Semiconductor laser elements

A semiconductor laser element having a GaAs substrate formed thereon with an active layer of a strained quantum well construction provided with an In.sub.x Ga.sub.1-x As strained quantum well layer and a GaAs barrier layer and clad layers arranged up and down of said active layer through an epitaxial growth means. A lattice mismatching rate of the clad layer with respect to the substrate is less than 10.sup.-3.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to semiconductor laser elements of a strained 
quantum well type and a method for the production of the semiconductor 
laser elements. 
2. Description of Prior Art 
A semiconductor laser element formed on a GaAs substrate with an active 
layer of a strained quantum well construction provided with an In.sub.x 
Ga.sub.1-x As (x=0.0 to 0.5) strained quantum well layer and a GaAs 
barrier layer is expected as a light source of wavelength of 0.9 to 1.1 
.mu.m which just comprises of a raving in a conventional lattice matched 
type laser such as GaAs/AlGaAs and InAsP/InP. 
In case of semiconductor laser elements, as a clad for confining carrier 
and light in an active layer, a semiconductor should be used which has a 
permeability with respect to light having an oscillation waveform, is 
smaller in refractive index than that of the active layer (or a layer for 
confining light near the active layer), and is large in energy gap. 
In the conventional semiconductor laser element of an In.sub.x Ga.sub.1-x 
As strained quantum well type, Al.sub.w Ga.sub.1-w As of w&gt;0.2 is used as 
a clad. 
FIG. 8 shows a conventional semiconductor laser element of an In.sub.x 
Ga.sub.1-x As strained quantum well type. 
In FIG. 8, an n-type GaAs substrate 1 having approximately 350 .mu.m of 
thickness is formed thereon with an n-type GaAs buffer layer 2 having 
approximately 0.5 .mu.m of thickness and an n-type Al.sub.0.3 Ga.sub.0.7 
As clad layer 3 having approximately 1.5 .mu.m of thickness through 
epitaxial growth means such as MBE method or MOCVD method. 
Further, in FIG. 8, the n-type GaAs substrate 1 is formed at its 
predetermined position with an essential portion 4 including an active 
layer provided with an In.sub.0.35 Ga0.65As strained quantum well layer, a 
GaAs barrier layer, etc., and a light confining layer, a p-type Al.sub.0.3 
Ga0.7As clad layer 5 having 1.5 .mu.m of thickness and a p-type contact 
layer 6 having 0.2 .mu.m of thickness. 
The details of the essential part 4 is clearly shown in FIG. 9. 
In FIG. 9, two upper and lower GaAs light confining layers 7 has 1500 .ANG. 
of thickness, a GaAs barrier layer 8 between these light confining layers 
7 has 100 .ANG. of thickness and an In.sub.0.35 Ga.sub.0.65 strained 
quantum well layer 9 has a thickness of 40 .ANG.. 
A double hetero construction formed on the GaAs substrate 1 is formed with 
a current restricting layer and an electrode and is applied with 
microworking such as element separation to prepare a laser chip. 
One example of prior art has been described above. The semiconductor laser 
element has various modes such as a ratio of composition of mixed crystal, 
the number of layers of strained quantum wells, thickness of the layers, 
etc. As the light confining layer, there is well known a GRINSHC 
construction using AlGaAs in which the ratio of Al composition is 
parabolic. 
Next, the process for working the bridge waveguide path into a strained 
quantum well type semiconductor laser element will be described 
hereinafter with reference to FIGS. 10(a) to 10(d). 
In the process shown in FIG. 10(a), a resist 10 is patterned on a p-type 
GaAs contact layer 6 through means such as photolithgraphy. 
In the process shown in FIG. 10(b), the resist 10 is used as a mask. The 
GaAs contact layer 6 of a double hetero construction and the upper clad 
layer 5 are subjected to etching till the depth of the etching reaches 
about 0.2 m of the active layer 4. 
As etching liquids in etching AlGaAs/GaAs type, there can be used a mixed 
solution of sulfuric acid and hydrogen peroxide, a mixed solution of 
tartaric acid and hydrogen peroxide, a mixed solution of ammonia and 
hydrogen peroxide or dry etching such as chlorine (for example, reactive 
ion beam etching). 
In the process of FIG. 10(c), means such as spattering is used to form a 
surface of an epitaxial film with an etching mask 11 in the form of a film 
such as SiO.sub.2, SiN, etc. 
In the process of FIG. 10(d), means such as photolithgraphy is used to form 
an etching portion 12 in a stripe-like SiO.sub.2 as a patterned resist 
mask. 
