Semiconductor device for passing current between a GaAs layer and an InGaAlP layer

A semiconductor device for passing electric current between a GaAs semiconductor layer (103) and an InGaAlP semiconductor layer (101) both having the same conductivity type. The device includes a higher carrier density region (102) with the carrier density equal to or more than 5.times.10.sup.17 cm.sup.-3 and thickness in a rannge from 400 .ANG. to 800 .ANG. in at least a part of the InGaAlP layer (101) adjoining the GaAs layer (103). As a result, good ohmic contact is achieved and the semiconductor device has a lower operating voltage and a satisfactory thermal characteristic.

BACKGROUND OF THE INVENTION 
The present invention relates to a semiconductor device and, more 
particularly, to a semiconductor device in which an electric current is 
passed between a GaAs semiconductor layer and an InGaAlP semiconductor 
layer both having the same conductivity type. 
InGaAlP materials now receive widespread attention as materials for light 
emitting devices in the short wavelength range, these materials having the 
largest energy gap among alloys of group III-V compound semiconductors, 
except for nitrides. In particular, those compositions capable of lattice 
matching with GaAs can offer satisfactory epitaxial growth, with fewer 
crystal defects, by the metal organic chemical vapor deposition method 
(hereinafter abbreviated as MOCVD method). 
When manufacturing light emitting devices and electronic devices which 
contain InGaAlP materials as active parts, it is a frequent practice to 
bring such materials into contact with metals through GaAs, which is 
capable of lattice matching therewith for obtaining good ohmic contact (as 
described in "Applied Physics Letters," 48 (1986) p. 207, for example). 
However, the difference in energy gap between GaAs and InGaAlP materials 
is so large that discontinuous energy bands at the interface cause large 
notches or spikes that obstruct ohmic injection of electric current. In 
particular, a significant effect is more likely to be observed in p-type 
heterojunctions in which holes of low mobility serve as carriers. 
One method for avoiding such an adverse effect is to dispose, between the 
GaAs layer and the InGaAlP layer, an InGaAlP layer having a lower Al 
composition ratio and an intermediate energy gap between those two layers, 
for the purpose of effecting the ohmic injection of electric current (as 
disclosed in Japanese Patent Laid-Open No. 62-200784 (1987), for example). 
However, the provision of such an InGaAlP intermediate energy gap layer 
does not necessarily offer an ohmic characteristic. As a result, the 
voltage drop at the interface can give rise to the problem of increasing 
the operating voltage of the device. When applied to semiconductor lasers, 
the resulting overheating creates high temperatures that impair the 
oscillation characteristics. 
Thus, in an attempt to achieve ohmic contact through GaAs in a 
semiconductor device having its active part made of InGaAlP, good ohmic 
contact between InGaAlP and GaAs is not achieved and, hence, the device 
operating voltage increases and its thermal characteristic is degraded. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide conditions 
necessary for good ohmic contact between semiconductor layers. 
This and other objects are accomplished by a semiconductor device that 
achieves good ohmic contact between semiconductor layers, a low operating 
voltage and a satisfactory thermal characteristic. An electric current is 
caused to pass between an InGaAlP layer and a GaAs layer of the 
semiconductor device, both layers being of the same conductivity type, by 
providing in at least a part of the InGaAlP layer adjoining the GaAs 
layer, a higher carrier density region with the carrier density equal to 
or more than 5.times.10.sup.17 cm.sup.-3 and a thickness in a range of 400 
.ANG. to 800 .ANG.. 
Thus, according to the present invention, in a semiconductor device in 
which electric current is caused to pass between an InGaAlP layer and a 
GaAs layer both being of the same conductivity type, it becomes possible 
to achieve good ohmic contact and, as a result, provide a semiconductor 
device which has a low operating voltage and a satisfactory thermal 
characteristic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Details of the present invention will be described below in connection with 
the illustrated embodiments. 
FIG. 1 is a conceptual view showing a first embodiment of the present 
invention. A semiconductor device 100 has an In.sub.1-x-y Ga.sub.y 
AL.sub.x P (0.ltoreq.x,y.ltoreq.1)layer 101, a higher carrier density 
region 102 in the In.sub.1-x-y Ga.sub.y Al.sub.x P layer 101, and a GaAs 
layer 103. The layers are of the same conductivity type and are 
illustrated as being p-type. It is important, particularly in light 
emitting devices or the like, that In.sub.1-x-y Ga.sub.y Al.sub.x P layer 
101 is capable of lattice matching with GaAs layer 103, to assure the 
epitaxial layer grown on the GaAs layer of possibly perfect crystallinity. 
