Liquid phase epitaxy method of forming a filimentary laser device

A semiconductor device of a filamentary laser is fabricated by a method including the steps of forming a first GaAlAs layer on a GaAs body, forming a laser active layer of GaAs on the first GaAlAs layer, etching the first GaAlAs layer and the laser active layer so as to have a mesa shaped structure and to expose the GaAs body, and forming a second GaAlAs layer on said exposed GaAs body and on the surface of said mesa shaped structure so that the mesa etched first GaAlAs layer and the mesa etched active layer are surrounded by the second GaAlAs layer.

This invention relates to a semiconductor device utilized for an optical 
device, such as a laser device, a light-emitting diode, a light-waveguide 
and a band-pass filter, and more particularly to a semiconductor device 
having a filamentary multi-layered semiconductor crystal including therein 
at least one heterojunction and a laser active layer adjacent to said 
heterojunction, and a method for fabricating the semiconductor device. 
It is commonly known that the advent of semiconductor devices of double 
hetero-structure junction lasers has enabled one to reduce remarkably the 
threshold current density required for lasing and to realize continuous 
wave operation at room temperature. The term "double hetero-structure" 
refers to a structure in which a laser active layer in the form of, for 
example, a layer of p-type GaAs is sandwiched between an n-type GaAlAs 
layer and a p-type GaAlAs layer to form a pair of heterojunctions 
therebetween. A structure in which a laser active layer in the form of 
Ga.sub.1-y Al.sub.y As is sandwiched between layers of Ga.sub.1-x Al.sub.x 
As (x&gt;y) also includes a pair of heterojunctions. 
In a semiconductor device such as a laser fabricated from a crystal having 
such a structure, the electrons and holes injected into the laser active 
layer during operation cannot advance any further and are confined within 
the laser active layer since the advancing movement thereof is obstructed 
by the potential barriers established by the outer layers having a large 
band gap. Thus, the radiative recombination of the electrons and holes can 
be effectively attained in such a laser structure. Further, the light 
produced by the recombination described above is confined within the laser 
active layer due to the fact that the refractive index of the laser active 
layer is higher than those of the layers disposed on opposite sides of the 
laser active layer. Confinement of the carriers and light within the laser 
active layer leads to a remarkable reduction of the threshold current 
density required for lasing. Due to the above manner of operation, a 
double hetero-structure laser can emit a laser beam with a threshold 
current density which is far lower than those of conventional homojunction 
lasers and single hetero-structure lasers. 
The principles and fundamental structures of the double hetero-structure 
junction lasers are described in detail in U.S. Pat. No. 3,691,476. 
However, a laser structure in which electrodes are merely deposited on 
opposite faces of a double hetero-strucutre crystal emits a laser beam 
which is not satisfactory in its monochromaticity and coherency due to the 
presence of minute non-uniformity in the crystal structure. In an effort 
to overcome such a defect, a stripe-geometry laser has been proposed. (J. 
C. Dyment et al., J. Appl. Phys., Vol. 40, Page 1802, 1969). 
This laser has a structure as described below. 
An electrical insulator layer is coated on an epitaxially grown surface of 
a double hetero-structure crystal; a narrow stripe portion of this 
insulator layer is removed by etching in a direction perpendicular to the 
cleavage plane of the crystal; and a metal layer is evaporated on the 
insulator layer to be in contact with the crystal surface at the narrow 
stripe portion above described. 
With such a structure, current flows in stripe form through the laser 
active layer during operation of the laser, resulting in a reduction of 
the lasing area. Thus, this laser emits a laser beam of better optical 
properties than when current flows through the entire laser active layer. 
On the other hand, an attempt to narrow the width of the stripe electrode 
in contact with the crystal surface in this laser structure results in an 
undesirable increase of the threshold current density required for lasing. 
This is attributable to the fact described below. 
