Hydrogen Assisted Nano-crystallization in TiO₂ Thin Film Prepared by Hot-Wire Chemical Vapor Deposition

Abstract

Preparations and structural studies of TiO 2 thin films using hot-wire chemical vapor deposition (CVD) (hot-filament CVD) are reported for the first time. Titanium tetra-isopropoxide [Ti(OC 3 H 7 ) 4 ] was used as a source gas and decomposed on a heated rhenium filament. The film deposited at the filament temperature (T f ) of 1300 C shows amorphous structure with the substrate temperature (T s ) of 300 C, and X-ray diffraction (XRD) peaks originated from nano-crystalline with anatase structure appeared over T s of 400 -700 C. The optical band gap energies of the nano-crystalline TiO 2 films with anatase structure were 3:4 eV. An increase of T s from 400 to 700 C enhanced the XRD peak intensity of (112) orientation. Meanwhile, an increase of T f up to 1500 C induces nano-crystalline TiO 2 with rutile structure. Furthermore, the hydrogen dilution realizes the nano-crystallite growth of rutile structure even in the deposition at T f ¼ 1300 C. During this deposition, the actual substrate surface temperature (T suf ) was 305 C. In bulk TiO 2 materials, the anatase structure changes to the rutile structure by thermal annealing up to about 800 C. We propose for the first time that atomic hydrogen contributes to the low temperature nucleation of rutile structure in the deposition of oxide system, TiO 2 films.