non-optical recombination elimination layer
accumulates a compressive strain therein. Further, the lattice constant of
the active layer is set larger than the lattice constant of the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer.
Thus, the active layer accumulates a compressive strain.
As noted previously, growth of an epitaxial layer occurs while reflecting
the information of the foundation on which the epitaxial growth is made.
Therefore, the defect tends to crawl up to the grown layer when there is a
defect existing on the substrate surface. On the other hand, it is known
that the crawling up of defects like this is effectively suppressed, when
there is provided a strained layer. When the above-noted defects have
reached the active layer, the efficacy of optical emission is degraded
inevitably. Meanwhile, there occurs a decrease of critical thickness in
the active layer when the active layer accumulates a strain. Because of
this, the problem that growth of the active layer with necessary thickness
is not possible arises, especially when growing the active layer
accumulating a compressive strain of 2% or more or when growing the
strained active layer with a thickness larger than the critical film
thickness. In such a case, because of the existence of defects, the growth
is not possible even when a low-temperature growth process or other
non-equilibrium growth processes are employed. In the present invention,
the lattice constant of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layer used for the non-optical
recombination elimination layer is set larger than the lattice constant of
the semiconductor substrate formed of GaAs. Because of this, the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1)
non-optical recombination elimination layer accumulates therein a
compressive strain, and the crawling up of defects noted above is
suppressed. Thereby, the efficiency of optical emission is improved. As a
result, growth of an active layer accumulating a compressive strain of 2%
or more or growth of the strained active layer beyond the critical
thickness thereof becomes possible.
Furthermore, it should be noted that the strain accumulated in the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer
works with the sense identical with the sense of the strain accumulated in
the active layer. Because of this, the strain accumulated in the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer
works so as to reduce the compressive strain, which the active layer
senses. In view of the fact that influence of external factor increases
with increasing strain, the construction of the present embodiment is
especially effective in the case the compressive strain of the active
layer is large, such as 2% or more, or in the case the thickness of the
active layer exceeds the critical thickness.
The vertical-cavity, surface-emission-type laser diode of 1.3 .mu.m band is
preferably formed on a GaAs substrate. Further, semiconductor multilayer
reflector is used frequently for the resonator. Thereby, it is necessary
to grow the semiconductor layers of 50-80 layers before the growth of the
active layer, so that the thickness becomes 5-8 .mu.m.
In such a case, even when a GaAs substrate of high quality is used, the
defective density at the surface on which the active layer is grown in the
state immediately before the growth of the active layer increases
inevitably over the defective density of the GaAs substrate surface, due
to various reasons. For example, the defect that has once occurred crawls
up in the direction of crystal growth. Also, defects can be formed at a
hetero interface. When the actual compressive strain that the active layer
senses is reduced, or when a strained layer is inserted in the state
before the growth of the active layer is started, it becomes possible to
reduce the influence of the defects existing on the surface ready for
growth of the active layer thereon.
Sixth Embodiment
FIGS. 3A and 3B are diagrams (FIG. 3B is an enlarged view of the active
region of FIG. 3A) showing a constitutional example of a vertical-cavity,
surface-emission-type laser diode according to a sixth embodiment of the
present invention. Also, FIGS. 4A and 4B are the diagrams (FIG. 4B is an
enlarged view of the active region of FIG. 4A) showing a second
constitutional example of the vertical-cavity, surface-emission-type laser
diode of the sixth embodiment of the present invention. It should be noted
that FIGS. 3A and 3B correspond to the vertical-cavity surface-emission
type laser diode of the first embodiment (FIG. 1), while FIGS. 4A and 4B
correspond to the vertical-cavity surface-emission type laser diode of the
second embodiment (FIG. 2).
Referring to FIGS. 3A and 3B and FIGS. 4A and 4B, the vertical-cavity,
surface-emission-type laser diode of the sixth embodiment includes current
confinement layers 8 and 18 formed by selective oxidization of a
selectively oxidized layer, formed primarily of AlAs, on the upper part of
the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer (designated by the reference numeral 6 in FIGS. 3A
and 3B, designated by the reference numeral 16 in FIGS. 4A and 4B), used
for the non-optical recombination elimination layer.
The GaInPAs system layer including P and used for the non-optical
recombination elimination layer in the present embodiment functions also
as an etching stopper layer with respect to the layer of AlGaAs system
that contains Al and As as primary components. Because of this, the height
of the mesa structure, which is formed for the purpose of selective
oxidation process of the current confinement layer 8 or 18, is controlled
exactly, in the case the current confinement layer 8 or 18, which is
subjected to selective oxidation process and contains Al and As as primary
components, is formed in the upper part of the Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layer (represented by the
reference numeral 6 in FIG. 3 and by the reference numeral 16 in FIG. 4).
In this way, the accuracy of control of device fabrication process is
improved in the sixth embodiment. Further, the homogeneity and
reproducibility of the device characteristic are improved. Further, the
fabrication cost is reduced.
Seventh Embodiment
In the seventh embodiment of the present invention, the active layer 2 or
12 in any of first through sixth embodiments described before, is formed
by any of GaInNAs or GaInAs.
By using GaInNAs or GaInAs in the active layer 2 or 12, it becomes possible
to construct a vertical-cavity, surface-emission laser diode of the
wavelength band of 0.9 .mu.m or longer on a GaAs substrate. In this case,
the material system of AlGaAs/GaAs can be used and a large refractive
index difference is achieved. Therefore, the total number of stacks in the
semiconductor distributed Bragg reflector is reduced as compared with the
case of forming the laser diode on an InP substrate, while simultaneously
realizing a higher reflectance. In view of the fact that a widegap
material can be formed to a GaAs substrate, it is possible in the present
embodiment to increase the band discontinuity with respect to the active
layer. As a result, the efficiency of carrier confinement is improved and
a vertical-cavity, surface-emission type laser diode of long wavelength
band is obtained with excellent temperature characteristics. It should be
noted that the semiconductor distributed Bragg reflector forming the lower
reflector 5 or 15 or the upper reflector 14 provides a larger refractive
index difference as compared with the case in which all the low-refractive
index layers are formed of the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layer. Because of this, a high
reflectance is obtained with fewer number of stacking. Thus, the laser
diode of the present embodiment can reduce the crystal defects originating
from and also the non-optical recombination, without increasing the number
of stacking of the reflector and the total film thickness of the device.
As a result, the reliability of the vertical-cavity, surface-emission type
laser is improved.
Eighth Embodiment
In the eighth embodiment of the present invention, the active layer 2 or 12
accumulates a compressive strain of 2.0% or more in any of the first
through seventh embodiments.
When a layer is grown on an underlying substrate with a lattice constant
different from that of the substrate, the lattice deforms elastically and
absorbs the energy. When a material having a lattice constant different
from the lattice constant of the underlying substrate is growth with large
thickness, on the other hand, there comes a point in which absorption of
strain energy by elastic deformation is no longer possible and a misfit
dislocation appears. This film thickness is called the critical thickness.
It is difficult to produce a good device when the misfit dislocation has
resulted.
Theoretically, the critical thickness (h.sub.c), in which the misfit
dislocation appears by a dynamic process, is given by the following
equation proposed by Matthews and Blakeslee (J. Crystal Growth. 27, (1974)
pp. 118-125)
##EQU1##
wherein .upsilon. represents Poisson's ratio (.upsilon.=C.sub.12 /(C.sub.21
+C.sub.12); C.sub.12 and C.sub.21 being elastic stiffness constant),
.alpha. is the angle formed by a Burger's vector and a line segment of a
dislocation line at the interface (cost .alpha.=1/2), .lambda. represents
the angle formed between an intersection line of a slip surface and the
interface and the Burger's vector (cos .lambda.=1/2), b is given as
b=a/2.sup.1/2 (a; lattice constant), f represents the degree of lattice
mismatching and hence strain represented as f=.DELTA.a/a, .DELTA.a being a
difference of lattice constant with respect to the underlying substrate. A
compressive strain appears for the case in which the lattice constant of
the material grown on the substrate is larger than the lattice constant of
the substrate. On the other hand, a tensile strain appears in the opposite
case. It should be noted that Eq. (1) is derived for the case in which a
single layer film is grown on a substrate of infinite thickness.
Hereinafter, the critical thickness hc given by Eq. (1) will be referred
to as critical thickness based on the theory of Matthews and Blakeslee.
FIG. 5 shows the critical film thickness of a GaInAs layer calculated by
the generally accepted theory of Matthews and Blakeslee noted above for
the case in which the GaInAs layer is formed on a GaAs substrate. It
should be noted that the lattice constant of Ga.sub.1-x In.sub.x N.sub.y
As.sub.1-y, a material system in which nitrogen (N) is added to Ga.sub.1-x
In.sub.x As, becomes equal to the lattice constant of Ga.sub.1-x In.sub.x
NAs in which the In content x is smaller by 3% (y=x-0.03) for addition of
nitrogen of every 1%. In the case of forming a GaInAs layer on the GaAs
substrate, increase of In content results in an increase of strain and the
critical film thickness, which is a film thickness in which a
two-dimensional growth is possible, is decreased. As a result of increase
of the In content, the oscillation wavelength of a semiconductor light
emission device (a laser diode) changes to long wavelength, in the case a
GaInAs layer grown on a GaAs substrate is used. However, there
simultaneously occurs an increase strain. The critical amount of strain in
this case is about 2%. Corresponding to this, there is a limit in the
increase of oscillation wavelength and it was said the wavelength of 1.1
.mu.m would be the limit. Reference should be made to IEEE Photonics
Technol. Lett. Vol. 9 (1197), pp. 1319-1321.
