Modulated strain heterostructure light emitting device

A heterostructure laser diode is provided with an active region that includes a ternary or quaternary semiconductor compound. The composition of the semiconductor compound forming the active region is modulated resulting in an active region with a modulated strain profile (.increment.a/a), e.g., a triangular sawtooth-like strain profile, perpendicular to the laser diodes epitaxial layers, i.e., parallel to the z-axis. This permits the present invention to increase strain and avoid formation of misfit dislocations by compensation, i.e., by inserting strained layers having opposing strains.

TECHNICAL FIELD 
The invention relates to semiconductor light emitting devices such as light 
emitting diodes and laser diodes, and in particular, to heterostructure 
diodes, having an active region with a modulated strain profile. 
BACKGROUND OF THE INVENTION 
Most semiconductor laser diodes available nowadays are 
double-heterostructure laser diodes. The manufacture of these 
heterostructure diodes is simpler and, thus, better understood when it is 
compared to the more complex quantum-well laser diodes. It also guarantees 
higher yields. 
The present invention relates to an improved heterostructure diode that 
only requires slight modifications to an existing manufacturing process, 
This is clearly preferable to starting a new manufacturing process and 
accepting risks of unknown reliability. Prior to switching from a 
conventional double-heterostructure laser diode to a quantum well laser 
diode, the intermediate step towards an improved heterostructure diode is 
deemed to be adequate for most manufacturers. 
Single-Heterostructure diodes (SH) and Double-Heterostructure diodes (DH) 
will, henceforth, be addressed independently of their material, structure, 
and layer configuration. Details on Single-Heterostructure and 
Double-Heterostructure laser diodes and light emitting diodes are provided 
in Chapter 12, 'LED and Semiconductor Lasers' in the book "Physics of 
Semiconductor Devices", S. M. Sze, Second Edition, John Wiley & Sons, New 
York, 1981. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved heterostructure semiconductor laser diode or light emitting 
diode. 
It is another object of the present invention to provide for a 
heterostructure laser diode having a reduced threshold current. 
It is a further object of the present invention to provide a 
heterostructure laser diode with increased differential gain. 
It is yet another object of the present invention to provide an improved 
heterostructure laser diode or light emitting diode with minimum 
modifications, such that the manufacturing can be carried out using 
existing manufacturing processes. 
SUMMARY OF THE INVENTION 
These and other objects of the invention are accomplished in accordance 
with the present invention by providing a heterostructure laser diode or 
light emitting diode with an active region that includes alternating 
compressive and tensile strained sublayers of quaternary semiconductor 
compounds, stacked on top of each other, resulting in an active region 
with a modulated strain profile perpendicular to the laser diode epitaxial 
layers. The net layer strain of this active region can either be zero or 
shifted towards a compressive or tensile strain, e.g., by providing the 
laser diode with a modulated strain profile and an adjustable net layer 
strain. It is thus shown how to increase strain and avoid the formation of 
misfit dislocations by compensation, i.e., by inserting strained layers 
with opposing strains.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Heterostructure diodes are characterized in that a potential barrier is 
introduced either on one side (Single-Heterostructure) or on both sides 
(Double-Heterostructure) of the diode active region. A typical 
Single-Heterostructure (SH) diode is described in "Physics of 
Semiconductor Devices", S. M. Sze, Second Edition, John Wiley & Sons, New 
York, 1981, cf. FIG. 27, page 709. It includes a p-doped GaAs active 
region embedded between an n-doped GaAs cladding layer and a p-doped 
AlGaAs cladding layer. This SH is characterized by a homojunction on one 
side, i.e., the junction between the p-doped GaAs active region and the 
n-doped GaAs cladding layer, and a heterojunction, i.e., the junction 
between the p-doped GaAs active region and the p-doped AlGaAs cladding, on 
the other side. This heterojunction provides the previously mentioned 
potential barrier. In addition, a step of the refractive index of the 
order of a few percent leads to better confinement of the light on this 
side of the active region. 
In Double-Heterostructure laser diodes, which were the first diodes to be 
operated continuously at room temperature, the recombination and the 
optical field are confined to an active region by the introduction of a 
second potential barrier to the Single-Heterostructure. A cross-sectional 
view of a prior art broad area Double-Heterostructure laser diode 15 is 
shown in FIG. 1. A similar structure is set out in FIGS. 26 and 27, on pp. 
