Semiconductor laser device

A semiconductor laser device including a first semiconductor layer having a strip waveguide structure to obtain optical confinement and a second semiconductor layer having a ridge waveguide structure for defining an electrical current passage region. The strip waveguide structure has a first width, and projects on the first semiconductor layer, extending over the central area of the layer in a longitudinal direction. The ridge waveguide structure projects on the second semiconductor layer and extends in the longitudinal direction with a second width which corresponds to the strip structure. The strip waveguide structure cooperates with the ridge waveguide structure to produce a difference between the refractive index of a center region which extends in the longitudinal direction of the second semiconductor and that of a neighboring region due to the difference in thicknesses between the two, so that the center region serves as an optical waveguide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a semiconductor laser device. It 
is specifically directed to an improved semiconductor device which, due to 
a difference between the refractive index of the waveguide portion and the 
neighboring region results in optical confinement of the emitted light so 
that the width of the region through which the electrical current passes 
can be reduced. 
2. Description of the Prior Art 
The prior art includes semiconductor laser devices having a ridged 
waveguide structure in which the width of the current passing region is 
reduced. Referring to FIG. 7, there is shown a distributed feedback 
semiconductor laser (DFB laser) in which a ridged waveguide structure is 
employed. The structure includes a semiconductor substrate 50 of a given 
conductivity type made, for example, of an n-type GaAs, having a major 
surface. A semiconductor cladding layer 52 having the same conductivity 
type as the semiconductor substrate 50 and composed, for example, of an 
n-type AlGaAs is formed on the major surface of the substrate 50 by means 
of epitaxial growth. An active semiconductor layer 54 made, for example, 
of GaAs, is then formed on the cladding layer 52 in the same manner. A 
semiconductor guiding layer 56 having a conductivity opposite to that of 
the cladding layer 52 and composed, for example, of a p-type AlGaAs is 
formed on the active layer 54 in the same manner. A periodically 
corrugated surface which serves as a diffraction grating 58 is formed on 
the surface of the guiding layer 56 opposite from the active layer 54. The 
diffraction grating covers the entire surface of the guiding layer and is 
composed of corrugations which extend laterally, are triangular in cross 
section, and have a regular pitch interval. After the diffraction grating 
58 is formed on the guiding layer 56, a semiconductor cladding layer 60 of 
the same conductivity characteristics as the guiding layer 56 and 
composed, for example, of a p-type AlGaAs is formed on the guiding layer 
56 by means of epitaxial growth. Then, a semiconductor cap layer of the 
same conductivity type as the cladding layer 60 and composed, for example, 
of a p-type GaAs is formed on the cladding layer 60 in the same manner. An 
etching process is performed selectively to remove both side regions of 
the cap layer 62 and the cladding layer 60, leaving a central region 
thereof extending in the longitudinal direction, and the entire lower 
region of the cladding layer 60, the cap layer 62 comprising only the 
central region. The cladding layer 60 has a T-shaped cross section. As a 
result, a stripe structure is formed by the cap and cladding layers 62 and 
60. The surfaces of the removed portions of the cap and cladding layers 62 
and 60 are covered with insulation films 64. The top surface of the cap 
layer 62 and the bottom surface of the substrate 50 are provided with 
counter electrodes 66 and 68 so as to establish ohmic contacts. 
In this type of semiconductor laser device, the flow of current can be 
restricted to a narrow current passing region. However, this type of 
device does not provide good optical confinement. In order to achieve good 
optical confinement, the device must be designed to have about 0.01 
difference in the refractive index between the central waveguide region 
and the neighboring region. This difference depends on the effective 
thickness of the guiding layer 56 (GL) and the thickness d of the 
neighboring region of the cladding layer 56. However, it is difficult to 
obtain the desired difference in refractive index by adjusting the 
thicknesses since the allowable error is so small. Therefore, a device of 
this type cannot be produced which has consistently predictable optical 
confinement characteristics. 
There has also been proposed a DFB laser having a waveguide which comprises 
a narrow strip for obtaining uniform and reproducible optical confinement 
characteristics. One such device is illustrated in FIG. 8 and includes a 
semiconductor substrate 70 of a given conductivity type composed, for 
example, of a n-type GaAs, having a major surface. A semiconductor 
cladding layer 72 of the same conductivity type as the substrate 70 and 
composed, for example, of n-type AlGaAs is formed on the major surface of 
the substrate 70 by means of epitaxial growth. An active semiconductor 
layer 74 composed, for example, of GaAs is then formed on the cladding 
layer 72 in the same manner. A semiconductor guiding layer 76 having 
conductivity characteristics opposite to that of the cladding layer 72 and 
made, for example, of a p-type AlGaAs is formed on the active layer 74 in 
the same manner. Then, a corrugated strip 78 having a regular period of 
repitition and which serves as a diffraction grating is formed on the 
surface of the guiding layer 76 opposite to the active layer 74. 
