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A19.13.1

Depth Profiling of Light-Induced Defects in Hydrogenated Amorphous Silicon by Transient Photocurrent Spectroscopy Steve Reynolds, Charlie Main and Rudi Brüggemann1 School of Computing and Advanced Technologies, University of Abertay Dundee, Bell Street, Dundee, U.K. 1 Fachbereich Physik, Carl von Ossietzky Universität Oldenburg, D-26111 Oldenburg, Germany.

ABSTRACT The sensitivity of transient photocurrent measurements to the spatial location of native and metastable electronic defects in hydrogenated amorphous silicon films is demonstrated. The technique utilises red and green laser excitation to generate excess carriers in the bulk and at the surfaces of the film, respectively. In annealed films the defect density is found to be higher in the surface regions. Following white light soaking, the metastable defect density at the surface at which the light is incident is greater than that in the bulk, which in turn is greater than that at the exit surface. This is attributed to the white light absorption profile within the film. Green light soaking creates metastable defects at the incident surface, with the bulk of the film largely unaffected.

INTRODUCTION Transient photoconductivity (TPC) is a sensitive probe of the energetic distribution of localized states in amorphous semiconductors. [1]. The technique is based on the interpretation of the form of the decay in current I(t) following a short flash of light, in terms of the multipletrapping transport model. Recently, the present authors have shown that a degree of spatial as well as energetic resolution can be achieved using TPC [2,3]. This variant of the technique utilises the differing absorption depths of the light flash depending on the wavelength used. Green (510 nm) and red (640 nm) laser dyes were used giving, in the case of a-Si:H, typical absorption depths of 0.1 and 1.5 µm, respectively. Thus for a film thickness of order 1 µm, carrier generation by the red flash is fairly uniform throughout the film, but the green flash generates carriers close to the surface at which it is incident. As the diffusion length of carriers prior to undergoing deep trapping is of the order of 0.1 µm, the photocurrent decay will bear the hallmark of the local density of states (DOS) where they are generated. Ghosh and Ganguly [4], using steady-state photoconductivity measurements, demonstrated a reduction in the ηµτ-product at short wavelengths in a-Si:H films following light soaking, in comparison with the annealed state. Although clearly visible following white light soaking, the effect was more pronounced following blue and violet illumination. Inspired by this observation, we have followed a similar approach, studying the decay in transient photocurrent following annealing and subsequent light-soaking with white light and green-filtered white light, using red and green laser pulses for carrier excitation, from both the silicon-air and the silicon-substrate sides of the film. This has enabled us to assess the correlation between defect creation and defect detect