NASA Contractor Report 3361 

GaAlAslGaAs Solar Cell 

Process Study 

David W. Almgren and Katinka I. Csigi 

CONTRACT NAS l- 15 5 1 G 

DECEMBER 1980 

NASA 

CR 

336 1 

c-1 
TECH LIBRARY KAFB, NM 

llnlllIllyTllIlliusl

00bll93~7 

NASA Contractor Report 3361 

GaA1Asf GaAs Solar Cell 

Process Study 

David W. Almgren and Katinka I. Csigi 

Arthur D. Little, Inc. 

Cambridge, Massachusetts 

Prepared for 

Langley Research Center 

under Contract NASl-15 5 16 

National Aeronautics 

and Space Administration 

Scientific and Technical 

Information Branch 

1980 

TABLE OF CONTENTS 

Page 

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . vi 

I. 

II. 

Ill. 

IV. 

SUMMARY. ......................... 1 

INTRODUCTION ....................... 3 

A. PURPOSE AND SCOPE ................... 3 

B. METHODOLOGY ..................... 3 

THESINGLE CRYSTALGALLIUMARSENIDE SOLAR CELL ..... 7 

A. PRINCIPLES OF OPERATION ................ 7 

B. CHOICE OF DOPANTS ................... 13 

C. CONTACTING ...................... 14 

D. RADIATION DAMAGE EFFECTS. .............. 15 

E. SUMMARY OF SIGNIFICANT CELL DESIGN PARAMETERS ... 18 

DEFINITION OF EPITAXIAL GROWTH PROCESSES ......... 19 

A. INTRODUCTION ..................... 19 

B. LIQUID PHASE EPITAXY .................. 19 

C. VAPOR PHASE EPITAXY .................. 21 

D. MOLECULAR BEAM EPITAXY. ............... 26 

E. OTHER GROWTH PROCESSES. ............... 29 

F. CHOICE OF EPITAXIAL GROWTH PROCESSES FOR FURTHER 

COMPARISON ...................... 29 

V. COMPARISON OF EPITAXIAL GROWTH PROCESSES ........ 31 

A. COMPARISON METHODOLOGY ............... 31 

B. COMPARISON FACTORS .................. 31 

1. Material Usage ..................... 31 

2. Energy Consumption .................. 33 

3. Labor Hours Required .................. 35 

4. Degree of Control or Reproducibility ............ 37 

5. Problems and Potential for Scale Up and Automation ...... 38 

6. Cost ........................ 42 

iii 
TABLE OF CONTENTS (Continued) 

Page 

VI. CONCLUSIONS AND RECOMMENDATIONS ............ 49 

APPENDIXA ........................... 51 

APPENDIX B ........................... 53 

iv 
LIST OF TABLES 

Table No. Page 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

Summary of Major Direct Costs Per Cell for Growth 

of the Epitaxial Layers of 10,000,2 cm x 2 cm (AIGa) 

As/GaAs Solar Cells . . . . . . . . . . . . . . . . 

Summary of Dopants Used with GaAs . . . . . . . . . 

Summary of Contacting Materials Currently Being Used 

with (AIGa)As/GaAs Solar Cells . . . . . . . . . . . 

Summary of Material Requirements to Fabricate 10,000 

2 cm x 2 cm GaAs Solar Cells Assuming No Recovery 

of Lost Material . . . . . . . . . . . . . . . . . 

Energy Consumption Per 2 cm x 2 cm Cell by Fabrication 

Process . . . . . . . . . . . . . . . . . . . . . 

Summary of Fabrication Times for Different Epitaxial 

Growth Procedures Using One Growth Chamber of Current 

Decision . . . . . . . . . . . . . . . . . . . . 

Statistical Summary of AM0 Efficiencies of Small Lots of 

(AIGa)As/GaAs Space Solar Cells . . . . . . . . . . . 

Unit Cost of Materials Utilized in Fabrication of (AIGa)As/ 

GaAs Solar Cells . . . . . . . . . . . . . . . . . 

Cost of Materials for Fabricating 10,000 2 cm x 2 cm 

(AIGa)As/GaAs Solar Cells . . . . . . . . . . . . . 

Summary of Energy Costs by Fabrication Process . . . . 

Summary of Unburdened Labor Costs to Grow Epitaxial 

Layers for 10,000 (AIGa)As/GaAs Solar Cells with a Single 

Growth Chamber . . . . . . . . . . . . . . . . . 

Summary of Capital Equipment Costs for Different Solar 

Cell Fabrication Processes Assuming a Minimum Number of 

Growth Chambers . . . . . . . . . . . . . . . . 

Summary of Major Direct Costs Per Cell for Growth of the 

Epitaxial Layers of 10,000, 2cm x 2cm (AIGa)As/GaAs 

&lar&lls . . . . . . . . . . . . . . . . . . . 

. . . 2 

. . . 13 

. . . 14 

. . . 34 

. . . 35 

. . . 37 

. . . 39 

. . . 43 

. . . 44 

. . . 45 

. . . 46 

. . . 47 

. . . 48 

V 
Figure No. 

LIST OF FIGURES 

Page 

8 

9 

10 

11 

12 

13 

Physical Characteristics of Space Designed 

(AIGa)As/GaAs Solar Cell Considered in this Study . . . . . . 4 

Number of Photons in AM0 Solar Spectrum Having 

Energies Greater than 1.43 eV . . . . . . . . . . . . . . 8 

Absorption Coefficient for GaAs and Gal-xAIxAs 

as a Function of Wavelength . . . . . . . . . . . . . . . 9 

Number of AM0 Photons Absorbed in Discrete Layers 

of GaAs as a Function of Depth (300K) . . . . . ‘. . . . . . 10 

Summary of Contacting Geometry Being Used with 

GaAlAs Layer . . . . . . . . . . . . . . . . . . . . . 1.2 

Maximum Output Power (4 cm2) Versus 1MeV 

Electron Fluence . . . . . . . . . . . . . . . . . . . 16 

Predicted (AIGa)As-GaAs Solar Cell Short Circuit Current 

Density Versus 1MeV Electron Radiation Fluence as a 

Function of Junction Depth . . . . . . . . . . . . . . . 17 

Typical Growth Apparatus for Infinite Melt Liquid 

Phase Epitaxial Process . . . . . . . . . . . . . . . . . 20 

Typical Growth Apparatus for Finite Melt Liquid Phase 

Epimxial Process . . . . . . . . . . . . . . . . . . . 22 

Schematic Drawing of a VPE Growth Reactor . . . . . . . . 23 

Simplified Schematic Diagram of Laboratory Type MO-CVD 

Apparatus for Growth of GaAs and GaAlAs . . . . . . . . . 25 

Schematic Diagram of a Molecular Beam Epitaxy System 

for the Growth of a AlxGal-xAs/Ga as Solar Cell 

(1 Torr= 133.3Pa). . . . . . . . . . . . . . . . . . . 27 

Temperature-Time Schedule for Each Effusion Cell for 

Fabricating a Graded Band-GAP AlxGal-xAs Solar Cell 

by Molecular Beam Epitaxy . . . . . . . . . . . . . . . 28 

vi 
I. SUMMARY 

This study was conducted to collect information on (AlGa)As/GaAs fabrication processes 

which could produce single crystal solar cells for use in space and to assess the relative merits of 

the different processes for pilot line production in the early 1980’s. The production rate assumed 

for this study was based on the launch of one 500 watt array per year, approximately 5,000 to 

10,000, 2cm x 2cm, apace-qualified cells (18% AMO). 

Discussions with several potential solar cell suppliers indicated that this production rate 

could be satisfied with existing facilities. The current production of gallium arsenide solar cells is 

typically only a small part of the total current production of gallium arsenide devices. For 

example, more than 100 times the current budget for solar cells is being spent on microwave 

devices by at least one company. This situation is the result of the very small current market for 

(AlGa)As/GaAs solar cells and the small future market perceived for space-qualified cells of this 

type. 

Three basic growth processes were identified which could be used to fabricate the epitaxial 

layers on a single crystal, (AlGa)Ae/GaAs solar cell. The processes are: 1) Liquid Phase Epitaxy 

(LPE), including infinite melt and finite melts, 2) Metal-Organic Chemical Vapor Deposition 

(MO-CVD) and 3) Molecular Beam Epitaxy (MBE). Table 1 summarizes the major direct costs 

per cell for growth of the epitaxial layers by three processes for a production quantity of 10,000, 

2cm x 2cm solar cells. 

The infinite melt LPE process, with 100% yield, has the lowest per cell cost because of its 

ability to fabricate all of the cells in a time period of 4 months, thereby reducing labor costs. This 

production rate requires holders with a capacity to hold 32, 2cm x 2cm substrates at one time. 

The current MO-CVD process, with an enlarged growth station and 100% yield, has the 

potential to produce cells at the same cost per cell as the infinite melt LPE process. A significant 

uncertainty with the MO-CVD process is the yield of the growth system, i.e., the fraction of the 

active material in the input gas stream that ends up in the epitaxial layers. 

The MBE process is capable of producing well defined epitaxial layers but it is a slow and 

expensive process because of the associated labor and capital equipment costs. 

Additional work is needed in the area of defining and documenting the detailed process 

parameters and yield for the MO-CVD system so that a more accurate determination of its 

economic potential to produce space quality (AlGa)As/GaAs solar cells can be made. 

I .111-11.---m.- mm,-- ..,,.- --.--.,..--,1..-..--,..-,.-.. ,--.-mm I.--_--.--,-,,,,-.-.-- .1.~.-..11.--..~1--~--1---I~-I 
TABLE 1 

SUMMARY OF MAJOR DIRECT COSTS PER CELL FOR GROWTH 

OF THE EPITAXIAL LAYERS OF 10,000,2cm x 2cm (AIGa)As/GaAs SOLAR CELLS 

l Excluding overhead and profit 

Process 

LPE - Infinite Melt 

- Finite Melt 

MO-CVD 

MBE 

Yield 

(%I 

100 

100 

10 

1 

100 

10 

1 

100 

10 

1 

50 

5 

50 

5 

Time to 

Fabricate 

(months) 

4 

63 

63 

63 

31 

31 

31 

3 

3 

3 

344 

344 

31 

31 

Material Unburdened 

costs Labor Costs 

$ .006 

$ .006 

$ .06 

$ .62 

$ .23 

$ 2.29 

$22.94 

$ .23 

$ 2.29 

s 22.94 

§l .03 

$ .30 

$ .03 

$ .30 

$ 1.54 

$14.11 

$14.11 

$14.11 

$ 6.94 

$ 6.94 

$ 6.94 

$ 1.15 

$ 1.15 

$ 1.15 

$77.06 

$77.06 

$ 6.94 

$ 6.94 

Capital 

Equipment 

COStS 

Energy cost 

costs per Cell’ 

S .32 $ .016 

s 2.00 $ .015 

s 2.00 $ .015 

% 2.00 $ .015 

$ 3.88 $ .002 

$ 3.88 $ 902 

$ 3.88 $ .002 

$ .38 $ .002 

$ .38 $ .002 

$ .38 $ .002 

$25.00 $ .79 

$25.00 $ .79 

$12.92 $ .72 

$12.92 $ .72 

$ 1.88 

$ 16.13 

S 16.19 

$ 16.75 

$ 11.05 

$ 13.11 

$ 33.76 

$ 1.76 

$ 3.82 

$ 24.47 

$102.88 

$103.15 

$ 20.61 

$ 20.88 

1. Does not include a $l.24/cm2 purchase price for polished substrates. 
II. INTRODUCTION 

A. PURPOSE AND SCOPE 

The purposes of this study were 1) to collect and organize available information on major 

crystal growing processes that could be used to fabricate single crystal, heteroface, gallium 

arsenide solar cells and 2) to assess the potential of these processes to produce space - qualified 

cells in the early 1980’s. The specific cell configuration that was examined (see Figure 11 has a 

three layer structure, namely a p-Ga,.,Al,As window, a p-GaAs layer and a n-GaAs layer (which 

includes the n-GaAs substrate). 

A similar study was performed in 1975 by the Air Force with the consulting assistance of A. 

E. Middleton”. At that time only a limited amount of experience had been accumulated using 

Mg and Be as dopants and results from the metal-organic chemical vapor deposition and 

molecular beam epitaxy growth techniques were not available. Since that time, additional 

measurements have also been made on the radiation resistance of the heteroface cell. The earlier 

work was concerned with a production rate of 10,000, 1 cm2 (AlGa)As/GaAs solar cells per day of 

which 100 cm2 were space qualified cells. 

The production goal assumed for the current study was the fabrication of enough single 

crystal, gallium arsenide solar cells to support the launch of one 500 W, space-qualified, solar cell 

array per year. For a nominal (unconcentrated) solar constant of 1353 W/ma and an 18% AM0 

(25°C) cell, the production goal translates into a minimum of 5200, 2cm x 2cm, space-qualified 

cells per year. Additional cells will be needed to account for production and handling losses and 

spares, as well as any decrease in operating cell efficiency due to elevated temperatures or 

radiation damage on-orbit. We assumed, therefore, an annual production capacity of 10,000,2cm 

x 2cm cells per year (40 cells per day for 250 working days per year). 

