Semiconductor laser manufacturing method

A double heterojunction semiconductor laser according to the invention includes first and third cladding layers sandwiching an active layer. The third cladding layer includes a mesa opposite a light emitting region of the active layer. The mesa is confined by a current blocking layer. A cap layer that is part of the mesa is used as a dopant diffusion source to dope a light emitting region of the active layer heavily. A second cladding layer may be present between the active layer and third cladding layer having the same conductivity type as the third cladding layer adjacent the light emitting region but the opposite conductivity type elsewhere. A semiconductor laser according to the invention may also include a stripe groove structure. The semiconductor lasers include pnpn structures outside the light emitting region and in window structures adjacent the facets of the semiconductor laser for suppressing leakage currents, thereby increasing laser efficiency and reducing threshold current while increasing power output.

FIELD OF THE INVENTION 
The present invention relates to a double heterojunction semiconductor 
laser having increased power output, reduced current leakage, and improved 
efficiency. 
BACKGROUND OF THE INVENTION 
In double heterojunction semiconductor lasers, an active layer in which 
carrier recombination occurs, resulting in the emission of light, is 
sandwiched between cladding layers of opposite conductivity type. The 
cladding layers have larger energy band gaps and smaller refractive 
indices than the active layer in order to confine light within the active 
layer. The laser structure includes two opposed, generally parallel facets 
that are generally perpendicular to the active layer. The facets are 
coated with a reflective material to produce, with the active layer, an 
optical resonator in which light resonates to sustain a laser oscillation. 
The coating on at least one of the facets permits some of the laser light 
to escape, producing the light output of the laser. 
A number of factors limit the power output of a semiconductor laser. 
Carrier recombination can occur more efficiently at the surfaces adjacent 
the facets than within the body of the laser. The increased carrier 
recombination and resulting increased charge carrier density at the facets 
results in increased light absorption there. That light absorption, in 
turn, increases the temperature at the facets. If the temperature rise is 
sufficient, localized melting of the semiconductor materials can occur, 
resulting in catastrophic optical damage (COD) that destroys the laser. 
The power output of a semiconductor laser can be increased without risking 
COD by providing a window structure as described by Yonezu et al in the 
Journal of Quantum Electronics, Volume QE-15, August 1979, pages 775-781, 
the disclosure of which is incorporated herein by reference. In the window 
structure described by Yonezu, the regions of the semiconductor laser 
adjacent the facets, i.e., the windows, are heavily doped n-type and the 
light emitting region, which lies between the windows in the central 
portion of the laser, is made p-type by overcompensation with a p-type 
dopant. As a result of this doping profile, the energy band gap in the 
central portion of the laser is decreased relative to the energy band gap 
in the windows. The increased energy band gap in the window structures 
results in reduced absorption of light near the facets, thereby increasing 
the power level that can be attained without risk of COD. 
Although the window structure increases the power output that can be safely 
produced by a laser, the relatively high doping concentrations associated 
with the window structure create other problems. For example, when the 
dopant concentration is relatively high in the light emitting region where 
carriers recombine and emit light, there is significant light loss due to 
free carrier absorption, i.e., the absorption of light by charge carriers. 
In addition, the relatively heavy dopant concentrations throughout the 
laser structure encourage the flow of leakage currents between the laser 
electrodes which are generally parallel to the active layer and transverse 
to the facets. These leakage currents reduce laser efficiency and 
effectively raise the current threshold at which laser oscillation begins. 
SUMMARY OF THE INVENTION 
The present invention is directed to solving the problems of internal light 
absorption in a double heterojunction semiconductor laser incorporating a 
window structure and to reducing current leakage in a double 
heterojunction semiconductor laser, particularly a laser incorporating a 
window structure. 