In the thereafter processes, electrodes are formed on both upper and lower 
surfaces of a laminate construction, and microworking such as element 
separation is applied thereto. 
The technical process with regard to the aforementioned semiconductor laser 
element include oxidation of Al, compressive stress from a substrate 
lattice and defective etching process, which will be described 
hereinafter. 
Oxidation of Al 
Main uses of laser having 0.9-1.1 .mu.m of waveform are excitation of a 
fiber amplifier in which rare earth such as Er is doped, or a visual light 
source in combination with SHG. In case of these semiconductor products, a 
prolonged service life is required at high output in excess of scores of 
mW. 
However, the conventional strained quantum well type semiconductor laser 
cannot fulfill such a requirement as described above since it uses the 
aforementioned AlGaAs as a clad layer, for the following reasons. 
Among elements (for example, In, Al, Ga, As, P, Sb, etc.) constituting a 
compound semiconductor, Al is an element which tends to be oxidized most 
easily. For example, when a regrowth surface is made by an embedding 
growth means during fabrication of a laser chip, oxidation of Al tends to 
occur. 
Such an Al oxidation results in the occurrence of a non-light emitting 
center and degradation of crystallization, resulting in a failure to 
obtain a semiconductor laser having excellent laser characteristics. 
Furthermore, in case where a plait surface is used as a laser end, 
oxidation of the end progresses during use of laser to bring forth a 
lowering of refractive index and an increase of absorption, deteriorating 
laser characteristic. 
The semiconductor laser increases its temperature particularly during laser 
operation at high pouring, the progress of oxidation is sped up. 
Means has been proposed to remove oxygen and water content in the process 
for the fabrication of laser in order to suppress oxidation of Al. 
However, this requires much labor and a device for protection of the end 
should be made. 
Because of this, in the prior art, it is not possible to easily obtain a 
laser having a long service life under the using conditions of high 
output. 
Compressive stress from substrate lattice 
In a double hetero construction for laser diode using a conventional 
IN.sub.x Ga.sub.1-x As/GaAs strained quantum well construction as an 
active layer, semiconductors having a larger lattice constant than that of 
GaAs substrate are laminated. 
Specifically, the lattice constant of the GaAs substrate is 5.65 .ANG. 
whereas Al.sub.w Ga.sub.1-w As has a large lattice constant, 0.14 w % and 
In.sub.x Ga.sub.1-x As has a large lattice constant, 7.3 x %. 
For instance, when compositions of In.sub.x Ga.sub.1-x As and Al.sub.w 
Ga.sub.1-w As are x=0.35 and w=0.3, respectively, lattice nonmatching 
rates with respect to GaAs are +2.65% and +0.04%, respectively. 
In this case, the Al.sub.w Ga.sub.1-w As layer is small in the lattice 
non-matching rate with respect to GaAs but is thick, 3 .mu.m (about ten 
thousand atom layer), and In.sub.x Ga.sub.1-x As layer is thin, 120 .ANG. 
(about 40 atom layer) but the lattice nonmatching rate with respect to 
GaAs is large and therefore, a laminate of the Al.sub.w Ga.sub.1-w As 
layer and In.sub.x Ga.sub.1-x As layer receives a compressive stress from 
the substrate. This compressive stress causes an occurrence of transition 
and slip in the active layer of the strained quantum well construction 
during high pouring and laser operation of high excitation. 
As a result, the semiconductor laser element tends to give rise to DLD 
(dark line defect), lowering the laser oscillation life. 
Defective etching process 
In order to control a lateral mode, in the semiconductor laser element it 
is necessary to employ a guide wave mechanism of either gain guide wave 
type or refractive index guide wave type. 
With respect to these guide wave types, various laser element constructions 
have been proposed. In case of the strained quantum well type 
semiconductor laser element, a lattice mismatching is present between a 
well and a barrier, and therefore, an active layer is subjected to mesa 
etching to have a stripe configuration, after which a layer is embedded 
therein and grown. 
However, in the laser element such as BH construction, a defect such as 
transition tends to occur in the embedded growth layer near the active 
layer, making it difficult to obtain a semiconductor laser element having 
a long life. 
On the other hand, in the ridge waveguide laser element, the element can be 
produced leaving an active layer to be flat, which is therefore one of 
laser elements suitable for the strained quantum well type. 
This ridge waveguide laser element can be produced by means illustrated in 
FIG. 10. 
Among the steps shown in FIGS. 10(a) to 10(b), the mesa forming process in 
FIG. 10(b) is important, in order to control the lateral mode, to 
accurately control the distance between the mesa bottom and the active 
layer and in order not to produce scattering of light, to finish the mesa 
bottom flat. 
