Germanium metal-insulator-semiconductor capacitors with La2O3 dielectrics deposited at high temperature or subjected to post deposition annealing show good electrical characteristics, especially low density of interface states Dit in the 1011eV−1cm−2 range, which is an indication of good passivating properties. However, the κ value is estimated to be only about 9, while there is no evidence for an interfacial layer. This is explained in terms of a spontaneous and strong reaction between La2O3 and Ge substrate to form a low κ and leaky La–Ge–O germanate over the entire film thickness, which, however, raises concerns about gate scalability. Combining a thin (∼1nm) La2O3 layer with thicker HfO2 degrades the electrical characteristics, including Dit, but improves gate leakage and equivalent oxide thickness, indicating a better potential for scaling. Identifying suitable gate dielectric stack which combines good passivating/interfacial properties with good scalability remains a challenge.

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Electrical data on ZrO2/GeO2 stacks prepared by atomic oxygen beam deposition on Ge at 225 °C reveal a relatively weak dependence of the stack equivalent oxide thickness upon the ZrO2 thickness. This trend points to a very high zirconia dielectric permittivity (k) value which is estimated to be around 44. This is indicative of zirconia crystallization into a tetragonal phase which is also supported by x-ray diffraction data. X-ray photoelectron spectroscopy analysis is in line with the assumption that due to a finite GeO2 decomposition, Ge is incorporated into the growing ZrO2, thus, stabilizing the high-k tetragonal phase.

The oxidation of a Ge surface by molecular oxygen in the presence of ultrathin La, Al, and Hf layers was examined by in situ x-ray photoelectron spectroscopy. Upon exposure to O2, clean bare Ge and Hf-covered or Al-covered Ge surfaces show no Ge–O bond formation. On the contrary, a La-covered Ge surface strongly reacts with O2 forming a stable germanate LaGeOx compound. This has a beneficial side effect for the interface because the formation of volatile GeO is suppressed, resulting in the good passivating properties of LaGeOx. The photoemission results are correlated with the oxygen density differences in the corresponding oxides.

Thin La2O3 (LaGeOx) passivating layers combined with ZrO2 caps form a chemically stable bilayer gate stack on Ge with good electrical properties. The most important observation is that a higher-κ tetragonal zirconia phase coexists with the most commonly observed monoclinic, increasing the κ value of the oxide to about 32, thus benefiting the measured stack equivalent oxide thickness. This indicates that the ZrO2/La2O3 combination could be a promising candidate gate stack for Ge metal-oxide-semiconductor devices in terms of scalability.