Semiconductor laser device having a buried heterostructure

A semiconductor laser device having a buried heterostructure which includes a multi-layered crystal structure, containing an active layer for laser oscillation, on a substrate, said multi-layered crystal structure having a striped mesa-portion, a multi-layered structure surrounding said mesa-portion and a burying layer disposed on an upper face of said striped mesa-portion. The unique structure results in a heterojunction at each side of the active layer.

BACKGROUND OF THE INVENTION 
1. Field of the invention: 
This invention relates to a method for the production of semiconductor 
laser devices, especially those having a buried heterostructure therein. 
2. Description of the related art: 
Semiconductor laser devices, which comprise a buried heterostructure 
resulting from surrounding the active layer for laser oscillation with a 
heterojunction, have excellent device characteristics in that provide for 
stabilized laser oscillation which can be attained in a transverse mode 
and a low oscillation threshold current level which can be maintained, and 
thus they are usable as laser light sources. 
In a conventional semiconductor laser device having a structrue such as 
that shown in FIG. 2, in which a p type crystal 10 is employed as a 
substrate for epitaxial crystal growth, the p-n junction of a current 
confining region has to withstand voltage at a high level, so that a high 
output power operation can be achieved by the application of a high 
voltage. This conventional semiconductor laser device is produced as 
follows: On the p-InP substrate 10, a p-InP buffer layer 20, a non-doped 
InGaPAs active layer 30, and an n-InP cladding layer 40 are successively 
grown by liquid phase epitaxy. Both sides of the resulting multi-layered 
crystal structure are then etched in a manner to reach the buffer layer 
20. On the post-etched portion, an n-InP current blocking layer 50 and a 
p-current confining layer 60 are grown as burying layers. According to the 
structure of the above-mentioned device, in order to block the 
electroconductivity between the cladding layer 40 and the current blocking 
layer 50, the top of the current blocking layer 50 must be positioned 
below the active layer 30. For this reason, both the depth of etching in 
the multi-layered crystal structure and the growth thickness of the 
current blocking layer 50 must be regulated with high precision. However, 
neither liquid phase epitaxy nor a wet-etching technique can attain such 
precise regulation, resulting in a low production yield of semiconductor 
laser devices of this type. 
SUMMARY OF THE INVENTION 
The method for the production of semiconductor laser devices of this 
invention which overcomes the above-discussed and numerous other 
disadvantages and deficiencies of the prior art, comprises (1) growing a 
multi-layered crystal structure, containing an active layer for laser 
oscillation, on a p-substrate, (2) etching said multi-layered crystal 
structure to form a striped mesa-portion, and (3) surrounding said 
mesa-portion of the multi-layered crystal structure with a p-n-p 
multi-layered structure, resulting in a heterojunction at each of both 
side faces of said active layer. 
The multi-layered crystal structure comprises, in a preferred embodiment, a 
p-buffer layer, a non-doped active layer, and an n-cladding layer, in 
sequence, on the p-substrate. 
The multi-layered crystal structure is, in a preferred embodiment, etched 
in the range of the upper face thereof to the middle of the p-buffer 
layer. 
The p-n-p multi-layered structure comprises, in a preferred embodiment, a 
p-buffer layer, an n-current blocking layer and a p-current confining 
layer, in sequence. The p-buffer layer of said p-n-p multi-layered 
structure electrically separates the n-current blocking layer from the 
cladding layer of the striped mesa-portion. The p-buffer layer of said 
p-n-p multi-layered structure is, in a preferred embodiment, a layer, the 
forbidden band width of which is greater than that of the active layer and 
the refraction index of which is smaller than that of the active layer. 
