Intrinsic limitations for gate stack applications of complex high-k oxides in advanced Si devices: band edge states

Valence and conduction band edge electronic states in high-k oxide dielectrics have been studied by X-ray absorption spectroscopy (XAS), ultra-violet photoemission spectroscopy (UPS), and vacuum ultra- violet spectroscopic ellipsometry (VUVSE) and photoconductivity (PC). These studies confirm results of \textit{numerous} theoretical studies which have demonstrated that valence and conduction band electronic states are comprised of transition metal/rare earth (TM/RE) atom d-states mixed with O-atom 2p states. Electronic states at the top of the valence band and bottom of the conduction band have a $\pi $-bonding symmetry, while those deeper in the valence band and higher in the conduction band have a $\sigma $-bonding symmetry. XAS studies of \textit{empty} TM/RE d-states by transitions from deep TM/RE p-states are combined with studies of conduction band edge states by transitions from the O-atom 1s state to provide qualitative and quantitative insights into electronic structure at the conduction band edge. This approach was first applied to HfO$_{2}$ and TiO$_{2}$, and then to the \textit{complex/binary oxides}: i) Zr$_{x}$Ti$_{1-x}$O$_{4}$, with x = 0.67 and 0.33, LaAlO$_{3}$, and LaScO$_{3}$. Thin films of these oxides are nano-crystalline as-deposited and/or after an anneal in an inert ambient at 500 to 1000\r{ }C. Analysis of the XAS spectra indicate that d- state degeneracies are completely removed for Hf in HfO$_{2}$, Ti in TiO$_{2}$ and the Zr titanates, La in LaAlO$_{3}$, and Sc in LaScO$_{3}$. This removal indicates a distorted local bonding arrangement for these TM/RE atoms, or equivalently \textit{Jahn-Teller term splittings }that increase the total binding energy. More importantly, the term split states identified in XAS spectra are directly correlated with d-state features at the conduction band edge by VUVSE and PC. These localized $\pi $-bonded states limit performance and reliability in scaled Si devices, and are associated with \textit{asymmetric} bias voltage dependent electron transport and trapping.