Note: Descriptions are shown in the official language in which they were submitted.
~173SSO
This invention relates to a semiconductor light emitting device, and
more particularly to i~provement in a semiconductor laser which is suitable
for use as a light source, for example, for optical communication.
BRIEF DESCRIPTIO~ OF THE DRAWINGS
Figure 1 is a sectional view showing the principal part of an
example of a conventional device;
Figure 2 is a sectional view showing the principal part of another
example of the conventional device;
Figure 3 is a sectional view showing the principal part of another
example of the conventional device;
Figure 4 is a sectional view showing the principal part of another
example of the conventional device;
Figure 5 is a sectional view showing the principal part of another
example of the conventional device;
Figure 6 is a sectional view showing the principal part of another
example of the conventional device;
Figure 7 is a sectional view explanatory of the principal part of
another example of the conventional device;
Figure 8 is a sectional view explanatory of the principal part of
an embodiment of the present invention;
Figure 9 shows the relationships between a refractive index and mode;
Figure 10 is a graph showing an effective refractive index and loss;
Figure 11 is a graph showing the relationship between R and h in
Figure 8;
Figure 12 is a sectional view explanatory of the principal part of
another embodiment of the present invention; and
Figure 13 is a sectional view explanatory of the principal part of
still another embodiment of the present invention.
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Description of the Prior Art
Heretofore, there has been known a semiconductor laser as shown in Figure 1.
In Figure 1, reference numeral 1 indicates an n type GaAs substrate; 2
designates an n type GaO 7A ~ 3As cladding layer; 3 identifies a GaAs àctive
layer; 4 denotes a p type GaO 7A~o 3As cladding layer; 5 represents a p type
GaAs layer; 6 indicates an electrode on the positive side; and 7 refers to an
electrode on the negative side.
In the conventional semiconductor laser shown in Figure 1, as cùrrent in-
creases, its light emitting region spreads, resulting in a lateral oscillation
mode for becoming unstable. The reason is that there is no mechanism for stab-
ilizing the lateral mode other than a difference in the gain of a current dis-
tribution.
In order to overcome the defect of the prior art, there has been proposed
a semiconductor laser of such a construction as shown in Figure 2, in which
parts corresponding to those in Figure 1 are identified b~ the same reference
numerals.
The semiconductor laser of Figure 2 differs from the semiconductor laser
of Figure 1 in that the cladding layer 2 has a pro~ecting portion 2G and in
that the cladding layer 2 outside of the pro~ecting portion 2G is made thin. ~ie
cap layer S is composed of an n type GaAs portion 5' and a p type GaAs region S'
for setting therein a current path.
In the semiconductor laser of Figure 2, light emitted from the active layer
3 is emitted out of the cladding layer 2 outside of the pro~ecting portion 2G
and absorbed and reflected by the n type GaAs substrate 1. That is, the effect-
ive refractive index in the portion outside of a stripe region except the pro-
jecting portion 2G is varied and a loss in that portion is made large, by which
the oscillation region is restricted to the portion corresponding to the pro-
jecting portion 2G to set up an otpical guide mechanism,
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stabilizing the lateral oscillation mode.
