Ballistic electron and photocurrent transport in Au-molecular layer-GaAs diodes

Abstract

We present a study on hot electron transport through Au∕molecule∕n-GaAs(001) diodes via ballistic electron emission microcopy (BEEM). The molecules in the structure form a monolayer of either octanedithiol [HS–(CH2)8–SH] or hexadecanethiol [HS–(CH2)15–CH3]. For the dithiol case, the presence of the molecular interlayer leads to undetectable BEEM transmission. Whereas a small photoinduced collector current is detected at random locations at a forward (reverse) scanning tunneling microscopy (STM) tip voltage of −1.43±0.01V (+1.50±0.02V). In comparison, with monothiol diodes, or diodes where the molecules are sandwiched between two Au films (Au∕molecule∕Au∕GaAs), the BEEM transmission remains a significant fraction of the reference diode signal (30%–80%) with a slight increase in the ballistic transport threshold voltage (−1.0to−1.1V) from that of the reference Au∕GaAs diodes (−0.89V). Auger depth profiling and cross-sectional transmission electron microscopy show that Au-molecule intermixing occurs in Au/hexadecanethiol/GaAs but not in Au/octanedithiol/GaAs diodes. The suppression of BEEM signal and the detection of STM-induced photocurrent in the Au/octanedithiol/GaAs case are consistent with an insulating monolayer containing pinholes or recombination centers with densities of 1 every 25×25nm2 or ∼2000μm−2.