Gallium and boron doped silicon solar cells, processed by ion-implantation followed by either laser or furnace anneal were irradiated by 1 MeV electrons and their post-irradiation recovery by thermal annealing determined. During the post-irradiation anneal, gallium-doped cells prepared by both processes recovered more rapidly and exhibited none of the severe reverse annealing observed for similarly processed 2 ohm-cm boron doped cells. Ion-implanted furnace annealed 0.1 ohm-cm boron doped cells exhibited the lowest post-irradiation annealing temperatures (200 C) after irradiation to 5 x 10 to the 13th e(-)/sq cm. The drastically lowered recovery temperature is attributed to the reduced oxygen and carbon content of the 0.1 ohm-cm cells. Analysis based on defect properties and annealing kinetics indicates that further reduction in annealing temperature should be attainable with further reduction in the silicon's carbon and/or divacancy content after irradiation

Brandhorst, H. W., Jr.

Mehta, S.

Swartz, C. K.

Weinberg, I.

English

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https://ntrs.nasa.gov/search.jsp?R=19820023567 2020-03-21T07:51:51+00:00Z
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A Technical M .morandum 82892
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Effects of Processing and Dopant
on Radiation Damage Removal in
Silicon Solar Cells
r`
(NASA-TM-82892)	 EFFECTS , F FLUCESSINU Atli) 	 N	 144j
LUPANI LN RAUTATIuN LAMAGG REMOVAL IN
SILICCN SULAfi CELLS (NASA)	 15 P
HL A021MF A01	 CSCL NA	 U11CIdS
H2/,J 2F8b3
I. Weinberg, H. W. Brandhorst, Jr., and C. K. Swartz
Lewis Research Center
Cleveland, Ohio
and
S. Mehta
Cleveland State University
Cleveland, Ohio
Prepared for ti.-
Third European Symposium on Photovoltaic generators in Soace
cosponsored by the Royal Aircraft Establishment,
U.K. Department of Industry, and European Space Agency
Bath, United Kingdom, May 4-6, 1982
A-
NASA
^x
"
11 1
0
N
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EFFECTS OF PROCESSING AND DOPANT ON RADIATION
DAMAGE REMOVAL IN SILICON SOLAR CELLS
1. b'einberg, H. W. Brandhorst, Jr., C. K. Swartz, and S. Mehta*
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
Gallium and boron doped silicon solar cells, processed by ion-implanta-
tion followed by either laser or furnace anneal were irradiated' by 1 MeV
electrons and their post-irradiation recovery by thermal annealing
determined. During the post-irradiation anneal, gallium-doped cells prepared
by both processes recovered more rapidly and exhibited none of the severe
reverse annealing observed for similarly processed 2 ohm-cm boron doped
cells. Ion-implanted furnace annealed 0.1 ohwcm-boron doped cells exhibited
the lowft post-irradiation annealing temperatures (200 * C) after irradiation
to 5x10 e-/cm2 . The drastically lowered recovery temperature is
attributed to the reduced oxygen and carbon content of the 0.1 ohwcm cells.
Analysis based on defect properties and annealing kinetics indicates that
further reduction in annealing temperature should be attainable with further
reduction in the silicon's carbon and/or divacancy content after irradiation.
INTRODUCTION
It is well known that degradation due to particulate space radiation is
the most significant factor in reducing solar cell output. One possible
solution to the degradation problem lies in at taining the capability to
periodically anneal and thus restore cell performance in space. However, the
currently-used silicon solar cells undergo reverse annealing and require
temperatures around 400 * C in order to restore cell performance. At this
latter temperature, irreversible damage to array components occurs.
Temperatures around 200 * C, or below, are required to prevent such damage.
With respect to possible increases in radiation resistance, gallium-doped
cells appear promising in contrast with cells whose p-dopant is boron (ref.
1). In the present case, our main interest lies in comparing the
annexlability of such cells. Because annealing of gallium-doped cells with
diffused junctions has been previously investigated (ref. 2), we focus our
attention on cells whose junctions are formed by phosphorus ion-implantation
followed by either furnace or laser annealing to remove implant damage. These
are compared to boron-doped cells prepared by identical processes from the
same 2 ohwcm starting material. Additional results are presented for cells
fabricated from 0.1 ohwcm boron doped silicon.
EXPERIMENTAL PROCEDURES
Irradiations and annealir, j
