- Email: [email protected]
III–V solar cell research at spire corporation
Solar Cells, 27 (1989) 107 - 120
107
III-V SOLAR CELL RESEARCH AT SPIRE CORPORATION S. M. VERNON, S. P. TOBIN, S. J. WOJTCZUK, C. J. KEAVNEY, C. BAJGAR, M. M. SANFACON, J. T. DALY and T. M. DIXON
Spire Corporation, Patriots Park, Bedford, MA 01 730 (U.S.A.)
Summary This paper reports the experimental results from several technical approaches aimed at achieving highly efficient solar cells for terrestrial and space-power applications. Efficiencies of up to 28.7% "(200X, AM1.5) and 24.8% (IX, AM1.5) have been achieved with homoepitaxial GaAs p/n cells. This one-sun AM1.5 efficiency value is believed to be the highest reported to date. Tandem solar cells using GaAs/Ge structures have been fabricated and tested to have efficiencies up to 24.1% (lX, AM1.5), and a GaAs/Si cell at 17.6% (IX, AM1.5) is reported. The use of InP as a radiation-tolerant material for solar cells with high end-of-life efficiency has drawn much interest lately. We report homoepitaxial n/p InP cells with an efficiency of 18.8% (lX, AM0); this is believed to be the best InP cell reported to date. Fabrication of heteroepitaxial InP solar cells, with one-sun AM0 efficiency values of 9.4% (on GaAs) and 7.2% (on silicon) is described. In addition, a new large-scale metal-organic chemical vapor deposition (MOCVD) reactor has been developed; studies involving computer simulation, flow visualization, and deposition experiments are described.
1. Introduction Two of the most promising materials for constructing high-efficiency solar cells are GaAs and InP. Unfortunately, GaAs and InP wafers are not ideal substrate choices for solar ceils; the problems that these two compound semiconductors have in this regard include high cost, high density, poor thermal conductivity, low mechanical strength, and availability in only relatively small diameters. One experimental approach being pursued is the heteroepitaxial growth of high-quality thin-film GaAs or InP solar cells onto silicon and/or germanium wafers. As a substrate, silicon has many attractive properties. Compared with both GaAs and InP, silicon is stronger, less expensive, less dense, more thermally conductive, and available in much larger wafers which could improve throughput and reduce costs in a photovoltaic manufacturing process. The major disadvantage of silicon in this application is the fact that 0379-6787/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
108 it is n o t well lattice matched to GaAs or InP; therefore the problem becomes the growth of heteroepitaxial layers with good minority-carrier properties despite the large n u m b e r of dislocations generated b y the lattice mismatch at the film-substrate interface. As a substrate for GaAs, germanium is an excellent choice because it is closely matched in both lattice constant and thermal expansion coefficient. A power-to-weight advantage can also be gained because the substrate thickness dictated by practical wafer-handling considerations is significantly less for germanium, because of its superior mechanical strength, than it is for GaAs. Present expectations are that germanium substrates will be available in larger diameters than those of GaAs substrates and at significantly lower cost. One important added advantage of using germanium is the possibility of forming monolithic tandems of very high efficiency in which the germanium acts as a b o t t o m cell and n o t simply a mechanical support. Because of the lattice match, GaAs of very high quality can easily be deposited on germanium without the crystal-defect problems mentioned above, so that cell designs of very high efficiency are a real possibility.
2. GaAs solar cells
2.1. Cell fabrication GaAs solar cells have been fabricated on GaAs, germanium, and silicon substrates. Epitaxial growth was carried o u t by metal-organic chemical vapor deposition (MOCVD) at atmospheric pressure using the Spire SPI-MO CVD T M 450 system; experimental deposition and fabrication details are shown in Tables 1 and 2, respectively. I m p o r t a n t features of our processing include image-reversal photolithography and thick-metal lift-off to define tall, narrow grid lines, and a triple-layer antireflection coating consisting of the A10.sGa0.2As window, ZnS, and MgF2. The deposited antireflection coatings are applied by thermal rather than electron-beam evaporation to prevent e-beam damage [ 1 ]. Spire's high-efficiency GaAs cells use the p-on-n double heteroface structure shown in Fig. 1, with a high band-gap A1GaAs window at the lightincident side of the cell and a lower band-gap A1GaAs layer at the rear of the active GaAs layers. (In some cases, a GaAs high-low doping structure is used at the rear of the cell in place of the AIGaAs layer.) Both heterointerfaces act as minority-carrier mirrors, preventing recombination at surfaces. The AIGaAs window is the most critical feature of the cell. It is very thin and has a high AlAs fraction to minimize absorption losses. It acts as part of the antireflection coating system, with a refractive index intermediate between that of GaAs and the deposited antireflection coatings, and its thickness, of the order of 30 rim, is chosen to minimize reflectance. Very low reflectances can be obtained with a polished-surface cell and a ZnS/MgF2 antireflection coating. The reflectance p o w e r loss across the AM1.5 spectrum is typically a b o u t only 1%.
109 TABLE 1 MOCVD Growth of GaAs-GaAlAs solar cell structures
Reactor Capacity Reactor pressure Sources Dopants Growth temperature Growth rate V - I I I ratio
TABLE
SPI-MO CVD 450 Five 2-in. wafers 760 Torr Trimethylgallium, trimethylaluminum, and arsine Silane and dimethylzinc, diluted in hydrogen 7 0 0 - 8 0 0 °C 4/~m h -1 20:1
2
High-efficiency GaAs solar cell device processing sequence 1. Coat wafer front with SiO2 2. Evaporate A u G e back ohmic contact 3. Alloy back contact 4. Evaporate gold back metallization 5. Image-reversal photolithography for front grid 6. Etch SiO2 in grid openings 7. Evaporate CrAu front contact 8. Dissolve photoresist to liftoff excess metal 9. Sinter contacts 10. Photolithography for mesa etch 11. Mesa etch 12. R e m o v e all SiO2 13. Selective cap removal etch 14. Evaporate ZnS/MgF2 antireflection coating
F o r GaAs/Si cells, a GaAs buffer layer is first deposited onto a silicon substrate b y the now-popular three-step m e t h o d outlined here: the silicon substrates (oriented 2 ° off (100) in the (011) direction) are heated in hydrogen to over 1000 °C to remove the oxide, and the temperature is then lowered to ca. 400 °C for nucleation and deposition of ca. 200 A of GaAs; finally, a thick GaAs layer (1 #m or more) is deposited at typical MOCVD conditions, at a growth temperature of 700 °C, a growth rate of 4/~m h - ' , and a V - I I I ratio of 15:1. In some experiments, the GaAs buffer layer is formed using our thermal-cycle-growth (TCG) method, described earlier [2]. This process has been shown to lead to lower defect densities and higher minority-carrier lifetimes [2]. Once the GaAs buffer is complete, the GaAs/Si wafer can be treated like a GaAs substrate; this permits the growth of high-efficiency GaAs solar cell structures onto both GaAs and GaAs/Si substrates for sideby-side comparisons. Monolithic tandem cells consisting of a standard GaAs p-on-n cell on a germanium p-on-n b o t t o m cell are grown and fabricated b y the methods
110 h t,
D METALLIZA" CrAu CONTACT LAYI GaAs p + ANTIREFLECTION COATING WINDOW
0.¢
EMITTER
0
BASE
3.0
gm
GaAs
n
BSF
1.o
#m
AIGaAs
n+
GaAs
n+
Sl
n+
BUFFER VARIABLE
SUBSTRATE
25mils
n METALLIZATION
Fig. 1. High-efficiency GaAs cell s t r u c t u r e . Device s h o w n here is o n silicon. GaAs s u b s t r a t e s use a A u G e m e t a l l i z a t i o n a n d g e r m a n i u m s u b s t r a t e s use a gold m e t a t l i z a t i o n .