One example of composition ratios x,y meeting the above conditions is 
given by: 
EQU x+y=0.5 
It has been found by experiment that whether a satisfactory current-voltage 
characteristic can be obtained between the GaAs layer 103 and the 
In.sub.1-x-y Ga.sub.y Al.sub.x P layer 101, is largely dependent on a 
carrier density p and a thickness d of the higher carrier density region 
102. FIG. 2 shows the magnitude of voltage drop across the interface 
between GaAs layer 103 and the higher carrier density region 102 with 
respect to the carrier density p, with an injection current density of 1 
ka/cm.sup.2 applied to the device. With the carrier density of 
p.gtoreq.5.times.10.sup.17 cm.sup.-3, there is no voltage drop effect, and 
hence no problems of increased operating voltage and generated heat. 
However, if the carrier density is increased to enter a range of 
p&gt;3.times.10.sup.19 cm.sup.-3, the number of defects formed in the higher 
carrier density region is drastically increased. A continuous performance 
test at 50.degree. C. and 3 mW showed that a device with such a higher 
carrier density undergoes degradation failure after approximately 100 
hours. This is quite inferior in reliability to the value of 1000 hours or 
more that is obtained with a carrier density of p.ltoreq.3.times.10.sup.19 
cm.sup.-3. The effect of the voltage drop resulting from a carrier density 
of p&lt;5.times.10.sup.17 cm.sup.-3 (FIG. 2) is so great as to bring forth an 
excessive increase in operating voltage and overheating. 
It has also been found that a similar voltage drop effect is experienced 
when the thickness d of higher carrier density region 102 is very thin. 
FIG. 3 shows the voltage drop between GaAs layer 102 and InGaAlP layer 101 
with respect to the thickness d of higher carrier density region 102, with 
an injection current density of 1 kA/cm.sup.2 applied to the device and 
the carrier density of the higher carrier density region equal to 
1.times.10.sup.18 cm.sup.-3. In a range of d&lt;400 .ANG., the voltage 
increases drastically. In a range of d.gtoreq.400 .ANG., the voltage drop 
gradually increases due to an increase in the series resistance with the 
increasing layer thickness. 
As described above, the current-voltage characteristic between GaAs layer 
103 and InGaAlP layer 101 is largely affected by the carrier density p and 
the thickness d of higher carrier density region 102 formed in InGaAlP 
layer 101 adjacent the interface with GaAs layer 103, the carrier density 
p and the thickness d being required to fall in ranges of 
p.gtoreq.5.times.10.sup.17 cm.sup.-3 and d.gtoreq.400 .ANG., respectively. 
As to the carrier density p, since the range of p&gt;3.times.10.sup.19 
cm.sup.-3 will cause the above-described problem, p is preferably 
maintained within a range of 5.times.10.sup.17 cm.sup.-3 
.ltoreq.p.ltoreq.3.times.10.sup.19 cm.sup.-3. 
FIG. 4 shows a semiconductor light emitting device according to a second 
embodiment of the present invention, and is a sectional view showing the 
schematic structure of a semiconductor laser in which a GaAs layer and an 
InGaAlP layer serve as an ohmic contact layer and a clad layer, 
respectively. Referring to FIG. 4, the device comprises an n-GaAs 
substrate 401. Over substrate 401, there is formed a double heterojunction 
comprising a n-In.sub.1-w-z Ga.sub.z Al.sub.w P (0.ltoreq.w,z.ltoreq.1) 
clad layer 402, an In.sub.1-s-t Ga.sub.t Al.sub.s P 
(0.ltoreq.s,t.ltoreq.1) active layer 403, and a p-In.sub.1-u-v Ga.sub.v 
Al.sub.u P (0.ltoreq.u,v.ltoreq.1) clad layer 404, the junction serving as 
a light emitting active part. Over the p-InGaAlP clad layer 404, there are 
formed a p-InGaP (In.sub.1-x-y Ga.sub.y Al.sub.x P; 0.ltoreq.x,y.ltoreq.1) 
cap layer 405 and n-GaAs current restricting layers 406, in the order 
described. It is preferred that cap layer 405 contain no aluminum, i.e., 
x=0. Therefore, cap layer 405 of the aperture portion between current 
restricting layers 406 is not oxidized even if cap layer 405 is exposed to 
the atmosphere. However, small amounts of aluminum may be present without 
adversely affecting the performance of the layer. A strip of n-GaAs 
current restricting layer 406 is selectively removed. A p-GaAs ohmic 
contact layer 407 is formed over the portion of p-InGaP cap layer 405 
where the overlying n-GaAs current restricting layer 406 has been removed, 
and also over the remaining portions of n-GaAs current restricting layer 
406. Furthermore, an electrode 408 is formed on the bottom of substrate 
401 and an electrode 409 is formed on the top of p-GaAs layer 407. 
