Ballistic electron and photocurrent transport in Au/organic/Si(001) diodes with PDI8-CN2 interlayers

The authors use ballistic electron emission microscopy (BEEM) to probe hot-electron and photocurrent transport in Au/organic/n-Si(001) diodes incorporating the n-type perylene diimide semiconductor PDI8-CN2. For the case of an ultrathin organic interlayer, hot-electron injection is weak and can be detected only at randomly distributed nanosized domains, where BEEM provides electronic barrier heights of ~0.67 and ~0.94 eV, respectively. No ballistic transport is detected for devices with a 10 nm-thick interlayer. Regardless of the organic layer thickness, BEEM reveals laterally uniform contributions due to scanning tunneling microscopy-induced photocurrent (STM-PC), with a characteristic energy onset at ~1.2 eV and a broad intensity peak in the 2-4 eV range. The authors give insight on such spectroscopic features by examination of temperature-dependent spectra and of literature data. This study shows that PDI8-CN2 limits the penetration of Au toward Si, likely due to stiff intermolecular interactions and reactivity of the cyano groups. Moreover, ballistic transmittance is remarkably suppressed and photocurrent transport takes place via defects or recombination centers. Our analysis of electronic and STM-PC fingerprints appears useful for the characterization of several organic-on-inorganic interfaces of interest for heterostructures and devices.