Semiconductor device

A semiconductor device includes a first conductivity type cladding layer; a second conductivity type cladding layer; an active layer of a semiconductor sandwiched between the first conductivity type cladding layer and the second conductivity type cladding layer; and a second conductivity type superlattice barrier layer sandwiched between the active layer and the second conductivity type cladding layer and having a superlattice structure including a first compound semiconductor having a larger energy band gap than the active layer and a second compound semiconductor having a smaller energy difference in the conduction band than the first compound semiconductor and a larger energy difference in the valence band than the first compound semiconductor, the first and second conductivity type compound semiconductors being alternatingly laminated in at least one pair of layers. The energy barrier provided between the active layer and the second conductivity type cladding layer has a sufficient height to prevent overflow of carriers and to improve LD characteristics, particularly operation at a high temperature.

FIELD OF THE INVENTION 
This invention relates to a semiconductor device, more particularly, to a 
semiconductor device having a superlattice structure between an active 
layer and a cladding layer. 
BACKGROUND OF THE INVENTION 
For semiconductor lasers, in order to improve characteristics, it is 
important to confine electrons and holes efficiently in an active layer 
sandwiched between cladding layers having a band gap higher than that of 
the active layer. However, in a semiconductor laser diode producing red 
light (hereinafter referred to as red LD) which cannot provide a barrier 
having a sufficient height between the active layer and a p-type cladding 
layer (hereinafter also referred to as p-), electrons injected from an 
n-type (hereinafter also referred to as n-) cladding layer into the active 
layer are likely to flow over the barrier into the p-type cladding layer. 
This flow of electrons into the p-type cladding layer is generally called 
an electron overflow, causing deterioration in LD characteristics, in 
particular, in operation at a high temperature. In order to particularly 
improve the operation at a high temperature, it is essential to provide a 
barrier having a sufficient height between the active layer and the p-type 
cladding layer. 
FIG. 3(a) is a cross-sectional view illustrating a structure of a prior art 
AlGaInP series red semiconductor laser, and FIG. 3(b) is an energy band 
diagram illustrating the energy band structure in the proximity of an 
active layer of the laser. In FIG. 3(a), reference numeral 1 designates an 
n-GaAs substrate, numeral 2 designates an n-AlGaInP lower cladding layer 
about 1.5 .mu.m thick, numeral 3 designates an active layer having a 
multiple-quantum-well structure comprising AlGaInP, numeral 4a designates 
a p-AlGaInP first upper cladding layer 0.2 to 0.25 .mu.m thick, numeral 4b 
designates a p-AlGaInP second upper cladding layer about 1.25 .mu.m thick, 
and the first upper cladding layer 4a and the second upper cladding layer 
4b constitute a p-type upper cladding layer 4. Reference numeral 5 
designates an etching stop layer, numeral 6 designates a band 
discontinuity relaxing layer comprising p-GaInP about 0.1 .mu.m thick, 
numeral 7 designates a current blocking layer comprising n-GaAs or the 
like, and numeral 8 designates a p-GaAs contact layer about 3.0 .mu.m 
thick. An n-side electrode on a rear surface of the substrate 1 and a 
p-side electrode on the contact layer 8 are not shown in the figure. In 
FIG. 3(b), reference numeral 11 designates electrons, numeral 11a 
designates electrons which have overflowed, and numeral 12 designates 
holes. 
FIG. 2 is a diagram representing two relations in an (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P layer: a relation between an Al 
composition proportion x and an energy difference in the conduction band, 
.DELTA.Ec; and a relation between the Al composition proportion x and an 
energy difference in valence band .DELTA.Ev. 
In the prior art semiconductor laser shown in FIGS. 3(a) and 3(b), in order 
to make higher the barrier between the active layer 3 and the p-type first 
upper cladding layer 4a, increasing the energy band gap of the p-type 
first upper cladding layer 4a is one of the most effective methods. 