The successful fabrication of space-qualified solar cell arrays involves additional steps after 

growing the epitaxial layers such as metallization, bonding to the array structure, interconnecting 

the cells and testing. The objective of this study was an assessment of the epitaxial growth 

processes so that a suitable cell fabrication process could be selected by NASA for further 

development of the necessary additional steps leading to space-qualified, gallium arsenide, solar 

cell arrays. 

B. METHODOLOGY 

The methodology established in the Statement of Work for the assessment of the various 

cell fabrication processes was to quantify, where possible, the following process parameters: 

0 materials usage 

l energy usage 

l manpower required 

l degree of control and repeatability 

l process yields 

l problems and potential for scale up and automation 

0 cost. 
p-(AIGa)As Layer 

Thickness: < 0.5pm 

Carrier Concentration: 10” cmS3 

Al Fraction: 0.85 to 0.95 

I 

----------------_- 

, p-GaAs Layer 

Thickness: 0.4 to 1 .Opm 

Carrier Concentration: < 10” cm9 

Minority Carrier Diffusion 

Length: > 3pm 

Y n-GaAs Layer 

Thickness: 250pm 

Carrier Concentration: 1017 cme3 

Minority Carrier Diffusion 

Length: > 2pm 

FIGURE 1 PHYSICAL CHARACTERISTICS OF SPACE DESIGNED 

(AIGa)As/GaAs SOLAR CELL CONSIDERED IN THIS STUDY. 
Because of the differences in funding levels that have been allocated to develop the various 

cell fabrication processes, it was assumed that today’s best documented cell efficiencies do not 

define a quality factor for comparing the different processes. The approach was to assume that all 

celrfabrication processes could produce an equally efficient cell (l&20%) and then define current 

and projected costs for these cells and additional development necessary to achieve these costs 

A study of this type is limited by the uncertainties inherent in trying to quantify production 

type data, e.g., process yields, from laboratory scale experiments. Much of the published literature on gallium arsenide solar cells deals with the fabrication of individual cell structures of 

small size. The investigators were primarily interested in defining cell growth parameters that 

would lead to more perfect crystalline structures and optimum doping profiles. Problems 

associated with materials and energy usage, manpower requirements, control of process parameters over large areas and economics of higher production rates were not typically considered. Our 

methodology was to document the available process parameters and to parametrically examine 

significant uncertainties when process data was not available. 

III. THE SINGLE CRYSTAL GALLIUM ARSENIDE ’ 

SOLAR CELL 

The purpose of this section is to define the basic cell structure before discussing the various 

cell fabrication procedures in Section IV. 

A. PRINCIPLES OF OPERATION 

The space-type, gallium arsenide solar cell under consideration for this study would be used 

to convert an unconcentrated, AM0 solar spectrum into electrical energy. The basic structure of 

the GaAs solar cell is a GaAs p-n junction covered by a window layer of (AlGa)As (to reduce 

surface recombination losses), covered by an anti-reflection coating and provided with ohmic 

electrical contacts. GaAs has a direct band gap of 1.43eV (3OOK) and, based on NASA’s standard 

AM0 solar spectrum datatzl there are 2.37 x 10” photons/(cm’-second) incident on a normal 

surface capable of creating a hole-electron pair in GaAs. Once the electron-hole pair has been 

created, the collection of these photogenerated carriers by movement across the p-n junction is in 

competition with the loss of these carriers by bulk and surface recombination. If every electronhole pair were collected by the p-n junction, a maximum short circuit current, I,, of 38 ma/cm2 

would be realized. 

Figure 2 shows a histogram of the number of incident photons in the AM0 solar spectrum 

having an energy of at least 1.43eV, broken down into discrete wavelength bands of 0.1 pm width. 

Using the absorption coefficient shown in Figure 3 for GaAs I’), the number of electron-hole pairs 

created in the material as a function of depth is shown in Figure 4. The data in Figure 4 assumes 

no reflection losses at the surface or absorption losses in the Gal.,A1,As window layer. 

The lifetime of a minority carrier (L, for electrons, L, for holes) in doped GaAs is in the 

range of 0.1 to 6.0 x 10-O sec. depending on the level of doping and method of crystal growth;“’ 

Experimentally, the lifetime in GaAs decreases with increasing doping level but, in practice, 

there is no unique lifetime at a given doping level. For a finished device, the realized lifetime 

depends on the surface treatment during fabrication, the diffusion temperature, the rate of 

cooling and the presence or absence of annealing steps; I” Closely associated with the lifetime of a 

minority carrier is the distance a free charge carrier can diffuse in one lifetime period (termed the 

diffusion length). I 

Typical values for diffusion lengths in a space type GaAlAs/GaAs solar cell are 2 pm for the 

n-GaAs bottom or substrate layer and 3 pm for the p-GaAs middle layer. The longer diffusion 

lengths obtained for the electrons in the p-type GaAs material is a reason to design the cell so that 

the majority of the hole-electron pairs are created in the p-type material. An n+ on p-type GaAs 

cell, with a top n+ layer thickness of only 200 A (without a GaAlAs window layer) has also been 

fabricated so that the majority of the electron-hole pairs are created below the junction in the ptype material.‘61 

As shown in Figure 4, for a junction depth of 0.6 pm below the surface of a GaAs cell 

(neglecting any absorption by the window layer), ‘78% of the incident photons would be absorbed 

in the p-type material before reaching the junction. As the junction depth is decreased to enhance 

the radiation resistance of the solar cell, more of the charge carriers will be generated below the 

junction in the n-type material. With a 0.2 pm junction depth, for example, approximately 57% of 

the electron-hole pairs would be created above the junction. 

7 
‘% 

?J v, 7.0 

a’ 6.0 

--& 

- 5.0 

Total Number of Arriving Photons 

t2.37 x 1017 

Having Energy Greater Than 

.l .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 

Wavelength (pm) 

l Divided into discrete spectral bands 

FIGURE 2 NUMBER OF PHOTONS IN AM0 

SOLAR SPECTRUM HAVING 

ENERGIES GREATER THAN 1.43 eV. 

8 
106 

- 

'6 

105 

E2 104 

0" 0 

5 .- ~103 

!? 

a 

2 

102 

10’ I I, I 

.4 .6 .8 

Wavelength (pm) 

FIGURE 3 ABSORPTION COEFFICIENT FOR 

GaAs AND Gal -,AI,As AS A 

FUNCTION OF WAVELENGTH. 
Depth Range 

0.0-O. 10 pm 

.04 .06 

a) 

II 1 

.08 .lO 

200 

100 

Depth Range 

0.0-l .O E.cm 

Percentage 

Breakdown of 

All Incident Photons 

78%122% 

200 

100 

- Depth Range 

0.0-5.0 pm 

.2 .4 .6 .8 1.0 0.0 1.0 2.0 3.0 4.0 5.0 

b) cl 

Depth (pm) 

0 All incident photons are assumed to enter GaAs material 

0 No window layer 

FIGURE 4 NUMBER OF AM0 PHOTONS ABSORBED IN 

DISCRETE LAYERS OF GaAs AS A FUNCTION 

OF DEPTH (300K). 

10 
For non-space applications the actual junction depth, i.e., the width of the p-GaAs region is 

not as an important parameter when the window layer is very thin.‘@) Only for a deep junction 

solar cell, > 1 .O pm, is the short circuit current, I.,, significantly reduced as the electron diffusion 

length is decreased. For space application it has been shown that the p-GaAs thickness is an 

extremely important parameter for radiation damage control.“’ At a given fluence of 1 MeV 

electron/cm2, the short circuit current is higher for cells with shallower junction, e.g., 0.5 pm. This 

situation is due to the shorter diffusion lengths caused by the radiation damage, The significance 

of this requirement for shallow junctions in the space-type cells is in the relative abi1it.y of the 

different cell fabrication processes to control the thin layer thicknesses and dopant profiles 

uniformly over large areas so that optimized (AlGa)As/GaAs solar cells can be produced in 

significant quantities with a high yield. 

The primary purpose of the p-GaAlAs window layer on the p-GaAs material is to reduce the 

recombination velocity at the GaAs “surface” (the interface between the two materials). Surface 

recombination is a significant cause of low power conversion efficiencies in conventional GaAs 

solar cells, an effect that is augmented by the high absorption coefficients of GaAs to the AM0 

solar spectrum. The recombination velocity at the front of a standard GaAs solar cell is in the 

range of lp - loo cm/set. At the Ga,.,Al.As-GaAs heterojunction the recombination velocity is 

typically 10’ cm/set or lower. 

Increasing the Al fraction in the Ga,.,Al.As increases the overall efficiency of the cell 

because of reduced optical absorption in the (AlGa)As. It is therefore desirable to fabricate cells 

with as much Al incorporation as is compatable with other factors which determine overall cell 

efficiency. When x > .96, for example, a visible oxide forms on the cell surface immediately upon 

removal from H,. When x - .95, several hours are required before the oxide film becomes visible; 

forx - .92, several days are needed.‘8’ 

Increased doping of the Ga,.,Al,As window lowers its sheet resistivity and thereby decreases 

the series resistance in the cell. The level to which the window layer can be doped while 

maintaining the desired junction depth in an LPE process is a function of the choice of dopant, 

the concentration of dopant in the melt and the time-temperature growth procedure. With Be, for 

example, there is an initial high diffusion rate of Be into the GaAs material until a few hundred 

angstrom thick layer of (AlGa)As forms on the surface.‘@’ 

The junction depth is particularly sensitive to solution concentration when slow growth 

rates are used. Slow growth rates are important to allow for orderly growth of the window layer to 

maintain good crystal perfection as well as avoiding dopant segregation at the growth interface. 

An increase in the dopant concentration in the (AlGa)As melt will cause a deeper junction and a 

decreased average lifetime in the p-type material. 

An approach to minimizing the effect of too high a doping level in the (AlGa)As layer, while 

also making low resistance metallic contacts directly to (AlGa)As, is to grow an additional heavily 

doped layer of GaAs on top of the (A1Ga)A.s. Ia) During the growth of this p-type GaAs contacting 

layer, diffusion occurs from that layer into the window layer underneath, thereby substantially 

reducing the (AlGa)As sheet resistivity without increasing the junction depth in the GaAs. After 

metallization is applied to the contacting layer, the uncovered p-type GaAs contacting layer is 

selectively etched away, stopping at the (AlGa)As layer (see Figure 5). 

11 
AR coating p-contact I 

w p-Gq+&As 

t 

--A--__ p-GaAs 

n-GaAs 

p-contact 

p-Gal-xAIxAs 

------___ p-GaAs 

n-GaAs 

--WA____ 

n-GaAs 

---__-_- 

n-GaAs 

AM0 

(Same grid pattern 

as used with 

2cm x 2cm silicon cell) 

Terrestrial 

Concentrator 

Cell 

Terrestrial 

Concentrator 

Cell 

Terrestrial 

Concentrator 

Cell 

0 Not to scale 

FIGURE 5 SUMMARY OF CONTACTING GEOMETRY BEING USED WITH GaAl As LAYER. 

12 
6. CHOICE OF DOPANTS 

Table 2 summarizes the p-type and n-type dopants that could be used with GaAs. N-type 

substrates are typically manufactured by the horizontal Bridgman technique with Si, Se, Te or 

Cr as the n-type dopant. The solar cell business of the substrate suppliers typically represents less 

than 15% of their total business so that generally the photovoltaic community buys the same 

material that is being made for FET’s or LED’s. 

TABLE 2 

SUMMARY OF DOPANTS USED WITH GaAs 

P-Type 

Be 

Cd 

Mg 

Mn 

Zn 

Amphoteric N-Type 

Ge Cr 

Si Se 

Sn Te 

Many fabricators of GaAs solar cells believe that the less than perfect quality of the n-type 

substrate material as purchased will degrade cell efficiency unless steps are taken to mask the 

substrate defects with a n-type buffer layer or to leach out the substrate impurities as a first step 

in the cell fabrication process. 