708-709, of the above-mentioned book of S. M. Sze. The laser diode 15 
consists of an active region 11 that includes preferably p-doped GaAs, an 
upper cladding layer 10 and a lower cladding layer 12. The upper cladding 
layer 10 consists preferably of p-doped AlGaAs and a lower cladding layer 
of n-doped AlGaAs. Additionally, the laser 15 has contact metallizations 
13 and 14. The bandgap (E.sub.g) profile of this laser 15 is illustrated 
in FIG. 2. As can be seen from this drawing, the active region 11 is 
embedded between two potential barriers provided by cladding layers 10 and 
12. One disadvantage of such a conventional heterostructure laser diode 
lies in its relative high threshold current (I.sub.th). 
The band structure theory anticipates reduced threshold current densities 
(J.sub.th) for quantum-well and heterostructure InP-based lasers 
containing compressive strained InGaAsP active regions, as reported in the 
article: "Band Structure Engineering for Low Threshold High-Efficiency 
Semiconductor Lasers", by A. R. Adams, Electronics Letters, Vol. 22, No. 
5, February 1986, pp. 249-250. Originally, only a compressive strain was 
considered for enhancing the laser performance, as disclosed in the 
above-mentioned study by A. R. Adams. However, an even more reduced 
threshold current arises from InP based laser diodes with highly tensile 
strained InGaAsP active region. This is confirmed by T. C. Chong et al. in 
the article: "Theoretical Gain of Strained-Layer Semiconductor Lasers in 
the Large Strain Regime", IEEE Journal of Quantum Electronics, Vol. 25, 
No. 2, February 1989, pp. 171-178. 
Fundamentally, the strain perturbs the valence band structure of the 
semiconductor, such as InGaAsP, so as to decrease the in-plane hole mass 
and the density of states near the valence band edge. This results in a 
reduction of carrier loss mechanisms, such as Auger recombination, and 
increases the differential gain of the laser. On the other hand, an 
intermediate tensile strain is predicted to increase J.sub.th due to 
valence band maxima outside the zone center, which encourage additional 
losses due to indirect transitions, as described in the article: 
"Theoretical Gain in Compressive and Tensile Strained InGaAs/InGaAsP 
Quantum Wells", by S. W. Corzine et al., Applied Physics Letters, Vol. 59, 
No. 5, July 1991, pp. 588-590. 
The dependency between strain and threshold current density (J.sub.th), is 
reported in the article: "Low-Pressure MOVPE Growth and Characterization 
of Strained-Layer InGaAs-InGaAsP Quantum Well Lasers", by P. J. A. Thijs 
et al., Microelectronic Engineering, Elsevier, Vol. 18, 1992, pp. 57-74, 
and in "Progress in Quantum Well Lasers: Application of Strain", P. J. A. 
Thijs, Proc. 13th IEEE Int. Semiconductor Laser Conference, 21-25 
September 1992, Takamatsu, Kagawa, Japan, paper A-1, pp. 2-5. This 
dependency is illustrated in FIG. 3. In it are shown the threshold current 
densities extrapolated for infinite cavity length Single-Quantum-Well 
(SQW) lasers plotted versus strain (.increment.a/a) in the well. The 
variations of the threshold current densities are ascribed to the strain 
and the quantum size effects on the valence subband structure. Theory 
predicts that the threshold current density is independent of the quantum 
well width. A dramatically reduced threshold current density for both 
increasing tensile and compressive strain can be observed. The lowest 
threshold current density was obtained from a 1.6% tensile strained 
In.sub.0.3 Ga.sub.0.7 TAs SQW structure which has a lattice constant 
a=0,5775 nm. The sharply increased threshold current density for the 2.1% 
tensile strained SQW structure arises most probably from crystallographic 
defects. No lasing at room temperature was observed for a QW laser diode 
with a tensile strain of about 0.9%. Comparable effects have also been 
reported for InGaAlP visible lasers in the article: "Effect of Strain on 
the Threshold Current of GaInP/AlGaInP Quantum Well Lasers Emitting at 633 
nm", by A. Valster et al., Proc. 13th IEEE Int. Semiconductor Laser 
Conference, 21-25 September 1992, Takamatsu, Kagawa, Japan, paper G-1, pp. 
152-153. 
