Characterization of microroughness parameters in gadolinium oxide thin films: A study based on extended power spectral density analyses

Spectral microroughness is a performance-limiting factor for optical thin films like Gd2O3, which have dedicative applications in ultraviolet or deep ultraviolet region of the electromagnetic spectrum. Such a morphological parameter of a thin film surface can be very well characterized by power spectral density (PSD) functions. The PSD provides a more reliable description to the topography than the RMS roughness and imparts several useful information of the surface including fractal and superstructure contributions. Through the present study it has been noticed that deposition parameters like evaporation rate and oxygen pressure can play very definite, dominant and predictable roles in the evolution of fractal and superstructures in thin film topographies recorded through atomic force microscopy (AFM). In this work, the PSD functions derived from morphologies of various gadolinia thin films have been fitted with a novel multi peak-shifting Gaussian model along with fractal and k-correlation functions, to extract characteristic parameters of the precision surfaces. Using such information, roughness contributions of the fractal components (substrate dominated), pure film and the aggregates have been successfully extracted. Higher spectral fractal strengths have depicted lower refractive index values. The microroughness and grain sizes of the pure film have been influenced very differently with deposition rate and oxygen pressure. The oxygen pressure strongly influenced the grain sizes where as the deposition rate influenced the microroughness of the gadolinia films.