Role of hydrogen addition in the plasma phase in determining the structural and chemical properties of RF sputtered ZnO films

In the present work, ZnO thin films were RF sputtered from a pure ZnO target, without external heating, in H2:Ar plasma at different H2 concentrations (0–50%). Aim of the study was the identification of the effects of H incorporation on the film growth and properties. During the deposition experiments, optical emission (OES) spectra were recorded to monitor any variation in the plasma chemical species relative to different process or gas mixture settings. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier-transformed infrared spectroscopy (ATR-FTIR) were used to study the bulk and surface chemical composition of the films, while X-ray diffraction (XRD) analysis allowed lattice structure and grain size determination. The introduction of hydrogen in the plasma phase appears to strongly affect the structural and chemical properties of ZnO films. Both FTIR spectra and X-ray diffraction patterns showed that all the films crystallized in the hexagonal würtzite form. Nevertheless, while samples deposited in pure Ar plasma are highly textured, presenting just one dominant preferred orientation along the [0 0 2] axis, films sputtered in H2:Ar atmosphere exhibit multiple growth directions with crystallites of noticeably reduced dimensions. Such a structural modification turns up together with clear variations in the films surface chemical state which appears to deviate from the pure oxide (Zn–O). By combining XPS, ATR-FTIR and OES data we could correlate such variations with the process induced H incorporation in the crystal structure in the form of hydroxide species.