Integrated semiconductor laser producing light of different wavelengths at respective active regions

A method of making an integrated semiconductor laser on a common substrate including at least two active regions, each active region oscillating at a respective, different wavelength, including producing a precursor laser structure by successively growing on a semiconductor substrate a first conductivity type semiconductor first cladding layer, an active layer including at least one compound semiconductor quantum well layer sandwiched between compound semiconductor quantum barrier layers, and a second conductivity type semiconductor second cladding layer, the quantum barrier layers having a larger energy band gap than and including at least one more element than the quantum well layer, annealing the precursor structure including controlling at first and second spaced apart regions the diffusion of the at least one more element from the quantum barrier layers into the quantum well layer to produce first and second spaced apart active regions in the active layer having different effective lasing energy band gaps, and forming respective electrical contacts to the first and second cladding layers on opposite sides of each of the first and second active regions.

FIELD OF THE INVENTION 
The present invention concerns an integrated semiconductor laser including 
at least two separate active regions, each active region producing light 
of a different wavelength, and to a method of making such an integrated 
semiconductor laser. 
BACKGROUND OF THE INVENTION 
Wavelength division multiplex optical communications systems are of growing 
importance in the transmission of large quantities of information, such as 
the transmission of images. Integrated lasers producing at least two light 
beams of different wavelengths are particularly useful in multiplexed 
transmission of information in optical communications systems. In those 
systems, an integrated laser producing at least two different wavelength 
laser beams simplifies optical alignments and optical matching. 
Semiconductor lasers including two or more active regions, each active 
region producing light of a different wavelength, are known. However, the 
lasers require complicated manufacturing processes that result in poor 
production yields. An example of such an integrated semiconductor laser is 
described in Japanese Published Patent Application 62-48917. The laser 
structure described in that publication is shown in a schematic 
perspective view in FIG. 7. A method of manufacturing that laser structure 
is shown in FIGS. 8(a)-8(d). 
The laser of FIG. 7 includes three active regions disposed on a base 
including an n-type indium phosphide substrate 21 and an n-type indium 
phosphide buffer layer 22 disposed on the substrate. In a first active 
region, at the left of the structure as shown in FIG. 7, a first indium 
gallium arsenide phosphide active layer 31 is disposed directly on the 
buffer layer 22. The relative proportions of the constituents of the 
indium gallium arsenide phosphide active layer 31 are adjusted for laser 
oscillation at a first wavelength. In a central active region, layer 31 is 
also present and successively disposed on it are a first intermediate 
n-type indium phosphide layer 41 and a second indium gallium arsenide 
phosphide active layer 32. The relative proportions of the constituents of 
the second active layer 32 are adjusted for laser oscillation at a second 
wavelength different from the wavelength at which laser oscillations are 
produced in the first active layer 31. In a third active region, shown at 
the right side of FIG. 7, the same layers are present as in the central 
active region. In addition, a second n-type indium phosphide intermediate 
layer 42 and a third indium gallium arsenide phosphide active layer 33 are 
successively disposed on the first intermediate layer 41. The relative 
proportions of the constituents of the third indium gallium arsenide 
phosphide active layer 33 are adjusted for laser oscillation at a third 
wavelength different from the wavelengths produced in the first and 
central active regions. Each of the active regions is confined at its 
sides by a p-type indium phosphide current blocking layer 23 and an n-type 
indium phosphide current confining layer 24 disposed on layer 23. The 
active regions and the current confining layer 24 are covered by a p-type 
indium phosphide layer 25. Finally, an n-type indium gallium arsenide 
phosphide contact layer 26 is disposed on the p-type indium phosphide 
layer 25. A silicon dioxide film 27 is selectively disposed on the contact 
layer 26 and includes openings opposite each of the three active regions. 