Composition ratios of In, Ga and Al are set so that the respective layers 
constituting the double heterojunction and p-InGaP cap layer 405 have 
their lattice constants substantially equal to that of the substrate, and 
clad layers 402,404 have their band gap energies larger than that of 
active layer 403. 
The strip of n-GaAs current restricting layer 406 selectively removed has a 
width of 7 .mu.m and a cavity length of 300 .mu.m. In the case of 
providing p-InGaP cap layer 405 with a carrier density of 
1.times.10.sup.18 cm.sup.-3 and a thickness of 500 .ANG., the 
semiconductor laser exhibited an oscillation threshold current of 70 mA in 
the pulse operation mode and 72 mA in the continuous operation mode. Thus, 
it has been proved that the change in oscillation threshold current caused 
by generated heat is very small as compared with the value in the pulse 
operation mode. There has also been achieved a maximum continuous 
oscillation temperature of 90.degree. C. This is attributable to the fact 
that no excessive voltage drop occurs across the heterojunction interfaces 
of p-GaAs/p-InGaP/p-InGaAlP, and hence the operating voltage at the 
oscillation threshold is held as low as 2.3 volts. Achievement of 
continuous oscillation at 90.degree. C. is a substantial improvement over 
the above described prior art devices. 
In the case of providing p-InGaP cap layer 405 with a carrier density equal 
to 4.times.10.sup.17 cm.sup.-3, the operating voltage exhibited a very 
large value of 3.0 volt. Correspondingly, the generated heat in the 
continuous operation mode was increased and the oscillation threshold 
current reached as high as 77 mA. Also, the maximum continuous oscillation 
temperature was as low as 60.degree. C. This may likely be attributable to 
the fact that the excessive voltage drop occurs across the heterojunction 
interfaces of p-GaAs/p-InGaP/p-InGaAlP. 
Further, in the case of setting the cap layer 405 thickness equal to 250 
.ANG., even with the carrier density of p-InGaP cap layer 405 set equal to 
1.times.10.sup.18 cm.sup.-3, the operating voltage exhibited a very large 
value of 3.0 volt. Correspondingly, the generated heat in the continuous 
operation mode increased and the oscillation threshold current reached as 
high as 77 mA. Further, the maximum continuous oscillation temperature was 
as low as 60.degree. C. In the case of increasing the p-InGaP cap layer 
thickness to the order of 1000 .ANG., instead of the above thickness, the 
maximum continuous oscillation temperature experienced was as low as 
30.degree. C. due to increases in both the thermal resistance and 
threshold current resulting from the current being more widely spread, as 
well as an increase in the generated heat resulting from the increased 
series resistance. 
FIG. 5 shows the dependence of the maximum continuous oscillation 
temperature on a thickness d of p-InGap cap layer 405. With the thickness 
falling in a range of 400 .ANG..ltoreq.d.ltoreq.800 .ANG., the maximum 
continuous oscillation temperature exhibited a high value of more than 
80.degree. C., the temperatures experienced outside that thickness range 
being substantially lower. Particularly, in a range of 400 
.ANG..ltoreq.d.ltoreq.600 .ANG., the maximum continuous oscillation 
temperature became more than 85.degree. C., so that a very satisfactory 
thermal characteristic was observed. The achievement of continuous 
oscillation at these temperatures is a substantial improvement over prior 
art devices. 
