Recrystallisation of Cadmium Sulphide Powder Films with a CO 2 Laser
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RECRYSTALLISATION OF CADMIUM SULPHIDE POWDER FILMS WITH A CO2 LASER P.E. BARDEN AND T.J. CUMBERBATCH Engineering Department, Cambridge University, England CB2 1PZ
Trumpington Street, Cambridge,
ABSTRACT Thin films (1-2ým) of cadmium sulphide, deposited by electrophoresis, consist of a close packed layer of randomly oriented cubic-phase microcrystalline particles with an average diameter of 30nm. A CW CO2 laser, operating at 10.6Am has been used to convert this into a polycrystalline structure with columnar crystals, of hexagonal phase, which extend through the thickness of the film and whose c-axis is perpendicular to the substrate. The recrystallised regions comprise aligned grains up to 300nm in diameter whose cathodoluminescence spectrum exhibits a narrow peak centred at 2.42eV with a half width identical to that for evaporated CdS (0.1eV). INTRODUCTION Cadmium sulphide holds considerable potential as a photovoltaic material due to the ease with which it can be deposited over large areas in the form of a thin film. Amongst the wet chemical techniques available, electrophoretic deposition from a colloid offers the simplest technology since it is a room temperature process with efficient materials utilisation. The primary disadvantage of this approach is that the deposited semiconductor is a cubic phase powder layer on a tin oxide coated glass substrate which needs to be converted into a polycrystalline hexagonal film for device applications. In accordance with techniques used for vacuum deposited films, conventional furnace heat treatments were investigated over a wide range of temperatures and with a wide range of "flux" materials [E]. In the absence of a flux, the temperatures required for recrystallisation (>700 C) are precluded by the thermal properties of the substrate and chemical properties of the cadmium sulphide. Procedures known to reduce the recrystallisation temperature for contiguous thin films were generally shown to be inoperative for the powder layers because of their particulate nature which reduces the contact area between adjacent microcrystallites and thus impedes material transfer. For this reason, laser and electron beam transient heating techniques have been investigated. Layers exposed to a scanned electron beam with a wide range of processing conditions remained cubic and no grain growth was observed. Auger depth profiling revealed a superficial sulphur loss, even at low beam Rowers, and was attributed to the reduced temperature of dissociation (^525 C) in a vacuum [2] which prevented grain growth. Other work has shown that recrystallisation is possible with lasers operating in the visible region of the spectrum [3], where the light is absorbed by bandgap excitation in the cadmium sulphide. Pulsed radiation with a duration of 300ns or less was shown to melt and recrystallise a very thin surface skin, the thickness of which was related to the pulse period. However, this material was damaged and significant changes occurred in the surface topography. Extending the pulse length to 3ps incr
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