Semiconductor laser producing visible light

A semiconductor laser for producing visible light includes a first conductivity type semiconductor substrate; a first conductivity type semiconductor first cladding layer, a semiconductor active layer of GaAs.sub.1-y P.sub.y (y.ltoreq.0.45), and a second conductivity type second cladding layer, the first cladding layer, the active layer, and the second cladding layer being successively disposed on the semiconductor substrate, the first and second cladding layers having substantially the same composition and a different composition from the active layer, thereby forming heterojunctions with the active layer, and having a lattice constant different from the lattice constant of the active layer and introducing stress into the active layer without producing dislocations in the active layer; and first and second electrodes electrically connected to the substrate and the second cladding layer, respectively.

FIELD OF THE INVENTION 
The present invention relates to a semiconductor laser that produces 
visible light with reduced threshold current density and particularly to a 
semiconductor laser with an active layer of GaAs.sub.1-y P.sub.y 
(0.4.ltoreq.y.ltoreq.0.45). 
BACKGROUND OF THE INVENTION 
Semiconductor lasers producing visible light have been known for some time. 
Generally, those lasers employ materials from the InGaAlP system of 
materials in the various layers within the laser. Typically, the active 
layer of these lasers is In.sub.0.5 Ga.sub.0.5 P. That material and all 
active layer materials in semiconductor lasers must have a "direct" 
transition energy band structure. In these kinds of materials, as 
illustrated in FIG. 2(a), the distribution of energy states in the 
conduction band 24 has a minimum 25 that is directly opposite a maximum 26 
of the energy state distribution in the valence band 27 when charge 
carrier energy is plotted as a function of wave vector. In this type of 
direct transition energy band structure, electrons and holes can directly 
recombine to radiate light efficiently without interaction with a phonon. 
In an indirect transition semiconductor material, i.e., when the energy 
density minimum and maximum are not directly opposite each other in the 
wave vector graph, the recombination of an electron and a hole requires a 
phonon interaction. Light emission is not efficient in direct transition 
semiconductor materials and laser oscillation has not been achieved in 
them. 
Although the minimum conduction band energy and the maximum valence band 
energy in a direct transition semiconductor material determine the energy 
band gap and the wavelength of radiated light when that material is used 
as the active layer in a semiconductor laser, the conduction band edge 
structure of binary, ternary, and quaternary III-V compound semiconductor 
materials is more complex than a single minimum of energy state densities. 
For example, as shown in FIG. 2(a), the conduction band edge 24 may 
include a second "valley" 28 displaced from the minimum 25. When the 
density of electrons sufficiently fills the energy states at the minimum 
conduction band edge energy 25, some of the carriers may overflow and 
begin to fill the lowest energy states in the nearby conduction band edge 
valley 28. Those overflowing electrons cannot participate in laser 
oscillation because they require phonon interactions in recombining with 
holes in the valence band. Therefore, current supplied to a semiconductor 
laser that results in filling of the valley 28 near the conduction band 
edge minimum energy 25 does not result in laser oscillation and reduces 
the efficiency of the conversion of electrical current to laser light. One 
of the external effects of that reduced efficiency is an apparent increase 
in the threshold current density at which laser oscillation begins. 
An example of the structures of a known semiconductor laser 300 producing 
visible light and employing materials of the InGaAilP material system is 
illustrated in FIG. 1. That semiconductor laser is described in Applied 
Physics Letters, Volume 56, Number 18, pages 1718-1719, (1990). Referring 
to FIG. 1, the laser 300 includes an n-type GaAs substrate 11, a one 
micron thick silicon doped n-type In.sub.0.5 (Ga.sub.0.3 
Al.sub.0.7).sub.0.5 P first cladding layer 12, a forty nanometer thick 
undoped In.sub.0.5 Ga.sub.0.5 P active layer 13, and a one micron thick 
zinc doped p-type In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P second 
cladding layer 14. Those three layers are sequentially disposed on the 
substrate 11 and the second cladding layer 14 includes a central mesa 19. 
A fifty nanometer thick zinc doped p-type In.sub.0.5 Ga.sub.0.5 P cap 
layer 15 is disposed on the top surface of the mesa of the second cladding 
layer 14. As well understood in the art, in manufacturing the 
semiconductor laser 300, the second cladding layer 14 and the cap layer 15 
are sequentially grown and the mesa is formed by masking parts of those 
layers and removing the unmasked parts of the layers by etching. A zinc 
doped p-type GaAs contact layer 16 is disposed on and contacts the second 
cladding layer 14 and the cap layer 15. The thickest part of the contact 
layer 16 is three microns thick. The semiconductor laser 300 also includes 
electrodes 17 and 18 on the contact layer 16 and the substrate 11, 
respectively. 
