Determination of optical constants and average thickness of inhomogeneous-rough thin films using spectral dependence of optical transmittance

The thickness variation, surface roughness, variation in refractive index, and illumination wavelength bandwidth strongly influence the optical transmittance of thin films. The consequences of linear and Gaussian distributions of the change in phase of radiation traversing a thin film on the spectral characteristics of optical transmittance are presented. A new method of determining the spectral dependences of the real part of the refractive index and absorption coefficient as well as thickness and standard deviation of its value is proposed for examinations of transmittance of thin films on thick substrates. This method takes into account the Gaussian distribution of the change in phase of radiation over the illuminated sample area. Its usefulness in investigations of the optical properties of a-Si:H is shown in comparison with the standard methods. The novel method gives film thickness more compatible with the result obtained using the multiple-beam interference method. The obtained values of optical parameters allow better fitting of the spectral characteristics of transmittance with the theoretical curve.