 were performed on n+p cells with 2 and 0.1
ohm,-cm base resistivities. The 2 ohm-cm silicon cells were processed from
vacuum float zone silicon whose boron doping was achieved by either
*NASA-Cleveland State University Intern.
introducing diborane gas during float zoning or implanting boron ions prior to
the float zone operation (refs. 1 and 3). Gallium doping was achieved by
introducing elemental gallium into the seed end of the rod prior to zone
melting (ref. 1). Additional details concerning the 2 ohm-cm starting silicon
are contained in reference 1 and the final report of the contract under which
the material was prepared (ref. 4). The p-n junctions were formed by
phosphorus ion implantation followed by either laser annealing or a multi-step
furnace anneal (ref. 5). The 0.1 ohm-cm cells were processed from Wacker,
Waso-S float zone, boron-doped silicon with the ion implantation step being
followed by a furnace anneal to remove implant damage. Infrared absorption
measurements deteine that the gallium-doped 2
	 ci silicon had a carbon
content of 2.5x10 /cm and an oxygen content 10 /cm while the bo^Qn
doped 0.1 ohm-cm silicon had oxygen and carbon concentrations 5x10 55/cO.
P hgsph rus ions were implanted at energies of 10 KeV to a fluence of
2.5x101 /cW to form the n+ cell regions. Laser annealing was performed
at a wavelength of 0.532 micrometer using a Nd:YAG laser. The furnace anneal
was a multistep process, carried out in nitrogen, in which the cells were
heated at 550 * C for 2 hours, then at 850 0 C for 15 minutes and finally at
550 C for 2 hours.
After processing and contact formation, the nco ted cells were
irradiated by 1 NeV electrons to a fluence of 101
 /cm7. All cells were
isochronally annealed in 50 *
 C steps with time at temperature being 20
minutes. Isothermal anneals were performed at temperatures where the
isochronal anneal indicated the possibility of significant low temperature
annealing.
EXPERIMENTAL RESULTS
Isochronal annealing data for the 2 ohm-cm cells are shown in figures 1
and 2 while the same data for the 0.1 oh cm cel 11gar shown in figure 3.
Data are shown for the fluences of 5x10 1 and 10 1 /cu. In each case,
fluence dependencies are observed. With respect to degradation, the laser
annealed gallium-doped cells showed the least degradation at both fluences.
However, the effect was more pronounced at the lower fluence. For the furnace
annealed cells, both the gallium-doped and diborane-doped cells showed the
least degradation at the lower fluence. The 2 ohm-cm gallium-doped cells
showed no reverse annealing while the remaining 2 ohm-cm cells exhibit the
reverse annealing usually observed with boron-doped cells of this resistivity
(ref. 6). The gallium-doped furnace annealed cells show full recovery on
annealing at 350 C and a slightly higher recovery temperature for the
similarly doped laser annealed cells. With the exception of the 2 ohm-cm,
boron doped furnace annealed cells, the remainin cells show recovery at 400*
C or higher. In contrast the 0.1 ohm-cm cells, ?fig. 3), show no reverse
annealing at both fluences (ref. 6) and a dramatically large reduction in
annealing temperature at the lower fluence.
Thus, both the 2 ohm-cm gallium-doped and the 0.1 ohm-cm boron-doped
cells, irradiated at the lower fluence,.show evidences of some de ref of
damage recovery when isochronally annealed at temperatures below 300 C.
Since our objective is to accomplish annealing temperatures of 200 0 C or
lower, isothermal annealing was carried out at this temperature. After 15
hours, little or no recovery was observed in the gallium-doped furnace
2
annealed 2 ohm,-cm cells at this temperature (fig. 4). A similar result was
obtained with the gallium-doped laser annealled cells. On the other hand, the
boron-doped 0.1 ohm-cm cells, irradiated at the lower fluence, showed
substantial recovery when annealed at 200 * C for 5 hours (fig. 5). This
represents the lowest temperature for which recovery is observed when
isothermally annealing boron-doped silicon solar cells.
DISCUSSION
Reverse Anneali ng
The reverse annealing observed in the 2 ohm-cm cells has been previously
considered and attributed to a defect which was tentatively identified as a
complex of boron, oxygen and a vacancy (ref. 6). For the gallium-doped cells,
a detailed picture of defect complexes is not available; thus we are unable to
relate the absence of reverse annealing to any defect or combination of
defects at this time.
Low Temperature Annealing
Although a fluence dependent reduction in isochronal anneali ng