This semiconductor laser, however, poses some problems in terms of
its manufacture. The cladding layer 2 is formed on the substrate 1 in which
a recess is made prior to the formation of the cladding layer 2 and since the
cladding layer 2 is very thin except the projecting portion 2G, there is the
possibility that the cladding layer 2 sags at the position corresponding to
the recess of the substrate 1 and consequently at the projecting portion 2G,
causing the active layer 3 to curve. If the cladding layer 2 is made thick
to avoid this problem, then the light guide effect is lost. Another problem
is a difficulty in maintaining, in a desired shape, the recess formed in the
substrate 1 for obtaining the projecting portion 2G. That is, in the case
where a recess lG is formed in the substrate 1 first, as shown in Figure 6,
and then the cladding layer 2 is formed by liquid phase epitaxy on the sub-
strate 1, as shown in Figure 7, the edge of the recess lG Cindicated by the
broken lines) is rounded into a gentle slope lG'. The reason is as follows:
T~e edge of the recess lG is high in surface energy but the other portion is
low, so that when the growth solution makes contact with the substrate 1, the
edge of the recess lG is liable to be etched back into the solution. In the
case where such a gentle slope lG' is formed and the projecting portion 2G of
the cladding layer 2 also conforms to the recess lG, the light emitting region
Becomes wider as current flows, making it impossible to maintain the unity of-
the oscillation de. Still another problem is that it is ~ery difficult to
form the p type GaAs region 5" for setting up a current path and the pro~ecting
portion 2G in alignment with each other. If they are not aligned, an ineffect-
ive current which does not contribute to oscillation increases threshold current
and operating current, introducing non-uniformity in the light emission in the
lateral direction and a change in the light emitting region.
Figure 3 illustrates another conventional semiconductor laser which
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is different in construction from the semiconductor laser of Figure 2. The
former differs from the latter in that the n type GaAs substrate 1 is formed
flat without forming therein the recess; a p type GaAs current preventing
layer 8 is formed on the substrate 1; after a groove is formed in the layer
8, the cladding layer 2 is grown on the layer 8 to form the projecting portion
2G in the groove.
This semiconductor laser also has the same defects as mentioned
above in connection with Figure 2 except the current confinement. In addition,
the p type GaAs current preventing layer 8 which will be considered to be
advantageous over the semiconductor laser of Figure 2 is of no use in practice.
In order for the laser device of Figure 3 to serve as one having an optical
guide mechanism, it is also necessary that the p type GaAs current preventing
layer 8 absorbs light of the active layer 3 emitted from the cladding layer 2.
Then, in the current preventing layer 8 electrons and holes are generated by
the light absorption and only the holes are gradually stored. This is equiva-
lent to the application of a bias voltage to the current preventing layer 8
in a forward direction with respect to the n type GaAs substrate 1 and the n
type GaA~As clad layer 2, and when the holes have been stored, the current
preventing layer 8 is biased to a potential substantially equal to a diffusion
potential between the substrate 1 and the cladding layer 2. In the case where
the current rejecting layer 8 is not sufficiently thick as compared with the
diffusion length of minority carriers, electrons in the substrate 1 flow into
the cladding layer 2 through the current preventing layer 8, so that the
current preventing layer 8 does not perform its function. The diffusion length
of minority carriers differ with the carrier concentration and it is 1 to 3~um
in the case of GaAs and, in order to ensure that the current preventing layer
performs its function, the layer must be 5 to 10 times as thick as the diffusion
length of minority carriers. In this case, it is difficult to select the thick-
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ness of the current preventing layer 8 in the range of up to 10 ~ m and form
the groove 1 to 6 ~m by means of etching, or to select the distance between
the active layer 3 and the current preventing layer 8 in the range of up to
0.4 ,~m and form the active layer 3 flat. If the current preventing layer 8
and the active layer 3 are spaced a distance of 1 ~m or more so as to prevent
the current preventing layer 8 from absorbing light, the layer 8 always per-
forms the current preventing function but the optical guide function is lost.
Furthermore, a conventional semiconductor laser shown in Figure 4 is
also known in the art. In Figure 4, parts corresponding to those in Figures
1 to 3 are identified by the same reference numerals.
In Figure 4, reference numeral 8 indicates a p type &aAQAs current
preventing layer; and 9 designates a p or n type GaAs layer.
One of the defects of this semiconductor laser is an increase in the
threshold current. That is, since use is made of what is called a loss guide
system in which light is absorbed by the GaAs layer 9 on the outside of the
stripe and consequently guided in the stripe alone, the threshold current
increases. Another defect is such that the active layer 3 becomes hollow and
cannot be made flat, as shown in Figure 5. The reason is that since the value
~ shown in Figure 4 must be selected to be, for example, 0~3,h~m or less for
guiding light, the active layer 3 is exposed directly to the influence of the
groove. Still another defect is that since the GaAs laye 9 is thick, the gentle
slope lG' described previously in respect of Figures 6 and 7 is produced as in
the cases of the other conventional devices.