However, severe control is required with respect to concentration of 
temperature of etching liquid in accurately controlling the depth of 
etching, and similarly, concentration and temperature of etching liquid 
should be maintained uniformly so that the depth of etching is not uneven 
within the etching surface even when etching is carried out. Therefore, 
the technical difficulty in the important process increases. 
As a result, it is difficult to produce a semiconductor laser element 
having an excellent laser characteristic with good reproduceability, 
lowering the yield of good products. 
In view of the aforesaid technical processes, the present invention 
provides a semiconductor laser element which exhibits an excellent laser 
characteristic for a long period of time and a method for the production 
of the semiconductor laser element. 
SUMMARY OF THE INVENTION 
According to a first feature (claim 1) of the present invention, there is 
provided a semiconductor laser element comprising an active layer of a 
strained quantum well construction provided with an In.sub.x Ga.sub.1-x As 
strained quantum well layer and a GaAs barrier layer and clad layers 
arranged up and down of said active layer, said active layer and said clad 
layers being formed on a GaAs substrate through an epitaxial growth means, 
said clad layer being formed of In.sub.z Ga.sub.1-z As.sub.y P.sub.1-y. 
In this case, it is desired that the lattice mismatching rate of the clad 
layer to the substrate be less than 10.sup.-3 as described in claim 2. 
According to a further feature (claim 3), there is provided a semiconductor 
(claim 1) wherein a stress relieving layer comprising In.sub.1-z Ga.sub.z 
P (x=0.51) is interposed between said active layer and said upper and 
lower clad layers. 
According to another feature (claim 4), there is provided a semiconductor 
laser element comprising an active layer of a strained quantum well 
construction provided with an In.sub.x Ga.sub.1-x As strained quantum well 
layer and a GaAs barrier layer and clad layers arranged up and down of 
said active layer, said active layer and said clad layers being formed on 
a GaAs substrate through an epitaxial growth means, said clad layer being 
formed of InGaP, said clad layer having a GaAs etching stop layer inserted 
therein. 
Also in this case, it is desired that the lattice mismatching rate of the 
clad layer to the substrate be less than 10.sup.-3 as described in claim 
4. 
According to another feature (claim 6) of the present invention, there is 
provided a method for the production of a semiconductor laser element 
comprising an active layer of a strained quantum well construction 
provided with an In.sub.x Ga.sub.1-x As strained quantum well layer and a 
GaAs barrier layer, InGaP clad layers arranged up and down of said active 
layer and a GaAs etching stop layer inserted into said clad layer, said 
active layer, said clad layers and said etching stop layer being formed on 
a GaAs substrate through an epitaxial growth means, the method comprising 
the steps of etching an InGaP layer to a GaAs layer with an etching liquid 
containing either sulfuric acid, tartaric acid or ammonia and hydrogen 
peroxide, and etching an InGaP layer to a GaAs layer with an etching 
liquid containing hydrochloric acid but not containing hydrogen peroxide. 
Function of the present invention will be described. 
(1) Semiconductor laser element of claim 1: 
In the semiconductor laser element, in order that clad layers provided up 
and down of the active layer may sufficiently confine light and carriers 
in the strained quantum well active layer and a light confining layer, it 
is necessary that the former is lattice matched with the GaAs substrate, 
the refractive index with respect to light having 0.9 to 1.1 .mu.m of 
wavelength is Al.sub.w Ga.sub.1-w As (w=0.5-0.6) and the energy gap is 
large equal to Al.sub.w Ga.sub.1-w As (w=0.5-0.6). 
In the semiconductor laser element according to claim 1, each of upper and 
lower clad layers of the active layer is formed of In.sub.z Ga.sub.1-z 
As.sub.y P.sub.1-y. 
The In.sub.z Ga.sub.1-z As.sub.y P.sub.1-y is fulfilled with the aforesaid 
condition as the clad layer by adjusting the composition, and does not 
contain Al, and therefore, problems caused by Al oxidation are not 
present. 
Of course, in the strained quantum well type semiconductor laser element 
not only the clad layer but also all the structural members do not contain 
Al, which is therefore desirable. 
(2) In the semiconductor laser element, if the mismatching rate of the clad 
layer with respect to the substrate is less than 10.sup.-3 as in claim 2, 
both the members become substantially completely lattice-matched. 
(3) In the semiconductor laser element according to claim 3: 
The lattice constant a of In.sub.1-z Ga.sub.z P is given by the following 
formula (1) according to Vegad rule. When z=0.51, lattice matching with 
GaAs is obtained, and when z&gt;0.51, the lattice constant is smaller than 
that of GaAs. 