However, concerning the energy band gap Eg of (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P, i.e., a material of the p-type first 
upper cladding layer 4a of the red LD, .DELTA.Ev increases with an 
increase in the Al composition proportion x within the range of 
0.ltoreq.x.ltoreq.1.0, as shown in FIG. 2, for example, .DELTA.Ev at x=1.0 
is larger by 96 meV than that at x=0.7. On the other hand, .DELTA.Ec also 
increases with an increase in the Al composition proportion x within the 
range of 0.ltoreq.x.ltoreq.0.7, the carrier transition between band edges 
changes from a direct transition (.GAMMA.valley) to an indirect transition 
(X valley) at x=0.7, .DELTA.Ec decreases with an increase in the Al 
composition proportion x when x&gt;0.7, and for example, .DELTA.Ec at x=1.0 
takes a value smaller by 70 meV than that at x=0.7. Therefore, when x&gt;0.7, 
the rate of increase of the energy band gap .DELTA.Eg due to the increase 
in the Al composition proportion x becomes smaller than the rate of 
increase when the composition proportion is from x=0 to x=0.7, the energy 
band gap .DELTA.Eg of (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P at 
x=1.0 is larger by about 26 meV at x=1.0 than that at x=0.7. Moreover, 
when the Al composition proportion increases, increases in the amount of 
absorbed oxygen and in resistance occur and an increase in the p-type 
carrier concentration becomes impossible. Hence, in the prior art 
semiconductor laser, for both of the p-type upper cladding layers 4a and 
4b, an (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P layer with an Al 
composition of x=0.7 is employed. 
In the semiconductor laser employing p-type upper cladding layers 4a and 4b 
comprising (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7), however, 
it is, as described above, impossible to provide a barrier between the 
active layer 3 and the p-type upper cladding layers 4a and 4b having a 
sufficient height. Hence, electrons injected from the n-type lower 
cladding layer 2 into the active layer 3 flow into the p-type cladding 
layers 4a and 4b over the barrier between the active layer 3 and the 
p-type first upper cladding layer 4a, resulting in an electron overflow, 
which in turn causes deterioration in LD characteristics, particularly, 
operation at a high temperature. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor device 
providing an energy barrier having a sufficient height between an active 
layer and a cladding layer, and preventing electron overflow. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given thereinafter; it should be understood, 
however, that the detailed description and specific embodiment are given 
by modifications within the scope of the invention will become apparent to 
those skilled in the art from the detailed description. 
According to a first aspect of the present invention, a semiconductor 
device comprises a first conductivity type cladding layer; a second 
conductivity type cladding layer; an active layer comprising semiconductor 
disposed between the first conductivity type cladding layer and the second 
conductivity type cladding layer; and a second conductivity type 
superlattice barrier layer sandwiched between the active layer and the 
second conductivity type cladding layer, and having a superlattice 
structure comprising a first compound semiconductor having a larger energy 
band gap than that of the active layer and a second compound semiconductor 
having a smaller energy difference in conduction band .DELTA.Ec than that 
of the first compound semiconductor and a larger energy difference in 
valence band .DELTA.Ev than that of the first compound semiconductor, the 
first and second compound semiconductors being alternatingly laminated in 
one or more pairs of layers. The energy barrier provided between the 
active layer and the second conductivity type cladding layer and having a 
sufficient height, prevents overflow of carriers, and improvements in LD 
characteristics, particularly operation characteristic at a high 
temperature, are realized. 
According to a second aspect of the present invention, in the semiconductor 
device of the first aspect, the first compound semiconductor includes Al 
and Ga in such a composition proportion that makes .DELTA.Ec of the first 
compound semiconductor the largest, and the second compound semiconductor 
includes Al and Ga in such a composition proportion that makes .DELTA.Ev 
of the second compound semiconductor larger than that of the first 
compound semiconductor. The energy barrier provided between the active 
layer and the second conductivity type cladding layer and having a 
sufficient height, prevents overflow of carriers, and improvements in LD 
characteristics, particularly operation characteristic at a high 
temperature, are realized. 
According to a third aspect of the present invention, in the semiconductor 
device of the first aspect, the first compound semiconductor comprises 
(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In .sub.0.5 P, and the second compound 
semiconductor comprises (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P 
(0.7&lt;x.ltoreq.1). The energy barrier provided between the active layer and 
the second conductivity type cladding layer and having a sufficient 
height, prevents overflow of carriers, and improvements in LD 
characteristics, particularly operation characteristic at a high 
temperature, are realized. 