Electrodes 51, 52, and 53 are disposed on the silicon dioxide film 27 and 
in contact with the contact layer 26 respectively opposite each of the 
active regions. To improve the quality of the contact, zinc is diffused 
through layer 26 and into layer 25 at regions 71, 72, and 73 respectively 
opposite the first, central, and third active regions. The electrodes 51, 
52, and 53 respectively contact regions 71, 72, and 73. A common electrode 
61 is disposed on the substrate opposite the buffer layer 22. 
The process for manufacturing the laser structure of FIG. 7 is relatively 
complex. Steps in that process are illustrated in FIGS. 8(a)-8(d). As 
shown in FIG. 8(a), the buffer layer 22, the first active layer 31, the 
first intermediate layer 41, the second active layer 32, the second 
intermediate layer 42, and the third active layer 33 are successively 
grown on substrate 21. As illustrated in FIG. 8(b), the grown films are 
selectively etched to expose active layers 31, 32, and 33 over respective 
lengths of about two hundred microns. The active regions of the laser are 
then prepared by etching ridges 81, 82, and 83 lying along the &lt;110&gt; 
direction and having a width of two to three microns as illustrated in 
FIG. 8(c). The ridges are defined by respective etching masks 91, 92, and 
93. Subsequently, as illustrated in FIG. 8(d), the current blocking layer 
23 and current confining layer 24 are successively grown adjacent the 
sides of the ridges. Finally, the p-type indium phosphide layer 25 and the 
n-type contact layer 26 are successively grown on the current confining 
layer 24 and each of the ridges. Thereafter, diffusion masks, such as the 
layer 27, are deposited on the contact layer. The masks each include an 
opening disposed opposite the ridges 81, 82, and 83, typically of a width 
of about ten microns. Zinc is diffused through the openings in the 
diffusion masks to a depth to reach the p-type layer 25 and establish 
contact to the respective uppermost active layers at each ridge. 
Electrodes 51, 52, and 53 are deposited on the diffusion masks in contact 
with the respective zinc-diffused regions 71, 72, and 73. A common 
electrode 61 is deposited on the reverse side of the substrate. 
In the resulting structure, each of the active regions can be separately 
forward biased and each oscillates at a different wavelength, providing 
three light beams that can be independently generated and modulated to 
increase the amount of information transmitted in an optical 
communications system. However, the method of manufacturing the integrated 
laser is so complicated that it is difficult to manufacture the structure 
economically. 
Another integrated semiconductor laser structure having three active 
regions and a single substrate is shown in a perspective, partially 
cut-away view in FIG. 9. The active regions 101, 102, and 103 interact 
with respective diffraction gratings 201, 202, and 203. The periods of the 
respective diffraction gratings are different in order to produce 
different wavelength light at each of the active regions. The diffraction 
gratings are produced by a conventional technique in which interference 
fringes illuminate a resist film before its development and subsequent 
etching. However, it is difficult to control the different periods of the 
three gratings and each of the laser sections includes a wavelength 
control adjustment portion 301, 302, and 303, respectively, for tuning the 
oscillation wavelengths. An insulating film 400 separates the respective 
electrodes 501, 502, and 503 from the substrate. A common electrode 600 is 
disposed on the opposite side of the substrate. The three active regions, 
i.e., laser elements, are mutually isolated by grooves 701 and 702. 
Like the structure of FIG. 7, the complex structure of FIG. 9 requires many 
complicated processing steps, particularly in the formation of several 
diffraction gratings, each having a different period. As a result, the 
production yield is very poor, resulting in high costs. 
Accordingly, it would be desirable to produce an integrated laser including 
at least two active regions, each active region producing laser light at a 
different wavelength in a relatively simple process providing good product 
yield at reasonable product cost. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a relatively simple 
method for producing an integrated semiconductor laser including at least 
two active regions, each active region producing laser light at a 
different wavelength. 
It is another object of the present invention to provide an integrated 
semiconductor laser including at least two active regions on a common 
substrate, each active region oscillating at a different wavelength. 