According to a first aspect of the invention, a semiconductor laser 
includes a semiconductor substrate of a first conductivity type, a 
semiconductor first cladding layer of the first conductivity type disposed 
on the substrate, a semiconductor active layer disposed on the first 
cladding layer and having a central light emitting region of a second 
conductivity type opposite the first conductivity type, a semiconductor 
third cladding layer of the second conductivity type disposed on the 
active layer including a mesa opposite and projecting away from the light 
emitting region of the active layer, a semiconductor current blocking 
layer of the first conductivity type disposed on the third cladding layer 
and adjacent the mesa, a semiconductor fourth cladding layer of the second 
conductivity type disposed on the current blocking layer and on the mesa, 
a semiconductor contacting layer of the second conductivity type disposed 
on the fourth cladding layer, and first and second electrodes respectively 
disposed on the substrate and the contacting layer wherein the laser 
includes generally parallel first and second facets transverse to the 
first and second electrodes for transmitting laser light outside the laser 
and a semiconductor cap layer in the mesa adjacent the fourth cladding 
layer, the cap layer having the first conductivity type proximate the 
first and second facets and the second conductivity type elsewhere. 
A method of manufacturing a semiconductor laser according to the invention 
includes successively growing a semiconductor first cladding layer of the 
first conductivity type, a semiconductor active layer, a semiconductor 
third cladding layer of a second conductivity type opposite the first 
conductivity type, and a semiconductor cap layer of the first conductivity 
type on a semiconductor substrate of a first conductivity type, diffusing 
a dopant producing the second conductivity type into the cap layer except 
at portions where each of two opposed facets of the semiconductor layer 
will be formed, thereby converting the cap layer in the diffused portion 
to the second conductivity type, removing portions of the third cladding 
layer and the cap layer to leave a mesa including portions of the third 
cladding layer and the cap layer projecting from a remaining portion of 
the third cladding layer, heating the substrate, first cladding layer, 
active layer, and mesa to diffuse the dopant from the cap layer through 
the mesa and into the active and third cladding layers adjacent the mesa, 
growing a semiconductor first conductivity type current blocking layer on 
the third cladding layer abutting the mesa, successively growing a 
semiconductor fourth cladding layer of the second conductivity type and a 
semiconductor contacting layer of the second conductivity type on the 
current blocking and cap layers, depositing first and second electrodes on 
the substrate and the contacting layer, respectively, and forming a pair 
of generally parallel opposed facets generally transverse to the first and 
second electrodes and spaced from the portions of the cap layer into which 
the dopant was diffused. 
According to another aspect of the invention, a semiconductor laser 
includes a semiconductor substrate of a first conductivity type, a 
semiconductor first cladding layer of the first conductivity type disposed 
on the substrate, a semiconductor active layer disposed on the first 
cladding layer and having a central light emitting region of a second 
conductivity type opposite the first conductivity type, a semiconductor 
third cladding layer disposed on the active layer, a semiconductor current 
blocking layer of the first conductivity type disposed on the third 
cladding layer, the current blocking layer including an opening extending 
to the third cladding layer, a semiconductor fourth cladding layer of the 
second conductivity type disposed on the current blocking layer and on the 
third cladding layer in the opening in the current blocking layer, a 
semiconductor contacting layer of the second conductivity type disposed on 
the fourth cladding layer, and first and second electrodes respectively 
disposed on the substrate and the contacting layer wherein the laser 
includes generally parallel first and second facets transverse to the 
first and second electrodes for transmitting laser light outside the 
laser, the third cladding layer is of the second conductivity type 
opposite the light emitting region of the active layer and of the first 
conductivity type proximate the facets and elsewhere outside the light 
emitting region of the active layer. 