described above. However, because of the " a u t o d o p i n g " effect of germanium atoms which evolve from the germanium back surface and are incorporated into the growing GaAs and GaA1As layers, an extra "capping" step is added. This capping is done b y depositing a few microns of GaAs by MOCVD o n t o the back of the germanium wafer in a separate run. The p - n junction in the Ge b o t t o m cell is formed by in-diffusion of gallium and arsenic into the germanium from the growing GaAs cell structure during the MOCVD run. The cell interconnect is formed by the leaky G a A s - G e highly d o p e d heterojunction. 2.2. Cell results
The data of Table 3 summarize the solar cell results for our best GaAs cells deposited on GaAs, g e r m a n i u m , and silicon substrates. The illuminated I - V curve of our best GaAs/GaAs cell is shown in Fig. 2. The GaAs/Ge cells are tandem structures in which the germanium b o t t o m cell contributes a b o u t 200 mV to the open-circuit voltage; the efficiency values shown may not, however, be entirely accurate because of the difficulties which exist in the measurement of series-connected tandem cells as reported by Hart et al. [3]. These tandem cell results are from preliminary studies, and much optimization is still possible.
111 TABLE 3 GaAs solar cell results achieved at Spire Substrate
Air mass Suns
Measured by
Voc (V)
Jsc (mA
FF
Efficiency (%)
cm -2)
GaAs GaAs GaAs GaAs
0 1.5 0 1.5
1 1 170 200
SERI SERI Sandia Sandia
1.03 1.03 1.14 1.14
33.2 27.9 33.6 28.8
0.864 0.864 0.875 0.875
21.7 24.8 24.5 28.7
Ge Ge
0 1.5
1 1
NASA SERI
1.20 1.19
28.6 23.8
0.849 0.849
21.3 24.1
Si Si Si
0 1.5 1.5
1 1 370
SERI SERI Sandia
0.90 0.89 1.09
29.8 25.5 20.8
0.777 0.777 0.816
15.2 17.6 18.5
SERI data courtesy of K. Emery, Sandia data courtesy of D. Ruby, NASA data courtesy of R. Hart. All data measured at 25 °C, total area.
Spire,GaAs, global, 1000W/m 2 Sample:
1552-3-5
Temperature
M a r . 6, 1 9 8 9 3 : 4 9 p m 8
'
I
'
I g
'
I
Area '
I
I
= 25.0°C
= 0.250 cm z '
I
'
I
t
I
6
4 Voc
= 1 .0 2 9 v o l t s
2 Jsc = 27.89 mA/cm ~ Fill factor
= 86.43 %
Efficiency
= 24.8 %
I
I
0
-2
-4
, I
,
I
~
~
I
•
,
I c;
VOLTAGE
(volts)
Fig. 2. Efficiency measurement data for best GaAs/GaAs cell at 1 Sun AM1.5 conditions.
T h e e f f i c i e n c y o f o u r G a A s / S i cells h a s b e e n g r e a t l y i m p r o v e d in t h e p a s t y e a r b y t h e use o f t h e T C G t e c h n i q u e . B e f o r e its use, v e r y t h i c k G a A s b u f f e r s (ca. 8 # m ) w e r e n e e d e d t o a c h i e v e r e a s o n a b l e m i n o r i t y - c a r r i e r l i f e t i m e s ; w i t h t h e T C G p r o c e s s , h i g h l i f e t i m e s ( o v e r 1 ns) are o b t a i n e d a t a b u f f e r t h i c k n e s s o f o n l y 2 /~m. T h i s i m p r o v e m e n t in t h e q u a l i t y o f o u r
112
GaAs/Si material, coupled with optimization of the GaAs cell design and processing, has caused the efficiency to increase from 11.6 to 17.6% (onesun, AM1.5, SERI data). For GaAs/Si concentrator cells, efficiencies up to 18.5% have been achieved at 370X AM1.5 on non-optimized GaAs/Si buffers; concentrator efficiencies of near 20% are predicted for cells to be made using our latest TCG GaAs/Si material. The best reported GaAs/Si cells are currently from NTT in Japan [4]. Ohmchi e t al. [4] reported GaAs/Si cells at 20% (one-sun, AM1.5), but their measurement conditions may n o t correspond exactly to those used for the data of Table 3. The dramatic improvements in GaAs/Si cell efficiency are a direct result of improved materials growth techniques which reduce the density of latticemismatch-related threading dislocations. The dislocations act as recombination sites, increasing space-charge recombination current and reducing minority-carrier diffusion lengths. Figure 3 shows the calculated and measured effect of dislocation density on solar cell efficiency [5, 6]. The data include recent Spire results where dislocations were deliberately introduced into GaAs cells on GaAs substrates using GaAsP lattice-mismatched buffer layers. Initial GaAs/Si material had dislocation densities in the range of 10 s cm -2. Through thermal cycling techniques, this was reduced to about 10 ~ cm -2. Recent work with strained-layer superlattices has reduced this still further to the low 106 c m - : range. Another factor of 10 decrease in dislocation density will give efficiencies (and dislocation densities) essentially equal to those on bulk GaAs substrates.
2 5
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103
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o
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104
'
105
[3] [36]
[13]
........
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10 6
........
'
........
107
108
109
Dislocotion Density (cm - 2 )
Fig. 3. Efficiency of GaAs and GaAs/Si cells as a function of threading dislocation density.
