Semiconductor quantum well structure and semiconductor device using the same

A semiconductor quantum well structure having at least two quantum wells, each having an electron quantum level, a heavy hole quantum level and a light hole quantum level. In the two quantum wells, only their respective heavy hole quantum levels or their respective light hole quantum levels coincide with each other. Further, there is a construction in which a barrier portion between the two quantum wells has a thickness and a band gap which allow connecting the wave functions of the respective electrons between the two quantum wells. Alternatively, the thickness and band gap of the barrier allow the connection between the quantum wells of the wave functions of those holes whose quantum levels coincide with each other. In order to set the hole quantum level to a desired quantum level, a specific construction imparts an appropriate strain to the quantum wells.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor quantum well structure 
and, more specifically, to a semiconductor quantum well structure formed 
by stacking semiconductor layers having different band gaps. The present 
invention further relates to a semiconductor device using such a quantum 
well structure. 
2. Description of the Related Art 
An example of the conventional semiconductor device using a strain quantum 
well is a semiconductor laser. In a semiconductor laser, to achieve an 
improvement in performance by, for example, reducing the effective mass of 
the holes in the valence band, a semiconductor whose lattice constant is 
larger than that of the substrate is used in the well portion of the 
quantum well forming the active layer, making it possible to introduce a 
biaxial compressive strain. By introducing a compressive strain, it is 
possible to reduce the threshold value of the transverse electric (TE) 
light oscillation of the semiconductor laser and, further, to attain an 
improvement in terms of modulation characteristics. 
However, in the above prior-art example, when the width of the quantum well 
is enlarged for the purpose of attaining an increase in gain, the quantum 
level of the heavy holes and that of the light holes approach each other, 
with the result that the effect of the introduction of a strain is 
diminished. Further, the value of the film thickness (the critical film 
thickness) which allows growth of a substance with a lattice constant 
different from that of the substrate is restricted, and, in a quantum well 
containing a material whose lattice constant is different from that of the 
substrate, there are limitations in terms of an increase in film thickness 
and degree multiplicity. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor quantum 
well structure in which the above problems have been solved and a 
semiconductor device including such a semiconductor quantum well 
structure. 
In accordance with the present invention, there is provided a semiconductor 
quantum well structure which includes at least two quantum wells, each 
having an electron quantum level, a heavy hole quantum level and a light 
hole quantum level, wherein, in these two quantum wells, only either their 
respective heavy hole quantum levels or their respective light hole 
quantum levels coincide with each other. In the present invention, it has 
been observed that an effect is obtained through an increase in quantum 
well width with respect to those holes whose quantum levels coincide with 
each other. To realize such an effect, the thickness and the band gap of 
the barrier layer portion between the two quantum wells are determined 
such that the wave functions of the electrons of the two quantum wells can 
be connected together and that the wave functions of those holes of the 
two quantum wells whose levels coincide with each other can be connected 
together. 
More specifically, the composition ratios of the semiconductors forming the 
two quantum wells may be different from each other. Alternately, the 
semiconductor forming one of the two quantum wells may contain at least 
one semiconductor material which is not contained in the semiconductor 
forming the other quantum well. 
As a specific construction for quantum level control, it is possible to 
adopt a construction in which a strain effect is only applied to one of 
the two quantum wells. The strain effect may be the effect of a 
compressive strain or the effect of a tensile strain. Further, in the 
above-described two quantum wells, it is also possible to adopt a 
construction in which a strain effect is applied to each of the two wells, 
with the strain effect applied to one well being different from that 
applied to the other well. In this case, the respective amounts of 
compressive strain applied to the quantum wells may be different, or the 
respective amounts of tensile strain applied to the quantum wells may be 
different. Alternatively, the strain effect applied to one of the two 
quantum wells may be the effect of a compressive strain, and the strain 
effect applied to the other being the effect of a tensile strain. 
Whether a strain effect in a semiconductor is a compressive strain effect 
or a tensile strain effect is determined by the deviation of the lattice 
constant of the semiconductor from a reference lattice constant; that is, 
it is determined according as to whether the lattice constant of the 
semiconductor is larger or smaller than the reference lattice constant. 
