Photoconductivity of Hf-based binary metal oxide systems

摘要

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 Al_2O_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 HfO_2 is prevented possibly by dilution of the second cation subnetwork. When Ce is admixed to HfO_2 an intermediate bandgap value (between the CeO_2 and HfO_2 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 HfO_2 are observed for all studied Si concentrations, suggesting phase separation to occur. The photoconductivity of SrHfO_3 exhibits two thresholds at 4.4 and 5.7 eV, which are close to the bandgaps of elemental SrO and 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.