The current flowing through the laser active layer in stripe form has a 
density distribution in the transverse direction such that the current 
density is lowest at portions adjacent to the opposite ends in the 
transverse direction of the stripe region. Thus, the current in these 
portions does not contribute to the lasing operation and is ineffective 
for producing a laser beam because of the low current density. The 
narrower the width of the stripe electrode, the greater is the degree of 
divergence of the current out of the laser active layer, and a larger 
current is required for the lasing operation due to the increase of wasted 
current portions. 
For eliminating undesirable losses due to such divergence of current, 
mesa-stripe geometry lasers have been proposed. (IEEE Journal of Quantum 
Electronics, Vol. QE-9, No. 2, February 1973, "Mesa-Stripe-Geometry 
Double-Heterostructure Injection Lasers", by T. Tsukada et al). These 
lasers have structures wherein a narrow stripe mesa is formed by etching 
to remove portions of a crystal until the crystal portions are etched to a 
depth beyond the depth of a laser active layer so that the laser active 
layer can be included in the narrow stripe mesa (High-Mesa-Stripe), or 
wherein a narrow stripe mesa is formed by etching to remove portions of 
layers overlying the laser active layer and not by etching the laser 
active layer (Low-Mesa-Stripe). According to these structures, current 
flows uniformly through the laser active layer without giving rise to 
undesirable losses as above described and this laser can lase with a very 
low threshold current density even if the width of the stripe is narrowed; 
and especially, the Low-Mesa-Stripe laser has shown good optical 
properties such as a single mode oscillation and a satisfactory 
polarization of a laser beam. 
However, all of the above-mentioned semiconductor devices of the lasers 
have drawbacks that they have not been fully satisfactory for high quantum 
efficiency and for affording good optical confinement because side walls 
are surrounded by an atmosphere of air or by the same material as that of 
the laser active layer. 
For obtaining a satisfactory result in the high quantum efficiency and 
confinement, it has been proposed that the laser structure wherein a laser 
active layer, such as GaAs, is bounded on all sides by a material, such as 
GaAlAs, having a broader band gap and higher refractive index than those 
of the laser active layer (Japanese laying open patent application No. 
8471 of 1972, (U.S. Pat. No. 3,780,358)). Hereinafter, this structure will 
be called a buried-heterostructure laser. 
This buried-heterostructure laser has a high quantum efficiency, since the 
laser active layer has a barrier against carriers on all sides, and 
affords very good optical confinement, since the laser active layer is 
bounded on all sides by a material having a lower refractive index than 
that of the laser active layer. 
Though it has been considered that the buried-heterostructure laser wherein 
the laser active layer of GaAs is surrounded by GaAlAs layer would be 
fabricated by forming a GaAs layer on a GaAlAs body, selectively etching 
the GaAs layer so as to remain a laser active layer, and forming the 
GaAlAs layer on the surfaces of said GaAlAs body and of said laser active 
layer of GaAs, this method has not been accomplished. 
It has been found by the inventor during his experiments extending over a 
long period of time, that GaAlAs or other materials could not be formed or 
could be formed only partially on a III-V compound semiconductor body 
including Al in its composition, which is exposed to air. The reason why 
GaAlAs or other materials could not be or could be partially formed on the 
body exposed to air is not apparent, but is considered by the inventor 
that the surface of the body is oxidized and becomes inadequate to secure 
wetting between a Ga solution to form GaAlAs on other materials, and said 
body. 
It is, therefore, an object of the present invention to provide a novel and 
improved semiconductor device utilized for an optical device, such as a 
laser device, a light-emitting diode, a light-waveguide and a band-pass 
filter, which affords a high quantum efficiency and a good optical 
confinement. 
Another object of the present invention is to provide a semiconductor 
device whose active region has not a dimensional unbalance between 
thickness and width and which is adequate as a light source in optical 
communication. 
A further object of the present invention is to provide a semiconductor 
device of a laser which is capable of continuous wave operation at room 
temperature. 
A still further object of the present invention is to provide a method for 
fabricating a semiconductor device having a high quantum efficiency and a 
good optical confinement, and whose active region has not a dimensional 
unbalance between thickness and width and which is adequate as a light 
source in optical communication and is capable of continuous operation at 
room temperature. 