The GaAs solar cells fabricated by one organization use Te doped substrate material having 

a dopant concentration of 5 x 10” cm- a.o) This material was chosen for economic reasons in that 

it was already developed for use in LED’s. The highly doped substrate also makes contacting with 

Au-Ge-Ni easier. A 2 10 pm buffer layer of n-GaAs is then grown on the substrate, doped to a level 

of 1 x 10” cmes using tin. Tin has a low vapor pressure and well behaved segregation coefficient 

and was chosen over Te because of the somewhat higher vapor pressure of Te. The level of doping 

was chosen to maximize the open circuit voltage as a function of carrier concentration in the n 

layer. The thickness of 2 10 pm was chosen because results have indicated that substrate 

“effects” are minimized at this thickness. This thickness is about three times the minority carrier 

diffusion length. The p-GaAs layer is then formed (2 x 1O’a cm-‘) by diffusion of the Be dopant 

from the melt used to grow the (AlGa)As window layer.‘@’ 

An alternative approach, used by another organization, is to grow their own high quality 

substrate material by a liquid encapsulation Czochralski technique which has been doped with 

either Sn or Te to a level of 10” cm-‘. A 30 pm thick, n-GaAs epitaxial layer (doped with Sn to a 

level of 8 x 10” cmsa) is then grown as a buffer layer. The p-GaAs layer is then formed by 

diffusion of Mg from the melt used to grow the (AlGa)As layer. Mg was chosen over Be because it 

is a less toxic material. Typical Mg concentrations are 7 x 1O1’ cm-’ in the p-GaAs layer and 

5 x 10” crnss in the (AlGa)As window layer. The p+-GaAs layer grown on top of the window layer 

to receive the metallization is doped to a level of 10” cm+ with Mg.‘al 

The LPE growth techniques being studied at a third organization were typically done with 

n-GaAs substrates doped to a level of 1.5-2.0 x 10” cm -I. Zn was a dopant used for many 

experimental runs however the incorporation of Mg as the p-type dopant in early 1978 allowed for 

13 
the growth of both p-type and n-type layers in the same run without appreciable cross doping 

effects. The use of Mg doping was considered to be very desirable since there is not an appreciable 

evaporation of Mg from the melt during growth. This is in contrast to the noticeable amount of Zn 

evaporation during growth. 

C. CONTACTING 

The methods currently used to provide ohmic contacts to the top and bottom of a 

Ga,.,Al,As/GaAs solar cell show several variations, due primarily to difficulty in making contacts 

that would adhere to (AlGa)As. Reference No. 10 tabulates sixty different materials that have 

been used as ohmic contacts to GaAs and Table 3 summarizes the materials currently being used 

in the United States for the fabrication of GaAs solar cells. 

TABLE 3 

SUMMARY OF CONTACTING MATERIALS CURRENTLY BEING USED WITH 

(AIGa)As/GaAs SOLAR CELLS 

Back Contact Front Contact 

Au-Ge-Ni (%OOO&/Ag (10 pm) Au-Zn (3000-4000&/Ag (10 pm) 

Mg(20&/Ti(2000& 

Mg(20-50a)Ti(300~/Pd(300~/Ag(50OOa) 

Au-Ge (200083 

Au-Ge 

Mg(20&/Ti(300~/Pd(300~lAg 

Au (20OOa)/Zn ( 16OO&/Au (200& 

Ag-Zn/Ag(S pm) 

The back ohmic contact generally covers the full area of the cell and is relatively easy to 

fabricate as compared to the front contact. (The use of Ni in the contacting material, however, 

degrades the diffusion lengths in VPE grown, n-GaAs.) By making the back contact out of Au-Sn, 

which exhibits a low contact resistance over a wide range of alloying temperatures and times, the 

contact alloying temperature cycle can be optimized specifically for the front contact. 

The front contact has the specific problems of difficult adhesion to the p-(AlGa)As and 

diffusion of the contact material to the shallow junction. Figure 5 shows some of the techniques 

used to solve the problems associated with the front contacts. 

For their space and terrestrial cells, one organization contacts directly to the (AlGa)As layer 

with a sputtered Au-Zn layer (.3-.4 pm). The finger thickness is then built up to 10 grn with a 

subsequent plating of silver. Annealing of the contacts on the space type cell at 450°C for three 

minutes caused a lowering of the V,, that was traced to a migration of either Au or Zn to the 

junction in localized areas of the cell. This problem did not occur when the junction depth was 

greater than z .25 pm below the window layer.‘el 

Another organization selectively etched through their ultrathin t< 5OOA) window layer end 

contacted directly onto relatively thick p-GaAs layer. Ag-Zn was the contacting material that was 

14 
evaporated onto the cell. The thickness of each finger was subsequently increased to 5 pm by 

electroplating with Ag.“” 

Researchers at a third organization solved the problems of front contacts by growing an 

additional p+-GaAs layer (1 pm thick) on top of the window layer prior to metallizing. Au and Mg 

were used as the contacting material. To avoid a-high Mg concentration at the GaAs surface, 

(known to cause surface damage), a 500 A layer of Au was first applied, followed by a 250 A layer 

of Mg and capped by a 3000 A layer of Au. Annealing at 460°C for 10 seconds in hydrogen causes 

the Mg to diffuse through the Au to the GaAs.‘8’ 

The production of reliable front contacts with a low contact resistance will be a significant 

problem in scaling up the production quantity of (AlGa)As/GaAs, space-type, solar cells (with 

shallow junctions). Difficulties have been noted by personnel at several organizations in this 

specific area. Variations in measured solar cell parameters from cells produced by one group did 

not correlate with (AlGa)As thickness or diffused junction depths leaving personnel to believe 

that the observed scatter may be due to variations in either GaAs diffusion lengths or specific 

contact resistance.‘81 

D. RADIATION DAMAGE EFFECTS 

One of the important concerns for space-type solar cells is their resistance to radiation 

damage. The end of life (EOL) efficiency of the cells determines the total array area that must be 

launched to satisfy payload power requirements for the lifetime of the mission. On-orbit radiation 

damage is caused by the high energy protons and electrons trapped by the earth’s magnetic field 

and by solar proton flares whose frequency is tied to the sun’s eleven year activity cycle. Protons 

and electrons create different types of damages in semiconductor materials. A proton interacts 

more strongly with the lattice because of its large size and, therefore, will not penetrate very 

deeply into the material unless it initially possesses very high energy, It creates considerable 

damage near the surface that it initially enters. The electron, because of its smaller size, 

penetrates more deeply into the cells and the damage it creates, therefore, is spaced more 

uniformly across the cell. 

For (AlGa)As/GaAs solar cells the most important location for radiation damage is in the 

vicinity of the shallow junction. Because GaAs has a large optical absorption coefficient and a 

short diffusion length, radiation damage beyond the active region near the surface of the cell has a 

negligible effect on cell performance. 

Work has been performed by personnel at several organizations to test the degradation in 

cell response for a limited range of electron and proton fluences. Figure 6 shows the reduction in 

maximum power of 2cm x 2cm, deep junction cells, not optimized for resistance to electron 

radiation damage, that were exposed to 1 MeV electrons at fluences in excess of 4 x 10” cm-‘.“’ 

Using a computer model to predict cell performance, short circuit current, L,, was predicted as a 

function of fluence of 1 MeV electrons for a range of junction depths. Figure 7 shows the 

significant beneficial effect of a shallow junction on resistance to electron radiation damage. 

Experimental data for a junction depth of .5 pm using 1 MeV electrons verified the analysis.“’ 

The (AlGa)As solar cells exhibited a higher maximum power output over the range of proton 

fluences (15.4 MeV and 40 MeV) than the deep junction (AlGa)As cells or the silicon cells that 

were also tested. 

15 
150 

3 

E, 100 

ii3 

3 

2 

E 

2 .- 

2 50 

5 

0 

Projected 18% (AlGalAs-GaAs 

/ 

Solar Cell 

Shallow Junction (0;5pm) 

Deep Junction (lctm) f \ 

(AlGalAs-GaAs Solar Cell 

\ 

10'2 10’3 10’4 10’5 

1 MeV Electron Radiation Fluence (E/cm21 

10’6 

FIGURE 6 MAXIMUM OUTPUT POWER 

(4 cm*) VERSUS 1MeV 

ELECTRON FLUENCE. 

16 
I ._-_.I I I 

10'2 10'3 10'4 10'5 1~316 

1 MeV Electron Radiation Fluence (E/cm21 

FIGURE 7 PREDICTED (AlGalAs-GaAs SOLAR CELL SHORT CIRCUIT 

CURRENT DENSITY VERSUS 1MeV ELECTRON RADIATION 

FLUENCE AS A FUNCTION OF JUNCTION DEPTH. 

17 
E. SUMMARY OF SIGNIFICANT CELL DESIGN PARAMETERS 

The significant design parameters which must be closely controlled during the cell fabrication processes include: 1) dopant concentration profiles, 2) junction depth and 3) thickness of 

the window layer. Because of the large absorption coefficient for GaAs, the p-n junction depth is 

on the order of 0.5 pm. A shallow junction depth is also beneficial from the viewpoint of increasing 

the resistance of the cell to radiation damange. The various epitaxial growth processes which 

could be used to fabricate the active layers of the cell will be discussed and compared in the 

following sections. 

18 
I 

IV. DEFINITION OF EPITAXIAL GROWTH PROCESSES 

A. INTRODUCTION 

The purpose of this section is to briefly describe the three basic crystal growing procedures 

that have been used to epitaxially grow doped GaAs and AlGa/As layers on a GaAs substrate. The 

three techniques are: Liquid Phase Epitaxy (LPE), Vapor Phase Epitaxy (VPE) which includes 

the Metalorganic Chemical Vapor Deposition (MO-CVD) process, and Molecular Beam Epitaxy. 

Because of the difference in funding levels that have been applied to develop (AlGa)As/GaAs, 

heteroface solar cells by the three different techniques, cell conversion efficiences achieved to 

date are not being used as a measure of the potential of each process to produce an optimized, 

space quality solar cell. 

B. LIQUID PHASE EPITAXY 

Liquid-phase epitaxy (LPE) has become an established technique for the growth of devices 

based on GaAs and other III-V compounds. The basic growth process is to saturate a melt of Ga 

m.p. with GaAs at a temperature of around 900°C and to bring this saturated solution in contact 

with a GaAs substrate. The substrate and solution are cooled at a rate of O.l-0.5”C/min. to cause 

the epitaxial layer to grow and the melt and substrate are decoupled when the desired layer 

thickness has been obtained. A dopant can be added directly to the melt and will be incorporated 

in the crystal structure in accordance with diffusion rates in the melt and its segregation 

coefficient. 

Variations in the LPE process include both 1) the design of the oven apparatus which holds 

the melt and substrate and controls contact between the two, and 2) the time-temperature profile 

followed during the growth process. 

The three basic growth techniques used in LPE are: 1) the “dipping” technique in which the 

substrate(s) are lowered and raised vertically into and out of a melt, 2) a rotating or sliding boat 

process in which, through the use of a multi-bin system the layers are grown by bringing the 

substrate into contact with different melts, or 3) a “tipping” technique in which the meltsubstrate contact is achieved by tilting the furnace to cause the solution to flow over the 

substrate. The “dipping” and “rotating or sliding” processes (will be discussed here) because 

they have been used to grow (AlGa) As/GaAs solar cells. The “dipping” process is typified by the 

infinite melt LPE process which is currently in use to grow space and terrestrial type (AlGa) 

As/GaAs solar cells. 

In the infinite melt LPE process, two separate furnaces are needed to maintain two growth 

melts at equilibrium conditions (near SCHFC) continuously. A 5000 melt would be maintained for 

the n-type GaAs melt, used to grow a 1Opm buffer layer on the as-received GaAs substrate, and a 

3000gr melt would be separately maintained for growing the SO.5pm, p-type window layer. The 

infinite melt LPE technique is currently being used in a limited production mode and has 

demonstrated the advantages of long term stability of the growth solution, a single melt having 

been maintained for a period of two years. w Once the solution is prepared, it is maintained in an 

ambient of palladium-diffused hydrogen. All operations for layer growth or maintenance of the 

melt are performed by passing the appropriate material through an entry chamber. The entry 

chamber can be independently evacuated and fitted with hydrogen before it is opened to the 

quartz growth tube. (See Figure 8). 

19 
Vacuum Valve 

Substrate Holder 

- Melt 

FIGURE 8 TYPICAL GROWTH APPARATUS FOR INFINITE MELT 

LIQUID PHASE EPITAXIAL PROCESS. 

20 

L __ -___ .._. .._._. -.. ..------.------’ 
A graphite substrate holder, used to hold the GaAs substrates while they are being dipped 

into the saturated GaAs solution, serves two basic purposes. First, it provides a backing for the 

substrate, thereby exposing only one side to the solution. Second, its retractable cover protects 

the substrate while the substrates and holder are equilibrating with the temperatures of the 

furnace. 

In the limited melt LPE process a rotating or sliding multiple-bin boat allows for sequential 

deposition of several layers during one growth cycle. (See Figure 9). The GaAs substrate is placed 

in a recessed area of the slide which, prior to initiation of layer growth, is not in contact with the 

reservoirs containing the melts. The various reservoirs are fitted with the components of the 

desired melts, as solids, and the whole apparatus is placed in an evacuated quartz tube which is 

located in a furnace. A bake-out at 900°C in hydrogen for 12 hours may be necessary prior to 

initiation of the growth process. 

Many different, carefully controlled, time-temperature profiles have been utilized to grow 

the epitaxial layers on the GaAs sybstrates. 

C. VAPOR PHASE EPITAXY 

The Vapor Phase Epitaxy (VPE) processes can be broadly subdivided into those that utilize 

inorganic compounds (HCI, A&l,, ASH,, Hz, etc.) and those that utilize the organometallic 

compounds (Ga(CH,),, Al(CH,),, Zn(C,H,),). The processes that use the inorganic compounds 

have demonstrated their ability to fabricate thin, n-type GaAs, and AlAs layers. These processes 

could be used as part of an overall process to fabricate space type GaAs solar cells. The inorganic 

processes can be further subdivided based upon the materials used as input to the system. There 

was some early work (1966) in which single crystal layers of AlAs-GaAs were grown by an iodidedisproportionation-type, closed-tube, vapor transport system”“). 

