Physics of Metal/Ge Interfaces; Interface Defects and Fermi-Level Depinning

Abstract

Germanium (Ge) has high carrier mobility compared to Si and is expected again to work as a promising key material in next-generation high-speed energy-saving devices. With respect to metal/Ge interfaces, however, there still exist serious issues to be overcome for applications; one which is the control of interface disorders and Schottky-barrier height (SBH). In this work, we discuss this issue based on fundamental properties of metal/Ge interfaces evaluated by theoretical calculations. It is well known that a variety of metal/Ge interfaces show a strong Fermi-level pinning (FLP); the Fermi energies of various metals are located at similar energy positions around the valence-band edge of Ge irrespective of the kind of metal, which induces the difficulty in controlling the SBH by simply changing the kind of metal. By the first-principles calculations, we found that this FLP is caused by the small band gap of Ge and the long penetration depth of metal-induced gap states (MIGS) into Ge layers even at clean interfaces [1]. Moreover, by evaluating the formation energies of various defects and dynamical atom motions around the interface, we showed that the MIGS are well hybridized with electronic states of defects and become the leading actors to increase the interface disorders (See Fig.1 in attached file for the case of vacancy defect in Ge layers). This result indicates important concept that the MIGS promote the disorder-induced gap states (DIGS). Correspondingly, even at weakly FL-depinned Fe3Si/Ge interface, the interface defects also promote the FLP [2]. One of recipes to release the FLP is to insert the third materials into the interfaces by the segregation of dopant atoms such as S and P, similar to the cases of metal/Si interfaces [3], and by the stacking of transition layers made of semiconductor/oxide. For example, we found that the appearance of ultrathin semiconducting α-Sn layers between metallic β-Sn and Ge layers works to increase the SBH for hole carriers, in agreement with recent experiments. Similarly, when GeO2 oxide is produced between metal and Ge layers, the MIGS penetration into Ge layers is prohibited and the SBH increases to the original position (See Fig.2 in attached file for the case of Al/Ge interfaces). These results are discussed in details, together with reviewing recent experiments and focusing on the fundamental physics at metal/Ge interfaces. <references>[1] T. Hiramatsu et al., Jpn. J. Appl. Phys. 53, 058006 (2014).[2] K. Kobinata and T. Nakayama, Jpn. J. Appl. Phys. 53, 035701 (2014).[3] T. Nakayama and K. Kobinata, Thin Solid Films 520, 3374 (2012). Figure 1