Photoconductivity of Hf-based binary metal oxide systems

To explore the possibility of bandgap engineering in binary systems of oxide insulators we studied photoconductivity of nanometer-thin Hf oxide layers containing different concentrations of cations of different sorts (Si, Al, Sr, or Ce) deposited on (100)Si. The lowest bandgap of the Hf:Al oxide is close to the value 6-6.2 eV of elemental amorphous Al2 O3 and insensitive to the Al content for concentrations of Al exceeding 36%. This result suggests that the Al oxide subnetwork with the largest bandgap preserves this energy width while development of a narrower gap of HfO2 is prevented possibly by dilution of the second cation subnetwork. When Ce is admixed to HfO2 an intermediate bandgap value (between the CeO2 and HfO2 bandgap widths) of 5.3+0.1 eV is observed for all concentrations of Ce, suggesting that the electronic structure of both elemental oxide subnetworks which form the binary metal oxide system, is affected. In Hf:Si oxide samples photoconductivity thresholds of 5.6-5.9 eV corresponding to the bandgap of HfO2 are observed for all studied Si concentrations, suggesting phase separation to occur. The photoconductivity of SrHfO3 exhibits two thresholds at 4.4 and 5.7 eV, which are close to the bandgaps of elemental SrO and HfO2, respectively, indicating, again, phase separation. Through this work we have illustrated photoconductivity as a feasible method to trace phase separation in nanometer-thin layers of binary systems of metal oxides.