Another method of manufacturing a semiconductor laser according to the 
invention includes successively growing a semiconductor first cladding 
layer of the first conductivity type, a semiconductor active layer, a 
semiconductor second cladding layer of the first conductivity type, a 
semiconductor third cladding layer of a second conductivity type opposite 
the first conductivity type, and a semiconductor current blocking layer of 
the first conductivity type on a semiconductor substrate of a first 
conductivity type, forming a diffusion mask including a central opening on 
the current blocking layer, the mask covering portions of the 
semiconductor layers proximate the locations where facets of the laser 
will be formed, diffusing a dopant producing the second conductivity type 
through the opening in the diffusion mask into the current blocking layer, 
heating the substrate, first cladding layer, active layer, second and 
third cladding layers, and current blocking layer to diffuse the dopant 
from the third cladding layer through the second cladding layer and into 
the active layer, removing the diffusion mask and depositing on the 
current blocking layer an etching mask having a central opening extending 
to the locations where the facets of the semiconductor laser will be 
formed and aligned with the location of the opening of the diffusion mask, 
removing the portion of the current blocking layer not covered by the 
etching mask by etching, removing the etching mask, successively growing a 
semiconductor fourth cladding layer of the second conductivity type and a 
semiconductor contacting layer of the second conductivity type on the 
current blocking layer and on the third cladding layer where a portion of 
the current blocking layer was removed, depositing first and second 
electrodes on the substrate and the contacting layer, respectively, and 
forming a pair of generally parallel opposed facets generally transverse 
to the first and second electrodes spaced from the portions of the second 
cladding layer into which the dopant was diffused. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter. The detailed description 
is given by way of illustration only, since various additions and 
modifications within the spirit and scope of the invention will be 
apparent to those skilled in the art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A double heterojunction semiconductor laser according-to an embodiment of 
the invention is shown in a perspective view in FIG. 1(a). In order to 
illustrate the internal structure of the laser, in FIG. 1(a), the central 
portion of the laser has been separated and moved to the right, away from 
the end sections of the laser. The end sections include the facets and are 
different from the central portion of the laser because of the presence of 
the window structures. In FIG. 1(b), a side sectional view of the laser of 
FIG. 1(a) taken along line 1(b)-1(b) of FIG. 1(a) is shown to illustrate 
further the window structures. 
In FIGS. 1(a) and 1(b), an n-type gallium arsenide substrate 1 has a number 
of semiconductor layers successively disposed on it, forming the laser 
structure. Those layers are a first cladding layer 2 of n-type aluminum 
gallium arsenide, an active layer 3 of gallium arsenide, a second cladding 
layer 4 of n-type aluminum gallium arsenide, and a third cladding layer 5 
of p-type aluminum gallium arsenide A current blocking layer 6 of n-type 
gallium arsenide is disposed on part of the third cladding layer. 
The double heterojunction laser of FIGS. 1(a) and 1(b) has a stripe 
groove-type construction. A stripe groove 11 extends through the current 
blocking layer 6 and exposes the third cladding layer 5 from one facet 18 
to the other. A fourth cladding layer 7 of p-type aluminum gallium 
arsenide is disposed on the current blocking layer and in the stripe 
groove 11 in contact with the third cladding layer 5. A contacting layer 8 
of p-type gallium arsenide is disposed on the fourth cladding layer 7. 
Electrodes 9 and 10 are respectively disposed on the substrate 1 opposite 
the other semiconductor layers and on contacting layer 8 to complete the 
laser. 
In the structure as described, three pn rectifying junctions are present 
between the electrodes 9 and 10 along a current path through the current 
blocking layer 6. One pn junction is present between the fourth cladding 
layer 7 and the current blocking layer 6 where those two layers are in 
contact, and a second pn junction is present between the current blocking 
layer 6 and the third cladding layer 5 where those two layers are in 
contact. Finally, a third rectifying junction is present between the 
second and third cladding layers 4 and 5. Leakage current flows, i.e., 
current flows other than through the active layer 3 at the stripe groove 
11, are strongly suppressed by these three pn junctions. A current path 
between the electrodes 9 and 10 passing through the stripe groove 11 and 
the asdeposited layers includes only one pn junction between second and 
third cladding layers 4 and 5 that lies on the side of the active layer 
toward electrode 10. 
