This talk outlines a simple process for the fabrication of n(+)-p solar cells in indium phosphide. Large area cells (greater than 0.25 sq cm) have been made by this process, with a photovoltaic conversion efficiency of 15.21 percent under AM0 conditions of illumination. An ideality factor of 1.1 and a saturation current density of 8 x 10 to the minus 15th power A/sq cm have been observed for these cells. The technique for cell fabrication involves the diffusion of sulfur into InP by an open tube process, and gives highly reproducible results from run to run. A vacuum-deposited layer of gallium sulphide (Ga2S3) was used as the source for sulfur diffusion, with a chemically vapor deposited SiO2 cap layer to prevent decomposition of the InP surface during heat treatment. Diffusions were carried out in a flowing nitrogen ambient at 585 to 708 C, and characterized by their surface carrier concentration and the diffusion constant. The diffusion profile for sulfur in InP is estimated to be of the complementary error function type. The activation energy of the diffusion was estimated to be 1.94 eV. The technique described here is ideally suited for the fabrication of shallow n(+)-p junctions in InP, and has been used for space-borne solar cells

Borrego, J. M.

Ghandhi, S. K.

Parat, K. K.

English

NASA Technical Reports Server

N89-24714 
Encapsulated Diffusion of Sulphur into InP 
K.K. Parat, J.M. Borrego, S.K. Ghandhi 
Electrical, Computer and Systems Engineering Department 
Rensselaer Polytechnic Institute 
%y, New York 12180 
Abst rac t  
This talk outlines a simple process for the fabrication of n+-p solar cells in indium phosphide. 
Large area cells (>0.25 cm2) have been made by this process, with a photovoltaic conversion efficiency 
of 15.21% under AM0 conditions of illumination. An ideality factor of 1.1 and a saturation current 
density of 8 x A/cm2 have been observed for these cells. 
The technique for cell fabrication involves the diffusion of sulphur into InP by an open tube 
process, and gives highly reproducible results from run to run. A vacuum-deposited layer of gallium 
sulphide (Ga2S3) was used as the source for sulphur diffusion, with a chemically vapor deposited 
Si02 cap layer to  prevent decomposition of the InP surface during heat treatment. Diffusions were 
carried out in a flowing nitrogen ambient at  585OC to 708OC, and characterized by their surface carrier 
concentration and the diffusion constant. The diffusion profile for sulphur in InP is estimated to be 
of the complementary error function type. The activation energy of the diffusion was estimated to  
be 1.94 eV. 
The technique described here is ideally suited for the fabrication of shallow n+-p junctions in 
InP, and has been used for space-borne solar cells. 
Introduction 
At the present time, there is considerable interest in indium phosphide as an active material 
for solar cells. InP is a direct bandgap semiconductor with an energy gap of 1.34 eV, which is close 
to  that required for maximum conversion efficiency at AM 1.5 [ref. I]. An important property of 
this material is its ability to  be surface passivated. Typically, the surface recombination velocity 
of n-type InP is about lo3 cm/sec as compared to 1 x lo7 cm/sec for GaAs [ref. 21. It has been 
reported that InP solar cells have a higher resistance to  gamma ray radiation degradation than Si 
or GaAs solar cells of comparable junction depths [ref. 31. Thus, these solar cells are of particular 
interest for space applications. 
Conventionally, diffusion of n-type impurities in III-V compounds has been carried out [refs. 
4-61 in sealed tubes, in order to  prevent evaporation of the more volatile group V element, which 
leads to  massive deterioration of the surface. These diffusions require both high temperature as well 
as long duration, due to  the low diffusion coefficient of donor impurities, with some vapor phase 
etching of the substrate. 
