Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Semiconductor Laser Device
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This invention relates to a semiconductor laser device.
In recent years, semiconductor laser devices have been
proposed as light sources for optical communication, laser
printers, instrumentation etc. In exciting such devices,
a method of carrier injection has been employed because it
has the advantages that a high light conversion efficiency
can generally be attained and the optical output can be
readily modulated by directly controlling the injection
current.
When exciting a semiconductor laser device by this
method of carrier injection, it is common practice to con-
struct the device as a so-called stripe-geometry laser
with a narrowed active region. This permits operation of
the device with a low threshold current and facilitates
control of the oscillation mode of the laser.
Concrete examples of a stripe-geometry laser are
described in U.S. Patent 3,920,491, issued November 18,
1975 to Hiroo Yonezu and U.S. Patent Re. 29,395 issued
September 13, 1977 to Hiroo Yonezu.
Such a semiconductor laser device includes a narrow
elongated semiconductor region of the same conductivity
type as that of another semiconductor region lying in the
vicinity of the active region of the device. The elongated
region extends in depth from the surface of the device to
the vicinity of the active region. A surface semiconductor
layer of the opposite conductivity type covers the entire
surface of the device except for the elongated region.
Summary of the Invention
An object of the present invention is to provide a
semiconductor laser device employing a stripe-shaped
impurity-diffused region for injecting current into an
active region wherein a novel structure makes the per-
centage of acceptable products of manufacture high, i.e. a
low reject rate.
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To this end the invention consists o~ a semiconductor
laser device having a stripe-shaped impurity-diffused
region disposed at least in parts of semiconductor layers
from a surface semiconductor layer of a semiconductor
layer assembly to a second semiconductor layer lying in
contact with a first semiconductor layer having an active
region, the impurity-diffused region having the same con-
ductivity type as that of the second semiconductor layer
and extending at least from the surface semiconductor layer
to a depth vicinal to the first semiconductor layer, an
electrode being disposed on the impurity-diffused region
so that a current may flow from the electrode to
the first semiconductor layer through the impurity-diffused
region; characterized in that a third semiconductor layer
in which a diffusion rate of an impurity for use in the
formation of said impurity-diffused region is lower than in
said second semiconductor layer is disposed between said
surface semiconductor layer and said second semiconductor
layer.
Since the diffusion rate of the impurity is low in the
third semiconductor layer, control of the depth of the
impurity-diffused region is facilitated.
In an example of a GaAs-GaAlAs-system, double-hetero-
structure injection laser, GaAlAs is usually used for the
second semiconductor layer, Zn or the like as the impurity
for forming the impurity-diffused region, and GaAs for the
surface semiconductor layer.
In order to realize a GaAs-GaAlAs-system semiconductor
laser device that emits visible radiation, the mole
fraction of AlAs (u) in Gal uAluAs needs to be made
large. This aims to enhance the transfer efficiency of
the visible radiation. However, when the mole fraction of
AlAs in GaAlAs is large, the diffusion rate of ~n increases
abruptly. Simultaneously, the depths of the impurity-
diffused regions disperse very greatly and become difficult
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to control in practice. Accordingly, troubles in which
the impurity region extends over the greater part of the
second semiconductor layer or even reaches the active
region occur frequently.
Especially in the GaAlAs material in which the mole
fraction of AlAs (u) is at least 0.45, not only the
diffusion rate increases, but also the dispersion of the
diffusion depths becomes conspicuous. When u is 0.5 or
greater, the dispersion becomes very conspicuous, and such
material is difficult to use in practice. Further, it
becomes difficult to form a grown layer of good quality.
In a case where the GaAlAs material described above is
used for the second semiconductor layer of the device, the
third semiconductor layer is inserted between the second
semiconductor layer and the surface semiconductor layer.
The impurity diffusion rate in this third semiconductor
layer is then selected to be lower than that in the second
semiconductor layer, whereby the diffusion depth of the
; impurity can be well controlled. In using GaAlAs for the
third semiconductor layer, u should preferably be 0.1 to
0.35.