As in conventional semiconductor lasers, when the semiconductor laser 300 
is forward biased, holes move from the contact layer 16 toward the active 
layer 13 and electrons move from the substrate 11 toward the active layer 
13. The holes and electrons are injected into the active layer 13 where 
they recombine to produce light. Because the material of the first and 
second cladding layers is different from the material of the active layer, 
heterojunctions are formed at the interfaces of the active layer 13 with 
the first and second cladding layers 12 and 14. The heterojunctions 
include potential barriers that assist in confining the charge carriers 
injected into the active layer 13 to an active region to stimulate 
recombination and the emission of light. Because of the differences in the 
composition of the first and second cladding layers and the active layer, 
those layers have different refractive indices. The first refractive index 
of the first and second cladding layers also assists in confining light 
produced by carrier recombination to the active layer, defining a resonant 
cavity for supporting laser oscillation. 
In this structure, because of the composition of the active layer 13, the 
laser light produced has a wavelength of about six hundred seventy 
nanometers. The light produced by the charge carrier recombinations and 
confined to the active region resonates between opposed facets 20 and 21 
of the laser that are transverse to the mesa 19, resulting in laser 
oscillation. Most of that laser light is produced along the direction of 
the mesa 19 because the holes preferentially flow through the mesa toward 
the active layer 13. In the semiconductor laser 300, since all the 
materials are selected from the InGaAiP system, partially to ensure 
relatively close matching between the lattice constants of the different 
materials, the maximum difference between the energy band gap of the 
cladding layers and of the active layer is about 0.2 eV. 
Although the InGaAlP system of materials offers the advantage of small 
lattice constant mismatches between various materials within that system, 
simplifying epitaxial growth processes, a limited amount of strain 
produced by lattice mismatching between contiguous layers of a 
semiconductor laser can have a beneficial effect, as illustrated in FIGS. 
2(a) and 2(b). As already described, the typical energy state distribution 
as a function of wave vector for a compound semiconductor material is 
illustrated in FIG. 2(a). In FIG. 2(a), the energy band edges are 
illustrated without the presence of any stress, for example, introduced by 
confining the semiconductor material as a layer between two other layers 
having different lattice constants. That stress produces a strain, locally 
altering the lattice constant of the semiconductor layer as compared to 
the unstressed lattice constant. In FIG. 2(b), the same energy band 
structure for the same material as in FIG. 2(a) is shown when that 
material is subjected to a moderate stress. By comparing FIGS. 2(a) and 
2(b), it can be seen that the stress causes the valence band edge 27 to 
become more parabolic, i.e., the density of energy states becomes more 
closely concentrated around the y axis, the wave vector value at which 
direct transitions of combining holes and electrons occur. As a result of 
this change in the distribution of the energy states, laser oscillation 
efficiency is improved and the threshold current is reduced. 
In the InGaAlP material system, it may be desired to introduce stress to 
achieve improved efficiency, applying the principle illustrated in FIGS. 
2(a) and 2(b). For example, the cladding layers might be AIGaP, the 
material within that system of materials having the largest energy band 
gap. Theoretically, that material would provide improved charge carrier 
confinement in the active layer because of increased potential barriers at 
the heterojunctions with the active layer as well as introducing stress 
because of the different lattice constants of AIGaP and of the In.sub.0.5 
Ga.sub.0.5 P active layer. However, because the difference in the lattice 
constants of AIGaP and In.sub.0.5 Ga.sub.0.5 P is so large, the resulting 
stress can produce crystalline dislocations within the active layer. 
Dislocations can be avoided for a particular stress by making the active 
layer sufficiently thin. For example, when the active layer is less than 
ten nanometers thick, a relatively large amount of stress can be tolerated 
without the appearance of dislocations. However, such a thin active layer 
causes rapid filling of the energy states at the minimum of the conduction 
band with electrons so that electrons easily overflow into an adjacent 
valley of the conduction band edge. Thus, the improved efficiency achieved 
by introducing stress is lost. Therefore, suitable materials cannot be 
selected from the InGaAlP system for constructing a semiconductor laser 
producing visible light with improved efficiency. The desirable effects of 
stress on the active layer require that the active layer be so thin that 
other losses, reducing efficiency, would be experienced. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor laser 
producing visible light that effectively confines charge carriers in the 
active layer and restricts the energy distribution of holes, resulting in 
laser oscillation at a reduced threshold current density. 