At present, many studies are being made so as to overcome the above-
said defects of the prior art. For example, there has been proposed to form
the cladding layer 2 to a thickness of up to 0.3 ~m except the projecting
portion 2G and define the supersaturation degree of the growth solution, the
cooling rate, the time for growing the active layer 3 and so forth in order to
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grow the layer 3 flat. However, the manufacture of the semiconductor laser
under such restricted conditions involves difficulties in terms of control.
For example, an increase in the supersaturation degree of the growth solution
suppresses the etching-back of the edge of the recess during the formation o
the cladding layer but causes an increase in the growth sp0ed; accordingly, the
cladding layer tends to become thick to increase the distance between the active
layer and the substrate, resulting in the loss of the optical guide function.
SUMMARY OF THE INVENTION
The present invention provides a semiconductor laser which is free
from the abovesaid defects of the prior art, which is equipped with a current
confining region setting function effective at all times and an optical guide
function unobtainable with the prior art and which is capable of stable oscil-
lation in the lateral mode and easy to manufacture.
According to the invention there is provided a semiconductor
light emitting device which comprises a semiconductor substrate; first cladding
layer of low refractive index disposed on said substrate; active layer of
high refractive index disposed on said first cladding layer; second cladding
layer of low refractive index disposed on said active layer; first semicon-
O n
ductor layer disposed ~* said second cladding layer and the outside portion
~7
of a light emitting stripe; second semiconductor layer disposed ~n~said
second cladding layer and on said first semiconductor layer, characteri7ed
in that said first semiconductor layer is thinner than said second semi-
conductor layer and has a higher refractive index than said second cladding
layer and said second semiconductor layer, and said second semiconductor
layer has an opposite conductivity type to said second cladding layer.
DESCRIPlION OF THE PREFERRED EMBODIMENTS
Figure 8 is a sectional view illustrating the principal part of an
embodiment of the semiconductor light emitting device of the present invention.
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~173550
In Figure 8, parts corresponding to those mentioned in the foregoing are
identified by the same reference numerals.
In the illustrated embodiment, an etch--back preventive layer 9 is
formed in addition to the current preventing layer 8 and the forbidden band
width of a predetermined layer is selected to be a certain value. The etch-
back preventive layer 9 is an n type Ga ~ layer and the amount of aluminum
contained is held at a certain value, as described later. Letting the for-
bidden band widths of respective parts be represented as follows:
Substrate 1 : El (a sixth semiconductor layer)
Current preventing layer 8 : E2 Ca fifth semiconductor layer)
Etch-back preventive layer 9 : E3 (a fourth semiconductor layer)
Cladding layer 2 (including the pro;ecting portion 2G) : E4 (a third
semiconductor layer)
Active layer 3 : E5 (a irst semiconductor layer)
Cladding layer 4 : E6 (a second semiconductor layer)
Cap layer 5 : E7
the forbidden band width E2 is selected larger than El and E3. The substrate
1 is identical in conductivity type with the cladding layer 2, whereas the
current preventing layer 8 and the cladding layer 4 are opposite in conduct-
ivity type to the substrate 1.
The current suppressing action of the present device is as follows:
When light emitted from the active layer 3 is absorbed by the etch-
back preventive layer 9, holes and electrons are produced in the layer 9 and
some of them flow into the current preventing layer 8 to bias it effectively
in a forward direction with respect to the substrate 1. Since the forbidden
band width of the current preventing layer 8 is larger than that of the
substrate 1, however, majority carriers in the substrate 1 cannot flow into
the current preventing layer 8. Accordingly, the ma~ority carriers in the
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substrate 1 flow into the active layer 3, passing through the projecting
portion 2G of the cladding layer 2, by which current can be con~rolled to flow
only in a stripe region defined by the pro~ecting portion 2G.