Another early process to grow n-GaAs on n+-GaAs substrate utilized a H,-H,O transport 

system with both the source and substrate were located in a fused silica reactor tube.“” The 15 

mm diameter substrate was maintained at lOOO”C-1050°C and the upstream source was held at a 

temperature 50°C higher, the hottest location in the reactor tube. Growth rates of .ll to .75 p/min 

were recorded. 

At about the same time (1966), vapor phase epitaxial growth was achieved using the 

complete hydrogen reduction of GaCl, in the presence of arsenic.‘lz’ 

Growth rates ranged from .07pmlmin to 0.45,ulmin depending on the flow rate of GaCl, and 

it was observed that for the higher gas flow rates, the downstream substrates showed a lower 

growth rate than the upstream substrates. The growth rates were more uniform at the lower flow 

rates. This effect was believed to be caused by the greater time available for the reaction products 

from the upstream substrates to diffuse away from the vicinity of the substrates with the lower 

flow rates. 

More recent work (1976) with a HCl transport VPE process has demonstrated the ability to 

grow n-type AlAs on p-GaAs substrates of a quality comparable to that grown by an LPE 

system”S’. Figure 10 shows a schematic diagram of the reported system. The temperature of the 

reactor varies along the length with a 1000°C - 1100°C temperature environment in the vicinity 

21 
Substrate Position A 

, Substrate Holder 

Melt Holder 

Substrate Position B 

(a) Rotating Substrate and Melt Holders. 

(b) Horizontal Sliding Boat. 

FIGURE 9 TYPICAL GROWTH APPARATUS FOR FINITE MELT 

LIQUID PHASE EPITAXIAL PROCESS. 

22 
Exhaust 

/ 

Ga, Be, Mn Source 

H2 + HCI 

H2 + HCI 

I @--- H2 + AsH3 

/ Perforated Tube and 

c 

Aluminum Source 

FIGURE 10 SCHEMATIC DRAWING OF A VPE GROWTH REACTOR. 

23 
of the end of the tube containing the aluminum. Growth of alternating layers of GaAs and AlAs as 

accomplished by drawing the substrate to an appropriate position in the 600-800°C range and 

switching’ the HCI flow from the Al source to the Ga source. Typical epitaxial growth rates were 

0.2 to l.Opm/min. 

Planar layers as thin as .05 I.trn have been grown and variations in thickness over the 4cm x 

4cm substrate were less than 10%. Growth of 20 pm thick layers at an area rate of 60 cm’lhour 

were reported and it was believed that the size of the apparatus could be readily expanded to 

allow 600 cm2/hour using commercially available components. 

Using the HCI transport process, n-AlAs/p-GaAs solar cells were fabricated on Zn doped, 

10cm2, GaAs substrates. A 20 pm layer of n-AlAs was grown after which the AlAs surface was 

anodized. AM1 conversion efficiencies were in the range of 13-18%. Work is continuing to 

fabricate more efficient contact grids and to increase the open circuit voltage. 

Another VPE process being utilized to grow a homojunction n+/p/p+ GaAs solar cell uses 

AsCl,-Ga-H, as reactants.‘6) The Hz flows through a A&l, evaporator (1S’C) and over a Ga boat 

(820°C) prior to arriving at the heated growth position (730°C). Dopants are introduced in the 

vapor phase by using n(CH,), and H,S respectively. The initial n+ layer thickness is about 2060 A 

and, after the anodization process, final n+ layer thicknesses as small as 200 A were obtained. A 

reported advantage of this ceil structure is that the surface n+ layer is sufficiently thin so that 

most of the hole-electron pairs are created below the junction in the p-type material thereby 

reducing surface recombination losses without using a Ga,.,Al,As window layer. AM1 efficiencies 

of 20% have been reported using GaAs and Ge substrates. 

The use of organometallic (metalorganic - MO) and hydride sources has permitted the 

growth of both p and n doped layers of GaAs and Ga,..Al.As on a GaAs substrate by a vapor phase 

epitaxy process, i.e., chemical vapor deposition (CID). Figure 11 is a simplified schematic 

diagram of a laboratory type MO-CVD apparatus for the growth of GaAs and (AlGa)As. 

A successfully grown three layer structure consisted of GaAlAs:Zn/GaAlAe:Zn/GaAs:Se on 

a GaAs:Si substrate.‘1a’ Typical layer thicknesses were GaAlAs:Zn-SOOA; GaAe:Zn-1-2pm; 

GaAe:Se 4-6 pm. The layers are grown at 750°C at a rate of - O.PSctmlmin over areas greater than 

12 cmz. Scaling of the process to large-area growth on 200 cm2 of substrates in a single run is 

believed possible. 

The compounds used in the reaction are trimethylgallium (TMGa), trimethylaluminum 

(TMAl) areine (ASH,) and H,Se or Zinc (DEZn), for the source of dopant. The organometallic 

compounds require care in their handling in that they will ignite and may detonate on exposure to 

air. Trimethylaluminum, however, is in common uee in industry as a catalyst in the preparation 

of polyethylene. 

Layers grown by the CVD process have shown small variations in thickness due to boundary 

layer effects in the gas stream, and the only reported growth efficiency (amount of material in 

layer/amount of material available in gas stream) was in the range of l-3% (for very small 

substrates) .‘l’) 

24 
I 

Flow Exhaust 

Controllers 

/ AsH3 

- 

I 

in H2 

I 

H2Se 

in H2 -u 

, Quartz 

Purified 

H2 

TMAI 

\I 

Dewar Flasks Vacuum 

FIGURE 11 SIMPLIFIED SCHEMATIC DIAGRAM OF LABORATORY TYPE MO-CVD 

APPARATUS FOR GROWTH OF GaAs and GaAIAs. 

25 
Experience in fabricating periodic structures using the MO-CVD process has shown that it 

is possible to grow individual layers at least as thin as 30-40 A”6) 

The various VPE processes offer the capability of growing thin epitaxial layers over large 

substrate surface areas and, in the case of the organometallic compounds, growing both p and n 

doped layers of Al,Ga,..As. The inorganic VPE processes using an HCl transport process would 

appear to be a technique for growing the 10pm thick, n-GaAe, buffer layer on large area, n-type 

GaAs substrates as the p-(A1Ga)As window layer has not been grown by this process. 

D. MOLECULAR BEAM EPITAXY 

The Molecular Beam Epitaxy (MBE) process achieves epitaxial growth in an ultra high 

vacuum through the reaction of multiple molecular beams of differing flux density and chemistry 

with a heated, single crystal substrate. An MBE system for the growth of doped (AlGa)As is 

shown schematically in Figure 12. The temperature of each furnace is chosen eo that the vapor 

pressures of the materials are sufficiently high for free evaporation generation of thermal energy, 

molecular “beams.” The furnaces are arranged eo that the central portion of the beam flux 

distribution from each furnace intersects the substrate. Individual shutters are interposed between each furnace and the substrate, permitting abrupt cessation or initiation of any given beam 

flux to the substrate. Figure 13 shows a typical temperature-time-shutter operation sequence for 

a graded band-gap, p-Al,Ga,., As/n-GaAe, heterojunction solar cell.(la’ 

The MBE technique is characterized by a relatively slow growth rate, approximately 1 pm 

per hour (one monolayer per second) and a low growth temperature for GaAs (500~600°C for MBE 

as compared to 700-900°C for LPE and -760°C for VPE). The slow growth rate makes precise 

thickness control possible and the low temperature minimizes thermally activated processes such 

as diffusion. The use of different beams for the constituent components, each with its own 

shutter, allows abrupt changes in composition or doping. 

The beams are created from sources contained in effusion ovens where thermal equilibrium 

is maintained. The deposition process at the substrate, on the other hand, is not a thermal 

equilibrium situation and is governed mainly by complex kinetic reaction processes; it not being 

obvious that growth of etoichiometric material is possible. It has been shown, however, that the 

growth of stoichiometric III-V compounds is poesible because of the surface chemical dependence 

of the sticking coefficient of the Group V elements (As). For temperatures at which epitaxial 

growth occurs, only that amount of the Group V element is absorbed which satisfies the available 

Group III bonding orbitale at the surface. (I’) The growth rate is therefore determined by the 

arrival rate of Group III elements while the condition of stoichiometry is satisfied by growing in an 

excess flux of the Group V elements. 

Another advantage of the MBE process, as compared to the LPE and VPE proceeees, is that 

the vacuum environment allows in situ analytical instrumentation to 1) monitor surface crystallographic and topographical characteristics prior to and during growth, 2) monitor gas composition in the work chamber including a portion of the actual molecular beam flux as well as 

background gases and 3) identifying the elemental composition of the films grown by the MBE 

process, including surface contaminants. 

The MBE process offers a procedure for fabricating (AlGa)As/GaAs heteroface solar cells 

with a high degree of uniformity and reproducibility of structure (layer thicknesses and doping 

profiles). 

26 
Single Crystal 

GaAs Substrate (- 500°C) 

Pressure - 10-loTorr ’ 

,’ ! 

(n-Q 

Y &vlolecular Beams 

cl 

Ga 

FIGURE 12 SCHEMATIC DIAGRAM OF A MOLECULAR BEAM EPITAXY 

SYSTEM FOR THE GROWTH OF A AIxGal-xAs/Ga As SOLAR 

CELL ( 1 Torr = 133.3 Pa). 

27 
0 30 60 90 

I Time (min) 

n-GaAs n-GaAs graded p-AIGaAs 

substrate 0.5pm l.lpm 

l Dashed line shows temperature of cells when shutters 

are closed. 

FIGURE 13 TEMPERATURE-TIME SCHEDULE FOR EACH 

EFFUSJON CELL FOR FABRICATING A GRADED 

BAND-GAP AIxGal_,As SOLAR CELL BY 

MOLECULAR BEAM EPITAXY. 

28 
E. OTHER GROWTH PROCESSES 

Other processes for the fabrication of compound semi-conductors include flash evaporation 

and sputtering.““’ In flash evaporation, the material to be evaporated is fed into the system as 

very fine grains. The temperature of the evaporator on which the grains fall is set high enough to 

evaporate the least volatile component, and the temperature of the substrate is kept low enough 

to prevent the re-evaporation of the more volatile component. GaAs has been grown by this 

technique using an evaporation temperature of approximately 1300°C and a range of substrate 

temperatures (3OO”C-600°C). No reference to the preparation of solar cells by this technique was 

found. 

Sputtering is a process in which an electrical discharge is passed between electrodes at a low 

gas pressure and the cathode electrode slowly disintegrates under the bombardment of the 

ionized gas molecules. Some of the liberated atoms are then condensed on surfaces surrounding 

the cathode. l-12pm GaAs films have been grown on Ge substrates by this technique, with 

substrate temperatures of 560-580°C and argon as the gas component (2.5-12x10-zTorr). No 

reference to solar cell fabrication using this technique has been found. 

Ion implantation is a process that has been used to fabricate p-n junctions in GaAs. Early 

work examined the use of Zn and Be as implanted p-type dopants. 

The energy of ions used for implantation in GaAs generally ranges from 20 to 400 keV and 

the amount of damage generated is dependent on the temperature of the substrate. For GaAs, 

there appears to be an appreciable in situ annealing in samples implanted above 150°C. 

Even with the use of heated substrates, temperatures in the range of 800~900°C are generally 

required to achieve maximum electrical activation of the implanted ions. At these temperatures, 

it is necessary to encapsulate the GaAs to prevent 1) out diffusion of the implanted species and 2) 

decomposition of the GaAs surface. During the high-temperature annealing step, there is diffusion of the implanted ions which can broaden and lower the distribution. 

Electrodeposition of thin film, polycrystalline GaAs has been demonstrated on various 

substrates, industry indium tin oxide-coated glass. Although electrical characterization has yet to 

be completed for the GaAs films or devices made from them, the process is believed to hold 

promise of lowering manufacturing costs and achieving reasonably high efficiencies. No reference 

to the preparation of single crystal solar cells by this technique was found. 

F. CHOICE OF EPITAXIAL GROWTH PROCESSES FOR 

FURTHER COMPARISON 

The three epitaxial growth processes chosen for further comparison of their relative merits 

for pilot line production are: LPE, MO-CVD and MBE. These process are capable of fabricating 

the required epitaxial layers and are sufficiently documented in the literature so that a comparison can be made. 

29 

V. COMPARISON OF EPITAXIAL GROWTH PROCESSES 

A. COMPARISON METHODOLOGY 

A significant factor currently restricting the increased use of gallium arsenide solar cells for 

space applications is the initial cost per cell, particularly in small quantities. An accurate 

assessment of the economic benefits of using gallium arsenide solar cells in space will require a 

system level study to trade off the initial cost per cell against the advantages of total array size. 