In accordance with one aspect of the present invention, there is provided a 
semiconductor device comprising a semiconductor body consisting of a III-V 
compound semiconductor material, except a III-V compound semiconductor 
material including Al in its composition, and having a major surface, a 
mesa-shaped multi-layered semiconductor crystal including therein a laser 
active layer and a first semiconductor material being different from and 
having a broader band gap than that of said laser active layer, which is 
disposed on a part of the major surface of said semiconductor body, and a 
second semiconductor material being different from and having a broader 
band gap than that of said laser active layer, which is disposed on the 
other part of said major surface of the body and side surfaces of said 
laser active layer. 
In another aspect of the present invention, there is provided a method for 
fabricating the semiconductor device of the present invention comprising 
the steps of preparing a semiconductor body consisting of a III-V compound 
semiconductor material, except a III-V compound semiconductor material 
including Al in its composition, forming a laser active layer on said 
semiconductor body, etching said laser active layer so as to have a 
mesa-shaped structure and to expose the surface of the semiconductor body, 
and forming a semiconductor layer consisting of a semiconductor material 
being different from and having a broader band gap than that of said laser 
active layer on said exposed surface of the semiconductor body, and on the 
surfaces of said laser active layer.

Referring to FIG. 1 showing schematically one embodiment of the 
semiconductor device utilized for a laser device according to the present 
invention in perspective view, on a part of a major surface of an n-type 
GaAs body 4 whose impurity concentration is about 2.times.10.sup.18 
cm.sup.-3, a mesa-shaped 2.mu. wide multi-layered semiconductor crystal 
consisting of an n-type Ga.sub.0.7 Al.sub.0.3 As layer 3 having a 
thickness of about 1.mu. directly disposed on the major surface of said 
GaAs body 4 and a p-type or undoped GaAs layer 1 having a thickness of 
about 0.5.mu. disposed on said Ga.sub.0.7 Al.sub.0.3 As layer 3, is 
disposed, and an n-type, undoped or insulated Ga.sub.0.7 Al.sub.0.3 As 
layer 2 is disposed on the surface of the GaAs body 4 and of the 
mesa-shaped multi-layered semiconductor crystal. A semiconductor region 5 
having a p-type conductivity is disposed in the Ga.sub.0.7 Al.sub.0.3 As 
layer 2 so as to reach said GaAs layer 1. An electrical insulating layer 6 
which may be a phospho-silicate glass layer is disposed on the surface of 
the Ga.sub.0.7 Al.sub.0.3 As layer 2, except the surface where the 
semiconductor region 5 is exposed, and metal layer 7 consisting of 
chromium and gold and metal layer 8 consisting of gold containing 
germanium and nickel are disposed on said insulating layer 6 so as to 
provide contact with said semiconductor region 5 and on the other surface 
opposing to said major surface of said GaAs body 4, respectively. Opposite 
end faces 9 and 10 are parallel to each other so that a cavity resonator 
for a laser is formed. 
The semiconductor laser having the above structure is fabricated in a 
manner as will be described below. 
Referring to FIGS. 2a through 2d showing in schematic vertical sections the 
structure of a hetero-structure crystal, an n-type Ga.sub.0.7 Al.sub.0.3 
As layer 3 (wherein the dopant is Sn) of 1.mu. in thickness and a p-type 
GaAs layer 1 (wherein the dopant is Ge) of 0.5.mu. in thickness are 
successively grown on an n-type GaAs body 4 (wherein the dopant is Te) by 
a well known liquid phase epitaxial method. (FIG. 2a). 
