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KAPL-P-000047 (K96048)
RECENT PROGRESS IN INGAASSB/GASB TPV DEVICES
Z.A. Shellenbarger, M.G. Ma&, L.C. DiNetta, G.W.
Charache
May1996
NOTICE
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that its use would not infringe privately owned rights.
KAPL ATOMIC POWER LABORATORY SCHENECTmY, NEW YORK 12301
Operated for the U. S. Department of Energy by KAPL, Inc. a
Lockheed Martin company
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This report was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor any agency thereof, nor any of their employees,
makes any warranty, express or implied, or assumes any legal
liability or responsibility for the accuracy, completeness, or use-
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Recent Progress in InGaAsSbGaSb TPV Devices Z.A. SHELLENBAROER,
M.G. MAUK, and L.C. DINETI-A
AstmPower, tnc. Solar Park, Newark, DE 19716-2CiCO G.W.
CHAF~ACHE
Lockhsed Martin Corp. P.O. Box 1072, Schenectady, NY I230 7 -1
072
SUMMARY
AstroPower is developing JnGaAsSb themphotovoltaic devices. Thls
photovoltaic cell is a two-layer apitaxfal InGaAsSb stfucture
formed by liquid-phase epitaxy on a GaSb substrate. The (direct)
bandgap ofthe fn,,Ga&,,Sb, atby is 0.50 to 0.55 eV. depending
on its exad alloy composition (x,y); and is closely latticematched
to the GaSb substrate. The use of the quaternary alfoy. as opposed
to a ternary alloy - such as, for example. InGaAdtnP -permits low
bandgap devtCes optimized for 1000 to 1500 'C tbermal sources mrh,
at the same time. near-emct lattice matching to the GaSb substrate.
Latt:ce matching is important since even a small degree of lattice
mismatch degrades device performance and reliabiiiy and increases
processing complexity.
internal quantum eflfcianctes a8 high 8s 95% have been measured
at a wavelength of 2 microns. At 1 micron wavelengths, internal
quantum efficiencies of 65% have been observed. The opencfrcuit
voltage at cunents sf 0.3 Akm' is 0.220 vofts and 0.280 V for
current densities of 2 Ncm'. Flll faciors of 56% have bean measured
at 60 mAlcm'. However. as current dens& increases them is sorne
decrease in fifi factor. Our results to data show Ut& the
GaSb-based quaternary compounds provide a viable and high
performance energy mnvmton solutfon for tnermophotovoitatc systems
operating with 1000 to 1500 'C source temperatures.
1. lNfRODUCTION
We report our latest reaults o n InGeAsSb thermophotovoltalc
(TPV) cella. TPVs are p-n junction semiconductor devices that
convert photons ernitled by a heated source directly into electkai
power. For T?V Systems utilizing therrnaf radiation from an omittar
hestad at 1000 to 1500 'C, there is a need for low-bandgap cells
with a hhh spectral response in the range of 1500 to 2500 nm
wavelength. this unplles a T W cell with a bandgap of -0.5 eV. One
important potenflat apph!ion is the radioisotap General Purpose
Heat Source (GPHS) where 1100 'C bhckbody radiation bn be used for
themnophotovoltaic energy conversion. In this paper we describe
high4flaency TPV devices based on lattice ,matched
InoMG~p2AS&b0.03 (EG = 0.53 eV) epitaxial layers on GaSb
SUbStrat~S. To our knowledge. this is the Srst report of the
InGaAsSb quaternary alloy apclied to TPV devices
Several theoretical studies have indicated that photovoltaic
cells based on ths InG3AsSb quaternary al!cy are gcod andidates for
TPV applications :ha: require high spadral response In the 1500 to
2500 nm waveleng:h range. Depending cn Its aiioy composition ( x ,
y ) . t h e direct bandgap of the In,.,Ga&,.,Sb, alloy varies
frcrn 0 18 eV (IflSb) to t 43 eV (GaAs) The quaternary alloy can be
closely tanicematched to the GaSb sabstrate pravrded the
composition IS restrained lo values Such that y = 0 7 + 0 9 x With
thrs lantce matchmg condl::cr:. 1P.4
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bandgap of the quaternary alloy ranges h m approximately 0.3 to
0.7 eV. However, there ts a further lirnltatjcn due to a wide
solid-phase miscibility gap in the quaternary at typical growth
temperatures The mIscibrlity gap ~vldently precludes bandgaps in
the range of 0.35 to 0.5 eV. Therefore, for the spectral range of
interest, we gg~ume the lowest attainable bandgap is 0.50 to 0.52
eV. This bandgap range conesponds to an o p b i absorptfOn edge of
2380 lo 2480 nanometers.
