Quantum well thermophotovoltaic energy converter

A thermophotovoltaic cell is provided containing strained or lattice-matched quantum wells that have a bandgap smaller than the bandgap of the InGaAs alloy. The alloy is lattice-matched to the substrate. These narrow bandgap quantum wells provide more efficient conversion of IR emission from a black body or other emitter by converting energy from a wider range of wavelengths than a conventional single junction cell. The thickness of the quantum well region and the individual thickness of the individual quantum wells are chosen to avoid lattice mismatch defects which cause degradation of thick conventional lattice mismatched devices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to conversion of infrared radiation to electrical 
energy. More particularly, an indium-gallium-arsenide p-i-n photovoltaic 
cell is modified by insertion of strained quantum wells while avoiding 
lattice defects to increase conversion efficiency. 
2. Description of Related Art 
Thermophotovoltaic (TPV) energy conversion is a potentially environmentally 
friendly approach to achieve high efficiency, compact and reliable sources 
of electrical energy. In TPV conversion, a source of energy such as 
concentrated sunlight, nuclear power, fossil fuel, or a radioisotope heat 
source is used to heat an intermediate thermal emitter. The emitter then 
radiates photons which impinge on a photovoltaic cell. 
There is now renewed interest in TPV energy conversion using nuclear energy 
sources or combustion driven systems operating at low temperatures 
(&lt;1500K). Possible uses include power generation for deep space 
exploration, silent portable gas or natural gas operated generators, 
non-polluting and silent energy generation for natural gas operated 
vehicles, power cogeneration (using conversion of waste heat into 
electricity), and peak electricity for power utilities or household use. 
Common to all TPV systems operating at moderate emitter temperatures is the 
desire for low-bandgap photovoltaic devices that can convert efficiently 
the infrared-rich spectrum emanating from the emitter. In conventional 
photovoltaic cells the electron and hole result from absorption of a 
photon with energy above the bandgap. These carriers rapidly thermalize to 
their respective band edges. The fundamental efficiency limitation in a 
conventional cell results from the trade-off between a low bandgap, which 
maximizes light absorption and hence the output current, and a high 
bandgap, which maximizes output voltage. The spectral energy peak of a 
1200 C (1500K) black body falls at a wavelength of 2 microns. As a result, 
for silicon photovoltaic devices only a very small portion (&lt;2%) of the 
emitted energy is above the bandgap and is available for PV conversion. 
Therefore the use of narrower bandgap semiconductors has been identified 
as a necessary condition to achieve higher efficiencies. Most of the 
existing development work is concentrated around two semiconductor 
systems: the ternary InGaAs cells fabricated on InP substrates and GaSb 
cells and the GaInAsSb quaternary alloys fabricated on GaSb substrates. 
However, currently, GaSb based technologies are only available in 2-inch 
diameter wafers, while the 3-inch diameter wafers used for InGaAs cells 
have twice the area, resulting in twice as many cells per processed wafer. 
High quality InP wafers are available from several competitive vendors due 
to the telecommunication industry's need for 1.5 microns detectors and 
lasers, while GaSb wafer production, limited to a much smaller TPV market, 
is still in a development stage. Consequently, U.S. companies and major 
government laboratories involved in TPV have mainly concentrated their 
research effort on the development of low-bandgap (0.55 to 75 eV) InGaAs 
devices fabricated on InP substrates. The initial effort was directed 
toward the fabrication of TPV cells using Ga.sub.0.47 In.sub.0.52 As with 
an energy bandgap of 0.75 eV and a material lattice constant matched to 
InP. These cells exhibit excellent PV characteristics; however, their 
efficiency for a 1500K spectrum is limited by transparency losses. The 
most recent research approach has promoted the use of narrower bandgap 
InGaAs (0.6-0.55 eV). In fact, for a 0.55 eV cell 35% of the black body 
energy is from photons with energy above the bandgap instead 14% for the 
Ga.sub.0.47 In.sub.0.52 As (0.75 eV) cell. However, these narrower bandgap 
cells are fabricated with materials presenting 1-2% lattice mismatch with 
respect to the InP substrates. The large lattice mismatch between the 
substrate and the device material leads to the generation of dislocations 
for thickness exceeding a few hundreds of Angstroms. A conventional P/N 
junction cell requires an active area thickness larger than 2 microns. The 
presence of dislocations results in a reduction of the minority carrier 
lifetime and hence leads to a poor performance. In order, to partially 
reduce the defect density in the device active region, 3-4 micrometer 
step-graded buffer layers or superlattices (U.S. Pat. No. 4,688,068) can 
be deposited prior to the active device growth. Incorporating these 
additional steps increases substantially the epitaxial process cost. 
