Ambient-Pressure XPS Study of GeO2/Ge(100) and SiO2/Si(100) at Controlled Relative Humidity
In-situ XPS measurements were performed at beamline 9.3.2 of the Advanced Light Source (ALS), of the Lawrence Berkeley National Laboratory.A differentially pumped electrostatic lens system separates the analysis chamber from a hemispherical photoelectron spectrometer. Water vapor was introduced up to a pressure of 1.0 Torr while the sample temperature was controlled from room temperature to around -5°C by a chiller. This enabled us to control relative humidity from 0 % to around 15 %.
A simple estimation from O1s and Ge3d (or Si2p) indicates that thicker water layers are formed on GeO2 than on SiO2 at RH higher than 10-4%. Namely, the water thickness reaches one monolayer on SiO2 at 10% whereas it is two or three monolayers in the case of GeO2. We also find that the energy separation between Ge4+ and Ge0+ signals in Ge3d spectra increases more drastically at RH higher than 10-4% than that between Si4+ and Si0+ signals in Si2p spectra. This means that GeO2 films are charged more positively than SiO2 above the critical RH. We present two possibilities for the positive charging of the ultrathin GeO2 films in the presence of water vapor. One simple explanation is positive charging of the oxide with adsorbed water molecules by X-ray irradiation. The other explanation is that water-related species emit electrons to the Ge bulk and positive charges are created in GeO2, which may be the cause of the reported negative shift of the flatband voltage in Ge-based MOS capacitors with air-exposed GeO2.
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