Electronic Transport in Hydrogenated Amorphous Silicon
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Electronic Transport in Hydrogenated Amorphous Silicon 5
N. SILVER , D. ADLER", M. P. SHAWL, V. CANNELLAt', AND J. McGILL" Dept. of Physics and Astronomy, University of North Carolina, Chapel NC 27514 Center for Materials Science
and Engineering,
Massachusetts
echnology, Cambridge, MA 02139 Department of Electrical and Computer Engineering, 1troit, MI 48202 Ovonic Display Systems, Troy, MI 48084
Hill,
Institute of
Wayne State University,
ABSTRACT The puzzles regarding the magnitude of the free electron mobility in hydrogenated amorphous silicon are examined. It is suggested that highlevel double injection produces a metastable increase in the carrier mobility by neutralizing positively and negatively charged defect states thereby eliminating long-range potential fluctuations. Since these defect states cannot be neutralized under low-level or single injection, they both contribute to the modulation of the conduction band and increase the freecarrier scattering. If the latter is the predominant scattering mechanism, the neutralization of charged defects directly leads to a mobility increase under double-injection conditions. We discuss the various implications of this model, and present recent experimental results in agreement with these ideas.
INrIIfUCTION In the previous paper [1], some of the difficulties with the conventional view of the electronic structure of hydrogenated amorphous silicon (a-Si:H) were discussed in detail, and some of the evidence for the existence of defects with a negative effective correlation enerjgy wa1 If such states are indeed present in concentrations ~ 10 1 cm presented. then: (1) their average separation would be of the order of 100 A, and (2) they would be charged either positively (T3+) or negatively (T3 ) in their lowelt energy states. 3 Beyond the random pair distribution function, P(r) 4 nr N exp [-(j n r ) N ±1 which itself indicates that a significant NT e ' -1/3 fraction of pairs with r < (NT) are present, it is clear that longrange potential fluctuations greater than kT/e in magnitude still exist. As pointed out by Overhof [2], the T ot-mean-square amplitude of the random pot ntiaj A, is proportional to NT , and is approximately 0.1 eV for NT - IT lV 10 cm . Clearly, long-range potential fluctuations of order 0.1 eV would give rise to significant spatial modulation of the electronic states and increase scattering, thereby resulting in a relatively low free-carrier mobility. However, if these random potential fluctuations are eliminated by filling the charged defect states, then the extended state mobility can increase. In this paper, we show that such an increase in band mobility due to high-level double injection is in fact a reasonable explanation of the multitude of data on a-Si:H. We also present some new data in support of this view and speculate that the presence of charged defect states is responsible for the lack of geminate recombination in a-Si:H.
THE PUZZLE Under
the assumption that the exponential energy distribution of the
Mat. Res. Soc. Symp. Proc.
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