Electrical conduction and band offsets in Si/Hf_(x)Ti_(1-x)O_(2)/metal structures

摘要

The electron energy band alignment in the Si/Hf_(x)Ti_(1-x)O_(2)/metal (Au,Al) structures is determined as a function of oxide composition using internal photoemission of electrons and photoconductivity measurements. For x≤0.5 the electron excitations with thresholds corresponding to the band-gap width of amorphous TiO_(2) (4.4 eV) and HfO_(2) (5.6 and 5.9 eV) are observed at the same time, suggesting formation of TiO_(2)- and HfO_(2)-like subnetworks. With respect to the Fermi level of Au the conduction band of TiO_(2) appears to be 1.4 eV below the conduction band of HfO_(2) which indicates that the valence bands of the two oxides are nearly aligned. This significant downshift of the conduction band due to Ti incorporation leads to low barriers for electrons at the interfaces of Hf_(x)Ti_(1-x)O_(2) with Si and Al (～1 eV or less) strongly impairing insulating properties of the oxide. Crystallization of TiO_(2) upon high-temperature annealing further enhances leakage currents because of a significantly lower band-gap width of crystallized TiO_(2) (3.1-3.4 eV).