Thermal properties of TiO2 films grown by atomic layer deposition

We study the thermal properties of amorphous TiO2 thin films of various thicknesses t, grown by atomic layer deposition. The thermo-optic coefficient dn/dT and the temperature coefficient d rho/dT of film density rho are determined from ellipsometric data in wavelength range 380<1800 nm with the Cauchy model and the Lorentz-Lorenz relation. It is found that dn/dT exhibits negative values for films with t<150 nm and positive'values for thicker films, while no significant changes in the two coefficients take place if t<200 nm. A qualitative physical explanation based on porosity of the thin films is suggested. Films with t=60 nm are illustrated in detail at lambda=640 nm: the room-temperature values of the coefficients are found to be dn/dT= 3.1x10(-5) degrees C-1 and d rho/dT=-4.8 x 10(-5)g cm(-3 degrees) C-1 (C) 2012 Elsevier B.V. All rights reserved.