Furthermore, the remaining dislocation density (&gt;10.sup.8 cm.sup.-2) may 
still lead to the aging of the TPV cell. 
Recently, in the context of solar cells, it has been proposed that the use 
of periodic layers (quantum wells) in the active region of the device can 
enhance conversion efficiency. (U.S. Pat. No.5,496,415) What is needed is 
a process which both increases IR conversion efficiency of a conventional 
InGaAs-lattice-matched-to-InP thermophotovoltaic cell and prevents the 
generation of dislocations in the device. 
SUMMARY OF THE INVENTION 
A thermophotovoltaic (TPV) converter is provided where one or several thin 
(several nm) strained or lattice-matched quantum wells having a bandgap 
smaller than that of the In.sub.0.52 Ga.sub.0.48 As alloy are introduced 
between the conventional p-conductivity and n-conductivity cell which is 
lattice matched to the substrate material. The presence of these narrow 
bandgap quantum wells allows for more efficient conversion of the IR 
emission emanating from a black body or a selective emitter over a wider 
range of wavelengths than a conventional single junction cell and 
decreases transparency losses of the conventional cell. The approach hence 
may be used to increase the cell current output and efficiency and make it 
comparable or better than that of a lattice mismatched bulk-like 
(0.55-0.65 eV) InGaAs cell. The total thickness of the quantum well region 
and the individual thickness of the individual quantum wells (few nm) can 
be chosen so that no lattice mismatch defects are generated. This approach 
circumvents the crystalline quality and electronic performance degradation 
associated with the fabrication of thick conventional lattice mismatched 
devices. Materials are selected based on physical properties determining 
energy bandgap and low-defect crystal growth. 
A method of fabrication of photovoltaic devices is provided where ternary 
strained and lattice matched (In, Ga) As alloys are used as well material 
in the quantum well region of a conventional (In,Ga) As cell 
lattice-matched to an InP substrate. Strained narrow bandgap In.sub.x 
Ga.sub.1-x As/In.sub.0.47 Ga.sub.0.53 As (x&gt;0.6) multiple quantum wells 
(MQW) are introduced within the intrinsic region of a conventional 
In.sub.0.47 Ga.sub.0.53 As p-i-n cell lattice-matched to InP. An 
appropriate choice of well and barrier thickness and number of wells in 
the i-region maintains the pseudomorphism (i.e., lack of crystal defects) 
and lattice-matching to InP, while the presence of narrow bandgap wells 
extends photon absorption up to that of confined energy states in the 
wells. For low-temperature black body emitters (1000-1500K) this new 
device conversion efficiency will exceed twice that of its conventional 
counterpart. The method of this invention can be used to select other 
material systems for thermovoltaic converters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, strained, thin In.sub.x Ga.sub.1-x As layers (quantum 
wells) are shown in the intrinsic region of a 0.75 eV InGaAs p-i-n cell. 
This materials system is shown as an example of the devices of this 
invention. The apparatus and method of this invention can be used with a 
wide variety of materials systems that have the required properties set 
out below. The cell is fabricated on substrate 12, the top layer of which 
may be lattice-matched to indium phosphide and the bottom layer of which 
is back contact 10 which is preferably a reflective or mirror surface. 
Alternatively, an in-situ mirror may be placed between the active layers 
and the cell substrate, as disclosed in U.S. Pat. No. 5,626,687 to 
Campbell. Back surface field 14, base 16, intrinsic region 18 and emitter 
20 are shown. Other alloys may be used if lattice-matched to substrate 12. 
