Reexamination of Fermi level pinning for controlling Schottky barrier height at metal/Ge interface

Abstract

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. © 2016 The Japan Society of Applied Physics G ermanium (Ge) is one of the promising semiconductor materials for high-performance complementary metal-oxide-semiconductors (CMOSs) in the next generation owing to its high electron and hole mobilities. Recently, high electron mobility in Ge n-channel metal-insulation-semiconductor field-effect transistors (n-MISFETs) with a sub-nm equivalent oxide thickness gate stack has been demonstrated, owing to appropriate gate stack designing, [1][2][3][4] in addition to the high hole mobility in Ge p-MISFET, which has already been reported. 5,6) Therefore, the most serious challenges for realizing practical scaled Ge CMOS are currently the reduction in parasitic resistance and the suppression of off-state leakage at the source=drain of MISFETs. In particular, to reduce the contact resistance at the metal=Ge interface, a reduction in Schottky barrier height (SBH) at the interface is definitely required. However, the Fermi level is almost perfectly pinned to the valence band edge of Ge at the metal=Ge interface for various element metals. 7,8) This fact suggests that the Fermi level pinning (FLP) on Ge may be limited by an intrinsic mechanism. Therefore, understanding the FLP mechanism and controlling the SBH on n-Ge are strongly required.Recently, FLP alleviation has been demonstrated in ultrathin-film [GeO 2 , 9) Al 2 O 3 , 9) GeN, 10) SiN,11) MgO,12) Si,13) ZnO, 14) TiO 2 , 15) W-encapsulating Si, 16) indium tin oxide (ITO) 17) ] insertion at the element metal=Ge interface. When the direct metal=Ge interface is replaced by the metal= ultrathin film and ultrathin film=Ge interfaces, the strong FLP at the direct metal=Ge interface becomes less significant. Nevertheless, since the interface layer resistance is added, the direct metal=Ge interface with a low SBH is more preferable from the viewpoint of contact resistance reduction. With respect to the direct metal=Ge interface, it has recently been reported that the SBHs at epitaxial Fe 3 Si, 18) α-TiGeN, 19) epitaxial Mn 5 Ge 3 , 20) epitaxial NiGe 2 , 21) graphene, 22) and Sn 23) =n-Ge interfaces are relatively low, which deviates from the trend of strong FLP. 7,8) Some of them 18,20,21,23) considered that the FLP on Ge might not be determined by an intrinsic but extrinsic origin. It is inferred that those two kinds of experimental results may include a key aspect to understand the FLP mechani...

Face Dependence of Schottky Barriers Heights of Silicides and Germanides on Si and Ge

“…Comparing experimental data with our calculated values in Fig. 5 , we see that experimentally the (111) SBHs lie above the (100) values for the Ge/MnGe x system 33 , for the range of germanides studied by Nishmura and Toriumi 22 and for Fe 3 Si on Ge 32 . In some cases, one facet has an epitaxial layer on it while the other has a polycrystalline layer.…”

“…However they are mostly opposite to experiment for Ge, where (111) SBH values often lie above (100) values as seen the experimental points in Fig. 5 for Nishimura 22 , Yamane 32 , Nishimura 33 and Deng 34 . This suggests that Ge/germanide interfaces might be a problem, being disordered, non-abrupt or multi-faceted.…”

“…This suggests that Ge/germanide interfaces might be a problem, being disordered, non-abrupt or multi-faceted.

Figure 5 Normalised calculated barrier height values on Si and Ge from Figs 3 and 4 , compared to experimental data for barrier heights from refs 12 , 22 , 30 , 32 – 36 .

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“… Normalised calculated barrier height values on Si and Ge from Figs 3 and 4 , compared to experimental data for barrier heights from refs 12 , 22 , 30 , 32 – 36 . …”

“…Recently Nishimura et al . 22 found that the SBH of germanides on Ge were also more weakly pinned than for elemental metals. They also found that their SBHs on Ge were face-dependent, whereas the SBHs of elemental metals were independent of face 3 .…”