There are two other problems associated with the sealed tube diffusion technique. First, the 
diffusion constants of impurities in III-V compounds are critically dependent upon the vacancy 
concentrations, and thus on the partial pressures of the host lattice elements. The inability to  
99 
https://ntrs.nasa.gov/search.jsp?R=19890015343 2020-03-20T01:43:21+00:00Z
maintain identical partial pressure conditions from run to run makes results hard to reproduce, 
80 that there is a large disagreement between different workers because of the different diffusion 
conditions involved. The second problem is the neceasity of handling the highly toxic substances 
such as arsenic, or hygroscopic materials such as phosphorus, which have to  be carefully weighed 
and sealed in tubes. 
The possibility of an open tube technique [ref. 71, where the diffusion can take place in a flowing 
inert ambient gas, is thus very attractive. Ease of implementation, adaptability to large scale batch 
processing, and potential low cost are its main advantages. 
This paper describes the technical details and the results obtained with the open tube diffusion 
of sulphur into InP. 
We have used a vacuum evaporated layer of gallium sulphide as the source for sulphur doping, 
capped by a chemical vapor deposited film of Si02 to  prevent the loss of phosphorus from the surface 
of InP. This results in excellent morphology even after high temperature processing. The equilibrium 
vapor pressure of phosphorus during diffusion does not depend upon extrinsic parameters as in the 
case of sealed tube diffusion, where the phosphorus vapor pressure is set by the amount of phosphorus 
in the tube. As a result, this technique is simple to implement, and is highly reproducible from run 
to run. Solar cells made by this method have demonstrated equal (or slightly superior) performance 
than those made by the sealed tube process [refs. 8, 91. 
Experimental 
The p-type substrates used for the fabrication of n+-p InP solar cells were pre-polished LEC 
grown 50 mm diameter slices, obtained from Crystacorn, Inc., Mountain View, CA 94043. These were 
(100) misoriented 2" towards (110), and doped with Zn to a concentration of 5 x 10l6/cc. The as- 
received material was cut into suitable pieces and degreased in successive hot baths of trichloroethy- 
lene, acetone and methanol. Next, they were etched in a 1% Br-methanol solution for 5 minutes 
to remove the residual polishing damage. This was followed by etching in HF:HCl:H202:HzO in a 
ratio of 2:2:1:8 by volume, and by a rinse in 10% H3P04 solution to remove the oxide present on 
top. Finally gallium sulphide (99.99% purity) was evaporated onto the frontside of the wafer using 
an alumina coated tantalum boat. 
After the deposition of the Ga2S3 film, the slices were transferred to a cold wall resistance 
heated chemical vapor deposition system operated at  atmospheric pressure. The transfer time was 
kept short (under 10 min.) to avoid any possible contamination from the atmosphere. The samples 
were first encapsulated on the frontside with 0.5 pm of SiOz, which was grown by the oxidation of 
SiH4 in an argon ambient. The partial pressures of oxygen and silane used for this purpose were 6.46 
x atm. respectively. The growth temperature was 325°C with an approximate 
growth rate of 100A/min. Next, the samples were turned over and the backside was also capped 
with 0.5 p m  of ,502. This is necessary to prevent phosphorus evaporation from the back of the 
slices during the diffusion step. Moreover, it provides a balancing of stress in the InP during heat 
treatment, and thus avoids dislocation formation, as well as damage to the cap layers. 
and 7.5 x 
Diffusions were carried out at  temperatures ranging between 585OC and 708OC in an open tube 
diffusion system with a flowing nitrogen ambient, for times varying from 5 minutes to 24 hours. 
After diffusion, the Si02 cap was removed in dilute HF. The Ga2S3 layer was then removed in the 
100 
etch formulation described earlier (HF:HCl:H202:H20=2:2:1:8 by vol.). No visual damage of the 
InP surface was observed after the entire diffusion process. 
