Interfacial-layers-free Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$/Ge MOS Diodes

High $\kappa $ dielectric Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$ films were deposited directly on Ge by Molecular-Beam-Epitaxy without the employment of GeON interfacial layer. Excellent electrical properties, such as a high $\kappa $ value of 14.5, a low leakage current density of only 3x10$^{-9}$ A/cm$^{2}$ at V$_{fb}$+1V, and well-behaved CV characteristics, were demonstrated, even being subjected to a 500$^{\circ}$C annealing in N$_{2}$ ambient for 5 min. An abrupt Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$/Ge interface without any interfacial layer was revealed by high-resolution transmission electron microscopy as well as \textit{in-situ} x-ray photoelectron spectroscopy (XPS). Detailed XPS studies indicate that the oxide/Ge interface consists of mainly Ge-O-Gd bonding, distinctly different from that of native oxide. Furthermore, the 500$^{o}$C annealing did not change the chemical bonding, implying a great thermodynamic stability of the hetero-structure. The outstanding electrical and thermodynamic properties qualified Ga$_{2}$O$_{3}$(Gd$_{2}$O$_{3})$ as a promising dielectric for Ge and proved the GeON interfacial layer to be unnecessary.