Low temperature growth study of nano-crystalline TiO2 thin films deposited by RF sputtering

Abstract

Precise control of the various structural phases of TiO2 at a low temperature is particularly important for practical applications. In this work, the deposition conditions for the growth of anatase and rutile phase at a low temperature (⩽300 °C) were optimized. TiO2 films were deposited by radio frequency (RF) sputtering of a ceramic TiO2 target in argon and argon-oxygen plasma (10 and 20% O2) at room temperature. For the films deposited in pure Ar and 20% O2, the growth temperature was varied from 25 to 400 °C. The plasma properties were investigated using optical emission spectroscopy (OES) in a wide range of values of gas composition (0–50% O2 in Ar-O2 mixture). The structural and chemical properties were characterized by means of x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The results indicate that O2 addition to the Ar-O2 gas mixture significantly changed the density of the plasma species (Ar, Ar+, Ti, Ti+ and O), which in turn influence the crystal structure and surface chemistry of the prepared films. Anatase phase was obtained for the films grown in Ar-O2 plasma over the whole range of temperature. In contrast, the films deposited in argon discharge largely persist in amorphous phase at temperature ⩽200 °C and revealed the formation of single rutile phase at ⩾300 °C. The oxygen vacancies detected by XPS analysis for the films deposited in Ar plasma facilitate the growth of a rutile phase at low temperature (∼300 °C). Our results demonstrate that oxygen negative ions, oxygen vacancies and surface energy conditions at the substrate are the key parameters controlling the phase of the prepared films at low temperature.