Figure 1 is a partial, vertical, cross-sectional view
of a semiconductor laser device according to one
embodiment of the invention;
Figure 2 is a perspective view of the device of Figure
l;
Figure 3 is a partial, vertical, cross-sectional view
of a semiconductor laser device according to one example
of the prior art;
Figure 4 is a graph showing the relationship between
the diffusion depth and the mole fraction of AlAs;
Figure 5 is a graph showing the threshold current
distribution of semiconductor laser devices; and
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Figures 6 and 7 are partial, vertical, cross-sectional
views of semiconductor laser devices according to other
embodiments of the invention.
Detailed Description of the Embodiments
In Figs. 1 and 2, on an n-GaAs substrate 1 which has
the (100) face as its upper surface, an n-GaO 4Alo 6As
layer 2 is formed to a thickness of 1.5 ~m, an
n-GaO 75Alo 25As layer 3 to a thickness of 0.1 ~m,
a p-GaO 4Alo 6As layer 4 to a thickness of 1.5 ~m, a
p-GaO 8Alo 2As layer 5 to a thickness of 1 ~m, and an
n-GaAs layer 6 to a thickness of 1 ~m. Each semiconductor
layer may be formed by the conventional liquid phase,
epitaxial growth process.
The n-GaAs layer 3 corresponds to the first semiconduc-
tor layer mentioned above, and has the active region. The
p-GaO 4Alo 6As layer 4 corresponds to the second semi-
conductor layer, the p-GaO 8Alo 2As layer 5 to the third
semiconductor layer, and the n-GaAs layer 6 to the surface
semiconductor layer.
In a double-heterostructure injection laser of the
GaAs-GaAlAs-system, the first semiconductor layer is made
of Gal xAlxAs (O < x < 0.5), and the cladding layers
holding it therebetween are made of Gal yAlyAs (0.2 ~ y
~ 0.8), x and Y being so related that x ~ y. Regarding the
thickness of the layers, the first semiconductor layer is
set at 0.05 ~m - 0.3 ~m, and the cladding layers at 1.0 ~m
- 3.0 ~m. The surface semiconductor layer is necessary (1)
for preventing the semiconductor layers under manufacture
from oxidizing, (2) for protecting the semiconductor layers
- 30 when washing the semiconductor layer assembly, and (3) for
reducing the contact resistance to an electrode disposed
thereon. For these purposes, GaAs is the most preferred,
and the thickness of the surface semiconductor layer is
usually made 0.5 ~m - 1.5 ~m.
On the n-GaAs layer 6, an A1203 layer 8 is formed.
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In this layer 8, a window which is 3.0 ~m wide is provided
by the well-known photolithographic process. Through the
window, zn is selectively diffused to be 2 ~m deep, that
is, to diffuse into the n-GaAs layer 6 and the p-GaO 8-
Alo 2As layer 5. The regions indicated by symbols 7 and7' correspond to these Zn-diffused region.
Thereafter, stacked layers of Au and Cr are formed as
a p-side electrode 9, and an Au-Ge-Ni alloy is deposited
as an n-side electrode ~0. The crystal is cloven at the
opposing (110) faces ( or faces equivalent thereto) to
form an optical resonator and to construct the semicon-
ductor laser device. The cavity length is 300~ m.
In an example of this embodiment, the laser could
oscillate at a threshold current density of approximately
2 kA/cm at room temperature. The oscillation wavelength
was 7,500 A , and the external quantum efficiency was
approximately 40 %. Arrows in Figure 2 indicate the
emerging directions of laser radiation.
The advantages of this construction will be described
by referring to an example of a prior-art structure shown
in Figure 3 which is a sectional view showing a typical
known example of a GaAs-GaAlAs-system semiconductor laser.
