The effect of substrate temperature on filtered vacuum arc deposited zinc oxide and tin oxide thin films

Abstract

Zinc oxide (ZnO) and tin oxide (SnO 2) thin films were deposited on commercial microscope glass and UV-fused silica (UVFS) substrates using a filtered vacuum arc deposition (FVAD) system. During the deposition, the substrates temperature was kept at room temperature (RT) or at 400 °C. The film structure, surface morphology, and composition were determined using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. The XRD patterns of the ZnO films deposited on RT substrates contained lines of strong c-axis orientation, whereas the intensity of the XRD lines of ZnO films deposited on 400 °C substrates was significantly stronger. The XRD patterns of SnO 2 films deposited on RT substrates did not contain any diffraction lines, indicating an amorphous film structure, whereas the XRD patterns of SnO 2 films deposited on hot substrates contained diffraction lines indicating that the films are polycrystalline. The ZnO and SnO 2 film thickness was in the range 100–363 nm. The surface roughness (RMS) of ZnO film was 1.3 nm at RT and increased to 5 nm at 400 °C, whereas that of SnO 2 films was 1.5 nm and decreased to 0.5 nm at RT and at 400 °C, respectively. The films’ optical constants in the 250–1100 nm wavelength range were determined by variable angle spectroscopic ellipsometry and by transmission measurements. The peak transmission of the ZnO and SnO 2 films in the VIS was 80–90%. The refractive index n of the films deposited on RT and hot substrates were in the range 1.72–2.23 for ZnO samples, and in the range 1.87–2.20 for SnO 2 samples, as function of wavelength. The extinction coefficient of all films in the VIS was in the range 0.001 to 0.05, depending on wavelengths and deposition parameters. The optical band gap ( E g) was determined from the dependence of the absorption coefficient on the photon energy at short wavelengths. Depending on the deposition conditions, the values of E g of ZnO and SnO 2 were in the range 3.25–3.30 and 3.60–3.98 eV, respectively.