High Field Electron Drift in a-Si:H
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HIGH FIELD ELECTRON DRIFT IN a-Si:H QING GU and ERIC A. SCHIFF Department of Physics, Syracuse University, Syracuse, NY 13244-1130, USA JEAN BAPTISTE CHEVRIER and BERNARD EQUER Ecole Polytechnique, Laboratoire de Physique des Interfaces et des Couches Minces, Palaiseau, FRANCE ABSTRACT We have measured the electron drift mobility in a-Si:H at high electric fields (E < 3.6 x 105 V/cm). The a-Si:H pin structure was prepared at Palaiseau, and incorporated a thick p+ layer to retard high field breakdown. The drift mobility was obtained from transient photocurrent measurements from 1 ns - 1 ms following a laser pulse. Mobility increases as large as a factor of 30 were observed; at 77 K the high field mobility depended exponentially upon field (exp(E/Eo), where Eo = 1.1 x i0o V/cm). The same field dependence was observed in the time range 10 ns - 1 /zs, indicating that the dispersion parameter change with field was negligible. This latter result appears to exclude hopping in the exponential conduction bandtail as the fundamental transport mechanism in a-Si:H above 77 K; alternate models are briefly discussed.
INTRODUCTION Nonlinear electron transport in a-Si:H has been observed by several groups [1-4]. Fairly small (twofold) increases were reported in initial work near room temperature at E - 3 x 10' V/cm [1,2]. Very much larger increases are seen at lower temperatures, where electron transport is dispersive due to bandtail trapping effects. The dispersion parameter a obtained at low fields depends upon temperature as a = kT/4o, where E6is the bandtail width. An important issue which emerged in these measurements is whether electric fields modify the dispersion parameter, as might be expected if electric fields modify the "effective temperature" of the photocarrier distribution [5,6]. Antoniadis and Schiff [3] reported no change of dispersion parameter at 90 K for fields up to 1.5 x 10i V/cm. Nebel, et al reported a substantial increase in the dispersion parameter in measurements at 40 K up to 4.5 x 105 V/cm [4]. A similar effect was also reported by Nebel and Street for holes [7]; again, this effect was not observed by Antoniadis and Schiff in their hole measurements [3]. This paper will further address the issue of field-dependent dispersion in the electron drift mobility of a-Si:H. The significance of field-dependent dispersion is that it may discriminate between several models with identical predictions for low-field transport. In particular, mobility-edge, hopping, and potential fluctuation approaches have all been successfully used to interpret dispersive transport in a-Si:H at low fields, where transport is linear. For carrier hopping between localized bandtail states, nonlinear transport sets in via a field-dependent dispersion parameter [4-8]. Tunneling to a mobility-edge can produce substantial nonlinearity without field-dependent dispersion [9]. The high-field behavior of the potential fluctation model is unknown. Although the low field mobilities agree fairly well between laboratories, the high field effects
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