FIG. 6 shows a semiconductor light emitting device according to a third 
embodiment of the present invention, and is a sectional view showing the 
schematic structure of a semiconductor laser in which a GaAs layer and an 
InGaAlP layer serve as an ohmic contact layer and clad layer, 
respectively. Referring to FIG. 6, the device comprises an n-GaAs 
substrate 601. Over substrate 601, there is formed a double heterojunction 
comprising an n-In.sub.1-w-z Ga.sub.z Al.sub.w P (0.ltoreq.w,z.ltoreq.1) 
clad layer 602, an In.sub.1-s-t Ga.sub.t Al.sub.s P 
(0.ltoreq.s,t.ltoreq.1) active layer 603, a p-In.sub.1-r-s Ga.sub.s 
Al.sub.r P (0.ltoreq.r,s.ltoreq.1) first clad layer 604, a p-In.sub.1-p-q 
Ga.sub.q Al.sub.p P (0.ltoreq.p,q.ltoreq.1) etching stopper layer 605 and 
a p-In.sub.1-u-v Ga.sub.v Al.sub.u P (0.ltoreq.u,v.ltoreq.1) second clad 
layer 606 being in strip form and convex in section, the junction serving 
as a light emitting active part. Over the convex portions of p-In-GaAlP 
second clad layer 606, there is formed a p-InGaP (In.sub.1-x-y Ga.sub.y 
Al.sub.x P; 0.ltoreq.x,y .ltoreq.1) cap layer 607. It is preferred that 
cap layer 607 contain no aluminum, i.e., x=0. However, small amounts of 
aluminum may be present without adversely affecting the performance of the 
layer. In areas except for the convex portions of p-InGaAlP clad layer 
606, there are formed n-GaAs current restricting layers 608. A p-GaAs 
ohmic contact layer 609 is formed over p-InGaP cap layer 607 and n-GaAs 
current restricting layers 608. Furthermore, an electrode 610 is formed on 
the bottom of substrate 601, and an electrode 611 is formed on the top of 
p-GaAs layer 609. Composition ratios of In, Ga and Al are set so that the 
respective layers constituting the double heterojunction and the p-InGaP 
cap layer 607 have their lattice constants substantially equal to that of 
the substrate, and the clad layers 602, 604, 606 respectively have band 
gap energies larger than that of active layer 603. 
The strip width and the cavity length were selected to be 5 .mu.m and 300 
.mu.m, respectively. The maximum continuous oscillation temperature of the 
semiconductor laser illustrated in FIG. 6 is dependent on both the carrier 
density and the thickness of p-InGap cap layer 607. This dependency was 
found to be substantially identical to that of the device having the 
structure of the second embodiment shown in FIG. 4. With respect to the 
device having the structure of the third embodiment, when forming 
p-InGaAlP clad layer 606 in strip form and convexed, it is necessary to 
selectively remove p-InGaP cap layer 607 except for areas corresponding to 
the convex portions of layer 606. When a mixed solution of Br.sub.2, HBr 
and H.sub.2 O is employed for etching p-InGaP cap layer 607, the etching 
rates of p-InGaAlP clad layer 606 and p-InGaP cap layer 607 tend to be 
partially increased in the vicinity of the strip projecting portions. For 
this reason, in order to achieve flat and satisfactory etching, it is 
desirable to minimize the thickness of p-InGaP cap layer 607 and hence 
make as short as possible the etching time necessary for removing p-InGaP 
cap layer 607. The thickness of p-InGaP cap layer 607 allowing such flat 
and satisfactory etching was found to be less than 600 .ANG.. 
Although a semiconductor laser has been described as using p-InGaP as the 
cap layer material in the foregoing embodiments, the cap layer may 
generally be formed of InGaAlP as well. In such a case, the cap layer is 
selected to have an energy gap smaller than that of the clad layer, so 
that the difference in energy gap between the cap layer and the GaAs layer 
is reduced, thereby making it easier to obtain an ohmic characteristic. 
Needless to say, the present invention is also applicable to any 
semiconductor devices, such as light emitting diodes and other electronic 
elements, which have a contact interface between an InGaAlP layer and a 
GaAs layer both having the same conductivity type, and having a function 
of passing an electric current through that interface. In addition, the 
present invention can be practiced in other various modified forms without 
departing from the scope of the present invention. 
As fully described above, according to the present invention, in a 
semiconductor device in which electric current is caused to pass between 
an InGaAlP layer and a GaAs layer both having the same conductivity type, 
it becomes possible to achieve good ohmic contact, and hence provide a 
semiconductor element which has low operating voltage and a satisfactory 
thermal characteristic. 
It will be apparent to those of ordinary skill in the art that various 
modifications and variations can be made to the above-described 
embodiments without departing from the scope of the appended claims and 
their equivalents.