The invention described herein makes possible the object of providing a 
method for the production of semiconductor laser devices by which 
semiconductor laser devices having a buried structure can be readily 
produced without a crystal growth process and an etching process under a 
highly precise regulation, and thus the production yield thereof can be 
increased.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1(A) and 1(B) show the production process of a semiconductor laser 
device according to a preferred embodiment of the present invention. As 
shown in FIG. 1(A) on a p-(100)InP substrate 1, a p-InP buffer layer 2 
having a thickness of 3 .mu.m, a non-doped InGaPAs active layer 3 having a 
radiation wavelength of 1.3 .mu.m and a thickness of 0.2 .mu.m, and an 
n-InP cladding layer 4 having a thickness of 0.5 .mu.m are successively 
grown by liquid phase epitaxy. On the cladding layer 4, a photo-resist 
stripe (not shown) having a width of approximately 3 .mu.m is then formed 
in the (011) direction by photolithography. The multi-layered crystal 
structure is then subjected to an etching treatment with a Br-methanol 
solution in the range of the upper face thereof to the middle of the 
buffer layer 2, resulting in a striped mesa-portion 11. When the striped 
mesa-portion 11 is designed with a stripe width W of 2-4 .mu.m and a 
height h of 2 .mu.m, the crystal growth can be prevented from proceeding 
over the mesa-portion 11 in the succeeding burying-growth process (Mito el 
al., TGOQE 80-116). The burying growth process is then carried out as 
shown in FIG. 1(B) in such a manner that a p-InP buffer layer 7 (the plane 
portion thereof having a thickness of 0.5 .mu.m), an n-InP current 
blocking layer 5, a p-InP current confining layer 6 (the plane portion 
thereof having a thickness of 0.5 .mu.m), and an n-InP burying layer 8 
(the plane portion thereof having a thickness of 3 .mu.m) are successively 
grown along the striped mesa-portion 11 by liquid phase epitaxy. Then, an 
n-sided ohmic electrode (not shown) and a p-sided ohmic electrode (not 
shown) are formed on the InP burying layer 8 and the substrate 1, 
respectively, followed by cleaving at the (011) face, resulting in a 
semiconductor laser device. 
In the semiconductor laser device having the above-mentioned structure, the 
InGaPAs active layer 3 for laser oscillation has a junction at the 
interface with each of the InP buffer layer 2 and the InP cladding layer 4 
in the thickness direction thereof and has a junction at the interface 
with the InP buffer layer 7 at each of both sides thereof, resulting in an 
active region which is limited to a striped form by the said 
heterojunctions. Since the buffer layer 7 tends to be grown on the sides 
of the striped mesa-portion 11, the sides of both the cladding layer 4 and 
the active layer 3 are covered with the p-InP buffer layer 7 so that the 
cladding layer 4 is electrically separated from the current blocking layer 
5 and leakage current never arises between the cladding layer 4 and the 
current blocking layer 5. Moreover, since the carrier concentration of the 
buffer layer 7 is selected to be at a low level, the buffer layer 7 
functions as an insulating layer which prevents leakage current from the 
cladding layer 4 to the current blocking layer 5. Moreover, since the 
forbidden band width of the buffer layer 7 is greater than that of the 
active layer 3 and the refraction index is smaller than that of the active 
layer 3, it functions to confine carrier and light within the active layer 
3. 
When a driving current is injected into the semiconductor laser device 
through the n-sided ohmic electrode and the p-sided ohmic electrode, the 
current flows into the electroconductive region corresponding to the 
striped mesa-portion 11 including the cladding layer 4 and the active 
layer 3 therein and is blocked from flowing outside of the striped 
mesa-portion 11 by the p-n-p multi-layered structure which is composed of 
the current confining layer 6, the current blocking layer 5 and the buffer 
layer 7. The carrier and light are confined within the active layer 3 by 
the junction at the interface between the buffer layer 7 and the active 
layer 3. The top of each p-n junction in the p-n-p multi-layered structure 
is positioned at the shoulder portion of the cladding layer 4 and the 
lower portion thereof is curved toward the lateral direction. This is 
because the buffer layer 7 is grown in the lateral direction during its 
epitaxial growth around the striped mesa-portion 11 and, on the buffer 
layer 7 which functions as an underlying layer, the current blocking layer 
5 and the current confining layer 6 are successively formed. Because of 
the above-mentioned structure, a current with a high density can be fed to 
the active layer 3, and the semiconductor laser device obtained exhibits 
excellent device characteristics since it can oscillate at a low 
oscillation threshold current level in the range of 15 to 20 mA at ambient 
temperature and the characteristic temperatures of the oscillation 
threshold current are 70 K or higher up to a temperature of about 
100.degree. C. Moreover, since the depth of etching and the thickness of 
crystal growth layers are not required to be regulated with accuracy, the 
production yield is greatly improved. 
The above-mentioned example discloses only the InGaPAs system materials, as 
an active layer, which exhibit a radiation wavelength of 1.3 .mu.m, but is 
not limited thereto. This invention is, of course, applicable to other 
InGaPAs system materials which have a radiation wavelength ranging from 
1.1 .mu.m to 1.7 .mu.m, and/or other semiconductor materials, as well. All 
of the four layers constituting the buried structure in the 
above-mentioned example are composed of InP, but they can be InGaPAs, the 
forbidden band width of which is greater than that of the active layer and 
the refraction index of which is smaller than that of the active layer. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty which reside in the present 
invention, including all features which would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.