Electronic States of Hafnium and Vanadium oxide in Silicon Gate Stack Structure

Vanadium oxide (VO$_{2})$ is a narrow band gap material with a metal-insulator transition (MIT) at less than 100C. Hafnium oxide (HfO$_{2})$ is currently the preferred high-k material for gate dielectrics. To utilize VO$_{2}$ in a charge storage device, it is necessary to understand the band relationships between VO$_{2}$, HfO$_{2}$, and Si substrate. In this study, a 2nm thick VO$_{2}$ layer is embedded in a dielectric stack structure between an oxidized n-type Si(100) surface and a 2nm HfO$_{2}$ layer. The in situ experiments are carried out in an UHV multi-chamber system. After each growth step, the surface is characterized using XPS and UPS. After the initial plasma cleaning and oxidation treatment the Si substrate displayed essentially flat bands at the surface. After deposition of the VO$_{2}$ layer, the Si 2p peak shifted to lower binding energy, and the Si 2p associated with the SiO$_{2}$ layer also was shifted, indicating an internal field in the SiO$_{2}$. The VO$_{2}$ valence band maximum (VBM) was identified at 0.6 eV below the Fermi level (E$_{F})$. This ultra thin VO$_{2}$ exhibits the metal-insulator transition at a temperature higher than thicker films. As a comparison, a 100nm thick film of VO$_{2}$ on Si showed a MIT at 60C. After the HfO$_{2}$ deposition, the Si 2p substrate feature returned to the initial value indicating a return to flat band conditions. The UPS indicated the VBM of HfO$_{2}$ at 4.0 eV below E$_{F}$. This work is supported by the NSF (DMR-0805353).