temperature is noted for the gallium-doped, 2 ohm-cm silicon cells, the
reduction is small when compared to the lowering of recovery temperature
observed at the lower fluence in the isochronal annealing of the 0.1 ohm-cm
boron-doped cells. The superior annealability of these latter cells is
manifested in the recovery observed when isothermally annealed of 200 0 C. In
this connection, it is noted that the carbon and oxygen content of the 2
ohm-cm cells is greater than that observed for the 0.1 ohm-cm cells.
Further insight into the reasons for the reduced annealing temperature
can be gained from the behavior of the radiation induced defects during
thermal annealing. At present, extensive information on such defect behavior
is available for boron-doped but not for gallium-doped silicon. Hence, we
r ,^;trict our consideration of defect behavior to the case of boron-doped
silicon. In particular, our interest lies in identifying the defects which
are removed by annealing at 300° C.
The temperatures at which major defects in boron -doped silicon appear and
disappear during isochronal anneal are summarized in Table I. The defects
were detected by Deep Level Transient Spectroscopy ( DLTS) and/or Electron
Paramagnetic Resonance ( EPR). In the table, the defects are characterized by
energy level and the temperature at which the defects appear and disappear
during the isochronal anneal. Several of the defects are unidentified and
there exists ambiguity with respect to defect identification. However,
pertinent for the present case is the fact that only two of the possible
defects shown anneal at 300 0 C. These are the carbon-carbon pair at
Ev+0.38eV and the divacancy at Ev+0.23 eV. hence, it is concluded that
isochronal annealing (figure 3) removes either the divacancy and/or the
carbon-carbon pair at 300 C. In order to investigate defect removal below
300 0 C, we have performed isothermal annealing at several points below this
temperature (fig. 6). Using this data we find (Appendix A) that the
activation energy for annealing is 1.5 t 0.2 eV. This agrees within
experimental error, with the activation energy for annealing of both the
divacancy and carbon-carbon pair (Appendix A). This supports the conclusion
obtained from the data of Table I. It also suggests that reduction in
annealing temperature below 200* C could be obtained by further reduction of
the carbon and/or divacancy concentrations in silicon.
Effects of Processing
From the data presented here, for the 2 ohm-cm cells, whether furnace or
laser annealed, the major difference in behavior between the gallium-doped and
boron-(or diborane) doped cells is a small reduction in annealing temperature
and the absence of reverse annealing. At the lower fluence, processing by
furnace annealing appears to result in lower annealing temperature for all 2
ohm-cm cells.
The 0.1 ohm-cm cells show a rather dramatic drop in isochronal annealing
temperature to 300 * C at the lower fluence. Furthermore, isothermal annealing
occurs at 200* C, a phenomenon which does not occur in the 2 ohm-cm cells. We
note that all cells for which data are shown have had p-n junctions formed by
ion-implantation. Thus, the major difference between the 0.1 ohm-cm and 2
ohwcm cells is the lowered oxygen and carbon content exhibited by the lower
resistivity cells.
CONCLUSION
From the results of the present study, for the 2 ohm-cm cells produced by
ion-implantation and either furnace or laser annealing, it is concluded that:
Gallium-doping results in lower annealing temperatures and the absence of
reverse annealing.
At low fluence (5x10 13/cm2 ), processing by furnace annealing yields a
slightly lower isochronal annealing temperature.
The 2 ohm-cm cells show no recovery when isothermally annealed at 200 * C
On the other hand, for the 0.1 ohm-cm cells, with reduced oxygen and
carbon content, processed by ion-implantation and furnace annealing:
A 200 9 C reduction in isochronal annealing temperature is observed at the
lower fluence.
Substantial recovery is achieved on isothermal annealing at 200 * C.
The defects removed by annealing below 300 * i,	 a the divacancy and/or
the carbon-carbon pair.
For both the 0.1 and 2 ohm-cm cells processed by ion-implantation, any
process steps which result in reduced carbon and oxygen content in the
starting silicon should lead to substantial damage removal by annealing below
300 * C.
4
APPENDIX A - Activation Energies
The activation energy for annealing is obtained from the relation (ref. 13)
K - Ko exp (-EA/kbT)
	