According to a first aspect of the invention, a method of making an 
integrated semiconductor laser on a common substrate including at least 
two active regions, each active region oscillating at a respective, 
different wavelength, comprises producing a precursor laser structure by 
successively growing on a semiconductor substrate a first conductivity 
type semiconductor first cladding layer, an active layer including at 
least one compound semiconductor quantum well layer sandwiched between 
compound semiconductor quantum barrier layers, and a second conductivity 
type semiconductor second cladding layer, the quantum barrier layers 
having a larger energy band gap than and including at least one more 
element than the quantum well layer, annealing the precursor structure 
including controlling at first and second spaced apart regions the 
diffusion of the at least one more element from the quantum barrier layers 
into the quantum well layer to produce first and second spaced apart 
active regions in the active layer having different effective lasing 
energy band gaps, and forming respective electrical contacts to the first 
and second cladding layers on opposite sides of each of the first and 
second active regions. 
According to a second aspect of the invention, an integrated semiconductor 
laser including at least two active regions, each active region 
oscillating at a respective, different wavelength, comprises a common 
semiconductor substrate of a first conductivity type, a semiconductor 
first cladding layer of the first conductivity type disposed on the 
substrate, at least two spaced apart active regions disposed on the first 
cladding layer within a common active layer, the active layer including at 
least one compound semiconductor quantum well layer sandwiched between 
compound semiconductor quantum barrier layers, the quantum barrier layers 
having a larger energy band gap than and including at least one more 
element than the quantum well layer, the at least one more element of the 
quantum barrier layers penetrating farther into the quantum well layer at 
the first active region than into the quantum well layer at the second 
active region, a second cladding layer disposed on each of the first and 
second active regions, and respective electrical contacts to the first and 
second active regions through the first and second cladding layers 
disposed on opposite sides of each of the first and second active regions. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter. It should be understood, 
however, that the detailed description and specific embodiments are given 
by way of illustration only, since various changes and modifications 
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 
FIGS. 1(a)-1(g) illustrate a method of making an integrated semiconductor 
laser including at least two active regions according to an embodiment of 
the invention. Initially, as illustrated in FIG. 1(a), a precursor laser 
structure is produced by successively growing, on a p-type gallium 
arsenide substrate 1, a p-type aluminum gallium arsenide cIadding layer 2, 
a gallium arsenide/aluminum gallium arsenide quantum well/quantum barrier 
structure as active layer 3, and an n-type aluminum gallium arsenide 
cladding layer 4. The quantum well structure includes at least one quantum 
well layer of gallium arsenide sandwiched between substantially identical 
barrier layers of aluminum gallium arsenide. While the embodiment of the 
invention described with respect to FIGS. 1(a)-1(g) and elsewhere herein 
includes a single quantum well layer sandwiched by two barrier layers, the 
invention encompasses a multiple quantum well structure including more 
that one quantum well layer sandwiched by respective quantum barrier 
layers. 
In FIG. 1(b), stripes 5a and 5b of dielectric materials are deposited on 
cladding layer 4 at two spaced apart regions. As described below, active 
regions of the integrated semiconductor laser are subsequently formed 
opposite the respective dielectric stripes. An important feature of the 
invention is the formation of the dielectric stripes 5a and 5b of 
different materials and/or of different thicknesses. While only two 
stripes 5a and 5b are shown in FIG. 1(b), resulting in the formation of 
two spaced apart active regions as described below, more than two 
dielectric stripes can be formed in order to form more than two active 
regions in the subsequent processing steps. 
After the formation of the dielectric stripes, the precursor laser 
structure is subjected to an annealing treatment, for example, at 
850.degree. C. for several hours in an arsenic ambient. During the 
annealing step, the aluminum that is present in the aluminum gallium 
arsenide barrier layers within quantum well structure diffuses across the 
interfaces of the quantum barrier layers and the quantum well layer and 
into the quantum well layer where no aluminum is present initially. This 
interdiffusion alters the energy band edge configuration of the quantum 
well structure. 
