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163 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society

For the system a-Si:H/c-Si the simplified Eq.(l) is valid: AS(t) =Ana(t)tnae+Anc(t)gtnce+Apc(t)tpce

(1)

where Ana(t) refers to the total number of excess electrons characterized by the mobility 4tna in a-Si:H and Anc(t) (Apc(t)) refers to the total number of excess electrons (holes) characterized by the mobility gtnc(jipc) in c-Si. Pulsed illumination (FWHM 12 ns) was carried out by a Nd:YAG laser at 1064 nm and at 532 nm on the a-Si:H (front) side of the samples. a-Si:H was deposited by Plasma Enhanced Chemical Vapor Deposition of silane on an n-doped Si wafer (p=50 Q2cm) of thickness 1=525 gim under standard conditions during 165 min. (deposition rate about 0.11 nms- 1), where only the deposition temperature was varied. Sample #681 was produced by deposition at 40 'C, sample #682 at 250 1C. Sample #696 is produced by 45min deposition at 120 'C, heating to 250 'C with a 30 min annealing of the film at 250 'C and subsequently 120 min deposition at 250 'C. Sample #699 was produced by 45min deposition at 40 "C, heating to 250 'C, a 30 min annealing at 250 °C and a 120 min deposition at 250 'C. For comparison sample #672 was produced by 165 min deposition at 250 'C on Coming glass. The back surface of the samples is characterized by a surface recombination velocity S>2x104 cms- 1, as deduced from TRMC measurements induced by 532nm light at the back side. RESULTS AND DISCUSSION In all four samples #681, #682, #696 and #699 the TRMC signals induced by 1064 nm light pulses decay exponentially over at least one order of magnitude in the signal amplitude, where the decay time does not depend on the excitation density between 2 nJcm- 2 and 1 jiJcm- 2 . In Fig. 1 a representative example for every sample is shown for 1=15 nJcm' 2 .This behaviour implies that after excitation in the c-Si part of the junction (as it is the case for 1064 nm excitation) the heterojunction reacts as a virtual surface characterized by a constant surface recombination velocity. No effects of charge carrier storage in the space charge region corresponding to a photovoltage are observed in contrast to the behaviour of the c-Si/SiO2 system. It must be concluded that electron-hole pairs arriving at the junction are not separated by the space charge field. In view of these considerations the TRMC-decay time of the four samples can only vary between (12/(Dpn 2) and (412 /(Dp7C2 ))[31, i.e. from 21ýts-84g.ts for the present wafer. The low limit, diffusion controlled surface recombination for both surfaces, is observed for sample #681 (decay time 21gts) and it can be concluded that the bad quality of the a-Si:H film

164

(because of low temperature deposition) leads to a very defective surface, s>2xl0 4 cms-l.The same behaviour is shown by a non-annealed 120 "C film on the same substrate.

S0.1 Cu C

S0.01 i-.

0.001 .

0

100

200

300

-6

400x10

time [s] Fig. 1: TRMC transients induced by 1064 nm excitation (15 nJ cm- 2 ) in the samples #681, #682, #696 and #699. The high li