3. InP solar cells
Recently, much effort has been devoted to the study of InP solar cells for space applications. This work was triggered by the discovery that
113 exposure to radiation, as in Earth orbit, causes less damage to the photovoltaic performance of these cells than to t h a t of GaAs or silicon cells. Furthermore, the damage which is done can be annealed at a relatively low temperature [7, 8]. The development of high-efficiency InP-based solar cell structures has proceeded quickly [9 - 15]; efficiencies up to 18.8% (AM0, 25 °C) have been measured [9]. Theory predicts an attainable beginning-oflife efficiency for this material nearly the same as that for GaAs. Because of the superior radiation resistance, this would correspond to a considerably higher end-of-life efficiency in typical space applications than for any other k n o w n material. One disadvantage of InP from a photovoltaic point of view is its cost. This could presumably be reduced in the face of a large demand for space solar cells, but is n o t expected to be much lower than for GaAs. The largest commercially available InP wafers are of 75-mm diameter. Another disadvantage of InP which is particularly important for space applications is its density (4.8 g cm-3). Although the active region of an InP solar cell is only about 3 pm thick, the low mechanical strength of the material would make handling of cells of less than 150 pm very difficult. This corresponds to 72 mg cm -2, or 2.7 g W-1. The use of silicon substrates addresses both of these problems. Silicon wafers are commercially available in sizes up to 200-mm diameter, at costs which would contribute negligibly to the final cost of space solar cells. Silicon has a lower density than InP (2.3 g cm-3), and is considerably stronger. Cells of 50-gm thickness, which weigh only 12 mg cm -2, are currently produced for space applications. 3.1. Cell fabrication InP solar cells have been fabricated in our laboratory on InP, GaAs, and GaAs-coated silicon substrates. The structure of these cells is shown in Fig. 4. The use of a GaAs buffer layer on the silicon substrates has been found to facilitate greatly the deposition of high-quality InP films [16]. The InP layers are grown by MOCVD, using trimethylindium and phosphine at a growth temperature of 600 °C. For the InP-GaAs/Si structures, the GaAs/Si film is deposited by our standard process as described above. The frontsurface ohmic contacts to InP use a Cr/Au/Ag metallization, whereas the back-surface contacts are formed from A u - Z n on InP or GaAs substrates, or from A1-Ti-Pd-Ag on silicon substrates.
3.2. Cell results Experimental results of InP cells fabricated at Spire are shown in Table 4. The efficiency value of 18.8% AM0 is the best InP cell reported to date. For this cell, the InP base layer was grown by MOCVD and the N ÷ emitter was formed by ion implantation. For the other cells of Table 4, the cell structures were formed totally by MOCVD. The higher efficiency of the implanted-junction cell vs. the alI-MOCVD cell reflects the degree of
114 Light
\
Front Grid
/
DLARC
Cr/Au/Ag ZnS/MgF
N+InP (.05~)
Emitter
Base
P InP (3ju}
Buffer
P+lnP ( l p )
P+InP, GnAs, G e A s ( l p ) SJ
Subatrate
Back C o n t a c t
~,/////////,-/A
Au-Zn, Au-Zn, A I - T i - P d - A g
Fig. 4. Diagram of InP solar cell structures.
TABLE 4 InP solar cells fabricated at Spire Substrate
Junction formation
Air mass
Measured by
Voc (V)
Jsc (mA cm -2)
FF
Efficiency (%)
InP InP GaAs GaAs/Si
I/I-MOCVD MOCVD MOCVD MOCVD
0 0 0 0
NASA NASA Spire Spire
0.873 0.868 0.672 0.636
35.7 33.9 27.4 25.6
0.829 0.838 0.701 0.601
18.8 17.9 9.4 7.2
All data measured at 1 Sun, 25 °C, total area. N A S A data from LeRC courtesy of R. Hart.
optimization of these t w o processes in our laboratory at the time of this work. The spectral response of InP solar cells fabricated on three different substrates is s h o w n in Fig. 5. The low red response for the cells on the GaAs and GaAs-coated silicon substrates is indicative o f the low minoritycarrier lifetime values for the heteroepitaxial layers. The poor material quality is due to the high number of dislocations caused by the lattice mismatch between InP and either GaAs or silicon. Plan-view TEM studies [16] have s h o w n that InP has ca. l 0 s dislocations cm -~ when deposited on GaAs or GaAs/Si substrates. Experiments are currently under way to improve material quality by the use of the thermal-annealing or strainedlayer growth techniques that have proven to be reasonably successful in the GaAs/Si field; defect densities in the low 10 6 cm -2 range are expected. If this is achieved, then cell efficiencies similar to those o f InP homoepitaxial
115 1.0 (J
.. ..... . s - " "
t-
0.8 E 0.6
/
-~ ~r 0.4 •"
-6
E
0.2
c
..... ::'::-
~
-~
/J ~"
|.
silicon substrate
- - - G(JAssubstrate • • • InP substrate " T " calculated fo; L = 0.2.5 #m
, 0.0 300
500
700 wavelength (nm)
I ~. 900
Fig. 5. Spectral response o f InP cells on GaAs and silicon substrates. The low red response is the result o f a high defect density. The response o f a homoepitaxial InP cell is shown for comparison. 22 20
g
"5 14 ~ 12
0
8 6
InP on Si l n P o n GoAs • InP on InP -calculated for beginning-of-life --- calculated for end-of-life
•
•
i
104
(1015
crn -2
e-
@ 1 MeV) i
i
i
105 106 107 108 dislocation density (cm -2)
•
..-
109
Fig. 6. Comparison of theoretical and experimental efficiencies of InP cells as a function of dislocation density. Not only are similar beginning-of-life efficiencies predicted, but also the end-of-life efficiency is less sensitive to the dislocation density.
cells are expected, as shown in Fig. 6. This figure shows the calculated and measured efficiencies of InP solar cells and is based upon a PC-1D analysis and the theory of Yamaguchi e t al. [17] relating lifetime to dislocation density.
4. MOCVD system scale-up for solar cell production In recent years, MOCVD has been established as a viable method for the production of high-quality III-V epitaxial materials. Photovoltaic GaAs devices are currently being produced commercially by this method. However, at present, high-quality material with good uniformity (less than 10% thickness and doping variation) has been realized only in relatively small reactors. For photovoltaic materials to be produced in the quantities
116
needed to become a significant alternative energy source, large deposition systems are becoming increasingly of interest. Therefore, we have developed a new, large-scale, barrel-type MOCVD reactor capable of growing highquality III-V solar cell structures on seven 3-in. diameter or 14 2-in. diameter substrates with good uniformity.
4.1. Apparatus Figure 7 presents a schematic illustration of the SPI-MO C V D TM 1200 system used for the growth of the GaAs layers. The reaction chamber features a hollow, barrel-type, SiC-coated graphite susceptor with seven facets tilted 10 ° from the vertical. Each facet has recesses which can hold either two 2-in. wafers or one 3-in. wafer. The quartz bell-jar has a 10-in. inside diameter, is ca. 27 in. tall, and is shaped to follow closely the contours of the susceptor. The bell-jar can be water cooled or air cooled as desired. The susceptor is inductively heated by an external r.f. coil to a preset temperature which is monitored by an optical pyrometer. The pyrometer is connected to a sapphire light pipe that is inserted into the center of the susceptor. Gas leaves the reactor through eight ports equally spaced around the base of the bell-jar and connected to a circular exhaust plenum. The GAS FLOW
Fig. 7. S c h e m a t i c i l l u s t r a t i o n o f t h e SPI-MO C V D TM 1 2 0 0 r e a c t i o n c h a m b e r .
