Tight-binding model and electronic structure of tetrahedral zirconium silicate

We present a tight-binding model of Zr silicate in the limit that every Zr and Si atom is bonded to four O atoms. We view the material as being compos ed of small, relatively uncoupled collections of atoms, called bonding unit s, with stoichiometries SiSiO4 and ZrSiO4. The SiSiO4 bonding unit constitu tes a model for pure SiO2, and the ZrSiO4 bonding unit represents the funda mental element distinguishing tetrahedral Zr silicate from pure SiO2. In th e first part of this article we look at the electronic structure of "ideal" bonding units in which the O atoms are arranged in a perfect tetrahedron a nd the M-O-Si angle is 180 degrees (M=Si or Zr). We find the valence levels of both bonding units to be dominated by O p states, and the lowest conduc tion levels of the ZrSiO4 bonding unit to derive primarily from Zr d states , whose energy depends sensitively upon the charge transfer to the O atoms. The energy gap of the ideal ZrSiO4 bonding unit is found to be 5.9 eV, com pared to 8.0 eV for the SiSiO4 bonding unit. Finally, for the ZrSiO4 bondin g unit, we present a simplification which allows the energy levels of the Z rSiO4 bonding unit to be obtained approximately in terms of decoupled Zr-O and Si-O interactions. In the next part of the article we investigate how b ond angle and bond length distortions affect the electronic structure of th e ZrSiO4 bonding unit. In particular, we note that significant distortions of the Zr-O-Si angle could produce Zr-based localized states that could act as traps for electrons tunneling through the material. In the last part of the article we discuss the basic principles governing band lineups for Si/ silicate interfaces constructed by substituting Zr atoms for Si atoms on th e SiO2 side of crystalline Si/SiO2 interfaces. We calculate the band lineup s as a function of Zr concentration for one particular interface. (C) 2001 American Institute of Physics.