The degree of strain effect is determined by the degree of this deviation. 
Thus, a strain effect as mentioned above can be appropriately imparted by 
controlling the composition ratio of the semiconductor forming the quantum 
well. The composition ratio of the semiconductor is controlled to the 
extent that the lattice constant of the semiconductor becomes a desired 
lattice constant. As the reference lattice constant, it is expedient to 
use the lattice constant of the substrate forming the quantum well 
structure. 
According to another aspect of the present invention, the above-described 
semiconductor quantum well structure is applicable to various 
semiconductor devices including a quantum well structure. For example, it 
is possible to construct a semiconductor laser including a semiconductor 
quantum well structure as described above in at least a part of the active 
layer thereof. Further, the above quantum well structure is also 
applicable to other semiconductor optical devices, such as a semiconductor 
light amplifier and semiconductor light modulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
(First Embodiment) 
FIGS. 1 and 2 are diagrams most clearly illustrating the features of the 
present invention. FIG. 2 shows a semiconductor laser to which the present 
invention is applied, and FIG. 1 is a band diagram of a part of FIG. 2. In 
FIG. 2, numeral 1 indicates a substrate which is formed, for example, of 
n-InP; numeral 2 indicates a buffer layer formed, for example, of n-InP; 
numeral 3 indicates a clad layer which is formed, for example, of n-InP; 
numeral 4 indicates an active layer; numeral 5 indicates a clad layer 
which is formed, for example, of p-InP; numeral 6 indicates an insulating 
layer which is formed, for example, of SiO.sub.2 ; numeral 7 indicates a 
cap layer which is formed, for example, of p-InGaAs; numerals 8 and 9 
indicate electrodes for supplying electric current to the active layer 4; 
and numeral 10 indicates an electric current. In this embodiment, a ridge 
waveguide is used as the lateral waveguide structure of the semiconductor 
laser. Of course, this waveguide structure may be of any other type, e.g., 
the buried type, as long as it can form a semiconductor laser. 
The construction of the active layer 4 is shown in FIG. 1. The active layer 
4 as shown in FIG. 1 in which there are provided two quantum wells between 
separate confinement hetero structure (SCH) layers 15 and 16 of p and 
n-type InGaAsP (1.3 .mu.m composition). The quantum wells comprise a well 
(1) consisting, for example, of a layer of In.sub.0.53 Ga.sub.0.47 As 
having a thickness of 10 nm, a well (2) consisting, for example, of a 
layer of In.sub.0.62 Ga.sub.0.38 As.sub.0.9 P.sub.0.1 having a thickness 
of 10 nm, and a barrier layer 17 situated between the wells (1) and (2) 
and consisting, for example, of a layer of InGaAsP (1.3 .mu.m composition) 
having a thickness of 7 nm. 
In this construction, the well (2) is formed of a material whose lattice 
constant is somewhat larger than that of the substrate. When, as in this 
embodiment, epitaxial growth is effected, an in-plane compressive stress 
is generated by a lattice mismatching of approximately 0.29%, resulting in 
a strain effect. 
A semiconductor crystal under an in-plane stress undergoes a change in band 
gap with respect to the light and heavy holes as shown below: 
Heavy Holes: 
EQU E.sub.gHH =E.sub.g0 +a(2.epsilon..sub.XX -2(c.sub.12 
/c.sub.11).epsilon..sub.XX)-b(.epsilon..sub.XX +2(c.sub.12 
/c.sub.11).epsilon..sub.XX) 
Light Holes: 
EQU E.sub.gLH =E.sub.g0 +a(2.epsilon..sub.XX -2(c.sub.12 
/c.sub.11).epsilon..sub.XX)+b(.epsilon..sub.XX +2(c.sub.12 
/c.sub.11).epsilon..sub.XX) 
where E.sub.g0 is the band gap when there is no in-plane stress; 
.epsilon..sub.XX is the degree of lattice mismatching 
(((a-a.sub.0)/a.sub.0 ; it will be assumed that a.sub.0 is the lattice 
constant of the substrate, and that a is the lattice constant of the 
semiconductor crystal under in-plane stress); a and b are deformation 
potentials; and a.sub.11 and C.sub.12 are stiffness constants. 