117
entire system is operated by a personal computer interfaced to an industrial controller and can be operated in either an atmospheric- or reduced-pressure mode. 4.2. Computer simulations The steady-state operating conditions of the new reactor were modelled using a numerical technique described earlier [18, 19]. The model simultaneously solved the equations of continuity, motion, differential thermal energy balance, and mass conservation for the various species in an axisymmetric system. In addition, allowance was made for variable property {temperature and composition dependent) values. The results of the simulation for atmospheric and low pressure operation are summarized in Figs. 8 and 9. Although both pressure regimes can lead to the attainment of good thickness uniformity, Fig. 8 shows a large difference in the flow patterns. The thermal convection loop shown for atmospheric pressure (Fig. 8a) does not permit the growth of abrupt interfaces and can cause difficulties in controlling the deposition of thin layers; this recirculation does, however, lead to a high source-utilization efficiency. At low-pressure operation (Fig. 8b), we see that the recirculation loop is eliminated; this flow pattern is optimal for depositing well-controlled thin layers and abrupt interfaces, but the source-utilization efficiency is somewhat lower. 4.3. Flow visualization Before the initiation of epitaxial growth experiments, the gas flow dynamics of the new reactor were studied experimentally using standard
i
;
1.0 atm
(a)
(b)
Fig. 8. C o m p u t e r simulation o f gas flow streamlines calculated for a susceptor temperature o f 650 °C, total gas flow rate o f 20 1 min -1, and (a) I atm and (b) 0.1 a t m p r e s s u r e s .
118 ._c E
0.10
....
I ....
_
E oo8
I ....
i ....
Wafer Deposition Zo°e
I
I
,~@ 0 . 0 6 .\ I= O
0.04
0 O. @
0.02
( a ) ...............................
I
0.00 0
I
I
I
I 0.05
I
I
I
I
I
0. I
i
i
i
m
(b)
i
I
0.15
I
I
I 0.2
Susceptor Flat Position (m)
Fig. 9. Predicted growth rate vs. susceptor facet position for each case depicted in Fig. 8.
flow visualization techniques [20]. A separate gas-handling system featuring titanium tetrachloride and de-ionized water bubblers, manual flow-meters, and helium as the carrier gas was connected to the reaction chamber for this purpose. Approximately equal amounts of the titanium tetrachloride and water (about 200 sccm) were injected into the carrier gas line just before the inlet to the bell-jar. This resulted in the formation of titanium dioxide " s m o k e " which, when illuminated by a He-Ne laser, revealed the details of the gas flow pattern inside the chamber. The susceptor temperature was kept at or slightly below 200 °C to prevent excessive clouding of the belljar by titanium dioxide deposition. The effects of reactor pressure and gas flow rate were investigated. Figure 10 illustrates the principal results of these experiments. At atmospheric pressure and for all flow rates from 10 to 40 standard liters per minute (slpm), a free convection loop was observed in the vicinity of the susceptor flat as seen in Fig. 10(a). On the other hand, at low pressure, streamlined laminar flow (also called Poiseuille flow) is observed for flow rates above about 15 slpm as shown in Fig. 10(b). These results confirm the predictions of the computer model. 4.4. Epitaxial growth experiments A number of GaAs growth runs have been performed and characterized in the new 1200 reactor. For all these experiments, a water-cooled bell-jar was used, and the chamber pressure was kept constant at 76 Torr. Although the growth parameters have not y e t been completely optimized, highly uniform layers over the area of a 3-in. wafer have been achieved; uniformity values of +3 and +8% (standard deviations) for thickness and doping, respectively, have been measured on our best wafers to date. Over the many experiments performed in this investigation, thickness and doping u n f o r m i t y were routinely observed to be better than -+6 and +12% over a wide range of operating conditions, with wafer-to-wafer uniformity being about the same.
119
,
i
b (a)
(b)
Fig. 10. Gas flow pattern at (a) atmospheric pressure and 40 slpm and (b) low pressure (0.1 atm) and 20 slpm.
5. Conclusions Experimental results from a number of solar cell structures have been described. For GaAs cells, AM1.5 efficiencies of over 24% have been achieved on both GaAs and germanium substrates. For InP cells, AM0 efficiencies of over 18% have been described and preliminary cell results for InP-GaAs and I n P - G a A s - S i structures have been presented. The MOCVD growth of lattice-mismatched heteroepitaxial structures shows promise for achieving high cell efficiency using low-cost/low-weight substrates, and research in this area is advancing. In addition, a large-scale MOCVD system has been developed and modelled, and preliminary smoke-test and growth data show the ability of this reactor to achieve excellent process control. Acknowledgments Funding for various portions of this work has come from SERI (Golden, CO), NASA (Cleveland, OH), Wright-Patterson AFB (OH), and
120
ARO (Research Triangle, NC), and their support is gratefully acknowledged. The authors also thank Drs. J. Szekely and A. Dilawari of the Massachusetts Institute of Technology for their computer modelling efforts.
References 1 S. P. Tobin, S. M. Vernon, C. Bajgar, C. J. Keavney, M. M. Sanfacon and V. E. Haven, Solar Cells, 24 (1988) 103. 2 S. M. Vernon, R. K. Ahrenkiel, M. M. AI-Jassirn, T. M. Dixon, K. M. Jones, S. P. Tobin and N. H. Kararn, Mater. Res. Soc. Syrup. Proc., 145, to be published. 3 R. E. Hart, Jr., D. S. Brinker and K. A. Emery, Conf. Rec. 20th I E E E Photovoltaic Specialists Conf., Las Vegas, NV, 1988, p. 764. 4 Y. Ohrnchi, Y. Kadota, Y. Watanabe and H. Okamoto, presented at Materials Research Society 1988 Fall Meeting, Boston, MA, 1988, Symp. W, to be published. 5 M. Yarnaguchi, A. Yamamoto and Y. Itoh, J. Appl. Phys., 59 (1986) 1751. 6 Y. Itoh, T. Nishioka, A. Yamarnoto and M. Yarnaguchi, Appl. Phys. Left., 52 (1988) 1617. 7 M. Yamaguchi and K. Ando, J. Appl. Phys., 63 (1988) 5555. 8 I. Weinberg, C. K. Swartz, R. E. Hart and R. L. Statler, Rec. 1 9 t h l E E E P h o t o v o l t a i c Specialists Conf., N e w Orleans, CA, 1987, p. 548. 9 C. J. Keavney and M. B. Spitzer, Appl. Phys. Lett., 52 (1988) 1439. 10 M. B. Spitzer, C. J. Keavney, S. M. Vernon and V. E. Haven, Appl. Phys. Lett., 51 (1987) 364. 11 M. Sugo, A. Yarnamoto and M. Yamaguchi, IEEE Trans. Electron. Devices, ED-34 (1987)772. 12 M. Yamaguchi, Y. Itoh and C. Uemura, Proc. 2nd Int. Photovoltaic Sci. Eng. Conf., Beijing, 1986, p. 319. 13 S. Bothra, N. G. Bhimnathwala, K. K. Parat, S. K. Ghandi and J. M. Borrego, 19th IEEE Photovoltaic Specialists Conf., New Orleans, LA, 198 7, p. 261. 14 K. Y. Choi, C. C. Shen and B. I. Miller, 19th IEEE Photovoltaic Specialists Conf., New Orleans, LA, 1987, p. 255. 15 T . J . Coutts and S. Naseem, Appl. Phys. Left., 46 (1985) 164. 16 S. J. Pearton, K. T. Short, A. T. Macrander, C. R. Abernathy, V. P. Mazzi, N. M. Haegel, M. M. A1-Jassim, S. M. Vernon and V. E. Haven, J. Appl. Phys., 65 (1989) 1083. 17 M. Yamaguchi, A. Yarnamoto, N. Uchide and C. Uemura, Solar Ceils, 19 (1986) 85. 18 A. H. Dilawari and J. Szekely, in J. Szekely, L. B. Hales, H. Henein, N. Jarrett, K. Rajarnari and I. Samarsekera (eds.), Proc. Math. Modelling o f Materials Processing Operations. Metallurgical Society AIME, Pittsburgh, PA, (1987), p. 933. 19 A. H. Dilawari and J. Szekely, J. Crystal. Growth, in the press. 20 C. A. Wang, S. H. Groves and S. C. Palmateer, J. Crystal. Growth, 77 (1986) 136.
