GaAsP on GaP top solar cells as an attachment to silicon bottom solar cells are being developed. The GaAsP on GaP system offers several advantages for this top solar cell. The most important is that the gallium phosphide substrate provides a rugged, transparent mechanical substrate which does not have to be removed or thinned during processing. Additional advantages are that: (1) gallium phosphide is more oxidation resistant than the III-V aluminum compounds, (2) a range of energy band gaps higher than 1.75 eV is readily available for system efficiency optimization, (3) reliable ohmic contact technology is available from the light-emitting diode industry, and (4) the system readily lends itself to graded band gap structures for additional increases in efficiency

Barnett, A. M.

Mcneely, J. B.

Negley, G. H.

English

NASA Technical Reports Server

N86 - 17847 J
GaAsP ON GaP TOP SOLAR CELLS
James B. McNeely, Gerald H. Negley, and Allen M. Barnett
Astropower Division
Astrosystems, Inc.
Newark, Delaware
GaAsP on GaP top solar cells as an attachment to silicon bottom
solar cells are being developed. The GaAsP on GaP system offers
several advantages for this top solar cell. The most important is that
the gallium phosphide substrate provides a rugged, transparent mechan-
ical substrate which does not have to be removed or thinned during
processing. Additional advantages are that I) gallium phosphide is
more oxidation resistant than the III-V aluminum compounds, 2) a range
of energy band gaps higher than 1.75 eV is readily available for system
efficiency optimization, 3) reliable ohmic contact technology is
available from the light-emitting diode industry, and 4) the system
readily lends itself to graded band gap structures for additional
increases in efficiency.
INTRODUCTION
The optimum band gap for a top solar cell on a silicon bottom cell
(i.i eV) lies between 1.75 and 2.07 eV [I]. Top solar cell layer
compositions corresponding to band gaps of 1.75 eV or higher are being
investigated to determine the optimum band gap as a function of real
device and system performance. For example, the potential gain in
minority carrier diffusion lengths at 2.1 eV, due to a reduced lattice
mismatch, may net a higher system efficiency.
GaAsP TOP SOLAR CELL DESIGN
The GaAsP on GaP top solar cell design is based on a model used to
calculate theoretical maximum efficiencies of tandem solar cell sys-
tems. The model that is being used follows that described by Fan [I].
Assuming unity quantum efficiency and no losses, the model
predicts a maximum tandem solar cell efficiency of 34.8% for the AMO
spectrum at a one sun insolation. This optimum performance is based on
a four-terminal configuration. The top solar cell has an energy gap of
1.97 eV and the calculated performance for both ideal solar cells is
shown in Table I.
*Partial support has been provided by the Air Force AeroPropulsion
Laboratory under Contract No. F33615-84-C-2486.
77
https://ntrs.nasa.gov/search.jsp?R=19860008377 2020-03-23T06:49:36+00:00Z
Table I.
' _ _. _, _ ! _'_de_al Tandem Solar Cell Model AMO
_ (Bottom Cell - Silicon)
Gap Jsc Voc F F Efficiency
(eV) (mA--/_m 2) (volts) (%)
1.97 20.75 1.50 0.91 21.i
1.12 32.68 0.67 0.84 13.7
34.8
The actual anticipated efficiencies can be modeled by considering
losses [2]. The one sun losses are tabulated below in Table II.
Table II.
Tandem Solar Cell Model with Losses:
- Optical and recombination losses which reduce Jsc as follows:
Top surface reflection -
Grid Shading - top cell
Absorption losses - top cell
bottom cell
Recombination losses - top & bottom cell
Bottom grid shading loss - bottom cell only
Reflection loss - bottom cell only
2.6%
4.0%
2.0%
9.0%
6.2%
4.0%
1.0%
- Electrical losses, which reduce fill factor:
Series resistance losses - both solar cells 2.0%
Gap Jsc Vo___!c F F Efficiency
(eV) (mA-7-cm 2) (volts) (%)
1.97 17.8 1.50 0.89 17.6
1.12 24.8 0.67 0.82 I0.i
27.7
Another useful case is based on an energy gap larger than the
optimum of 1.97 eV. For example, at 2.07 eV, the arsenic concentration
can be reduced from x = 0.54 to x = .40, leading to a reduced lattice
mismatch of 1.5% from 2.0%. The energy conversion efficiency reduction
from the optimum may be offset by longer actual diffusion lengths based
on reduced lattice mismatch. This design clearly merits further
examination. The analysis of this design (2.07 eV) leads to a predict-
ed tandem solar cell efficiency of 34.5% which is similar to that
predicted by the optimized design at 1.97 eV.
Finally, the GaAsP on GaP system lends itself to graded energy
gaps on both sides of the junction. It has been shown that this can
lead to as much as an 11% increase in energy conversion efficiency
78
[3]. This graded energy gap design will be studied to maximize system
conversion efficiency.
A range of energy gaps between 1.75 eV and 2.07 eV for the top
solar cell junctions is being explored during this program.
DEVICE PREPARATION