This crystal is maintained at about 450.degree. C. in a chemical vapor 
deposition apparatus and a layer of an oxide such as SiO.sub.2 about 5,000 
A thick is coated on the GaAs layer 1 by a chemical vapor deposition 
method. In lieu of the oxide such as SiO.sub.2, phosphosilicate glass may 
be utilized. The oxide layer is then etched away by a photo resist etching 
technique except a stripe portion about 5.mu. wide extending in a 
direction perpendicular to the (110) plane of the crystal, thereby 
exposing the surface of the GaAs body 1. The exposed surface portions of 
the semiconductor crystal are etched by an etchant which may consist of a 
4:1:1 mixture of H.sub.2 SO.sub.4, H.sub.2 O.sub.2 and H.sub.2 O in volume 
ratio until these 
A mesa-shaped multi-layered semiconductor crystal (a stripe mesa) about 
2.mu. wide is thus formed on the surface of the GaAs body 4 by the above 
steps. The oxide layer covering the surface of the mesa-shaped crystal is 
removed by an etchant which may be a 6:1 mixture of NH.sub.4 F and HF in 
volume ratio. (FIG. 2b). 
An n-type Ga.sub.0.7 Al.sub.0.3 As layer 2 wherein the dopant is Sn is 
grown on the surface of the GaAs body 4 and of the mesa-shaped crystal by 
a well known liquid phase epitaxial method. (FIG. 2c). 
On the surface of the grown n-type Ga.sub.0.7 Al.sub.0.3 As layer 2, 
phospho-silicate glass 6 having a thickness of about 5,000A is formed by a 
conventional chemical vapor deposition method, and a part of the phospho 
silicate glass 6 corresponding to the mesa-shaped portion is selectively 
etched by an etchant of NH.sub.4 F and HF. A ZnAs.sub.2 atmosphere heated 
at 700.degree. C. is contacted to the surface of said Ga.sub.0.7 
Al.sub.0.3 As layer 2 exposed in the stripe region, for 5 minutes, whereby 
a p-type semiconductor region 5 is formed into the Ga.sub.0.7 Al.sub.0.3 
As layer 2 so as to reach to the GaAs layer 1. Finally gold containing 
germanium and nickel is evaporated under vacuum on the surfaces of the 
phosphosilicate glass 6 and the semiconductor region 5, and chromium and 
gold on the surface of the GaAs body 4 for providing metal layers about 
1.mu. thick as shown in FIG. 2d. 
The crystal is then cleaved along the plane perpendicular to the 
mesa-shaped (filamentary) multi-layered semiconductor crystal, that is, 
the plane parallel to the (110) plane so as to obtain a crystal piece 
which has a longitudinal width of the order of 200.mu.. These cut faces of 
the crystal pieces, that is, the end faces 9 and 10 provide the reflecting 
faces of the Fabry-Perot resonator. This laser device is commonly combined 
with a heat sink for a lasing operation. 
In the above-described method, the side surface of the Ga.sub.0.7 
Al.sub.0.3 As layer 3 is exposed to air. However, the Ga.sub.0.7 
Al.sub.0.3 As layer 2 is grown also on the side surface of the Ga.sub.0.7 
Al.sub.0.3 As layer 3, because of the high density of nucleation centers 
on the side walls of said layer 3. 
The semiconductor laser shown in FIG. 1 and formed by the steps shown in 
FIGS. 2a through 2d has such advantages that the threshold current density 
for lasing becomes very low, since the GaAs layer 1 is completely 
surrounded by the Ga.sub.0.7 Al.sub.0.3 As layers 2 and 3 having a broader 
band gap than that of GaAs, and hence carriers to be recombined with each 
other are locked within the GaAs layer 1, that is, the laser active layer, 
and the laser beam is also locked within the GaAs layer 1, since the 
refractive index of the GaAs layer 1 is higher than that of the Ga.sub.0.7 
Al.sub.0.3 As. 
