The Electron Mobility in Amorphous Silicon Under Double Injection
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THE ELECTRON MOBILITY IN AMORPHOUS SILICON UNDER DOUBLE INJECTION D. GOLDIE, P.G. LeCOMBER AND W.E. SPEAR Carnegie Laboratory of Physics, University of Dundee,
Dundee,
Scotland,
UK
ABSTRACT The transport of a pulse of photogenerated excess electrons has been investigated in a-Si p+-i-n+ junctions under forward bias in the presence of strong double injection. It is found that the electron drift mobility 2 is completely independent of forward current densities of up to 0.33 Acm , in disagreement with the work of Silver et al. We suggest that the experimental disagreement arises from problems inherent in the voltage step technique of mobility measurement.
1.
INTRODUCTION
It is now generally agreed that the extended state electron mobility 2 1 1 )Lc in amorphous silicon lies in the range from 10 to 20 cm V- s(1,2,3). These values are deduced from transient drift mobility measurements on the basis of the multi-trapping model which relates the observed drift mobility Ae to jLc" In the experiments one observes the transit of a pulse of excess electrons generated by a 3ns laser flash close to the top electrode of a reverse biased a-Si p -i-n+ junction or Schottky barrier. As the generated holes are extracted at the top electrode this experiment involves essentially single carrier injection. In several recent papers (4,5,6,7) Silver and his collaborators claimed that under high-level double injection the value of )1 in a-Si becomes appreciably larger than the values quoted above. Their conclusions are based on the results of double injection experiments in forward biased a-Si junctions. )Le was deduced from the transient response to a small voltage step superimposed on the forward current. We shall return to this experimental approach in section 3. In explanation Silver et al suggest that the extended state mobility •c in a-Si is normally limited by long-range potential fluctuations arising from pairs of positively and negatively charged defects. This is expressed in the form
where )oo is the extended state mobility in the absence of potential fluctuations. A represents the r.m.s. amplitude of the fluctuations which according to the work of Overhof and Beyer (8) is approximately given by
/X
.1.v
(2)M-7
N± denotes the density of charged defect pairs within the sample. Silver et al then proposed that under high level double injection excess electrons and holes are captured by these defect centres, thereby decreasing the fluctuations and the parameter A in eqn.(l). )tc increases and so does 1 the observed drift mobility Le which is proportional to Mc" It has also been suggested that the defects may be the T3+-T3- dangling bond pairs with negative correlation energy, originally considered by Adler (9). Mat. ReA. Soc,Symp. Proc. Vol. 11. - 1M Materials Research Society
490
The purpose of the present work has been to provide independent Our results on electron transport in a-Si under strong double injection. experimental approach differs from that of the earlier work by the use of transient photogeneration in the d
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