Germanium metal-insulator-semiconductor capacitors with ${\mathrm{La}}_2{\mathrm{O}}_3$ dielectrics deposited at high temperature or subjected to post deposition annealing show good electrical characteristics, especially low density of interface states $D_{\mathrm{it}}$ in the ${10}^{11}{\mathrm{eV}}^{-1}{\mathrm{cm}}^{-2}$ range, which is an indication of good passivating properties. However, the $\kappa$ 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 ${\mathrm{La}}_2{\mathrm{O}}_3$ and Ge substrate to form a low $\kappa$ and leaky La–Ge–O germanate over the entire film thickness, which, however, raises concerns about gate scalability. Combining a thin $(\sim 1\mathrm{nm})$ ${\mathrm{La}}_2{\mathrm{O}}_3$ layer with thicker $\mathrm{Hf}{\mathrm{O}}_2$ degrades the electrical characteristics, including $D_{\mathrm{it}}$ , 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.

Metal-oxide-semiconductor field effect transistors (MOSFET) with a thin high-$k$ dielectric were fabricated on bulk $n$ -type germanium substrates. Surface oxides were thermally desorbed in situ by heating the substrates under ultrahigh vacuum conditions. First an ultrathin passivating layer was formed by evaporating germanium in the presence of atomic oxygen and nitrogen supplied from a remote radio frequency plasma source. Subsequently, the ${\mathrm{HfO}}_2$ dielectric was deposited by evaporating hafnium in the presence of atomic oxygen. An in situ $\mathrm{TaN}$ metal gate was similarly deposited. Long channel devices were fabricated using a standard process flow. These devices exhibited a low equivalent oxide thickness (EOT) of $0.7\mathrm{nm}$ with gate leakage less than $15\mathrm{mA}∕{\mathrm{cm}}^2$ at $V_{\mathrm{FB}}+1\mathrm{V}$ . Device mobility was extracted from $I_s\text{-}{V}_g$ and split $C\text{-}V$ characteristics. Results indicate a $2\times$ mobility enhancement in Ge $p$ -MOSFET devices compared to Si control devices. The demonstration of subnanometer EOT suggests that high-$k$ gate dielectrics on germanium are scalable to low EOT and suitable for use in ultrascaled MOSFET devices.

Electrical data on ${\text{ZrO}}_2/{\text{GeO}}_2$ stacks prepared by atomic oxygen beam deposition on Ge at $225°\text{C}$ reveal a relatively weak dependence of the stack equivalent oxide thickness upon the ${\text{ZrO}}_2$ thickness. This trend points to a very high zirconia dielectric permittivity $\left(k\right)$ 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 ${\text{GeO}}_2$ decomposition, Ge is incorporated into the growing ${\text{ZrO}}_2$ , thus, stabilizing the high-$k$ tetragonal phase.