Next, a description will be given of the optical guide function in
the present device.
Figure 9 shows refractive index and loss distributions in the direct-
ion A-A in the stripe region dsfined by the projecting portion 2G and in the
direction B-B vertical to junctions outside of the stripe region in Figure 8.
Flgure 9(a) shows the distribution of the refractive indexes of the layers 2
to 4 in the stripe region, Figure 9(b) the distribution of the refractive in-
dexes of the layers 2 to 4, 8 and 9 outside of the stripe region, and Figure
9(c) the loss distributions. On account of such refractive index distributions,
an even and an odd mode exist outside of the stripe region, as seen in Figures
9(e) and ~f~, and light in the stripe region is coupled mainly with the odd
mode. In the odd mode, by reducing the thickness h of the etch-back preventive
layer 9, the propagation constant i8 made small as compared with the case of
the mode in the stripe region (see Figure 9~d)) even if the thickness ~ of the
cladding layer 2 is large. By this, the effective refractive index outside of
the stripe region is made smaller than the effective refractive index in the
stripe region~ As a consequence, light is confined to the stripe region.
Figure 10 is a graph showing the effective refractive index and the
loss calculated from the odd mode and the mode propagation constant in the
stripe region, with d = h = 0.1 J~m, d being the thickness of the active layer
3. The refractive indexes of the respective layers were selected as follows:
Refractive index of active layer 3 nO = 3.63
Refractive index of cladding layer 4 nl = 3.44
Refractive index of cladding layer 2 n2 = 3 39
Refractive index of etch-back preventive layer 9 n3 = 3.64
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Refractive index of current preventing layer n~ = 3.44
In general, it is said that a refractive index difference necessary for control-
ling the lateral mode is required to be a relative effective refractive index
difference of about 3xlO 3. In Figure 10 the solid-line curve A and the
broken-line curve B respectively indicate the effective refractive index and
the 10s8 in the conventional device described previously in connection with
Figures 4 and 5. In thi.s conventional device, when the thickness ~ of the
cladding layer 2 is selected to be 0.3~m, the effective refractive indexes
inside and outside of the stripe region are substantially equal. On the other
hand, the loss outslde of the stripe region is as large as 800 cm 1, which
is in excess of 500 cm 1 which is a loss necessary for guiding light. Accord-
ingly, light is guided on the basis of the loss on the outside of the stripe
region. In contrast thereto, in the present invention, as will be evident
from Figure 10, when the thickness ~ of the cladding layer 2 is in the range
of 0.2 to O.4~m, the loss is smaller than 450 cm 1 and no li~ht is guided
oa the basis of the loss on the outside of the stripe region. In other words,
as the thickness Q of the cladding layer 2 decreases, the effective refractive
index on the outside of the stripe region lowers to increase the effective
refractive index difference between the inside and the outside of the stripe
region, by which light is guided.
Thus, the abovesaid conventional device utilizes the loss on the out-
side of the stripe region for guiding light whereas in the present invention
the optical guide is dependent on the effective refractive index difference
between the inside and the outside of the stripe region. In the present
invention, a change in the effective refractive index is large with respect
to the thickness R of the cladding layer 2, and in the case where the thick-
ness h of the etch-back preventive layer 9 is 0.1 ~ m, even if the thickness
J~ of the cladding layer 2 is 0.5~u m, the odd mode provides a relative
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effective refractive index difference of 3xlO 3. Accordingly, even if the
thickness R of the cladding layer 2 is large, effective confinement of light
is possible. The fact that the thickness ~ of the cladding layer 2 can be
made large enables the active layer 3 to be formed flat, and hence is effect-
ive for enhancement of the yield in terms of crystal growth. With the thick-
ness h of the etch-back preventive layer 9 selected large, the optical guide
mechanism becomes similar to that of the conventional device shown in Figure
2; namely, with h 0.4 J~m, the optical guide mechanism is substantially the
same as that of the prior art device. Therefore, the ~hickness h of the etch-
back preventive layer 9 should not be made so large. The relationship between
h and R necessary for obtaining the relative effective refractive index of
3xlO 3 is such as shown in Figure 11.
