Ballistic electron emission microscopy and internal photoemission study on metal bi-layer/oxide/silicon, high-k oxide/silicon, and 'end-on' metal contacts to vertical silicon nanowires.

摘要

In this thesis, three separate experiments involving electronic materials are described. Nanometer-spatial resolution ballistic electron emission microscopy (BEEM) is used to study the inhomogeneity of metal-bialyer/SiO2 interface, and small size and restricted geometry effect of "end-on" metal contacts to vertical Si NWs embedded in spin-on glass. BEEM and internal photoemission (Int-PE) are also combined for the first time to study conduction band and valence band offsets between Si and promising high-k (high dielectric constant) oxides: Sc2O3 and Lu 2O3.;In the first experiment, a comparison of the dependence on gate voltage of the average energy barrier measured by BEEM of a metal bi-layer Pt/Al/SiO 2/Si sample and a Pt/SiO2/Si sample suggests that the metal/oxide interface of the Pt/Al/SiO2/Si sample is laterally inhomogeneous at nm length scales. However, BEEM images of the bi-layer sample do not show significantly larger lateral variations than observed on a (uniform) Pt/SiO 2/Si sample, indicating that any inhomogeneous "patches" of lower energy barrier height have size smaller than the lateral resolution of BEEM, estimated for these samples to be ~10 nm. Finite element electrostatic simulations of an assumed inhomogeneous interface with nm size patches of different effective work function can fit the experimental data of the bi-layer sample much better than an assumed homogenous interface, indicating that bi-layer film is in fact laterally inhomogeneous at the nm scale.;Int-PE measurements on 20 nm-thick epitaxial Sc2O3 and Lu2O3 film on Si (111) show the existence of a lower "tail state" conduction band (CB) extending ~1 eV below the upper CB (similar to that reported for amorphous Sc2O3 and Lu2O3 films), indicating that these states are not simply due to disorder in amorphous films. BEEM measurements on epitaxial Sc 2O3/Si also show that this lower CB supports elastic hot-electron transport even against an applied electric field, indicating transport via extended rather than localized states.;BEEM measurements on "end-on" Au contacts to vertical Si NWs show strong suppression of hot-electron injection at higher injected current in the NWs (produced either by increasing the tip voltage or the tunneling current) comparing to a regular Au/Si junction, suggesting that this current suppression is due to a steady-state charge build-up in the NW that increase as more current is injected into the NW. The BEEM current suppression of most NWs was also found to increase strongly as the temperature was decreased, indicating more charge build-up at lower temperature. Time-dependent current suppression due to changing steady-state charge build-up in the NWs was observed when the tunnel current was abruptly increased and decreased, directly supporting a model in which the BEEM current suppression is due to (temperature-dependent) steady-state charge build up. Those electrons might be trapped at the Si NW/SiO 2 interface close to the metal/Si NW contact, which is consistent with finite element electrostatic simulations. Our BEEM measurements also show that the local Schottky barrier height (SBH) at the edge of two separate NWs is ~20 meV lower than at the NWs center. Finite element simulation suggests that the lower SBH at the contact edge might be due to the NW/oxide interface states charge together with the geometry enhanced electric field effect (the NWs protruding ~8 nm out of the spin-on glass).