Effect of coating thickness on the growth rates, optical properties and microstructure of TiO2 thin films grown via thermal and plasma enhanced atomic layer deposition

Abstract

Titanium dioxide (TiO₂) thin films were synthesized on single-crystalline silicon (111) substrates using thermal (TALD) and plasma-enhanced atomic layer deposition (PEALD) methods in a self-limited growth regime with different numbers of deposition cycles (250, 500, 750, 1000, 1500 and 2000). The synthesized TiO₂ layers were studied by ellipsometry, X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The findings of the study indicated that during the initial stages of growth, the silicon substrate exerted an inhibitory effect on the degree of nucleation of crystalline titanium dioxide films, resulting in the formation of coatings consisting of nanocrystallites in an amorphous matrix. Moreover, for titanium dioxide coatings synthesized by the PEALD method, compared to TiO₂ films obtained by the TALD method, this effect was more pronounced. As the thickness of the titanium dioxide films increased, an increase in the nanocrystallite sizes, the crystallization of the amorphous regions and the formation of a continuous polycrystalline coating were observed. Moreover, upon reaching the critical layer thickness, a transformation in the type of internal mechanical stress of the structure was identified, shifting from negative compressive to positive tensile stress, associated with an increased influence of the substrate on the structure of the titanium dioxide coating. It was observed that an increase in the thickness of the coating, at a number of cycles exceeding 1000, resulted in the stabilization of the internal mechanical stress and the cessation of the inhibition process.