It is worth emphasizing that be use of the q&J&maly
alloy. as OpPOSed to a ternary alfoy-tiuch as. !or exarnpte.
InGaAs-provides the needed bafidgap with. at the same time,
nearaxact lattice matching to the GaSb substrate. LatUce-matching k
important aince even a Small degree of lattice mismatch degrades
device prfomance and relisbility Although there afe epitaxy
techniques to partially ameliorate effects associated with lattice
mismatch of ternary ahoy layers on binary subs.trateS (64.
defect-filtering superlattices. InktNpkd srct& regimens, etc.).
We believe the use of the quatarnary alley to avoid lattice
mismatch altogether IS a simpler ar.d more effective approach.
The TPV device we are making Is a hvo-layer epitaxial InGaAsSb
structure formed by liquid-phase epitaxy on a GaSb substrate at a
gr&h temperature of 515 "C. Liquid-Phase Epitaxy (LPE) is 8
w9lkStabllshed technology for Ill-V compound semiconductor devfces.
A major advantage of W E for this application is the high mazerfat
quallty, and more specifically, the long minoriiy canter diffusion
lengths, that can be achieved. nis results In devices which are
equal or superior In periomrance to those made by other epitaxy
processes S U C ~ 2s molecular beam epitaxy (NBE) of metal organfc
chemical vapor deposlffon (MOGVD). Another major advantage is that
LPE is a simple. inexpensive, and safe method for smkonductot
device tabticatton. Significantly, ttre LPE process does not
requfre or produce any hlghjy toxic or dangerous substmces-in
contrast to MOCVD. Also, tfie epitaxiat growth rate with InGsAsSb
LPE is -2 rnicmnslminute which is ten to hundred times faster than
MOCVO or MBE. MfO have succassfulfy scaled up the LPE process for
epitaxial growth In a semiconlinuous made on 3inch diameter wafers.
This. combined with the high growth rates. will dramaffcaliy
improve the manufacturing throughput tmnpared to traditional and
more c0stJ-y eptW procssses. Our objective is to davelop an
epitaxiai growth technology to produce lowcost. largearea. hlgh
eff~crency TPV devices.
2. EPITAXIAL GROWTH AND FABRICATION OF IrrGaAsSb TPV CLi lS
tnGaAsSb photodlodes, Itght9rnitting diodes. and double
heterostructure injection lasers made by liquid- phase epitaxy have
been previously reponed. We have adapted this technofogy for the
production of InGaAsSb TPV cells.
We use a standard honzontaf stideboat techntque for the
liquid-phase epitaxial growth of the InGaAsSb. The graphite
slideboat is situated in a sealed quartz tube placed in a
micfoprocessorcontroll~. progfarnmabte. threetone tube furnace. The
growth ambient b palladiumdiffused hydmgen at atmospheric pressure
wth a flow rate of 300 mUmin.
The substrates ace 500-micfon thick, chern'btly polished (100)
oriented, n-type GaSb wafers obtatned from MCP Wafer Technology,
Ltd. (Milton Keynes. UK) or Firebird Semiconductor, Ltd. (Trail,
BC, Canada). Substrates are doped to 3-5 x lo" ~ r n ' ~ with
tellurium. The substrate resistivity is 0 x lo4 Q UTI. and the
avenge etch-pit density is appmxmately 1000 cm-' *-
(x,,=O.Ot). The melts are formulated with 3- to 5-mm shot Of
high punty (99.9999%) indium. gallium. and antimony metals and
arsenic added as undoped In& polycrystalline mateflal. The
total weight of the melt IS about 10 g Pnor to gtowth. the melts
are baked out at 7CO "C for fiffeen hours under flow~ng hydrogen to
deoxidize the metallic melt components and outgas restduat
impunties. Aner b a k e a t . aPpropnats dopant impunties are added
IO each melt The first melt far the growth of the o-tyce InGaAsSb
base yyer contains tin or tellur!um. The small a m ~ ~ n ! cf Te
needed t~ dope the layer (a:amrc fraCd@n ln the melt z 10- ) is
orobiernatic For r2yoduC:bIe doping. a weighable amount of Te is
added as 100 to 2co mg of TMcped GaSb (C?.=10'9 cm ') Tin is added
:c
The growth solutlons are indium (x,,=0.59}. gallium {X,=O. 19).