On a GaAs substrate 1, an n-GaO 4Alo 6As layer 2 is
formed to a thickness of 1.5 ~m, an n-GaO 75Alo 25As
layer 3 to a thickness of 0.3 ~m, a p-GaO 4Alo 6As
layer 4 to a thickness of 1.5 ~m, and an n-GaAs layer 6 to
a thickness of 1 ~m. In parts of the n-GaAs layer 6 and
the p-GaO 4Alo 6As layer 4, a Zn-diffused region at 7
and 7' is formed. The n-GaAs layer 3 corresponds to the
first semiconductor layer described before, the
p-Gao 4A10.6As layer 4 to the second semlcond~ctor layer, and
the n-GaAs layer 6 to the surface semiconductor layer.
Numeral 8 designates an insulator layer, and numerals 9
and 10 respectively designate electrodes.
In a case where the diffusion depth of the impurity-
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diffused layer 7 is precisely controlled, as illustrated
in Figure 3, no problem occurs. As stated before, however,
when the mole fraction of AlAs in the p-GaO 4Alo 6As layer
4 becomes 0.45 or above, the diffusion rate of Zn increases
and the diffusion depth thereof disperses greatly. In
practice, accordingly, control becomes difficult.
Figure 4 illustrates the relationship of the diffusion
depth to the mole fraction of AlAs at the time when Zn was
diffused into a Gal xAlxAs crystal. Curves 11, 12 and 13
respectively indicate the characteristics obtained when Zn
was diffused for 50 minutes at 700 C, 670 C and 640 C.
From this graph, it is understood that the diffusion
depths disperse when the mole fraction of AlAs is 0.45 or
greater. This tendency to dispersion was similarly noted
in experiments that were conducted at diffusion tempera-
tures of 600 - 800 C and for diffusion times of 1 - 300
minutes.
Shown in Figure 5 are the distributions of the thresh-
old currents of manufactured samples of lasers having the
structure of Figure 3, compared with a laser according to
this invention. The curve 20 corresponds to a laser of
this invention, while the curve 21 refers to the prior-art
laser of Figure 3. The specifications of the respective
lasers are as given above. It is shown that the disper-
sion of the characteristics of the laser products becomesvery small with application of the present invention.
In the embodiment of Figs. 1 and 2, the third
semiconductor layer 5 is made of p-type GaAlAs, and the
surface semiconductor layer 6 is made of n-type GaAs.
As a result, when current is caused to flow through the
laser in the forward direction, the interface between the
p-GaAlAs layer and the n-GaAs layer is reverse-biased, so
that leakage current is prevented. A greater band gap can
be established at the junction between the GaAs and GaAlAs
layers, than at a p-n junction in an identical material.
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The arrangement is thus useful for prevention of leakage
current.
In addition, since the mole fraction of AlAs decreases
in the order of the second semiconductor layer, the third
semiconductor layer and the surface semiconductor layer,
- crystal lattices are matched more easily.
In a case where the mole fraction of AlAs in the third
semiconductor layer is less than 0.1, the advantage is
also realized that the difference between the third
semiconductor layer and the GaAs (crystal) as the surface
semiconductor layer formed thereon becomes indistinct.
More specifically, although the diffusion depth needs to
be measured from the surface of the GaAs surface layer
formed on the GaAlAs layer, the boundary between the
GaAlAs and GaAs layers cannot be distinguished in this
case. Herein, the boundary between the GaAs and GaAlAs
layers is able to be observed visually with a microscope
when the polished portion of the crystal is etched with,
for example, fluoric acid, hydrogen peroxide and water (at
a mixing ratio of 1 : 1 : 5). The visual discrimination
thus realized is very convenient and practical in the
inspection of mass-produced articles.
The third semiconductor layer 5 is made at least 0.5 ~m
thick. However, it is unnecessary to make the layer very
thick. This is because a resistance which is connected in
series with the active region of the semiconductor laser
increases with the thickness of the layer.
Although a GaAs-GaAlAs system has been employed in the
foregoing example, the invention is applicable to other
material systems, for example, a Ga-Al-As-Sb system, a
Ga-Al-As-P system, a Ga-As-P system and an In-Ga-As-P
system. The technical idea of this invention is also
applicable to a semiconductor laser construction having
conductivity types opposite those of the example.