However, in practice, it is possible to form a material layer with In
content of 30% or more, in other words, a material that accumulates a
strain of 2% or more, in a GaInAs quantum well layer formed on a GaAs
substrate, to a thickness exceeding the critical film thickness hc of
Matthews and Blakeslee by using a non-equilibrium growth process such as
MOCVD process or MBE process under a strongly non-equilibrium conduction
such as lower temperature process conducted at about 600.degree. C. or
less. By doing so, the actual critical thickness (hc') exceeds the
critical thickness of Matthews and Blakeslee, and it becomes possible grow
a highly strained GaInAs quantum-well active layer having the strain of 2%
with a thickness larger than the thickness conventionally possible.
Because of this, a laser oscillation in the long wavelength band exceeding
1.2 .mu.m is obtained. Laser oscillation in such long wavelength band has
not been possible in conventional laser diodes and other semiconductor
light emission devices. It should be noted that a Si semiconductor
substrate is transparent with respect to this wavelength. In this way, it
becomes possible to achieve optical transmission through the Si substrate
in a circuit chip in which electron devices and optical devices are
integrated on the Si substrate. Further, highly efficient HEMT (High
electron mobility transistor) can be obtained.
FIG. 5 also represents experimental results.
Referring to FIG. 5, in the case the In content is 32% and the thickness is
8.6 nm, it can be seen that PL (photoluminescence) central wavelength is
1.13 .mu.m. In the case the In content is 36% and the thickness is 7.8 nm,
the PL central wavelength is 1.16 .mu.m. Also, in the case the In content
is 39% and the thickness is 7.2 nm, it can be seen that the PL central
wavelength is 1.2 .mu.m. In these cases, the critical thickness h.sub.c
calculated on the basis of theory of Matthews and Blakeslee Eq. (1) is
exceeded.
Further, FIG. 6 shows the relationship between the PL central wavelength
and PL intensity for a GaInAs single quantum well layer, wherein the In
content x of the GaInAs well layer (represented by continuous line part
the drawing) is set to 31-42%. Further, the thickness of the well layer is
decreased to 9-6 nm simultaneously with the increase of the In content x.
A strong PL intensity was obtained for the quantum well layer up to the
wavelength of about 1.2 .mu.m. It can be seen that the PL intensity
decreases gradually up to the wavelength of 1.2 .mu.m, while when the
wavelength of 1.2 .mu.m is exceeded, the PL intensity starts to fall off
sharply. This corresponds to the flatness of the layer surface. More
specifically, a mirror surface was obtained up to the wavelength of 1.2
.mu.m. From these results, it is interpreted that the above kind of sharp
decline of PL intensity is resulted because the thickness of the quantum
well layer has exceeded the critical thickness h.sub.c ' substantially. In
the case that a layer is grown with high growth rate by a strong
non-equilibrium growth process such as MOCVD process or MBE process
conducted at a low temperature, it is reported that there occurs an
increase in experimentally obtained critical thickness. Further, there is
a report that there occurs a three-dimensional growth, and hence
roughening of surface, even when the layer thickness is smaller than the
critical thickness based on the theory, depending on the growth condition
(as in the case of high-temperature growth). Therefore, it is understood
that the foregoing result reflects the situation in which the actual
critical film thickness h.sub.c ' realized in the case of non-equilibrium
growth at low-temperature exceeds the critical thickness hc based on the
theory, and because of this, it was possible to grow a two-dimensional
thick film without causing misfit dislocations.
In the case of the GaInNAs active layer, increase of strain means that it
is necessary to increase the In content for obtaining the same wavelength,
and because of this, it is possible to reduce the nitrogen concentration.
FIG. 7 shows the relationship between the threshold current density and
nitrogen content for a GaInNAs laser diode (edge-emission type) for the
case of the In content of 10%. From FIG. 7, it can be seen that the
threshold current density increases sharply with increase of the nitrogen
content. The reason of this is attributed to degradation of crystal
quality of the GaInNAs layer caused with the increase of the nitrogen
content.
FIG. 8 shows the relationship between the oscillation wavelength and the
threshold current density for the GaInAs/GaAs-MQW laser diode of the
present invention having a strong compressive strain. The oscillation
wavelength of a GaInAs/GaAs laser diode reported since before was about
1.1 .mu.m. According to the experimental results by the inventor of the
present invention, it was confirmed that operation up to the wavelength of
1.225 .mu.m is possible at room temperature. Moreover, it can be seen that
a low threshold is realized up to the wavelength of about 1.2 .mu.m. It
should be noted that these results reflect the PL characteristics shown in
FIG. 6. In the case nitrogen is added to the highly strained GaInAs
quantum well layer oscillating at about 1.2 .mu.m in the nitrogen-free
state, to obtain the wavelength of 1.3 .mu.m, an amount of 0.5% of
nitrogen is sufficient. It should be noted that about 3% of nitrogen had
to be added in the case of the lattice-matched thick active layer of FIG.
7 in which the In content is 10%. Accordingly, it will be understood that
the necessary amount of added nitrogen is reduced substantially.
Associated therewith, the degradation of crystal quality is suppressed,
and a highly efficient laser diode of the 1.3 .mu.m band is realized.
Thus, according to the present invention, the nitrogen content can be
reduced and the crystal quality is improved. As a result, the
characteristic of the GaInNAs laser diode is improved substantially.
Also, a vertical-cavity surface-emission-type laser diode of the 1.3 .mu.m
band is realized on a GaAs substrate by using a highly strained GaAsSb
active layer.
Conventionally, there was no material suited for a laser diode of 1.1-1.3
.mu.m wavelength bands. According to the present invention, this becomes
possible by using a highly strained GaInAs, GaInNAs or GaAsSb having a
strain of 2.0% or more. By using GaInNAs, especially, a further longer
wavelength becomes possible.
It should be noted that such highly strained materials are extremely
sensitive and are easily influenced by the strain of other layers, which
may not cause a problem in the case of active layers of smaller strain,
when the highly strained active layer is applied to a vertical-cavity
surface-emission-type laser diode consisting of a plural layers. However,
it becomes possible to eliminate this adversary influence by interposing
the Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) non-optical recombination elimination layer. For example,
the Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer is capable of adjusting the strain thereof by
adjusting the composition thereof. Because of this, the above kind of
adversary influence can be corrected by interposing the layer between the
reflector and the active layer. Further, Al is very reactive and tends to
become the cause of defects. On the other hand, it is said that the
problem of crawling up of the defects from a foundation side (substrate)
can be blocked when a layer containing In is provided. Accordingly, the
present invention eliminates this problem by providing such a layer
between the reflector containing Al and the active layer.
Ninth Embodiment
A ninth embodiment of the present invention provides a vertical-cavity,
surface-emission-type laser-diode array by arranging a plurality of
vertical-cavity, surface-emission-type laser diodes of the sixth
embodiment.
In the case that an array is formed by using the vertical-cavity,
surface-emission-type laser diode, in-plane homogeneity influences the
device-to-device variation of characteristics. In view of the fact that
AlGaAs system can be used as an/the etching stopper for a GaInPAs system,
the height of the mesa structure formed for selective oxidation process is
controlled exactly over the devices in the array. As a result, the
accuracy of process control at the time of device fabrication is improved,
and homogeneity and reproducibility of the device characteristic are
improved over the devices forming the array.
Tenth Embodiment
In a tenth embodiment of the present invention, an optical transmission
module is provided that uses a vertical-cavity, surface-emission-type
laser diode of any of first through ninth embodiments as a light source.
In the tenth embodiment, a low-cost, highly quality and reliably optical
transmission module is realized by using low-cost, high-quality and
reliable vertical-cavity, surface-emission-type laser diode.
Eleventh Embodiment
In an eleventh embodiment of the present invention, an optical transceiver
module is provided that uses a vertical-cavity, surface-emission-type
laser diodes of any of first through ninth embodiments as a light source.
In the eleventh embodiment, a low-cost, high-quality and reliable optical
transceiver module is realized by using the low-cost, high-quality and
reliable vertical-cavity, surface-emission-type laser diode of any of
first through ninth embodiments.
Twelfth Embodiment
In a twelfth embodiment of the present invention, an optical
telecommunication system is provided that uses a vertical-cavity,
surface-emission-type laser diode of any of first through ninth
embodiments as a light source.
In the twelfth embodiment, low-cost, high-quality and reliable
optical-fiber telecommunication system, optical interconnection system,
and other optical telecommunication systems are realized by using a
low-cost, high-quality and reliable vertical-cavity, surface-emission-type
laser diode of any first through ninth embodiments.
Thirteenth Embodiment
In a thirteenth embodiment of the present invention, a fabrication process
of vertical-cavity, surface-emission-type laser diodes of first through
sixth embodiment will be provided for the case in which the active layers
2 and 12 are formed with a semiconductor layer containing nitrogen. In the
process of the present embodiment, a process of removing Al source
material, Al product, Al compound or Al remaining from a site, such as the
gas supply line or the growth chamber in which the nitrogen source
compound or the impurity contained in the nitrogen source compound may
make a contact, is provided after the growth of the semiconductor layer
that contains Al but before the start of growth of the growth of the
active layer containing therein nitrogen.
It turned out that this invention has an effect especially in the case of
the active layer 2 or 12 is an active layer containing nitrogen such as
GaNAs, GaPN, GaNPAs, GaInNAs, GaInNP, GaNAsSb, or GaInNAsSb.