In the laser structure of FIG. 1(a), the pn junction between the second and 
third cladding layers is eliminated only in the vicinity of the stripe 
groove 11 and only in the central portion of the laser by a dopant that 
produces p-type conductivity. That dopant is disposed in the active and 
second and third cladding layers opposite the stripe groove 11 in a region 
12 in FIG. 1(a). The p-type dopant overcompensates the second cladding 
layer 4, converting it to p-type conductivity in region 12 opposite stripe 
groove 11 in the central portion of the laser. Thereby, in the central 
portion of the laser there exists a current path between electrodes 9 and 
10 in which only one pn junction, which sandwiches the active layer 3, is 
present. That pn junction, when properly biased, produces carrier 
recombinations that result in laser light generation. 
As clearly shown in sectional side view in FIG. 1(b), region 12 is limited 
to the central portion of the laser. At each end of region 12, adjacent 
one of the facets 18 in a window region 13, the p-type dopant that 
overcompensates second cladding layer 4 is absent from that layer and 
layers 3 and 5. The absence of the overcompensating p-type dopant from the 
window regions reduces current flow, surface recombination, and light 
absorption so that the laser can produce higher output power, without COD, 
than is possible when the window structures are absent. 
A method for manufacturing the laser structure of FIGS. 1(a) and 1(b) is 
illustrated in FIGS. 2(a)-2(d). Each of those figures is a cross-sectional 
view taken in the central portion of the laser. The diffusion steps 
illustrated in FIGS. 2(a) and 2(b) in which p-type impurities are disposed 
in the active layer and second and third cladding layers do not take place 
in the window regions 13 adjacent facets 18. 
Turning to FIG. 2(a), the first cladding layer 2, the active layer 3, the 
second and third cladding layers 4 and 5, and a current blocking layer 6 
are successively grown by a known process, such as metal organic chemical 
vapor deposition (MOCVD), molecular beam epitaxy (MBE), liquid phase 
epitaxy (LPE), or the like. At this stage, the stripe groove 11 is not yet 
formed in current blocking layer 6. Thereafter, a film 15 that is 
effective as a diffusion mask against p-type impurities that will be 
diffused into the precursor of the laser structure is deposited on the 
current blocking 6. When the p-type dopant is zinc, silicon nitride 
deposited in a thermal chemical vapor deposition process may be employed 
as the film 15. After the deposition of the film 15, a central aperture is 
opened in it as indicated in FIG. 2(a). That aperture is not present in 
the window regions. Since the laser facets are usually formed after the 
laser structure is complete, the aperture in the diffusion mask is 
generally rectangular with the short ends of the rectangle spaced from the 
locations where the laser facets 18 will be subsequently formed. 
In the specific embodiment described, zinc is diffused through the 
diffusion mask aperture into and penetrating the current blocking layer 6. 
The source of the zinc atoms may be a vapor or a solid phase diffusion 
source, such as a film containing a mixture of zinc compound and another 
compound, for example, zinc oxide and silicon dioxide. In the illustrated 
method, a vapor phase zinc source is employed. The precursor of the laser 
structure, usually in a wafer form, is heated in a sealed quartz ampoule 
in the presence of zinc arsenide and elemental zinc to about 700.degree. 
C., vaporizing the zinc. The zinc arsenide provides an excess arsenic 
pressure that protects the compound semiconductor layers including arsenic 
from deterioration. The zinc concentration in the current blocking layer 
opposite the diffusion mask aperture reaches about 1.times.10.sup.20 
cm.sup.-3. The resulting high p-type dopant concentration is undesirable 
in the completed laser because it strongly encourages free carrier 
absorption, resulting in light losses as already described. In keeping 
with achieving that goal, the time and temperature of the diffusion, 
considering the thickness of the current blocking layer 6, are limited so 
that the diffusion front of zinc in this first diffusion step does not 
reach the second cladding layer 4. 