According to a fourth aspect of the present invention, in the semiconductor 
device of the first aspect, the first compound semiconductor comprises 
Al.sub.0.48 Ga.sub.0.52 As, and the second compound semiconductor 
comprises Al.sub.x Ga.sub.1-x As (0.48&lt;x.ltoreq.1). The energy barrier 
provided between the active layer and the second conductivity type 
cladding layer and having a sufficient height, prevents overflow of 
carriers, and improvements in LD characteristics, particularly operation 
characteristic at a high temperature, are realized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Embodiment 1! 
FIG. 1(a) to 1(c) are diagrams for explaining a semiconductor laser 
according to a first embodiment of the present invention, and FIG. 1(a) 
shows a cross-sectional view of the structure, FIG. 1(b) shows an energy 
band diagram in the proximity of an active layer, and FIG. 1(c) 
illustrates energy band gaps of a main part of the semiconductor laser. In 
the figures, reference numeral 1 designates an n-GaAs substrate, numeral 2 
designates an n-AlGaInP lower cladding layer about 1.5 .mu.m thick, 
numeral 3 designates a single GaInP active layer or an AlGaInP active 
layer having a multiple-quantum-well structure, numeral 4a designates a 
p-AlGaInP first upper cladding layer, numeral 10 designates a superlattice 
barrier layer which comprises an (Al.sub.x Ga.sub.(1-x)).sub.0.5 
In.sub.0.5 P (x=0.7) layer 10a ten angstroms thick and an (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layer 10b ten angstroms thick, 
the layers 10a and 10b being alternatingly laminated in one or more pairs 
of layers. The number of pairs of laminated layers is selected in a range 
of 1 to 20 or so, preferably in a range of approximately 5 to 10. The 
thicknesses of the respective layers are adjusted, as required, so as not 
to exceed their critical layer thickness. The sum of the thickness of the 
superlattice barrier layer 10 and the thickness of the first upper 
cladding layer 4a is designed to be approximately 0.2 to 0.25 .mu.m. 
Reference numeral 4b designates a p-AlGaInP second upper cladding layer 
about 1.25 .mu.m thick, numeral 5 designates an etching stop layer, 
numeral 6 designates a band discontinuity relaxing layer comprising 
p-GaInP about 0.1 .mu.m thick, numeral 7 designates a current blocking 
layer comprising n-GaAs or the like, numeral 8 designates a p-GaAs contact 
layer about 3.0 .mu.m thick. An n-side electrode on a rear surface of the 
substrate 1 and a p-side electrode on the contact layer 8 are not shown in 
the figures. This semiconductor laser is an AlGaInP series semiconductor 
laser emitting a red laser light. 
FIG. 2 is a diagram representing two relations in an (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P layer: a relation between an Al 
composition proportion and an energy difference in the conduction band, 
.DELTA.Ec; a relation between the Al composition proportion and an energy 
difference in the valence band, .DELTA.Ev. 
A description is given of a function of the semiconductor laser. In the 
first embodiment, the superlattice barrier layer 10 is disposed between 
the p-type cladding layer 4a and an active layer 3, and the superlattice 
barrier layer 10 comprises alternatingly arranged (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) layers 10a and (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layers 10b. 
As shown in FIG. 2, when an Al composition x of (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P is more than 0.7, an energy difference 
in the valance band, .DELTA.Ev increases with the Al composition 
proportion x while an energy difference in the conduction band, .DELTA.Ec, 
decreases with an increase in the Al composition proportion x. Therefore, 
the energy band gaps Eg of the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 
P (x=0.7) layer 10a and the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P 
(x=1.0) layer 10b are 2.32 eV and 2.35 eV, respectively, as shown in FIG. 