Under increasing compressive strain, the threshold current density 
decreases due to the reduction of the in-plane heavy hole effective mass 
and a reduction of the loss mechanisms, as already mentioned. As shown in 
FIG. 3, the threshold current density (J.sub.th) is found to decrease 
monotonically for compressive as well as tensile strain, in the range of 
0%&lt;.increment.a/a&lt;1.5% and -0.9%&lt;Aa/a&lt;-1.6%, respectively. 
In the case of compressive strain, it has been demonstrated that similar 
trends exist for broad area 1.3 .mu.m wavelength InGaAsP-InP 
Double-Heterostructure (DH) lasers. J.sub.th of these structures 
(identified by circles) decreases monotonically with compressive strain, 
but strongly increases for the moderate tensile strain, as shown in FIG. 
4. The trends shown in FIGS. 3 and 4 indicate that an even higher 
compressive and/or tensile strain in the active region should further 
decrease J.sub.th. However, further improvements of J.sub.th in these DH 
structures cannot be achieved with a higher strain because misfit 
dislocations are generated in the structure when exceeding the critical 
layer thickness. In the particular case of a GainAsP layer thickness of 
120 nm (1200 A %), the maximum strain value is found to be 
-0.14%&lt;.increment.a/a&lt;+0.28% indicated by the hatched regions in FIG. 4. 
These values were determined from estimates provided in: "Defects in 
Epitaxial Multilayers", J. W. Matthews et al., Journal of Crystal Growth, 
Vol. 27, 1974, pp. 118-125. Alternatively, the versatility of most 
quaternary materials as well as other semiconductor material with more 
than four components, such as AlGaAsP, A1GaInP, InGaAsP, ZnCdSSe, 
Cu(AlGa)(SSe), ZnSiP, and AlGaInSb, just to name a few, allows for the 
independent adjustment of the bandgap and stress, and allows for different 
embodiments of the present invention, three of which will be described 
hereinafter. The teaching of the present invention and the hereinafter 
described embodiments relating to laser diodes can also be advantageously 
used in light emitting diodes. Details on the different materials being 
well suited for the application of the present invention are described in: 
"Heterostructure Lasers", H. C. Casey, Jr., and M. B. Panish, Academic 
Press, Inc. Orlando, 1978, Part B, `Materials and Operating 
Characteristics`; 
"Material Parameters of In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y and Related 
Binaries", S. Adachi, Journal of Applied Physics, Vol. 53, No. 12, 
December 1982, pp. 8775-8792; 
Landolt-Boernstein, "Numerical Data and Functional Relationships in Science 
and Technology", Group III: Crystal and Solid State Physics, Vol. 22, 
Semiconductors, Supplements and Extensions to Volume III/17, Subvolume a, 
Springer Verlag, Berlin, in particular chapter 2.16.3. 
Referring now to the several embodiments of the present invention, the 
first embodiment is described in connection with FIGS. 5 and 6. The layers 
of the laser diode 38 are not drawn to scale. Some of the dimensions are 
scaled in order to show all the details. A stripe-geometry InGaAsP laser 
diode 38 emitting at 1.3 .mu.m with double-heterostructure is shown in 
FIG. 5. This laser diode 38 includes different epitaxial layers 
subsequently grown on top of an n.sup.+- doped InP substrate 35, i.e., an 
n-doped InP lower cladding layer 32, followed by an undoped InGaAsP active 
region 31, and a p-doped InP upper cladding layer 30 which has been etched 
to form a shallow ridge. As indicated in FIG. 5, the active layer 31 
comprises several sublayers 31a-31g. The ridge itself is covered by a 
p.sup.+- doped InGaAs cap layer 36. The remaining surfaces of the upper 
cladding layer 30 are covered by an insulation layer 37 consisting of 
Si.sub.3 N.sub.4. Contact metallizations 33 and 34 are positioned on both 
sides of the laser diode 38. 
As shown in FIG. 6, the bandgap (E.sub.g) of the active region 31, as a 
function of z, is about 0.95 eV (.tbd..lambda..apprxeq.1.3 .mu.m), the InP 
cladding layers 30 and 32, with E.sub.g =1.35 eV 
(.tbd..lambda..apprxeq.918 nm), providing for potential barriers at both 
sides of the active region 31. According to the invention, the active 
region 31 comprises strained sublayers 31a-31g, providing for a modulated 
strain profile perpendicular to the epitaxial layers of the laser 38, 
i.e., parallel to the z-axis shown in FIG. 5. This modulated strain 
profile is illustrated in FIG. 6. The lattice constant of the InGaAsP 
active region 31 is a function of z (a=f(z)). The lattice constant can be 
modified by varying or modulating the composition of InGaAsP, (i.e., the 
so-called solid solution). The composition of compound semiconductors, 
such as InGaAsP, is usually indicated by indices x and y. Generally, 
quaternary compounds can be written as A.sub.1-x B.sub.x C.sub.y 
D.sub.1-y, InGaAsP being known as In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y. 
