Nm-resolution studies of Au/molecular-film/GaAs junctions using ballistic electron emission microscopy (BEEM)

BEEM was used to image and quantify lateral homogeneity and energy band alignments at molecule/electrode interfaces in Au/dC-X/GaAs structures, where dC-X are dicarboxylic ligands with X= H, OCH$_{3}$, CF$_{3}$, CN, or CH$_{3}$ [1]. Transport through such junctions was proposed to be dominated by ``pinholes'' in the dC-X film, with the Au/GaAs Schottky barrier height (SBH) at pinholes modified by the surrounding molecular film dipole [1]. BEEM images of dC-CH$_{3}$ with V$_{tip }< \quad \sim $1.38 eV indeed revealed isolated 20-40 nm sized ``pinholes'' with measured local SBHs ranging from 0.90 -- 1.0 eV, consistent with the model [1]. However, between the pinholes we also observed a new conduction channel for V$_{tip }> \quad \sim $1.38 eV, possibly due to transport through the LUMO of the dC-CH$_{3}$ film itself. BEEM measurements for the other --X groups also showed non-uniform, film-dependent SBH, but for those films the pinholes were too dense or the films too transparent to resolve isolated pinholes. All dC-X films were stable under the BEEM hot-electron flux. \newline \newline [1] H. Haick \textit{et al}., Adv. Mater. \textbf{16}, 2145 (2004).