1(c). When making a comparison between the energy band gap of the 
(Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) layer 10a and the 
energy band gap of the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P 
(x=1.0) layer 10b, .DELTA.Ec of the (Al.sub.x Ga.sub.(1-x)).sub.0.5 
In.sub.0.5 P (x=0.7) layer 10a is larger than that of (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layer 10b, and .DELTA.Ev of the 
(Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layer 10b is larger 
than that of the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) 
layer 10a. When the semiconductor layers having these band structures are 
arranged to form the superlattice barrier layer 10 having a superlattice 
structure, the superlattice barrier layer 10 has an energy band structure 
shown in FIG. 1(c). This superlattice structure such as that of the 
superlattice barrier layer 10 is commonly classed as II type. 
In the superlattice barrier layer 10, the energy difference in the 
conduction band, .DELTA.Ec, is determined by the (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P layer 10a with the Al composition of 
x=0.7, and the energy difference in the valence band, .DELTA.Ev, is 
determined by the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P layer 10b 
with the Al composition of x=1.0. Therefore, the effective band gap of the 
superlattice barrier layer 10 is 2.42 eV, a value larger by 96 meV than 
that of a single (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) 
layer, and the semiconductor laser has an energy band structure in the 
proximity of the active layer 3 as shown in FIG. 1(b). 
In the prior art AlGaInP series red LD, the p-(Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P cladding layer with the Al composition 
of x=0.7 is used, and therefore the band gap Eg of the p-type cladding 
layer is as small as 2.324 eV. This causes electron overflow, resulting in 
deterioration in LD particularly, operation characteristic at a high 
temperature, as described above. On the other hand, in the first 
embodiment, the superlattice barrier layer 10 having a higher energy 
barrier than the p-(Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P with the 
Al composition proportion of x=0.7 is disposed between the active layer 3 
and the p-type first upper cladding layer 4a, thereby preventing electron 
overflow. 
Thus, in the semiconductor laser according to the first embodiment, the 
superlattice barrier layer 10 comprising alternatingly laminated (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) layers 10a and (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layers 10b is provided between 
the active layer 3 and the p-type first cladding layer 4a. Therefore, the 
energy band gap Eg of the superlattice barrier layer 10 is determined by 
the energy difference in the conduction band, .DELTA.Ec, in the (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) layer 10a and the energy 
difference in the valence band, .DELTA.Ev in (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layer 10b. As a result, an 
energy barrier having a sufficient height is provided between the active 
layer 3 and the p-type upper cladding layers 4a and 4b, preventing the 
overflow of carriers. This results in improvements in LD particularly, 
operation characteristic at a high temperature. 
While in the first embodiment the layers constituting the superlattice 
barrier layer 10, i.e., the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P 
(x=0.7) layer 10a and the (Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P 
(x=1.0) layer 10b, are alternatingly laminated in such a way that the 
layer in closest proximity to the active layer 3 is the (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=0.7) layer 10a, as shown in FIG. 
1(b), these layers may be alternatingly laminated in such a way that the 
layer in closest proximity to the active layer 3 is the (Al.sub.x 
Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P (x=1.0) layer 10b, and the same effects 
as in the first embodiment are obtained. 
While in the first embodiment the Al composition proportion x of the 
(Al.sub.x Ga.sub.(1-x)).sub.0.5 In.sub.0.5 P layer 10b is 1.0, the Al 
composition proportion x may be larger than 0.7 and equal to or smaller 
than 1.0 in the present invention and the same effects as in the first 
embodiment are obtained. 
While in the first embodiment the respective layers which constitute the 
superlattice barrier layer 10 have an In composition proportion of 0.5, 
the In composition proportion may be varied as required in the present 
invention and the same effects as in the first embodiment are obtained. 
Embodiment 2! 
FIG. 4 is a diagram for explaining a semiconductor laser according to a 
second embodiment and the figure shows an energy band structure in the 
proximity of the active layer of the semiconductor laser. In the figure, 
reference numeral 23 designates an Al.sub.x1 Ga.sub.(1-x1) As active 
layer, numeral 22 designates an n-Al.sub.x2 Ga.sub.(1-x2) As (x1&lt;x2) lower 
cladding layer, numeral 24 designates a p-Al.sub.x2 Ga.sub.(1-x2) As 
(x1&lt;x2) upper cladding layer, numeral 20 designates a superlattice barrier 
layer, numeral 20a designates an Al.sub.x Ga.sub.(1-x) As (x=0.48) layer, 
numeral 20b designates an Al.sub.x Ga.sub.(1-x) As (0.48&lt;x.ltoreq.1) 
layer. The semiconductor laser is an AlGaAs series short wavelength LD 
and, in the second embodiment, a superlattice barrier layer described as 
the first embodiment is applied to the AlGaAs series short wavelength LD. 
