Evolution of Defect-Associated Subband Energy States in Nanocrystalline TiO₂ Films on Si and Ge Substrates

Abstract

We identified the electronic states in the conduction- and valence-band edges associated with intrinsic defects in nanocrystalline TiO2 layers on Si and Ge substrates. This was accomplished through spectroscopic study with soft X-ray photoemission spectroscopy and visible-ultraviolet spectroscopic ellipsometry. The interpretation of the spectra based on molecular orbital (MO) theory well explains the origin of empty and occupied states of band edge in TiO2 with the correct assignment of MO states. The evolution of these band-edge states under thermal nanocrystalline growth and interfacial chemical mixing as a function of type of substrate was investigated, taking into consideration the asymmetric local bonding distortion and fast atomic diffusion at the grain boundary. The engineering solution to utilize TiO2 as a gate dielectric on Ge substrate is demonstrated by implanting SiON/Si interfacial layer. This study suggests that the control of electronically active defect density and energy level in nanoscale TiO2 thin films is strongly affected by thermal grain expansion and interfacial chemistry, depending on the semiconductor substrates used.