EQU a=5.869--0.42.times.z . . . (1) 
In case of the semiconductor laser element according to claim 3, an 
In.sub.1-z Ga.sub.z P stress relieving layer having z which is smaller in 
lattice constant than that of GaAs substrate is provided near the strained 
quantum well active layer. 
It is desired that the In.sub.1-z Ga.sub.z P stress relieving layer be 
designed so that an average lattice constant a of an epitaxial layer given 
by the following formula (2) is equal to the lattice constant of the GaAs 
substrate. 
EQU a=.SIGMA.a.sub.i t.sub.i /.SIGMA.t.sub.i . . . (2) 
a.sub.i :lattice constant of each epitaxial layer 
t.sub.i :thickness of each epitaxial layer 
Since the thus designed epitaxial layer is lattice-matched to the 
substrate, stress in an interface between the substrate and the epitaxial 
layer hardly occurs, and occurrence of transition at the interface can be 
suppressed. 
Accordingly, the case of strained quantum well type semiconductor laser 
element, the service life is long. 
(3) Semiconductor laser element of claim 4: 
The semiconductor laser element according to claim 4 is configured by 
making use of properties that InGaP is lattice-matched to GaAs and InGaP 
and GaAs can be subjected to selective etching at different speeds. 
That is, in a double hetero construction for laser having an active layer 
of a strained quantum well construction with a ridge waveguide type, a 
clad layer is formed of InGaP, and A GaAs etching stop layer is inserted 
at a control position of an etching depth in a mesa forming step into the 
InGaP clad layer, and therefore, when such a double hetero construction is 
subjected to selective etching processing, it is possible to accurately 
control the distance between the mesa bottom and the active layer. 
(5) Also in the case of the semiconductor laser element, if the lattice 
mismatching rate of the clad layer to the substrate plate is less than 
10.sup.-3 as in claim 5, both the members become substantially completely 
lattice-matched. 
(6) Method for the production of a semiconductor laser element according to 
claim 6: 
The method according to claim 6 is the method for producing the 
semiconductor laser element. 
In the mesa forming step, either a mixed liquid of sulfuric acid and 
hydrogen peroxide, a mixed liquid of tartaric acid and hydrogen peroxide 
or a mixed liquid of ammonia and hydrogen peroxide is used to apply 
etching to a GaAs contact layer, and an etching liquid containing 
hydrochloric acid but not containing hydrogen peroxide is used to apply 
etching to an InGaP clad layer. 
In case of one of said etching liquids containing hydrogen peroxide, the 
etching speed varies depending on concentration and temperature. However, 
the etching speed with respect to GaAs is normally 20 times or more of the 
etching speed with respect to InGaP. 
In case of the other etching liquid not containing hydrogen peroxide, the 
etching speed with respect to GaAs is normally one ten-thousandth of the 
etching speed with respect to InGaP. 
Accordingly, in the mesa forming step in the method according to claim 6, 
first, the aforementioned one etching liquid containing hydrogen peroxide 
can be used to selectively etch the GaAs layer alone. Subsequently, the 
other etching liquid can be used to selectively etch the InGaP clad layer 
up to the GaAs etching stop layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A method for the production of a semiconductor laser element according to 
the present invention will be described in connection with the embodiments 
shown. 
FIGS. 1 and 2 show a first embodiment of a semiconductor laser element 
according to the present invention. A semiconductor laser element of the 
first embodiment has a structure which will be described below. 
In FIG. 1, an n-type GaAs substrate 1 having about 350 .mu.m of thickness 
is formed thereon with a n-type GaAs buffer layer 2 having about 0.5 .mu.m 
of thickness and an n-type In.sub.0.49 Ga.sub.0.51 As clad layer 3a having 
about 1.5 .mu.m of thickness through epitaxial growth means such as MBE 
method and MOCVD method and further at a predetermined position thereof 
with a main portion 4 including an active layer provided with an 
In.sub.0.35 Ga.sub.0.65 As strained quantum well layer, a GaAs barrier 
layer and the like and a light confining layer, a p-type In.sub.0.49 
Ga.sub.0.51 As clad layer 5a having about 1.5 .mu.m of thickness and a 
p-type contact layer 6 having about 0.2 .mu.m of thickness. 
