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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[3] K. Kita et al., Jpn. J. Appl. Phys. 47, 2349 (2008).

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[5] T. Hosoi et al., Appl. Phys. Lett. 94, 202112 (2009).

[6] Y. Oniki et al., Appl. Phys. Express 4, 081101 (2011).

[7] S. Ogawa et al., Appl. Phys. Lett. 99, 142101 (2011).

[8] M. Salmeron et al., Surf. Sci. Rep. 63, 169 (2008).

[9] M.E. Grass et al., Rev. Sci. Instrum. 81, 053106 (2010).

[10] A. Mura et al., J. Phys. Chem. C 117, 165 (2013).

[11] K. Arima et al., ECS Transactions 64, 8 (Semiconductors, Dielectrics, and Metals for Nanoelectronics 12), 77 (2014).

[12] D. Mori et al., J. Appl. Phys. 120, 095306 (2016).