Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

TL;DR

This study demonstrates that charge transport in metal/alkyl monolayer-Si junctions is primarily governed by the LUMO level, as evidenced by spectroscopic analysis and tunneling measurements, challenging the traditional HOMO-based understanding.

Contribution

The paper provides experimental evidence that electron tunneling through alkyl monolayers is controlled by the LUMO level, not the HOMO, supported by spectroscopic and transport measurements.

Findings

Fermi level is closer to LUMO than HOMO in the junctions.

Charge transport is dominated by electron tunneling via the LUMO.

LUMO-based energy difference correlates with tunneling barrier height.

Abstract

We compare the charge transport characteristics of heavy doped p- and n-Si-alkyl chain/Hg junctions. Photoelectron spectroscopy (UPS, IPES and XPS) results for the molecule-Si band alignment at equilibrium show the Fermi level to LUMO energy difference to be much smaller than the corresponding Fermi level to HOMO one. This result supports the conclusion we reach, based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction, that for both p- and n-type junctions the energy difference between the Fermi level and LUMO, i.e., electron tunneling, controls charge transport. The Fermi level-LUMO energy difference, experimentally determined by IPES, agrees with the non-resonant tunneling barrier height deduced from the exponential length-attenuation of the current.