Corrugated strip 78 extends over the central region of the guiding layer 
76 in a longitudinal direction. The strip 78 defining the diffraction 
grating is composed of corrugations having a regular pitch and extending 
perpendicularly to the longitudinal axis thereof. After the diffraction 
grating 78 is formed on the guiding layers 76, a semiconductor cladding 
layer 80 of the same conductivity characteristics as the guiding layer 76 
and formed, for example, of a p-type AlGaAs is formed on the guiding layer 
76 by way of epitaxial growth. Then, a semiconductor cap layer 82 of the 
same conductivity characteristics as the cladding layer 80 and composed, 
for example, of a p-type GaAs is formed on the cladding layer 80 in the 
same manner. Thereafter, ion implantation is performed by injecting ions 
such as boron ions or the like from the cap layer 82. High resistance 
current restricting regions 84 are formed on both sides of the cap layer 
82 so as to insulate the sections adjacent to central region extending in 
the longitudinal direction. A pair of counter electrodes 86 and 88 are 
provided on the top surface of the cap layer 82 and the bottom surface of 
substrate 70 to establish ohmic contacts therebetween. 
This type of semiconductor laser device effectively achieves good optical 
confinement due to the differences in the refractive indices of the 
respective sections thereof. However, the current passing region cannot be 
made narrow so as to increase the reactive current since there is no 
mechanism for restricting the flow of current to a well defined area 
within the cladding layer 80. 
The aforementioned disadvantages of the semiconductor laser device having a 
ridged waveguide structure or the narrow strip can also be observed in 
conventional Fabry-Pe'rot semiconductor lasers. 
SUMMARY OF THE INVENTION 
The present invention seeks to eliminate the aforementioned disadvantages 
and to provide a semiconductor laser device which can effectively achieve 
good optical confinement due to differences in refractive indices, and 
which also has a narrow current passing region. These effects can be 
consistently obtained in the devices of the present invention. 
In order to accomplish these results, a semiconductor laser of the present 
invention includes ridge structures and a strip which is defined in the 
light guide. 
More specifically, the semiconductor laser device of the invention may 
include a semiconductor substrate having a cladding layer thereon of the 
same conductivity type. A laser active layer is disposed on the cladding 
layer on the side opposite from the semiconductor substrate. A second 
semiconductor layer of the opposite conductivity type is disposed on the 
laser active layer and has a strip waveguide structure for obtaining 
optical confinement. The strip waveguide structure projects from the 
second semiconductor layer on the opposite side from the laser active 
layer and extends to the central area of the second semiconductor layer in 
a longitudinal direction. A third semiconductor cladding layer of the 
second named conductivity type is disposed on the strip waveguide. A third 
semiconductor cladding layer having a ridged waveguide structure for 
defining the current passage region extends in the longitudinal direction 
with a width which corresponds to the strip structure. A fourth 
semiconductor layer of the second conductivity type is disposed on the 
ridge waveguide structure, and a pair of electrodes is included for 
supplying a bias voltage, one being connected to the semiconductor 
substrate and the other to the fourth semiconductor layer. 
The refractive index of the third semiconductor cladding layer differs from 
that of the neighboring region due to the difference in thicknesses so 
that the center region serves as an optical waveguide. The difference 
between the refractive indices of the center and neighboring regions may 
be approximately 0.01 and is preferably in the range from 0.008 to 0.015. 
The difference in refractive index due strictly to the strip waveguide 
structure is preferably from 0.007 to 0.013. The ridged structure may 
project in a perpendicular direction to the third semiconductor cladding 
layer. The third semiconductor layer may have a plate portion and a ridged 
portion and has a T-shaped cross section. The thickness of the plate 
portion may be approximately equal to or less than 5,000A. The thickness 
of the third semiconductor is approximately 15,000A. The strip waveguide 
structure may project in a perpendicular direction with respect to the 
second semiconductor layer. The semiconductor laser device may comprise a 
Fabry-Pe'rot laser device. The strip waveguide structure may have a 
periodically corrugated surface which serves as a diffraction grating. The 
grating may be composed of laterally extending corrugations of regular 
pitch and of essentially triangular cross section. 
In accordance with another phase of the present invention, there is 
provided a distributed feedback semiconductor laser device which includes 
a semiconductor substrate of a first conductivity type and a semiconductor 
cladding layer of the same conductivity type located on the major surface 
of the semiconductor substrate. A laser active layer is disposed on the 
cladding layer on the side opposite from the semiconductor substrate. A 
second semiconductor layer of opposite conductivity type is disposed on 
the laser active layer and includes a strip waveguide structure having a 
periodically varying corrugated surface which serves as a diffraction 
grating. The strip waveguide structure projects from the second 
semiconductor layer on the side opposite from the laser active layer and 
extends over the central area of the second semiconductor layer in a 
longitudinal direction. The corrugated surface has corrugations which 
extend in a lateral direction perpendicular to the longitudinal direction. 