107
III-V SOLAR CELL RESEARCH AT SPIRE CORPORATION S. M. VERNON, S. P. TOBIN, S. J. WOJTCZUK, C. J. KEAVNEY, C. BAJGAR, M. M. SANFACON, J. T. DALY and T. M. DIXON
Spire Corporation, Patriots Park, Bedford, MA 01 730 (U.S.A.)
Summary This paper reports the experimental results from several technical approaches aimed at achieving highly efficient solar cells for terrestrial and space-power applications. Efficiencies of up to 28.7% "(200X, AM1.5) and 24.8% (IX, AM1.5) have been achieved with homoepitaxial GaAs p/n cells. This one-sun AM1.5 efficiency value is believed to be the highest reported to date. Tandem solar cells using GaAs/Ge structures have been fabricated and tested to have efficiencies up to 24.1% (lX, AM1.5), and a GaAs/Si cell at 17.6% (IX, AM1.5) is reported. The use of InP as a radiation-tolerant material for solar cells with high end-of-life efficiency has drawn much interest lately. We report homoepitaxial n/p InP cells with an efficiency of 18.8% (lX, AM0); this is believed to be the best InP cell reported to date. Fabrication of heteroepitaxial InP solar cells, with one-sun AM0 efficiency values of 9.4% (on GaAs) and 7.2% (on silicon) is described. In addition, a new large-scale metal-organic chemical vapor deposition (MOCVD) reactor has been developed; studies involving computer simulation, flow visualization, and deposition experiments are described.
1. Introduction Two of the most promising materials for constructing high-efficiency solar cells are GaAs and InP. Unfortunately, GaAs and InP wafers are not ideal substrate choices for solar ceils; the problems that these two compound semiconductors have in this regard include high cost, high density, poor thermal conductivity, low mechanical strength, and availability in only relatively small diameters. One experimental approach being pursued is the heteroepitaxial growth of high-quality thin-film GaAs or InP solar cells onto silicon and/or germanium wafers. As a substrate, silicon has many attractive properties. Compared with both GaAs and InP, silicon is stronger, less expensive, less dense, more thermally conductive, and available in much larger wafers which could improve throughput and reduce costs in a photovoltaic manufacturing process. The major disadvantage of silicon in this application is the fact that 0379-6787/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
108 it is n o t well lattice matched to GaAs or InP; therefore the problem becomes the growth of heteroepitaxial layers with good minority-carrier properties despite the large n u m b e r of dislocations generated b y the lattice mismatch at the film-substrate interface. As a substrate for GaAs, germanium is an excellent choice because it is closely matched in both lattice constant and thermal expansion coefficient. A power-to-weight advantage can also be gained because the substrate thickness dictated by practical wafer-handling considerations is significantly less for germanium, because of its superior mechanical strength, than it is for GaAs. Present expectations are that germanium substrates will be available in larger diameters than those of GaAs substrates and at significantly lower cost. One important added advantage of using germanium is the possibility of forming monolithic tandems of very high efficiency in which the germanium acts as a b o t t o m cell and n o t simply a mechanical support. Because of the lattice match, GaAs of very high quality can easily be deposited on germanium without the crystal-defect problems mentioned above, so that cell designs of very high efficiency are a real possibility.
2. GaAs solar cells
2.1. Cell fabrication GaAs solar cells have been fabricated on GaAs, germanium, and silicon substrates. Epitaxial growth was carried o u t by metal-organic chemical vapor deposition (MOCVD) at atmospheric pressure using the Spire SPI-MO CVD T M 450 system; experimental deposition and fabrication details are shown in Tables 1 and 2, respectively. I m p o r t a n t features of our processing include image-reversal photolithography and thick-metal lift-off to define tall, narrow grid lines, and a triple-layer antireflection coating consisting of the A10.sGa0.2As window, ZnS, and MgF2. The deposited antireflection coatings are applied by thermal rather than electron-beam evaporation to prevent e-beam damage [ 1 ]. Spire's high-efficiency GaAs cells use the p-on-n double heteroface structure shown in Fig. 1, with a high band-gap A1GaAs window at the lightincident side of the cell and a lower band-gap A1GaAs layer at the rear of the active GaAs layers. (In some cases, a GaAs high-low doping structure is used at the rear of the cell in place of the AIGaAs layer.) Both heterointerfaces act as minority-carrier mirrors, preventing recombination at surfaces. The AIGaAs window is the most critical feature of the cell. It is very thin and has a high AlAs fraction to minimize absorption losses. It acts as part of the antireflection coating system, with a refractive index intermediate between that of GaAs and the deposited antireflection coatings, and its thickness, of the order of 30 rim, is chosen to minimize reflectance. Very low reflectances can be obtained with a polished-surface cell and a ZnS/MgF2 antireflection coating. The reflectance p o w e r loss across the AM1.5 spectrum is typically a b o u t only 1%.