In Al.sub.x Ga.sub.(1-x) As, which is used as a material of a cladding 
layer in the AlGaAs series short wavelength LD, a charge carrier 
transition changes from a direct transition (.GAMMA.valley) to an indirect 
transition (X valley) at the Al composition x=0.48. Therefore, when this 
is applied to the superlattice barrier layer of the first embodiment and a 
superlattice barrier layer 20 comprising alternatingly laminated Al.sub.x 
Ga.sub.(1-x) As (x=0.48) layers 20a and Al.sub.x Ga.sub.(1-x) As 
(0.48&lt;x.ltoreq.1) layers 20b is disposed between an active layer 23 and a 
p-type cladding layer 24, the superlattice layer 20 has a superlattice 
structure called the II type, and the energy band gap Eg of the 
superlattice barrier layer 20 is larger than that in the case where the Al 
composition x2 of the p-Al.sub.x2 Ga.sub.(1-x2) As (x1&lt;x2) upper cladding 
layer 24 is 0.48. Therefore, an energy barrier having a sufficient height 
is provided between the active layer 23 and the p-type upper cladding 
layer 24, preventing overflow of carriers. As a result, the same effects 
as in the first embodiment are obtained. 
Although in the second embodiment the layers constituting the superlattice 
barrier layer 20, i.e., the Al.sub.x Ga.sub.(1-x) As (x=0.48) layer 20a 
and the Al.sub.x Ga.sub.(1-x) As (0.48&lt;x.ltoreq.1) layer 20b, are 
alternatingly laminated in such a way that the layer in closest proximity 
to the active layer 23 is the Al.sub.x Ga.sub.(1-x) As (x=0.48) layer 20a, 
as shown in FIG. 4, these layers may be alternatingly laminated in such a 
way that the layer in closest proximity to the active layer 23 is the 
Al.sub.x Ga.sub.(1-x) As (0.48&lt;x.ltoreq.1) layer 20b, and the same effects 
as in the second embodiment are obtained. 
Although the first and second embodiment refer to an AlGaInP series red LD 
and an AlGaAs series short wavelength LD, the present invention is 
applicable in semiconductor lasers comprising other materials. In such 
cases, a second conductivity type superlattice barrier layer having a II 
type superlattice structure is provided between an active layer and a 
cladding layer. The II type superlattice structure comprises a first 
compound semiconductor having a larger energy band gap than that of the 
active layer and a second compound semiconductor having a smaller energy 
difference in the conduction band, .DELTA.Ec, than that of the first 
compound semiconductor and a larger energy difference in the valence band, 
.DELTA.Ev, than that of the first compound semiconductor, the first and 
second compound semiconductors being alternatingly laminated in one or 
more pairs of the layers. Thus, the same effects as in the first and 
second embodiments are obtained. 
Although the first and second embodiments refer to a case where a 
superlattice barrier layer is disposed between an active layer and a 
p-type cladding layer, the present invention is applicable to a case where 
a superlattice barrier layer is disposed between an active layer and an 
n-type cladding layer, and the same effects as in the first and second 
embodiments are obtained. 
Although the first and second embodiments refer to a case where a 
superlattice barrier layer is provided between an active layer and a 
p-type cladding layer, in the present invention, a thin layer of the same 
material as that of the p-type cladding layer may be sandwiched between 
the active layer and the superlattice barrier layer, and the same effects 
as in the first and second embodiments are obtained. 
Although the first and second embodiments refer to a semiconductor laser, 
the present invention is applicable to other semiconductor devices having 
a structure where an active layer is sandwiched between a first 
conductivity type cladding layer and a second conductivity type cladding 
layer, and the same effects as in the first and second embodiments are 
obtained.