With the structure of the main portion 4 clearly shown in FIG. 2, two upper 
and lower GaAs light confining layers 7 have 1500 .ANG. of thickness, a 
GaAs barrier layer 8 between the light confining layers 7 has 100 .ANG. of 
thickness, and an In.sub.0.35 Ga.sub.0.65 As distortion quantum well layer 
9 has 40 .ANG. of thickness. 
The double hetero construction uniformly formed on the GaAs substrate 1 is 
formed with a current restricting layer and an electrode as is known, and 
is applied with microworking such as element separation to prepare a laser 
chip. 
The semiconductor laser element of the present invention illustrated in 
FIGS. 1 and 2 is different from that shown in FIGS. 8 and 10 in that one 
clad layer 3a is formed of the p-type In0.49Ga0.51As and the other clad 
layer 5a is formed of the n-type In.sub.0.49 Ga.sub.0.51 As. 
The front electrode type semiconductor laser element having the 
construction as described above has 140 A/cm.sup.2 of oscillation 
threshold current density during the pulse drive under room temperature. 
This value is equal to that in which both clad layers are formed of 
Al.sub.0.50 Ga.sub.0.5 As. 
FIG. 3 shows a second embodiment of a semiconductor laser element according 
to the present invention, and an embedded type semiconductor laser element 
of the second embodiment has the structure which will be described below. 
In FIG. 3, an n-type GaAs substrate 11 is formed thereon with a main 
portion 14 including an n-type GaAs buffer layer 12, an n-type InGaP clad 
layer 13, an active layer of a strained quantum well construction and a 
light confining layer, a p/n junction p-type InGaP blocking layer 15 and 
an n-type InGaP blocking layer 16, a p-type InGaP clad layer 17 and a 
p-type GaAs cap layer 18 through the aforementioned epitaxial growth means 
and etching means and further on the back side thereof with a p-type 
electrode 19 of AuZn type, the p-type GaAs cap layer 18 being formed on 
the upper surface thereof with an n-type electrode 20 of AuGeNi type. 
In FIG. 3, the main portion 14 including an active layer of distortion 
quantum well construction and a light confining layer has 1.5 .mu.m of 
width and 600 .mu.m of length of resonance unit, and a cleavage surface 
thereof is in the form of a mirror without a protective film. 
When the semiconductor laser element of the second embodiment shown in FIG. 
3 is operated under room temperature, CW oscillation is obtained at 
pouring of 7 mA, and output of 100 mW is obtained at pouring of 600 mA. 
After laser drive 50 mW and 200 hours, the I-L characteristic remains 
unchanged. A long service life is maintained under the high output state. 
While in the second embodiment, InGaP has been used as a clad layer, it is 
to be noted that even if InGaAsP having 1.5 eV or more of energy gap is 
used as a clad layer, light and carrier can be confined. 
Since InGaP as well as InGaAsP contain no Al, oxidation of regrowth 
interface and cleavage surface is hard to occur and accordingly a laser of 
long service life can be prepared. 
With respect to InGaAsP, lattice matching with the GaAs substrate may be 
provided to a degree that transition does not occur. 
In the second embodiment, the film thickness of layers, composition, and 
the number of strained quantum well layers are not limited to the 
aforementioned contents and the illustrated examples. 
In the second embodiment, the GRIN construction in which the composition of 
InGaAsP is stepwisely changed is sometimes employed as a light confining 
layer. 
In case of the semiconductor laser element of the second embodiment 
illustrated in FIG. 3, the cleavage surface is sometimes subjected to 
coating for the purpose of improving low threshold, efficiency, and 
output. 
FIG. 4 shows a third embodiment of a semiconductor laser element according 
to the present invention. The semiconductor laser element of the third 
embodiment has the structure described below. 
In FIG. 4, an n-type GaAs substrate 31 having 350 .mu.m of thickness formed 
thereon with an n-type GaAs buffer layer 32 having 0.5 .mu.m of thickness, 
an n-type In.sub.0.49 Ga.sub.0.51 As clad layer 33 having 1.5 .mu.m of 
thickness, a p-type In0.47Ga.sub.0.35 P stress relieving layer having 0.12 
.mu.m of thickness, a main portion 40 including an active layer and a 
light confining layer, an n-type In.sub.0.47 Ga.sub.0.53 stress relieving 
layer 42 having 0.12 .mu.m of thickness, a p-type In.sub.0.49 Ga.sub.0.51 
As clad layer 35 having 0.5 .mu.m of thickness, and a p-type contact layer 
36 having 0.2 .mu.m of thickness in order through the aforementioned 
epitaxial growth means. 
The structure of the main portion 40 in FIG. 4 is the same as that 
mentioned in FIG. 2. Parts constituting the main portion 40 are indicated 
by reference numeral with () in FIG. 2. 
