Au/Ag and Au/Pd molecular contacts to GaAs

The hot electron transport through Au/Pd or Au/Ag/molecular layer/n-GaAs diodes has been studied by using ballistic electron emission microscopy (BEEM). The monolayers consisted of either octanedithiol (HS-(CH2)(8)-SH) or hexadecanethiol (HS-(CH2)(15)-CH3) linear alkane chains and were deposited from ethanol solutions onto molecular beam epitaxially grown GaAs (001) surfaces. The metals were electron-beam evaporated onto the molecular layer. Similar to previous results for Au diodes, Au/Pd metallization onto an octanedithiol monolayer resulted in a large reduction in the BEEM signal, compared to Au and Au/Pd reference diodes (without monolayers). However, unlike Au/octanedithiol diodes, the BEEM signal was stronger and the threshold was found to be 1.2 eV instead of 1.4 eV. We attribute this to ballistic electron transmission rather than to the photonic signal that we previously reported for Au/octanedithiol. In comparison, the Au/Pd/hexadecanethiol diodes showed little change from the reference Au/Pd diodes, suggesting monolayer disruption by interdiffusion of Pd or Au through the molecules to the interface. Ag/GaAs or Au/Ag/GaAs diodes, with or without a molecular layer, did not result in reliable tunneling, likely the result of a tip-induced reaction. The octanedithiol layer on GaAs provides a free thiol group for bonding with the top metal contact, minimizing Au or Pd penetration and the displacement of the molecule-GaAs thiol bond but not completely eliminating these effects in the case of Pd. (C) 2008 American Vacuum Society.