Spatial and Temporal Variations in Electronic Transport Through a CdTe-Based Schottky Barrier
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Spatial and Temporal Variations in Electronic Transport Through a CdTe-Based Schottky Barrier Diana Shvydka, V. Parikh, V.G. Karpov and A.D. Compaan Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, U.S.A. ABSTRACT We study the electric current through metal-semiconductor junctions of a type used in thin-film PV for back contacts. To concentrate on one type of junction we used the symmetric structures of rf-sputtered CdTe layer sandwiched between two Cr contacts. Along with the conventional measurements, the current-sensing contact mode AFM was employed to measure the current-voltage characteristics and current variations with time under fixed voltage. We found that (i) the electric current flow is laterally strongly nonuniform; (ii) it chaotically varies over time; (iii) this behavior did not correlate with surface topography. We interpret our observations in terms of defect assisted tunneling through time-dependent defect pathways. INTRODUCTION Applying a metal contact on a semiconductor device results in a Schottky barrier which affects current collection. This phenomenon known as the back barrier or back surface field can affect all major photovoltaics (PV), such as CdTe, silicon, and CIGS [1]. Due to the lack of crystallinity, the electronic transport through the back barrier in thin-film can be laterally nonuniform. Even for the case of crystal-based structures, it has been long realized that the barrier transport is inherently nonuniform. The model of laterally fluctuating barrier height VB was put forward by Tung [2] and verified in subsequent work (see [3] and references therein). Independently, it was pointed out that in amorphous thin films [4] and Schottky barriers [5], defect assisted tunneling can dominate the transport, which then becomes strongly non-uniform with small area patches responsible for the whole current flux. Such laterally nonuniform defect assisted tunneling (hopping) transport is illustrated in Fig. 1 including possible thermal activation. Some implications of the back barrier transport nonuniformity were addressed in Ref. [6]. Our present work is aimed at experimental study of microscopically nonuniform barrier transport and relating it to the macroscopic barrier parameters. We show that local current through a polycrystalline Schottky barrier strongly varies between different local spots and exhibits large temporal fluctuations at any given spot. These strong spatial and temporal variations average out into much smaller yet practically important fluctuations of the device macroscopic parameters. EXPERIMENTAL RESULTS The main junction effects hamper direct study of back contact transport in complete PV device. To screen against this factor we conducted our experiments on sandwich
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lB (a) (b) (c) Figure 1. (a) Electron hopping through a barrier in the real space; open circles represent defects. Pathways 1, 2, and 3 correspond to the regions of abnormally low, high, and average transparency respectivel
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