FIG. 13 is a diagram showing the room temperature photoluminescence
spectrum of the active layer having the GaInNAs/GaAs double quantum well
structure that consists of a GaInNAs quantum well layer and a GaAs barrier
layer and produced by an MOCVD apparatus of the inventor of the present
invention. In FIG. 13, the curve A represents the spectrum for the
specimen in which the double-quantum well structure is formed on an AlGaAs
cladding layer with a GaAs intermediate layer intervening therebetween. On
the other hand, the curve B represents the spectrum for the specimen in
which a double quantum well structure is formed continuously on a GaInP
cladding layer with an intervening GaAs intermediate layer. FIG. 14, on
the other hand, shows the fundamental structure of the specimens A and B.
Referring to FIG. 14, the specimens A and B are basically formed on a GaAs
substrate 501 by laminating a lower cladding layer 502, an intermediate
layer 503, an active layer 504 containing nitrogen, an intermediate 503,
and an upper cladding layer 505.
As represented in FIG. 13, the intensity of photoluminescence spectrum
falls off more than one-half in the specimen A as compared with the
specimen B. Thus, there has been a problem in that the emission intensity
the active layer is degraded when the active layer of GaInNAs containing
nitrogen is formed continuously on the semiconductor layer of AlGaAs that
contains Al as a constituent element while using a single MOCVD apparatus.
As a result, the threshold current density of the laser diode of the
GaInNAs system formed on the AlGaAs cladding layer become more than twice
as compared with case in which the laser diode is formed on the GaInP
cladding layer.
The inventor of the present invention conducted investigation about this
problem. FIG. 15 shows the depth-distribution profile of nitrogen (N) and
oxygen (O) in an exemplary laser diode of FIG. 14 that uses AlGaAs for the
cladding layer 502, GaAs for the intermediate layer 503 and GaInNAs/GaAs
double quantum well structure for the active layer 504, while using a
single epitaxial growth (MOCVD) apparatus. The measurement was made by
secondary ion mass spectroscopy. The measurement condition is shown in
Table 1.
TABLE 1
primary ion specie Cs.sup.+
primary acceleration voltage 3.0 kV
sputtering rate 0.5 nm/s
measurement area 160 .times. 256 .mu.m.sup.2
degree of vacuum <3 E-7 Pa
polarity of measured ions --
In FIG. 15, there can be seen two nitrogen peaks in the active layer 504 in
correspondence to the GaInNAs/GaAs double quantum well structure. Further,
a peak of oxygen is detected in the active layer 504. However, it can be
seen that the oxygen concentration in the intermediate layer 503 that does
not contain Al is about one order lower than the oxygen concentration of
the active layer 504.
On the other hand, in the case the depth-distribution profile of the oxygen
concentration was measured for the laser diode device that uses GaInP for
the cladding layer 502, GaAs for the intermediate layer 503, and
GaInNAs/GaAs double quantum well structure for the active layer 504, it
was confirmed that the oxygen concentration in active layer 504 background
level.
In other words, it became clear by the experiment of the inventor that
oxygen is taken into the active layer 504 that contains nitrogen, when the
laser diode is grown continuously by a single epitaxial growth apparatus
while using a nitrogen compound source material and a metal-organic Al
source material continuously such that the laser diode has the
semiconductor layer 502 containing Al between the substrate 501 and the
active layer 504 that contains nitrogen. Oxygen thus taken into the active
layer 504 forms a non-optical recombination state, and because of this,
the efficacy of optical emission of the active layer 504 is decreased
substantially. Further, it became clear that oxygen thus taken into the
active layer 504 becomes the cause of decrease of the efficacy of a laser
diode for the case the laser diode includes the semiconductor layer 502
containing Al between the active layer 504 and the substrate 501. It is
thought that origin of the oxygen contamination would be the material
containing oxygen and remaining in the apparatus or a material containing
oxygen and included as impurity in the nitrogen compound source material.
Next, an investigation was made about the cause the oxygen incorporation.
FIG. 16 shows the depth distribution profile of in the same sample as FIG.
15. It should be noted that the measurement was made by secondary ion mass
spectroscopy under the condition represented in Table 2.
TABLE 2
primary ion specie O2+
primary acceleration voltage 5.5 kV
sputtering rate 0.3 nm/s
measurement region 60 .mu.m.phi.
degree of vacuum <3 E-7 Pa
polarity of measured ion +
From FIG. 16, it will be understood that Al is detected in the active layer
504 in which no Al source material is used. On the other hand, in the
intermediate layer (GaAs layer) 503 adjacent to the semiconductor layer
(the cladding layer) 502 or 505 containing Al, it will be noted the Al
concentration level is lower than the Al concentration of the active layer
504 by one order or more. This indicates that the Al contamination in the
active layer 504 is not caused by diffusion and substitution of Al from
the semiconductor layer (cladding layer) 502 or 505 that contains Al.
On the other hand, no Al was detected in the active layer in the case the
active layer containing nitrogen is grown on the semiconductor layer not
containing Al, such as GaInP.
Thus, it is concluded that Al detected in the active layer 504 as
represented in FIG. 16 originates either from Al remaining in the gas
supply line or growth chamber, or from Al source, Al product, Al compound
or Al, taken into the active layer 504 by combining with a nitrogen source
compound or with the impurity (water) contained in such a nitrogen source
compound. In other words, it was newly discovered by the inventor that Al
is taken naturally into the active layer containing nitrogen, when a laser
diode that includes a semiconductor layer containing Al between the
substrate and the active layer containing nitrogen continuously in a
single epitaxial growth apparatus while using a nitrogen source compound
and a metal-organic Al source material.
When comparison is made with respect to the depth distribution profile of
nitrogen and oxygen concentration in a laser diode having a construction
identical with that of FIG. 15, it will be noted that the two oxygen peak
profiles in the double quantum-well active layer 504 do not correspond
with the peak profile of nitrogen but correspond to the Al concentration
profile of FIG. 16. From this, it became clear that the oxygen impurity in
the GaInNAs well layer is not incorporated together with the nitrogen
source compound but is incorporated into the well layer in the form
coupled with Al. In other words, Al causes coupling with a material
containing oxygen such as water contained in the nitrogen compound source
or water remaining in the gas line or reaction chamber, as the Al source
material, Al product or Al compound or Al remaining in the processing
chamber causes contact with a nitrogen source compound. In this way, Al
and oxygen are taken into the active layer 504. It was this oxygen that
was taken into active layer 504 in this way that has caused decrease of
efficacy of optical emission in the active layer 504. The above became
clear for the first time by the experiments of the inventor of the present
invention.
Thus, in order to eliminate this problem, it is necessary to provide a
process for removing the Al source material, Al product, Al compound or Al
remaining in the processing chamber or a site where there is a chance that
these Al-containing material makes a contact with impurity in the nitrogen
source compound.
By providing such a process after the growth of the semiconductor layer 502
that contains Al but before the start of growth of the active layer 504,
which contains nitrogen, the concentration of the impurity that contains
Al and oxygen and incorporated into the active layer through the mechanism
of the nitrogen source compound or impurity contained in the nitrogen
source compound causing a reaction with the residual Al source material,
Al product, Al compound or Al, is reduced effectively. Further, the
adversary influence on the non-optical recombination in active layer 504
is successfully reduced even when the carriers are injected in the active
layer 504 by current injection, provided that the residual Al is removed
before the end of the growth process of the non-optical recombination
elimination layer.
For example, a room temperature continuous oscillation became possible by
reducing the Al concentration in the active layer 504 containing nitrogen,
to the level of 1.times.10.sup.19 cm.sup.-3 or less.
Furthermore, an optical emission characteristic equivalent to the one for
the case in which the active layer is formed on a semiconductor layer not
containing Al is obtained by reducing the Al concentration in the active
layer 504 containing nitrogen to 2.times.10.sup.18 cm.sup.-3 or less.
Table 3 shows the result of evaluation of threshold current density for a
broad stripe laser diode having a GaInNAs double quantum well structure
(layer containing nitrogen) and a cladding layer (layer containing Al) of
AlGaAs.
TABLE 3
Al in active O in active threshold current
cladding layer layer [/cm3] layer [/cm3] density [kA/cm2]
AlGaAs >2 E + 19 >1 E + 18 >10
AlGaAs 8-9 E + 18 9 E + 17 2-3
AlGaAs <1 E + 18 <2 E + 17 0.8
GaInP <2 E + 17 <2 E + 17 0.8
From Table 3 it can be seen that Al of 2.times.10.sup.19 cm.sup.-3 or more
and oxygen of 1.times.10.sup.18 cm.sup.-3 or more are incorporated into
the active layer in the structure, in which an active layer containing
nitrogen is grown continuously on a semiconductor layer that contains Al.
Thus, the threshold current density takes a remarkably high value of 10
kA/cm.sup.2 or more. However, the oxygen concentration in the active layer
is reduced to 1.times.10.sup.18 cm.sup.-3 or less when the Al
concentration in the active layer is reduced to 1.times.10.sup.19
cm.sup.-3 or less. Along with this, the broad stripe laser diode can
oscillate at the threshold current density of 2-3 kA/cm. When the active
layer has a crystal quality characterized by the threshold current density
of several kiloamperes/cm.sup.2 or less, room temperature continuous
oscillation of the broad stripe laser diode becomes possible. Accordingly,
it is concluded that by controlling the Al concentration in the active
layer containing nitrogen to be 1.times.10.sub.19 cm.sup.-3 or less, it
becomes possible to produce a laser diode that can oscillate continuously
at room temperature.