That is, in FIG. 2 also showing the main portion 40 of FIG. 4, two GaAs 
light confining layers 37 positioned up and down have 1500 .ANG. of 
thickness, each GaAs barrier layer 38 between these light confining layers 
7 has 100 .ANG. of thickness, and each In.sub.0.35 Ga.sub.0.65 As strained 
quantum well layer 39 has 40 .ANG. of thickness. 
An average lattice constant between an In.sub.0.35 Ga.sub.0.65 As strained 
quantum well layer 39 having 40 .ANG. thickness and +2.5% of lattice 
mismatching rate and an In.sub.0.47 Ga.sub.0.53 P stress relieving layers 
41 and 42 having 1200 .ANG. thickness and -0.13% of lattice mismatching 
rate is substantially equal to that of the GaAs substrate 31. 
Let .DELTA. a/ao be the lattice mismatching rate of the expitaxial growth 
layer with respect to the substrate 31, this value is very small as shown 
by the formula (3) below. 
EQU .DELTA.a/ao=1.2.times.10.sup.-6 . . . (3) 
As a comparative example 1, as shown in FIGS. 8 and 9, on an n-type GaAs 
substrate 1 having 350 .mu.m of thickness were laminated in order of an 
n-type GaAs buffer layer 2 having 0.5 .mu.m of thickness, an n-type 
Al.sub.0.3G Ga.sub.0.7 As clad layer 3 having 1.5 .mu.m of thickness, a 
main portion 4 including an active layer and a light confining layer, a 
p-type Al.sub.0.3 Ga.sub.0.7 As clad layer 5 having 1.5 .mu.m of 
thickness, and a p-type contact layer having 0.2 .mu.m of thickness to 
prepare a predetermined specimen. 
In this case, the GaAs light confining layer 7 of the main portion 4 has 
1500 .ANG. of thickness, the GaAs barrier layer 8 between the light 
confining layers 7 has 100 .ANG. of thickness, and the In.sub.0.35 
Ga.sub.0.65 As strained quantum well layer 9 has 40 .ANG. of thickness. 
Each double hetero construction of the third embodiment and comparative 
example 3 was worked into a complete electrode type laser element having 
300 .mu.m of cavity length and they were pulse oscillated under room 
temperature. The oscillation threshold current density was measured to 
obtain results given in Table 1 below. 
TABLE 1 
______________________________________ 
Threshold current density (A/cm.sup.2) 
______________________________________ 
Third Emdodiment 
280 .+-. 20 
Comparative 280 .+-. 20 
Example 1 
______________________________________ 
FIG. 5 shows a fourth embodiment of a semiconductor laser element according 
to the present invention. 
The semiconductor laser element of the fourth embodiment shown in FIG. 5 is 
provided with a SiO.sub.2 insulating film 43 and alloy electrodes 44 and 
45 while in the third embodiment, the element is prepared in the form of 
ridge wave guide type strained quantum well type. 
In FIG. 5, reference numeral 46 denotes a flow of current. 
In Comparative Example 2 to be compared with the fourth embodiment, the 
element of the Comparative Example 1 is formed into the strained quantum 
well type of the ridge wave guide type similar to that of the fourth 
embodiment. 
With respect to the elements of the fourth embodiment and Comparative 
Example 2, the threshold current density and life were measured to obtain 
the result shown in Table 2 below. 
TABLE 2 
______________________________________ 
Threshold current density 
(mA) Life 
______________________________________ 
Fourth Embodiment 
12 2000 hr or 
more 
Comparative 13 500 hr 
Example 2 
______________________________________ 
The life in Table 2 was determined according to 10% (30.degree. C.) rise of 
drive current by APC of 50mW. 
It is found from the above measured results that the threshold current and 
life of the strained quantum well type semiconductor laser element 
according to the present invention were remarkably improved. 
In case of the third and fourth embodiments, as the clad layer, Al.sub.w 
Ga.sub.1-w As of w&gt;0.3 is sometimes used. 
In the third and fourth embodiment, it is desired that the average lattice 
constant of the epitaxial layer is equal to that of the GaAs substrate. 
However, an allowable range of the mismatching rate is approximately 
.vertline..DELTA.a/ao.vertline.&lt;1.2.times.10.sup.-4. 
Thickness of layers, composition and number of strained quantum well layers 
are not limited thereto but the GRIN construction is sometimes employed as 
a light confining layer. 
FIG. 6 shows a fifth embodiment of a semiconductor laser element according 
to the present invention. The semiconductor laser element of the fifth 
embodiment has the structure described below. 
