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EFFECT OF HEATING SAMPLES DURING PULSED
ELECTRON BEAM ANNEALING ON THE
OPEN-CIRCUIT VOLTAGE OF SILICON SOLAR
CELLS
A. Laugier, Damien Barbier, M. Doghmane, G. Chemisky
To cite this version:
EFFECT OF HEATING SAMPLES DURING PULSED ELECTRON BEAM ANNEALING ON THE OPEN-CIRCUIT VOLTAGE OF S I L I C O N SOLAR CELLS
A . Laugier, D. B a r b i e r , M.S. Doghmane and G. Chemisky
Laboratoire de Physique de La Matisre (LA 35581, I n s t i t u t National des Sciences AppZique'es de Lyon, 20, avenue AZbert Einstein, 69622 ViLZeurbanne Ceder, France
On a recherche les causes d e s faibles tensions e n circuit ouvert associees a u recuit pulse ilectronique dans l a reallsation des photopiles s i l ~ c i u m implantees phosphore. Les defauts induits ont
6te
analyses sur l a base d'observations e n microscopic electronique, e n D.L.T.S. e t d ' i t u d e de dbgradation d e diodes Schottky. On definit une methode p e r m e t t a n t d'ameliorer mnotablement les valeurs d e Voc e n associant des d e c t r o n s d e faible energie cinetique moyenne-
(10 keV), des fluences faibles
(5
1 ~ / c m ' ) sur cibles portees&
450°C. Des valeurs d e Voc comparablesi
celles obtenues par recuit thermique conventionnel sont observkes.ABSTRACT
D e f e c t s associated t o pulsed electron beam annealing of P implanted Si solar cells lead t o poor Voc ( < 500 mV). Their n a t u r e is discussed on t h e basic of S.E.M. observations, D.L.T.S. study and degradation of electrical characteristics of Schottky barriers. An improved PEBA process is determined, characterized by a low mean energy electron beam (10 keV)
2
associated t o a starting temperature of 450°C and low fluences
(5
I J / c m ). Values of Voc similar t o conventional t h e r m a l annealing a r e obtained.I. INTRODUCTION
Solar cells made by Pulsed Electron Beam Annealing (PEBA) of phosphorous implanted silicon a r e characterized by a very good short-circuit c u r r e n t and a rather poor open-circuit voltage V /1,2/. As for laser annealing a significative improvement (typically
OC
40 mV) is obtained with a post thermal t r e a t m e n t at T" 400°C.
In this work e f f e c t s of heating t h e samples during t h e pulse a r e investigated. In addition a n optimization of t h e PEBA process is determined.
II.
PEBA SOLAR CELLSThe starting material was phosphorous implanted (100) C.Z. boron doped silicon 16 3
(NA= 10 cm- ). In order t o give some basis for t h e PEBA t r e a t m e n t optimization, t h e results of t w o different solar cells processes a r e compared in this section.
n O I : p+ annealed with a 10 keV mean energy electron beam pulse /1,3/
n02 : p+ annealed with a higher mean electron energy (15 keV), leading t o a more penetrating dose profile /2/.
JOURNAL DE PHYSIQUE
Details about t h e PEBA process a r e also given in ref.121. In both cases, t h e pulse
-
duration was about 5 0 ns and fluences w e r e in t h e range 0.8-1.2 3/cmL.A computer simulation of PEBA induced thermal e f f e c t s have been performed /4/ in order t o investigate t h e particular f e a t u r e s of t h e melting and freezing kinetics a s a function of t h e substrate starting temperature.
For process n O 1 a t 20°C crystalline silicon is fully molten over 0.35 micron with only
-
1 3/cmL. For process n02 with 1.2 3/cmL t h e enthalpy required for complete melting of crystalline silicon is not reached and a n inhomogeneous physical state is obtained in t h e melting layer.
The amorphous f r o n t layer (0.2 micron) is fully molten while a "melt like" state is 2
achieved in t h e underlying crystal up t o 0.65 micron. So annealing a t 1 J / c m i s more e f f e c t i v e with process n 0 l than with process n02.