According to one aspect of the invention, a semiconductor laser for 
producing visible light comprises a first conductivity type semiconductor 
substrate; a first conductivity type semiconductor first cladding layer, a 
semiconductor active layer of GaAs.sub.1-y P.sub.y (y.ltoreq.0.45), and a 
second conductivity type second cladding layer, the first cladding layer, 
the active layer, and the second cladding layer being successively 
disposed on the semiconductor substrate, the first and second cladding 
layers having substantially the same composition and a different 
composition from the active layer, thereby forming heterojunctions with 
the active layer, and having a lattice constant different from the lattice 
constant of the active layer and introducing stress into the active layer 
without producing dislocations in the active layer; and first and second 
electrodes electrically connected to the substrate and the second cladding 
layer, respectively. 
According to a second aspect of the invention, a semiconductor laser for 
producing visible light comprises a first conductivity type semiconductor 
substrate; a first conductivity type semiconductor first cladding layer, a 
semiconductor active layer including alternating layers of GaAs.sub.1-y 
P.sub.y (y.ltoreq.0.45) and GaP, and a second conductivity type second 
cladding layer, the first cladding layer, the active layer, and the second 
cladding layer being successively disposed on the semiconductor substrate, 
the first and second cladding layers having substantially the same 
composition, forming heterojunctions with the active layer and introducing 
stress into the active layer without producing dislocations in the active 
layer; and first and second electrodes electrically connected to the 
substrate and the second cladding layer, respectively. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter. The detailed description 
of specific embodiments is for illustration only since various additions 
and modifications of the embodiments within the spirit and scope of the 
invention will become apparent to those of skill in the art from the 
detailed description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 3 is perspective, schematic view of a semiconductor laser 100 for 
producing visible light according to an embodiment of the invention. The 
semiconductor laser 100 includes an n-type GaP substrate 1 on which there 
are successively disposed a one micron thick n-type Al.sub.0.3 Ga.sub.0.7 
P first cladding layer 2, a twenty nanometer thick undoped GaAso.sub.0.55 
P.sub.0.45 active layer 3, and a 0.7 micron thick p-type Al.sub.0.3 
Ga.sub.0.7 P second cladding layer 4. The semiconductor laser 100 has a 
groove-type structure. A 0.2 micron thick n-type Al.sub.0.7 Ga.sub.0.3 P 
current blocking layer 5 is disposed on the second cladding layer 4 and 
has a central longitudinal groove exposing part of the second cladding 
layer 4. The groove is formed conventionally during the manufacture of the 
laser 100 by masking and etching the current blocking layer 5, for 
example, with an etchant including H.sub.2 SO.sub.4, H.sub.2 O.sub.2, and 
water mixed in a ratio of 5:1:1. A 0.2 micron thick p-type GaP cap layer 6 
is disposed on the current blocking layer 5 and on the second cladding 
layer 4 in the groove in the current blocking layer 5. Electrodes 7 and 8 
are disposed on and are in electrical contact with the cap layer 6 and the 
substrate 1, respectively. The semiconductor laser 100 includes front and 
rear facets 22 and 23 transverse to the layers 2, 3, and 4 forming a 
resonator including the double heterojunction structure of layers 2, 3, 
and 4. In addition, the groove in the current blocking layer 5 aids in 
concentrating the current flow between the electrodes 7 and 8 into an 
active region of the active layer 3, assisting in defining the resonant 
cavity of the laser 100. 
The semiconductor laser 100 is manufactured using conventional techniques 
including the successive epitaxial growth of the compound semiconductor 
layers 2-6 on the substrate 1 by metal organic chemical vapor deposition 
or molecular beam epitaxy. The electrodes 7 and 8 may be formed by 
depositing an alloy of gold and germanium followed by the deposition of 
gold. The facets 22 and 23 are preferably formed by cleaving. 
GaAl.sub.1-y P.sub.y has a direct transition energy band structure provided 
y is no larger than 0.45. Therefore, in order to produce laser 
oscillation, y cannot exceed 0.45. With that composition of the active 
layer 3 and Al.sub.0.3 Ga.sub.0.7 P cladding layers 2 and 4, the lattice 
constant mismatch between the active layer 3 and the cladding layers 2 and 
4 is about two percent. That mismatch is sufficient to introduce stress 
and alter the distribution of the density of states in the valence band of 
the active layer 3, as illustrated in FIGS. 2(a) and 2(b), thereby 
improving the electricity-to-light conversion efficiency of the laser. 
When, as in the specific example described, the active layer 3 is twenty 
nanometers thick, the stress does not produce dislocations in the active 
layer 3 that adversely affect laser performance. When y is decreased below 
0.45, the lattice constant mismatch and the introduced stress increase, 
increasing the risk of generating dislocations in the active layer. 