Referring to FIG. 3 which is a schematic perspective view of another 
embodiment of the present invention, on an n-type GaAs body 4, a 
mesa-shaped multi-layered crystal consisting of an n-type Ga.sub.0.6 
Al.sub.0.4 As.sub.0.98 P.sub.0.02 layer 11 disposed on the body 4, a 
p-type GaAs layer 1 disposed on said Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 
P.sub.0.02 layer 11, and a p-type Ga.sub.0.6 Al.sub.0.4 As .sub.0.98 
P.sub.0.02 layer 12 disposed on said GaAs layer 1, is disposed, and an 
n-type or undoped or insulated Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 
P.sub.0.02 or Ga.sub.0.4 Al.sub.0.6 As.sub.0.97 P.sub.0.03 layer 13 is 
disposed on the surfaces of the body 4 and of the mesa-shaped 
multi-layered crystal. A p.sup.+ -type semiconductor region 14 is disposed 
within the p-type Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 P.sub.0.02 layer 12 
and the n-type or undoped or insulated Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 
P.sub.0.02 or Ga.sub.0.4 Al.sub.0.6 As.sub.0.97 P.sub.0.03 layer 13 as 
shown in FIG. 3. A SiO.sub.2 layer 15 is disposed on the layer 13 except a 
part of the semiconductor region 14, and metal layers 7 and 8 are disposed 
on the SiO.sub.2 layer 15 so as to contact with the semiconductor region 
14 and on the surface of the body 4, respectively. 
This layer device is fabricated by the steps as follows. 
An n-type Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 P.sub.0.02 layer 11, an undoped 
GaAs layer 1 and a p-type Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 P.sub.0.02 
layer 12 are formed successively on an n-type GaAs body 4 by a 
conventional liquid phase epitaxy method. (FIG. 4a). The layers 11, 1 and 
12 are selectively etched so as to have a mesa-shaped crystal structure 
and to expose the surface of the GaAs body 4. (FIG. 4b). An n-type layer 
13 is then formed on the surfaces of the body 4 and of the mesa-shaped 
crystal. According to this step, the layer 13 is not formed on the layer 
12, since on III-V semiconductor compounds including Al in its 
composition, it is very hard to form crystals. (FIG. 4c). A SiO.sub.2 
layer 15 is formed on the surfaces of the n-type layer 13 and of the 
p-type layer 12, and is selectively etched so as to expose at least a part 
of the surface of said p-type layer 12. Zn is diffused into the Ga.sub.0.6 
Al.sub.0.4 As.sub.0.98 P.sub.0.02 layer through the exposed surface, 
thereby forming a p-type semiconductor region 14. Conductive layers 7 and 
8 are formed on the SiO.sub.2 layer 15 and the p-type semiconductor region 
14, and the surface of the GaAs body 4, respectively. 
This device has the following advantages, as compared with the device shown 
in FIG. 1. In the device shown in FIG. 1, a diffusion front of the 
semiconductor region 5, which tends to increase defects in the region 1, 
reaches the laser active layer 1, and hence the threshold current density 
becomes high and the deterioration of the device occurs rapidly; on the 
other hand, in the device shown in FIG. 3, since the diffusion front does 
not reach the laser active layer 1, the threshold current density does not 
become high and a deterioration of the device hardly occurs. And also, in 
a fabrication of the device shown in FIG. 3, there is an advantage that 
the control of the time period and the temperature in forming the 
semiconductor region 14 becomes easy compared with the control in 
fabricating the device shown in FIG. 1, since the duffusion front may be 
stopped either in the Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 P.sub.0.02 layer 
12 or in the GaAs layer 1. When the diffusion front is stopped in the 
Ga.sub.0.6 Al.sub.0.4 As.sub.0.98 P.sub.0.02 layer 12 as in the device 
shown in FIG. 3, advantages such as in the device shown in FIG. 3 
described before can be obtained. 
FIGS. 5 through 9 are schematic sectional views of further embodiments of 
the present invention. 
