Semiconductor laser

A semiconductor laser includes a body having parallel end faces and a substrate having a ridge in a major surface thereof which extends between the end surfaces, an active layer overlying the ridge and which tapers in thickness from that portion of the active layer which overlies the ridge and a confinement layer overlying the active layer. The invention also includes a method of forming a semiconductor laser which includes a substrate having a ridge thereon. The method includes the steps of coating a portion of a flat surface of the substrate with an etch resistant material, etching the surface with an anisotropic etchant thereby forming a mesa therein, removing the etch resistant material, further etching the substrate to round the mesa to form a ridge and depositing the active and confinement layers over the surface and the ridge.

The invention relates to a semiconductor laser having a substrate with a 
rounded ridge in a surface thereof extending between the end faces of the 
laser and a method of making this laser. 
BACKGROUND OF THE INVENTION 
A semiconductor laser includes a body of semiconductor material, generally 
composed of group III-V compounds, having a thin active layer between 
layers of opposite conductivity type, i.e., a layer of P-type conductivity 
on one side of the active layer and a layer of N-type conductivity on the 
other side of the active layer. Such a laser, however, typically emits 
light in more than one optical mode which limits its utility. Botez, in 
U.S. Pat. No. 4,215,319 issued July 29, 1980 and entitled SINGLE FILAMENT 
SEMICONDUCTOR LASER, has disclosed a laser having a stable, single mode, 
output light beam. The control over the output light beam from this laser 
arises from the tapering in thickness of the layers. This laser is 
prepared by deposition of the confinement and active layers onto a 
substrate having a pair of substantially parallel grooves therein. The 
tapering is caused by the difference in growth rate of the layers over a 
land between the grooves and over the grooves when the layers are prepared 
by liquid or vapor phase epitaxy techniques. 
However, if the layers are deposited on an indium phosphide substrate 
having such a pair of parallel grooves, using either liquid phase or vapor 
phase epitaxy, flat, planar surfaces are observed with the grooves filling 
faster than the flat substrate portions until a continuous, smooth surface 
is obtained. This growth habit of InP limits the utilization of the 
structure disclosed by Botez for lasers composed of InP and related 
alloys. It would be desirable to have a laser composed of InP and related 
alloys which exhibits the tapered layer structure characteristic of the 
laser disclosed by Botez. 
SUMMARY OF THE INVENTION 
We have discovered that when a laser is formed on a substrate having a 
ridge therein, the deposited layers have curved surfaces with the desired 
taper in thickness. The semiconductor laser of the invention includes a 
body of semiconductor material having a pair of end surfaces and a 
substrate having one or more ridges therein extending between the end 
faces. An active layer overlies the surface of the substrate and the 
ridges and tapers in thickness in the lateral direction (a direction in 
the plane of the surface of the substrate and perpendicular to the axis of 
the ridges). A confinement layer overlies the active layer. 
The method of forming this laser includes the steps of depositing a layer 
of an etch resistant material on a portion of the substrate, etching the 
uncovered portions of the surface of the substrate, removing the etch 
resistant material and leaving a mesa in the substrate surface, further 
etching the surface thereby forming a ridge therein and depositing the 
active and confinement layers sequentially over the surface of the 
substrate and the ridge.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a semiconductor laser 30 includes a semiconductor body 
32 having spaced parallel end faces 34, at least one of which is partially 
transmissive of light at the wavelength of the output laser beam, and a 
pair of side surfaces 36 extending between the end faces 34. The 
semiconductor body 32 includes a substrate 38 having a pair of opposed 
major surfaces 40 and 42. A buffer layer 44 overlies the major surface 40 
and has a rounded ridge 46 in a surface 48 thereof which extends between 
the end faces 34 of the body 32. An active layer 50 overlies the ridge 46 
and the surface 48 of the buffer layer 44 and tapers in increasing 
thickness in the lateral direction from the portion thereof over the peak 
of the ridge 46 toward the base of the ridge. A confinement layer 52 
overlies the active layer 50 and a capping layer 54 overlies the 
confinement layer 52. An electrically insulating layer 56 overlies the 
capping layer 54 and has an opening extending therethrough in the form of 
a stripe 58 which is over the ridge 46 in the buffer layer 44. A first 
electrically conducting layer 60 overlies the electrically insulating 
layer 56 and the surface of the capping layer 54 in the region of the 
stripe 58. A second electrically conducting layer 62 overlies the second 
major surface 42 of the substrate 38. The first and second electrically 
conducting layers 60 and 62 respectively form the electrical contacts to 
the body 32. 
