Quantitative analysis of chemical interaction and doping of the Si(111) native oxide surface with tetrafluorotetracyanoquinodimethane

Abstract

The charge-transfer states and the carrier concentration of the native oxide Si(111) surface adsorbed with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) were investigated by X-ray photoelectron spectroscopy (XPS) and independently driven four-probe electrical conductivity measurements. The XPS results show that F4-TCNQ molecules are involved in charge transfer with the SiO2/Si(111) surface. The Si 2p XPS spectra and the surface photovoltage shift provide the evidences of (i) change in the oxidation states at the SiO2-Si(111) interface region and (ii) formation of a p-type space charge layer (SCL) with a hole concentration of 1.7 × 1010 cm−2, respectively. The four-probe I–V measurements also support the formation of the p-type SCL, and the estimated hole concentration of 2.0 × 1010 cm−2 agrees well with the XPS results. The estimated SCL hole concentrations were much smaller than the excess charge density in the F4-TCNQ layer, of the order of 1013 cm−2, suggesting that most of charges were localized as the oxidation states at the SiO2-Si(111) interface region. The present quantitative methods ensure precise determination of the doping concentration near the surface region.