engleski

The influence of post deposition plasma treatment on SnOx structural properties

Thin SnOx films deposited on glass substrate are widely used as transparent electrodes (TCO) in thin film Si solar cells. In standard production procedure, the thin Si films are deposited on SnOx using high frequency discharge in gas mixtures with substantial hydrogen content. In order to study influence of this kind of plasma on SnOx structural properties, the films with thickness of 0.7 µ m were deposited on glass substrate by APCVD (Atmospheric Pressure Chemical Vapor Deposition) on glass substrate, using tin tetrachloride as precursor. The deposition condition was chosen to satisfy common demands on TCO layer that are transparency in the visible part of electromagnetic radiation close to 90%, diffuse to direct transmittance ratio between 20 and 30% and sheath resistance between 5 and 10 Ω / . After the deposition, the films were exposed to RF hydrogen plasma under the conditions that are typical for crystalline thin silicon film deposition and the structural changes caused by this treatment were examined by X-ray diffraction (XRD) and Grazing Incidence Small Angle X-ray Scattering (GISAXS). The analysis of XRD line profile within Rietveld method shows that the as deposited films are nanocrystalline, with typical average crystallite size between 15 and 40 nm and slightly preferred orientation, depending on parameters of deposition. By exposure to hydrogen plasma, the degree of preferential orientation increase, the average crystal size lowers and the micro-strain increase. GISAXS results suggest that as deposited samples are built of nanocrystals in shape of non truncated pyramids, whose typical size agrees well with XRD results. After plasma treatment, their shape close to the surface is greatly distorted and the GISAXS signal becomes isotropic. The observed structural changes are discussed as a consequence of the preferential plasma etching of surface, accompanied with defect formation which allows the in depth hydrogen diffusion.

tin-oxide; TCO; GISAXS; XRD; SEM; XPS

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