The development, fabrication, and failure modes of AlxGa(1-x)As-GaAs heteroface solar cells are described. Crystal growth, the diffusion of Zn into the GaAs layer to form the p-n junction, SEM studies of the diffusion length of GaAs, and procedures for making ohmic contacts are discussed

Feucht, D. L.

Milnes, A. G.

English

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Sena-Annual Status Report
For Period
October 15, 1974 - April 30, 1975
STUDIES OF HMMFACE SOLAR CELL PE RFOR'WCE
BY
J,	 D. L. Feucht and A. G. Mimes
Aye ^`	 Electrical Engineering Department
Cn
Carnegie 4lellon University
Pittsburgh, PA 15213
NASA Grant NGR 39-087-021
This work was performed for the National
Aeronautics and Space A& nistration, Langley
Research Center, Hampton, Virginia 23365.
(NASA-CF-145E93)	 SIUEIES CF HETEFCFACE	 F76-13594
SCIAB CELL FERFCEPAKI Seriannual Statut
Re Fort, 15 Cct. 1974 - 30 Apr. 1975
(Carnegie-Fellon Univ.)	 11 p HC $3.5i.	 Unclas
CSCL 10A G3/44 v563J
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Page
1. Fabrication of Alpal_x As-GaM Feteroface Solar Cells
1.1 Inproverents in the Growth Technique
1.2 Zinc Diffusion Depth Meas=ermts	 4
1.3 Ohmic Contacts	 7
2. Scanning Electron Mcroscope Diffusion Length Measurements 	 7
1. Fabrication of A! Ga1_ As-GaAs Heteroface Solar Cells
1.1 Improvement in the Growth Technique
The groVtth system modifications noted in the last report did not result
in acceptably smooth succeeding grown layers. Experiments were done to iden-
tify the causes of non-smooth growth. Three distinct types of growth fail-
ure were apparent, all due to sore kind of ---gradation of the substrate sur-
face. Therefore, even if a cc ! .`_inuous aluminum-gallium arsenide layer is
formed, good solar cell performance cannot be expected fram a non-smooth sam-
ple because of the importance of the substrate gallium arsenide in the opera-
tion of the desired structure.
The first type of failure is a many-micron deep furrowed, or "worm track",
appearance which is somewhat smoothed over by the aluminum-gallium arsenide
layer. M-en the layer is removed by etchina in hydrochloric acid, the features
of the substrate are sharper. A temperature program in which the seed is
brought into contact with the melt ten minutes before the furnace is cooled
for growth which was adopted to improve the quality of the heterointerface,may
be the cause. It is postulated that instabilities in the furnace temperature
during the time when the melt is in contact with the seed may result in non-
uniform etch back. Changing the temperature program by cooling the :Welt by
one degree before bringing the seed into contact and continuing to cool the
furnace has eliminated this problem,but may not give as good an interface.
Reducing the temperature of growth, from 950°C to 850°C or 800 0C,should re-
duce the amount of arsenic lost from the substrate surface and tend to counter-
act the elin ination of the cleaning period (melt in contact with substrate be-
fore g=% h to remove arsenic deficient surface layersl
i
-2-
The second failure mode is a very fine roughness, less than one micron
deep, in the substrate surface. This af, ,ears to have been caused by a de-
fective growth tube since replacing the tube has cured the problem. The growth
tubes are quartz and trend to devitrify after extensive operation at high temp-
eratures. They can leak at growth temperatures but not at room temperature.
This leakage is not detectable with a helium mass spectrometer leak detector
since the helium will diffuse through the wall of a sound quartz tube at the
growth temperature.
The third type of failure is one that has appeared at random as long as
the program of research has been in operation. It has the appearance of very
deep scratches in the substrate surface, obviously caused by the substrate
rubbing the top lamination of the boat. The scratches may be widely separated
or crowded together so no undamaged sr face appears between them. The latter
behavior is usually seen at a high spot on the substrate surface, created by
etching the substrate for many minutes in the cleaning procedure. Growth of
aluminum-gallium arsenide does not usually cover scratches, especially when
they are crowded together.
A recent growth run with s^.:ratches all the way across the substrate but
with an aluminum gallium arsenide layer on only three quarters of the region
