Crystallinity Control of Silicon Thin Films for High-Efficiency Solar Cells by Two-Step Growth Method

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To control crystallinity of silicon thin films, microcrystalline silicon thin films were prepared by hydrogen radical CVD method and their structure and properties were evaluated. Especially, to clarify effect of amorphous initial layer and crystalline growth process, crystallinity and electrical properties of thin films were estimated in detail. Substrate dependence of film structure was compared by using glass and Al substrates. Moreover, two-step growth was done by changing gas phase reaction with substrate position. As a result, the following matters were clarified.The microcrystalline films prepared at 200℃ showed (111) preferential orientation and the grain size estimated from XRD was 30nm in maximum. However, the average grain size obtained from AFM was changed from 90nm to 270nm and simultaneously the surface roughness was increased. These results suggest that microcrystalline grains grow, collide and coaggregate. The vertical electrical conductivity was very small compared to the lateral one and nearly equal to the values of amorphous films. This result shows the effect of amorphous initial layer. By raising the substrate temperature from 200 to 350℃, the thickness of amorphous initial layer was decreased. The experiments of two-step growth showed that, by regulating SiH4 flow rate, growth rate can be increased and crystallinity of films can be controlled.