The solar cell structure is being grown using liquid phase
epitaxial growth. The advantages of liquid phase epitaxy are excellent
transport properties, minimum contamination, uniform thickness control,
and reductions in point defects and dislocations. Liquid phase
epitaxial growth has, in general, demonstrated devices that are
superior in performance to those grown by the other methods [4,5].
These superior performance devices include light emitting diodes,
semiconductor lasers, magnetic garnet bubble memories and GaAs solar
cells. The improved performance of liquid phase epitaxial, LPE,
devices when compared to vapor phase or diffused devices can be
attributed to the exact stoichiometry control, longer diffusion
lengths, fewer deep levels and the tendency of impurities to segregate
to the liquid rather than the solid.
Ideally, one would like to grow epitaxial layers on lattice-
matched substrates. While this is not possible with the GaAsP on GaP
devices, we have demonstrated lattice-mismatched liquid phase epitaxial
growth of GaAsP on gallium phosphide substrates. Recently multiple
graded layers of GaAsP on GaP with a composition of GaAs.68P.32
corresponding to a band gap of 1.81 eV have been grown with smooth
morphologies in our laboratories. The overall degree of lattice
mismatch between GaAs.68_32 (a o = 5.5886) and GaP (a o = 5.45117) is
2.52%. This growth was accomplished by three individual grading layers
each accommodating 1.86%, 0.33%, and 0.33% of the lattice mismatch,
respectively. Grading was readily accomplished by melt depletion.
Moon and co-workers [6] have independently shown feasibility of liquid
phase epitaxial grading to obtain smooth layers that are mismatched
relative to the substrate for the GaAsP and other ternary and quater-
nary systems. The GaAsI_P x graded layer from Moon's work shows uniform
compositional grading for GaAsP system and smooth growth morphology for
a GaAso.6Po. 4 layer on which corresponds to a 2.22% lattice mismatch in
a single layer.
These results are indeed encouraging. The optical transmission of
the three GaAsP layers grown by us are shown in Figure i.
Mismatch dislocations result from compositional discontinuities
and from composition variations that are too large for the layers to
accommodate the mismatch within the graded layer. A method of reducing
misfit dislocations to zero has been described [7]. This method uses a
two-stage top layer growth from similar melts to achieve zero misfit
dislocation growth. Misfit dislocations are pinned at the termination
of the first layer growth, making extension of the misfit dislocation
in the second homo-epitaxy layer difficult.
Characterization of these layers is by optical transmission,
optical microscopy, spectral response, scanning electron microscopy,
79
EDAX and misfit dislocation decoration and analysis. Minority carrier
diffusion length will be optimized based on variations in growth
parameters and compositions.
Solution growth of multi-layer structures of GaAs-GaP will be
performed using the slider method developed for liquid phase epitaxial
growth [8]. The slider apparatus serves as a substrate holder and melt
container for the growth solutions. Advantages of the slider appar-
atus over other techniques, such as dipping, are I) the substrate wafer
can be brought in and out of contact with the melts, 2) several melts
can be used in sequence, 3) growth is restricted to a single side of
the wafer, 4) substrate-solution contact is from the bottom of the melt
where there are no floating oxides or other contaminants, 5) excess
solution can be wiped off the wafer by the slider action of the boat,
and 6) thermal equilibrating and temperature profiling are greatly
facilitated. The graphite slide apparatus fits into a cooling or
temperature gradient furnace, as appropriate. The furnace zones can be
controlled to better than l°C. We use a furnace atmosphere based on a
high purity mixture of hydrogen or 15% hydrogen and 85% nitrogen, which
is continuously sweeping the furnace and slider apparatus during the
furnace operation.
A feature of the slider boat apparatus particular to this program
is the grading of the width of the growth wells shown in Figure 2.
This graded well width design permits individual access to each of the
grown layers after the substrate is removed from the boat.
Based on thermodynamic considerations, the preferred solvent for
the p-layer is gallium, and for the n-layer, gallium or tin. Both of
these materials are available commercially with purities in excess of
99.9999%. Both p-type and n-type single crystal gallium phosphide are
available commercially with various dopants and doping levels, consis-
tent with device design considerations. Melts are composed of about I0
grams of solvent and appropriate amounts of GaP and GaAs. Growth is
accomplished by placing the GaP substrates under the first melt to grow
a transition layer of GaAsxPl_ x by controlling the temperature level,