In order to  make solar cells on these diffused structures, back contacts to the p-substrates were 
made by evaporating Au/5% Zn and alloying a t  42OOC in forming gas (80% H2, 20% N2) for 20 
seconds. Next, the front contact grids were defined photolithographically. Electroplated gold was 
used to make contacts to the top n+-layer. These top contacts were well adhering, and ohmic (as 
plated) without any heat treatment. This is because the top n+-layer is very heavily doped. A total 
cell area of 0.31 cm2 was delineated by photolithography and mesa etching in an iodic acid (10% by 
wt.) bath,  with the aid of a second photomask. 
Diffused junctions were characterized using the van der Pauw technique. A clover leaf pattern 
was delineated on the InP slices for this purpose by etching in 1% Br-methanol. Ohmic contacts 
to the n+ layers were formed by alloying tin dots at 26OOC for 1 minute in a flowing forming gas 
ambient. The  sheet conductance and the effective Hall mobilities on these samples were measured 
at room temperature and at 77K. Following this, samples were etched in a 3% iodic acid etch with 
an approximate etch rate of 2OOA/min for short durations of time. After each successive etch (i.e., 
the removal of a thin n+ layer), Hall measurements were made until the sheet conductance of the 
remaining layer was less than 1% of the initial value. The etch rate of the individual samples was 
determined by simultaneously etching part of an identically diffused sample and then measuring 
its etched depth by a multiple beam interferometer. An etch rate of approximately 2OOA/min was 
measured in this manner. 
Results and Discussion 
Figure 1 shows the sheet conductance of the diffused layers, plotted as a function of t,he square 
root of the diffusion time for three different thicknesses of the Ga& layer. All the diffusions were 
carried out a t  a fixed temperature of 625OC. The straight line nature of the graphs shows that ,  
for the diffusion durations considered here, the Ga2S3 layer always served as an infinite source of 
sulphur [ref. lo]. The  effective mobilities of the diffused layers were found to  be independent of the 
GazSg thickness. If we assume that the general nature of the sulphur diffusion profile into InP is the 
same for all these diffusion cases, and that the mobility of these diffused layers is limited by impurity 
scattering, and hence doping dependent, it follows that  the surface concentration is independent of 
the Ga2S3 source layer thickness. For a given doping profile (e.g., a complementary error function 
type) with a fixed surface concentration, the sheet conductance changes in proportion to the change 
in (Dt)'/'. The  parallel nature of the three sheet conductance vs. (time)'I2 graphs leads to the 
conclusion that  the diffusion constant of sulphur into InP is the same for all three cases, and hence 
is independent of the Ga2S3 thickness. 
The  reason for the initial delay could be due to strain enhanced diffusion, as has been observed 
[ref. 111 during Zn diffusion into GaAs. However, this explanation does not seem to hold here. Note 
the absence of a slow diffusion regime in the beginning; rather, the sheet conductance rises abruptly 
after a finite time delay. Moreover, mobility values measured on these samples are sufficiently high 
so as to  rule out the possibility of significant stress in the diffused layer. This has been reaffirmed 
by the fact than n+-p diodes formed by this diffusion technique have ideality factors close to unity, 
as seen in Fig. 2. 
101 
Figure 2 shows the delay as a function of the (Ga& thickness)'. The character of this curve 
is indicative of the  fact tha t  S diffusion into InP  is taking place through a barrier. We propose tha t  
atomic sulphur required for the sulphur diffusion is primarily generated at the Ga2S3/Si02 interface, 
and then diffuses through the Ga& layer to reach the InP. This would introduce a specific time 
delay before any substantial diffusion is observed. In addition, this delay should be proportional t o  
the  square of the Ga2S3 thickness, as seen in Fig. 3. 
Figure 4 shows the sheet conductance and room temperature Hall mobiIity, measured after 
successive etching and removal of thin layers of the n+ region. The  sample shown here had 200A of 
Ga& as the dopant source and was diffused for 4 hours at 625OC. The  mobility values shown here 
are the  effective values for the remaining n+ layer. Local mobility values were determined from these 
by a technique described elsewhere [ref. 121. Here, we assumed tha t  the drift mobility of electrons in 
these n+ layers is the same as the measured Hall mobility, since the  layers were degenerately doped 
[ref. 131. 
