(Invited) Reactivity of Water Vapor with Ultrathin GeO2/Ge and SiO2/Si Structures Investigated by Near-Ambient-Pressure X-ray Photoelectron Spectroscopy
Authors: Arima Kenta , Hosoi Takuji , Watanabe Heiji , Crumlin Ethan J 
Published: February 2020

Germanium (Ge) is a promising channel material for future MOSFETs because of its higher holes and electron mobilities than those of Si.[1] Another advantage of Ge is its process compatibility with Si-based MOS technologies. Because of these advantages, various attempts have recently been made to fabricate Ge-based MOS devices. Although Ge oxide (GeO2) is regarded as a key material in Ge-based MOSFETs,[2] it decomposes at low temperatures[3] and is soluble in liquid water[4], unlike the more familiar Si oxide (SiO2). Thus it is demanded to grasp the relationship between the physical/chemical properties of GeO2 and its dielectric properties. What seems to be lacking, however, is to understand the effect of the microscopic interaction of water vapor on the quality of the GeO2/Ge structure. Several groups reported the electrical characteristics of metal/GeO2/Ge structures and revealed a negative shift of the flat-band voltage as well as anomalous hysteresis and a minority career response upon exposure to air.[5, 6] Diverse physical analyses such as by thermal desorption spectroscopy, infrared spectroscopy and secondary ion mass spectrometry revealed the origin to be infiltration of adsorbed water or organic molecules into the GeO2 film.[7] However, the interaction between water molecules and GeO2 layers is still unclear.
We carried out near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) measurements with synchrotron radiated light[8, 9] to investigate the reactivity of water molecules in the gas phase with ultrathin GeO2 films on Ge substrates. And the results were compared with those for SiO2 films on Si. The GeO2 and SiO2 films were formed by dry oxidation in O2 ambient at 550°C and 1000°C, respectively. After transferred to a NAP-XPS chamber, the samples were annealed at 300°C for 30 min in vacuum for surface cleaning. Then we introduced water vapor into the chamber at a pressure of up to 1.0 Torr. We also controlled the sample temperature from room temperature to approximately -4°C using a chiller. This enabled us to obtain NAP-XPS spectra (ex. O1s, Ge3d, Si2p) at any relative humidity (RH) values of up to ~15%. We show that the GeO2/Ge structures attract more water molecules than the SiO2/Si structures at RH above ~10-4%. This is probably because water molecules infiltrate the GeO2 films to form hydroxyls. Then we reveal positive charging of the water-adsorbed SiO2 films by their interaction with X-rays. For the water-adsorbed GeO2 films, we find greater positive charging of the films. Together with the X-ray induced effect, this seems to be caused by a factor unrelated to X-ray irradiation. We consider this non-X-ray effect to be the emission of electrons from adsorbed water species in the GeO2 films to the Ge bulk, resulting in the formation of immobile cations in GeO2.[10-12]

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