Thus, in the thirteenth embodiment of the present invention, oxygen taken
into the active layer that contains nitrogen at the time of growth of the
active layer is reduced successfully, by providing the process of removing
residual Al source material, Al product, Al compound or Al from the site,
such as gas supply line or growth chamber, in which the nitrogen source
compound or impurity contained therein may make a contact, after the
growth of the semiconductor layer containing Al but before the start of
growth of the active layer. As a result, it becomes possible to grow a
semiconductor light-emitting device having an active layer containing
nitrogen formed on or above a semiconductor layer containing Al, without
reducing the efficacy of optical emission.
Fourteenth Embodiment
In a fourteenth embodiment of the present invention, a process for purging
a carrier gas is provided in the fabrication process of a vertical-cavity,
surface-emission-type laser diode of the thirteenth embodiment after the
growth of the semiconductor layer containing Al but before the end of the
growth process of the non-optical recombination elimination layer, for
removing residual Al source material, Al product, Al compound or Al from a
site, such as gas supply line or growth chamber, in which a nitrogen
source compound or impurity contained therein may make a contact.
Thus, according to the fourteenth embodiment, there is provided a purging
process, after the process of growing the semiconductor layer 502
containing Al but before the start of growth of the active layer 504
containing nitrogen, for purging the residual Al source material, Al
product, Al compound or Al from a site in which the nitrogen compound
source material or impurity contained therein may make a contact,-by using
a carrier gas.
Here, the time of the purging process is defined as a time interval after
interruption of supply of the Al source material to the growth chamber
with the termination of growth of the semiconductor layer 502 containing
Al but before starting supply of the nitrogen source compound for
commencing the growth of the active layer 504 that contains nitrogen.
As noted previously, the Al source material, Al product, Al compound or Al
may remain in the growth chamber, when a semiconductor layer containing Al
as constituent element. However, it is possible to decrease the
concentration of residual Al gradually remaining in the growth chamber, by
purging the gas line and the growth chamber with the carrier gas.
Specifically, the Al concentration in the active layer can be reduced to
1.times.10.sup.19 cm.sup.-3 or less with the purging process of about 10
minutes. Thus, the efficacy of optical emission in the active layer is
improved and it becomes possible to construct a laser diode device that
oscillates continuously at room temperature.
By providing the purging process over 30 minutes or more, it is possible to
decrease the Al concentration level to 1.times.10.sup.18 cm.sup.-3 or
less.
As a purging method, there is a process to interrupt the growth of the
intermediate layer that does not contain Al and carry out the purging with
the carrier gas. In the case that process of interrupt growth and purge is
to be used, such an interruption of growth can be provided after the
growth of the semiconductor layer containing Al up to midway of growth of
the non-optical recombination elimination layer.
FIG. 17 shows an example of the laser diode according to the fourteenth
embodiment of the present invention. The laser diode of FIG. 17 is formed
on the substrate 501 by consecutively laminating thereon the semiconductor
layer 502 containing Al as a constituent element, a first lower
intermediate layer 601, a second 2nd lower intermediate layer 602, an
active layer 504 containing nitrogen, the upper intermediate layer 503
and, the second semiconductor layer 505.
In the crystal growth of the laser diode of FIG. 17, an epitaxial growth
apparatus that uses a metal-organic Al source material and an organic
nitrogen source material is used. Thereby, a growth interruption process
is provided after the growth of the first lower intermediate layer 601 but
before the start of growth of the second lower intermediate layer 602.
During the growth interruption process, the Al source material, Al
product, Al compound or Al remaining in the site where the nitrogen source
compound or the impurity contained in the nitrogen source compound in the
growth chamber may make a contact is removed by purging with hydrogen that
is used for a carrier gas.
FIG. 18 shows the result of measurement of depth distribution of Al to the
laser diode device for the case the growth interruption is provided
between the first lower intermediate layer 601 and the second lower
intermediate layer 602 for a duration of 60 minutes. As can be seen in
FIG. 18, the Al concentration in the active layer 504 can be reduced to
the level of 3.times.10.sup.17 cm.sup.-3 or less. This value of Al
concentration is generally the same as the Al concentration in the
intermediate layer.
FIG. 19 shows the result of measurement of the depth distribution profile
of nitrogen (N) and oxygen (O) for the same device as the device of FIG.
18. As shown in FIG. 19, the oxygen concentration in active layer 504 was
reduced the background level of 1.times.10.sup.17 cm.sup.-3.
It should be noted that the oxygen peak appearing in the lower intermediate
layers 601 and 602 is caused by segregation of oxygen to the interface in
which the growth was interrupted. Therefore, it is preferable to provide
such a growth interruption after the growth of the semiconductor layer
containing Al and before the end of the growth process of the non-optical
recombination elimination layer. This is because the non-optical
recombination elimination layer can have an increased bandgap energy as
compared with the quantum-well active layer or the barrier layer and that
it is because the adversary effect of non-optical recombination caused by
oxygen that segregated to the growth interruption interface is suppressed
when the carriers are injected to the active layer by current injection.
In the illustrated example, growth of the laser diode is interrupted
between the first lower intermediate layer 601 and the second lower
intermediate layer 602. By conducting the purging process for 60 minutes,
impurity concentration level of Al, oxygen, and the like, in the active
layer 504 containing nitrogen was reduced. With this, the efficacy of
optical emission of the active layer 504 containing nitrogen was improved.
During the purging process of the growth chamber conducted with the carrier
gas, the efficiency of removing the Al source material or reaction product
adsorbed on a susceptor or periphery of the susceptor is improved by
conducting the purging process while heating the susceptor.
When the substrate is heated simultaneously, it should be noted that the
group V source gas such as AsH.sub.3 or PH.sub.3 has to be supplied
continuously to the growth chamber during the growth interruption so as to
prevent that the outermost surface of the semiconductor layer 505
experiences thermal decomposition.
When purging the growth chamber with the carrier gas, it is also possible
to move the substrate to another chamber from the growth chamber. In this
case in which the substrate is moved to another chamber from the growth
chamber, it is not necessary to supply the group V source gas of AsH.sub.3
or PH.sub.3 to the growth chamber during the process of purging conducted
while heating the susceptor. Accordingly, thermal decomposing of the
reaction product of Al deposited on the susceptor or periphery of the
susceptor is facilitated, and the Al concentration inside the growth
chamber is reduced effectively.
Further, it is also possible to carry out the purging during the growth
process of the intermediate layer. Because of the fact that the
non-optical recombination elimination layer is provided between the
reflector of the AlGaAs system, which contains Al, and the active layer
containing nitrogen, the distance between the active layer containing
nitrogen and the layer containing Al is increased. Because of this, there
is a merit that the duration of purging can be increased when the purging
process is conducted simultaneously to the growing process. In this case,
it is preferable to reduce the growth rate for securing sufficient purge
time.
Fifteenth Embodiment
In a fifteenth embodiment of the present invention, the vertical-cavity,
surface-emission-type laser diodes of the thirteenth and fourteenth
embodiments are formed by a crystal growth process that uses an MOCVD
process while using at least a metal-organic Al source material and a
nitrogen source compound.
In a semiconductor light-emitting device having a semiconductor layer
containing Al between a substrate and an active layer that contains
nitrogen, no remarkable decrease of efficacy of optical emission is
reported when the semiconductor device is produced it with a crystal
growth process that does not use a metal- organic-metal Al source material
and a nitrogen source compound such as an MBE process. In the case an
MOCVD process is used, on the other hand, decrease of efficacy of optical
emission is reported for a GaInNAs active layer formed on a semiconductor
layer containing Al. According to the article in Electron Lett., 2000,36
(21), pps1776-1777, it is reported that the intensity of photoluminescence
decreases remarkably when a GaInNAs quantum well layer is grown
continuously on an AlGaAs cladding layer in the same MOCVD growth chamber,
even in the case an intermediate layer of GaAs with is provided. In the
above report, therefore, the AlGaAs cladding layer and the GaInNAs active
layer are grown in different MOCVD growth apparatuses for improving the
photoluminescent intensity. Accordingly, this problem is thought to be a
problem that cannot be avoided in the case of conducting a crystal growth
process while using a metal-organic source of Al and nitrogen source
compound, as in the case of MOCVD process.
In an MBE process, the crystal growth is carried out in a highly vacuum
environment. Contrary to this, an MOCVD process is conducted under a
process pressure much higher than the process pressure of an MBE process
of several ten Torr to atmospheric pressure. Because of this, the mean
free path of the gas molecules is overwhelmingly short in the MOCVD
process. Thus, there are many opportunities that the supplied source
material molecules and the carrier gas molecules cause contact and
reaction with the gas line, reaction chamber or other parts. This is the
reason why it is preferable to provide the process for removing the
residual Al source material, Al product, Al compound, or Al, in the case
of a process such as MOCVD process that uses a high pressure for the
growth chamber or for the gas line, from the site in which the nitrogen
source compound or impurity contained in the nitrogen source compound may
cause a contact, after the growth of the semiconductor layer containing Al
before the growth of the active layer containing nitrogen (more preferably
before the end of the growth process of the non-optical recombination
elimination layer). By doing so, the effect of reducing the amount of
oxygen taken into the active layer containing nitrogen is increased.
EXAMPLES
Below, explanation will be made on various examples of the present
invention.
