Ambient-Pressure XPS Study of GeO2/Ge(100) and SiO2/Si(100) at Controlled Relative Humidity

Germanium (Ge) is a promising channel material for electronic devices in the next generation because it has higher mobilities for both holes and electrons than those of Si. Germanium oxide (GeO₂) is one of the key materials in Ge-based transistors. However, GeO₂ is permeable and soluble in water. This implies that GeO₂ films will react with water vapor in air. In this study, water growth on ultrathin GeO₂ films on a Ge(100) substrate and their effect on the electronic properties of GeO₂ films are investigated using in-situ X-ray photoelectron spectroscopy. In addition, the wetting property of GeO₂/Ge is compared with that of more familiar SiO₂/Si.

In-situ XPS^[1] measurements were performed at beamline 9.3.2 of the Advanced Light Source (ALS), of the Lawrence Berkeley National Laboratory.^[2]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.^[3]

[1] M. Salmeron and R. Schlögl, Surf. Sci. Rep. 63, 169-199 (2008).

[2] M.E. Grass, P.G. Karlsson, F. Aksoy, M. Lundqvist, B. Wannberg, B.S. Mun, Z. Hussain and Z. Liu, Rev. Sci. Instrum. 81, 053106 (2010).

[3] A. Mura, I. Hideshima, Z. Liu, T. Hosoi, H. Watanabe and K. Arima, J. Phys. Chem. C 117, 165-171 (2013).