In FIG. 6, an n-type GaAs substrate 51 having 350 .mu.m of thickness is 
formed thereon with an n-type GaAs buffer layer 52 having 0.5 .mu.m of 
thickness, an n-type In.sub.0.49 Ga.sub.0.5 As clad layer 53 having 1.5 
.mu.m of thickness, a main portion 60 including an active layer and a 
light confining layer, a p-type In.sub.0.51 Ga.sub.0.49 P clad layer 55a 
having 0.2 .mu.m of thickness and a p-type In.sub.0.51 Ga.sub.0.49 P clad 
layer 55b having 1.3 .mu.m of thickness, a p-type GaAs etching stop layer 
61 interposed between both the p-type clad layers 55a and 55b, and a 
p-type contact layer 56 having 0.2 .mu.m of thickness through the 
aforementioned epitaxial growth means. 
The structure of the main portion 60 in FIG. 6 is the same as that 
mentioned in FIG. 2. Parts constituting the main portion 60 are indicated 
by reference numerals with [ ] in FIG. 2. 
That is, in FIG. 2 also showing the main portion 60 in FIG. 4, two GaAs 
light confining layers 57 positioned up and down have 1500 .ANG. of 
thickness, each GaAs barrier layer 58 between the light confining layers 7 
has 100 .ANG. of thickness, and each In.sub.0.35 Ga.sub.0.65 As distortion 
quantum well layer 59 has 40 .ANG.. 
It is desired that the thickness of the etching stop layer 61 be less than 
0.2 .mu.m so as not to impair a light confining effect caused by the clad 
layer. 
As a modified form of the fifth embodiment, in the same construction as 
that of the fifth embodiment, only the thickness of the etching stop layer 
61 is set to 50 .ANG., and an absorption end of the etching stop layer 61 
is sometimes shifted to a short wave side by the quantum effect. 
In the method for production of a semiconductor laser element according to 
the present invention, the step of mesaforming a semiconductor laser 
element of the fifth embodiment is executed as described hereinafter. 
First, when a GaAs contact layer 56 is subjected to etching, a mixed liquid 
of sulfuric acid and hydrogen peroxide is used. 
The etching solution of the mixed solution varies with the conditions such 
as temperature, mixing ratio, stirring state of the solution. 
However, since the etching speed of the GaAs by the mixed solution is 20 
times or more of InGaP, such a mixed solution is used whereby only the 
GaAs can be etched. 
Next, when an InGaP clad layer 57 is subjected to etching till reaching the 
etching stop layer 51, hydrochloric acid of 36% (weight percentage) is 
used. 
The etching speed of InGaP by hydrochloric acid at 20.degree. C. is 
approximately 0.1 .mu.m/sec., and an InGaP layer having 1.3 .mu.m of 
thickness can be etched in 13.+-.2 seconds by using the hydrochloric acid. 
On the other hand, the etching speed of GaAs by hydrochloric acid is so 
small that it cannot be measured, such as 0.1 A/sec.. For example, in case 
of GaAs layer of 50 .ANG., even if the etching time of 10 minutes has 
passed, it cannot be etched. 
In this manner, by carrying out etching processing for 15 seconds using 
hydrochloric acid, the etching depth of the InGaP clad layer is less than 
a single atomic layer (3 .ANG.). 
Furthermore, if the aforementioned double hetero construction is prepared 
by a precise crystal growth method such as MCVD method and MBE method, a 
very flat film is obtained. 
Incidentally, in the case where a device size is about 300 .mu.m square, 
the flatness of atomic layer level can be obtained, and the error in film 
thickness is less than 1%. 
Accordingly, in the mesa forming step of the present method, if such a 
double hetero construction is used, the depth of mesa can be designed 
within the range of an error less than 1%, and a ridge waveguide laser 
element of a strained quantum well construction having a flatness in which 
an etching bottom is at a level of an atomic layer can be easily prepared. 
Double hetero constructions of the aforementioned fifth embodiment, the 
modified form and the prior art are prepared three times by use of the 
MOCVD method, and 100 ridge waveguide laser elements having a strained 
quantum well construction were prepared from wafers thereof. 
With respect to the semiconductor laser elements of the fifth embodiment, 
the modified example and prior art, the oscillation threshold currents in 
0.93.+-.0.101 .mu.m of oscillation wavelength were measured to obtain the 
results shown in Tables 3-1, 3-2 and 3-3 below. 
