Thin Film Polycrystalline Si by Cs Solution Growth Technique
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ABSTRACT A deposition process has been developed which allows the growth of large grain (20' pm) polysilicon films on SiO 2 substrates at a growth temperature of 6500 C. A thin layer of liquid Simetal solution is formed on the substrate surface as the growth medium. This layer is kept saturated by Si flux from a DC magnetron sputter gun. XRD analysis of the deposited films show a strong (111) preferred orientation, with increasing integrated peak intensities with increasing depositon temperature and solution layer thickness. Films deposited using an In-Si solution are p-type, with carrier concentrations in the mid 1016 cm-3 range. Conductivities of -. 2 (IQcm)1 were measured, with activation energies for both carrier generation and conductivity of about 135meV. The hole mobility was found to be - 30 cm2 V's-. A wetting layer is used which may have a detrimental effect on the minority carrier lifetime.
OBJECTIVES The objective of this work was to investigate deposition techniques for large-grain polysilicon films, suitable for large-area photovoltaic energy conversion. Low cost per unit area was of highest importance. This dictated a low temperature process , allowing a low cost substrate (e.g. glass, sheet metal). A grain size of at least the projected film thickness was desired (-25 pm), allowing carrier transport parallel to, instead of across, the grain boundaries. A high rate process was needed - discouraging the use of UHV environments or very long annealing times. DEPOSITION TECHNIQUE A thin film solution growth technique has been developed, using a sub-micron thick layer of Si-metal solution to mediate the delivery of Si from a sputter source to the substrate surface. This is very similar in concept to vapor-liquid-solid (VLS) growth', with the exception that the melt forms a continuous layer instead of a discrete droplet, and that the Si is delivered by a sputter gun instead of CVD. The primary solvent metals investigated to date have been In and Sn. High quality epilayers have been deposited from both In-Si 3'4 and Sn-Si melts5 . Our earlier work focussed on Al based solutions, but problems with film uniformity and the poor electrical results reported elsewhere2 caused this appproach to be abandoned. The present deposition sequence is outlined in Figure 1. This technique differs from bath-type solution growth in that melt subcooling is achieved by composition rather than temperature; hence, the process has been dubbed CS growth (for Compositional Subcooling). All depositions were performed in an oil diffusion - pumped chamber. Base pressures 883 Mat. Res. Soc. Symp. Proc. Vol. 358101995 Materials Research Society
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