A third semiconductor cladding layer of the opposite conductivity type is 
disposed on the strip waveguide structure and has a ridge waveguide 
structure for defining an electrical current passing region. The ridge 
structure projects from the side opposite to the second semiconductor 
layer and extends in the longitudinal direction. A fourth semiconductor 
layer of the opposite conductivity type is disposed on the ridge waveguide 
structure and a pair of electrodes is included for supplying bias voltage, 
one of the electrodes being connected to the semiconductor substrate and 
the other to the fourth semiconductor layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, particularly to FIGS. 1 and 2, the preferred 
embodiment of a distributed feedback semiconductor laser according to the 
present invention includes a semiconductor substrate 10 having one 
conductivity type, such as an n-type GaAs which has a major surface. A 
semiconductor cladding layer 12 of the same conductivity type as the 
substrate 10, and preferably consisting of n-type AlGaAs is formed on the 
major surface of the substrate by means of epitaxial growth. An active 
semiconductor layer 14 composed, for example, of intrinsic GaAs, is then 
formed on the cladding layer 12 in the same manner. A semiconductor 
guiding layer 16 of the second conductivity type opposite to that of the 
cladding layer 12 is formed on the active layer 14 in the same manner. The 
guiding layer 16 may consist, for example, of a p-type AlGaAs. A 
periodically corrugated strip 18 serving as a diffraction grating is 
formed on the surface of the guiding layer 16 opposite from the active 
layer 14. The corrugated strip extends over the central area of the 
surface of the guiding layer 16 in a longitudinal direction whose width is 
W.sub.2. The guiding layer 16 consists of a thicker central portion 16a on 
which the corrugated strip 18 is formed and a pair of thinner plate 
portions 16b formed on either side of the corrugated strip 18. The 
effective thickness of the central portion 16a of guiding layer 16 which 
defines the waveguide is thicker than that of the plate portion 16b. The 
corrugated strip 18 is composed of corrugations which are essentially 
triangular in cross section, have a regular pitch, and extend 
perpendicular to the longitudinal axis of the strip. After the corrugated 
strip 18 is formed on the guiding layer 16, a semiconductor cladding layer 
20 of the same conductivity as that of the guiding layer 16 and composed 
of a p-type AlGaAs, for example, is formed on the guiding layer 16 by way 
of epitaxial growth so as to cover the entire surface including the 
thicker central portion 16a and the thinner plane surface of the guiding 
layer 16. The band gap of the cladding layer 20 is larger than that of the 
guiding layer 16 and the active layer 14. A semiconductor cap layer 22 
having the same conductivity as that of the cladding layer 20 and made of 
a p-type GaAs, for example, is formed on the cladding layer 20 in the same 
manner. Thereafter, an etching process is performed to selectively remove 
the side portions of the cap layer 22 and the cladding layer 20 to a 
predetermined depth. A central portion of the cap layer 22 and a portion 
of the cladding layer 20 having a T-shaped cross section remain after the 
etching process. The cladding layer 20 comprises a ridged portion 20a 
having a width W.sub.1 and a plate portion 20b having a thickness d. The 
ridged portion 20a projects upwardly from the guide layer 16 at a location 
corresponding to the corrugated strip 18 and extends in the longitudinal 
direction. The cap layer remains only at the top of the ridged portion 
20a. As a result, a ridged waveguide structure is formed by the cap and 
cladding layers. The surfaces of the removed portions of the cap and 
cladding layers 22 and 20 are covered with insulation films 24. Moreoever, 
the top surface of the cap layer 22 and the bottom surface of the 
substrate 10 are provided with electrodes 26 and 28 so as to establish 
ohmic contacts therebetween, respectively. 
The DFB laser of the present invention has characteristics of both the 
ridged and strip structures. The characteristics of such a laser device 
will now be described. 
The thickness of the ridge portion should be two or more times that of the 
plate portion in order to sufficiently prevent the flow of electrical 
current from spreading laterally and to keep the current flow restricted 
to a narrow area. The ridged structure differs from a mesa electrode 
structure in that the thickness d of the plate portion 20b of the cladding 
layer 20 is equal to or less than about 5,000A and the thicknesses of the 
ridge portion 20a of cladding layer 20 and the cap layer 22 are about 
15,000A and 5,000A respectively, whereas the thickness d is more than 
about 10,000A in the mesa structure. In an embodiment, the thickness of 
the third semiconductor layer is 15,000A. As will be described below, the 
magnitude of difference between the refractive indices of the ridged and 
plate portions 20a and 20b of the cladding layer begins to be significant 
when their thicknesses have the aforementioned values. The ridged 
structure may restrict the flow of electrical current to a narrow region 
more effectively than does the mesa structure. 