Referring to FIG. 2 the identification of the elements common to a 
semiconductor laser 70 and the semiconductor laser 30 of FIG. 1 is the 
same. The semiconductor laser 70 differs from the semiconductor laser 30 
in that there are a pair of rounded ridges 46 in the substrate 38 with the 
buffer layer 44 overlying the substrate and ridges. The active layer 50 
then overlies the buffer layer 44 and tapers in increasing thickness in 
the lateral direction toward the bases of the ridges from the portion 
thereof over the center of a land 72 between the ridges 46. 
The substrate 38 is typically composed of a binary group III-V compound or 
an alloy of such compounds having a surface 40 which is parallel to the 
(100) or (110) crystallographic plane. The substrate may be slightly 
misoriented from one of these orientations but preferably either a (100) 
or (110) plane is used. However, it is to be understood that other 
substrate orientations may also be used. In the selection of the substrate 
and the layers deposited thereon, it is desirable that the layers be 
lattice matched to the substrate. Preferably the substrate is composed of 
N-type InP. 
The buffer layer 44 is typically composed of the same material as the 
substrate and is used to provide a high quality surface upon which the 
overlying layers can be deposited. Typically this layer is between about 3 
and about 10 micrometers thick. If the ridges 46 are in the substrate 38 a 
buffer layer may be interposed between the substrate 38 and active layer 
50. 
The rounded ridges 46 are shown in FIGS. 1 and 2 as being in the buffer 
layer 44 and the substrate 38 respectively. The ridges may be between 
about 5 and about 20 micrometers wide at their base and between about 0.2 
and about 10 micrometers in height. The height and width are chosen so as 
to provide the desired curvature of the layers deposited thereon. If more 
than one ridge is present, the spacing between the ridges as well as the 
height and width of the individual ridges is chosen so as to provide the 
desired curvature of the layers deposited thereon. Typically, the 
center-to-center spacing of the ridges is between about 10 and about 100 
micrometers. 
The ridges may be formed using the sequence of steps shown in FIG. 3. In 
FIG. 3(a) the substrate 102 is coated with a buffer layer 104. Portions of 
the surface 106 of the buffer layer 104 are then coated with a masking 
layer 108 of etch resistant material such as an oxide of silicon, using 
standard photolithographic and deposition techniques. The surface 106 is 
then etched with an anisotropic etchant such as 0.1 to 1.0 percent bromine 
in methanol which etches the exposed portion of the buffer layer 104 and 
forms mesas 110 in the surface 112 of the buffer layer 104 as shown in 
FIG. 3(b). The masking layer 108 is then removed leaving the mesas 110 and 
the surface 112 as shown in FIG. 3(c). The mesas 110 and surface 112 are 
then further etched using the same or a different etchant to round off the 
mesas thereby forming the rounded ridges 120 in the surface 122 of the 
buffer layer 104 as shown in FIG. 3(d). The active, confinement and 
capping layers are then sequentially deposited over the ridges 120 and 
surface 122. It is clear that the ridges could equally well have been 
formed in the substrate itself followed by the sequential deposition of 
the layers. 
The various epitaxial layers may be deposited on the substrate 38 of FIG. 1 
using techniques of liquid phase epitaxy such as are disclosed by H. F. 
Lockwood et al in U.S. Pat. No. 3,753,801 entitled METHOD OF DEPOSITING 
EPITAXIAL SEMICONDUCTOR LAYERS FROM THE LIQUID PHASE, issued Aug. 21, 1973 
and which is incorporated herein by reference. Alternatively, the layers 
may be deposited by vapor phase epitaxy using techniques such as are 
disclosed by Olsen et al in U.S. Pat. No. 4,116,733 entitled VAPOR PHASE 
GROWTH TECHNIQUE OF III-V COMPOUNDS UTILIZING A PRE-HEATING STEP, issued 
Sept. 26, 1978 and incorporated herein by reference. Using these 
techniques, layers which taper in thickness can be deposited since the 
local growth rate of an individual layer will vary with the local 
curvature of the surface upon which it is grown; the greater the amount of 
local positive curvature of the surface, the higher the local growth rate. 