(1)
where K is the reaction rate constant, EA is the activation energy, T is the
temperature and KO
 is a constant which is frequently taken to be equal to
the maximum lattice vibrational frequency. If the annealing kinetics are
second order, then;
N ^N t	 (2)0	 0
where N is the defect concentration at time t and No the defect
concentration at the beginning of the reaction. In lieu of defect
concentrations we use diffusion lengths, a quantity which we routinely
measure. Assuming the presence of one dominant defect, it follows that (ref.
14)
1	 1	
BN	 (3)
- 77 '
0
where Lo is the pre-irradiation diffusion length, L is the diffusion length
at time t during the anneal and B is a constant at constant temperature.
Hence from equations 2 and 3
1	 1 _1
	
C 
+ kt	 (4)
-^
0
where C is a constant
Plots of equation (4) for # = 5x10 13/cm2 are shown in figure 1 for T =
200, 250, 215, and 300 * C. A similar straight line is obtained for 200 * C.
Because an attempt to fit the data to first order kinetics was unsuccessful,
it follows that the annealing is reasonably described by second order kinetics.
Equation 1 can be rewritten as
11 = exp ^
K	 E
b
	 (5)
A plot of equation 5 using the present data is shown in figure 8 from which,
by a least squares fit, we obtain
EA = 1.5 t 0.2 eV
The activation energy for divacancy annealing is found to be 1.2 eV from
infrared studies (ref. 15) while that obtained from EPR is 1.3 eV (ref. 11).
5
For the carbon-carbon pair the activation energy for defect re-orientation is
found to be 1.2 eV from EPR studies (ref. 12). No activation energy for
annealing is available for the carbon-carbon pair. However, the activation
energy for atomic re-orientation has, in the case of divacancy, been found
equal to that for annealing (ref. 15). If we assume this to be the case for
the carbon-carbon pair, then the activation energy obtained from the current
annealing kinetic studies is consistenj with the presence of either the
divacancy or the carbon pair below 300 C. The annealing process below 300 0 C
is thus one in which either or both of these defects is annihilated with
temperatures above 200 C being necessary to remove this defect in the present
low resistivity silicon.
6
REFERENCES
1. Fodor, J.; and Opjorden, R.: Advanced Silicon Material for Space Solar
Cells, 14th IEEE Photovoltaic Specialists Conference, San Diego 7-10 Jan.
1980, 882-886.
2. Scott-Monck, J. A.; and Anspaugh, B. E.: Effect of Dopants on Annealing
Performance of Silicon Solar Cells, Conference on Solar Cell High
Efficiency and Radiation Damage, NASA Conference Public iallon 2097, 1979,
.
3. The 2 ohm-cm wafers were supplied by Dr. Patrick Rahilly of the Aero
Propulsion Lab., Wright Patterson Air Force Base, Ohio, U.S.A.
4. AFWAL-TR-81-2013 (Wright-Patterson AFB) 1981, Low Resistivity—High
Lifetime Single Crystal Silicon Investigation, by Fodor, .; and Opjorden,
5. The ion-implantation and subsequent anneal, to remove implant damage,
were performed by the Spire Corporation, Bedford, Massachusetts, U.S.A.
6. Weinberg, I.; and Swartz, C. K.: Origin of Reverse Annealing in
Radiation-Damaged Silicon Solar Cells, Appl. Phys. Lett. 36, 1980,
693-695.
7. Mooney, P. M.; et al.: Defect Energy Levels in Boron-Doped Silicon
Irradiated with 1-MeV Electrons, Phys. Rev. B: Solid State 15(8), 1977,
3836-3843.
8. Lee, Y. H.; et al.: EPR of a Carbon-Oxygen-Divacancy Complex in
Irradiated Silicon, Phys. Status Solidi, A 41(2), 1973, 637-647.
9. Brower, K. L.: EPR of a Jahn-Teller Distorted 111 Carbon Interstitial in
Irradiated Silion, Phys. Rev. B: Solid State 9, 1973, 2607-2617.
10. Kimerling, L. C.: Defect States in Electron Bombarded Silicon:
Capacitance Transient Analyses, Radiation Effects in Semiconductors,
Dubrovnik 6-9 Sept. 1976, Conference Series - InstiFuXe—ot Physics, no.
31, 1973, 221-230.
11. Watkins, G. D.; and Corbett, J. W.: Defects in Irradiated Silicon:
Electron Paramagnetic Resonance of the Divacancy, Phys. Rev. 138(2),
1965 1 543-555.
12. Brower, K. L.: EPR of a 001 Si Interstitial Complex in Irradiated
Silicon, Phys. Rev. B: Solid State, 14(3), 1976, 872-883.
13. Corbett, J. W.: Electron Radiation Damage in Semiconductors and Metals,
New York, Academic Press, 1966, 36-43.
7
14. Shockley, W.; and Read, W. T., Jr.: Statistics of the Recombinations of
Holes and Electrons, Phys. Rev. 87(5), 1952, 835-841.
15. Chen9, L. J.; et al.: 1.8-, 3.3-, and 3.9-Micron Bands in Irradiated
Silicon: Correlations with the Divacancy, Phys_. Rev. 152(2), 1966,
761-774.
8
TABLE I - ANNEALIN6 TEMPERATURES OF MAJOR
DEFECTS IN BORON-DOPED SILICON
Energy Detect Annealing Detection
level. tenrature. wethod
eV C
In	 Out
Ev + 0.38 V-0-C 00 400 EPRg. OLTS6
or
C I -CS <30 300 EPR 10 . DLTS11
v	 0.23 Divacancy <30 300 EPR 12 . OLTS6.1
Ec - 0.27 81-0I <30 200 DLTS6
or
BIBS <30 180 DLTS11
Ev + 0.30 8-0-V 100 400+ DLTS6
or
SI-SI 200 500 EPR13 , DLTS11
Ev + 0.26 --	 - 270 400 DLTS6
Ev + 0.43 ---	 - <30 200 DLTS6
ORIGINAL PAGE i$
OF POOR QUALITY
9
INAL PAGEoRi
pow QUALITYof
iI
O • a DOPE
O • WION DOPED
ELECTRON D • OlIORAIE DoPEa
log
3^ lo"►c^
^f
v 90 f
f	 i
IO
1.lxlo131t	 2
fa
^	 f
70 ff
i
0 100	 200	 !00	 100	 gO0
TEMPERATURE, 4	 j
Figure 1. - Isothronal anneal d 2 ohm-cm Ian-laiplanbd.
laser annWed calls.
O • OI DOPED
O n BORON DOPED
1-MW A • DIIORANE DOPED
ELECTRON
100
oc
U	
90
V
so
N
f]
70
An
0	 100	 200	 300
	