In the thermophotovoltaic cell (TPV) of this invention, intrinsic region 
18 includes one or more quantum wells 19. Contact layer 22, front contact 
24 and window or antireflective coating 26 complete the TPV. 
A method for predicting the optical and electrical characteristics of such 
devices for low-temperature (for example, approximately 1000 to 
2000.degree. K) TPV applications is described below. The InGaAs material 
in the quantum wells (QWs) is undergoing compressive strain. Therefore, it 
is important to identify both the strain modifications to the band 
structure and the design parameters to avoid lattice relaxation. A method 
for assessment of possible QW thickness and number of QWs is provided. 
Results of the assessment can be checked experimentally using techniques 
such as X-ray diffraction or transmission electron microscopy to determine 
the absence of lattice relaxation using techniques described below. A 
description of the modeling of the optical properties and device 
characteristics of the MQW TPV cell follows. 
The ground confined state energies for different compositions can be 
calculated using well-known techniques set out in the paper "Modeling p-i 
(Multi Quantum Well)-n solar cells: a contribution for a near optimum 
design." (IEEE, First World Coef. On Photovoltaic Energy Conversion, 
1994). Calculations show that with 90% Indium composition and 60 .ANG. 
wells, bandgaps can be reduced to a level allowing absorption of 
wavelengths up to almost 2.4 microns. The MQW region contribution to the 
photocurrent depends on the diffusion length of the majority carrier 
photogenerated in a well. This diffusion length is expressed as 
L=.mu.F.tau., where F is a perpendicular electric field in the MQW created 
by the ionized impurities at the junction, .mu. is the carrier mobility, 
and .tau. its recombination time. The effective carrier mobility for 
perpendicular transport in microstructures depends on several processes. 
Most importantly, the effective mobility depends on thermionic emission, 
which dominates when the barrier is thick; and multi-hopping when the 
barrier is thin (phonon-assisted tunneling). The effective mobility of 
carriers (electrons and holes) versus barrier and well widths, electric 
field, and temperature can be estimated. Following the modeling of MQW 
photovoltaic cells, we estimate the diffusion length of carriers in the 
intrinsic zone (x.sub.i) as 
EQU L.sub.e =10.sup.-9 cm.sup.2 V.sup.-1 .multidot.F and L.sub.b 
=4.multidot.10.sup.-9 cm.sup.2 V.sup.-1 .multidot.F (Eq. 1) 
where F is the electric field in the intrinsic region in V/cm) for the 
regions where the quasi-Fermi level is close to the valence or conduction 
band, respectively. For the typical electric field of 15 kV/cm in the MQW 
region of a photovoltaic cell, this gives L.sub.e =0.15 and L.sub.b =0.6 
microns. (L.sub.e and L.sub.b refer to emitter and base, respectively. 
FIG. 2 shows the normalized external quantum efficiency of a MQW TPV cell 
as a function of incident photon wavelength, for a cell having 20 periods 
of 90 angstrom wells with 190 angstrom barriers between, with 90 per cent 
indium in the well, a 0.2 micron emitter and 2.5 micron-thick base. The 
spectral response of the i(MQW) zone is obtained by solving the continuity 
equation provided in the paper by Leavitt and Bradshaw (App. Phys. Lett. 
59, 19 (1991). 
Several mechanisms contributing to the dark current of a p-i(MQW)-n 
photodiode were considered: a diffusion current I.sub.s due to majority 
carriers diffusing through the junction and recombining in the opposite 
quasi-neutral region; a generation-recombination (GR) current associated 
with Shockley-Read-Hall recombination on impurities in the depletion and 
intrinsic regions; and a tunneling current due to the probability of 
band-to-band tunneling of carriers through the junction. This current, 
usually neglected in moderately doped diodes, becomes significant in a MQW 
under an electric field. This effect becomes more noticeable due to the 
reduced bandgap of a MQW region and is also influenced by the barrier 
width. 
The device characteristics such as open circuit voltage(V.sub.oc) and short 
circuit current (I.sub.sc) were calculated as a function of In 
composition, well/barrier thickness and number of wells in the MQW region 
for a 1000 to 1500 K black body emitter. It is shown that the insertion of 
narrow bandgap wells extends the spectral response toward the infrared 
(FIG. 2). A rough estimate shows that for a 1500 K black body up to 40% of 
the incident power is available for conversion by such a MQW device. 