The active layer is typically between about 0.05 and about 2.2 micrometers 
thick and is preferably between about 0.1 and about 0.5 micrometer thick. 
This layer is either undoped or lightly P- or N-type conducting and may be 
composed of an InGaAsP or InGaAs alloy where the relative concentration of 
the elements is chosen to provide an approximate lattice match to the 
buffer layer and an output light beam of the desired wavelength, as 
disclosed, for example, by Olsen et al in the Journal of Electronic 
Materials 9, 977 (1980). 
The confinement layer 52 is typically composed of P-type InP and is between 
about 0.5 and about 3 micrometers thick. The capping layer 54 may be used 
to improve the quality of the electrical contact made to the laser 30. It 
is typically between about 0.2 and about 0.5 micrometer thick and is 
composed of InGaAsP or InGaAs having the same conductivity type as the 
confinement layer 52. 
It is to be understood that the devices of the invention can be fabricated 
using other combinations of group III-V alloys. 
The electrically insulating layer 56 is preferably composed of silicon 
dioxide which may be deposited on the capping layer 38 by pyrolytic 
decomposition of a silicon-containing gas, such as silane, in oxygen or 
water vapor. The stripe 58 is formed through the electrically insulating 
layer 56 down to the capping layer 54 using standard photolithographic and 
etching techniques and is preferably located over the ridge 46 when a 
single ridge is present. Alternatively, if two ridges are used, then the 
stripe 58 is located over the land between the ridges. 
The electrically conducting layer 60 is preferably composed of titanium, 
platinum and gold and is deposited by sequential evaporation. One skilled 
in the art would realize that it is only necessary that the electrically 
conducting layer overlie the confinement layer in the region over the 
ridge 46 in a device having a single ridge. 
Alternatively, the electrically insulating layer 56 may be eliminated by 
depositing on the confinement layer 52 a blocking layer of opposite 
conductivity type to the confinement layer 52 which has a region therein 
of the same conductivity type as the confinement layer. The electrically 
conducting layer 60 then may overlie the entire surface of this blocking 
layer. Upon application of a bias voltage to the laser 30 the p-n junction 
between the blocking layer and the confinement layer is reverse biased 
except in the region of this layer which has been converted to the same 
conductivity type as the confinement layer 52. 
The electrically conducting layer 62 on the second major surface 42 of the 
substrate 38 may be formed by vacuum deposition and sintering of tin and 
gold. 
An end face 34 of the laser 30 is typically coated with a layer of aluminum 
oxide or similar material having a thickness of about one half wave at the 
lasing wavelength. Such a layer has been disclosed by Ladany et al in U.S. 
Pat. No. 4,178,564 issued Dec. 11, 1979 and entitled HALF WAVE PROTECTION 
LAYERS ON INJECTION LASERS. The opposed end face 34 may be coated with a 
mirror which is reflecting at the lasing wavelength. Such as disclosed by 
Caplan et al in U.S. Pat. No. 3,701,047 issued Oct. 24, 1972, entitled 
SEMICONDUCTOR LASER DEVICES UTILIZING LIGHT REFLECTIVE METALLIC LAYERS and 
Ettenberg in U.S. Pat. No. 4,092,659 issued May 30, 1978 and entitled 
MULTI-LAYER REFLECTOR FOR ELECTROLUMINESCENT DEVICE. 
Refering to FIG. 4 a photomicrograph of a cross section of a laser 150 
constructed according to the principles of the invention and having the 
desired taper includes an InP substrate 152 having an InP buffer layer 
thereon which has a ridge 154 therein. An InGaAsP active layer 156 which 
is about 300 nanometers thick overlies the surface of the buffer layer. An 
InP confinement layer 158 overlies the active layer and an InGaAsP capping 
layer 160 overlies the confinement layer. The layers are distinguished 
from one another by the use of staining techniques which are well known in 
the art. A demarkation between the substrate 152 and the buffer layer 
cannot be seen because they are composed of the same material and thus the 
staining will affect both in the same way. The ridge 154 in the buffer 
layer is asymmetric because the substrate surface was slightly misoriented 
from the (110) direction.