Therefore, as y is decreased, the thickness of the active layer 3 is 
decreased to avoid the generation of dislocations. As a practical 
limitation, once the thickness of the active layer 3 is reduced to about 
ten nanometers, the improved efficiency achieved through the introduction 
of stress into the active layer declines because of charge carrier 
overflow. Therefore, when the thickness of the active layer 3 is reduced 
to about ten nanometers, i.e., when y equals about 0.4, to control the 
stress introduced by lattice constant mismatching so that dislocations are 
not generated, a practical limit is reached. 
In the semiconductor laser 100 described above, the energy band gap of the 
first and second cladding layers 2 and 4 ranges from 2.26 to 2.46 eV and 
the energy band gap of the active layer 3 is about 2.15 eV. The energy 
band gap difference between the cladding and active layers ranges from 0.1 
to 0.3 eV, providing relatively large charge carrier confining potential 
barriers at the heterojunctions. The refractive indices of the second and 
first cladding layers 2 and 4 fall within a range from about 3.03 to 3.45, 
efficiently confining light in the active layer 3. 
The operation of the semiconductor laser 100 is similar to that of the 
operation of the prior art semiconductor laser 300. In response to a 
forward bias, resulting in the flow of a current exceeding the threshold 
current density of the laser, holes move from the cap layer 6 in the 
direction of the active layer 3 and electrons move from the substrate 1 
toward the active layer 3. The holes and electrons are injected into that 
active layer 3 and are confined in that layer by the potential barriers at 
the heterojunctions of the first and second cladding layers 2 and 4 and 
the active layer 3. The injected, confined charge carriers recombine to 
produce red light having a wavelength of about six hundred fifty 
nanometers when the active layer has a composition of GaAs.sub.0.55 
P.sub.0.45. The differences in refractive indices of the cladding layers 
and the active layer confine the light generated by charge carrier 
recombination to the portion of the active layer opposite the groove, 
causing the light to resonate between the facets 22 and 23 of the 
resonator structure, producing laser oscillation. 
FIG. 4(a) is a schematic, perspective view of a semiconductor laser 200 
producing visible light in accordance with another embodiment of the 
invention. The structure of the semiconductor laser 200 is identical to 
that of the structure of the semiconductor laser 100 with the exception of 
the active layer 3a which, in the semiconductor laser 200, comprises a 
multi-quantum well structure. As illustrated more clearly in FIG. 4(b), 
that active layer 3a includes ten or more alternating layers 30a and 30b 
of different materials. The alternating layers 30a, for example, are 
undoped five nanometer thick layers of GaAs.sub.0.55 P.sub.0.45 and the 
alternating layers 30b are twenty nanometer thick layers of GaP. The 
undoped GaP layers 30b have a much higher energy band gap than the 
GaAs.sub.1-y P.sub.y layers 30a. Most preferably, the layers of the active 
layer 3a that are adjacent and in contact with the first and second 
cladding layers 2 and 4 are the narrower energy band gap GaAs.sub.1-y 
P.sub.y layers 30a. The multi-quantum well active layer 3a functions in 
the same way as the active layer 3 of the semiconductor laser 100. 
In the semiconductor laser 200, the total thickness of the layers 30a and 
30b, i.e., of the active layer 3a, if at least ten of each of those layers 
is present, is larger than the thickness of the active layer 3 of the 
semiconductor laser 100. Nevertheless, stress is desirably present, 
altering the effective valence band edge structure of the multi-quantum 
well structure. Because of the alternating layer structure, dislocations 
are not introduced in the thicker active layer 3a even though stress is 
present so that the desirable effects of stress are achieved without 
overflow of charge carriers from the relatively thick active layer. All of 
these characteristics result in improved light emission efficiency. 
Although the semiconductor lasers 100 and 200 were described as including a 
substrate 1 of GaP, because its lattice constant is nearly the same as 
that of silicon, a less expensive substrate of silicon may be employed in 
place of the GaP substrate. When a silicon substrate is used, it may 
extend beyond the semiconductor laser structure, enabling the production 
of other semiconductor devices, such as memory elements or transistors, in 
the substrate that may be associated with the semiconductor laser. 
While the specific embodiments of the invention illustrated in the figures 
employ a groove-type semiconductor laser structure, the invention is not 
limited to that particular semiconductor laser structure. A mesa structure 
of the type illustrated in FIG. 1 may also be employed in the invention 
with an active layer of GaAs.sub.1-y P.sub.y and Al.sub.x Ga.sub.1-x P 
cladding layers, provided the active layer is a direct transition 
semiconductor material and that the stress introduced into the active 
layer by the first and second cladding layers is not sufficient, 
considering the thickness of the active layer, to produce dislocations in 
the active layer.