Therefore the short circuit current I.sub.sc of the p-i(In.sub.x 
Ga.sub.1-x As/In.sub.0.53 Ga.sub.0.47 As MQW)-n cell may be substantially 
increased compared to that of a baseline p/n In.sub.0.53 Ga.sub.0.47 As 
cell. Device open circuit voltage remains comparable (the voltage drop 
associated with carrier diffusion, tunneling and reduced mobilities in the 
well is compensated by the operation at a higher current output). 
The introduction of additional QW periods in the intrinsic region increases 
photocurrent up to a point and a saturation is reached at around 20 
periods, as shown in FIG. 3. This can be explained by a decrease in the 
electric field (in the depletion region) and carrier collection due to the 
increasing intrinsic region thickness. The I.sub.sc of a p/n single cell 
is shown for reference (zero number of QW periods). This figure shows the 
short circuit current I.sub.sc (A cm.sup.-2) as a function of number of 60 
.ANG. In.sub.0.9 Ga.sub.0.1 As wells in the i-region, illuminated with a 
1500 K black body. Solid circles show results of a calculation for a 
p-i(MQW)-n with no back surface reflector. Open squares show results of a 
calculation for a 100% back surface reflector. Solid square shows results 
of calculation of I.sub.sc of a conventional p/n cell. 
Since a metalization at the back of polished InP substrate acts as an 
efficient light reflector, we have estimated I.sub.sc for both 100% 
reflective and non-reflective substrates (FIG. 4). For the sake of 
comparison in our calculations, we fixed the total period thickness (well 
and barrier) at 0.028 micron. The I.sub.sc general behavior with changing 
number of QW periods should be valid for wells of different thicknesses, 
provided the period thickness is fixed and that the barrier thickness 
prevents coupling of the wave functions of individual wells. 
The calculated short circuit current versus InGaAs composition in the well 
shows substantial enhancement with increasing Indium concentration. FIG. 5 
shows the short circuit current I.sub.sc as a function of Indium 
composition in InGaAs well, for a 20-period 90 .ANG. well/190 .ANG. 
barrier MQW cell for a.1500 K black body emitter. 
The open circuit voltage of a MQW cell is expected to be less than that of 
a baseline cell because of the MQW region dark current increase. However, 
our calculations showed comparable or slightly higher V.sub.oc for the MQW 
cells when increasing the In composition in the well. In fact, the voltage 
drop associated with dark current in MQW is compensated by the operation 
at a higher current due to increased photoabsorption. 
As a result, the projected efficiency of this 20-period 90 .ANG. well/190 
.ANG. barrier In.sub.0.9 Ga.sub.0.1 As well MQW cell (9% for a 1500 K 
black body) can exceed twice that of a single p/n In.sub.0.53 Ga.sub.0.47 
As photovoltaic cell (4.5%). 
Returning to the calculations of the short circuit current, we can see that 
90 .ANG. is a critical thickness for 84% indium composition in InGaAs. 
Therefore, we project the corresponding cell I.sub.sc to be limited to 7.0 
A/cm.sup.2 at 6.1% efficiency. This can be further increased by doubling 
the photon path through the MQW region, e.g. using a reflective 
metalization on the back of the device. 
We have presented the results calculated for a 1500 K black body. To give a 
more complete picture, for a 0.75 eV bandgap p/n cell, about 3% of the 
power of a 1000 K black body is emitted above its bandgap. This ratio 
increases to 16% for a 1500 K, and 35% for a 2000 K black body. Assuming 
no transparency and recombination losses, it leads to a theoretical 
efficiency limit of 2%, 13% and 26%. For a 0.47 eV bandgap, which is close 
to the excitonic absorption edge of a strained 90 .ANG. InAs well with 
In.sub.0.53 Ga.sub.0.47 As barriers, these figures are 20%, 48% and 67% of 
the black body radiant power, respectively. The upper limit of achievable 
cell efficiency for this MQW approach are projected to be 16%, 34% and 42% 
for the 1000, 1500 and 2000 K black body respectively. 