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Needless to say, this invention can be applied to
various modified semiconductor lasers. Figure 6 is a
sectional view showing another embodiment of the invention.
This embodiment differs from the embodiment of Figure 1 in
that the substrate 1 is provided with a beltlike recess 15.
It is intended to improve mode control in the lateral
direction by exploiting an optical characteristic change
at the boundary of the recess 15. Mode control means is
disclosed in, for example, Japanese published Patent
Application No. 52-143787 (HITACHI).
In an example of the embodiment in Figure 6, a photo-
resist film having a window 10 ~m wide was formed by the
conventional photoresist process on an n-GaAs substrate 1
which had the (100) face as its upper surface. The surface
; 15 of the substrate was chemically etched through the window
at 20 C by the use of, for example, phosphoric acid :
hydrogen peroxide : ethylene glycol = 1 : 1 : 3, whereby
the groove 15 concave in the depth direction was formed.
The width of the groove was made about 10 ~Im (usually,
5 - 20 ~m), and the depth 1.5 ~m (usually 0.8 - 2.5 ~m).
Subsequently, the layers 2, 3, 4, 5 and 6 as in Figure 1
were grown on the resultant substrate by the continuous
liquid phase growth method. Such method may conform with
well-known parameters. However, the solution compositions
and growth times that were used for forming the respective
semiconductor layers are listed in Table 1 by way of
example.
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Table 1
L ayer 2¦Layer 3¦Layer 4¦Layer 5¦Layer 6 ¦
Ga (gr) l 6 6 6 6 6
GaAs (mg) 400 400 400 400 400
SolutionAl (mg) 10 3 10 2 _
Composition Sn (mg) _ _ _ 200
Te (mg) 0.5 _ _
Ge (mg) _ _ _ 200
zn (mg) _ _ 30 _
10Growth time 2 min.2 sec. 8 min.3 min.1 min.
The saturated solution had its temperature lowered at
a rate of about 0.4 C/min. from 780 C and was overcooled
for 3 minutes. Thereafter, the solutions were successively
brought into contact with the substrate. Thus, the layer 2
had its thicker part made 2 ~m thick and had its thinner
part made 0.3 ~m thick. The thicknesses of the layers 3,
4, 5 and 6 were 0.1 ~m, 2 ~m, 2 ~m and 1 ~m respectively.
As dopant impurities, Sn was used for the n-type layers,
and Ge for the p-type layers. Subsequently, through a
window in the A12O3, formed by the same photoresist
process as in the previous case, Zn was diffused at 700C
for 10 minutes, to form the p-type diffused regions at 7
and 7' which were 1.0 - 3.0 ~m deep. Thereafter, Au and
Cr, and an Au-Ge-Ni alloy were respectively deposited as
the positive electrode 9 and the negative electrode 10.
Lastly, the crystal was cloven at the (110) faces so as to
obtain opposite parallel surfaces. A reflector was then
formed to construct the laser device. The laser length
was 300 ~m.
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Such a semiconductor laser could osc;llate at a thresh-
old current density of approximately 2 kA/cm2 at room
temperature. The oscillation wavelength was approximately
7,500 A, and the external quantum efficiency was approxi-
mately 40 %.
This invention is also applicable to a so-called buried
heterostructure, injection laser whose active region is
buried in a different kind of semiconductor region. Figure
7 is a sectional view showing such an embodiment. The
first semiconductor layer 3 is held between burying side
layers 16 and 16'. This structure is described in, for
example, U.S. Patent No. 4,121,177 issued October 17, 1978
to Toshihisa Tsukada. Also in this case, the objective can
be satisfactorily accomplished by diffusing Zn into the
third semiconductor layer 5 and the surface semiconductor
layer 6 and thus forming the impurity-diffused region at 7
and 7'. In the figure, parts assigned the same numerals
as in Figure 1 are the same.
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