The total array size with gallium arsenide solar cells benefits from the inherent cell advantages of 

beginning of life (BOL) efficiency and resistance to radiation damage (with prospects for thermal 

annealing of incurred damage). 

The comparison between the several processes for fabricating single crystal (AlAa)As/GaAs 

solar cells developed in this report is primarily a cost comparison, resulting in a tabulation of the 

major direct costs per cell for the growth of the epitaxial layers on 10,000, 2cm x 2cm 

(AlGa)As/GaAs solar cells. The elements of the total cost are: 1) material usage 2) energy 

consumption 3) labor hours required and 4) capital equipment. In addition to these cost elements 

two additional comparison factors are also discussed: 1) the degree of control or reproducibility 

and 2) problems and potential for scale up and automation. 

This section of the report contains a discussion of the cell fabrication comparison factors 

and concludes with a comparative summary of the direct costs associated with each of the cell 

fabrication processes. 

B. COMPARISON FACTORS 

1. Material Usage 

The various solar cell fabrication processes utilize different amounts of input materials to 

grow the same llpm thick epitaxial layers, (including the 1Opm buffer layer). The relative 

differences are reduced, however, because of the amount of material contained in the 250pm thick 

substrate common to all processes. 

An annual production rate of 10,000, space-qualified, 2cm x 2cm solar cells would require 

40,000 cm2 (6200 in*) of single crystal GaAs substrates per year. The substrate material is 

typically grown by a horizontal Bridgman technique and sawed to a thickness of 500 pm (with 

50% saw kerf losses), polished to a thickness of 3OOpm (with 40% polishing losses) and cleaved to a 

2cm x 2cm solar cell shape (with 45% cleaving losses). Therefore, approximately 73,000 cm’ 

(11,300 in’) of polished, boat shaped wafers (250 - 3OOpm thick) would need to be purchased per 

year to support an annual production rate of 10,000 2cm x 2cm cells. Because only 17% of the 

GaAs material in the original boat grown crystal ends up in the final substrates for the flight 

qualified cells, 15.1 kg of high purity gallium and 16.2 kg of high purity arsenic would be required 

as input material (assuming 250pm thick substrates polished from 417pm thick wafers). Total 

material requirements for the fabrication of the substrates is reduced by recovery of the various 

material losses. 

The additional gallium and arsenic material contained in the 10pm thick, n-GaAs, buffer 

layer, the 0.5pm thick p-GaAs and the 0.5pm thick p-(AlGa)As epitaxial layers, for 40,060 cm* of 

31 
cell area, is 108 grams of gallium, 121 grams of arsenic and approximately 2 grams of aluminum. 

The total required input of high purity gallium and arsenic (or compounds containing Ga or As) 

for growing the epitaxial layers is then dependent on the yield of the specific epitaxial growth 

process. 

The infinite melt LPE growth system is the most conservative of material in that the only 

material that intentionally leaves the growth chamber is in the epitaxially grown layers. The 

annual consumption of material is approximately 108 grams of Ga and 121 grams of As. An 

infinite melt growth chamber contains approximately 5090 grams of a high purity Ga solution in 

inventory and two growth solutions are needed, one for the n-GaAs buffer layer (5000 grams) and 

one for the p-(AlGa)As window layer (a 3000 gram melt which also forms the p-GaAs layer by 

diffusion). 

The limited melt LPE process has a high consumption of Ga and As material because of the 

additional losses associated with “disposing” of the solutions remaining in the small melt 

chambers after the epitaxial layers are grown. Because these additional losses have not been 

quantified in the literature we treated this uncertainty in material consumption as a variable. 

Yield is defined as the ratio of the amount of material in the epitaxially grown layers to the 

amount of material used as input to the growth process. For a 10% yield, limited melt LPE 

process, 1080 grams of high purity gallium and 1212 grams of high purity arsenic would be 

consumed per year. For a 1% yield process, 10,800 grams of Ga and 12,122 grams of As would be 

needed annually. The advantages of 1) conservation of material and 2) reproducibility of solution 

composition from run to run favor the infinite melt LPE system over the limited melt LPE growth 

system. 

The MO-CVD process uses high purity trimethylgallium [Ga(CH,),] and amine (AsH,) as 

the input compounds to produce the GaAs material. It has been experimentally observed that 

better surface morphology and higher electron mobility are obtained when the ratio of ASH, to 

Ga(CH,), in the input stream is in the range of 5-10. We’ve assumed a ratio of 10 for our 

computation of material requirements. Therefore, 108 grams of gallium in the epitaxial layers 

requires at least 178 grams of Ga(CH,), and 121 grams of arsenic requires a minimum of 1260 

grams of ASH,. A significant uncertainty in determining the total amount of input material 

required is the yield of the MO-CVD process, i.e., the fraction of the Ga and As in the input 

compounds that ends up in the epitaxially grown layers. 

The amine uses purified hydrogen as a transport gas and a purchased concentration of 10% 

ASH, in hydrogen was used for our cost computations. (L.E.D. purity). 

Material losses with a cold wall reactor include the portion of the gas stream that flows past 

the substrate without reacting, as well as gas losses associated with starting, stopping and venting 

the system. Because of a lack of published information on process yield, we treated this uncertainty as a variable. For a 10% yield MO-CVD process with a As/Ga ratio of 10, 1779 grams of 

Ga(CH,), and 12,600 grams of ASH, would be needed per year. For a 1% yield process, 17,790 

grams of Ga(CH,), and 126,000 grams of ASH, would be needed annually. 

Additional experimental effort would be needed to quantify and optimize the process yield 

with the MO-CVD and to determine the feasibility of collecting, repurifying and/or recycling the 

exhaust gas stream. Current practice is to burn the exhaust gas stream as it is vented to the 

32 
atmosphere. The collection of the organometallic compounds from the exit gas stream would have 

to be done with care because the compounds are pyrophoric, i.e., they will burn spontaneously, or 

may even detonate on exposure to air. There is a difference in the boiling points of the compounds 

that could be utilized for separation of the active materials from the exhaust stream. 

The MBE growth process has a potential for low consumption of cell material depending on 

the ratio of the total cross sectional area of the molecular beams to the area of the substrate 

measured at the plane of the substrate. Without properly designed crucible nozzles, economical 

use of the source material and good flux uniformity over the substrate are not obtained simultaneously and the emphasis has generally been placed on uniformity. With properly designed crucible 

nozzles, however, collimation of the molecular beams can be achieved in a predictable pattern. 

Good uniformity then becomes a matter of crucible engineering. An additional loss comes from 

the intentional excess of As at the growing surface. For growth of device quality films, the 

accepted As/Ga ratio at the substrate is in the range of 5-10. 

If the MBE process were to be used to grow only the 0.5pm thick p- GaAs layer and the 

0.5pm thick p- (AlGa)As window layer, and assuming that the total molecular beam area is twice 

the substrate area, with an As/Ga ratio of 10, then the annual consumption of material would be 

21.3 grams of Ga, 220 grams of As and less than 1 gram of Al. If the MBE process were used to also 

grow the 10 m thick n-GaAs buffer layer, the total annual consumption of cell material would be 

216 grams of Ga, 2424 grams of As, and less than 1 gram of Al. These MBE material consumption 

figures do not include any losses associated with replenishing the crucibles between runs. 

The MBE process also requires the consumption of approximately 0.044m” (13 gallons) of 

liquid nitrogen per 8 hour day for the LN, cryopump. 

Table 4 summarizes the annual consumption of cell materials for the various cell fabrication 

processes. 

2. Energy Consumption 

Starting with cut and polished substrates and purified input materials, the basic energy 

consuming processes associated with fabricating the crystalline structure for (AlGa)As/GaAs 

solar cells, are the: 1) melting (vaporizing) and subsequent temperature control of the input 

material plus any associated heating of the chamber(s) holding a liquid melt, 2) heating of the 

substrate, substrate holder and other parts of the furnace region, i.e., the walls of the furnace in a 

hot wall CVD reactor, 3) maintaining of a high vacuum in an MBE system and 4) operating the 

process control system, e.g., control electronics or mass flow controllers. 

In the infinite melt LPE process, which maintains 5000 grams of Ga solution in a molten 

state for an extended period of time, the continuous power required is dependent on the designed 

thermal isolation of the heated region. An order of magnitude calculation based on three 

radiation shields in a vacuum shows an operating power requirement of 500 W for an 800°C melt 

in a three-inch diameter chamber. The initial energy consumption to melt the 5060 grams of Ga 

solution and raise it to 800°C (4.5 kWh) plus the energy required to simultaneously bring the 

furnace hardware up to operating temperature is neglected in this comparison of processes 

because the amount of energy is small when compared to the 360 kWh of energy required monthly 

to maintain the melt at 800°C. At a production rate of 32,2cm x 2cm cells every 2 hours during an 

33 
TABLE 4 

SUMMARY OF MATERIAL REQUIREMENTS TO FABRICATE 

10,000 2 cm x 2 cm GaAs SOLAR CELLS ASSUMING 

NO RECOVERY OF LOST MATERIAL 

0 250pm n-GaAs Substrate 

0 10pm n-GaAs Buffer Layer 

l 0.5 pm pGaAs Layer 

l 0.5 pm p-(AQGa) As Window Layer 

Process Yield 

(%I 

Substrate Preparation 

LPE - Infinite Melt 

- Limited Melt 

17 

100 

100 

10 

CVD-- Ga (CH3 I3 

- AsH3 

MBE - AslGa = 10 

100 

10 

1 

50 

5 

Gallium or Gallium Arsenic or Arsenic 

Containing Compound Containing Compound 

(9) (cl) 

15,084 

108 

108 

1,080 

10,800 

178 

1,779 

17,790 

216 

2,160 

16,210 

121 

121 

1,212 

12,120 

126 

1,261 

12,611 

2,424 

24,240 

eight hour work day, five days per week (512 cells/week), .16 kWh of energy is required per cell, 

per melt, assuming each melt is maintained at 800°C for the full week. At least two melts (5000 

grams and 3000 grams) are required, one for the n-doped buffer layer and one for the p-doped 

window layer, yielding a total energy requirement of .32 kWh per cell. This assumes that the 

furnace for the 5000 gram melt is identical in construction to the furnace holding the 3000 gram 

melt. 

For the limited melt LPE process, the energy consumption is related to heating the 

substrate(s), substrate holder and input materials up to operating temperatures (-SOO”C), 

maintaining 800°C “overnight” (12 hours) to bake out the system and then executing a timetemperature growth program near 800°C for approximately 1 hour. This sequence is following by 

cooling of the apparatus. In this process the p-(AlGa)As layer is being grown directly on the 

substrate. Assuming a thermal isolation design for the furnace identical to that of the infinite 

melt apparatus, and neglecting the change in energy stored in the components of the system, the 

total energy consumption is the 4.4 kWh energy loss during the 13-hour heater power cycle. 

Assuming a single 2cm x 2cm cell is grown per cycle, the energy requirement per cell is 4.4 kWh. If 

the 12-hour bake out is not required for subsequent cycles the energy requirement is reduced to .3 

kWh per 2cm x 2cm cell, 

For the MO-CVD process, with a cold wall reactor, the basic energy input is to the RF 

system which is heating the substrate holder to approximately 750°C during a growth period of 

34 
approximately 30 minutes. In the absence of published experimental data we have assumed an 

average energy consumption of .l kWh per 2cm x 2cm cell. 

The MBE system has a significant consumption of energy for the maintenance of the high 

vacuum (lo-” Torr). A standard Molecular Beam Epitaxy System uses 14.4 kW of energy 

continuously while it is operating. With a 3.06cm (1.2 inch) diameter wafer, producing one, 2 cm x 

2 cm cell every 11 hours (growing a 1Opm n-type buffer layer as well as two 0.5wm p-type layers at 

a rate of lpm/ hour) 158 kWh of energy would be required per 2cm x 2cm cell. If only the top lrm 

layers were grown by MBE, 14.4 kWh of energy would be required per cell. 

Second and third generation MBE systems will probably reduce the power requirements per 

cell because of their capability to hold a 5.08.cm (!&inch) or a 7.62cm (3-inch) diameter wafer at 

the growing station. 

Table 5 summarizes the energy consumption per cell for the various cell fabrication processes as required to maintain the proper thermal and vacuum environment. The smaller power 

required to operate the process control system has not been included. At an assumed level of 100 

W for control systems the additional power requirement would vary from .O2 kWh/cell with the 

infinite melt LPE process (1,024 cells/week) to 1 .l kWh/cell with the MBE process (1 cell every 11 

hours). 

TABLE 5 

ENERGY CONSUMPTION PER 2 cm x 2 cm CELL BY 

FABRICATION PROCESS 

0 Only That Energy Directly Related to Maintaining 

Thermal and Vacuum Environment is Considered. 

Process 

Energy Consumption 

Per Cell 

fkWh) 

Comment 

LPE - Infinite Melt 

- Limited Melt 

MO-CVD 

MBE 

.32 

4.4 

.3 

(.l) 

158 

14.4 

Two melts required/assumed maintained at 800°C continuously. 

