Optical Constants and Band Gap Evolution with Phase Transition in Sub-20-nm-Thick TiO2 Films Prepared by ALD

Yue-Jie Shi; Rong-Jun Zhang; Hua Zheng; Da-Hai Li; Wei Wei; Xin Chen; Yan Sun; Yan-Feng Wei; Hong-Liang Lu; Ning Dai; Liang-Yao Chen

doi:10.1186/s11671-017-2011-2

Optical Constants and Band Gap Evolution with Phase Transition in Sub-20-nm-Thick TiO₂ Films Prepared by ALD

Nanoscale Res Lett. 2017 Dec;12(1):243. doi: 10.1186/s11671-017-2011-2. Epub 2017 Mar 31.

Authors

Yue-Jie Shi¹, Rong-Jun Zhang², Hua Zheng¹, Da-Hai Li¹, Wei Wei³, Xin Chen⁴, Yan Sun³, Yan-Feng Wei³, Hong-Liang Lu⁵, Ning Dai³, Liang-Yao Chen¹

Affiliations

¹ Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Department of Optical Science and Engineering, Fudan University, Shanghai, 200433, China.
² Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Department of Optical Science and Engineering, Fudan University, Shanghai, 200433, China. rjzhang@fudan.edu.cn.
³ National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083, China.
⁴ National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083, China. xinchen@mail.sitp.ac.cn.
⁵ State Key Laboratory of ASIC & System, Fudan University, Shanghai, 200433, China.

Abstract

Titanium dioxide (TiO₂) ultrathin films with different thicknesses below 20 nm were grown by atomic layer deposition (ALD) on silicon substrates at 300 °C. Spectroscopic ellipsometry (SE) measurements were operated to investigate the effect of thickness on the optical properties of ultrathin films in the spectra range from 200 to 1000 nm with Forouhi-Bloomer (F-B) dispersion relation. It has been found that the refractive index and extinction coefficient of the investigated TiO₂ ultrathin film increase while the band gap of TiO₂ ultrathin film decreases monotonically with an increase in film thickness. Furthermore, with the purpose of studying the temperature dependence of optical properties of TiO₂ ultrathin film, the samples were annealed at temperature from 400 to 900 °C in N₂ atmosphere. The crystalline structure of deposited and annealed films was deduced by SE and supported by X-ray diffraction (XRD). It was revealed that the anatase TiO₂ film started to transform into rutile phase when the annealing temperature was up to 800 °C. In this paper, a constructive and effective method of monitoring the phase transition in ultrathin films by SE has been proposed when the phase transition is not so obvious analyzed by XRD.

Keywords: Atomic layer deposition; Phase transition; Rapid thermal annealing; Spectroscopic ellipsometry; TiO2 ultrathin film.