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

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.