TABLE 3-1 
______________________________________ 
(Fifth Embodiment) 
1st time 2nd time 2nd time 
______________________________________ 
- I th 12.3 mA 12.4 mA 12.6 mA 
.sigma. Ith 
0.40 mA 0.40 mA 0.35 mA 
______________________________________ 
TABLE 3-2 
______________________________________ 
(Modified Example of Fifth Embodiment) 
1st time 2nd time 2nd time 
______________________________________ 
- I th 10.2 mA 9.7 mA 10.1 mA 
.sigma. Ith 
0.35 mA 0.35 mA 0.40 mA 
______________________________________ 
TABLE 3-3 
______________________________________ 
(Prior Art) 
1st time 2nd time 2nd time 
______________________________________ 
- I th 25.2 mA 18.0 mA 30.4 mA 
.sigma. Ith 
3.2 mA 2.9 mA 4.1 mA 
______________________________________ 
As will be apparent from the above tables, the double hetero construction 
according to the embodiments of the present invention is superior in the 
average value of the threshold current to that of prior art. Particularly, 
in the modified example of the fifth embodiment, since the absorption of 
the etching stop layer at 0.93 .mu.m of oscillation wavelength is small, 
the threshold value is smaller that than of the fifth embodiment. 
Moreover, the double hetero construction in the embodiment of the present 
invention shows favorable results as compared with prior art with respect 
to irregularities of thresholds, between batches as well as chips. 
While in the above-described embodiment, the ridge waveguide laser has been 
employed, it is to be noted that as a sixth embodiment, an etching stop 
layer can be provided also in the laser construction of SAB type 
illustrated in FIG. 7. 
In FIG. 7, an n-type GaAs substrate 31 is formed thereon with an n-type 
InGaP clad layer 33, an etching stop layer 38, a p-type InGaP blocking 
layer 37, an active layer 34, an n-type InGaP clad layer 35 and an n-type 
GaAs contact layer 31. 
In the etching step for preparing a SAB type semiconductor laser element of 
FIG. 7, as an etching liquid for selectively etching InGaP, a mixed 
solution of hydrochloric acid and phosphoric acid or hydrochloric acid and 
acetic acid is sometimes used. 
The semiconductor laser element according to the present invention has the 
following effects. 
According to a first effect, in a semiconductor laser element having a GaAs 
substrate formed thereon with an active layer of a strained quantum well 
construction provided with an In.sub.x Ga.sub.1-x As strained quantum well 
layer and a GaAs barrier layer, and clad layers arrange up and down of 
said active layer through epitaxial growth means, said clad layer is 
formed of In.sub.z Ga.sub.1-z As.sub.y P.sub.1-y and therefore strained 
quantum well type semiconductor laser element having a large output a long 
service life can be obtained. 
In this case, if the lattice mismatching rate of the clad layer with 
respect to the substrate is less than 10.sup.-3, more favorable effect is 
obtained. 
According to a further effect, in the aforementioned double hetero 
construction, since a stree relieving layer formed of In.sub.1-z Ga.sub.z 
P is provided adjacent to the active layer, a strained quantum well type 
semiconductor laser element having a long service life can be obtained in 
view of the foregoing. 
According to another effect, in a semiconductor laser element having a GaAs 
substrate formed thereon with an active layer of a strained quantum well 
construction provided with an In.sub.x Ga.sub.1-x As strained quantum well 
layer and a GaAs barrier layer and clad layers arranged up and down of 
said active layer througy an epitaxial growth means, said clad layer is 
formed of InGaP, and a GaAs etching stop layer is inserted into said clad 
layer, and therefore, such a double hetero construction is applied with 
etching processing whereby a distance between a mesa bottom and the active 
layer can be accurately controlled, and as a semiconductor, an 
irregularity of threshold current is reduced and a yield of product is 
enhanced. 
Also in this case, if the lattice mismatching rate of the clad layer with 
respect to the substrate is less than 10.sup.-3, more favorable effect is 
obtained. 
Moreover, according to the method for the production of the aforementioned 
semiconductor laser element according to the present invention, the method 
comprises the steps of etching a GaAs layer to an InGaP layer with an 
etching liquid containing either sulfuric acid, tartaric acid and ammonia 
and hydrogen peroxide and etching an InGaP layer to a GaAs layer with an 
etching liquid containing hydrochloric acid but not containing hydrogen 
peroxide. Therefore, first, said one etching liquid containing hydrogen 
peroxide can be used to selectively etch only the GaAs layer, and then, 
said other etching liquid not containing hydrogen peroxide is used to 
selectively etch the InGaP clad layer up to the GaAs etching stop layer 
whereby a semiconductor having an excellent characteristic can be easily 
prepared.