109 TABLE 1 MOCVD Growth of GaAs-GaAlAs solar cell structures
Reactor Capacity Reactor pressure Sources Dopants Growth temperature Growth rate V - I I I ratio
TABLE
SPI-MO CVD 450 Five 2-in. wafers 760 Torr Trimethylgallium, trimethylaluminum, and arsine Silane and dimethylzinc, diluted in hydrogen 7 0 0 - 8 0 0 °C 4/~m h -1 20:1
2
High-efficiency GaAs solar cell device processing sequence 1. Coat wafer front with SiO2 2. Evaporate A u G e back ohmic contact 3. Alloy back contact 4. Evaporate gold back metallization 5. Image-reversal photolithography for front grid 6. Etch SiO2 in grid openings 7. Evaporate CrAu front contact 8. Dissolve photoresist to liftoff excess metal 9. Sinter contacts 10. Photolithography for mesa etch 11. Mesa etch 12. R e m o v e all SiO2 13. Selective cap removal etch 14. Evaporate ZnS/MgF2 antireflection coating
F o r GaAs/Si cells, a GaAs buffer layer is first deposited onto a silicon substrate b y the now-popular three-step m e t h o d outlined here: the silicon substrates (oriented 2 ° off (100) in the (011) direction) are heated in hydrogen to over 1000 °C to remove the oxide, and the temperature is then lowered to ca. 400 °C for nucleation and deposition of ca. 200 A of GaAs; finally, a thick GaAs layer (1 #m or more) is deposited at typical MOCVD conditions, at a growth temperature of 700 °C, a growth rate of 4/~m h - ' , and a V - I I I ratio of 15:1. In some experiments, the GaAs buffer layer is formed using our thermal-cycle-growth (TCG) method, described earlier [2]. This process has been shown to lead to lower defect densities and higher minority-carrier lifetimes [2]. Once the GaAs buffer is complete, the GaAs/Si wafer can be treated like a GaAs substrate; this permits the growth of high-efficiency GaAs solar cell structures onto both GaAs and GaAs/Si substrates for sideby-side comparisons. Monolithic tandem cells consisting of a standard GaAs p-on-n cell on a germanium p-on-n b o t t o m cell are grown and fabricated b y the methods
110 h t,
D METALLIZA" CrAu CONTACT LAYI GaAs p + ANTIREFLECTION COATING WINDOW
0.¢
EMITTER
0
BASE
3.0
gm
GaAs
n
BSF
1.o
#m
AIGaAs
n+
GaAs
n+
Sl
n+
BUFFER VARIABLE
SUBSTRATE
25mils
n METALLIZATION
Fig. 1. High-efficiency GaAs cell s t r u c t u r e . Device s h o w n here is o n silicon. GaAs s u b s t r a t e s use a A u G e m e t a l l i z a t i o n a n d g e r m a n i u m s u b s t r a t e s use a gold m e t a t l i z a t i o n .
described above. However, because of the " a u t o d o p i n g " effect of germanium atoms which evolve from the germanium back surface and are incorporated into the growing GaAs and GaA1As layers, an extra "capping" step is added. This capping is done b y depositing a few microns of GaAs by MOCVD o n t o the back of the germanium wafer in a separate run. The p - n junction in the Ge b o t t o m cell is formed by in-diffusion of gallium and arsenic into the germanium from the growing GaAs cell structure during the MOCVD run. The cell interconnect is formed by the leaky G a A s - G e highly d o p e d heterojunction. 2.2. Cell results
The data of Table 3 summarize the solar cell results for our best GaAs cells deposited on GaAs, g e r m a n i u m , and silicon substrates. The illuminated I - V curve of our best GaAs/GaAs cell is shown in Fig. 2. The GaAs/Ge cells are tandem structures in which the germanium b o t t o m cell contributes a b o u t 200 mV to the open-circuit voltage; the efficiency values shown may not, however, be entirely accurate because of the difficulties which exist in the measurement of series-connected tandem cells as reported by Hart et al. [3]. These tandem cell results are from preliminary studies, and much optimization is still possible.
111 TABLE 3 GaAs solar cell results achieved at Spire Substrate
Air mass Suns
Measured by
Voc (V)
Jsc (mA
FF
Efficiency (%)
cm -2)
GaAs GaAs GaAs GaAs
0 1.5 0 1.5
1 1 170 200
SERI SERI Sandia Sandia
1.03 1.03 1.14 1.14
33.2 27.9 33.6 28.8
0.864 0.864 0.875 0.875
21.7 24.8 24.5 28.7
Ge Ge
0 1.5
1 1
NASA SERI
1.20 1.19
28.6 23.8
0.849 0.849
21.3 24.1
Si Si Si
0 1.5 1.5
1 1 370
SERI SERI Sandia
0.90 0.89 1.09
29.8 25.5 20.8
0.777 0.777 0.816
15.2 17.6 18.5
SERI data courtesy of K. Emery, Sandia data courtesy of D. Ruby, NASA data courtesy of R. Hart. All data measured at 25 °C, total area.
Spire,GaAs, global, 1000W/m 2 Sample:
1552-3-5
Temperature
M a r . 6, 1 9 8 9 3 : 4 9 p m 8
'
I
'
I g
'
I
Area '
I
I
= 25.0°C
= 0.250 cm z '
I
'
I
t
I
6
4 Voc
= 1 .0 2 9 v o l t s
2 Jsc = 27.89 mA/cm ~ Fill factor
= 86.43 %
Efficiency
= 24.8 %
I
I
0
-2
-4
, I
,
I
~
~
I
•
,
I c;
VOLTAGE
(volts)
Fig. 2. Efficiency measurement data for best GaAs/GaAs cell at 1 Sun AM1.5 conditions.
T h e e f f i c i e n c y o f o u r G a A s / S i cells h a s b e e n g r e a t l y i m p r o v e d in t h e p a s t y e a r b y t h e use o f t h e T C G t e c h n i q u e . B e f o r e its use, v e r y t h i c k G a A s b u f f e r s (ca. 8 # m ) w e r e n e e d e d t o a c h i e v e r e a s o n a b l e m i n o r i t y - c a r r i e r l i f e t i m e s ; w i t h t h e T C G p r o c e s s , h i g h l i f e t i m e s ( o v e r 1 ns) are o b t a i n e d a t a b u f f e r t h i c k n e s s o f o n l y 2 /~m. T h i s i m p r o v e m e n t in t h e q u a l i t y o f o u r
112
GaAs/Si material, coupled with optimization of the GaAs cell design and processing, has caused the efficiency to increase from 11.6 to 17.6% (onesun, AM1.5, SERI data). For GaAs/Si concentrator cells, efficiencies up to 18.5% have been achieved at 370X AM1.5 on non-optimized GaAs/Si buffers; concentrator efficiencies of near 20% are predicted for cells to be made using our latest TCG GaAs/Si material. The best reported GaAs/Si cells are currently from NTT in Japan [4]. Ohmchi e t al. [4] reported GaAs/Si cells at 20% (one-sun, AM1.5), but their measurement conditions may n o t correspond exactly to those used for the data of Table 3. The dramatic improvements in GaAs/Si cell efficiency are a direct result of improved materials growth techniques which reduce the density of latticemismatch-related threading dislocations. The dislocations act as recombination sites, increasing space-charge recombination current and reducing minority-carrier diffusion lengths. Figure 3 shows the calculated and measured effect of dislocation density on solar cell efficiency [5, 6]. The data include recent Spire results where dislocations were deliberately introduced into GaAs cells on GaAs substrates using GaAsP lattice-mismatched buffer layers. Initial GaAs/Si material had dislocation densities in the range of 10 s cm -2. Through thermal cycling techniques, this was reduced to about 10 ~ cm -2. Recent work with strained-layer superlattices has reduced this still further to the low 106 c m - : range. Another factor of 10 decrease in dislocation density will give efficiencies (and dislocation densities) essentially equal to those on bulk GaAs substrates.