The  carrier concentration can be determined by differentiating the sheet conductance curve; 
however, unless the da t a  is smoothed o u t ,  the error introduced by this process will be very large. 
Because of this, we preferred to assume a profile and specific values of the surface concentration and 
(Dt)'/', and have iteratively fit the sheet conductance da ta  points using the mobility values obtained 
by the  technique outlined above. The  best fit (as denoted by the dark continuous line in Fig. 4) was 
obtained for a complementary error function (erfc) type of profile, with a surface concentration of 
3.4 x 101"/cm3 and a (Dt)'I2 of 633A. Similar graphs were obtained for samples diffused at different 
temperatures and different times. In all cases, erfc doping profiles were obtained, independent of 
the time and temperature of the diffusion. Similar results have been obtained for sulphur diffusion 
profiles in GaAs [ref. 141, which are also of the complementary error function type. 
Figure 5 shows the sheet conductance versus (diffusion time)'/' for diffusions carried out at 
585OC, 625OC, and 708°C. The  thickness of the Ga2S3 layer was kept constant at 200A for all these 
diffusions. The  conventional practice here is to plot junction depth vs. (diffusion time)'/2, which ne- 
cessitates diffusion into a substrate of opposite polarity, in this case, p-type. However, diffusion into 
p-type I n P  introduces the problem that heat treatment of Zn doped InP alters the carrier concen- 
tration in the substrate [ref. 151, thus seriously affecting the interpretation of junction dept,h data.  
Consequently, we have made our diffusions into semi-insulating InP. Here if we assume (as shown 
above) tha t  the doping profile is of the erfc type irrespective of the diffusion time and temperature, 
and tha t  the  surface carrier concentration is a function only of the diffusion temperature, then the 
ra te  of change of sheet conductance should be directly proportional to the change in (Dt) ' /2 .  This 
is indeed the case, as seen in this figure. 
Doping profiles were measured on several samples diffused at these temperatures, for different 
durations of time. The  surface concentration and diffusion constants were determined by the method 
outlined before. Table 1 presents the da ta  on sulphur diffusion into InP a t  these temperatures. These 
include the surface concentration, D and the effective Hall mobility of the diffused layer as a function 
of the diffusion temperature. Finally, Fig. 6 shows the log of the diffusion constant as a function 
of 1/T. The  activation energy of sulphur diffusion into InP  was calculated to he 1.94 eV. The  pre- 
exponential term was calculated to be 3.6 x cm2/sec. 
102 
Conclusion 
Open tube diffusion of sulphur into InP was studied in the temperature range of 585OC to 
708OC. The diffusion profile was found to be of the complementary error function nature. Diffusion 
was dependent on the thickness of the Ga& layer, which was used as the source sulphur diffusion. 
It was proposed that diffusion occurs through a barrier, which in this case is the Ga2S3 layer itself. 
A capped diffusion, of the nature described here, results in a constant overpressure of phosphorus 
and sulphur at any given temperature. This is a distinct advantage over conventional diffusion 
methods, from the point of obtaining reproducible diffusions as required in device applications. 
Acknowledgement 
The authors would like to thank J .  Barthel for technical assistance and P. Magilligan for 
manuscript preparation. This work was performed on Grant No. NAG 3-604 from the National 
Aeronautics and Space Administration, Cleveland, Ohio. This support, as well as discussions with 
Dr. I. Weinberg, have been of much profit and hereby acknowledged. 
References 
[ 11 S.M. Sze, Physics of  Semiconductor Devices, 2nd Ed., New York: Wiley, 1981, p. 798. 
[ 21 H.C. Casey, Jr .  and E. Buehler, Appl. Phys. Lett., 30, 247 (1977). 
[ 31 A. Yamamoto, M. Yamaguchi and C. Uemura, Appl. Phys. Lett., 44, 611 (1984). 