With 12 hour bake out at 900°C 

Without 12 hour bake out at 900°C 

Assumed Value 

Growing 10 pm buffer layer as well 

as two p-type 0.5 pm layer. 

Growing only two, p-type, 0.5 /.frn 

layers. 

3. Labor Hours Required 

Only the labor hours required for the growth of the epitaxial layers by the different 

fabrication processes is considered here. 

35 
For the production of 10,000, 2cm x 2cm, space-qualified solar cells, the control of the 

epitaxial growth process parameters is assumed to be automated, e.g., time-temperature profiles, 

shutter positions and gas flow rates. This assumption is based on the observation that laboratory 

scale GaAs fabrication processes are being developed by research personnel to operate under 

microprocessor control. This technique permits the transfer of an optimized production sequence 

to operating personnel with the research (and more expensive) personnel serving as part time 

inspectors or monitors. In this production model the operating personnel would be responsible for 

1) inspecting the incoming wafers, 2) cleaning the wafers, 3) loading the wafers into the growth 

chamber, 4) initiating the growth sequence, 5) withdrawing the grown material from the chamber, 6) inspecting the final material and 7) maintaining the apparatus, e.g., replenishing the 

input materials as needed. 

Our manpower model assumes one or two full time operating persons depending on the 

length of time required to grow the epitaxial layers and one part time (20%) professional person 

monitoring the production. The basic manpower differences between the epitaxial growth processes are derived from how long it takes to fabricate 10,000 cells. 

The fabrication of 10,000 solar cells per year may not require the dedication of personnel and 

equipment for a full 12 months. The manhours required per cell will depend on production rates 

with the different fabrication processes and the number of operating and supervisory personnel 

involved. The optimum number of growth chambers and operating personnel dedicated to the 

production of space-qualified, GaAs solar cells in a particular company would depend on the 

availability of equipment and people that are being shared with other contracted and in-house 

programs. With the capability for one person to operate several growth chambers simultaneously, 

the manpower required per cell could vary by a factor in the range of 2-10 depending on the 

availability of equipment. A redesign of a laboratory scale or early production model of a growth 

chamber could also increase the number of substrates that could be processed simultaneously in a 

single growth chamber. 

The comparison of the manpower requirements for the different epitaxial growth processes 

initially assumed that one growth chamber would be utilized with each process. The one chamber 

would have the capacity for holding a number of substrates that was obtained from current 

information. 

For our calculation of the growing time required using the MBE process an MBE system 

with a growth station capable of holding a 5.08cm (!&inch) substrate was used. At a growth rate of 

1 pm per hour on a 5.08-cm (2-inch) diameter substrate, approximately 10 hours would be 

required to grow a n-GaAs buffer layer (10 pm) plus another hour or two to grow the p-GaAs layer 

and p-AlxGa,.,As window. In the area of a 5.0~cm C&inch) diameter substrate holder, two 2 cm x 

2 cm solar cells could be fabricated simultaneously, yielding a production rate of approximately 

four cells per day from one machine. Scaling up the production would require additional machines or the design of a multiple growing station MBE system. Approximately ten MBE systems 

working in parallel would be required to satisfy a daily production goal of 40 2 cm x 2 cm cells. 

The daily throughput per system could be significantly increased if a higher quality 

substrate were available, thereby eliminating the need for the 10 pm thick buffer layer. It is also 

possible that the buffer layer could be grown by another process, e.g., LPE or VPE, and the final 

36 
layers added by the MBE process. If only a 1 pm layer were needed to be grown by MBE, the daily 

production rate, using a single MBE system, would increase to approximately 48 cells per day, 

sufficient to satisfy an annual production rate of 10,000 cells. If a planned third generation MBE 

system were available with the capability of growing on a 7.62-cm (3-inch) diameter wafer, 

epitaxial layers could be gron on seven 2 cm x 2 cm celle simultaneouely. If only a 1 pm layer were 

needed on each cell, a daily throughput of approximately 168 cells/24 hour day or 66 cells/8 hour 

day could be achieved. 

Table 6 summarizes the fabrication times for the different epitaxial growth procedures 

assuming one growth chamber of current design. It ia obvious from the calculation cf the number 

of months required to produce 10,000 cells that only the equipment for the infinite melt LPE 

process is currently sized to produce the required output within 1 year. The other processes would 

require more than one growth chamber or an increase in the size of the current reactor. Published 

data for the MO-CVD system suggests that the current 12 cm’ of growth area (one 2cm x 2cm cell 

in an assumed circular wafer) could be increased to 200 cm’ (thirty-six 2cm x 2cm cells) with a 

change in reactor size. This would reduce the total time required to grow 10,000 cells by the MOCVD process to less than 1 month. A commercially available, four loop MO-CVD system has the 

capability of holding two 7.62.~cm (3-inch) diameter wafers simultaneously. This system would 

reduce the total time required to grow 10,000 cells to 9 weeks using a 1 shift operation. 

TABLE 6 

SUMMARY OF FABRICATION TIMES FOR DIFFERENT EPITAXIAL GROWTH 

PROCEDURES USING ONE GROWTH CHAMBER OF CURRENT DESIGN 

Process Cycle Time No. of Cells 

(hours) (2 cm x 2 cm) 

Time Per Cell 

(hours) 

Time for’ 

10,000 Cells 

(months) 

LPE - Infinite Melt 2 32’ -06 4 

- Finite Melt 1 1 1.0 63 

MO-CVD 0.5 1 0.5 31 

M8E - 1 lpm growth 11 2 5.5 344 

- lpm growth 1 2 0.5 31 

1. Based on 160 hours of operation per month. 

2. Based on a system that has been developed to satisfy (limited) production requirements. 

The potential for scale up and automation of each process will determine the possible 

reduction in fabrication time per cell that could be achieved. This is discussed in a later section. 

4. Degree of Control or Reproducibility 

Specific parameters of the epitaxially grown solar cell material that need to be controlled 

are: 1) thickness of the p-(AlGa)As window layer, 2) junction depth, 3) concentration of desired 

dopants, 4) contamination and 5) cry&al defects. The production of apace-qualified solar cells 

having reproducible performance character&ice will require that these parameters be controlled 

to within specified tolerances by controlling the applicable process parameters. 

37 
All processes require high purity input materials. The infinite melt LPE process has an 

advantage over the other fabrication procedure in maintaining purified and reproducible material 

because large volumes of material are maintained under closely controlled conditions as a melt 

over an extended period of time. The slowly changing state of the melt can then be monitored, 

and modified as necessary, by testing the characteristics of the solar cell material on a continuing 

basis. 

The limited melt LPE process has a large surface-to-volume ratio for the growth solution, 

making it more difficult to control contamination of the growth solution. The limited melt LPE 

process also requires replenishment of the growth material between runs. For these reasons the 

controllability of the limited melt LPE process is more difficult than with the infinite melt. LPE 

process. 

The MO-CVD process will require high purity metalorganic compounds in quantities as 

high as 18 kg per year for trimethylgallium (assuming a 1% yield process). This quantity of 

material is approaching the annual production limits of the major U.S. producer at a purity of 

99.999%. A purity level of 99.9999% is more difficult to produce and is achieved in approximately 

only one out of three batches. The availability of high purity metalorganic compounds for the 

CVD process in increasing larger quantities will require a more detailed examination of the 

sources for this material and the economic incentives to increase production. 

Cleaning of the CVD system may have to be done on a regular basis; however there was no 

published data on degradation in cell performance as a function of length of time the chamber 

had been in use. 

The MBE process offers the greatest degree of process control because of the low growth 

rates and the ability to individually control each component of the process (temperatures of ovens 

and substrates and shutter position). The vacuum chamber will need to be baked out at 200°C for 

24 hours every couple of months to minimize the residual vapor pressure of contaminants in the 

chamber. 

Table 7 shows the distribution of cell efficiencies in small lots of space type gallium arenside 

solar cells fabricated by an LPE process and delivered to the Air Force. A typical range of 

variation in conversion efficiency in a size lot is f0.5% about the mean. Additional efforts would 

be required to assess the system level effects due to this variation in individual cell efficiency and 

to determine the range in cell efficiency of all of the cells fabricated in a given production run. 

5. Problems and Potential for Scale Up and Automation 

The magnitude of the scale up being considered for space qualified gallium arsenide solar 

cells is going from current production levels (-O/day) to an annual production rate of 10,000,2cm 

x 2cm cells. This is equivalent to 40 cells per day for 250 working days per year. The analyses are 

based on a single 8 hour shift per day. 

LPE was the only process identified in the literature as currently being in use for the 

(limited) production of terrestrial and space quality, heteroface, gallium arsenide solar cells. 

Approximately 100-200 cells have been delivered under government contracts as of June 1978. 

The total number of cells fabricated during this time is estimated at being an order of magnitude 

38 
TABLE 7 

STATISTICAL SUMMARY OF AM0 EFFICIENCIES OF SMALL LOTS OF 

(AIGa)As/GaAs SPACE SOLAR CELLS 

l As received by WPAFB under Contract F33615-76-C-2121 

l Cell size 2 cm x 2 cm 

January 1976: 5 cells 

Mean AM0 7): 

Mode of distribution: 

Range of distribution: 

Standard deviation: 

June 1976: 

January 1977: 

8 cells 

Mean AM0 77: 

Mode of distribution: 

Range of distribution: 

Standard deviation: 

7 cells 

Mean AM0 q: 

Mode of distribution: 

Range of distribution: 

Standard deviation: 

June 1978: 10 cells 

Mean AM0 77: 

Mode of distribution: 

Range of distribution: 

Standard deviation: 

14.12% 

14.0% 

1.30% 

.48% 

15.58% 

15.5% 

0.2% 

.l% 

16.53% 

15.9% 

1.4% 

.66% 

16.77% 

16.50% 

1.40% 

0.43% 

(2) 

(5) 

(2) 

(2) 

higher (1000-2000 cells). The MO-CVD and MBE processes have been used to produce only a 

small number of ceils for research and development purposes. 

A basic decision in scaling up a laboratory sized solar cell production process is whether to 

duplicate the laboratory hardware or to develop larger pieces of equipment, capable of fabricating 

more cells at a single growth station. The development of larger area growth stations may, in 

essence, be only an extension of the original laboratory research and development program. 

39 
The basic questions to be answered in making this decision as to how to scale up are: 1) is it 

technically feasible to develop larger growth stations for a particular cell fabrication process and, 

if technically feasible, 2) does it reduce the cost of the final, flight qualified, solar cell? 

The requirement for an annual production rate of 10,000, 2cm x 2cm, flight qualified, 

gallium arsenide solar cells to satisfy NASA’s assumed future needs does not guarantee that any 

one company would receive all of NASA’s future orders. There are also other government funded 

programs requiring gallium arsenide solar cells for space (DOD) and terrestrial applications 

(DOE) that could impact the number of cells a single company would have to produce in a given 

year. 

The economics of allocating the cost of new capital equipment over a limited production run 

of space qualified solar cells (with a basic uncertainty in market projections for cells of this type) 

is a business strategy decision affecting the cost and price of each cell. It is to be expected that the 

two or three companies in a position to bid on the fabrication of flight qualified gallium arsenide 

solar cells would, if possible, schedule the use of existing facilities to meet near term requirements 

rather than building or procuring new equipment. 

The scope of this section of the report is to compare the LPE, MO-CVD and MBE processes 

on the basis of their technical problems and potential for scaling up. In a following section, the 

costs associated with scaling up will be assessed. 

The infinite melt LPE process can be scaled up by 1) increasing the volume of the molten 

solution and 2) increasing the number and size of the substrate holders so that more cells can be 

fabricated in a single melt at one time. The infinite melt LPE process has already been scaled up 

to stored solution masses of 5000g and 3OOOg with holders capable of inserting thirty-two, 2cm x 

2cm substrates simultaneously, With the pair of growth chambers, one for the 1Okm thick ndoped buffer layer and one for the 0.5pm thick p-(AlGa)As window layer, the epitaxial layers for 

10,000 cells could be grown in approximately 4 months (160 hours per month). The existing 

equipment could satisfy NASA’s assumed annual production requirement with only a 33% 

annual utilization of the facility. The excess capacity could then be allocated for other uses. 

Therefore no further scale up would be needed. 

The current MO-CVD process typically grows one or two wafers per run with the maximum 

device size being limited by the size of the reactors employed and the size of the GaAs substrates. 

It is believed that large area growth on 200 cm2 of substrates can be achieved in a single deposition 

run. 

The quality of the MO-CVD grown material depends on such process parameters as 

temperature, gas phase stoichiometry, quality of the reactant materials and reactor geometry. As 

part of the development effort to scale up MO-CVD production capacity by increasing the size of 

the growth station, one has to assess the compositional changes in the boundary layer of the gas 

flowing over the large area of substrates. The boundary shape and its gas composition are affected 

by gas pressure, by reactor geometry, i.e., mixing of the gases prior to and surrounding the 

substrates and by the orientation and spacing of the substrates in the gas stream. The scaling up 

of reactor size could also affect the yield of the fabrication process, i.e., the portion of the input 

material that ends up as epitaxially grown layers on the substrates. 