cooling rate, and time of exposure, and continuing, in turn, to each
melt shown in the growth apparatus in Figure 2 until all requisite
layers are grown.
Phase equilibria data have been determined for both the gallium
and tin solvent systems.
The specific multi-layer structure being grown is based on our
demonstrated growth to date tailored to have a uniformly graded
structure. The structure is shown in figure 3 and is composed of four
basic layers. The first three layers are graded in composition in a
controlled manner either as finite composition steps, or as continuous
compositional variations shown in Figure 4. The continuous composi-
tional variations are achieved by melt depletion. The fourth layer is
a thin cap layer. These layers, the method for preparation, and
methodology for optimization are described in the next four paragraphs.
This structure can be produced readily in the LPE apparatus with
equal facility for either n on p or p on n structures. Either zinc or
8O
magnesium will be used in the top GaAsxPl_ x layer to form the p/n
junction, and tellurium will be used to form the n/p junction. The
salient feature of this design is that the band gap of top of each
layer is 2.07 eV, 1.95 eV, and 1.75 eV band gaps. Growth may be
truncated at individual layers of interest, and comparison of otherwise
equivalent material may be made with only the stated variations in band
gap. This permits experimental optimization of band gap during
preparation of the structure. The added benefit of this structure is
that each successive layer has only about 0.6% lattice mismatch, and at
no point in the top two graded layers does the structure grade more
than 2 mole % GaAs per micron of growth. This allows the top layers to
be reasonably thin and insures their equivalency for the optimization
experiments.
This graded layer structure does not represent an excessive number
of layers even with intermediate layers. Our current boat design
operates with up to nine melts.
The compositional variation with layer thickness is shown in
Figure 4. For the top two layers, the curvature is very slight and the
compositional variation is nearly a linear function. Indeed, the
differential or slope of composition with respect to thickness
increases only slightly as the final compositions with each layer are
approached. Other variables, growth time, temperature level, growth
rate and melt size, can be varied based on detailed comparison of the
experimental results. Initial and final temperature and melt composi-
tions establish the grown layer composition. Melt size and wafer area
determine layer thickness. Cooling rate and growth time are dependent
on each other since initial and final temperatures are fixed for a
given growth situation. For example, slowing the actual growth rate
can be done to improve crystal morphology, leading to increased growth
time. Intermediate dislocation blocking layers are also planned after
the first layer and the second layer are grown if necessary.
The cap layer is a thin grown layer. The cap serves to reduce
surface recombination and can also serve as a source of dopant for a
diffused junction [9] . This layer will be either GaP or
GayAll_yASxPl_ x material with initial experiments using gallium
phosphide. GayAll_yASxPl_ x will be used if interface dislocations due
to GaP lattice mismatch prove to be a problem. Gallium phosphide has a
high oxidation resistance, and it has been studied extensively in
terms of metallic ohmic contacts. The cap layer will be thin enough so
that it minimizes the effect on the crysta] structure in the underlying
active region. The cap layer is basically a conducting layer and
serves to reduce the surface recombination velocity of the adjacent
layer; it is probably not necessary that this layer possess the
crystalline perfection of the preceding layers. Therefore, a inter-
facial compositional discontinuity may be tolerated between the cap
layer and the top active layer. However, aluminum additions to the
structure will be used, if needed, to optimize lattice matching for the
cap layer. The cap layer may be grown from a tin solution for n-type
layers, or from gallium for p-type layers.
81
CONTACTS
With the grown multi-layer wide band gap structure described
above, the next task is to form ohmic contacts on the top and bottom
surfaces of the wafer. No thinning prior to contact formation is
required with this solar cell design because the gallium phosphide
substrate is transparent to photons less energetic than the band gap of
the active junction of the top cell. The structure is, in fact, quite
sound mechanically and, therefore, is expected to have low breakage and
high fabrication yields during subsequent processing.
The n-type contact consists of thermally evaporated Au/Ni or
Au/Ge/Ni on a clean wafer surface. Surfaces are etched prior to
evaporation using I:i HCI-H20 and are rinsed thoroughly in 18-meg ohm
