(Invited) Schottky Barrier Height Control at Metal/Ge Interface by Insertion of Nitrogen Contained Amorphous Layer
Authors: Yamamoto Keisuke , Wang Dong , Nakashima Hiroshi 
Published: July 2021

1. Introduction
Ge is attracted great attention as a candidate of channel material for future CMOS devices and near-infrared optical devices, owing to its high intrinsic carrier mobility and narrow bandgap. One of the difficulties to realize Ge-based devices is to control of Schottky barrier height (SBH) at metal/Ge interface. It is widely known any metal/Ge contact shows high electron barrier height (ΦBN > 0.5 eV) due to Fermi level pinning at metal/Ge interface. Our group found a method to alleviate the FLP, by which sputter-deposited TiN/Ge and ZrN/Ge contacts showed low ΦBN (< 0.10 eV) and high hole barrier height (ΦBP > 0.56 eV) [1]. From detailed structural analyses, it was clarified that the FLP alleviation was induced by a nitrogen-contained amorphous interlayer (a-IL) formed during TiN(ZrN) sputter-deposition. If we can change the kind of metal on a-IL, we may extend the control range of SBH. In this paper, we demonstrate the wide range SBH control for metal/a-IL/Ge contacts.
2. Experimental, Results, and Discussions
Figure 1 shows the fabrication procedure of the metal/a-IL/Ge structure. An a-IL was formed by the ZrN film sputter deposition and subsequent etching by dilute HF solution. Ag, Al, and Cu, were respectively deposited using thermal evaporation and patterned by the removal of photoresist. In figure 1, a dark field-STEM image of an Ag/a-IL/Ge contact is shown. An a-IL with dark contrast was clearly observed between the Ag and the Ge, implying that the ZrN film can be removed by a dilute HF solution while the a-IL can be retained on the Ge substrate.
Figures 2 shows J–V characteristics at 200 K for Cu/a-IL/n-Ge contacts, respectively. Here, higher rf power during the ZrN deposition makes thicker a-IL. It was found that the insertion of the a-IL significantly modulated the J–V characteristics. The rectifying feature on n-Ge becomes weak with an increase in rf power. Other metals also show a similar tendency (data are not shown).
Figure 3 represents the ΦBN versus work function of the metal (ΦM) plot. In this figure, slope factor (S) and effective charge neutrally level (ΦCNL,eff) are also indicated for each rf power. The weaker rf power leads to the larger S value. Interestingly, a S for contacts with a rf power of 50 W was around 0.26, which is close to S (0.27) of Si [2], suggesting that it is possible to widely control the SBH by forming contacts with N-contained a-IL.
It is well known that the ΦCNL of metal/Ge interface is located near the E
V edge, as shown in Fig. 4(a) [3]. To explain the dependences of S andΦCNL,eff in this study, two effects should be considered, which are the decrease in the intrinsic states and the creation of additional extrinsic states. We consider the decrease in the intrinsic states is dominated by the passivation of surface dangling bonds by a-IL. On the other hand, the creation of additional localized extrinsic states are associated with the interface dipoles related Ge-N bond [4], i.e., the extrinsic states cause a shift of ΦCNL to compensate the positively charged dipoles at the Ge side of the interface, causing the decrease in ΦCNL,eff. When the rf power was as low as 50 W (thin a-IL), the density of extrinsic states was low, as shown in Fig. 4(b). Therefore, the Fermi level can be modulated depending on the ΦM, as shown in Fig. 3 (50 W, blue line). In the case of a thick a-IL (rf power of 200 W), high-density extrinsic states could be generated. As a result, the modulation of ΦM became impossible, as shown in Fig. 4 (200 W, black line).
3. Conclusion
In conclusion, we succeeded in wide controlling SBH of metal/Ge contacts with N-contained a-ILs. The SBH swing and CNL position were strongly depend on the rf power during ZrN sputter deposition, i.e., the thickness of a-IL. Under the low power condition, a S-value of 0.26 was achieved, which is comparable to that of Si. The dependences of S and ΦCNL on the rf power were explained by an extrinsic states model associated with N-related interfacial dipoles.
References
[1] K. Yamamoto et al., J. Appl. Phys. 118 115701 (2015).
[2] S. M. Sze, Physics of Semiconductor Devices 2nd ed. (New York: Wiley, 1981).
[3] A. Dimoulas et al., Appl. Phys. Lett. 89 252110 (2006).
[4] T. Nishimoto and T. Nakayama, Jpn. J. Appl. Phys. 58 061007 (2019).

Figure 1