2 5
. . . . . . . .
,
. . . . . . . .
o
~ " 20
,
. . . . . . . .
o
,
. . . . . . . .
,
~N'i-F M
o
~
. . . . . . . .
Oe
'
......
0
'15
LO
,
•
(')
"6
. . . . . . . .
ooo
0
5
0
103
........
S p i r e Go
•
Spire GaAs/Si
o
N"Cr OL O a A e / S i
•
NTTAELGoAs/Si '
........
104
'
105
[3] [36]
[13]
........
'
10 6
........
'
........
107
108
109
Dislocotion Density (cm - 2 )
Fig. 3. Efficiency of GaAs and GaAs/Si cells as a function of threading dislocation density.
3. InP solar cells
Recently, much effort has been devoted to the study of InP solar cells for space applications. This work was triggered by the discovery that
113 exposure to radiation, as in Earth orbit, causes less damage to the photovoltaic performance of these cells than to t h a t of GaAs or silicon cells. Furthermore, the damage which is done can be annealed at a relatively low temperature [7, 8]. The development of high-efficiency InP-based solar cell structures has proceeded quickly [9 - 15]; efficiencies up to 18.8% (AM0, 25 °C) have been measured [9]. Theory predicts an attainable beginning-oflife efficiency for this material nearly the same as that for GaAs. Because of the superior radiation resistance, this would correspond to a considerably higher end-of-life efficiency in typical space applications than for any other k n o w n material. One disadvantage of InP from a photovoltaic point of view is its cost. This could presumably be reduced in the face of a large demand for space solar cells, but is n o t expected to be much lower than for GaAs. The largest commercially available InP wafers are of 75-mm diameter. Another disadvantage of InP which is particularly important for space applications is its density (4.8 g cm-3). Although the active region of an InP solar cell is only about 3 pm thick, the low mechanical strength of the material would make handling of cells of less than 150 pm very difficult. This corresponds to 72 mg cm -2, or 2.7 g W-1. The use of silicon substrates addresses both of these problems. Silicon wafers are commercially available in sizes up to 200-mm diameter, at costs which would contribute negligibly to the final cost of space solar cells. Silicon has a lower density than InP (2.3 g cm-3), and is considerably stronger. Cells of 50-gm thickness, which weigh only 12 mg cm -2, are currently produced for space applications. 3.1. Cell fabrication InP solar cells have been fabricated in our laboratory on InP, GaAs, and GaAs-coated silicon substrates. The structure of these cells is shown in Fig. 4. The use of a GaAs buffer layer on the silicon substrates has been found to facilitate greatly the deposition of high-quality InP films [16]. The InP layers are grown by MOCVD, using trimethylindium and phosphine at a growth temperature of 600 °C. For the InP-GaAs/Si structures, the GaAs/Si film is deposited by our standard process as described above. The frontsurface ohmic contacts to InP use a Cr/Au/Ag metallization, whereas the back-surface contacts are formed from A u - Z n on InP or GaAs substrates, or from A1-Ti-Pd-Ag on silicon substrates.
3.2. Cell results Experimental results of InP cells fabricated at Spire are shown in Table 4. The efficiency value of 18.8% AM0 is the best InP cell reported to date. For this cell, the InP base layer was grown by MOCVD and the N ÷ emitter was formed by ion implantation. For the other cells of Table 4, the cell structures were formed totally by MOCVD. The higher efficiency of the implanted-junction cell vs. the alI-MOCVD cell reflects the degree of
114 Light
\
Front Grid
/
DLARC
Cr/Au/Ag ZnS/MgF
N+InP (.05~)
Emitter
Base
P InP (3ju}
Buffer
P+lnP ( l p )
P+InP, GnAs, G e A s ( l p ) SJ
Subatrate
Back C o n t a c t
~,/////////,-/A
Au-Zn, Au-Zn, A I - T i - P d - A g
Fig. 4. Diagram of InP solar cell structures.
TABLE 4 InP solar cells fabricated at Spire Substrate
Junction formation
Air mass
Measured by
Voc (V)
Jsc (mA cm -2)
FF
Efficiency (%)
InP InP GaAs GaAs/Si
I/I-MOCVD MOCVD MOCVD MOCVD
0 0 0 0
NASA NASA Spire Spire
0.873 0.868 0.672 0.636
35.7 33.9 27.4 25.6
0.829 0.838 0.701 0.601
18.8 17.9 9.4 7.2
All data measured at 1 Sun, 25 °C, total area. N A S A data from LeRC courtesy of R. Hart.
optimization of these t w o processes in our laboratory at the time of this work. The spectral response of InP solar cells fabricated on three different substrates is s h o w n in Fig. 5. The low red response for the cells on the GaAs and GaAs-coated silicon substrates is indicative o f the low minoritycarrier lifetime values for the heteroepitaxial layers. The poor material quality is due to the high number of dislocations caused by the lattice mismatch between InP and either GaAs or silicon. Plan-view TEM studies [16] have s h o w n that InP has ca. l 0 s dislocations cm -~ when deposited on GaAs or GaAs/Si substrates. Experiments are currently under way to improve material quality by the use of the thermal-annealing or strainedlayer growth techniques that have proven to be reasonably successful in the GaAs/Si field; defect densities in the low 10 6 cm -2 range are expected. If this is achieved, then cell efficiencies similar to those o f InP homoepitaxial
115 1.0 (J
.. ..... . s - " "
t-
0.8 E 0.6
/
-~ ~r 0.4 •"
-6
E
0.2
c
..... ::'::-
~
-~
/J ~"
|.
silicon substrate
- - - G(JAssubstrate • • • InP substrate " T " calculated fo; L = 0.2.5 #m
, 0.0 300
500
700 wavelength (nm)
I ~. 900
Fig. 5. Spectral response o f InP cells on GaAs and silicon substrates. The low red response is the result o f a high defect density. The response o f a homoepitaxial InP cell is shown for comparison. 22 20
g
"5 14 ~ 12
0
8 6
InP on Si l n P o n GoAs • InP on InP -calculated for beginning-of-life --- calculated for end-of-life
•
•
i
104
(1015
crn -2
e-
@ 1 MeV) i
i
i
105 106 107 108 dislocation density (cm -2)
•
..-
109
Fig. 6. Comparison of theoretical and experimental efficiencies of InP cells as a function of dislocation density. Not only are similar beginning-of-life efficiencies predicted, but also the end-of-life efficiency is less sensitive to the dislocation density.
cells are expected, as shown in Fig. 6. This figure shows the calculated and measured efficiencies of InP solar cells and is based upon a PC-1D analysis and the theory of Yamaguchi e t al. [17] relating lifetime to dislocation density.