deionized water, dried, and placed immediately in the evaporator.
Evaporation is done using tungsten wire baskets to contain the individ-
ual sources, and layer thickness is controlled by evaporation to
completion (Au: 1200-1600A°; Ni: 50-150A°). alloying is accomplished
at an optimum established to be 525oc for two minutes. Contacts are
processed using standard photolithography techniques. Pulse-plating of
1-3 microns of gold or silver to the base metallization pattern will be
employed to assure sharp vertical plating features and to reduce
unnecessary shadowing or overlap of metallization patterns.
P-contacts will be fabricated similarly to the n-contacts except
gold-zinc alloy is used instead of Au/Ge/Ni or Au/Ni during the
evaporation step. Stable contacts to semiconductor laser and LED GaP
and GaAsP structures have become a well-controlled manufacturing
science and we do not expect this area to be a problem.
EXPERIMENTAL RESULTS
The growth of smooth, uniform layers of GaAsxPl_ x on GaP sub-
strates with x of the top layers ranging from 0.4 to 0.68 and energy
gaps of 2.07 eV to 1.81 eV has been successfully demonstrated. These
layers were grown using a combination of step grading and depletion
grading.
A GaAsP structure has been designed which will allow the explor-
ation of a range of energy band gaps between 1.75 eV and 2.07 eV for
the top solar cell. The higher energy gaps may lead to improved "real"
system performance in a four terminal configuration•
The solar cell structure is being grown using liquid phase
epitaxial growth techniques. Three-layer structures grading from
GaAs.4P.6 (2.07 eV) to GaAs.68P.32 (1.81 eV) have been produced in our
laboratories using LPE.
The solar cell development to date has been most encouraging. The
overall goal is to produce GaAsP top cell having Voc 1.50 volts, Jsc
17 8 mA/cm 2 a fill factor of 0 89, and an overall efficiency of 17.6%,
• t •
based on the details shown in Table II. To date we have achieved a
maximum Jsc of 10.6 mA/cm 2 (corrected for reflection losses), a maximum
Voc of 1.394 volts and fill factors greater than 0.75. The present
plan is to optimize each parameter separately using simplified col-
82
lector structures to optimize Voc, and Schottky and MIS structures to
optimize, measure, and improve diffusion lengths and Jsc. Ohmic
contacts for these structures, which are reproducible and reliable and
have shown good mechanical adhesion in tests to date, have been
developed.
CONCLUSIONS
Preliminary results are encouraging for the development of a wide
band gap GaAsP top solar cell for attachment to conventional silicon
solar cells• Solar conversion efficiency increases of over 50% may be
expected with the successful development of this technology•
References:
[1] J. C. C. Fan, B.- Y. Tsaur, B. J. Palm, "Optical Design of High
Efficiency Tandem Cells" Conf. Rec. 16th IEEE Photovoltaic
Specialists Conference, p. 692, (1982)•
[2] A. M. Barnett, M. G. Mauk, J. C. Zolper, I. W. Hall, W. A. Tiller,
J. B. McNeely and R. B. Hall, "Thin Film Silicon and GaAs Solar
Cells", Conf. Rec. 17th IEEE Photovoltaic Specialists Conf., 747,
Orlando (1984).
[3] G. Sassi, "Theoretical Analysis of Solar Cells based on Graded
Band-Gap Structures" J. Appl Phys , 54, (9), 5421, (1983)
, • • -- •
[4] L. R. Dawson, "Liquid Phase Epitaxy", Progress in Solid State
Chemistry, Chapter 4, _, 117, Pergamon Press (1972)•
[5] R. L. Moon, "Liquid Phase Epitaxy" Chapter II, Crystal Growth,
ed. B. Pamplin, 2nd ed., p. 421, Pergamon Press, (1980)•
[6] R. L. Moon, G. A. Antypas, Y. G. Chai, "Continuous Compositional
Grading by LPE in AI-Ga-V Quaternary Alloys", Proceedinss of the
7th International Symposium on GaAs and Related Compounds, p. 78,
ed. C. M. Wolfe, Institute of Physics, London, (1978).
[7] S. Yamazaki, K. Nakajima, S. Komiya, Y. Kishi and K. Akita,
"Liquid Phase Epitaxial Growth of InP/InGaAsP/InP Double-Hetero-
structure Wafers Free of Misfit dislocations", Appl. Phys. Lett.
43, (I), 82 (1983).
[8] H Nelson, "Depositing Successive Layers from the Liquid Phase"
U. S. Patent 3,565,702, February 23, 1971.
[9] H. J. Hovel and J. M. Woodall, "An Isothermal Etchback-Regrowth
Method for High Efficiency Ga I xAlxAs-GaAs Solar Cells" Appl
Phys. Lett., 30, 492 (1977).
83
F i g u r e  1. GailsP on  G a P  T r a n s n i s s i o n  Ca ta  
r! : i i g u r e  2 .  S l i d e r  b o a t  g r o w t h  a p p a r a t u s  s h o w i n g  t h e  g r a d e d  well w i d t h .  
84 
TOP L_YER -\
COL L -_CrO_,\
_,,_ _,
-'1 - - -----------_---_-_-_-_--_'-_-_--_-_
! 1 t''r GRA'DED _AL, p.., L/LYER
Figure 3. _ulti-layer GaAsP on GaP solar cell structure (not to
scale). Each successive layer is narro_:er than its prede-
cessor, which al]ows easy access to areas of the individual
growth surfaces for diagnostic purposes.
60,
|G ' _ I ' _ ' _*
Figure 4. Compositional graded three-layer GaAsP on GaP f;tructure
85


GaAsP on GaP top solar cells

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