Relationship between scaling behavior and porosity of plasma-deposited TiO2 thin films

Abstract

The growth of TiO2 thin films prepared by plasma enhanced chemical vapor deposition has been studied by analyzing their roughness with the concepts of the dynamic scaling theory. Differences in the growth and roughness exponents have been found depending on the composition of the plasma by using either O2 or mixtures Ar+ O2 as plasma gas and titanium isopropoxide as the precursor. The slope of the representations of the film roughness against the deposition time yielded values of the exponent β of 0.45 and 0.32 for, respectively, thin films prepared with plasmas of O2 or mixtures Ar+ O2. Meanwhile, values of the exponent α of 1.15 and 1.89/0.35 were deduced from the power spectral density representations for the films prepared under these two experimental conditions. These values are congruent with growth processes dominated, respectively, by shadowing or diffusion processes. A columnar microstructure was observed by scanning electron microscopy for the thin films prepared with pure oxygen. Meanwhile, homogeneous films were obtained with mixtures of Ar+ O2. The open porosity of the films was determined by measuring water adsorption-desorption isotherms with a quartz crystal monitor. This analysis showed that in the samples prepared with mixtures of Ar+ O2 the porosity consisted exclusively of micropores (d<2 nm), while in the films prepared with an oxygen plasma there were micro- and meso-pores (d>2 nm). It is concluded that the different growth mechanisms found by just changing the chemistry of the plasma are responsible for the quite distinct microstructures, porosities, and optical properties obtained for the films.