An embodiment shown in FIG. 5 comprises an n-type GaAs body 4 disposed on 
an n-type GaAs substrate 16, a mesa-shaped multi-layered crystal 
consisting of an n-type GaAlAs layer 3, a p-type GaAs layer 1, a p-type 
GaAlAs layer 12 and a p-type GaAs layer 17 disposed, successively, on a 
part of the surface of the n-type GaAs body 4, an undoped GaAlAs layer 19 
disposed on the surfaces of the n-type GaAs body 4 and of the mesa-shaped 
multi-layered crystal, a p-type GaAs layer 18 disposed on the surface of 
the undoped GaA1As layer 19, and a semiconductor region of a p-type 
conductivity disposed in a part of the GaAs layer 18, a part of the GaA1As 
layer 19, the GaAs layer 17, and a part of the GaA1As layer 12. Electrodes 
are disposed so as to contact the semiconductor region 20 and the GaAs 
substrate 16. 
The fabrication of this device is fundamentally the same as that of the 
device shown in FIG. 1. That is, the fabrications of the mesa-shaped 
multi-layered crystal and the GaA1As layer 19 are the same except the 
mesa-shaped multi-layered crystal in FIG. 5, consisting of four layers 3, 
1, 12 and 17, and it is different from the fabrication of the device shown 
in FIG. 1 in that, in the fabrication of the device shown in FIG. 5, only 
the n-type GaAs substrate 16 is prepared and the GaAs layer 18 is formed. 
The n-type GaAs substrate 16 is utilized for strengthening the GaAs body 
4, and the GaAs layer 18 is utilized for easily forming an ohmic contact 
with the electrode to be disposed thereon. 
In the fabrication of the device shown in FIG. 5, since the layers 1, 12 
and 17 are of p-type conductivity, the diffusion front of the 
semiconductor region 20 may be stopped in either one of said layers 1, 12, 
and 17, and hence the fabrication becomes easy. 
FIG. 6 shows a sectional view of another embodiment of the present 
invention, which comprises a mesa-shaped multi-layered crystal consisting 
of an n-type GaAlAs layer 3, a p-type GaAs layer 1 and a p-type GaAlAs 
layer 12, disposed on an n-type GaAs body 4, an n-type GaAlAs layer 2 
disposed on the surface of the GaAs body 4 and on the side surfaces of the 
mesa-shaped multi-layered crystal, and a p-type semiconductor region 21 
disposed in the n-type GaALAs layer 2 and in the p-type GaAlAs layer 12. 
Attention should be directed that after forming the mesa-shaped 
multi-layered crystal, the GaAlAs layer 2 cannot be formed on the GaAlAs 
layer 12, but on the surface of the GaAs body 4 and the side surfaces of 
the mesa-shaped multi-layered crystal. 
FIG. 7 shows a sectional view of another embodiment of the present 
invention, which comprises an n-type GaAs body 4, a mesa-shaped 
multi-layered crystal consisting of a p-type GaAs layer 1 and a p-type 
Ga.sub.0.7 Al.sub.0.3 As layer 12, disposed on the part of the surface of 
the body 4, and an undoped Ga.sub.0.7 Al.sub.0.3 As layer 19 disposed on 
the surface of the body 4 and the side surface of the mesa-shaped 
multi-layered crystal. 
FIGS. 8 and 9 show sectional views of still another embodiment of the 
present invention which are utilized for a light-waveguide. 
An embodiment of the present invention shown in FIG. 8 comprises an n-type 
GaAs substrate 16, an n-type GaAs body 4, a mesa-shaped multi-layered 
crystal being a triangular shape in cross-section consisting of an n-type 
GaAlAs layer 3 and a p-type GaAs layer 1, and an n-type GaAlAs layer 2. 
Since the GaAs layer 1 is completely surrounded by the GaAlAs layers 2 and 
3 whose refractive index is lower than that of the GaAs layer 1, light is 
locked in the GaAs layer 1, and hence the GaAs layer 1 is utilized for a 
light-waveguide. 
Another embodiment of the present invention shown in FIG. 9 is almost the 
same as that shown in FIG. 8. The difference between the embodiments shown 
in FIGS. 8 and 9 is that the device shown in FIG. 8 has a mesa-shaped 
multi-layered crystal being a triangular shape in cross-section, and, on 
the other hand, the device shown in FIG. 9 has a mesa-shaped multi-layered 
crystal being a rectangular shape in cross-section. 
