Recombination of Excess Carriers at High Excitation Density in Amorphous Silicon
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RECOMBINATION OF EXCESS CARRIERS AT HIGH EXCITATION DENSITY IN AMORPHOUS SILICON M.KUNST ,C.HAFFER AND C.SWIATKOWSKI Hahn-Meitner-lnstitut, Si Solare Energetik, Glienicker Str. 100, 1000 Berlin 39, Germany ABSTRACT Excess electron-hole recombination at high excitation densities in a-Si:H is investigated by comparison of the transient photoconductivity during and just after excitation in the nanosecond time range to numerical models. A simple one parameter recombination model fits the experimental data satisfactorily. An extension of the model where deep trapping is taken into account explains also the transient photoconductivity in Boron doped a-Si:H. INTRODUCTION Experimental results from different techniques have shown that the excess carrier decay rate in a-Si:H at high excess carrier densities increases strongly with the excess carrier density. This is not only evident from recent studies of the decay behaviour of signals obtained with (sub)picosecond pump-and-probe measurements (1,2) but also from the sublinear behaviour of the signal amplitude on the excitation density observed with time resolved Photoinduced Absorption (PA) and Photoconductivity measurements (3,4) with nanosecond time resolution. A wealth of experimental evidence indicates that this decay channel at high excitation density is due to electron-hole recombination
(1-4). Because this recombination is active for excess carrier densities larger than about 101 8 cm- 3 , it does certainly not play an important role in the performance of a-Si:H devices. However, investigation of this process appears to be of considerable importance for the theory of charge carrier transport in a-Si:H (1,2). Besides, it is shown that a-Si:H films can be characterized by the transient photoconductivity measured under conditions where this recombination is active (5).This makes an analysis of this process interesting for study of the theoretical background of this characterization. EXPERIMENTAL Films of a-Si:H were produced by glow discharge from silane under standard conditions yielding films of state-of-art quality (6).Time Resolved Microwave Conductivity (TRMC) measurements were Mat. Res. Soc. Symp. Proc. Vol. 297. 01993 Materials Research Society
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performed in a Ka Band (28.5-40GHz) equipment. TRMC signals were induced by IOns (FWHM) pulses at 532 nm from a Nd:YAG laser. The time resolution of the measurements is determined by the duration of the excitation pulse. The TRMC signal at time t (AP(t)/P) is proportional to the product of the number of mobile excess charge carriers at t and their respective mobility. For a-Si:H films only the contribution of excess electrons above the conduction band edge characterized by a mobility Ign and that of excess holes under the valence band mobility edge characterized by a mobility lp will be taken into account. This yields: AP(t)/P=A (An (t~btn +Ap(t)ýL~p)
( 1)
where A is a proportionality constant and An(t) and Ap(t) are the number of excess electrons, holes respectively, at time t after the start of the excitation. F
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