In FIG. 2, the alteration of the energy band configuration is illustrated 
for two different interdiffusions. Initially, the quantum well band edge 
has the idealized rectangular configuration shown by the dashed lines of 
FIG. 2. After a relatively small diffusion, the idealized rectangular 
potential well is altered to the gently curved band edge marked as A in 
FIG. 2. After still further diffusion, greater curvature is present in the 
energy band edge configuration, resulting in the energy band edge marked B 
in FIG. 2. These interdiffusions alter the effective depth of the 
potential well by making it shallower as the amount of interdiffusion 
increases, increasing the effective lasing energy band gap of the 
corresponding active region. As illustrated in FIG. 2, after the initial 
diffusion, the effective quantized energy level of the potential well is 
L-A, and, after a greater diffusion, the effective quantized energy level 
of the potential well is L-B. In a laser employing a quantum well 
structure active layer, the wavelength of the emitted light depends upon 
an energy transition between the energy level of electrons and the energy 
level of heavy holes. This energy transition is increased as the diffusion 
of aluminum into the quantum well layer increases, i.e., as the quantized 
energy level shifts from the bottom of the rectangular potential well to 
the level L-A, to L-B and so forth. The increase in the energy of the 
transition shifts the wavelength of laser light oscillation toward shorter 
wavelengths, i.e., higher energies, as the amount of interdiffusion 
increases. 
It has been demonstrated experimentally that the degree of interdiffusion 
of an extra element from the quantum barrier layers into the quantum well 
layer for fixed time and temperature conditions depends upon several 
factors. For example, the rate of the interdiffusion is affected by the 
degree of passivation of the external surface closest to the quantum well 
structure. The degree of passivation depends upon whether the surface is 
protected from the ambient and, if protected, the material and thickness 
of the material providing the protection. For example, it has been 
experimentally demonstrated that different degrees of interdiffusion are 
produced when the surface protection is provided by silicon nitride, 
aluminum nitride, and silicon dioxide films. Amongst these three films, 
silicon nitride delays the interdiffusion the most and silicon dioxide 
delays the interdiffusion the least. The relative interdiffusion constants 
for aluminum from the barrier layer into the quantum well layer with 
surface passivation films of silicon nitride, aluminum nitride, and 
silicon dioxide in units of 10.sup.-18 cm.sup.2 /s are .ltoreq. to 3, 4, 
and 17, respectively. 
The mechanism that controls the rate of the interdiffusion of aluminum from 
aluminum gallium arsenide into gallium arsenide in the quantum well 
structure is not yet fully understood. It is believed that, in the gallium 
arsenide series of materials, arsenic is evaporated from the exposed 
surfaces during annealing. The resulting arsenic vacancies induce gallium 
vacancies that encourage aluminum diffusion to produce an aluminum 
vacancy. As a result, diffusion rates are affected by the relative 
difficulty of the creation of an arsenic vacancy at the exposed surfaces. 
Since the dielectric films increase the difficulty of creating arsenic 
vacancies, the rate of aluminum diffusion is affected by the presence, 
type, and thickness of each of the films. In the invention, this 
phenomenon is exploited by choosing the dielectric film materials of 
stripes 5a and 5b to produce different degrees of interdiffusion at 
different parts of the laser precursor structure, producing active regions 
having different effective lasing energy band gaps and, therefore, 
different wavelength light emissions. For example, the stripes 5a and 5b 
are chosen from silicon nitride, aluminum nitride, and silicon dioxide 
using a different material and/or different thicknesses of the same or 
different materials for each active region location. 
After the annealing step, as illustrated in FIG. 1(c), the dielectric 
stripes 5a and 5b are removed. The dielectric stripes have resulted in 
different degrees of aluminum diffusion into the quantum well layers in 
the active layer 3 opposite each of the locations where the dielectric 
stripes had been present and where the active regions will be present. 