4. MOCVD system scale-up for solar cell production In recent years, MOCVD has been established as a viable method for the production of high-quality III-V epitaxial materials. Photovoltaic GaAs devices are currently being produced commercially by this method. However, at present, high-quality material with good uniformity (less than 10% thickness and doping variation) has been realized only in relatively small reactors. For photovoltaic materials to be produced in the quantities
116
needed to become a significant alternative energy source, large deposition systems are becoming increasingly of interest. Therefore, we have developed a new, large-scale, barrel-type MOCVD reactor capable of growing highquality III-V solar cell structures on seven 3-in. diameter or 14 2-in. diameter substrates with good uniformity.
4.1. Apparatus Figure 7 presents a schematic illustration of the SPI-MO C V D TM 1200 system used for the growth of the GaAs layers. The reaction chamber features a hollow, barrel-type, SiC-coated graphite susceptor with seven facets tilted 10 ° from the vertical. Each facet has recesses which can hold either two 2-in. wafers or one 3-in. wafer. The quartz bell-jar has a 10-in. inside diameter, is ca. 27 in. tall, and is shaped to follow closely the contours of the susceptor. The bell-jar can be water cooled or air cooled as desired. The susceptor is inductively heated by an external r.f. coil to a preset temperature which is monitored by an optical pyrometer. The pyrometer is connected to a sapphire light pipe that is inserted into the center of the susceptor. Gas leaves the reactor through eight ports equally spaced around the base of the bell-jar and connected to a circular exhaust plenum. The GAS FLOW
Fig. 7. S c h e m a t i c i l l u s t r a t i o n o f t h e SPI-MO C V D TM 1 2 0 0 r e a c t i o n c h a m b e r .
117
entire system is operated by a personal computer interfaced to an industrial controller and can be operated in either an atmospheric- or reduced-pressure mode. 4.2. Computer simulations The steady-state operating conditions of the new reactor were modelled using a numerical technique described earlier [18, 19]. The model simultaneously solved the equations of continuity, motion, differential thermal energy balance, and mass conservation for the various species in an axisymmetric system. In addition, allowance was made for variable property {temperature and composition dependent) values. The results of the simulation for atmospheric and low pressure operation are summarized in Figs. 8 and 9. Although both pressure regimes can lead to the attainment of good thickness uniformity, Fig. 8 shows a large difference in the flow patterns. The thermal convection loop shown for atmospheric pressure (Fig. 8a) does not permit the growth of abrupt interfaces and can cause difficulties in controlling the deposition of thin layers; this recirculation does, however, lead to a high source-utilization efficiency. At low-pressure operation (Fig. 8b), we see that the recirculation loop is eliminated; this flow pattern is optimal for depositing well-controlled thin layers and abrupt interfaces, but the source-utilization efficiency is somewhat lower. 4.3. Flow visualization Before the initiation of epitaxial growth experiments, the gas flow dynamics of the new reactor were studied experimentally using standard
i
;
1.0 atm
(a)
(b)
Fig. 8. C o m p u t e r simulation o f gas flow streamlines calculated for a susceptor temperature o f 650 °C, total gas flow rate o f 20 1 min -1, and (a) I atm and (b) 0.1 a t m p r e s s u r e s .
118 ._c E
0.10
....
I ....
_
E oo8
I ....
i ....
Wafer Deposition Zo°e
I
I
,~@ 0 . 0 6 .\ I= O
0.04
0 O. @
0.02
( a ) ...............................
I
0.00 0
I
I
I
I 0.05
I
I
I
I
I
0. I
i
i
i
m
(b)
i
I
0.15
I
I
I 0.2
Susceptor Flat Position (m)
Fig. 9. Predicted growth rate vs. susceptor facet position for each case depicted in Fig. 8.
flow visualization techniques [20]. A separate gas-handling system featuring titanium tetrachloride and de-ionized water bubblers, manual flow-meters, and helium as the carrier gas was connected to the reaction chamber for this purpose. Approximately equal amounts of the titanium tetrachloride and water (about 200 sccm) were injected into the carrier gas line just before the inlet to the bell-jar. This resulted in the formation of titanium dioxide " s m o k e " which, when illuminated by a He-Ne laser, revealed the details of the gas flow pattern inside the chamber. The susceptor temperature was kept at or slightly below 200 °C to prevent excessive clouding of the belljar by titanium dioxide deposition. The effects of reactor pressure and gas flow rate were investigated. Figure 10 illustrates the principal results of these experiments. At atmospheric pressure and for all flow rates from 10 to 40 standard liters per minute (slpm), a free convection loop was observed in the vicinity of the susceptor flat as seen in Fig. 10(a). On the other hand, at low pressure, streamlined laminar flow (also called Poiseuille flow) is observed for flow rates above about 15 slpm as shown in Fig. 10(b). These results confirm the predictions of the computer model. 4.4. Epitaxial growth experiments A number of GaAs growth runs have been performed and characterized in the new 1200 reactor. For all these experiments, a water-cooled bell-jar was used, and the chamber pressure was kept constant at 76 Torr. Although the growth parameters have not y e t been completely optimized, highly uniform layers over the area of a 3-in. wafer have been achieved; uniformity values of +3 and +8% (standard deviations) for thickness and doping, respectively, have been measured on our best wafers to date. Over the many experiments performed in this investigation, thickness and doping u n f o r m i t y were routinely observed to be better than -+6 and +12% over a wide range of operating conditions, with wafer-to-wafer uniformity being about the same.
119
,
i
b (a)
(b)
Fig. 10. Gas flow pattern at (a) atmospheric pressure and 40 slpm and (b) low pressure (0.1 atm) and 20 slpm.
5. Conclusions Experimental results from a number of solar cell structures have been described. For GaAs cells, AM1.5 efficiencies of over 24% have been achieved on both GaAs and germanium substrates. For InP cells, AM0 efficiencies of over 18% have been described and preliminary cell results for InP-GaAs and I n P - G a A s - S i structures have been presented. The MOCVD growth of lattice-mismatched heteroepitaxial structures shows promise for achieving high cell efficiency using low-cost/low-weight substrates, and research in this area is advancing. In addition, a large-scale MOCVD system has been developed and modelled, and preliminary smoke-test and growth data show the ability of this reactor to achieve excellent process control. Acknowledgments Funding for various portions of this work has come from SERI (Golden, CO), NASA (Cleveland, OH), Wright-Patterson AFB (OH), and
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ARO (Research Triangle, NC), and their support is gratefully acknowledged. The authors also thank Drs. J. Szekely and A. Dilawari of the Massachusetts Institute of Technology for their computer modelling efforts.
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