40 
The MO-CVD process appears promising for scaling up by increasing the size of the growth 

station; however, there are development difficulties which need to be quantified. Sales literature 

lists a commercially available, four loop MO-CVD system with a production capability of two, 

7.62cm (3 inch) diameter wafers per run (a maximum of 14,&m x 2cm cell substrates). There was 

no information obtained during the course of this study to validate that larger MO-CVD systems 

were already in existence. At this point in time no solar cells have been reported in the literature 

using a large growth area MO-CVD system. A technology development effort to an,alyze, design, 

fabricate and/or test the performance of a MO-CVD system with a large area growth station 

(including continuous or batch feed) is a logical extension of current efforts to understand the 

effects that variations in growth parameters have on the quality of the grown material. 

The basic conclusion is that, if an annual production rate of 10,000, 2cm x 2cm gallium 

arsenide solar ceils were to be achieved using this larger, commercially available MO-CVD 

equipment, and assuming a growth cycle of 0.6 hour per run, the full production run of 10,000 

cells could be completed in less than 3 months at 160 hours per month of running time. This can 

be compared to the 31 month fabrication time for the MO-CVD process used in the comparison 

ofthe different growth processes. 

The decision as to how a particular company would allocate personnel and MO-CVD 

equipment to satisfy the production requirement of 10,000 solar cells would depend on their 

availability of resources and other commitments at the time the proposal was prepared. The 

availability of a proven reactor with a large area growth station could lessen the impact on a 

company’s on-going MO-CVD activities due to the receipt of an order for 10,000 2cm x 2cm solar 

cells. The availability of a large area MO-CVD reactor could also reduce the additional capital 

equipment costs associated with expanding annual production rates to levels higher than 10,000 

cells. The MO-CVD process could also be used to grow only the two, p-type layers while the 10pm 

buffer layer could be grown by anot.her process, e.g., LPE or VPE. 

Scale up of the MBE process could use a third generation MBE system currently in the 

planning stage. This machine will be able to hold a 7.62cm (3-inch) diamet.er wafer at the growing 

station and could produce enough material for seven cells after an 11 hour growth cycle, assuming 

that both the lOpm, n-type buffer layer as well as the two, O.$m, p-type layers are being grown. 

The cycle time reduces to 1 hour if only the two O.&m thick layers are grown, however, additional 

equipment would then be needed to grow the buffer layer. Using this third generation MBE 

system, at a rate of 7 cells every 11 hours, would require 22 months of continuous running to 

fabricate 10,000 cells using one growth chamber. The operalion of two or more MBE systems in 

parallel in order to satisy an Ennual production rate of 10,006 cells is technically feasible but is an 

expensive alternative. (Our comparison of the different cell fabrication processes was based on a 

second generation MBE system which is currently available.) 

The advantage of the MBE process over the other process is in its ability to precisely control 

the thickness and dopant concentration in the two p-type epitaxial layers. A potential alternative 

to using the MBE process would be to use a continuous or batch VPE process for the 10pm thick, 

n-type, buffer layer and then use t.he MBE process to grow the closely controlled p-layers. 

Figure 8 shows a potential solar cell fabrication process based on these two processes. This is an 

example of a conceptual approach (presently unproven) which may lead to lower cost GaAs solar 

cells. 

41 
- 

In summary, the infinite melt LPE process is the only cell fabrication process which has 

currently been up to satisfy an annual reduction of 10,000,2cm x 2cm (AlGa)As/GaAs solar cells. 

6. Cost 

In the comparison of the costs associated with the different cell fabrication processes only 

the major direct costs of materials, labor, capital equipment and energy consumption were 

assessed. Whereas materials, labor and energy are assumed to be steady state operating costs, the 

initial capital equipment costs are amortized over the economic life of the facilities. For this cost 

comparison we assumed that the cost of the final cell must include the amortized cost of the 

equipment only during the actual time it is being utilized to fabricate cells. The useful life of the 

equipment can be either the physical life of the hardware or the expected life of the technology. 

For the limited production of flight qualified gallium arsenide solar cells technical obsolescence 

will probably occur before physical obsolescence. A 5 year total useful life for the equipment was 

assumed and then the fraction of that time that was to be allocated to fabricating 10,000 solar 

cells was determined. During the &year period minor modifications to the equipment and process 

parameters are expected, but with no replacement of the basic components, 

There is a difference between the cost numbers developed here for comparing the different 

processes and the total cost and selling price of the solar cells. There are additional steps in the 

fabrication of the flight qualified solar cells, e.g., metallization, application of an AR coating and 

testing, that are not included in our comparison as well as such indirect costs as company 

overhead and profit. The Solar Array Manufacturing Industry Costing Standards (SAMICS) 

developed for the Low-Cost Solar Array Project at the Jet Propulsion Laboratory provides a 

consistent and detailed methodology for estimating array costs and comparing production technology costs. The use of SAMICS was beyond the scope of this present work as detailed process 

information must be provided for all steps in the production sequences in order to be able to 

calculate direct costs. The thorough documentation of all direct processes requires a significant 

initial effort. Also the use of SAMICS is more applicable to dedicated facilities and plants rather 

than the part time use of laboratory scale equipment. 

The cost comparison documented in this section assumes that the facilities and personnel 

being utilized to fabricate 10,000 2cm x 2cm solar cells are being shared with other company 

programs. The time required to fabricate the 10,000 cells is an important parameter as it defines 

the fraction of annual costs, e.g., salaries and depreciation, that are to be paid by the solar cell 

fabrication effort. 

a. Material 

Table 8 summarizes the unit cost of the basic chemicals utilitized in the fabrication of 

gallium arsenide solar cells. Based on the quantity of material required for each process (see 

Table 4), Table 9 summarizes the total material costs for each of the three processes. The 

conclusion to be drawn from Table 9 is that the process yield with the MO-CVD fabrication 

procedure is a major uncertainty in material costs. Only the material cost associated with the 

substrate is comparable to the MO-CVD costs with a 10% or 1% yield factor. The comparisons 

shown in Table 9 clearly show the material cost advantage of the 100% yield, infinite melt LPE 

process. The 8OOOg of melt held in inventory was not included as a material cost factor. Only the 

cost of replenishing the melt for the fabrication of the solar cells was included. 

42 
TABLE 8 

UNIT COST OF MATERIALS UTILJZED IN FABRICATION OF 

(AIGa)As/GaAs SOLAR CELLS 

Material 

Arsenic (99.9999%) 

Arsine (10% concentration in HZ 

L.E.D. purity) 

Beryllium (99.95%) 

Diethyl Zinc (99.999%) 

Gallium (99.9999%) 

Germanium (99.9999%) 

Magnesium (99.99%) 

Selenium (99.999%) 

Tellurium (99.999%) 

Tin (99.9999%) 

Trimethyl Aluminum (99.9999%) 

Trimethyl Gallium (99.9995%) 

Zinc (99.9999%) 

*Cost to refill cylinder. 

Quantity 1979 Purchase Price Source 

10 kg 8122.20lkg 

500 kg $ 93.80/kg 

.2 kg $723.56/kg 

1.8 kg $563.61 /kg 

109 

25 9 

1009 

500 g 

10 kg 

500 kg 

109 

50 9 

25 9 

log 

5g 

25 9 

IOOg 

500 g 

259 

1009 

500 g 

50 9 

$ 10.20/g 

$ 8.00/g* 

$ 4.30/g* 

$ 2.95/g* 

$534.OO/kg 

$464.00/kg 

$ 5.00/g 

$ .74/g 

$ 1.50/g 

$i 2.4619 

$ 5.36/g 

§i 7.20/g* 

$ 3.60/g* 

S’ 2.28/g* 

$ 16.00/g* 

$ 10.80/g* 

$ 8.90/g’ 

$ .78/g 

1 

1 

2 

2 

3 

3 

3 

3 

1 

1 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

Source Code: 1. Cominco American. 

2. Matheson. 

3. Ventron Corporation, Alfa Catalog 1979-1980. 

43 
Process 

Substrate Preparation’ 

LPE - Infinite Melt 

LPE - Finite Melt 

MO-CVD AslGa = 10 

MBE AslGa = 10 

TABLE 9 

COST OF MATERIALS FOR FABRICATING 

10,000 2 cm X 2 cm (AtGa)As/GaAs SOLAR CELLS 

Yield 

(%I 

17 

100 

100 

10 

1 

100 

10 

1 

50 

5 

Compound 

15.1 g Ga 

16.2g AS 

108g Ga 

121g As 

1OSg Ga 

121 g As 

1,080 g Ga 

1,212g As 

10,800 g Ga 

12,120g As 

178 g Ga(CH,j3 

1,260 g Ad-I3 

1,779 g Ga(CH3j3 

12,600 g Adi3 

17,790 g Ga(CH3 13 

126,000 g AsH3 

2169 Ga 

2,424 g As 

2,160g Ga 

24,240g As 

Unit Cost 

§mxoo/kg 

$ 93.8Olkg 

$ .464/g 

$ .094/g 

$ .464/g 

$ .094/g 

$ 8.90/g 

$ .564/g 

$ .464/g 

$ .094/g 

Total Cost Cost Per Call 

% 7.006.40 

S 1.519.56 S .85 

$ 50.11 

$ 11.37 > 

<$ .Ol 

$ 50.11 

$ 11.37 <$ .Ol 

f 501.12 113.93 1 $ .06 

$ 5,011.20 1 

$ 1,139.28 j $ .62 

$ 1.584.20 

$ 710.64 > $ .23 

$ 15.833.10 $ 7.106.40 ) $ 2.29 

$158,331 .oo 

$ 71.064.00 $22.94 

$ 100.22 

$ 227.86 % .03 

$ 1.002.20 

$ 2.278.60 $ .32 

1. Assuming no recovery of lost material. 

44 
b. Energy 

The cost of the energy required to fabricate the epitaxial layers was computed based on a 

cost of electricitiy of $.05 per kWh. Table 10 shows the energy consumption per cell by t.he various 

fabrication processes and the cost of the energy. With an estimated energy consumption of. I kWh 

per cell with the MO-CVD process for running the RF heater, the MO-CVD fabrication procedure 

is the most energy conservative of all the processes. 

TABLE 10 

SUMMARY OF ENERGY COSTS BY FABRICATION PROCESS 

l Assumed cost of electricity is $.05 per kWh 

Process 

Energy Consumption 

Per Cell 

kWh) 

Cost of Energy 

Per Cell 

LPE - Infinite Melt 0.32 $ .016 

- Finite Melt 4.4 §I .22 

0.3 $ .015 

MO-CVD (.l) $ .002 

MBE 15.8 $ .79 

14.4 $ .72 

c. Labor 

For each of the fabrication processes we noted that the wafers would have to be cleaned and 

dried prior to loading into the growth chamber and handled subsequent to removal from the 

growth station. We assumed that for growth cycle times less the 0.5 hours, (LPE-infinite melt and 

MO-CVD processes) two operating persons would be needed, one to prepare and remove the 

wafers and a second to operate the growth chamber. For all other processes a single operating 

person would have time to perform both functions. 

We assumed an unburdened rate of $lO/hour for the operating personnel and $20/hour for 

the supervisor. This translates into a monthly unburdened cost of $1600 for each operator (160 

hours) and $640 for the supervisor (32 hours). 

Table 11 summarizes the total unburdened labor costs for each process and the unit labor 

cost per cell, based on the total time required to fabricate 10,000 cells. The infinite-melt LPE and 

MO-CVD processes (with a larger area growth station) have the lowest labor cost based on their 

ability to process a number of wafers simultaneously. The labor costs for the finite-melt LPE 

process could be reduced by adding more growth chambers in parallel. 

d. Capital Equipment 

The operating costs due to the utilization of capital equipment were calculated based on a 5 

year straight line depreciation schedule. The capital equipment costs charged to the solar cell 

fabrication processes were then computed as a fraction of the total equipment costs based on the 

45 
TABLE 11 

SUMMARY OF UNBURDENED LABOR COSTS TO GROW EPITAXIAL LAYERS FOR 

10,000 (AIGa)As/GaAs SOLAR CELLS WITH A SINGLE GROWTH CHAMBER 

l One or two full time operating persons at $10.00 per hour 

l One part time (20%) supervisor at $20.00 per hour 

Process 

Total Time’ No. of 

(months) Operators 

Total 

Labor Cost 

Labor Cost 

per Cell 

LPE - Infinite Melt 4 2 $ 15,360 $ 1.54 

- Finite Melt 63 1 $141,120 $14.11 

MO-CVD 31 1 $ 69,440 $ 6.94 

3 2 $ 11,520 $ 1.15 

MBE 344 1 $770,560 $77.06 

31 1 $ 69,440 $ 6.94 

1. Assuming a single 8 hour shift per day. 

fraction of the five-year (60-months) depreciation schedule that the equipment was in use 

fabricating solar cells. 