For typical low temperature TPVs, the conversion efficiency of strained 
p-i(In.sub.0.53 Ga.sub.0.47 As/In.sub.x Ga.sub.1-x As MQW)-n cells is 
predicted to exceed twice that of their conventional counterparts. This 
results from improved short circuit current, with no degradation of the 
open circuit voltage. The expected increase is limited by the number of QW 
periods that can be grown without relaxation. It is possible to avoid 
strain limitation on the number of periods by using In.sub.x Ga.sub.1-x 
As, x&lt;0.53, to balance the well strain. The device concept and model 
described above can be extended to other TPV semiconductor photovoltaic 
material systems. 
The lattice misfit which occurs during hetero-epitaxy imposes critical 
limitations on the well thickness and the number of periods which can be 
grown before the onset of dislocation formation in the epilayer. The 
critical thickness is estimated by equating the force exerted by misfit 
strain to tension in the misfit dislocation line. For a single epitaxial 
film on a substrate of an infinite thickness, the critical thickness 
h.sub.c for the epilayer grown on (001) InP can be found by solving the 
following equation: 
##EQU1## 
It was assumed in the derivation that the maximum value of the strain 
.epsilon. is equal to the lattice misfit .function.=(a.sub.s 
-a.sub.0)/a.sub.0 of the overgrowth layer. Here .nu. is the Poisson ratio, 
a.sub.s and a.sub.0 are the substrate and overgrowth lattice constants 
respectively, and .vertline.b.vertline.=a.sub.0 /.sqroot.2 is the 
magnitude of Burger's vector of dislocation for the (001) growth plane. 
Critical thickness dependence on indium composition for the epilayer grown 
on (001) InP is represented in FIG. 6. This figure shows critical 
thickness (.ANG.) as a function of Indium composition in In.sub.x 
Ga.sub.1-x As grown on (001) InP substrate. 
For the individual quantum well buried under a barrier layer lattice 
matched to the substrate, the critical thickness is different from that of 
a single epitaxial layer. Since the well material strain is partially 
accommodated by the barrier, the well material critical thickness for the 
(001) substrate orientation in the MQW is higher than for a single 
epitaxial layer: 
##EQU2## 
The thicknesses calculated according Equations 2 and 3 are compared in 
FIG. 6. For a given In composition x in the epitaxial In.sub.x Ga.sub.1-x 
As layer, the thickness has to be less than the critical value. A 
structure containing several strained layers may impose further 
restrictions. 
For a MQW made up of alternating strained wells and unstrained barriers, in 
addition to the well thickness limitation the overall (average) strain in 
the heterostructure imposes a limitation on the number of periods. To 
estimate the maximum number of periods in a MQW, the whole stack of layers 
is treated as a single epitaxial layer, with the average lattice constant 
a defined by well (L.sub.W) and barrier (L.sub.B) layer thicknesses and 
shear moduli G.sub.W and G.sub.B : 
##EQU3## 
where a.sub.W and a.sub.B are well and barrier lattice constants. (In our 
case a.sub.B =a.sub.S, the substrate lattice parameter) 
A force due to the misfit strain in N periods is given by [10, 11]: 
##EQU4## 
where G is the average shear modulus of a MQW structure, and .epsilon. is 
strain. 
For the dislocation line starting at the first interface of a stack of N 
periods the average tension is approximately: 
##EQU5## 
Equating F.sub..epsilon., a force accumulated in the multiple periods due 
to the misfit strain, to the tension in the misfit line F.sub.l, and 
substituting .epsilon.=.function.=(a.sub.S -a)/a, we obtain this equation 
to solve for the critical number of periods N.sub.C : 
##EQU6## 
This estimate shows that it would be possible to realize roughly 10-period 
structures. However, the number of periods can be substantially increased 
if Q.S. are alternated with tensively strained 
In.sub.x Ga.sub.1-x As barriers (x&lt;0.5), whose thickness is 
##EQU7## 
where indices W and B stand for well and barrier. FIG. 7 shows the 
critical number of periods as a function of Indium composition in a well 
for a 60 .ANG. well with 100, 300 and 500 .ANG. barriers. 