Example 1
FIGS. 9A and 9b show the constitutional example of a vertical-cavity,
surface-emission-type laser diode of Example 1, wherein FIG. 9B shows an
enlarged view of an active region of FIG. 9A. The vertical-cavity,
surface-emission-type laser diode of FIGS. 9A and 9b are formed on an
n-GaAs substrate 101 having a (100) surface orientation and includes an
n-semiconductor distributed Bragg reflector (AlAs/GaAs lower reflectors)
104 formed on the substrate 101, wherein the n-semiconductor distributed
Bragg reflector includes alternate repetition of an n-Al.sub.x Ga.sub.1-x
As (x=1.0) and an n-Al.sub.y Ga.sub.1-y As (y=0) repeated for 35 times,
with respective thicknesses of 1/4 times the oscillation wavelength
.lambda.(.lambda./4 thickness) in the respective media. Further, an
n-Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (x=.sub.0.5, y=1) layer (a
non-optical recombination elimination layer) 103 is laminated at the top
with the thickness of .lambda./4. In the Example 1, it should be noted
that the n-Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (x=0.5, y=1) layer 103
is a low-refractive index layer and forms a part of the lower reflector
104.
On the non-optical recombination elimination layer 103, a lower undoped
GaAs spacer layer 105, a multiple quantum-well active layer 106 formed of
three Ga.sub.x In.sub.1-x As quantum well layers (quantum-well active
layer) 106a and corresponding GaAs barrier layers 106b (thickness of 20
nm), and an upper GaAs spacer layer 107 are laminated. In this way, an
optical cavity having a thickness corresponding to one wavelength
(thickness of .lambda.) of the oscillation wavelength measured in the
medium is formed.
Further, a periodical structure (1 period) is formed thereon by laminating
a C-doped p-Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (x=0.5, y=1) layer
(non-optical recombination elimination layer) 108 and a Zn-doped
p-Al.sub.x Ga.sub.1-x As (x=0) layer alternately with a 1/4 thickness of
the oscillation wavelength as measured in respective media. Further, a
semiconductor distributed Bragg reflector (Al.sub.0.9 Ga.sub.0.1 As/GaAs
upper reflector) 109 is formed thereon by alternate deposition of a
C-doped p-Al.sub.x Ga.sub.1-x As (x=0.9) layer and a Zn-doped p-Al.sub.x
Ga.sub.1-x As (x=0) layer to form a periodical structure (25 periods) with
the thickness of 1/4 times the oscillation wavelength in each of the
media. In the Example 1, it should be noted that the p-Ga.sub.x In.sub.1-x
P (x=0.5, y=1) layer 108 is a low-refractive index layer and forms a part
of the upper reflector 109.
It should be noted that the uppermost p-Al.sub.x Ga.sub.1-x As (x=0) layer
110 has a role also of a contact layer (the p-contact layer) contacting
with a p-side electrode 112.
Here, it should be noted that the In content x of the quantum-well active
layer 106a is set to 39% (Ga.sub.0.61 In.sub.0.39 As). Further, the
thickness of the quantum-well active layer 106a is set to 7 nm. The
quantum-well active layer 106a accumulated a compressive strain of about
2.8% with respect to the GaAs substrate 101.
In the Example 1, the growth of the entire vertical-cavity,
surface-emission-type laser diode is conducted by an MOCVD process. In
this case, lattice relaxation was not observed. TMA (trimethyl aluminum),
TMG (trimethyl gallium), TMI (trimethyl indium), AsH.sub.3 (arsine) and
PH.sub.3 (phosphine) are used for the source materials of the layers that
constitute the laser diode. Further, H.sub.2 is used for the carrier gas.
In the case the strain in the active layer (quantum-well active layer)
106a is large as in the present case of the Example 1, it is preferable to
use a low-temperature growth process that proceeds under a non-equilibrium
state. In Example 1, the GaInAs layer (quantum-well active layer) 106a is
grown at 550.degree. C. It should be noted that an MOCVD process is suited
for crystal growth of highly strained active layer due to large the degree
of saturation. Further, an MOCVD process is advantageous in the point that
it does not require high vacuum environment as in the case of an MBE
process. Further, an MOCVD process is advantageous for mass production in
view of the fact that the process is easily controlled by controlling flow
rate and supply time of source gases.
In Example 1, a current confinement structure is formed also by forming an
insulation layer (high-resistance region) 111 outside the current path by
irradiating of proton (H.sup.+)
Further, a p-side electrode 112 is formed on the p-contact layer 110
forming a part of the upper reflector in Example 1 as an uppermost of
layer, except for an optical beam exit region 114. Also, an n-side
electrode 113 is formed on the back surface of the substrate.
In Example 1, it should be noted that the active region sandwiched between
the upper and lower reflectors 104 and 109 and causing recombination of
the carriers injected thereto (resonator formed of the upper and lower
spacer layers 105 and 107 and the multiple quantum-well active layer 106
in Example 1), do not use a material containing Al (the proportion of Al
with regard to the group III element is 1% or more). Furthermore, the
layers 103 and 108 in the lower reflector 104 and the upper reflector 109
located closest to the active layer 106 have a composition of Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1). It
should be noted that the carriers are confined between the low-refractive
index layers of the upper reflector 109 and the lower reflector 104 having
a widegap and located closest to the active layer 106. Because of this,
when the low-refractive index layer (wide gap layer) of the reflector that
contacts with the active region contained Al, there would occur
non-optical recombination of carriers at the interface with injection of
carriers, even in the case the active region is formed of a layer not
containing Al (the proportion of Al with respect to other the group III
elements is 1% or less). As a result, the efficacy has of optical emission
would fall off inevitably. In view of the object of the present invention,
it is preferable to form the active region by layers not containing Al.
In Example 1, it should be noted that the active region and the interface
between the active region and the reflectors 104 and 109 do not contain
Al. Because of this, the problem of non-optical recombination of carriers
caused by crystal defects, which in turn originate from Al, at the time of
the carrier injection is eliminated. As a result, non-optical
recombination is successfully reduced.
It is naturally preferable to apply the construction that does not contain
Al at the interface between the reflector and the active region as in the
case of Example 1 to both of the upper and lower reflectors 104 and 109.
However, the effect is obtained when using the construction in only one of
the reflectors. In Example 1, the semiconductor distributed Bragg
reflector is used in both of the upper and lower reflectors 104 and 109.
However, it is also possible to form only one of the reflectors by the
semiconductor distributed Bragg reflector and form the other reflector by
a dielectric reflector. In the abovementioned example, it is further noted
that only the layer in the low-refractive index layers forming the
reflectors 104 and 109 and located nearest to the active layer 106 has the
composition of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1). However, plural Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layers may be used in the reflectors
104 and 109.
In Example 1, the present invention is applied to the lower reflector 104
between the GaAs substrate 101 and the active layer 106. Because of this,
problem of crawling up of the crystal defects caused by Al to the active
layer 106, which appears at the time of growth of the active layer 106,
and the adversary effects associated therewith are suppressed. As a
result, the crystal quality of the active layer 106 is improved and the
efficacy of optical emission is improved also. Thereby, a sufficient
reliability for practical use is obtained. In view of fact that the
Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1)
layer not containing Al is used not in the entire low-refractive index
layers constituting the semiconductor distributed Bragg reflector but at
least in the part located close to the active region. Because of this, it
is possible to achieve the abovementioned effect without increasing the
number of stacking in the reflector.
The vertical-cavity, surface-emission-type laser diode thus fabricated
oscillates at the wavelength of was about 1.2 .mu.m. In a GaInAs layer
formed on a GaAs substrate, there occurs increase of wavelength as a
result of increase of the In content. However, such an increase of In
content is accompanied with increase of strain. Thus, it has been thought
that the wavelength of 1.1 m would be the upper limit. Reference should be
made to IEEE Photonics Technol. Lett. Vol. 9 (1997), pp. 1319-1321. In the
present invention, it becomes possible to grow the GaInAs quantum-well
active layer accumulating a large strain coherently with a thickness not
possible hitherto, by using a highly non-equilibrium process such as a
low-temperature growth process conducted at 600.degree. C. or less. As a
result, a wavelength of 1.2 .mu.m is realized. It should be noted that a
Si semiconductor substrate is transparent at this wavelength. Thus, it
becomes possible to achieve optical transmission through the Si substrate
in a circuit chip in which an electron device and an optical device are
integrated on a common Si substrate. Thus, it becomes possible to
construct a vertical-cavity, surface-emission-type laser diode of long
wavelength band on a GaAs substrate by using a GaInAs layer having a large
In content and associated large compressive strain for the active layer.
While the vertical-cavity, surface-emission-type laser diode of FIG. 9 can
be grown by an MOCVD process, the same can be grown also by an MBE
process. In Example 1, the example of triple quantum well structure (TQW)
was shown as the layered structure of the active layer 106. However, I is
also possible to use a structure (SQW, MQW) in which the number of the
wells used the quantum well is different. Further, the laser diode may
have a different structure. While the resonator in Example 1 has a
thickness of .lambda., it is also possible to use the thickness of integer
multiple of .lambda./2, preferably an integer multiple of .lambda.. In
Example 1, GaAs was used for the semiconductor substrate 101. However, the
present invention is applicable also to the case in which other
semiconductor substrate such as InP is used. Further, the repetition
period of the reflectors 104 and 109 may be changed to other repetition
period.
In example 1, GaInAs was used for the active layer 106. However, GaInNAs
can be used in place thereof. In this case, the vertical-cavity,
surface-emission-type laser diode of longer wavelength of 1.3 .mu.m band
or 1.55 .mu.m band or longer becomes possible by changing the composition
of the GaInNAs active layer. Also, the vertical-cavity,
surface-emission-type laser diode of the 1.3 .mu.m band can be realized on
the GaAs substrate 101 by using GaAsSb for the active layer 106.