The infinite-melt LPE process requires the use of two growth chambers. The growth 

chamber for the n-type buffer layer holds 5000 grams of molten solution in inventory and the 

growth chamber for the p-type window layer, 3000 grams. We estimated, based on duplicating an 

existing design, that each growth chamber, with substrate holders and associated plumbing and 

wiring, could be fabricated for $20,000. The total capital equipment costs for the infinite melt 

LPE system would be $40,000, or $666.67 per month for a 5 year depreciation schedule. The 8006 

grams of solution in inventory represents another $7,424 in fixed costs that we depreciated over 

five years. 

The hardware required for an finite melt LPE process is a relatively simple sliding boat 

growth apparatus that is positioned in an evacuated, temperature controlled oven. We estimated 

the total cost of the apparatus including various pumps with associated plumbing and wiring to 

be $20,000.00. 

A four loop, commercially available, MO-CVD cabinet has a retail price of $72,000 excluding the RF generator. This number is in relatively close agreement with the cost estimate of 

$60,000 provided by several U.S. research organizations who built their own MO-CVD system. 

We assumed a total capital equipment cost of $75,000 for the installed MO-CVD hardware 

including a RF generator and the necessary vent lines to the atmosphere. 

The second generation, commercially available MBE system is being produced as a pilot 

production unit. This basic system currently sells for approximately $208,000 and accepts 5.0&m 

(&-inch) diameter wafers. There is a single growth station with a capability of azimuthal rotation 

for a more uniform growth over the total substrate area. An optional chamber for substrate 

46 
preparation is available (.$45,000) containing a 6 station carousel, 2 of which are heated. For our 

cost model we assumed a second generation MBE system, at a total capital equipment cost of 

$250,000. 

Table 12 summarizes the total capital equipment costs, the total cost allocated to the 

fabrication of 10,000 solar cells and the resulting cost per cell for the different fabrication 

processes. The high cost of the MBE process in the capital equipment category does not permit a 

cost savings to be realized by running several chambers in parallel to meet the annual production 

requirements. The infinite melt LPE process and MO-CVD process with a large area growth 

station are the most cost competitive from the view-point of capital equipment. This is due 

primarily to the short period of time in which the equipment is being used. 

TABLE 12 

SUMMARY OF CAPITAL EQUIPMENT COSTS FOR DIFFERENT SOLAR CELL 

FABRICATION PROCESSES ASSUMING A MINIZIUM NUMBER OF GROWTH CHAMBERS 

l Assuming a five year straight line depreciation schedule 

Process 

LPE - Infinite Melt 

- Finite Melt 

MO-CVD 

MBE 

Total Capital 

Equipment Costs 

$ 47,424’ 

$ 20,000 

$ 75,000 

$250,000 

Fabrication Time 

(months) 

4 

63 

31 

3 

344 

31 

Total Equipment 

Cost Allocated 

to Solar Cells 

$ 3,162 

$ 20,000 

$ 38,750 

$ 3,750 

$250,000 

$129,167 

’ Includes $7,424.00 for 8,000 grams of melt held in inventory 

in two growth chambers 

Equipment 

Cost per 

Cell 

$ .32 

$ 2.00 

$ 3.88 

$ .38 

$25.00 

$12.92 

Table 13 summarizes all of the direct costs associated with the growth of the epitaxial layers 

by the three processes. 

47 
TABLE 13 

SUMMARY OF MAJOR DIRECT COSTS PER CELL FOR GROWTH 

OF THE EPITAXIAL LAYERS OF 10,000,2cm x 2cm (AIGa)As/GaAs SOLAR CELLS 

l Excluding overhead and profit 

Process 

Yield 

1%) 

LPE - Infinite Melt 100 

- Finite Melt 100 

10 

1 

MO-CVD 

MBE 

100 

10 

100 

10 

50 

5 

50 

5 

Time to 

Fabricate 

(months) 

4 

63 

63 

63 

31 

31 

31 

3 

3 

3 

344 

344 

31 

31 

Material Unburdened 

costs Labor Costs 

S .006 

$ .006 

% .06 

$ .62 

$ .23 

s 2.29 

s 22.94 

S .23 

s 2.29 

s 22.94 

s .03 

s .30 

s .03 

s .30 

$ 1.54 

$14.11 

$14.11 

$14.11 

S 6.94 

t 6.94 

S 6.94 

s 1.15 

$ 1.15 

$ 1.15 

$77.06 

$77.06 

$ 6.94 

S 6.94 

Capital 

Equipment 

costs 

Energy cost 

costs per Cell’ 

S .32 J .016 

s 2.00 8 .015 

s 2.00 s .015 

s 2.00 $ .015 

s 3.88 $ ,002 

S 3.88 t .002 

s 3.88 s .002 

S .38 s .002 

$ .38 $ .002 

$ .38 0 .002 

$25.00 $ .79 

$25.00 s .79 

$12.92 S .72 

$12.92 S .72 

S 1.88 

S 16.13 

S 16.19 

S 16.75 

s 11.05 

S 13.11 

S 33.76 

S 1.76 

S 3.82 

S 24.47 

$102.88 

$103.15 

$ 20.61 

S 20.88 

1. Does not include a S1.24/cm2 purchase price for polished substrates. 

48 
VI. CONCLUSIONS AND RECOMMENDATIONS 

The basic conclusions derived from this study are: 

1. The infinite melt LPE process currently has the lowest projected cost for the growth 

of the epitaxial layers on a single crystal, (AlGa) As/GaAs solar cell. 

2. The MO-CVD process has a potential for low cost fabrication of (AIGa) As/GaAs 

solar cells. There currently, however, is a significant uncertainty in the yield of the 

growth system, i.e., the portion of active material in the input gas stream that ends 

up as part of the epitaxially grown layers. 

3. Additional work is needed to optimize and document the process parameters for the 

MO-CVD process so that a detailed economic assessment of its potential to fabricate space quality (AlGa) AslGaAs solar cells can be made. 

4. The MBE process would be a useful epitaxial growth process for research and 

development activities related to optimizing the structure of a (AlGa)As/GaAs 

solar cell for space applications. 

49 

APPENDIX A 

51 
BIBLIOGRAPHY 

1. Stuerke, C. Investigation of GaAs Solar Cell Potential Performance and Cast. Rep. No. 

AFAPL-TR-76-100, U.S. Air Force, 1977. (Available from DTIC as AD A040 736.) 

2. Solar Electrumagnetic Radiation. NASA SP-8006, 1971. 

3. Hovel, H., and Woodall, J.M. Optimization of Solar Cells for Air Mass Zero Operation and a 

Study of Solar Cells at High Temperatures. Final Report, Phase I. NASA CR-145008, 1976. 

4. Hovel, H.J. Solar Cells. Vol. 11 of Semiconductors and Semimetals. Edited by R.K. Willardson and Albert C. Beer. New York: Academic Press 1975. 

5. Bozler, C.O.; Fan, J.C.C.; and McClelland, R.W. “High-Efficiency GaAs Shallow-Homojunction Solar Cells.” Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, 

MA. Solid State Research, Quarterly Technical Summary Report, November 1978-31 January 1979, Issued 13 June 1979, 41-46. 

6. Sahai, R.; Edwall, D.D.; and Harris, J.S., Jr. “High-Efficiency AlGaAs/GaAs Concentrator 

Solar Cells.” Rockwell International Science Center, Thousand Oaks, CA. Applied Physics 

Letters, Vol. 32, No. 2, 15 January 1979, 147-149. 

7. Loo, R.; Goldhammer, L.; Anspaugh, B.; Knechtli, R.C.; and Kamath, G.S. “Electron and 

Proton Degradation in (AlGa)As-GaAs Solar Cells.” The Conference Record of the 

Thirteenth IEEE Photovoltaics Specialists Conference - 1978, 562-570. 

8. James, L.W., VanderPlas, H.A., and Moon, R.L. High Performance GaAs Photovoltaic Cells 

for Concentrator Applications. Final Report SAND 78-7018, Sandia Laboratories, 1977. 

9. Kamath, S., and Wolff, G. High Efficiency GaAs Solar Cell Development. Final Report 

AFAPL-TR-78-96, U.S. Air Force, 1979. 

10. Milnes, A.G., and Feucht, D.L. “Heterojunctions and Metal-Semiconductor Junctions. ” 

Academic Press, New York and London, 1972. 

11. Lawley, K.L. “Vapor Growth Parameters and Impurity Profiles on N-Type GaAs Films 

Grown on N+-GaAs by the Hydrogen-Water Vapor Process.” Journal of the Electrochemical 

Society, Vol. 113, No. 3, March 1966, 240-245. 

12. Rubenstein, M., and Myers, E. “Epitaxial Synthesis of GaAa Using a Flow System.” Journal 

of the Electrochemical Society, Vol. 113, No. 4, April 1966, 366-367. 

13. Johnston, W.D., Jr. “Vapor-Phase-Epitaxial Growth of n-AlAs/p-GaAs Solar Cells.” Journal 

of Crystal Growth, Vol. 39, 1977, 117-127. 

14. Stringfellow, G.B., and Horn, G. “Increase in Luminescence Efficiency of Al,Ga,., As Grown 

by Organometallic VPE.” Applied Physics Letters, Vol. 34, No. 11, 1 June 1979, 794-796. 

15. Dupuis, R.D., and Dapkus, P.D.; and Garner, C.M.; Su, C.Y.; and Spicer, W.E., “Abrupt 

Gal., Al,As-GaAs Quantum-Well Heterostructures Grown by Metalorganic Chemical Vapor 

Deposition.” Applied Physics Letters, Vol. 34, No. 5, 1 March 1979, 335-337. 

16. Matsunaga, N., and Takahashi, K. “Graded Band-Gap pAl,Ga,.,As-nGaAs Heterojunction 

Solar Cells Prepared by Molecular Beam Epitaxy.” International Journal of Electronics, Vol. 

45, No. 3, September 1978, 273-282. 

17. Luscher, P.E. “Crystal Growth by Molecular Beam Epitaxy.” Solid State Technology, 

December 1977, 40-52. 

52 
APPENDIX B 
1. 

2. 

AERO PROPULSION LAB 

CRYSTAL SPECIALTIES 

3. HUGHES RESEARCH LAB 

4. IBM CORPORATION 

5. MIT - LINCOLN LAB 

6. NASA LANGLEY RESEARCH CENTER 

7. 

8. 

9. 

ROCKWELL INTERNATIONAL/ 

ELECTRONICS RESEARCH CTR. 

ROCKWELL INTERNATIONAL/ 

SCIENCE CENTER 

VARIAN ASSOCIATES Palo Alto, CA 

LIST OF PERSONNEL INTERVIEWED FOR 

GaAIAdGaAs SOLAR CELL PROCESS STUDY 

WPAFB, OH 

Monrovia, CA 

Malibu, CA 

Yorktown Heights, NY 

Lexington, MA 

Hampton, VA 

Anaheim, CA 

Thousand Oaks, CA J. S. Harris 

C. Stuerke 

W. Alfred 

G. S. Kamath 

R. C. Knechtli 

H. J. Hovel 

J. C. C. Fan 

E. J. Conway 

J. A. Hutchby 

G. H. Walker 

P. D. Dapkus 

R. Ruth 

R. Bell 

B. Cairns 

L. W. James 

54 
1. Report No. 2. 

NASA CR-3361 

4. Title and Subtitle 

GaAIAs/GaAs Solar Cell Process Study 

7. Author(s) 

David W. Almgren and Katinka 1. Csigi 

L Performing Organizrrios Name and Address 

Arthur D. Little, Inc. 

Acorn Park 

Cambridge, MA 02140 

12. Sponsoring Organization Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546 

15. Supplemcnmry Notes 

Langley Technical Monitor: Edmund J. Conway 

Final Report 

i. Report Dote 

December 1980 

6. 

C-82340 

8. Performing Organiurion Report No. 

ADL 82340 

10. Project/Tark/Work Unit No. 

11. Contract or Grant No. 

NASl-15516 

13. Type of Repon 

Contractor Report 

!6. Abstract 

This report summarizes available information on liquid phase, vapor phase (including chemical vapor deposition) 

and molecular beam epitaxy growth procedures that could be used to fabricate single crystal, heteroface, (AIGa) 

As/GaAs solar cells, for space applications. A comparison of the basic cost elements of the epitaxy growth processes 

shows that the current infinite-melt LPE process has the lower cost per cell for an annual production rate of 10,000 

cells. The metal organic chemical vapor deposition (MO-CVD) process has the potential for low cost production of 

solar cells but there is currently a significant uncertainty in process yield, i.e., the fraction of active material in the 

input gas stream that ends up in the cell. Additional work is needed to optimize and document the process parameters 

for the MO-CVD process. 

17. Originator’s Key Words 

Solar Cell, Gallium Arsenide, 

GaAs, Space, 

LPE, CVD, MBE Growth Processes 

18. Availability Smremenr 

Unclassified - Unlimited 

Subject Category 44 

r 

19. U.S. Securiry Classif. of the Report 20. U.S. Security Ckuif. d Tbir Page 21. No. of Pag” 22. Price 

Unclassified Unclassified 58 A04 

For sale by the National Technical Information Servrce, Sprrngfield. Virgrnra 22161 

NASA-Lang1 eY. 1980