Moreover, t h e e f f e c t of heating silicon before PEBA consists essentially in a n increase of t h e induced melting layer thickness f o r a given fluence and in a smaller thermal gradient in t h e solid leading t o a longer freezing kinetic. With a 450°C starting temperature t h e
-
s a m e melting depths-
as at 20°C a r e obtained with only 0.85 3/crnZ for t h e process n 0 l and 1 3/cmL for t h e process n 0 2 respectively.2
Excellent annealing i s obtained with process n O l a t only 0.8 J / c m and with process 2
n"2 at 1.2 J / C T ~ T h 3 junction depth i s less t h a n 200 nm with a
'
N
layer dopant concentration higher than 10 /cm.
Solar cells w e r e realized with PEBA wafers according t h e following process.Ti-Pd-Ag grid contacts w e r e then evaporated and a 8 0 nm thick T i 0 2 A.R. coating was deposited a t 325OC. The transparency of t h e grid was 94.5 %. An aluminium layer was deposited on t h e back side of t h e wafers and annealed during 20 mn a t 450°C. Assuming a uniform junction model t h e expected short circuit c u r r e n t
-
Isc and Voc for our starting material a r e respectively 3 3 m ~ / c m ' and 580 mV (AM1). The best experimental value obtained f o r-
Isc- -
was 32.4 m ~ l c m ' . This value agrees with t h e theorical o n e and with t h e spectral response shown on Fig.1 which i s consistent with a base minority carrier diffusion length of 100 microns. So t h e overall process does not a f f e c t t h e starting material properties. Values of Voc a r e plotted on table I and compared t o values obtained with process n O l .
Table I
-
Open circuit voltage of P implanted Si versus PEBA process AnnealingProcess no
Voc (mv)
As other pulsed annealing technics PEBA induces d e f e c t s in silicon. In a recent paper a thorough study of extended d e f e c t w e r e performed by mean of electron microscopy 151. Comparison made between conventional t h e r m a l annealing and PEBA have shown t h a t t h e l a t t e r removes d e f e c t s associated with thermal annealing. However observations has revealed t h e existence of dislocations and subgrain boundaries with a fluence increasing density. Polygonization seems t o indicate t h a t climb a s well a s glide a r e a c t i v e in t h e dislocation mobility. Fig.2 shows t h e S.E.M. micrograph of a bevelled sample pulsed with 1.2 ~ / c r n ~ a t 20°C with t h e 15 keV mean energy electron beam pulse. One remarks f i r s t t h e a r r a y of dislocation emergence a s previously described 151. Secondly, t h e e x t e n t of t h e dislocated zone nearly
SPECTRAL RESPONSE bA/W)
300 500 700 900 1 1 0 0
WAVELENGTH (nm)
Fig. 1
-
Spectral response of PEBA Si solar cell.coincide with t h e melting zone thickness a s calculated in sec.2. A t 450°C no extended defects
-
appear on samples t r e a t e d up t o 0.9 ~/cm'. This c a n be related t o t h e smaller thermal gradients and longer freezing kinetics in t h a t case.In t h e other hand quenched-in point defects a r e associated t o pulsed annealing /6,7/. In a previous work /7/ d e e p level transient spectroscopy (DLTS) performed with Schottky diodes m a d e on PEBA processed P-type virgin silicon revealed typical d e f e c t s associated t o PEBA depending on t h e irradiation characteristics.
In this work, w e have performed DLTS study on d / n junction made a s t h e above discussed solar cells (n02 process). A typical spectrum is shown on F i g 3 . Two discrete hole t r a p levels appear C (=0.22 eV) and C (- 0.48 eV) for which t h e biasing conditions correspond t o a n
1 3
-
analyzed s u r f a c e layer thickness of about 0.6 micron. The c a p t u r e cross sections a r e respectively 3 . 1 0 - ' ~ c m - ~ and 1 0 - ' ~ c m - ~ . The broad s t r u c t u r e C2 partially superimposed t o t h e peak C 3 is likely t o be due t o a band of levels. The hole t r a p levels observed cannot be assigned t o already known d e f e c t in Si.