Conventionally, there has been no material suitable for a laser diode of
1.1-1.3 .mu.m wavelength band. By using a highly strained GaInAs, GaInNAs
or GaAsSb layer formed on the GaAs substrate 101, a laser diode of 1.1-1.3
.mu.m wavelength band becomes possible. By using a material system capable
of crystal growth on a GaAs substrate 101, a highly efficient
vertical-cavity, surface-emission-type laser diode operable at the
wavelength band of 1.3 .mu.m or 1.55 .mu.m or longer wavelength, which has
hitherto been difficult to realize, is successfully realized.
Example 2
FIGS. 10A and 10B show the constitutional example of a vertical-cavity,
surface-emission-type laser diode according to Example 2, wherein FIG. 10B
is an enlarged view of the active region of FIG. 10A. The vertical-cavity,
surface-emission-type laser diode of FIGS. 10A and 10B is formed on n-GaAs
substrate 201 having a surface orientation of (100) and includes an
n-semiconductor distributed Bragg reflector (Al.sub.0.9 Ga.sub.0.1 As/GaAs
lower reflector) 204 formed on the substrate 201, wherein the
n-semiconductor distributed Bragg reflector 204 includes alternate
lamination of an n-Al.sub.x Ga.sub.1-x As (x=0.9) layer and an n-Al.sub.x
Ga.sub.1-x As (x=.sub.0) layer repeated for 35 periods with respective
thicknesses of 1/4 times the oscillation wavelength (.lambda.) as measured
in each media. Further, an n-Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(x=0.5, y=1) layer (non-optical recombination elimination layer) 203
having a thickness of .lambda./4 is laminated thereon. In Example 2, the
n-Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (x=0.5, y=1) layer 203 is a
low-refractive index layer and forms a part of the lower reflector 204.
Further, a lower GaAs spacer layer 205 is formed on the lower reflector
204, and a multiple quantum-well active layer 206 (triple quantum well
(TQW) in Example 2) is formed on the lower GaAs spacer layer 205, wherein
the triple quantum well active layer 206 consists of three undoped
Ga.sub.x In.sub.1-x N.sub.y As.sub.1-y quantum well layers 206a acting as
an active layer (quantum-well active layer) and corresponding GaAs barrier
layers 206b (15 nm). Further, an undoped upper GaAs spacer layer 207 is
laminated thereon. As a result, an optical cavity having a thickness of
one wavelength of the oscillation wavelength as measured in the media
(thickness of .lambda.) is formed.
Further, a p-semiconductor distributed Bragg reflector (the upper
reflector) 209 is formed on the multiple quantum-well active layer 206.
The upper reflector 209 includes a low-refractive index layer of the
thickness of 3 .lambda./4, wherein the low-refractive index layer includes
an AlAs layer 230 that becomes a selectively oxidized layer such that the
AlAs layer 230 is sandwiched by a GaInP layer 208 and an AlGaAs layer. The
GaInP layer 208 is formed of a C-doped p-Ga.sub.x In.sub.1-x P.sub.y
As.sub.1-y (x=0.5, y=1) layer (non-optical recombination elimination
layer) having a thickness of .lambda./4-15 nm, while the AlGaAs layer is
formed of a C-doped p-Al.sub.x Ga.sub.1-x As layer (x=0.9) having a
thickness of 2 .lambda./4-15 nm. Further, a C-doped p-Al.sub.z Ga.sub.1-z
As (z=1) is provided for the selectively oxidized layer 203 with a
thickness of 30 nm. On the low-refractive index layer thus formed, a GaAs
layer having a thickness of .lambda./4 is laminated for one period, and a
C-doped p-Al.sub.x Ga.sub.1-x As layer (x=0.9) and a p-Al.sub.x Ga.sub.1-x
As (x=0) layer are formed on the GaAs layer alternately for 22 periods
with respective thicknesses of 1/4 times the oscillation wavelength as
measured in each medium. Thus, a semiconductor distributed Bragg reflector
of Al.sub.0.9 Ga.sub.0.1 As/GaAs structure is formed as the upper
reflector 209.
Further, an uppermost layer 210 of p-Al.sub.x Ga.sub.1-x As (x=0) is formed
thereon, wherein the uppermost layer 210 functions as a contact layer
(p-contact layer) that makes a contact with a p-side electrode 212.
Here, it should be noted that the In content x of the quantum-well active
layer 206a is made to 37% and the N (nitrogen) content of the quantum-well
active layer 206a is made to 0.5%. Further, the thickness of the
quantum-well active layer 206a is made to 7 nm. In the present Example 2,
the growth of the vertical-cavity, surface-emission-type laser diode was
conducted by an MOCVD process. Thereby, DMHy (dimethyl hydrazine) was used
for the source material of nitrogen, together with the source materials of
TMA (trimethyl aluminum), TMG (trimethyl gallium), TMI (trimethyl indium),
AsH.sub.3 (arsine), or PH.sub.3 (phosphine). It should be noted that DMHy
decomposes at low temperature. Thus, the material is particularly suitable
for low-temperature growth at 600.degree. C. or less, and hence to the
growth of highly strained quantum well layer that requires a growth at
low-temperatures. In Example 2, H.sub.2 was used for the carrier gas. In
Example 2, the GaInNAs layer (quantum-well active layer) 206a was grown at
540.degree. C. In view of the fact that high degree of saturation is
realized, an MOCVD process is thought suitable for crystal growth of
material layers that contain N in addition to other group V elements.
Further, there is an advantage that an MOCVD process does not require high
vacuum environment contrary to the case of an MBE process. As the growth
process can be conducted by merely controlling the flow rate and supply
time of the source gases, an MOCVD process is particularly suitable for
mass production of semiconductor devices.
In Example 2, the mesa structure of a predetermined size is formed by an
etching process until the p-Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (x=0.5,
y=1) layer 208 is reached and the sidewall surface of the p-Al.sub.z
Ga.sub.1-z As (z=1) selectively oxidized layer 230 is exposed. The
Al.sub.z Ga.sub.1-z As (z=1) layer 230 having the sidewall surface thus
exposed is then oxidized by water vapor, starting from the sidewall
surface, and there is formed an Al.sub.x O.sub.y current confinement part
220 as a result of the oxidation process. Next, the etched region is
buried with a polyimide insulation film 221 for planarization, and the
polyimide film covering the upper reflector 209 is removed. Further, a
p-side electrode 212 is formed on the p-contact layer 210 except for an
optical beam exit region 214, and an n-side electrode 213 is formed to the
rear surface of the GaAs substrate 201.
In Example 2, the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1,0<y.ltoreq.1) layer 208 is provided under the
selectively oxidized layer 230 as a part of the upper reflector 209.
Thereby, the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer 208 functions as an etching stopper, provided that
sulfuric acid etchant is used in the etching process to form the mesa
structure. It should be noted that the material of the GaInPAs system
functions as an etching stopper with regard to the material of the AlGaAs
system. By inserting the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layer 208, the height of the mesa
structure used for the selective oxidation process is controlled exactly.
Because of this, homogeneity and reproducibility are improved for the
laser diode. Further, the cost is reduced also.
By using the vertical-cavity, surface-emission-type laser diode of Example
2 to form a one-dimensional or two-dimensional array, the process control
during the device fabrication process is improved, together with
improvement of homogeneity and reproducibility of device characteristics
between the elements within the array.
In Example 2, the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layer 208 that doubles acting also as an etching stopper
layer, was provided on the side of the upper reflector 209. However, the
layer 208 may be provided on the side of the lower reflector 204.
In Example 2, the material containing Al was not used in the active region
(optical cavity consisting of the upper spacer layer 207, the lower spacer
layer 205 and the multiple quantum-well active layer 206 in Example 2)
that is sandwiched between the upper and lower reflectors 204 and 209 and
causing recombination of carriers injected thereto. Further, the
low-refractive index layer in the lower reflector 204 and the
upper-reflector 209 located closest to the active layer is Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1). Thus, Al
is not contained in the active region or in the interface region between
the active region and the reflector in Example 2. Because of this, the
problem of non-optical recombination caused by crystal defects originating
from Al is reduced even when carrier injection is made. In view of the
object of the present invention, it is preferable to form the active
region from a layer not containing Al.
As set forth in Example 2, it is preferable to use the construction of
eliminating Al from the interface between the reflector and the active
region for both of the upper and lower reflectors 204 and 209. However,
the use of such a construction to only one of the reflectors is also
effective. In example 2, it is also noted that both of the upper and lower
reflectors 204 and 209 are formed of the semiconductor distributed Bragg
reflector. However, it is possible that only one of the reflectors is
formed of the semiconductor distributed Bragg reflector. In this case, the
other reflector may be formed of a dielectric reflector.
In example 2, it should be noted that the present invention is applied to
the lower reflector 204 located between the GaAs substrate 201 and the
active layer 206. Because of this, the problem of crawling up of crystal
defects originating from Al to the active layer 206 at the time of growth
of the active layer 206 and associated various adversary effects are
suppressed. Because of this, the active layer 206 can be grown with high
crystal quality, and the efficacy of optical emission in the
vertical-cavity, surface-emission-type laser diode is improved. Associated
therewith, a reliability sufficient for practical use is achieved for the
laser diode. In view of the construction of Example 2 in which not the
entire low-refractive index layers in the semiconductor distributed Bragg
reflectors 204 and 209 are formed of the Al-free Ga.sub.x In.sub.1-x
P.sub.y As.sub.1-y (0<x.ltoreq.1,0<y.ltoreq.1) layer but the
Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1)
layer is used only for the layers 203 and 208 located closest to the
active region, it becomes possible to obtain the above-mentioned effect
without increasing the number of stacking of the reflector. Because of
this, planarization process by using the polyimide layer can be achieved
easily. Associated therewith, the problem of interconnection pattern
(p-side electrode 212 in Example 2) being discontinued at a stepped part
is successfully avoided, and there occurred no degradation in the yield
device production.