As with other solutions, the properties such as the bandgap and lattice 
constant of a solid solution depend on the composition of the components. 
A typical diagram, showing the bandgap (E.sub.g) versus lattice constant 
(a) of In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y, is shown in FIG. 7. This 
simplified diagram has been extracted from the previously mentioned 
reference by H. C. Casey, Jr., and M. B. Panish, cf. FIG. 5.1-2 thereof. 
The border lines 70-73 represent the following ternary compounds: In 
As.sub.u P.sub.1-y (70), In.sub.1-x Ga.sub.x As (71), Ga As.sub.y 
P.sub.1-y (72), In.sub.1-x Ga.sub.x P (73). The binary compounds InP, 
InAs, GaAs and GaP are placed at cross-points of these border lines. 
The lattice constant (a) of the active region can be varied without 
influencing the bandgap, as shown in FIG. 6, by subsequently growing 
InGaAsP sublayers 31a-31g having a different composition. The bandgap 
(E.sub.g in eV) and lattice constant (a in nm) as functions of x and y are 
defined by equations (1) and (2), respectively. 
EQU E.sub.g (x,y)=1.35+0.668y+0.758x.sup.2 +0.078y.sup.2 -0.069xy-0.322x.sup.2 
y+0.03xy.sup.2 (1) 
and (2) 
EQU a(x,y)=0.58688-0.04176x+0.01896y+0.00125xy 
With these equations, it is possible to generate a first matrix giving the 
bandgap (E.sub.g in eV) as a function of x and y, and a second matrix 
showing the dependency between the lattice constant (a in nm) of this 
quaternary compound and the indices x and y. As can be seen from these 
matrices, as well as from three-dimensional figures known in the art, cf. 
FIG. 5.5-5 on page 39 of H. C. Casey, Jr., and M. B. Panish, different 
compositions of InGaAsP exist having approximately the same bandgap, 
either with a tensile or a compressive strain. 
In the present case, a sawtooth-like strain profile has been realized by 
varying the InGaAsP composition of the active region 31. In the first 
embodiment this modulated strain profile is created by ramping (i.e., 
rapidly varying) the composition of the InGaAsP solid solution from 
In.sub.0.5 Ga.sub.0.5 As.sub.0.8 P.sub.0.2, a compound having a lattice 
constant of about 0.58167 nm, to In.sub.0.95 Ga.sub.0.05 As.sub.0.4 
P.sub.0.6, a compound having a lattice constant of about 0.5924 nm. These 
In.sub.0.5 Ga.sub.0.5 As.sub.0.8 P.sub.0.2 sublayers 31a, 31c, 31e, and 
31g (lattice constant a=0.58167 nm) have a maximum lattice mismatch 
(.increment.a/a) of -0.9% when they are grown on a binary InP substrate 
(a=0.587 nm) resulting in a tensile strain. The In.sub.0.95 Ga.sub.0.05 
As.sub.0.4 P.sub.0.6 sublayers 31b, 31d, and 31f are compressively 
strained with a maximum lattice mismatch of +0.9% (lattice constant 
a=0.5924 nm). The sawtooth-like strain profile shown in FIG. 6, is 
characterized in that the net layer strain, i.e., the total strain of the 
active region 31 is .+-.0%. This can be achieved by the sublayers 31a, 
31c, 31e, and 31g with tensile strain compensate the sublayers 31b, 31d, 
and 31f with a compressive strain, as indicated by the `+` and `-` signs 
in FIG. 6. More details of the first embodiment are illustrated in Table 
1. 
TABLE 1 
__________________________________________________________________________ 
Illustrative example of the first embodiment 
Doping 
Width 
Bandgap 
.DELTA.a/a 
Layer 
No. 