In order to convert the portion of the second cladding layer 4 opposite the 
aperture in the diffusion mask film 15 from n-type to p-type, zinc must be 
further diffused to penetrate that layer. That diffusion is accomplished 
in a drive-in diffusion step. That drive-in diffusion is carried out at a 
higher temperature, for example, approximately 900.degree. C., that the 
first diffusion in an arsenic atmosphere. The arsenic is necessary to 
avoid deterioration of the exposed surface of the gallium arsenide current 
blocking layer 6. As a result of this second diffusion, some of the zinc 
present in the current blocking layer 6 reaches and penetrates the second 
and third cladding layers 4 and 5, opposite the aperture in the diffusion 
mask film 15, and also enters the active layer 3. The resulting zinc 
impurity concentration in region 12 is approximately 10.sup.18 to 10 
.sup.19 cm.sup.-3. 
In order to expose the third cladding layer for deposition of the fourth 
cladding layer, the zinc diffused region 14 within the current blocking 
layer must be removed. The diffusion mask film 15 covers the window 
regions of the structure. However, to produce the desired laser structure, 
the stripe groove 11 must extend the full length of the laser resonator, 
i.e., completely between facets 18. Therefore, the diffusion mask is not 
suitable as an etching mask for forming the stripe groove and is removed. 
A photoresist film 16 is deposited on current blocking layer 6 and a 
central aperture is opened in it, fully extending between the facet 
locations and aligned with the region 14 in which zinc has been diffused. 
As shown in FIG. 2(c), the stripe groove 11 is formed by selectively 
etching the gallium arsenide current blocking layer 6 through the aperture 
in the photoresist film 16 without substantially etching the aluminum 
gallium arsenide third cladding layer 5. Thereby, the most highly doped 
zinc diffusion region 14 that would produce free carrier absorption and 
light loss in the completed laser is removed. 
The laser is completed by removing the etching mask 16 and successively 
growing the fourth cladding layer 7 and the contacting layer 8 using 
MOCVD, MBE, LPE, or the like. Electrodes 9 and 10 are applied to opposite 
sides of the device and the facets 18 are formed, for example, by cleaving 
at the preselected locations to preserve the window structure at the 
opposite ends of the double heterojunction semiconductor laser. 
The laser of FIG. 1(a) provides improved performance. When forward biased, 
relatively high power laser light can be produced by the laser without 
damage to the facets because of the window structure. In addition, leakage 
currents, i.e., currents flowing between electrodes 9 and 10 other than 
through the active layer at the stripe groove, are suppressed. The only 
current paths between the electrodes outside the stripe groove include a 
pnpn structure, i.e., three rectifying junctions, that is highly effective 
in concentrating the current flow in the stripe groove region. A pn 
junction is present in the stripe groove 11 between second third cladding 
layers 4 and 5 in the window regions 13. That junction is electrically 
connected in parallel with the relatively heavily doped p-type region 12. 
Therefore, current flow at the facets 18 through the stripe groove and a 
rectifying junction is suppressed in favor of a flow through the single 
conductivity type region 12, further discouraging current leakage. The 
improved current concentration achieved in the laser structure of FIG. 
1(a) reduces the threshold current at which laser oscillation occurs and 
increases laser efficiency. 
In the manufacturing steps illustrated in FIGS. 2(b) and 2(c), it is 
important to align the aperture of the etching mask formed with film 16 
with the former location of the aperture of the diffusion mask formed with 
film 15. The alignment is desirable to ensure that all of the highly doped 
zinc region 14 in the current blocking layer 6 is removed when the stripe 
groove is formed. Preferably, the aperture in the etching mask is wider 
than the aperture in the diffusion mask to ensure that any portion of the 
current blocking layer in which zinc has laterally diffused is removed in 
the etching step. If these conditions are not met, the effectiveness of 
the structure in concentrating the current flow through the active layer 
only at the stripe groove is reduced. 
FIGS. 3(a) and 3(b) illustrate the structures that can result when desired 
alignment between the diffusion and etching masks is not achieved. In FIG. 
3(a), there has been a slight misalignment between the apertures of the 
diffusion and etching masks. As a result, a residual portion of the highly 
doped zinc region 14 has been left in part of the current blocking layer 
adjacent the stripe groove 11. A current leakage path is thereby provided, 
resulting in a non-linear relationship between light output and the 
current flowing through the laser as well as reduced efficiency. In FIG. 
