Oxidation of the Pt/HfO2 interface:: The role of the oxygen chemical potential

Using first-principles calculations we investigated the oxidation properties of model Pt/HfO2 interfaces as a function of oxygen partial pressure. A wide range of interfacial oxygen concentrations were explored, varying from an oxygen-free interface to the case of 1 oxygen ML separating the Pt(111) slab from the first Hf plane in the monoclinic HfO2(001) slab. In all cases the interfaces were optimized using ab initio molecular dynamics. It was found that 1 ML of oxygen at the Pt/HfO2 interface is only possible for chemical potentials equivalent to an oxygen pressure of tens of bars. With respect to silicon oxidation only the oxygen-free interface is stable. Depending on the anneal conditions, intermediate concentrations of similar to 0.25 or similar to 0.75 ML of oxygen at the interface are possible, while the concentration of 0.5 ML is only stable over a narrow range of conditions. The band offset and work of separation were calculated for Pt/HfO2 interfaces as a function of the oxygen concentration at the interface. The valence band offset drops sharply with increasing oxygen chemical potential, from similar to 3.0 to similar to 1.0 eV. The same trend is observed for the work of separation, which decreases from similar to 6 J/m(2) for an oxygen-free interface to similar to 1 J/m(2) for one oxygen monolayer. These findings may shed new light into recent experimental data reporting exceedingly small values for the effective work function of Pt on HfO2 and its dependence on the oxygen partial pressure during high temperature annealing. (c) 2007 American Institute of Physics.