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Microstructural and optical study of inhomogeneous SnO2 thin films

SnO2 is an important material in energy-related research because of its possible application in thin film solar cells. The spray pirolysis using Atmospheric Pressure Chemical Vapor Deposition (APCVD) is becoming popular in many industrial production lines because of its low cost. The growth of solar cell structures in most cases is a “transport limited growth”, e.g. the film growth mimics the surface roughness of SnO2 surface, which is determined by the crystal structure, orientation, crystallite size and microstrain. In this study, SnO2 thin films were deposited by APCD on the glass substrate in two sequential steps. In the first step the precursor that contains only Sn and O atoms is sprayed on the surface of hot glass and after that heated in oven. In the second step, the precursor contains fluorine as a dopant. The type of precursor affects the crystal size and other structural properties. As result, the films are inhomogeneous across depth. Due to the complex nature of as-deposited material, a careful analysis through the thin-film depth is needed in order to collect information about the local materials microstructure. One possible approach to accomplish this goal is XRD probing of the material at the grazing incidence angle (GIXRD), as the resulting synchrotron pattern yields information at specific depth depending on the incident angle. As-obtained GIXRD patterns are fully processed with the Rietveld refinement in order to get information about crystallite size, microstrain and preferred orientation. Such information, particularly the gradient of crystallite size distribution perpendicular to the film surface, is a valuable feedback regarding the thin film deposition conditions. The thickness, the absorption coefficient and the refraction index of SnO2 films were estimated upon reflectance measurements. Obtained thicknesses were between 0.24 and 0.62 m. The optical gap varied between 2.4 and 3.2 eV while the refraction index was less than 2 in the visible part of the spectrum, indicating a porous structure. Grazing incidence small angle X-ray scattering (GISAXS) was done on Austrian beam line at synchrotron Trieste and analyzed using Guinier approximation. In this way obtained sizes of “particles” were larger than sizes of crystals obtained by GIXRD indicating the coalescence of crystals.

thin films; electrode materials; vapor deposition; optical properties; X-ray diffraction

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