Energy level alignment at the Si(111)/RCA–SiO2/copper(II) phthalocyanine ultra-thin film interface

Abstract

Abstract The photoemission experimental techniques (i.e. ultraviolet photoelectron spectroscopy—UPS and X-ray photoelectron spectroscopy—XPS) were used to investigate the charge–rearrangement–related phenomena occuring at organic–inorganic semiconductor interface. Examined samples were copper phthalocyanine (CuPc) ultra-thin (up to 16 nm) layers deposited onto oxidized silicon Si(1 1 1) of n- and p-type of conductivity. The 1.3-nm-thick silicon oxide was prepared by means of RCA wet cleaning procedure. The analysis of the photoemission data (mainly UPS) suggested the existance of the polarization layer within first 3 nm of CuPc layer thickness. Basing on the UPS and XPS results the energy level diagrams of examined structures have been constructed. In present paper it is suggested that the existance of the polarization layer could be assigned to the disordered adsorption and continous molecular reorientation of the CuPc molecules during the interface formation process. In the terms of the lack of the charge transfer via substrate/organic overlayer interface and disordered adsorption the fluctuations of CuPc electronic parameters were detected. Moreover the ionization energy and the work function parameters of final CuPc layer were affected. The values were more consistent with those obtained for much thicker (over 500 nm) CuPc layers. Performed studies showed that contrary to CuPc layers deposited on native substrates (where the charge transfer via tunnelable oxide – determined as dipole effect – has been detected), the thicker RCA-prepared oxide seems to be non-tunnelable hence the possibility for Si(1 1 1)/RCA–SiO2/CuPc structures in organic transistor application remains open; however the substrate surface should be tailored prior the deposition in order to avoid disordered adsorption.