Nanometer-scale properties of metal/oxide interfaces and ``end-on'' metal contacts to Si nanowires studied by ballistic electron emission microscopy (BEEM)

BEEM is a hot-electron (HE) technique based on scanning tunneling microscopy that can probe buried metal/semiconductor and metal/dielectric interfaces with nm-scale spatial resolution and energy resolution of a few meV. BEEM is a three-terminal technique, so the HE energy and interface electric field can be varied independently. I will discuss two studies of interest for future transistor technologies. The first concerns the band structure and alignments in a 20 nm-thick film of the high-k dielectric material Sc$_{2}$O$_{3}$ grown epitaxially on Si(111). Sc$_{2}$O$_{3}$ and related rare-earth/transition metal oxide films on Si were found to have similar band alignments and bandgap, and also ``tailing'' conduction band (CB) states extending $\sim $1 eV below the primary CB. We combined BEEM with internal photoemission to measure the band alignment and to study electron transport through these ``tail'' states.\footnote{W. Cai, S. E. Stone, J. P. Pelz, L. F. Edge, and D. G. Schlom, Appl. Phys. Lett \textbf{91}, 042901 (2007).} Surprisingly, these tail states were found to form a robust band of extended states that supports elastic hot-electron transport even \textit{against} an applied electric field. The second study concerns HE injection and transport through ``end-on'' metal contacts made to $\sim $100 nm diameter vertical Si nanowires (NWs) embedded in a SiO$_{2}$ dielectric. At low HE flux, We observed \textit{lateral variations} of the local Schottky Barrier Height (SBH) across individual end-on Au Schottky contacts, with the SBH at the contact edge found to be $\sim $25 meV lower than at the contact center. Finite-element electrostatic simulations suggest that this is due to a larger interface electric field at the contact edge due to positively charged Si/native-oxide interface states near the Au/NW contact, with this (equilibrium) interface state charge induced by local band bending due to the high work function Au contact. We also observed a strong \textit{suppression} of the hot-electron transmission efficiency at larger HE flux, likely due to (non-equilibrium) \textit{steady-state negative charge accumulation} in metastable traps at the Si/oxide interface located near the injecting metal contact. Ongoing BEEM measurements of metal contacts to SrTiO$_{3}$ substrates and films may also be discussed.\\[4pt] In collaboration with W. Cai, Y. Che, L. F. Edge, D. G. Schlom, E. R. Hemesath, and L. J. Lauhon.