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 ${\text{Al}}_2{\text{O}}_3$ 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 ${\text{HfO}}_2$ is prevented possibly by dilution of the second cation subnetwork. When Ce is admixed to ${\text{HfO}}_2$ an intermediate bandgap value (between the ${\text{CeO}}_2$ and ${\text{HfO}}_2$ bandgap widths) of $5.3+0.1\text{eV}$ 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 ${\text{HfO}}_2$ are observed for all studied Si concentrations, suggesting phase separation to occur. The photoconductivity of ${\text{SrHfO}}_3$ exhibits two thresholds at 4.4 and 5.7 eV, which are close to the bandgaps of elemental SrO and ${\text{HfO}}_2$ , 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.