Modifying electronic properties at the silicon–molecule interface using atomic tethers

The electronic properties at the semiconductor–molecule interface can be altered by changing the nature of covalent attachment. We examine the change in work function of the silicon surface after formation of Si–O–C, Si–C–C, and Si–S–C bonded alkyl monolayers and separate charge transfer and dipolar contributions. The chemical state, monolayer structure, and electronic properties of aliphatic monolayers with oxygen, carbon, and sulfur covalent linkages to the Si(1 1 1) surface were investigated with contact angle wetting, spectroscopic ellipsometry, infrared vibrational spectroscopy, X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy. Vibrational spectra indicate aliphatic films tethered to Si with few gauche defects in agreement with hydrophobic contact angles and ellipsometric thickness measurements. Core level electronic spectra taken as a function of semiconductor doping reveal shifts in binding energy attributed to molecular bonding. Valence band spectra reveal the work function of the molecule–Si composite as a function of semiconductor doping and atomic tether. By combining valence band spectra with core level spectra, the electronic properties of the molecule–Si system can be understood. In particular, the relative contribution of charge transfer due to surface band bending and the polarization due to molecular dipoles were determined. The O, C, and S atomic tethers induce differing amounts of band bending and interface dipoles which can be utilized to engineer the electronic properties of molecule–semiconductor junctions.