3(b), the aperture in the etching mask was smaller than the aperture in 
the diffusion mask. As a result, residual portions of the highly doped 
region 14 are left at each side of the stripe groove in the current 
blocking layer 6. These two p-type regions provide a still larger current 
leakage path than in the structure of FIG. 3(a), again resulting in 
lowered laser efficiency as well as non-linearity between laser current 
and light output. 
In forming the stripe groove 11 in the process step illustrated in FIG. 
2(c), the aluminum gallium arsenide third cladding layer 5 is exposed to 
the ambient. That exposure may result in the oxidation of the exposed 
aluminum in the third cladding layer 5. The oxidation at the regrowth 
interface may interfere with the deposition of the fourth cladding layer. 
The potentially oxidized surface is positioned directly opposite the 
stripe groove where current is concentrated for the laser oscillation. 
This close proximity of an oxidized layer to the light emitting region may 
cause premature deterioration of the semiconductor laser. 
A second embodiment of the invention is shown in perspective and sectional 
views in FIG. 4(a) and in a side sectional view in FIG. 4(b). In this 
embodiment, there is no regrowth interface subject to oxidation located 
near the light emitting region or through which a concentrated current 
flows for producing laser light. Moreover, the possibility of a leakage 
current path in the current blocking layer resulting from misalignment of 
mask apertures is eliminated. 
In FIGS. 4(a)-5(g), the same elements are given the same reference numbers 
as in FIGS. 1(a)-2(d). Therefore, it is not necessary to identify again 
the elements that have already been described with respect to the other 
figures. 
The double heterojunction laser embodiment of FIG. 4(a) employs a reverse 
mesa structure rather than the stripe groove structure of the laser 
embodiment of FIG. 1(a). The reverse mesa includes side walls that 
converge when moving in the direction from the electrode 10 toward the 
active layer 3. The reverse mesa is an extension of the third cladding 
layer 5 and is disposed within an opening in the current blocking layer 6. 
The reverse mesa includes, in the central portion of the laser, a 
relatively highly doped region 14 containing a dopant producing p-type 
conductivity. In addition, a cap layer 21 of n-type gallium arsenide in 
the window regions 13, but overcompensated to p-type in the central 
portion of the laser, is present at the top of the reverse mesa, i.e., 
sandwiched between the mesa and the fourth cladding layer 7 and by the 
current blocking layer 6. As in the embodiment shown in FIG. 1(a), a 
p-type region 12 is present in the active layer 3 in the light emitting 
region in the central portion of the laser but not in the window 
structures adjacent the facets of the laser. The laser embodiment of FIG. 
4(a) includes, outside the light emitting region, both in the central 
portion of the laser and at the facets 18, a pnpn structure that is 
effective in concentrating current flow in the light emitting region and 
reducing leakage currents that may flow between the electrodes 9 and 10 
outside that light emitting region. 
A method for manufacturing the double heterojunction semiconductor laser of 
FIG. 4(a) is illustrated in FIGS. 5(a)-5(g). FIG. 5(a) is a side sectional 
view, like FIG. 4(b), whereas FIGS. 5(b)-5(g) are transverse sectional 
views taken in the central portion of the laser structure. Turning to FIG. 
5(a), the n-type aluminum gallium arsenide first cladding layer 2, the 
gallium arsenide active layer 3, the n-type aluminum gallium arsenide 
second cladding layer 4, and the p-type aluminum gallium arsenide third 
cladding layer 5 are successively grown using a known process, such as 
MOCVD, MBE, or LPE. Unlike the structure illustrated in FIG. 2(a), in FIG. 
5(a), the third cladding layer 5 is relatively thick, for example, 1.5 to 
2 microns thick. The increased thickness is required for the formation of 
the mesa structure, as explained below. An n-type gallium arsenide cap 
layer 21 is grown on the third cladding layer 5 as the final step in the 
initial growth process. 
