Photoconductivity of Hf-based binary metal oxides

In order to explore the possibility of bandgap engineering in binary oxide insulators we studied photoconductivity of nanometer-thin Hf oxide layers containing different fractions of cations of another sort (Si, Al, Sr, or Ce) deposited on (1 0 0)Si. The smallest bandgaps of the Hf:AI and Hf:Ce oxides are close to the values found in elemental Al2O3 (6-6.2 eV) and HfO2 (5.6 eV), respectively, and show little sensitivity to the concentration of Al or Ce. This result suggests that the oxide sub-network with the largest bandgap preserves its gap energy, while development of a narrower gap is prevented, likely, by dilution of the second cation sub-network. In Hf:Si oxide samples photoconductivity thresholds of 5.6-5.9 eV, corresponding to the bandgap of HfO2, were observed for all studied Si concentrations, suggesting phase separation to occur during deposition. Photoconductivity of SrHfO3 exhibits two thresholds, at 4.4 and 5.7 eV, which are close to the bandgaps of elemental SrO2 and HfO2, respectively. These gap values indicate the phase separation also to occur in this binary oxide. Through this work photoconductivity is demonstrated to be a feasible method to trace phase separation in binary oxides, even in nanometer-thin layers. (C) 2008 Elsevier B.V. All rights reserved