The strain-induced modifications of the (001) InGaAs band structure in the 
MQW were modeled using the parameters of Krijn (Semicond. Sci. Tech. 6, 
27-31 (1991)). The unstrained ternary compound bandgap was interpolated 
from the binary components. Following Pikus and Bir's theory of 
deformation potential and spin-orbit interaction (Symmetry and 
Strain-Induced Effects in Semiconductors (1974), Van der Walle's `model 
solid` theory (Phys. Rev. B 39 (3) 1871 (1989)) was applied in describing 
the strain dependence of the valence and conduction band edges of strained 
InGaAs alloy. The effective masses for electrons and heavy and light holes 
of the unstrained material were linearly interpolated from GaAs and InAs 
values. The presence of (001) uniaxial compressive strain in the InGaAs 
results in an increase of the apparent bandgap of the semiconductor. The 
tetragonal deformation results in a splitting of heavy hole and light hole 
(.GAMMA..sub.7) valence band: 
##EQU8## 
where E.sub.S is the splitting energy, .beta. is the tetragonal 
deformation potential, S.sub.11, and S.sub.12 are the elastic compliances, 
and .epsilon.=(a.sub.s -a.sub.0)/a.sub.0 is the magnitude of the in-plane 
strain. The modifications of the light hole and electron effective masses 
induced by strain along the quantification axis of the quantum well should 
also be taken into account in the calculation. This was followed for the 
electron, but neglected for the light hole, since the well for it is very 
shallow and its ground state is resonant with the barrier energy. 
The calculation of the MQW structure absorption coefficient is based on 
determining the confinement energies of the electrons and holes in quantum 
wells and on estimating the absorption rate. The confined energies of the 
electrons and holes are computed within a one-band model, since it can be 
shown that in the present case the correction from band non-parabolicity 
to the confined energies can be neglected. The exciton binding energies 
are calculated using the method of Leavitt and Little (Phys. Rev. B 42, 
1177 and 1184 (1990)) and parameters in Properties of lattice matched and 
strained Indium Gallium Arsenide, INSPEC, London (1993). The absorption 
coefficient is calculated from Fermi's golden rule (Bastard, Wave 
Mechanics applied to semiconductor heterostructures. 1990). The first 
three confined energy states and excitonic effects for the ground state 
are accounted for. 
For the assessment of the lowest energy absorption edge achievable in a MQW 
structure, the QW confined energy as a function of the well thickness for 
different indium compositions. was plotted. The QW critical thickness was 
estimated using Equations 2 and 7. The ground confined state energies for 
different compositions are shown on FIG. 8. As can be seen, with 90% 
Indium composition and 60 .ANG. wells, almost 2 microns confinement can be 
reached. 
The following additional TPV cell design and modeling parameters were used 
in the calculations above: 
TABLE I 
______________________________________ 
Thickness, emitter (microns) 
0.2 
Thickness, base (microns) 2.5 
Doping, emitter (cm.sup.-3) 2 .multidot. 10.sup.18 
Doping, base (cm.sup.-3) 10.sup.17 
Period, well + barrier, thickness 0.028 
(microns) 
Generation-recombination time, (s) 10.sup.-6 
Surface Recombination Velocity 10.sup.4 
(cm.sup.-2 s.sup.-1) 
In.sub.X Ga.sub.1-X As barrier In composition 0.53 
In.sub.X Ga.sub.1-X As well In composition 0.6-0.9 
Temperature (K.) 300 
______________________________________ 
In the approach of this invention, strained narrow bandgap In.sub.x 
Ga.sub.1-x As/In.sub.0.53 Ga.sub.0.47 As (x&gt;0.6) multiple quantum wells 
(MQW) are introduced within the intrinsic region of a conventional 
In.sub.0.53 Ga.sub.0.47 As p-i-n cell lattice matched to InP. The TPV cell 
parameters are as follows: 
TABLE II 
______________________________________ 
Most 
AMETER possible Preferred preferred 
______________________________________ 
Growth temperature (CBE 
300-650 C. 400-550 450-520 
process) 
Thickness, emitter (microns) 0.02-1 0.05-0.5 0.2 
Thickness, base (microns) 0.2-6 1-3 2.5 
Doping, emitter (cm.sup.-3) 10.sup.17 .multidot. 10.sup.19 1-5x 
.multidot. 10.sup.18 2 .multidot. 