Elsewhere in the active layer in the semiconductor precursor structure, 
larger and substantially identical interdiffusion has taken place, 
"smearing" or obliterating the quantum well structure. 
As illustrated in FIG. 1(d), a diffusion mask including three stripes 11a, 
11b, and 11c is formed on cladding layer 4. Stripes 11a and 11b are 
aligned with and disposed on the same locations as dielectric stripes 5a 
and 5b. Diffusion mask stripe 11c is disposed intermediate of stripes 11a 
and 11b. A p-type dopant, such as zinc, producing the same conductivity 
type as that of substrate 1 is then diffused through the openings between 
stripes 11a, 11b, and 11c, penetrating through the cladding layer 4 and 
the active layer 3, and into the cladding layer 2 to form diffused regions 
6. Where the regions 6 intersect the active layer, the zinc disorders the 
quantum well structure of the active layer 3. The disordered regions aid 
in confining light in the active regions. The zinc establishes electrical 
contact from the surface of cladding layer 4 to the cladding layer 2 and 
forms pn junctions with the portions of cladding layer 4 that have been 
protected from doping by the diffusion mask, i.e., stripes 11a, 11b, and 
11c. 
An electrically insulating layer 7, such as silicon dioxide, is then 
deposited, as shown in FIG. 1(f). As shown there, the silicon dioxide film 
may be deposited over the diffusion mask stripes 11a-11c. Preferably, 
stripes 11a-11c are removed before the electrically insulating film 7 is 
deposited. If those diffusion mask stripes are not initially removed, they 
are selectively removed, lifting off the electrically insulating film 7 
and providing self-aligned access to n-type layer 4 at the regions in 
which zinc is not diffused. If, as preferred, the diffusion mask stripes 
are first removed, stripe apertures, narrower than the stripe masks, are 
opened in film 7 at each of the locations where a diffusion mask stripe 
11a-11c was present. 
Finally, as illustrated in FIG. 1(g), electrodes 8a and 8b are deposited on 
electrically insulating film 7 respectively in contact with cladding layer 
4 where dielectric stripes 5a and 5b and diffusion masks 11a and 11b had 
been present, i.e., between two of the diffused regions 6. A common 
electrode 9 is deposited on the rear surface of substrate 1 opposite 
cladding layer 2 to complete the structure. 
The completed integrated laser includes a common substrate and two active 
regions, one opposite each of electrodes 8a and 8b, that respectively 
produce light of different wavelengths. The two active regions, i.e., 
laser elements, are totally electrically independent. By omitting the 
diffusion mask 11c shown in FIG. 1(e), the separation between two of the 
diffused regions 6 can be eliminated, simplifying the structure but 
increasing the potential for interaction between the two spaced apart 
active regions of the integrated semiconductor laser. 
In operation, each of the laser structures has charge carriers injected 
into the respective active regions in active layer 3 from one of 
electrodes 8a and 8b and common electrode 9 through the respective 
cladding layers 4 and 2. The resulting charge carrier recombinations 
produce light that is confined by waveguides defined by the zinc diffused 
regions 6, resulting in laser oscillation at a wavelength determined by 
the degree of interdiffusion of aluminum into the respective quantum well 
layers. 
The processing steps employed to produce the integrated semiconductor laser 
embodiment of FIG. 1(g) are relatively simple and far less complex than 
those required to produce the prior art structures of FIGS. 7 and 9. 
Moreover, the structure of the novel integrated semiconductor laser can be 
easily altered. The embodiment shown in FIG. 1(g) is a so-called vertical 
laser structure in which electrical current flows through and generally 
perpendicular to substrate 1 when the laser is in use. In an integrated 
circuit, it is desirable to provide all of the electrodes at the same 
surface. That horizontal-type laser can be obtained by altering the steps 
described with respect to FIGS. 1(a)-1(g). Instead of forming diffused 
regions 6 of the same conductivity type, diffused regions of alternating 
conductivity types can be produced with the electrodes for each laser 
structure being respectively disposed on the p-type and n-type diffused 
regions on opposite sides of a particular active region. Such a structure 
is shown in FIG. 3(f) and described below with respect to a different 
technique for establishing separate active regions oscillating at 
different wavelengths. 