In the above-mentioned semiconductor devices, when the devices are utilized 
for the laser, they must have a cavity resonator which usually comprises 
parallel planes disposed on the end surfaces of the laser active layer 1 
so as to be perpendicular to the laser beam to be emitted from the laser 
active layer 1. 
A still further embodiment of the present invention which does not 
necessitate the cavity resonator is shown in FIG. 10a to 10c. 
This device comprises an n-type GaAs body 4, a mesa-shaped multi-layered 
crystal consisting of a n-type GaAlAs layer 3 and a p-type GaAs layer 1 
having a periodically corrugated surface, disposed on a part of the 
surface of the body 4, and a n-type GaAlAs layer disposed on the surfaces 
of the body 4 and the mesa-shaped multi-layered crystal. 
Fabrication of this device will be explained according to FIGS. 10a through 
10c. 
On the n-type GaAs body 4, an n-type GaAlAs layer 3 and a p-type GaAs 1 are 
formed successively. Then a photo-resist layer is formed on the p-type 
GaAs 1, and ultra-violet light is directed onto said photo-resist layer so 
that interference fringes are formed on the surface of said photo-resist 
layer. When the photo-resist layer is developed, a photo-resist having a 
periodically corrugated surface is obtained. After that, the resultant 
crystal is introduced into an ion milling machine and is etched by ions, 
thereby forming a resultant crystal shown in FIG. 10a having a 
periodically corrugated surface on the GaAs layer 1. This resultant 
crystal is mesa-etched as shown in FIG. 10b. At this stage, the surface of 
the GaAs body 4 must be exposed. An n-type GaAlAs layer 2 is formed on the 
surfaces of the GaAs body 4 and of the mesa-shaped multi-layered 
crystal.(FIG. 10c). 
This device acts as a laser device when an electric field is applied to the 
GaAs layer 1, and a laser wavelength .lambda. of the laser device becomes 
EQU .lambda.=2Sn/m, 
wherein S is a period of the corrugation formed on the surface of the GaAs 
layer 1, n is a refractive index of the GaAs layer 1, and m is an integer. 
This device also acts as a light-waveguide having a filtering function, in 
other words, a band-pass filtering function. That is, only a light whose 
frequency is coincident with said .lambda. passes through the device. The 
band width of the device is very narrow, and hence this device may be 
utilized for separating modes of the laser beam. 
FIG. 11 is a schematic perspective view for explaining a still further 
embodiment of the present invention. 
This semiconductor device is utilized for a light-emitting diode, and 
comprises an n-type GaAs body 4, a circular mesa-shaped multi-layered 
crystal consisting of an n-type Ga.sub.0.5 Al.sub.0.5 As layer 22, an 
n-type Ga.sub.0.7 Al.sub.0.3 As layer 23 which is a light-emitting 
portion, and a p-type Ga.sub.0.5 Al.sub.0.5 As layer 24 disposed on a part 
of the surface of the body 4, an n-type Ga.sub.0.5 Al.sub.0.5 As layer 25 
disposed on the surface of the body 4 and on the side surface of the 
crystal, an insulating layer 26 such as SiO.sub.2 disposed on the surfaces 
of the crystal and of the layer 25, which has a through hole at a portion 
corresponding to a part of the surface of the crystal, and electrodes 27 
and 28 disposed on the insulating layer 26 and the exposed surface of the 
crystal through the through hole and on the body 4, respectively. 
This light-emitting diode emits a light having a wavelength of about 7,000 
A with a high conversion efficiency. 
In the above embodiment, though the crystal has the circular-shaped mesa 
structure, any shapes of the crystal, such as numbers and letters, may be 
utilized. 
While the invention has been explained in detail, it is to be understood 
that the technical scope of the invention is not limited to that of the 
foregoing embodiments. For example, though embodiments shown in FIGS. 1, 
3, 5, 6, 7 and 10 are explained for the laser devices, these are utilized 
as light-waveguides when no electric fields are applied to the mesa-shaped 
multi-layered crystal, and as light-emitting diodes when no cavity 
resonators are applied to the laser active layer, and other embodiments 
shown in FIGS. 8 and 9 may be utilized as the laser device or as the 
light-emitting diode. 