Next, a description will be given of the prevention of etch-back in
the present embodiment.
In the present embodiment, the etch-back preventive layer 9 is
provided, as shown in Figure 8. In concrete terms, the etch-back preventive
layer 9 is a Gal xA~xAs layer, where O.l)x~0. In the prior art, no attention
i8 paid to a difference between GaAs and Gal x~ As containing such a slight
amount of aluminum as indicated by the abovesaid condition. In our experi-
ments in which a GaAs substrate having formed therein a recess and a
Gal xARxAs (O.l>x~0) substrate similarly having formed therein a recess were -
subjected to the liquid phase epitaxial growth using an equilibrium solution,
a supersaturated solution having a supersaturation degree of 0.5 (C) and a
supersaturated solution having a supersaturation degree of 1 (C), the recess
of the GaAs substrate remained unchanged without causing the etch-back only
in the case of the solution having the supersaturation degree of 1 (C),
whereas the recess of the Gal AQ As substrate remained unchanged in the cases
of all the solutions and the so-called sag was not caused. This was also
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found in the case where after forming a Gal AQ As (O.l~x~O) layer on a GaAs
substrate and then forming a recess in the layer to extend down to the sub-
strate, the GaAs substrate was subjected to the liquid phase epitaxial growth
using each of the abovesaid solutions. Only in the case where the thickness
h of the etch-back preventive layer 9 is selected to be smaller than 0.4 J~m,
even if no aluminum is added, the etch back is very slight; namely, the edge
of the recess in the etch-hack preventive layer 9 is rounded only slightly.
Further, since the mixed crystal ratio of aluminum is large, the current
preventing layer 8 is not etched back and does not substantially change its
entire configuration.
Next, a description will be given of the manufacture of the semi-
conductor laser of the embodiment shown in Figure ô.
A p type GaO 7A~o 3As current preventing layer 8 is formed by the
liquid phase epitaxy to a thickness of O.5 to 1 ~m on a silicon-doped GaAs
substrate 1 which has a (100~ plane and an electron concentration of 1x1017
to 5X1013~ Then, an n type GaO 99A~o olAs etch-back preventive layer 9 is
similarly formed to a thickness of 0.1 to 0.4 ~Am on the current preventing
layer 8.
By known photo lighography techniques, a groove 2 to 10 ~m wide,
such as shown in Figure 6, is formed in the substrate assembly to extend down
to the substrate 1.
After this, an n type GaO 7ARo 3As cladding layer 2, a p type
GaO 95A~o 05As active layer 3, a p type GaO 65A~o 35As claddlng layer 4 and
a p type GaAs cap layer 5 are formed by liquid phase epitaxy to thickness of
0.2 to 0.5 Jlm (the portion indicated by ,~), 0.1 ~m, lf/m and 1 ~m, respect-
ively, on the substrate assembly.
Thereafter, electrodes 6 znd 7 are deposited by known method on the
cap layer 5 and the underside of the substrate 1, respectively. Then, the
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assembly is cleaved into individual chips about 300 J~m long.
When the groove was 6 ~m wide, the laser thus obtained exhibited a
threshold current density of 1.2 KA/cm2 at room temperature. Further, the
laser oscillated stably in the lateral mode of the least order and no kink
appeared in the current vs. optical output characteristic. Moreover, when the
etch-back preventive layer 9 was made thin, an excellent characteristic was
obtained even if the thickness ~ vf the cladding layer 2 was about 0.5~Um
thick.