For example, the well depth for the heavy holes is obtained as follows, 
taking the band offset ratio into consideration: 
EQU .alpha.E.sub.gb -.alpha.E.sub.g0 -.alpha.a(2.epsilon..sub.XX -2(c.sub.12 
/c.sub.11).epsilon..sub.XX)+b(.epsilon..sub.XX +2(c.sub.12 
/c.sub.11).epsilon..sub.XX) 
where .alpha. is the band offset ratio, and E.sub.gb is the band gap of the 
barrier (The barrier is in lattice matching). 
Similarly, the well depth for the light holes can be obtained as follows: 
EQU .alpha.E.sub.gb -.alpha.E.sub.g0 -.alpha.a(2.epsilon..sub.XX -2(c.sub.12 
/c.sub.11).epsilon..sub.XX)-b(.epsilon..sub.XX +2(c.sub.12 
/c.sub.11).epsilon..sub.XX) 
In this embodiment, due to the in-plane compressive stress, the well (2) 
has a band gap between first level of conduction band 19 and first level 
of heavy hole 20, which is approximately the same as that of the well (1) 
and a band gap between first level of conduction band 19 and first level 
of light hole 21 is larger than the same as that of the well (1) by 
approximately 0.02 eV (that is, the well (2) is shallower than the well 
(1) with respect to the light holes). 
Due to the effect of the lattice mismatching as described above, the 
prepared quantum well has a double quantum well structure with respect to 
the heavy holes, and an asymmetrical double quantum well structure with 
respect to the light holes. Moreover, when the thickness of the barrier 17 
is small, as in this construction, the two wells (1) and (2) are connected 
together. In that case, the level of the conduction band of the same 
energy level and the level of the heavy holes of the valence band are 
connected, and the level of the light holes is not connected. The quantum 
levels at this time are as shown in FIG. 3; a just level of conduction 
band 19, a just level of heavy holes 20 (both shown in broken lines) and a 
just level of light holes (shown in dotted broken lines) 21. As a result, 
when electric current is supplied, the transition between the first level 
of the conduction band and the first level of the heavy holes of the 
valence band is considerably predominant. The transition in which the 
light holes are involved is diminished. 
Due to the above construction, it is possible to reduce the threshold value 
of the transverse electric (TE) light oscillation of the semiconductor 
laser of FIG. 2 and, further, an improvement in modulation characteristics 
can be achieved. In this way, the level of the light holes and that of the 
heavy holes do not approach each other, so that it is possible to achieve 
an increase in gain without diminishing the effect of strain introduction, 
whereby the width of the quantum well can be substantially increased. 
(Second Embodiment) 
FIG. 4 is a diagram for illustrating a second embodiment of the present 
invention. The drawing, which corresponds to FIG. 1 illustrating the first 
embodiment, shows a band diagram of a quantum well portion. Apart from 
this portion, the construction of the ridge waveguide type semiconductor 
laser of the first embodiment, shown in FIG. 2, is used. 
In this embodiment, the well (1) consists, for example, of a layer of 
In.sub.0.905 Ga.sub.0.095 As.sub.0.3 P.sub.0.7 having a thickness of 8 nm, 
the well (2) consists, for example, of a layer of In.sub.0.8 Ga.sub.0.2 
As.sub.0.4 P.sub.0.6 having a thickness of 8 nm, and the barrier 27 
between the two wells consists, for example, of a layer of In.sub.0.91 
Ga.sub.0.09 As.sub.0.2 P.sub.0.8 (1.0 .mu.m composition). In the case of 
this construction, a compressive strain of approximately 0.30% is applied 
to the well (1) due to a difference in lattice constant between the well 
layer and the substrate (InP). Similarly, a tensile strength of 
approximately 0.117% is applied to the well (2). As a result of these 
strains, the band gaps of the wells undergo a variation. In FIG. 4, the 
band end of the heavy holes 28 is indicated by solid lines, and the band 
end of the light holes 29 is indicated by broken lines. The SCH layer on 
either side has the construction of InGaAsP (1 .mu.m composition). 