Another embodiment of the invention is described with respect to FIGS. 
3(a)-3(f). Those figures illustrate process steps for making a so-called 
horizontal integrated semiconductor laser in which all electrodes are 
accessible from the same surface. As illustrated in FIG. 3(a), the same 
laser precursor structure shown in FIG. 1(a) is first prepared by a 
conventional epitaxial growth process, such as metal organic chemical 
vapor deposition (MOCVD), molecular beam epitaxy (MBE), and the like. The 
same elements in FIGS. 3(a)-3(f) that appear in FIGS. 1(a)-1(g) are given 
the same reference numbers. As shown in FIG. 3(a), a silicon nitride film 
10a has been deposited on the cladding layer 4 and a stripe of the film 
has been removed opposite the location where one of the active regions is 
to be formed. Thereafter, a heat treatment, i.e., annealing step, is 
carried out with a fixed arsenic ambient pressure. As well known in the 
art, an excess pressure of arsenic must be present when annealing gallium 
arsenide and similar arsenic-containing compound semiconductor materials 
at high temperatures to avoid decomposition of those materials. Within the 
semiconductor laser precursor structure, very little interdiffusion of 
aluminum from the barrier layers into the well layer occurs where the 
silicon nitride film is present, as already described. On the other hand, 
where the cladding layer 4 is exposed through the opening in the silicon 
nitride layer, aluminum diffuses relatively rapidly from the quantum 
barrier layers into the quantum well layer, all within the quantum well 
structure 3. 
It is well known that the rate of aluminum interdiffusion into the well 
layer is dependent upon the arsenic ambient pressure. While the mechanism 
of this dependence is not fully understood, as discussed above, it is 
believed that the creation of arsenic vacancies affects the rate of 
aluminum interdiffusion. As a result, differences in ambient arsenic 
pressures during separate annealing steps result in different amounts of 
aluminum interdiffusion. In other words, referring again to FIG. 2, 
annealing at similar temperatures and times, but at different ambient 
arsenic pressures, produces different amounts of interdiffusion and 
different changes in the band edge configuration of the quantum well 
structure. Thereby, the effective lasing energy band gap of an active 
region can be controlled by controlling the arsenic ambient pressure 
during annealing. In FIG. 4, the relationship between changes in the 
wavelength of oscillation of an active region with arsenic ambient 
pressure during annealing steps for the same time and temperature is 
plotted. As shown there, the oscillation wavelength declines as the 
arsenic ambient pressure is increased until the pressure reaches about 100 
Torr. At higher arsenic pressures, the wavelength increases. 
In the first annealing step illustrated in FIG. 3(a), the arsenic ambient 
pressure is about 100 Torr to produce the maximum shift in laser 
wavelength for the active region opposite the stripe opening in the mask 
10a. After the first annealing step, the silicon nitride film 10a is 
removed and is replaced by a second silicon nitride film 10b including a 
second stripe where the film 10b is missing. The position of the stripe 
opening in the film 10b determines the location of the second active 
region. This second silicon nitride film is illustrated in FIG. 3(b). The 
structure is annealed a second time, this time at a lower arsenic ambient 
pressure, for example, 80 Torr, than during the first annealing step to 
produce greater interdiffusion of aluminum from the quantum barrier layers 
into the quantum well layer at the second active region and a smaller 
change in wavelength than achieved in the first annealing step. Since the 
silicon nitride film inhibits diffusion of aluminum into the quantum well 
layer, the two annealing steps produce significant changes in the band 
edge configuration of the quantum well structure in active layer 3 only 
opposite the respective missing stripes, i.e., openings, in the films 10a 
and 10b. 
