Nanooptics and photonics

Structure and optical properties of Cu7GeS5I-based thin film deposited by HiTUS
Bendak A.V., Lopushanska B.V., Rybak S.O., Izai V.Yu., Studenyak I.P.
Uzhhorod National University, Pidhirna Str., 46, 88000, Uzhhorod, Ukraine 
E-mail: andrii.bendak@gmail.com

Cu7GeS5I belong to the wide family of the compounds with argyrodite structure and are known as the highly effective superionic conductors. Now the preparation and investigation of thin films based on superionic conductors are of great interest. Thus, the paper is devoted to studies the preparation, structural and optical properties of Cu7GeS5I-based thin films deposited by HiTUS  (high target utilisation sputtering) technique.
Cu7GeS5I-based thin film was deposited on silicate glass substrate at room temperature by means of HiTUS technique (PQL HiTUS S500 sputter coating system) from Cu7GeS5I target (30mm in Dia.) in radiofrequency Ar discharges. The plasma source power density was fixed at 2000 W yielding a deposition rate of approximately 1.5 nm/min. Target pulsed DC power was fixed at 70 W. Substrates were placed parallel above the target at a distance of 180 mm without additional heating. All depositions were carried out at floating plasma potential. The base pressure in the chamber was below 7×10-4 Pa and the total working gas pressure during deposition was kept at 0.65 Pa. 
Structural studies were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). EDX measurements were used to determine the thin film chemical composition (chemical formula calculated from EDX measurements is of Cu5.9Ge2.6S5.3I0.2). Thus, thin film obtained using HiTUS technique appeared to be non-stoichiometric by chemical composition, enriched with Ge and S atoms and depleted with Cu and I atoms, with sufficiently low I content. It should be noted that the diffraction pattern of Cu5.9Ge2.6S5.3I0.2 thin film shows the film to be amorphous.
The refractive index and extinction coefficient for the Cu5.9Ge2.6S5.3I0.2 thin film were obtained from the spectral ellipsometer measurements. Thus, the refractive index for Cu5.9Ge2.6S5.3I0.2 thin film is 3.475 at room temperature and λ=400 nm while the extinction coefficient is 0.426. Based on the optical transmission spectrum the spectral dependence of the absorption coefficient was calculate for Cu5.9Ge2.6S5.3I0.2 thin film. The energy pseudogap (energy position of exponential absorption edge at a fixed absorption coefficient) of Cu5.9Ge2.6S5.3I0.2 thin film deposited by HiTUS technique is 2.435 eV.