Low-temperature preparation of rutile-type TiO2 thin films for optical coatings by aluminum doping

Abstract

A rutile-type TiO2 thin film with a high refractive index (n), a low extinction coefficient (k) and small surface roughness (Ra) is required for use in a variety of optical coatings to improve the controllability of the reflection spectrum. In this study, Al-doped TiO2 thin films were prepared by pulsed laser deposition, and the effects of Al doping on their phases, optical properties, surface roughness and nanoscale microstructure, including Al distribution, were investigated. By doping 5 and 10mol%Al, rutile-type TiO2 was successfully prepared under a PO2 of 0.5Pa at 350–600°C. The nanoscale phase separation in the Al-doped TiO2 thin films plays an important role in the formation of the rutile phase. The 10mol%Al-doped rutile-type TiO2 thin film deposited at 350°C showed excellent optical properties of n≈3.05, k≈0.01 (at λ=400nm) and negligible surface roughness, at Ra≈0.8nm. The advantages of the superior optical properties and small surface roughness of the 10mol%Al-doped TiO2 thin film were confirmed by fabricating a ten-layered dielectric mirror.