suggests that the scratches were made when the substrate was loaded into the
boat and were enhanced by evaporation of arsenic as the furnace was heated.
The scratches on the uncovered part of the substrate, although several micro--
meters deep, have smooth rounded walls and intersect rectangular pits filled
with gallium along their length (no melt was in contact with this part of the
surface). The rectangular pits are also seen around the edges of the substrate,
where evaporation of arsenic is expected. The original scratches were not vis-
ible in the glove box when the substrate was examined after loading.
}
-3-	
T
New  boat components were fabricated from boron nitride (Union Carbide
Corporation) in an attempt to alleviate the scratching problem. Boron nitride
is chemically inert at our growth temperatures and has the advantage of being
easier to machine than vitreous carbon. A slider and top lamination were
made fm-n boron nitride and used in several growth runs. It was found that
the pockets for the seed and source crystals in tle slider could be milled with
much flatter bottoms than with vitreous carbon, alt2x)ugh there was a tendency
for the material to break off in laminar pieces from the narrow ridge between
the pockets and the side of the slider. This breakage does not affect the
operation of the boat.
Along with the better control and uniformity of pocket depths in the slider,
it was hoped that boron nitride would make a tap lamination that was less brit-
tle and therefore less prone to crumble at the edge of the melt ' _pole. The
boron nitride top lamination did not give better results than the one made of
carbon since the places where e.e seed rubbed against the top lamination still
had a few scratches and rough growth. The old carbon top lamination was put
back and the boron nitride slider retained.
Another approach to the scratch problem was a reduction in the nwinumt
temperature during the growth cycle, although this was primarily done in an
attempt to improve the current-voltage characteristics of our cells. A re-
duction in riaximum temperature has two benefits: first, it allows a wider gap
between the seed surface and the top lamination without degradation of the sur-
face from arsenic loss, and second, it causes less widening of existing scrat-
ches. Both effects are due to the lowering of the equilibrium arsenic vapor
pressure c-rer the seed with the lowering of temperature. The scratch problem
was not entirely eliminated by lowering the growth temperature.
-4-
The growth procedure as shown schematically in Fig. 1 has been modified
slightly since the last time it was described in a status report. The gal-
lium for the melt is baked in a separate carbon crucible immediately before
the growth run. Only the gallium is baked,since at growth temperatures be-
low 900°C no extra pieces of gallium arsenide are necessary to insure sat-
uration of the melt. The crucible is necessary because the source crystal is
now loaded at the same time as the seed to avoid partial evaporation of the
source crystal during the gallium bake. The seed is loaded through the right-
hand opening to reduce the possibility of scratching the seed surface.
1.2 Zinc Diffusion Depth Measurerxnts
Studies of the diffusion of zinc into the GaAs to form the p-n junction
indicate that the zinc diffuses primarily during the heatup time of the boat
prior to the growth. Run 88A (Status Report, 4/16/73-10/15/73) sties done to
conclusively demonstrate this fact. In that experiment a melt and seed were
loaded and the fuYnace heated to 850°C and held there for 15 minutes without
the seed coming into contact with Lhe melt. A junction depth of 0.83 microns
was measured with the SDI, see Table 1. The results of other junction depth
measurements done since then are surmarized in Table 1. Although the time and
temperature cycles of the growths vary considerably, it is possible to see ex-
pected effects between tm runs where only one parameter varied between the
runs. For instance, the junction depth in run 156 is about one-half of that
of run 154, for a zinc concentration which was approximately halved.
The experience represented by Table 1 shows that it is possible to con-
trol the junction depth down to a value of about 0.5 microns, but below this
level control is difficult. This reflects both the fact that the true junc-
tion depth depends on the zinc concentration profile and the substrate doping
