Origin of flatband voltage shift and unusual minority carrier generation in thermally grown GeO2/Ge metal-oxide-semiconductor devices

Abstract

Improvement in electrical properties of thermally grown GeO2/Ge metal-oxide-semiconductor (MOS) capacitors, such as significantly reduced flatband voltage (VFB) shift, small hysteresis, and minimized minority carrier response in capacitance-voltage (C-V) characteristics, has been demonstrated by in situ low temperature vacuum annealing prior to gate electrode deposition. Thermal desorption analysis has revealed that not only water but also hydrocarbons are easily infiltrated into GeO2 layers during air exposure and desorbed at around 300 °C, indicating that organic molecules within GeO2/Ge MOS structures are possible origins of electrical defects. The inversion capacitance, indicative of minority carrier generation, increases with air exposure time for Au/GeO2/Ge MOS capacitors, while maintaining an interface state density (Dit) of about a few 1011 cm−2 eV−1. Unusual increase in inversion capacitance was found to be suppressed by Al2O3 capping (Au/Al2O3/GeO2/Ge structures). This suggests that electrical defects induced outside the Au electrode by infiltrated molecules may enhance the minority carrier generation, and thus acting as a minority carrier source just like MOS field-effect transistors.

Comparative study of GeO2/Ge and SiO2/Si structures on anomalous charging of oxide films upon water adsorption revealed by ambient-pressure X-ray photoelectron spectroscopy

“…42 More recently, the generation of positive charges in GeO 2 films after exposure to air has been confirmed by a negative shift of V FB in the capacitance-voltage (C-V) characteristics of metal/GeO 2 /Ge diodes. [12][13][14] Because the C-V characteristics can be used to sense electrical properties, especially at an oxide/semiconductor interface, these results indicate that water molecules in air infiltrate the GeO 2 film, reaching the GeO 2 /Ge interface, where they create positive charges. These reports agree with our data in Figs.…”

“…Several groups have so far investigated this issue. Hosoi et al 12 and Oniki et al 13,14 reported the electrical characteristics of metal/GeO 2 /Ge structures and revealed a negative shift of the flat-band voltage (V FB ) as well as anomalous hysteresis and a minority career response upon exposure to air. 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 GeO 2 film.…”

“…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 GeO 2 film. [12][13][14][15] X-ray photoelectron spectroscopy (XPS) is a powerful tool for identifying various properties of oxide/semiconductor structures. Analysis of the chemical shift of oxidation states is especially important for determining the oxide thickness, suboxide structure, and band offset.…”

Insight into unusual impurity absorbability of GeO2 in GeO2/Ge stacks

“…Recently, theoretical studies showed that a low interface defect (Ge dangling bond) was obtained for the GeO 2 /Ge interface due to its viscoelastic properties, 5,6 and some experimental studies demonstrated good electrical properties of the Ge-MOS. [7][8][9][10] For example, Hosoi et al reported that the electrical properties of Ge-MOS capacitors could be improved by in situ vacuum annealing at 300 C prior to metal gate electrode deposition, and an increase in the inversion capacitance, depending on its air exposure time, could be suppressed by an Al 2 O 3 layer capped on the GeO 2 surface. 8 Meanwhile, Kutsuki et al reported that surface nitridation on ultrathin GeO 2 was an effective way to improve the electrical properties (drastic suppression of leakage current, etc.)…”

“…[7][8][9][10] For example, Hosoi et al reported that the electrical properties of Ge-MOS capacitors could be improved by in situ vacuum annealing at 300 C prior to metal gate electrode deposition, and an increase in the inversion capacitance, depending on its air exposure time, could be suppressed by an Al 2 O 3 layer capped on the GeO 2 surface. 8 Meanwhile, Kutsuki et al reported that surface nitridation on ultrathin GeO 2 was an effective way to improve the electrical properties (drastic suppression of leakage current, etc.) compared to the GeO 2 /Ge stacks, 9 inferring that the nitride (Ge 3 N 4 ) film on GeO 2 works as a barrier layer against the chemical reaction of GeO 2 with air.…”