Effects of annealing and quenching treatments on reconstruction of rutile thin films on sapphire substrates

Morphology change in rutile TiO₂ thin films on sapphire substrates prepared by pulsed laser deposition under reduced oxygen environment was investigated as a function of film thickness, temperature and cooling treatments with atomic force microscopy, X-ray diffraction and scanning electron microscopy equipped with X-ray spectroscopy (SEM/EDX). The deposited TiO₂ was determined as epitaxially grown rutile films whose crystallographic correlation with substrates was (1 0 0)_rutile//(0 0 0 1)_sapphire. As increasing thickness of TiO₂ films, smooth surface changed to island structure. In addition, the morphology of TiO₂ film on α-Al₂O₃(0 0 0 1) varied drastically by annealing treatment from 973 to 1123 K. In case of ∼5 nm thickness films, morphology strongly depended on annealing and cooling treatments. We found interesting order structure of TiO₂ islands at annealing temperature (∼1073 K) and subsequent quenching (∼1.3 K/s). Formation process of TiO₂ particles on α-Al₂O₃(0 0 0 1) substrates is modeled based on instability of substrate at elevated temperatures.