In the embodiment of Figure 8, the thickness of the etch-back pre-
ventive layer 9 is selected to be 0.1 to 0.4 J~m and the value x 0.01. The
reason for selecting the thickness of the layer 9 in such a range is as
follows: With a thickness less than 0.1 ~ m, the layer is too thin and diffi-
cult to form. And the thickness of the layer 9 with which it is possible to
obtain a relative effective refractive index difference of 3xlO 3 when the
thickness R of the cladding layer 2 is 0.2 J~m, which is a feasible minimum
value, is 0.4 l~m. With a thickness exceeding this value, the abovesaid
specific refractive index difference cannot be obtained and the lateral mode
is unstable. The value x is not limited specifically to 0.01 mentioned in
the above embodiment and can be selected in the range of 0.1 to 0. If the
value x exceeds 0.1, an aluminum oxide film becomes rigid and the crystal
to be grown thereon is of poor quality.
Moreover, in the foregoing embodiment, the thickness of the cladding
layer 2 is selected to be 0.5 ,L-m but this thickness can be selected in the
range of 0.2 to 0.5~ m. With R< 0.2 ~m, the active layer 3 cannot be formed
flat, and with ~ > 0.5~ m, even if the etch-back preventive layer 9 is formed
to the feasible minimum thickness, i.e. 0.1 ~m, it is impossible to obtain the
relative effective refractive index difference 3xlO 3 which is necessary for
stabilizing the lateral mode.
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Figure 12 illustrates in section the principal part of another embodi-
ment of the present invention, in which parts corresponding to those in Figure
8 are marked with the same reference numerals.
This embodiment differs from the embodiment of Figure 8 in that a cap
layer 5' is an n type GaAs layer, and in that Zn is introduced in the cap layer
5' in a stripe pattern to form therein a p type GaAs region. In this embodi-
ment, a slight misalignment of the p type GaAs region 5" and the projecting
portion 2G does not present such problems as referred to previously with
respect to the conventional devices shown in Figures 2 and 3. The reason is
that since the current preventing layer 8 in the present invention effectively
performs its function at all times, the current path setting function is not
so strictly required of the p type GaAs region 5". Accordingly, the manu-
facture of the device of this embodiment is not so difficult, as compared with
the abovesaid prior art devices. In accordance with the present embodiment,
when the groove was 6 Jlom wide, a threshold current density of 1.1 KA/cm
was obtained and the lateral oscillation mode was also stable.
Further, a modified form of the embodiment of Figure 8 was produced
and, in this case, the current preventing layer 8 was a p type GaO 8AQo 2As
layer and the same characteristics as those of the embodiment of Figure 8 were
obtained~
In the foregoing embodiments, the so-called ternary compound semi-
conductor is employed, but it is a matter of course that a quaternary compound
can also be used. Figure 13 illustrates an example of a semiconductor laser
using the quaternary compound. In Figure 13, reference numeral 11 indicates an
n type InP substrate (100 ~m thick); 12 designates an n type InP layer
~1.5 ~u m thick); 13 identifies an n type InGaAsP active layer (whose thickness
is indicated by d); 14 denotes a p type InP layer (whose ~hickness is indi-
cated by R ); 15 represents an n type InGaAsP layer (whose thickness is
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indicated by h); 16 shows an n type InGaAsP (or InP) layer (1 ~m thick); 17
refers to a p type InP layer (1.5~11m thick); 18 indicates a p type InGaAsP
layer (1 ~lm thick); 19 designates a Ti-Pt-Au electrode; 20 identifies an
Au-Ge-Ni electrode; and S denotes the width of a stripe. In the ~bove, the
thicl~nesses d, ~ and h are 0.04 to 0.2, 0.2 to 0.5 and 0.1 to 0.5 ~m,
respectively.
As will be appreciated from the foregoing description, according to
the present invention, the current preventing layer and the etch-back preventive
layer are added to a semiconductor laser provided with a cladding layer comp-
osed of a pro~ecting portion for confining light from an active layer and a
portion for emitting light, and the forbidden band width and the conductivity
type of each layer are specified. In this way, it is possible to provide a
~emiconductor laser having an excellent current limit function and optical
guide function, and in addition, this semiconductor laser is easy to manu-
facture.
It will be apparent that many modiflcations and variations may be
effected without departing from the scope of the novel concepts of this
invention.
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