anbmmy (w3,=0.21). and arsenic
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the meit as 10 Lo 2CO mg of high purity shot OLir prefiminar]
rasults (Section 3) suggest that high n-bjpe dcpfng concanvations
can be achieved more readily vnth tin than with !ellunurn. However,
the reiattvely high Ilqula-pnase concanttation of tln abr3 the melt
mrnposi:ion needed to grow the latticematched lnGaAsSb quaternary
wrth the desired bandgap. For higher bn doping levels, we will need
to rwptirnize the melt compostbons to include the effects of
dilution with additional tfn. This will requ!re a phase equtllbrb
analysts and model of a Scomponent system (In-Ga-As-Sb-Sn). The
second melt for the growth of the ptype emitter contains 5 to 100
mg germanium. Presentiy, we are beginning a more detatled and
systematic characterization of impurity segragation and doping in
the In-Ga-As-Sb quaternary system with the aim of achieving better
control and a greeter range of doping concentrations.
Tha melts are equilibrated for 1 hour at 530 "C afid thefi coded
s! a rata 3f 0.7 *C/rnin. 41 555 "C. the sobstra:e is contastfA
wlth the first melt for Pm nli7cit6s to grow a 5-micron thick
n-type InGdsSb bass !apt Next. !he substrate is moved to the second
melt fer 5 seohds to grow a 0.3-micron thick p-type InG&sSb
emieer layer.
Front and back ohmic wntacts ara !armed on the epitaxlal
InGdsSblGaSb shuchrre by standard processing techniques. The back
of the substrate is metalked by plating with a 200-nrn thick
elecfron-beam evaporated Au:Ge:Au:Ni layer and alloyed at 300 "C.
The frmt contact is a gdd of 1O-miCfOn wide ffleta:lki%hn lfnes
with IOO-micron spacing and a singte 1-mm wide center busbar. The
grid is formed by a photolithography lift- off process with a
200-nm thick electron-baam evaporated Au:Zn*Au rnetaibation. The
front grid is thickensd to 5 mlcrons by gold electroplating. The
front contact is not sintered. The substrate is masked and
patterned to define a 1 cm x 1 cm devlcg and isoisrtiun etched with
a potassium iod!de - iodine 'gold' etch. Most of cur TPV cells art3
1 cm x 1 cm in are&: although larger cells (2 cm x 2 cm) with
CQmpaGIble performance have also been made. in order to airnplifj
the spectral rs3l;cnse anafysis, we e!ec?ad not b apply any
anti-refiecticn coetings to the cells. FIGURE 1 is a top-view
photograph of a 1 crn x 1 cm InGaAsSb TPV cell.
3. TPV QESIGN AND OPllMltATION
FIGURE 2 shows the TPV device design in cross-section. The
fabricated cells have s 0.3 to 0.5 micron thick p-type emitter wdh
8 Ge concentration of approximately lo'' cm". as indicated by
Secondary Ion Mass SpeCtrOscopy (SIMS). A thicker, more heavily
doped player will reduce the sheet resistance of the emitter and
therefore improve the ffil-factor, but will tend to reduce spectral
response due to higher free-camer absorption and increased
sensitivity to front surface mtnontj carrier recombination.
The base thickness in our cells ranges from 3 to 5 microns with
a Te or Sn concentration of about 1015 to cm4, as determined from
cspadtsnce-voltage measurements and SIMS. FIGURE 3 shows the SlMS
depth
I
profile indicating the abruptness of the p n junctbn and the
depth uniformity of the doping concentrations. Them Is apparently
very liffle smearing of the doping proflle due to diffusbon or
segngatlon of dopants. Dlsaepancies between the Te dopant
concentratfon measured by SIMS (totar impurity concentration) and
that implied by capacitancevoltage measurements {net donor
concentration) indicate that much of the Te is either not ionized
of else is compensated. This is a common problem In Te doping of
Ill-V semlconductors. especially In GaSb-based matarfais. and is
probably due to the formation of elec!nca!ly inactive telluride
complexes or compounds in the material. Inyeasing the Te
concentration in the melt showed a 'saturation effeCr in that the
Te doping level did not increase in propoftmn to the le
concentratton in the liquid phase. Our most recent devices
incorporate tin as the n-type base dopant and have base dopings
targeted around IO" ~ r n - ~ . Modeling indicates that base
dopings in this range wtll yield the optimum opencircuit voltages
and short-wavelength quantum effiCfenC:eS.