While the foregoing steps have been described with respect to changing the 
ambient arsenic pressure only, the same results can be obtained even if 
the ambient arsenic pressure is identical during the two annealing steps 
if other variables, such as the times and/or temperatures of the 
respective annealing steps, are varied. 
After the two annealing steps, the second silicon nitride film 10b is 
removed, leaving the structure of FIG. 3(c). As shown in FIG. 3(d), a 
third silicon nitride film 16 is deposited on the cladding layer 4. Film 
16 includes two stripe openings where the film 16 is missing. Each stripe, 
in the embodiment illustrated in FIG. 3(d), is disposed to one side of a 
respective active region in which aluminum interdiffusion has taken place. 
In other words, each of the stripe openings in film 16 lies to one side, 
the left side as illustrated in FIG. 3(d), of one of the openings that was 
present in films 10a and 10b, i.e., where the active regions will be 
located. Film 16 functions as a diffusion mask and a dopant is diffused 
through the openings in the film to form diffused contact regions 12, 
penetrating through cladding layer 4 and active layer 3 and reaching into 
cladding layer 2. The dopant may be zinc if p-type regions are formed. 
Thereafter, film 16 is removed and a fourth silicon nitride film 17 is 
deposited as a diffusion mask. Film 17 also includes stripe openings, as 
shown in FIG. 3(e). Each of those openings lies on the opposite side of a 
respective active region from a diffused contact region 12. A dopant 
producing the opposite conductivity type from that of regions 12 is 
diffused through the openings in film 17, penetrating through cladding 
layer 4 and active layer 3 and reaching into cladding layer 2 to form 
diffused contact regions 13. When diffused contact regions 12 are p-type, 
silicon may be diffused to form n-type diffused contact regions 13. These 
steps produce p-type and n-type regions on the opposite side of each 
active region. The diffused contact regions also disorder the active layer 
3 adjacent to the active regions, forming a light-confining cavity at each 
of the respective active regions. 
Finally, as shown in FIG. 3(f), film 17 is removed and an insulating film, 
such as silicon dioxide, is deposited on cladding layer 4 and an opening 
is made in the insulating film 7 opposite each of the diffused contact 
regions 12 and 13. Electrodes 14a and 14b are deposited respectively in 
contact with one of the p-type contact regions 12 and electrodes 15a and 
15b are deposited respectively in contact with one of the n-type contact 
regions 13, completing the laser structure. 
As in the embodiment of FIG. 1(g), each laser element in the integrated 
laser can be operated independently. Current flows between pairs of 
diffused contact regions 12 and 13 laterally, i.e., generally parallel to 
substrate 1. The current flow passes through one diffused contact region 
into the cladding layer of the same conductivity type as that diffused 
contact region, through the active region to the cladding layer of the 
other conductivity type, and then through the other diffused contact 
region which is the same conductivity type as the other cladding layer. 
Thus, the electrical contacts are made through the respective cladding 
layers as in the embodiment of FIG. 1(g) but the direction of current flow 
is different. The currents flowing through the respective active regions 
result in carrier recombinations that produce laser light. 
Although the p-type and n-type diffused contact regions 12 and 13 are 
disposed in an alternating arrangement in FIG. 3(f), the openings in the 
diffusion masks 16 and 17 can be altered so that neighboring boring 
diffused contact regions have the same conductivity type. If extreme 
isolation between different laser elements is not essential, adjacent 
common conductivity type diffused contact regions can be merged and a 
single electrode may be commonly employed in driving more than one of the 
active regions. Since the embodiment of the invention shown in FIG. 3(f) 
provides access to all electrodes of the laser structure from one side of 
the substrate, it is convenient for use in monolithic circuitry. 
