INFLUENCE OF THICKNESS INHOMOGENEITY ON THE DETERMINATION OF OPTICAL-CONSTANTS OF AMORPHOUS-SILICON THIN-FILMS

Real thin films, deposited by vacuum methods, exhibit thickness inhomogeneities which influence the optical transmission and reflection spectra. This is the cause of drastic shrinking of the interference fringes of both transmittance and reflectance and serious errors occur if one calculates refractive and extinction indices with the assumption of film uniformity. Influence of thickness inhomogeneity on the transmission spectrum only was discussed by several authors. In this work one obtains the analytical formula for reflectance of an inhomogeneous, wedge shaped film. Using expressions for envelopes around the maxima and minima for both transmittance and reflectance of the nonuniform films, it was possible to calculate accurate values of optical constants and thickness variation. The calculation procedure was applied to the optical spectra of a-Si:H films deposited by RF sputtering and glow discharge methods and the obtained results were compared with those for films considered to be uniform.