After the initial growth process, as illustrated in FIG. 5(a), a p-type 
dopant, such as zinc, is diffused into the cap layer 21 except at the 
window regions 13 at opposite ends of the structure where facets 18 are to 
be formed. Although not illustrated in FIG. 5(a), a diffusion mask, such 
as a silicon nitride layer with a rectangular aperture exposing the 
central portion of the cap layer 21, while protecting the regions at which 
the facets 18 will be formed, is disposed on the cap layer before the 
diffusion step. The zinc dopant may be supplied in a vapor form through 
the aperture or from a solid diffusion source disposed on the cap layer 21 
and produces a relatively high p-type dopant concentration within the cap 
layer 21 except at the facet regions. Unlike the zinc diffusion mask and 
step described with respect to FIG. 2(a), the opening in the diffusion 
mask need not be limited transversely, i.e., along a direction parallel to 
the facets 18 and the active layer 3, to the central portion of the laser 
structure. Because the cap layer 21 is subsequently removed between the 
facets, except opposite the light emitting region, it is not necessary to 
protect those other areas of cap layer 21 from zinc. It is only necessary 
to protect from zinc the regions at which the facets will be formed in 
order to produce the window structures. 
Following the zinc diffusion, the diffusion mask is removed and an 
additional mask 22 is applied. Mask 22 may be silicon nitride and extends 
longitudinally in the direction between and beyond the two facets, i.e., 
to and covering the window regions 13. The silicon nitride film 22 is 
formed by a conventional process, such as a thermally driven chemical 
vapor deposition process, and patterned by conventional photolithographic 
and selective etching steps. Initially, the mask 22 is employed as an 
etching mask. Where not protected by etching mask 22, the third cladding 
layer 5 is selectively etched to leave reverse mesa 23 in place. As is 
well known in the art, this reverse mesa structure can be obtained by 
proper crystallographic orientation of the substrate 1 and use of a 
crystallographically preferential etch. For example, the substrate 
orientation may be (100) and a reverse mesa may be formed along the &lt;011&gt; 
direction by use of an etch that is an aqueous solution of sulfuric acid 
and hydrogen peroxide. The manufacturing steps of FIGS. 5(b)-5(g) and the 
structure shown in FIG. 4(a) employ a reverse mesa structure which is 
preferred. However, the invention also encompasses a forward mesa 
structure in the &lt;011&gt; direction. In a forward mesa, the side walls 
diverge, moving in the direction from electrode 10 toward the active layer 
3. 
The etching step in the formation of mesa 23 exposes surfaces of aluminum 
gallium arsenide layer 5 that are subject to oxidation. However, as will 
be described below, the concentrated current flow that passes through the 
active layer to produce laser oscillation does not pass through those 
oxidized surfaces. In other words, the potentially oxidized surfaces are 
remote from the light emitting region so that they do not produce 
premature deterioration of the semiconductor laser. 
The relatively heavily doped region 14 provides a diffusion source for a 
drive-in diffusion step illustrated in FIG. 5(c). During the drive-in 
diffusion process, as in the initial diffusion doping region 14, an excess 
pressure of arsenic is employed to avoid deterioration of the gallium 
arsenide and the aluminum gallium arsenide layers. During the drive-in 
diffusion, zinc diffuses from region 14 under mask 22 through mesa 23, 
i.e., third cladding layer 5, into second cladding layer 4 and active 
layer 3. Lateral diffusion of the dopant atom is prevented by the side 
walls of the reverse mesa 23. As a result, the misalignments illustrated 
current blocking layers cannot occur in the step illustrated in FIG. 5(c) 
and the doped region 12 at the active layer 3 is precisely aligned with 
the base of the reverse mesa 23. During this drive-in diffusion step, mask 
22 protects the top surface of the cap layer from damage or deterioration. 
