A study on structural, optical, electrical and microstructural properties of thin TiOx films upon thermal oxidation: Effect of substrate temperature and oxidation temperature

Abstract

Influences of both substrate temperature, T s (∼305, 473 K) and oxidation temperature, T a (∼623–973 K) on the structural, optical, electrical and microstructural properties of thin TiO x ( x ≤2) films obtained by thermal oxidation of sputtered titanium thin films have been investigated. T s is found to be an important parameter that affects both the as deposited film morphology and phase evolution of TiO x films during oxidation. As deposited and oxidized films processed at T a ∼623 K exist in TiO form. Formation of anatase (TiO 2 ) phase takes place at T a ∼723 K. As the T a increases above 723 K, degree of crystallinity of the film improves and rutile (TiO 2 ) phase appears along with anatase phase at T a ∼873 K. Further increase in the T a enhances the contribution of rutile phase at the expense of anatase contribution. Apparent crystallite size, L , and refractive index of the TiO x ( x ≈2) films increase with T a but band gap energy, E g decreases from ∼3.4 to 3.35 eV. Scanning electron microscopic study reveals that both film densification and grain size improve with T a . As the T a increases above 873 K, rutile phase contribution as well as grains of the oxidized films deposited at a lower T s grow at a faster rate than that of the TiO x films prepared at a higher T s . Room temperature resistivity of the as deposited films is found to be dependent on T s . Film-resistivity increases with oxidation temperature and at T a ∼723 K, resistivity of the film increases drastically. Temperature coefficient of resistivity (TCR) for all the as deposited and oxidized films processed at T a ∼623 K is found to be negative and lie between ∼1.2×10 −3 –2.1×10 −3 K −1 . Thermal activation energy, E a , of the as deposited and oxidized ( T a ∼623 K) TiO x ( x ≈1) films is estimated to vary over the range ∼0.015–0.027 eV. Observed change in the film electrical properties with T a is discussed in the light of oxygen incorporation in the TiO x structure.