In order to achieve the desired electrical current restricting effect, the 
difference .DELTA.N between the refractive indices of the ridged and plate 
portions should be at least about 0.01. Since the allowable range of 
refractive index difference .DELTA.N is, from experience, from 0.08 to 
0.015, a difference .DELTA.N in this range will be considered below. 
FIG. 3 shows the relationship between the thickness d of the plate portion 
of the cladding layer and the magnitude of difference .DELTA.N in 
refractive index, with respect to various effective thicknesses of guiding 
layer (GL) in the ridge waveguide structure semiconductor laser device. 
FIG. 4 shows the difference .DELTA.N in refractive index related to the 
effective height of the strip structure with respect to various effective 
thicknesses of guiding layer in the strip waveguide structure 
semiconductor laser. 
FIG. 5 shows the relationship between the difference .DELTA.N in refractive 
index to the ridged waveguide structure and the thickness d of the plate 
portion of cladding layer 20 on the basis of FIGS. 3 and 4, in which the 
total difference .DELTA.N in the refractive index due to the ridged and 
strip waveguide structures is within the allowable range. In FIG. 5, the 
curved lines a and b correspond to .DELTA.N equals 0.08 and .DELTA.N 
equals 0.015, respectively. The thickness d of the plate portion of the 
cladding layer is assumed to be less than 5,000A in the ridged waveguide 
structure. When the thickness d is greater than about 5,000A, the 
difference .DELTA.N in refractive index begins to be observed. As seen 
from FIG. 5, when there is no effect due to the strip waveguide structure, 
the allowable range represented by the shaded area of the plate portion 
thickness d of the cladding layer 20 is very narrow. Although it is 
possible to prevent the flow of electrical current from spreading, 
relatively large stress is applied to the active layer 14 since the 
thickness d of the plate portion must be thin when the waveguide strip is 
not present. When this structure is produced by an etching process, the 
etching depth is about 19,000A plus or minus 200A and the allowable error 
is plus or minus 1%, so that very great accuracy is required. 
In the preferred embodiment of the present invention, the range of the 
difference .DELTA.N in the refractive index due to the strip waveguide 
structure is about 0.007 to 0.013, and the total difference .DELTA.N in 
the refractive index may be from 0.008 to 0.015. The width W.sub.2 of the 
diffraction grating may be equal to the width W.sub.1 of the ridged 
structure. However, it is preferably larger than the width W.sub.1 in 
order to assume that all of the electrical current passes through the 
diffraction grating. 
In the use of this structure, when the current passage restricting effect 
of the ridged structure is achieved, the waveguide effect produced by the 
combination of the strip and ridged structures can also be achieved. It is 
also possible to extend the allowable error in the plate thickness to 
permit the ridged structure to be formed by an etching process since the 
waveguide effect is mainly achieved by the strip. In cases where the 
difference .DELTA.N in the refractive index due to the strip is about 
0.007 to 0.013 when the total difference .DELTA.N between the refractive 
indices of the central waveguide and the circumference thereof is in the 
range of 0.008 to 0.015, the permissible error in formation of the ridged 
structure is greatly extended, so that the etching depth may vary within 
1,500A of 16,000A. Therefore, the permissible error of etching is about 
10% so that uniform results can be achieved. 
The degree of current restricting effect is determined mostly by the width 
W.sub.1 of the ridged structure and the degree of waveguide effect is 
determined by the width W.sub.2 of the strip. According to the preferred 
embodiment of the invention, the widths W.sub.1 and W.sub.2 can be 
controlled independently of each other Therefore, when the width W.sub.1 
the ridged structure is less than the width W.sub.2 of the strip, the 
degree of current restricting effect can correspond to the degree of 
waveguide effect, i.e., the area in which current flows can be made to 
correspond to the area of the diffraction grating so that effective high 
frequency modulation characteristics can be achieved. 
Due to the ridged structure, the area of the electrodes 26 and 28 can be 
decreased so that the volume of the device can be decreased thereby making 
high speed modulation possible. Furthermore, the threshold voltage is 20% 
less than that of a mesa laser. In addition, the device of the present 
invention provides superior reliability and durability since etching of 
the active layer is not carried out. 
FIG. 6 shows another embodiment of a semiconductor laser device according 
to the present invention in which after etching of the cladding layer 20 
of FIG. 1 is performed, a flush layer 30 composed of n-type AlGaAs is 
formed on the removed portion of the cladding layer by an epitaxial 
process. In this embodiment, the flush layer 30 is provided to prevent 
structural stress from being concentrated at the central portion of the 
cladding layer 14 by the ridged structure. 
It should be evident that various modifications can be made to the 
described embodiments without departing from the scope of the present 
invention.