10.sup.18 
Doping, base (cm.sup.-3) 1-100 .times. 10.sup.16 1-50 .times. 10.sup.16 
10.sup.17 
Period, well + barrier, 0.028 
thickness (microns) 
Well Thickness 1-20 nm 1-9 nm 1-5 nm 
Barrier Thickness 5-100 nm 10-50 nm 15-30 nm 
Number of wells 1-50 1-30 1-15 
In.sub.X Ga.sub.1-X As barrier In 0.35-0.55 0.45-0.53 0.51-0.53 
composition 
In.sub.X Ga.sub.1-X As well In 0.6-1 0.6-0.9 0.7-0.9 
composition 
______________________________________ 
The well thickness and composition are adjusted to provide maximum current 
without relaxation of the structure 
Epitaxial growth runs of TPV cells studied in this example were 
accomplished by Chemical Beam Epitaxy (CBE) using trimethylindium (TMIn) 
and pre-cracked arsine (AsH.sub.3) and phosphine (PH.sub.3) as growth 
precursors. Si and Be solid sources were used as n-type and p-type dopants 
respectively. However it is believed that any conventional techniques 
III-V epitaxial technique that allows growth of p- and n-type materials 
and quantum wells (such as MOCVD, VPE, MBE or any combination of these 
techniques) will be adequate to fabricate these devices. Such processes 
are described in U.S. Pat. No. 5,407,491, which is incorporated herein by 
reference. One of the advantages of Chemical Beam Epitaxy over MOCVD 
(MOVPE) is the lower growth temperatures allowing sharper interfaces and 
reducing dopant cross diffusion This issue is crucial to avoid the 
degradation of the MQW region during the subsequent growth of the InGaAs 
emitter/InGaAs contact layer. Structures were grown in the 480-520.degree. 
C. temperature range on highly S-doped InP (001) substrates at a typical 
growth rate of 1 .mu.m/h (about 1 monolayer/sec.). The undoped 1-10 nm QWs 
were incorporated in the intrinsic region of p-i-n structures with each 
well separated by 10-1000 nm thick or thin InGaAs lattice matched to InP 
barrier to prevent the effect of wave function coupling on carrier 
transport. The well and barrier thickness and number of periods were 
chosen so that critical thickness was never exceeded. Interruption growth 
intervals were monitored and improved in terms of species transient by 
using mass spectrometry and in situ real time reflection high-energy 
electron diffraction (RHEED) observations. Between the growth of 
successive layers, the TMIn and TMG flow were interrupted. To prevent 
surface decomposition, the AsH.sub.3 flow was maintained at a constant 
rate throughout the growth. Under the optimum growth conditions, a two 
dimensional 2.times.1 RHEED pattern was maintained throughout the growth 
the MQW region. The QW region is preferably grown at a growth rate of 
0.3-3 micron per hour and most preferably at a rate of 0.7-1 microns/hour. 
