Impact of Growth Conditions on the Phase Selectivity and Epitaxial Quality of TiO2 Films Grown by the Plasma-Assisted Atomic Layer Deposition

Plasma-enhanced atomic layer deposition (PE-ALD) is a promising method to grow conformal, epitaxial films at low temperatures (<300 °C). In this study, PE-ALD was used to deposit high-quality, epitaxial anatase and rutile TiO 2 thin films on sapphire substrates with various orientations ( c -, m -, and a -Al 2 O 3 ). For all substrate orientations, the influence of ALD growth temperature and plasma gas chemistry on the film morphology, phase, and crystallinity was investigated. On c -Al 2 O 3, using a mixed Ar–O 2 plasma, the phase selectivity between anatase and rutile TiO 2 could be achieved by simply controlling the growth temperature from 150 to 350 °C. By using a pure O 2 plasma, single-phase rutile TiO 2 was grown independent of temperature or substrate orientation. Additionally, using a pure O 2 plasma resulted in higher crystalline quality films as growth temperatures were reduced, resulting in single-phase crystalline films at 150 °C. Although high-quality rutile films were attained on c - and m -Al 2 O 3, only low-quality crystalline rutile films were produced on a -Al 2 O 3 . These results demonstrate the benefits of a PE-ALD approach toward depositing epitaxial TiO 2 under low-temperature conditions that is of high quality, phase and strain selective.