[ 41 A. Yamamoto, M. Yamaguchi and C. Uemura, IEEE Trans. Electron Dev., Ed-32, 2780 (1985). 
[ 51 H.C. Casey, Jr., “Diffusion in Compound Semiconductors”, Atomic Diffusion in Semiconductors, 
[ 61 J.J.  Coleman, Appl. Phys. Lett., 31, 283 (1977). 
[ 71 S.K. Ghandhi and K.K. Parat, Appl. Phys. Lett., 26, 209 (1987). 
[ 81 K.K. Parat, S. Bothra, J.M. Borrego and S.K. Ghandhi, Solid State Electron., 30, 383 (1987). 
[ 91 I. Weinberg, C.K. Swartz, R.E. Hart, Jr., S.K. Ghandhi, J.M. Borrego, K.K. Parat, M. Yam- 
[ 101 Ref. to basic straight line differential theory. 
[ 111 C.H. Ting and G.L. Pearson, J .  Electrochem. SOC., 118, 1454 (1971). 
[ 121 H. Weider, Thin Solid Films, 31, 123 (1976). 
[ 131 W. Walukiewicz, L. Lagowski, L. Jaatrzebski, P. Rava, M. Lichtensteiger, C.H. Gatos, and 
[ 141 A.B.Y. Young and G.L. Pearson, J .  Phys. Chem. Solids, 31, 517 (1970). 
[ 151 K. Tsubaki and K. Sugiyama, Jpn. J .  Appl. Phys., 19, 1789 (1980). 
Ed. D. Shaw, Plenum Press, New York (1973). 
aguchi, Solar Cells, 22, 123 (1987). 
H.C. Gatos, J .  Appl. Phys., 51, 2659 (1980). 
103 
TABLE 1 
D ~ ,  I n P  
EFFECTIVE 
NS"RF MOBILITY 
2 2 OC (/cm 1 (cm /V-sec) (cm /set) 
Temperature 3 
485 1 . 9  x 10 2050 1 . 6  1 0 - l ~  
18 
62 5 3 . 4  x lOl8  1800 5 . 0  1 0 - l ~  
670 5 . 6  x 10" 1600 1 . 1  1 0 - l ~  
1350 5 . 2  1 0 - l ~  708 7 . 7  x 1018 
14.( 
12.c 
- 1o.c 
4 I
.gj 
m 
I 8 . 0  
,-I 
X 
w 
- 
6.0 
0 
8 
E 4 . 0  
r 
2 . 0  
0.0 
1.0 2 . 0  3 . 0  4 . 0  5 . 0  0.0 
( D i f f u s i o n  Time)'" (hr) 'I2 
Fig. 1. Sheet conductanc f the diffused n+ layer as a function of 
(diffusion time)?/?. Different curves are for different Ga S 
layer thicknesses. 2 3  
104 
REVERSE BIAS 
(GazS3 Thickness)' ( x  lo5 A"')  
Fig. 3. Initial delay vs. square of the Ga2S3 
Diffusions at 625".  
layer thickness. 
105 
7.0 
6.0 
- 5 .0  
A 
2200 
2100 ;i 
: . .  
I 
d 
n 
'5 4 . 0  
X - 
8 
B 
8 
a 2 3.0 
1.0 
0.0 
b 
.rl I 
2000 il 
1900 
1800 
0.0 4 . 0  8.0 12.0 20.0 
Depth (x  LOO A') 
F i g .  4 .  Sheet conductance and effective mobility of the diffused layer 
after successive etching. 
16.0 24.0 32.0 40.0  0.0 8 . 0  
(Diffusion Time) 'I2 (min)'12 
F i g .  5 .  Sheet conductance vs. for samples diffused at 
different temperatures. 
106 
1.00 1.05 1.10 1.15 1.20 
J/T (x  K-') 
Fig. 6. Diffusion constant of sulphur in InP vs. 1 / T .  
107 


Encapsulated diffusion of sulphur into InP

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