High-$\kappa$ dielectrics on Ge have recently attracted much attention as a potential candidate to replace planar silicon transistors for sub-32-nm generations. However, the instability of the high-$\kappa /\text{Ge}$ interface, especially the desorption of germanium monoxide (GeO), hampers the development of Ge-based devices. Therefore, the typical ${\text{GeO}}_2/\text{Ge}$ structure was chosen to investigate GeO desorption. In this contribution, we describe the desorption kinetics of GeO, including $\text{Ge}/{\text{GeO}}_2$ interface reaction, the diffusion process during GeO desorption, the desorption activation energy of GeO, the different mechanisms of GeO desorption, and the active oxidation of Ge. Through annealing ${\text{GeO}}_2/\text{Ge}$ in an ultrahigh vacuum (UHV), direct evidence for the consumption of Ge substrate has been shown by atomic force microscopy (AFM) measurements of the consumption depth. By using thermal desorption spectroscopy (TDS) measurements and studying oxygen-18 isotope tracing, we have clarified that the GeO desorption is not caused by the GeO direct-diffusion process. Isothermal TDS measurements and morphology investigation have revealed a transition from a uniform desorption mechanism to a nonuniform one as ${\text{GeO}}_2$ thickness was reduced. On the basis of the experimental results in this study, a GeO desorption model has been developed. For GeO desorption, about 2 eV of activation energy has been obtained from kinetic calculation on the basis of TDS analysis. Different from the UHV annealing, the active oxidation of Ge has been observed by AFM measurements under low oxygen partial pressures.

We have systematically investigated Ge interface passivation methods, and the highest electron (1920 cm²/Vs) and hole mobility (725 cm²/Vs) have been demonstrated by dramatic reduction of D_it through the collaboration of self-passivation and valency passivation. In Si passivation, it is found that Si contributes to the upper half (worse) and lower one (better) in the bandgap differently. This study strongly suggests us that high performance Ge CMOS is really feasible.

GeO disproportionation into GeO₂ and Ge is studied through x-ray photoelectron spectroscopy. Direct evidence for the reaction 2GeO → GeO₂ + Ge after annealing in ultra-high vacuum is presented. Activation energy for GeO disproportionation is found to be about 0.7 ± 0.2 eV through kinetic and thermodynamic calculations. A kinetic model of GeO disproportionation is established by considering oxygen transfer in the GeO network. The relationship between GeO disproportionation and GeO desorption induced by GeO₂/Ge interfacial reaction is discussed, and the apparent contradiction between GeO desorption via interfacial redox reaction and GeO disproportionation into Ge and GeO₂ is explained by considering the oxygen vacancy.

This paper overviews the present status of Ge MOSFET technology, particularly focusing on n-FETs in terms of materials science of GeO₂/Ge gate stacks and device physics of inversion layer mobility, and then discusses future prospects and fundamental challenges for further miniaturization from the viewpoint of Ge/high-k gate stacks and ET-GeOI MOSFETs.

GeO desorption from a GeO₂/Ge stack is a critical concern in Ge metal oxide semiconductor field effect transistors (MOSFETs). In this contribution, we focus on a uniform-desorption region and unveil the GeO desorption mechanism from a GeO₂/Ge stack by ⁷³Ge and ¹⁸O isotope tracing in thermal desorption spectroscopy (TDS) analysis, in which the Ge and O diffusion kinetics in GeO₂ and the interfacial reaction kinetics have been investigated. Through ⁷³Ge isotope tracing, we have clarified that Ge in the desorbed GeO dominantly comes from the GeO₂ surface. Moreover, the self-diffusivity of oxygen was evaluated to be much larger than Ge in GeO₂. Furthermore, owing to the difference among GeO desorptions from GeO₂/Ge stacks with various substrate orientations, the reaction at the GeO₂/Ge interface was attributed to the redox reaction kinetics. On the basis of our experimental findings, we have proposed an oxygen vacancy diffusion model of the GeO desorption mechanism.