Following the drive-in diffusion, as illustrated in FIG. 5(d), the n-type 
current blocking layer 6 is grown by MOCVD on the third cladding layer 5 
including abutting the side walls of the reverse mesa 23 and on the 
longitudinal surfaces of third cladding layer 5 that were exposed during 
the etching step. The current blocking layer 6 is grown to a sufficient 
height to bury the reverse mesa but not the silicon nitride film 22 on 
which there is no deposition of gallium arsenide when the MOCVD process is 
used. 
Turning to FIG. 5(e), the silicon nitride film 22 is removed by selective 
etching, exposing the cap layer 21. Thereafter, as shown in FIG. 5(f), the 
fourth cladding layer 7 of p-type aluminum gallium arsenide and the 
contacting layer 8 of p-type gallium arsenide are successively grown by 
MOCVD or another conventional process on the cap layer 21 and current 
blocking layer 6. Finally, metal electrodes 9 and 10 are deposited on 
substrate 1 and contacting layer 8, respectively. 
In the structure of FIGS. 4(a) and 5(g), the only regrowth interface 
through which current producing laser oscillations flows is between cap 
layer 21 and fourth cladding layer 7. That interface is not susceptible to 
oxidation because cap layer 21 does not contain aluminum. That interface 
is relatively widely separated from the light emitting region by at least 
the thickness of the current blocking layer 6, i.e., about the same 
separation as the original thickness of third cladding layer 5, 1.5 to 2 
microns. In the stripe groove structure of FIG. 2(d), the regrowth 
interface between third cladding layer 5 and fourth cladding layer 7 is 
separated from the active layer 3 only by the thickness of second and 
third cladding layers 4 and 5, a much smaller distance than in the 
structure of FIG. 5(g). 
The operation of the structure of FIGS. 4(a) and 5(g) is essentially the 
same as that of the structure of FIG. 1(a). Both structures provide 
increased power output without facet damage because of the window 
structure. In addition, both structures have reduced threshold currents 
for laser oscillation and increased current efficiency because of the pnpn 
structure outside the light emitting region of the active layer that 
suppresses leakage currents and enhances the concentration of current flow 
through the light emitting region of the active layer. In addition, in the 
structure of FIG. 4(a), a pnpn structure is also present at each of the 
window regions in a current path passing through the reverse mesa. The 
n-type cap layer 21 retains its n-type conductivity proximate the facets 
in the window structures since it is protected from the diffusion step 
establishing the p-type region 14 and is not affected by the drive-in 
diffusion step. The cap layer 21 produces the additional pn junctions in 
the window region at the mesa. Therefore, a further improvement in 
concentrating the current flow through the active layer at the light 
emitting region and suppressing leakage currents is achieved in the 
structure of FIG. 4(a) compared to the structure of FIG. 1(a). 
The described structures employ gallium arsenide as the active layer 3. 
However, if shorter wavelength light laser is desired, the active layer 3 
may be aluminum gallium arsenide. The zinc diffusion step illustrated in 
FIGS. 2(a) and 5(a) may employ a solid source of dopant, such as a zinc 
oxide film or a film of zinc oxide mixed with another material, such as 
silicon dioxide. After the initial diffusion, a solid diffusion source is 
removed by etching. Dopant atoms other than zinc producing p-type 
conductivity in compound semiconductors, such as cadmium, magnesium, and 
beryllium, may be employed in place of zinc. In addition, compound 
semiconductor materials other than those described here, such as indium 
gallium arsenide phosphide or aluminum galliuim indium phosphide, may be 
employed. When these materials are used, the proportions of various 
components are adjusted in the semiconductor layers to produce a double 
heterojunction structure including an active layer of smaller energy band 
gap and larger refractive index than the cladding layers in order to 
obtain light confinement and laser oscillation. 
In the structures described, the second cladding layer 4 is preferably 
included because it adds a pn junction that assists in confining current 
flow to the light emitting region of the active layer, reducing leakage 
currents, increasing laser efficiency, and reducing the threshold current. 
However, the second cladding layer 4 is not essential to every embodiment 
of the invention.