We report on the impact of TiN interfaces on the ferroelectricity of nondoped HfO₂. Ferroelectric properties of nondoped HfO₂ in TiN/HfO₂/TiN stacks are shown in capacitance–voltage and polarization–voltage characteristics. The Curie temperature is also estimated to be around 500 °C. The ferroelectricity of nondoped HfO₂ clearly appears by thinning HfO₂ film down to ∼35 nm. We directly revealed in thermal treatments that the ferroelectric HfO₂ film on TiN was maintained by covering the top surface of HfO₂ with TiN, while it was followed by a phase transition to the paraelectric phase in the case of the open surface of HfO₂. Thus, it is concluded that the ferroelectricity in nondoped HfO₂ in this study was mainly driven by both of top and bottom TiN interfaces.

The element metal/germanium (Ge) interface exhibits a strong Fermi level pinning (FLP), which is usually characterized on the basis of Ge side semiconductor properties. In this work, we demonstrate that metal properties significantly affect the Schottky barrier height (SBH) on Ge. Metallic germanides show both FLP alleviation and a clear substrate orientation dependence of SBH on Ge, despite the nearly perfect FLP and very slight orientation dependence in the element metal case. As a result, ohmic characteristics are observed at germanide/n-Ge (111) junctions. The metal properties required to alleviate the FLP on Ge are also discussed.

Germanium (Ge) is a promising semiconductor as an alternative channel material to enhance performance in scaled silicon (Si) field-effect transistor (FET) devices. The gate stack of Ge FETs has been much improved based on extensive research thus far, demonstrating that the performance of Ge FETs is much superior to that of Si FETs in terms of the on-state current. However, to suppress the performance degradation due to parasitic contact resistance at the metal/Ge interface in advanced nodes, the reduction of the Schottky barrier height (SBH) at the metal/Ge interface is indispensable, yet the SBH at the common metal/Ge interface is difficult to control by the work function of metal due to strong Fermi level pinning (FLP) close to the valence band edge of Ge. However, the strong FLP could be alleviated by an ultrathin interface layer or a low free-electron-density metal, which makes it possible to lower the SBH for the conduction band edge of Ge to less than 0.3 eV. The FLP alleviation is reasonably understandable by weakening the intrinsic metal-induced gap states at the metal/Ge interface and might be a key solution for designing scaled Ge n-FETs.

In this work, we investigated the band alignment at bismuth (Bi)/germanium (Ge) and Bi/silicon (Si) interfaces to understand the mechanism of strong Fermi level pinning (FLP) at element metal/Ge and/Si interfaces. Bi/Ge and/Si interfaces exhibit almost ideal alignment deviating from the trend of strong FLP at element metal/Ge and/Si interfaces. This result suggests that the strong FLP at element metal/Ge and/Si interfaces is mainly caused by the metal-induced gap states (MIGS) in case of the free electron density of metal, and that the weak FLP at direct metal/Ge and/Si interfaces including germanide/Ge and silicide/Si interfaces is comprehensively understandable from the MIGS in case of low electron density. Furthermore, we also discuss impacts of interface structures on the band alignment at the MIGS-weakened interface.