Further, in the embodiments, though GaAs is mainly utilized as the laser 
active material, other materials capable of lasing function may be 
utilized for the laser active material; and also, though semiconductor 
materials including Al in its composition are utilized for materials 
surrounding the laser active layer, other materials having broader band 
gaps than those of the laser active materials, such as Ga.sub.1-x Al.sub.x 
As(0&lt;x.ltoreq.1) and Ga.sub.1-x Al.sub.x As.sub.1-y P.sub.y (0&lt;x, 
y.ltoreq.1), may be utilized. 
According to the semiconductor laser of the present invention, a laser beam 
which has only a lowest-order transverse mode, i.e. TE.sub.00 mode can be 
obtained with a high reproducibility, when the laser active layer has 
certain conditions. 
It has been recognized in the art that the laser beam having higher order 
modes, such as TE.sub.01, TE.sub.02, etc. therein are not desirable when 
the laser beam is coupled with other optical components such as an optical 
fiber, or when the laser is used as a light source in reconstructing a 
holographic image, and hence the laser which emits the laser beam having 
only the lowest-order transverse mode TE.sub.00 has been desired. The 
semiconductor laser of the present invention can satisfy such desire, and 
hence is very useful. 
The conditions are A&lt;1 .mu.m.sup.2 and 1/2(a+b)&lt;1 .mu.m, wherein A is an 
area of a section, of the laser active layer, being perpendicular to a 
direction in which a light generated in the active layer is propagated 
therein and a and b are lengths of sides, of the cross section of the 
active layer, one of which is facing toward a surface of the body of the 
semiconductor laser and the other of which is far from the surface of the 
body. 
These conditions are based on the results of the present inventor's 
experiments. 
When the mode is designated as TE.sub.MN, M=0 is obtained when the distance 
between said two sides of the active layer is not larger than 1 .mu.m, 
preferably 0.8 .mu.m, since the probability of the generation of the mode 
wherein M=1, i.e. TE.sub.1N, becomes very low, and N=0 is obtained when 
the lengths of sides a and b are in the relation of 1/2(a+b)&lt;1 .mu.m, 
which will be apparent from FIG. 12 wherein the relation between the mode 
order N and the length of 1/2(a+b) (.mu.m), which is one of the results of 
the present inventor's experiments, is shown. As is apparent from FIG. 12, 
the laser beam having the mode wherein N=0 is obtained when the length of 
1/2(a+b) is less than 1 .mu.m. When the length of 1/2(a+b) is 1 .mu.m, 
though the laser beam having the mode wherein N=0 is often obtained, the 
laser beam having the mode wherein N= 1 is sometimes obtained solely or 
with the laser beam having the mode wherein N=0. Therefore, the 
reproducibility is rather low. 
On the other hand, when the length of 1/2(a+b) is 0.9 .mu.m, only the laser 
beam having the mode wherein N=0 is obtained, and hence the 
reproducibility to obtain the laser beam having the mode N=0 becomes very 
high. 
According to the inventor's experiments, therefore, it is preferable for 
obtaining a laser beam having a mode of M=0 and N=0, i.e. TE.sub.00 to 
make the active layer so that the area A of the section is less than 1 
.mu.m.sup.2 and the length of 1/2(a+b) is less than 1 .mu.m. 
A preferable range of the length of 1/2(a+b) is from 0.3 .mu.m inclusive to 
1 .mu.m exclusive, and more preferably from 0.5 .mu.m inclusive to 1 .mu.m 
exclusive, since it becomes easy to fabricate when the length of 1/2(a+b) 
becomes long. 
While various embodiments of the present invention and modifications 
thereof have been described in detail by way of example, it is apparent to 
those skilled in the art that many changes and modifications may be made 
therein without departing from the spirit and scope of the appended 
claims.