As can be seen from FIG. 4, there are formed wells of the same depth with 
respect to the heavy holes and electrons (double quantum well), whereas, 
with respect to the light holes, there are two wells of different depths 
(asymmetrical double quantum well). In such a construction, the transition 
between the heavy holes and electrons is predominant, as described with 
reference to the first embodiment. 
FIG. 5 shows another example of quantum well construction. In this case, 
the well (1) is formed of Ga.sub.0.14 In.sub.0.86 As.sub.0.3 P.sub.0.7, 
which is a material in lattice matching with the substrate 1. The well (2) 
is formed, for example, of Ga.sub.0.8 In.sub.0.2 As and has a smaller 
lattice constant than the substrate, and a tensile strain of approximately 
2.2% is applied to the well (2). As a result, in the well (2), the band 
gap with respect to the heavy holes 33 is different from the band gap with 
respect to the light holes 34 (The broken line shown in FIG. 5 represents 
the band end for the light holes). Accordingly, as in the embodiment 
described above, the transition between the heavy holes and electrons is 
predominant. The SCH 30 and 31 on either side of the barrier 32 has the 
construction InGaAsP and a thickness of 0.95 .mu.m. 
(Third Embodiment) 
In the above described embodiments, the heavy hole levels substantially 
coincide with each other between a plurality of wells, whereas the light 
hole levels do not coincide. A construction example which is the reverse 
to the above case, that is, a case in which the light hole levels coincide 
with each other and in which the heavy hole levels do not coincide will 
now be described. The basic construction is the same as those shown in 
FIGS. 1 and 2 except for the construction of the active layer 4. The well 
(1) consists, for example, of a layer of In.sub.0.53 Ga.sub.0.47 As having 
a thickness of 10 nm, and the well (2) consists, for example, of a layer 
of In.sub.0.28 Ga.sub.0.72 As having a thickness of 10 nm. In the case of 
this construction, the well (2) has a lattice mismatching to a degree of 
1.7% and is under an in-plane tensile stress, the band gaps with respect 
to the heavy and light holes being different from each other. In this 
construction, the quantum wells are formed such that the wells (1) and (2) 
have the same depth with respect to the light holes, and different depths 
with respect to the heavy holes (The well (2) is shallower by 
approximately 0.13 eV). 
In FIG. 6, the band end of the heavy holes is indicated by solid lines 39, 
and the band end of the light holes is indicated by broken lines 38, (the 
solid line 39 and the broken line 38 is overlapping at well (1)). 
Although the above embodiments have been described with reference to 
devices in which the quantum well structure of the present invention is 
applied to a semiconductor laser structure, the range of application of 
the present invention is not limited thereto. The present invention is 
also applicable to any other type of conventional device as long as it 
contains a quantum well structure, for example, a light modulator using a 
quantum well structure. Further, by forming an anti-reflection film on 
either end surface of the semiconductor laser shown with reference to the 
embodiments, it is possible to construct a traveling-wave-type 
semiconductor laser amplifier having a high amplification factor with 
respect to transverse electric (TE) light, transverse magnetic (TM) light, 
etc. 
Further, while the above embodiments have been described with reference to 
the case in which two wells are used in constructing the structure, it is 
also possible for these two wells to constitute one of a plurality of 
pairs of wells forming a well structure. Further, the present invention is 
also easily applicable to a structure having three or more wells. 
As described above, in accordance with the present invention, a coincidence 
of electron levels is achieved with respect to a plurality of wells, and a 
coincidence of levels is achieved with respect to only either light or 
heavy holes, whereby it is possible to separate the level of heavy holes 
from that of light holes in a quantum well structure of a larger thickness 
and a higher degree of multiplicity than in the prior art. 
Accordingly, while the present invention has been described with respect to 
what is presently considered to be the preferred embodiments, it is to be 
understood that the invention is not limited to the disclosed embodiments. 
To the contrary, the invention intended to cover the various modifications 
and equivalent arrangements included within the spirit and scope of the 
appended claims. The scope of the following claims is to be accorded to 
the broadest interpretation so as to encompass all such modifications and 
equivalent structures and functions.