Metastability in Undoped Microcrystalline Silicon Thin Films Deposited by HWCVD

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A9.10.1

Metastability in Undoped Microcrystalline Silicon Thin Films Deposited by HWCVD S.K. Persheyev 1, K.A. O’Neill 1, S.Anthony 1, M.J. Rose 1, V. Smirnov 2, S. Reynolds 2 1

Carnegie Laboratory of Physics, Electronic Engineering and Physics Division University of Dundee, Dundee, DD1 4HN, Scotland, UK. 2 EPICentre, School of Computing and Advanced Technologies, University of Abertay Dundee, Dundee DD1 1HG, Scotland, UK.

ABSTRACT Films deposited by the Hot Wire CVD method were studied by means of dark conductivity, FTIR, Hydrogen Evolution, SEM and AFM surface characterization. Three types of metastability were observed: a) long term irreversible degradation due to oxidization processes on the film surface, b) reversible degradation determined by uncontrolled water adsorption, c) fast field switching effect in the film bulk. Oxygen and hydrogen content and its bonding configurations have been analyzed by hydrogen evolution and infrared spectroscopy methods on the films deposited on glass substrates and silicon wafers subsequently. It has been found that metastable processes close to the film surface are stronger than in the bulk. The switching effect is the fast increase of charge carrier density observed on bottom chromium contacts under a condition of air admittance. We propose this effect is associated with morphology changes during film growth and electrical field induced by adsorbed atmospheric components on the film surface.

INTRODUCTION For more than a decade there has been high interest in the Hot Wire CVD deposition technique [1-4] to obtain silicon related materials at low temperatures without the use of plasma to decompose source gas. Metastability in microcrystalline thin silicon films deposited by high hydrogen dilution is probably one of the major issues limiting its applications for solar cells, TFT and other devices. It is known that the properties of thin silicon films are affected by atmospheric adsorption processes, aging. Both reversible (short term) and irreversible (long term) effects in conductivity on different kinds of thin film materials exposed to atmospheric air have been reported [5-8]. The irreversible effect was related to surface chemical reactions of oxygen incorporation process [5] in amorphous silicon films. Reversible effects are related to physical adsorption of atmospheric components on the film surface. However, it is still unclear to what extent the material bulk is involved in aging process, or is this only a surface effect? In the present work we investigate the influence of exposure to atmospheric gases on both bulk and surface of hot wire microcrystalline silicon films and propose a model of the atmospheric components adsorption mechanism, based on obtained results. The results of the work can be used to improve the stability of HW microcrystalline silicon films to atmospheric processes.

A9.10.2

EXPERIMENTAL A deposition system [1] incorporating independently controlled heated substrate holders on both sides of the filaments have been built as an improvement to th