JOURNAL DE PHYSIQUE
2 .
on virgin silicon a f t e r irradiation a t 1.2 J / c m w ~ t h a 1 5 keV mean energy electron beam pulse. T h e results a r e shown o n t a b l e 2. O n e c a n observe a strong degradation of t h e diodes a f t e r PEBA at 20°C. A post t h e r m a l t r e a t m e n t is not sufficient t o completely recover t h e r e f e r e n c e diode characteristics. An improvement is observed f o r t h e 450°C PEBA processed samples. However d e f e c t s a r e always present a s evidenced by t h e large value of t h e saturation current.
SURFACE
- - -
. . -. i - .!
c,.
.. .
1
fl.65
pm
t
'; ..,
. . " ._ _ - - -
. .5
BULK
8.. .Fig. 2 - Scanning electron microscopy on bevelled sample of PEBA silicon. Dislocation e m e r g e n c e in whole t h e melting zone.
Table 2
-
,.
Evolution of Schottky barrier characteristics made on PEBA processed Si s u r f a c e PEBA PEB A +450°C 30mn PEBA R e f e r e n c e (1.2 J/cmL) n : ideality f a c t o r ; I : saturation c u r r e n t ; C : c a p a c i t a n c e a t - I V a t 1 MHz ; VBn : barrier height. s IV. CONCLUSION 20°C 20°C 450°CPate1 et a1 /8/ have shown t h a t dislocations could be annealed only a t t e m p e r a t u r e s higher than 700°C. So t h e residual Schottky diode degradation and f o r t h e solar cells t h e value of Voc a f t e r PEBA a t 450°C could be related t o dislocations. The poor value of Voc a f t e r PEBA at 20°C could be related t o a high concentration of quenched-in point defects.
2.90
1.96
1.78
1.16
The lower quenching r a t e induced by PEBA on silicon h e a t e d up t o 450°C is likely t o r e d u c e t h e point d e f e c t generation mechanism and t o improve t h e value of V
oc'
An improved PEBA t r e a t m e n t of phosphorous implanted Si solar cells is determined by using a low mean energy electron beam (10 keV) associated t o a sample t e m p e r a t u r e of 450°C. Then a fluence3.35
1.03
4.51
6.60 l o m 7
2
below 1 J / c m is convenient a n d values of Voc similar t o conventional t h e r m a l annealing a r e obtained. ACKNOWLEDGEMENT 165.2 124.2 110.4 89.7
This work was supported by A.F.M.E. and by C.N.R.S. (PIRSEM). 0.570
0.600
0.621
0.791
BIBLIOGRAPHY
/ I /
GREENWALD
A.c., KIRKPATRICK A.R., LITTLE R.G., MINNUCCI J.A., J . A ~ ~ I . P ~ Y S . 5 0 , 2 (1979) 783/2/
LAUGIER
A., BARBIER D., CHEMISKY G., Proc. 4th E.C. Photovoltaic Solar Energy Conf. Reidel edit. (1982) 1007/3/ GREENWALD A.C. P r i v a t e Communication
/4/ CHEMISKY G., BARBIER D., LAUGIER A., this conference
5 THOLOMIER M., PITAVAL M., AMBRI M., BARBIER D., LAUGIER A., J.Appl.Phys.
3
(1983) 1588161 CHANTRE A., KECHOUANE M., BOIS D., Laser and Electron Beam Processing of Materials, M.R.S. Proc.Acad.Press (L981) 385
/7/ BARBIER D., KECHOUANE M., CHANTRE A., LAUGIER A., Laser-Solid Interactions and Transient Thermal Processing of Materials. M.R.S. Proc. Elsevier (1982) t o b e published