The vertical-cavity, surface-emission-type laser diode thus produced
oscillated at the wavelength of was about 1.3 .mu.m. As a result of use of
GaInNAs in the active layer 206 in Example 2, it became possible to form a
vertical-cavity, surface-emission-type laser diode operable at a long
wavelength band on the GaAs substrate 201. Further, as a result of
formation of the current confinement structure by selective oxidation of
the selectively oxidized layer 230 that contains Al and As as the
principal component, it becomes possible to reduce the threshold current.
By using the current confinement structure that uses the current
confinement part 220 of Al oxide formed by selective oxidation of the
selectively oxidized layer 230, the current confinement part 220 can be
formed closer to the active layer 206 and the spreading of the electric
current is suppressed. Thus, the carriers are confined efficiency into a
minute region not exposed to the atmosphere. Further, the Al oxide film
formed as a result of oxidation process is characterized by a small
refractive index. Because of this, the optical beam is confined
efficiently by a convex lens effect into the minute region in which the
carriers are confined. Thereby, the efficiency of the vertical-cavity,
surface-emission-type laser diode is improved further and the threshold
current is reduced further. According to Experiment 2, it should be noted
that the current confinement structure can be formed easily. Because of
this, the fabrication cost of the laser diode is reduced. Thus, according
to Example 2, a low-cost vertical-cavity, surface-emission-type laser
diode of the 1.3 .mu.m band having a reduced electric power consumption is
provided.
The vertical-cavity, surface-emission-type laser diode of FIG. 10 can be
grown by the MOCVD process as noted before. However, it is also possible
to grow the laser diode by other process such as MBE process. Further, it
is noted that DMHy is used as the source material of nitrogen in Example
2. However, other nitrogen compound such as activated nitrogen or NH.sub.3
can also be used. In Example 2, a triple quantum well structure (TQW) was
shown as the layered structure of the active layer 206. However, it is
possible to use a structure (SQW, DQW, MQW) in which the number of the
wells used the quantum well is different from the triple quantum well
structure. Further, it is possible to use a different laser diode
structure.
In the vertical-cavity, surface-emission-type laser diode of FIG. 10,
operation at a longer wavelength such as 1.55 .mu.m band or more becomes
possible by adjusting the composition of the GaInNAs active layer 206a.
Further, it should be noted that the GaInNAs active layer 206a may contain
other III-V elements such as Tl, Sb, P etc. Further, it is possible that
the vertical-cavity, surface-emission-type laser diode of 1.3 .mu.m band
can be realized also on the GaAs substrate 201 by using GaAsSb for the
active layer 206a. In the case of using GaInAs for the active layer 206a,
it is possible to grow a highly strained GaInAs quantum-well active layer
with a large thickness not possible hitherto by using a low-temperature
growth of 600.degree. C. or less similarly as before. Thereby, an
oscillation wavelength of 1.2 .mu.m can be attained, while this wavelength
is longer than the wavelength of 1.1 .mu.m, which was hitherto thought as
being the limit. Conventionally, no suitable material has been known for
realizing a laser diode operable at the wavelength of 1.1-1.3 .mu.m. By
using highly strained GaInAs, GaInNAs or GaAsSb, a laser diode operable at
the wavelength of 1.1-1.3 .mu.m band becomes possible. Further, it becomes
possible to realize a highly efficient vertical-cavity,
surface-emission-type laser diode operable at long wavelength of the 1.3
.mu.m band or 1.55 .mu.m band, in which wavelength band, it has hitherto
been difficult to operate the laser diode with high performance.
Example 3
Example 3 relates to an optical transmission module. FIG. 11 shows the
general construction of the optical transmission module in which a quartz
optical fiber is coupled with a 1.3 .mu.m band GaInNAs vertical-cavity,
surface-emission-type laser diode of Example 2. Referring to FIG. 11, the
optical signal (laser beam) is injected into the optical fiber from the
laser diode, and the optical signal thus injected is transmitted along the
optical fiber. Thereby, the transmission rate can be increased by using a
wavelength multiple transmission technique that uses a plurality of laser
diodes having different oscillation wavelengths configured in
one-dimensional or two-dimensional array. Further, it is possible to
increase the transmission rate I by using an optical fiber bundle
consisting of a plurality of optical fibers corresponding to each laser
diode forming a one-dimensional or two-dimensional array together with
other laser diodes.
By using the vertical-cavity, surface-emission-type laser diode of the
present invention in the optical telecommunication system, the cost of the
optical telecommunication system that uses an optical transmission module,
in which a transmitter laser diode and an optical fiber are coupled, is
reduced substantially, in view of the low cost of the vertical-cavity
surface-emission type laser diode of the present invention. In view of the
fact that the temperature characteristics of the GaInNAs vertical-cavity
surface-emission type laser diode is excellent and in view of the laser
diode is characterized by low threshold of laser oscillation, the heat
generation associated with operation of the laser diode is reduced and the
system can be used without cooling up to high temperatures. Further, it is
possible to couple the GaInNAs vertical-cavity, surface-emission-type
laser diode with a fluorine added POF (plastics optical fiber) that shows
a low optical loss at the wavelength longer than the 1.3 .mu.m band. In
this case, the optical fiber is low cost and the diameter of the optical
fiber is large enough for facilitating optical coupling. Thus, the
mounting cost is reduced and the optical module can be realized with
extremely low cost.
Example 4
Example 4 is related to an optical transceiver module. FIG. 12 shows the
general construction of the optical transceiver module, in which the 1.3
.mu.m band GaInNAs vertical-cavity, surface-emission-type laser diode of
Example 2 and a receiver photodiode are coupled with an optical fiber of
example 2.
By using the vertical-cavity, surface-emission-type laser diode of the
present invention in the optical telecommunication system, the cost of the
optical telecommunication system that uses an optical transceiver module,
in which a transmitter laser diode, a receiver photodetector and an
optical fiber are coupled, is reduced substantially, in view of the low
cost of the vertical-cavity surface-emission type laser diode of the
present invention. In view of the fact that the temperature
characteristics of the GaInNAs vertical-cavity surface-emission type laser
diode is excellent and in view of the fact that the laser diode is
characterized by low threshold of laser oscillation, the heat generation
associated with operation of the laser diode is reduced and the system can
be used without cooling up to high temperatures. Further, it is possible
to couple the GaInNAs vertical-cavity, surface-emission-type laser diode
with a fluorine added POF (plastics optical fiber) that shows a low
optical loss at the wavelength longer than the 1.3 .mu.m band. In this
case, the optical fiber is low cost and the diameter of the optical fiber
is large enough for facilitating optical coupling. Thus, the mounting cost
is reduced and the optical module can be realized with extremely low cost.
It should be noted that the optical telecommunication system that uses the
vertical-cavity, surface-emission-type laser diode of the present
invention is by no means limited to a long-distance telecommunication
system that uses an optical fiber. For example, the optical
telecommunication system of the present invention is applicable to
short-distance telecommunication including device-to-device data exchange
in computers and local area networks (LAN). Furthermore, the present
invention is applicable to data exchange between large-scale integrated
circuits or inside a large-scale integrated circuit. Further, the present
invention is applicable to data exchange between printed circuit boards.
In recent years, the processing performance of large-scale integrated
circuits has been improved remarkably. Thus, the transmission rate in the
part connecting large-scale integrated circuits is becoming a bottleneck
of overall system performance. Thus, by replacing the conventional
electric interconnection by optical signal interconnection inside a system
by way of using optical transmission modules or optical transceiver
modules, examples of which may be device-to-device optical interconnection
inside a large-scale integrated circuit, optical interconnection between
large-scale integrated circuits in a circuit board, or optical
interconnection between circuit boards in a computer system, a ultrahigh
speed computer system becomes possible. Further, a ultrahigh speed network
system is realized when a plurality of computer systems, and the like, are
interconnected by using the above-mentioned optical transmission modules
or optical transceiver modules.
Especially, a vertical-cavity, surface-emission-type laser diode consumes
incommensurably small electric power as compared with an edge-emission
type laser diode and is particularly suited for constructing a
two-dimensional array. Because of this, the laser diode is suitable for
constructing a parallel optical telecommunication system.
Example 5
In Example 5, a vertical-cavity, surface-emission-type laser diode is
provided having a construction similar to that of the laser diode of
Example 2 except that the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layers 203 and 208 in the constitution
of FIGS. 10A and 10B acting as a non-optical recombination elimination
layer has a composition of Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (x=0.55,
y=0).
Thus, in the vertical-cavity, surface-emission-type laser diode of Example
5, the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layers 203 and 208 accumulate therein a strain. Because
of this, there is achieved an effect, in addition to the effect of Example
2, to suppress the crawling up of the defects existing in the substrate or
formed during the growth process, at least partially at the time of growth
of the epitaxial layers. As a result, the efficacy of optical emission is
improved substantially. Further, even in the case the crystal quality of
the n-side multilayer reflector (lower reflector) 204 is moderate, it was
possible to grow an active layer having a large strain easily. Thus, in
this example, it is possible to grow an active layer accumulating a
compressive strain of 2% or more. Further, it becomes possible to grow a
strained layer with a thickness exceeding the critical thickness.