Following the growth process non-alloyed Au/Ge and Au metallic layers, 
deposited by vacuum evaporation, were used as n-type (on substrate) and 
p-type ohmic contacts respectively. The Au-top contact grid was obtained 
using a lift off technique and an intermediate InGaAs layer (lattice 
matched to InP) was used to lower the top contact resistivity. the InGaAs 
contact layer was removed from the cell active area using H.sub.2 SO.sub.4 
/H.sub.2 O.sub.2 /H.sub.2 O solution. The InP window layer between InGaAs 
emitter and contact layer act as a selective etch stop. A mesa etching of 
the structure provided a total area per cell of 5.times.5 mm.sup.2. The 
grid shadowing was estimated to be about 10%. A conventional single or 
bilayer ARC can be implemented to reduce reflection losses at the device 
top surface. The device described here is fabricated on conductive surface 
growth on insulating substrates and processing the device in MIM is also 
possible using standard MIM recipes developed for lattice matched InGaAs 
cells 
The proposed approach can be extended to the use of any type of narrow 
bandgap conventional p-n cell such as GaSb, GaInAsSb, InAsP, InGaAs, 
InGaAsP, and InGaAsN. The well material can be substituted by InGaAsN, 
GaSbN, GaInAsSb, InAsPN InGaAsSb or any combination of these materials 
that will allow the growth of a MQW region that is pseudomorphically 
strained to the conventional device base region 
Modeling data indicate that for a typical 1200 C black body radiation the 
single junction InGaAs quantum well solar cell will reach a current output 
generally observed for narrow bandgap cell such as Ga.sub.x In.sub.1-x As 
(x&lt;25%) with a voltage comparable (or greater) to the one of a 
conventional 0.75 eV Ga.sub.0.47 In.sub.0.52 As cell. In addition, the 
existing InGaAs TPV cells are already manufactured with techniques that 
are compatible with Multi-Quantum Well (MQW) growth so quantum wells can 
be added during growth with little expense. The increase in the efficiency 
(by nearly a factor of two) will then divide the cost of the kW/hours and 
the size of modules by a factor of two compared to the present art. 
The anticipated advantages of this device over the present art include: 
higher efficiencies (due to extended spectral photoconversion), lattice 
matching to InP, no or low dislocations density (reduced dark current and 
improved survivability), compatibility with standard 0.74 eV InGaAs 
technology, no limitation on the window material thickness and reduced 
processing cost compared to heteroepitaxial InGaAs low bandgap cells. 
Table III shows a comparison of various existing technologies and the 
anticipated characteristics of the present invention. 
TABLE III 
__________________________________________________________________________ 
This 
technology 
InGaAs 
Technology Silicon GaSb InGaAs InGaAsSb InGaAs MQW 
(bandgap) (1.1 eV) (0.74 eV) (0.75 eV) (0.55 eV) (0.55 eV) (0.74-0.49 
eV) 
__________________________________________________________________________ 
Wafer size (2 inch) 
3 inch 
(2 inch) 
3 inch 3 inch 
Energy density for 2% 15% 14% Up to 35% Up to 35% 35%-50% 
absorbed photons 
(1500K) 
Epi cost NA NA Moderate High, Need High need Moderate 
for very thick for thick 
active area graded buffer 
Window/ NA None/ InP window/ None/ InAsP InP window, 
(recombination (30-50%) (5-10%) (50%) window (5-10%) 
losses) (10-20%) 
Operating +(&gt;0.3 v) +(&gt;0.3) (&lt;0.3 V) -(&lt;0.3) +(&gt;0.3) 
voltage 
Possibility of No Yes No Yes Yes 
fabricating 
Monolithically 
integrated 
modules 
Projected &lt;&lt;1% 4-5% 4-5% 7-8% 9-12% 5-16% 
Efficiency at 
cell level 
For 1500K black 
body radiator 
Defect None None none Highly None 
density/aging possible 
Watt/cm2/cost None 1 Normalized 1.5 2 
to 1 
__________________________________________________________________________ 
The proposed device is expected to enhance the reliability and the 
efficiency of the thermophotovoltaic converters without the usual 
shortcomings of the existing heteroepitaxial &lt;0.6 eV InGaAs devices 
(increased epitaxial process cost, aging, defects etc.). If demonstrated 
this technology will result in a revolutionary improvement in 
survivability, performance and manufacturability for nearly all-low 
temperature thermophotovoltaic commercial applications. Such application 
includes the 1100 C radioisotope heat sources for deep space missions, and 
variety of military and civil terrestrial applications ranging from 1200 
solar thermophotovoltaics for commercial power grid use to small natural 
gas TPV portable generators for home use. 
The first strained MQW TPV cell structure has been fabricated. As could be 
deduced from the x-ray characterization data, a 15 period 60/300 Angstrom 
well/barrier with 0.79 Indium composition in the well successfully 
maintained pseudomorphism. Similar data were provided for a MQW solar cell 
in U.S. Pat. No. 5,851,310.