level and a difficulty in accurately measuring distances below 0.5 microns
with our Sal.
1
a[ET
Top View
M o 1 f =—•,^	 -
(1) Load Ga in crucible and bake with moat at 1000°C.
/ Seed Crystol
(2) Load Source Crystal Ga, Zn, Al, Load seed through right hand opening.
^'_ n n
(3) Heat to 850°C for 15 min; cool to 800°C at 10°C/min with seed covered
by carbon..
(4) Move slider at 800°C so seed is below melt and cool at 10°C/min to 797°C.
1-0	 i 4
(5) Terminate growth by moving slider in revErse direction; open furnace.
Fig. 1 Schematic diagram of growth procalure
i	 '
-6-
Table 1. Zn Diffusion Depth Measurements in Fieteroface Solar Cells
Fm Substrate Zn/Ga Max.Temp. Time at Diffusion
No.	 (arr3 ) wt. ratio (°K) Tmax (min) Depth (G1m)
88A 2.5x1016 - 1123 16 0.83
137 5x1017 1.81.10 4 1225 40 0
151 3x1016 2.14x1074 1123 12 0.24
154 2x1017 2.19x10 3 1174 12 0.7
156 3x1017 1.29x10 3 1174 12 0.56
157 3.5x1017 1.04x10 3 1.174 12 0.61
158 1x1017 1.42K,6-3 1174 12 0.61
160 8x1016 1.46x10 3 1174 12 0.89
161 8x1016 5.45x10 3 1123 20 1.0
162 1x1017 2.91x10 3 1123 20 0.87
164 7.5x1014 2.91x10 3 1123 20 1.30
165 6x1014 2.91x].0 3 1123 20 1.47
166 5x1016 2.74x1G 3 1123 20 0.98
mow
1
-7-
1.3 Ohmic Contacts
The procedure for making an ohmic contact to the front, or aluminum-
gallium arsenide, side of the cell by evaporation of zinc followed by evap-
oration of silver and heating, has resulted in poor cell performance. The
heating required to form the contact results in diffusion of silver into the
junction region of cells with thin (a micron or less) aluminunrgallium arse-
nide layers. Cells with this kind of contact have a uniformly low performance
as first noticed by H. Hovel of IBM. According to a paper by Boltaks and
Shishiyanu, (translated in Soviet Physics-Solid State, Vol. 5, p. 1680), the
rapid diffusion coefficient D for silver in gallium arsenide at 550°C is 5x10-'
csn2/sec. Thus, a heat treatment of ten minutes gives a Dot product whose squaye^
root is ten microns. Silver is therefore seen to be nearly as bad as gold for
use as a contact metal where any heating of the device takes place after the
contact is applied. There is some dispute in the literature about the value
of D but there appears to be a correlation between poor cell performance and
a silver bearing front contact which involves subsequent heat treatment.
The contact procedure presently being used is the old one. Zinc is first
diffused into the aluminum-gallium arsenide in a closed ampoule. The p region
is removed from the back and the back contact to the gallium arsenide is made
by the evaporation of tin followed by silver and a heat treatment at 350°C
for one minute. The front contact to the aluminum gallium arsenide is then
made by the evaporation of silver through a mask with no subsequent heat treat-
ment.
2. Scanning Electron Microscope Diffusion. Length Rbasureniants
Table 2 sc uriarizes the scanning electron microscope measuremnts of dif-
fusion length made for Mr. Walter and Dr. Stirn. Stirn samples of gallium
arsenide--Phosphide from I--)plied Materials Corporation, with phosphorus frac-
tions of 0.3, were reported to have good diffusion lengths in the last rclort.
-8^
Measurements done in this period showed that AMC is capable of growing epi-
taxial gallium arsenide samples with diffusion lengths greater than three
microns. There is disagreement over the net electron concentration in these
samples as it is measured by our 1 MHz capacitance bridge, Stirn's capacitance
technique using lower frequencies, and AMC's van der Pauw technique on separate
check layers. The AMC electron concentration measurements are usually greater
than ours and Stizn ' s by nearly an order of magnitude.
-9-
Table 2. S'unrmy of Sal Diffusion Length Measurements
Material	 LP	 ND-NA
Walker TML
B-4-45 trop GaAs (bulk) 0.5 microns -
B-4-45 be ttom GaAs (bulk) 1.8 -
Walker AMC
1772 GaAs (bulk) 0.5 2.7x1017 3
1944 A GaAs (bulk) 0.48 2.7s.'U17
1944 B CaAs (bulk) 0.51 2.7x1017
GI-18 GaAs (epi) 1.0-3.0 1.3x10 17*
GI-19 GaAs (epi) 2.4 2.4Y1017 *
GI-20 GaAs (epi) 2.9-2.1) 2.6x1017 *
GI-22 GaAs (epi) 3.1 1.0x1017 *
Stirn AMC
T-245B2 GaAs (epi) 4.5 2.8x1015
71 235A7  GaM .5p .5 (epi) 1.0 1.5-1.9x1016
R-643A7 GaAs .3p .7 (epi) 0.42-1.0 5.8x1016
R-630A13 GaP (bulk) 0.22 1.0x1017
T-237 GaAs (epi) 4.5 4.0x1015
*As measured by AMC


Studies of heteroface solar cell performance

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