It is apparent from a comparison of FIGS. 1(c)-(g) to FIGS. 3(c)-3(f) that 
the electrode structures shown in FIGS. 1(g) and 3(f) can be alternatively 
employed with structures made by either the processes illustrated in FIGS. 
1(a) and 1(b) or FIGS. 3(a) and 3(b). In other words, the monolithic 
structure of FIG. 3(f) can employ active regions prepared by selectively 
masking portions of cladding layer 4 during annealing and the two-sided 
structure of FIG. 1(g) can be prepared with active regions formed in 
separate annealing steps at different arsenic ambient pressures, times, 
and/or temperatures. 
The embodiments of the invention described above include gallium arsenide 
substrates and layers and a gallium arsenide quantum well layer sandwiched 
between aluminum gallium arsenide barrier layers. However, the invention 
is not limited to those materials. For example, indium gallium arsenide 
and aluminum gallium indium arsenide may be employed as the quantum well 
and quantum barrier layer materials, respectively. Indium gallium 
phosphide and aluminum gallium indium phosphide may also be employed as 
quantum well and quantum barrier layer materials, respectively. Aluminum 
gallium indium phosphide advantageously provides a good lattice match to a 
gallium arsenide substrate and these materials do not exhibit a change in 
lattice constant when aluminum interdiffuses from the barrier layer into 
the quantum well layer. Indium phosphide and indium gallium arsenide 
phosphide may be employed as quantum well and quantum barrier materials, 
respectively. Indium gallium arsenide phosphide provides a good lattice 
match to an indium phosphide substrate, although some lattice constant 
change upon diffusion of arsenic from the quantum barrier to the quantum 
well layer takes place, resulting in some stresses. An important feature 
of each of these pairs of materials is the presence in the barrier layer 
material of at least one more element than is initially present in the 
quantum well material. That additional element increases the band gap of 
the quantum barrier material relative to the quantum well material and 
diffuses, at an elevated temperature, into the quantum well layer to 
change its energy band gap configuration. 
While emphasis has been placed upon the structures of FIGS. 1(g) and 3(f), 
the invention is not limited to those structures. Alternative embodiments 
of integrated lasers including at least two spaced apart active regions 
are illustrated in FIGS. 5 and 6. In FIG. 5, two separate active regions 
formed from a common active layer 3 are each surrounded by n-type and 
p-type layers successively disposed on substrate 1. The n-type layer 18 is 
disposed directly on substrate 1 in contact with cladding layer 2. P-type 
layer 19 is in contact with both cladding layers 2 and 4 as well as the 
active layer 3 at both sides of the active region. The individual laser 
elements are further isolated by a groove 20 extending through p-type 
layer 19 and into n-type layer 18. A common electrode 9 is disposed on the 
rear side of the common substrate 1. Each active region has a separate 
second electrode 8a and 8b. 
The integrated laser structure shown in FIG. 6 also includes a groove 20 
isolating the two active regions within active layer 3. The groove 20 
extends through cladding layer 4 and active layer 3 and into cladding 
layer 2. Each laser element in the integrated structure of FIG. 6 includes 
a ridge opposite its respective active region. Otherwise, the elements of 
the integrated semiconductor laser shown in FIG. 6 are the same as those 
already identified by the same reference numbers in describing other 
embodiments of the invention. 
The integrated laser structures of FIGS. 1(g) and 3(f) each include two 
active regions. However, the invention is not limited to an integrated 
laser with only two active regions. Three or more active regions can be 
present in an integrated laser according to the invention. Additional 
active regions can be formed in the processing steps of FIGS. 1(a)-1(g) by 
including additional masking stripes of different materials and/or 
thicknesses in the step illustrated in FIG. 1(b). Additional masking and 
annealing steps like those of FIGS. 3(a) and 3(b) would be employed in the 
processing according to FIGS. 3(a)-3(f) to produce more than two active 
regions in an integrated laser structure. Additional electrodes would be 
applied to either embodiment for driving the additional laser elements.