Material (cm.sup.-3) 
(nm) 
(eV) (%) 
__________________________________________________________________________ 
substrate 
35 InP 6 .times. 10.sup.18 (n) 
10.sup.5 
1.35 n.a. 
cladding 
32 InP 1 .times. 10.sup.18 (n) 
1500 
1.35 0 
active 
31 In.sub.1-x Ga.sub.x Asy.sub.y P.sub.1-y 
-- 150 
0.95 -0,9 -- +0,9 
region modulated 
cladding 
30 InP 1 .times. 10.sup.18 (p) 
1500 
1.35 0 
cap layer 
36 In.sub.0.5 Ga.sub.0.5 As 
2 .times. 10.sup.19 (p) 
500 
0.75 0 
__________________________________________________________________________ 
In a second embodiment of the present invention, a broad area DH laser 
diode 86 with modulated strain profile, is illustrated in FIG. 8. The 
layers of the laser diode 86 are not drawn to scale. Some of the 
dimensions have been scaled up in order to show all the details. This 
laser diode 86 includes a quaternary InGaAsP active region 81 with 
rectangular sawtooth-like strain profile parallel to the z-axis. This 
active region 81 is embedded in between a lower n-doped InP cladding layer 
82 and an upper p-doped InP cladding layer 80. The active region 81 with 
cladding 80 and 82, is grown on top of an InP substrate 85 being n-doped. 
Contact metallizations 83 and 84 are positioned on both sides of this 
broad area laser diode 86. 
The rectangular sawtooth-like strain profile perpendicular to the laser 
diode layers, i.e., parallel to the z-axis, is plotted in FIG. 9. As shown 
in this figure, the active region 81 comprises four In.sub.0.55 
Ga.sub.0.45 As.sub.0.75 P.sub.0.25 sublayers 81a, 81c, 81e, and 81g with 
tensile strain (.increment.a/a=-0.73%) and three In.sub.0.95 Ga.sub.0.05 
As.sub.0.4 P.sub.0.6 sublayers 81b, 81d, and 81f with compressive strain 
(.increment.a/a=0.91%). Similar to the first embodiment, the bandgap of 
active region 81 is approximately constant, namely, .apprxeq.1 eV. The 
thickness of the sublayers 81a-81g is chosen such that the stress of the 
compressive strained sublayers 81b, 81d, and 81f compensates the strain of 
the tensile strained sublayers 81a, 81c, 81e, and 81g, providing for an 
active region 81 with net layer strain .increment.a/a=.+-.0%. The second 
embodiment differs from the first in that it includes no sublayers with 
intermediate tensile strain. Additionally, no ramping is necessary from 
one composition to another. More details of the second embodiment are set 
out in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Illustrative example of the second embodiment 
Doping 
Width 
Bandgap 
.DELTA.a/a 
Layer 
No. 
Material (cm.sup.-3) 
(nm) 
(eV) (%) 
__________________________________________________________________________ 
substrate 
85 InP 3 .times. 10.sup.18 (p) 
10.sup.5 
1.35 n.a. 
cladding 
22 InP 1 .times. 10.sup.18 (p) 
2000 
1.35 0 
active 
81 In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y 
-- 160 
1 -0.73 -- +0.9 
region modulated 
cladding 
80 InP 1 .times. 10.sup.18 (n) 
2000 
1.35 0 
__________________________________________________________________________ 
The modulated stress profile of a third embodiment, e.g., having a 
sinusoidal or cosinusoidal strain profile, is illustrated in FIG. 10. This 
modulated strain profile is characterized in that the net layer strain is 
shifted towards compressive strain, i.e., .increment.a/a&gt;0%. 
The above structures have been tested and have shown a strongly improved 
threshold current density (J.sub.th) in comparison to conventional DH 
laser diodes either totally unstressed, or strained compressively, or 
under tensile strain. The reduction in threshold current density is about 
25% and demonstrates a significant improvement of the modulated strain 
profile heterostructure lasers. In addition, strain modulated 
heterostructure lasers, SH laser as well as DH lasers, are characterized 
by an increase of 70% in differential gain leading to an about 30% higher 
modulation speed. 
The bandgap is not required to be constant. The InGaAsP and other compound 
semiconductor systems provide a degree of freedom allowing to adjust the 
bandgap almost independently from the lattice constants. Embodiments are 
conceivable having a modulated strain profile and bandgap. Furthermore, 
the modulated strain profile does not even have to be an aperiodic 
function or an analytical function all together. To realize the previously 
mentioned embodiments, a growth process is required which allows spatial 
compositional variations leading to an appropriate modulated strain 
profile in a controlled environment. Suitable processes include MOVPE, 
MBE, CBE or VPE. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that the various changes in the form and detail 
may be made without departing from the spirit and scope of the invention.