It should be noted that the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y
(0<x.ltoreq.1, 0<y.ltoreq.1) layers 203 and 208 make a contact with
the active region. In view of the fact that there occurs an increase of
bandgap energy with decrease of lattice constant in the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layers
203 and 208, the height of the hetero barrier formed between the active
region and the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) is increased. As a result, the efficiency of carrier
confinement is improved and the threshold current is reduced. Thereby, the
temperature characteristics are improved. More specifically, in Example 5,
the bandgap became larger by about 70 meV as compared with the device of
Example 2 (Ga.sub.0.5 In.sub.0.5 P).
Example 6
In Example 6, the vertical-cavity, surface-emission-type laser diode is
different from the device of Example 2 in the point that the Ga.sub.x
In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1, 0<y.ltoreq.1) layers
203 and 208 acting as a non-optical recombination elimination layer in the
construction of FIGS. 10A and 10B are formed of Ga.sub.x In.sub.1-x
P.sub.y As.sub.1-y (x=0.45, y=1)
Thus, in the vertical-cavity, surface-emission-type laser diode of Example
6, the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layers 203 and 208 accumulate therein a strain. Because
of this, there is achieved an effect, in addition to the effect of Example
2, to suppress the crawling up of the defects existing in the substrate or
formed during the growth process, at least partially at the time of growth
of the epitaxial layers. As a result, the efficacy of optical emission is
improved substantially. Further, even in the case the crystal quality of
the n-side multilayer reflector (lower reflector) 204 is moderate, it was
possible to grow an active layer having a large strain easily. Thus, in
this example, it is possible to grow an active layer accumulating a
compressive strain of 2% or more. Further, it becomes possible to grow a
strained layer with a thickness exceeding the critical thickness.
Furthermore, in Example 6, it should be noted that the sense of the strain
in the Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y (0<x.ltoreq.1,
0<y.ltoreq.1) layers 203 and 208 is the same as the sense of the strain
in the active layer 206a. Because of this, one obtains an advantageous
effect in that the actual compressive strain that the active layer 206a
senses is reduced, in addition to the effect that the strained layer is
inserted. Because of this, the influence of the defects existing on the
surface of the foundation layer in the state immediately before the growth
of the active layer is reduced. As a result, the active layer was grown
with improved quality and the characteristics of the laser diode were
improved.
It turned out that the present invention is particularly effective in the
case of the vertical-cavity, surface-emission-type laser diode of long
wavelength band that requires a thick film growth process. For example, in
the case of the 1.3 .mu.m band GaInNAs vertical-cavity,
surface-emission-type laser diode formed on a GaAs substrate, it is
necessary to grow 50-80 semiconductor layers with a total thickness of 5-8
.mu.m before the growth of the active layer, in view of the use of the
semiconductor multilayer reflector for the optical cavity. In the case of
an edge-emission type laser diode, on the contrary, the total thickness
before the growth of the active layer is about 2 .mu.m. In this case, it
is sufficient to grow only about three semiconductor layers. In the case
of the vertical-cavity, surface-emission-type laser diode of such a long
wavelength band, the defect density of the foundation layer surface, on
which the growth of the active layer is made, increases inevitably in the
state immediately before the growth of the active layer, as compared with
the defect density of the GaAs substrate surface by various reasons, even
in the case a GaAs substrate of high quality is used. (It should be noted
that the defect once occurred generally crawls up in the direction of
crystal growth. Further, there can be defect formation, and the like, at
the hetero interface.) By reducing the actual compressive strain that the
active layer senses or by inserting a strained layer before the growth of
the active layer, the adversary influence caused by the defects existing
on the foundation layer surface immediately before the growth of the
active layer is reduced. Because of this, it becomes possible to conduct a
thick film growth such as the one used in the vertical-cavity,
surface-emission-type laser diode, easily with high crystal quality.
Example 7
FIGS. 20A and 20B show the construction of a vertical-cavity,
surface-emission-type laser diode according to Example 7, wherein FIG. 20B
shows an enlarged view of the active region of FIG. 20A.
Referring to FIGS. 20A and 20B, the vertical-cavity, surface-emission-type
laser diode of Example 7 is formed on an n-GaAs substrate 301 and includes
a lower reflector 302, a non-optical recombination elimination layer 303,
a multiple quantum-well active layer 306, and an upper reflector 309
laminated consecutively on the GaAs substrate 301. Here, the lower
reflector 302 is formed by alternate lamination of an n-Al.sub.x
Ga.sub.1-x As (x=0.9) layer and an n-Al.sub.x Ga.sub.1-x As (x=0) layer.
Further, the multiple quantum-well active layer 306 is formed of three
active layers each formed of a Ga.sub.x In.sub.1-x N.sub.y As.sub.1-y
quantum well layer (quantum-well active layer) 306a and corresponding GaAs
barrier layers 306b. In other words, in the vertical-cavity,
surface-emission-type laser diode of Example 7, the lower reflector 302
contains Al, while the active layer 306 contains nitrogen. In FIGS. 20A
and 20b, the reference numeral 340 designates a low-refractive index layer
having the thickness of 3 .lambda./4.
In the production of the vertical-cavity, surface-emission-type laser diode
of FIGS. 20A and 20B, the crystal growth was conducted by an MOCVD
process. Thereby, TMA was used as the Al source material when growing a
layer containing Al, while DMHy was used as the source material of
nitrogen when growing a layer (GaInNAs layer) containing nitrogen.
The difference between the device Example 7 and the device of Example 2 is
that the growth of the GaAs layer 310 located underneath the non-optical
recombination elimination layer 303 is interrupted midway of growth
thereof with (at the part shown with a dashed line B). During the
interruption of the growth, the Al source material, Al product, Al
compound or Al remaining in the growth chamber where the nitrogen source
compound or impurity therein may make a contact therewith is removed by a
purging process that uses a hydrogen carrier gas. In Example 7, the
purging process was conducted for the duration of 60 minutes. During the
interruption of growth in Example 7, the wafer was left in the reaction
chamber.
In this way, the Al concentration in the GaInNAs active layer 306 was
reduced to 3.times.10.sup.17 cm.sup.-3 or less, and the oxygen
concentration level in the GaInNAs active layer 306 was reduced to
1.times.10.sup.17 cm.sup.-3, or background level. Thereby, the efficacy of
optical emission of the active layer 306 containing nitrogen was improved
and the threshold current of the device was reduced.
In Example 7, it is noted that the interruption of growth was made during
the growth of the GaAs layer 310 located underneath the non-optical
recombination elimination layer 303. However, such an interruption growth
may be made during the growth of the non-optical recombination elimination
layer 303.
In Example 7, the growth of the crystal layer was interrupted and purging
of the Al source material, Al product, Al compound or Al was conducted.
However, it is also possible to reduce the growth rate without doing
interrupting the growth. In this case, the duration between the growth of
the layer containing Al and the growth of the layer containing nitrogen is
increased, and the purging can be made while continuing the grow process.
In example 7, the upper non-optical recombination elimination layer is not
provided unlike the device of Example 2. Further, the non-optical
recombination elimination layer is not used as an etching stopper.
However, it is as well possible to construct as in the case of Example 2.
Example 8
FIGS. 21A and 21B show the example of a vertical-cavity,
surface-emission-type laser diode according to Example 8, wherein FIG. 21B
shows an enlarged view of the active region of FIG. 21A.
Referring to FIGS. 21A and 21B, the vertical-cavity surface-emission type
laser diode of Example 8 is formed on a n-GaAs substrate 401 and includes
an n-semiconductor distributed Bragg reflector (lower reflector) 410, an
optical cavity part 413, and a p-semiconductor distributed Bragg reflector
(upper reflector) 412 laminated consecutively on the n-GaAs substrate 401.
It should be noted that the uppermost layer of the lower reflector 410 is
formed of an AlGaAs low-refractive index layer 409. Further, the lowermost
layer of the upper reflector 412 is formed of an AlGaAs low-refractive
index layer 411. Further, the optical cavity part 413 is formed of an
active layer 406 which in turn is formed of three GaInNAs quantum well
layers 406a and corresponding GaAs barrier layers 406b, first GaAs barrier
layers 405 and 407, and GaInP non-optical recombination elimination layers
(second barrier layer) 403 and 408.
The difference between the vertical-cavity, surface-emission-type laser
diode of Example 8 and the device of Example 2 is that the non-optical
recombination elimination layers 403 and 408 are formed inside the optical
cavity 413. Further, in Example 8, the thickness of the optical cavity 413
is set to one wavelength.
In the structure of FIGS. 21A and 21B, it should be noted that the GaInP
non-optical recombination elimination layers (the second barrier layer)
403 and 408 have a bandgap larger than the bandgap of GaAs first barrier
layers 405 and 407. Further, the active region in which carrier injected
is made extends substantially up to the GaAs first barrier layers 405 and
407. Because of this, the effect similar to the device of Example 2 is
obtained.
In the of interrupting the growth the crystal layers like the device of
Example 7, it is possible to conduct such an interruption process midway
of the growth of lower non-optical recombination elimination layer 403.
Alternatively, the interruption may be made during the growth of a GaAs
layer is provided between the lower non-optical recombination elimination
layer 403 and the layer containing Al (such as the layer 409).
Further, the present invention is not limited to the embodiments described
heretofore, but various variations and modifications may be made without
departing from the scope of the present invention.
Present invention is based on Japanese patent applications 2000-286477
filed on Sep. 21, 2000, 2001-068588 filed on Mar. 12, 2001 and 2001-214930
filed on Jul. 16, 2001, the entire contents of which are incorporated
herein as reference.
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