Investigation of Time of Flight Photocurrents in a-Si:H
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INVESTIGATION OF TIME OF FLIGHT PHOTOCURRENTS IN a-Si:H R.P.BARCLAY, G.SEYNHAEVE*, G.J.ADRIAENSSENS* AND J.M.MARSHALL Department of Materials Engineering, University College, Swansea SA2 8PP, U.K. *Laboratorium voor Vaste Stof en Hoge-Drukfysika, K.U. Leuven, Belgium ABSTRACT Transient photoconductivity can provide extensive information concerning the characteristics and energy distribution (g(E) ) of trapping centres in disordered solids. In this paper, we use both pre-transit (extraction free) and post-transit current decays, from the TOF experiment, to probe g(E) in a-Si:H of intentionally reduced quality.
INTRODUCTION It is now generally accepted that the transport of excess charge carriers in many amorphous semiconductors involves a multiple-trapping mechanism. When the localised states with which carriers interact have a broad range of release time constants, 'anomalouslydispersive' characteristics arise[I]. Studies of the transport of excess carriers have been used to explore the energy distribution (g(E)) of trapping centres, to profile the depletion layer electric field, and to estimate deep trapping lifetimes. In the 'time-of-flight' (TOF) experiment, photocarriers are generated close to one electrode. Their drift induces a current which initially reflects interactions with trapping centres. Subsequently, a more rapid decay of current occurs as the carriers reach the extraction electrode in significant numbers. Such data can yield information about localised states in various ways. For example, where band bending, deep trapping and diffusion effects do not distort the underlying response, the detailed shape of the transit pulse provides a means for probing g(E) [2]. The currents in the pre-transit and extraction regimes may be written as I1 (t) - t -(1-(al)
(pre-transit regime)
t -( +a2 )
(extraction regime)
12(t)
-
(1)
where al and a2 are force-fitted parameters describing the approximately power-law characteristics. Marshall, Street and Thomson [3] have examined the temperature dependences of al and (x2, plus the temperature/field dependence of the transit time, for a-Si:H of high quality. They infer a g(E) which is approximately linear from 0.08 eV to 0.15 eV below the conduction band. Another technique[41 utilises the fact that under appropriateanomalouslydispersive circumstances , the pre-transit current can be related to g(E) via the expression g(E)
-
(II(t).t)-I
Mat. Res. Soc. Symp. Proc. Vol. 118. c 1988 Materials Research Society
(2)
520
where E = kTln(iot). Although this method has been used successfully for a-As 2 Se3 , it has not yet been applied to electron transport in good a-Si:H. This is not surprising, since it is difficult to obtain a pre-transit current over an appreciable time range and great care must be taken to avoid distortions of the transit pulse shape. Also, except at low temperatures, 11(t) is not appropriately dispersive within the accessible time range, so that the procedure is invalid. A further possibility concerns the post-transit regime. Here, the current i
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