300
TEMPERATURE. oC
Fiqure 7. - Isoc hronal annul d 2 ohmtm IoMmplaMsd,
furnace annealed cells.
ulieN
D
N
2
,i
100
90
u
v^
00
TO
60
-V0
la1011licm2
I-Mev
ELECTRON
FLUENCE
1x101 M2
600
TEMPERATURE, aC
Figure 9. - Isochronal anneal of 0.1 ohm-cm Ion-
Implanted, furnace annealed ails.
2-MW
ELECTRON
FLUENCE
5c1ti1 CM2
T • 2000C
	
80L	 i	 I
	
0	 510
ANNEALING TIME, hr
Figure L - Isothermal anneal of 2 ohm-cm Go doped,
Ion-Implanted, furnace annealed all.
100
90
M .	 1W
T - NIP C
FIUENCE: 3N10131an2
1 MW ELECTRONS
l5
!0
	
0	 10	 20	 30	 40	 SO	 40
ANNEALMIG TIME, hr
Figure i - Isothermal annealing at NIP C d 111
0-cm bn-implenbd cell.
	
100 r	 M& C	 2750 C
Et	 ZSIP C
d
	
90	 FLUENCE: Sid0131cm2
I MIV ELECTRONS
as ^^^
	0 	 1	 2	 3	 4	 5
ANNEALING TINE, hr
Figure 4. - Isothermal) annul ing d ion-implenUd
410- CM  cells.


Effects of processing and dopant on radiation damage removal in silicon solar cells

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