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4. TPV DEVlCE EVALUATION
We present external and internal spectral response and
current-voltage charactenstics for 1 cm x 1 cm
p.ln,,,G~.s&.&3bo.S:Ge / n-In,, asQa,,01ASg&3bo.m: Te
(or Sn) epitaxial cells on an n-GaSb:Te substrate produced as
described above. The external spectral response of a typical
InGaAsSb TPV call is shown in FIGURE 4. FIGURE 5 shows the
corresponding internal spectral response. The lower external
spectral response IS due to grid shading and renectlon of incident
fight from the uncoated InQaAsSb emitter surface. The grid shading
is 18.2%. The absorpffon edge implied by the apectfal response
measurements of a nun;&? of samples ranged fiom approxlmatefy
2200 to 2250 nm. At a wavelength of 2000 nm, internal quantum
effhencies as high as 95% have been measured, and at a waveiength
at I micron, internal quantum efffciencies of almost 55% have been
observed. The internal quantum eRdenq avereged over the spectral
region from 1 to 2 microns wavelength is 60%. (It should be noted
that for the intended TPV applications. the reaponse of the cell
fcr wavelengths less than 1.5 microns is not important)
The 1 cm x 1 cm InGaAsSb TPV cells were tested under simulated
infrared light using a ZnSe-filtered tungsten source (Carley Lamps,
Inc.. Torrance, CA) with a spectral emission in the 800 Lo 3000 nm
wavelength range. Under an lllurnlnation intensity corresponding to
a short-circuit current density of 2 Ala', opencircuit voltages as
htgh as 0.260 volts have been measured. FIGURE 6 shows the
current-voltage characteristics of a 1 cm x 1 cm InGaAsSb TPV cell
under an infrared illumination intensity that yields a
short-circuit current densrty of 62.4 mAlun' and a open-circuit
voltage of 0.178 V. The fill-factor is 0.57. To date. the best
fill-factors observed are less than 0.6. We believe that one cause
of the somewhat tow fflt-facton is asria8 resistanca, which is
discussed further In the next section FIGURE 7 shows open-circuit
voltage vs. shortcircuit current for varying light intensity The
open-circuit voltage increases logarithmically with iIIurnination
intensity and an open-circuit voltage of -0.250 V IS reached for
current densities of 1 Atcm'. The diode ideality factor in the
voftage range of 0.1 to 0.25 V is dose to 2. implying that high
injection is dominant in vlis voltage range.
5. CONCLUSION AND DISCUSSION
Our resub to date have demonstrated the potential of InGaAsSb
TPV devices made by Iiquid-phase eprtw We believe there is still
room for substantial efftclency enhancements in these davlces by
optirnuaffon of the do ng levels and layer thicknesses. Further
improvements might Include wide bandgap lattice-matched AlGaAsSb
window layers for front surface passtvatm. and AlGaAsSb
back-surfaca field cladding layers to reduce the reverse saturation
anent and thereby increase the open-circuit voltage. Highly doped
contact layers will provide lower sene8 resistance, as will
substrate thinning. Lower serles resistance will lead to higher
fill factors. Thinning the substrate will a180 improve heat sinking
of the device.
The requid performance of a TPV device is dependent on its
system application. Spectral control of thermal emitters. the use
of selective filters and refledom. heat transfer, and photon
recycling effects need to be included in the device design and
system optirnuatfon. These consMerationa am not usually relevant
for conventional photovoltaic devices and therefore the design and
opbrnzation ales for TPVs will be significantly different than
those for solar calla. For example. grid obscuraffan and reffaction
are not necessartly losses in TPV systems if photons reflected from
the front surface are reabsorbed by the emitter. Our next
generation of InGaAsSb TPV devices will incorporate design features
to fulw exploit photon recycling effects
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