Optical spectra and interfacial band offsets of pulse-laser-deposited metal-oxides: SnO2, TiO2, and ZnO

Abstract

Transparent conducting oxides are electrically conductive materials with high optical transmittance in the visible region of the spectrum and are useful in a wide range of applications. In this study, the optical spectra of a set of single-phase transparent conducting oxides TiO2, ZnO, and SnO2 grown by pulse laser deposition are measured by vacuum ultraviolet spectroscopic ellipsometry and the optical bandgaps are determined to be 3.30 ± 0.05 eV, 3.13 ± 0.05 eV, and 3.95 ± 0.05 eV, respectively. Differences between these values and previous measurements are discussed. SnO2 and ZnO optical responses at the bandgap reveal that they are a direct bandgap, while TiO2 appears to show an indirect type. For the interfacial electronic characteristics, the internal photoemission measurement shows that the electronic barriers of these naturally n-type-doped metal oxides adjacent to an Al2O3 layer originate from the Fermi level in their conduction bands. The band offset determination shows that the barrier heights are similar and have a small internal field dependence. The work functions are then estimated from the measured barrier heights.