Ambipolar Ballistic Electron Emission Microscopy (BEEM) Studies of Gate-field Modified Schottky Barriers(SBs).

Gate-field modified SBs are important for ``Schottky Barrier FETs'' [1], and could be used to control spin and charge injection into other semiconductor device structures. We have made the first \textit{ambipolar} BEEM measurements on Au/Si SBs that can be changed from effective $n$-type to $p$-type by applying a positive or negative back-gate bias, respectively. Samples were fabricated using SIMOX silicon-on-insulator wafers (35nm Si/150nm SiO$_{2}$/p-Si substrate), with Ti/Au and Pt pads as ohmic contacts for $n$-type and $p$-type operation, respectively. The local SB heights at 80K for electrons and holes were measured \textit{at the same location} to be $\sim $0.785eV and $\sim $0.323eV respectively, which correspond to intrinsic SBHs of $\sim $0.84eV and $\sim $0.36eV after accounting for image force lowering. These sum to 1.20eV, close to the $\sim $1.17eV Si bandgap at 80K. We will discuss on-going measurements of the dependence of the local SBH on temperature, back-gate bias, Si film thickness, and bias between the Schottky and ohmic contacts. Future work will investigate local variations of the conduction and valence bands due to local ``geometry-induced'' electric fields in nanostructured contacts. Work supported by National Science Foundation Grants No. DMR-0505165 and DMR-0805237.[1] S. Heinze, et al., Phys. Rev. Lett. 89, 106801 (2002).