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:Al 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.