Ge is of great interest as a channel material in future CMOS devices. A bottleneck for achieving Ge-CMOS is the Fermi-level pinning (FLP) phenomenon. Recently, the S/D formation for the Ge n-MOSFET has remarkably progressed, in which several group have reported the low contact resistance for metal/n+-Ge contacts. Manik et al. have demonstrated the low contact resistivity (ρC) of 1.4×10−7 Ω·cm2 on epitaxial n+-Ge (2.5×1019 cm−3) using a thin ZnO interlayer capped with Ti . Chen et al. have reported a low ρC of 3×10−7 Ω·cm2 on epitaxial n+-GeSn (1019 cm−3) with the O2 plasma treatment .We also reported a low ρC of 4×10−7 Ω·cm2 on n+-doped Ge (3.9×1019 cm−3) using a thin Zr-N-Ge amorphous interlayer capped with Ti and with appropriate annealing .
In this presentation, we focus on the ZrN/Ge contact and demonstrate the unique feature of this contact. First, we show that thin amorphous interlayer (a-IL) is formed between ZrN and Ge during the sputter deposition, which induces the strong FLP alleviation. Second, we show that only the a-IL can be retained on Ge surface, which enables us to fabricate metal/a-IL/Ge contacts with different metals. We demonstrate electrical properties of metal/a-IL/Ge and metal/a-IL/n+-Ge contacts with post metallization annealing (PMA) and the a-IL remarkably modulate the barrier height and ρC.
2) Electrical properties of ZrN/Ge contacts
ZrN films were directly deposited on (100) Ge by rf magnetron sputtering. The contacts with no-PMA and 450°C-PMA showed hole barrier heights (ΦBP) of 0.40 and 0.56 eV, respectively. The HAADF-STEM images showed a-ILs with thicknesses of 1.4 and 1.8 nm exist at interfaces in ZrN/Ge contacts with no-PMA and 450°C-PMA, respectively, leading to large FLP alleviation. The ρC of ZrN contact on n+-Ge (3.9×1019 cm-3) was as high as 6.3×10-4 Ω·cm2, suggesting that the series resistance of the a-IL is high. Fortunately, a ZrN film is easily etched by dilute HF solution, and the a-IL can be retained on Ge surface. This enables us to fabricate metal/a-IL/Ge contacts with different metals.
3) Electrical properties of metal/a-IL/Ge contacts
We tried to fabricate the low ρC contact on the n+-Ge using this chemical feature. For ρc measurements, we used the circular transmission line measurement (CTLM). The substrate was p-Ge (100) with a resistivity of 0.2 Ω·cm. We used the thermal diffusion for fabricating the n+-Ge. The surface P concentration (ND) and the P diffused depth were 3.9×1019 cm-3 and 0.5 μm from SIMS measurements, respectively. Then, a ZrN film was deposited on the substrate with ring-patterned photoresist using a ZrN target, followed by removal by dilute HF solution. Then Zr, Ag, Al, Ti, or Ni was deposited on the a-IL/n+-Ge. After the metal deposition, the PMA was carried out in the temperature range of 350-450°C for 10 min. It was confirmed from a dark field-STEM image of a Ag/a-IL/Ge contact without PMA that an a-IL was clearly observed between the Ag film and the Ge substrate. In this study, metal/n+-Ge contacts without a-IL were also fabricated. For electron barrier height (ΦBN) estimation, the metal/n-Ge contacts and metal/a-IL/n-Ge contacts were also formed.
All metal/Ge contacts without a-IL showed the rectified features, which are the feature of usual metal/Ge contacts. On the other hand, Ag/a-IL/n-Ge contact showed ohmic behavior, and Ti/a-IL/n-Ge showed weak rectifying feature. These results suggest that a-IL contributes the low ΦBN. All contacts without a-IL showed higher ρc (>10-5 Ω·cm2). On the other hand, contacts with a-IL showed lower ρc. It was found that Ag and Ti contacts on n+-Ge represent extremely low ρc (4.4 – 7.2×10-7 Ω·cm2). In particular, the ρc of Ti/a-IL/n+-Ge contact was three orders of magnitude lower than that without a-IL. These results suggest that the reaction between metal and a-IL causes to be thinning the a-IL, and a result, the series resistance of a-IL is decreased while maintaining the low ΦBN.
This work was supported by (JSPS) KAKENHI (grant No. 26289090) and was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks.
 P. P. Manik, R. K. Mishra, V. P. Kishore, P. Ray, A. Nainani, Y.-C. Huang, M. C. Abraham, U. Ganguly, and S. Lodha, Appl. Phys. Lett. 101, 182105 (2012).
 K. Y. Chen, C. C. Su, C. P. Chou, and Y. H. Wu, 2016 IEEE Electron Device Lett. 37, 827 (2016).
 H. Okamoto, K. Yamamoto, D. Wang, H. Nakashima, Ext. Abstr. Solid State Devices and Materials, p. 643 (2016).
 K. Yamamoto, R. Noguchi, M. Mitsuhara, M. Nishida, T. Hara, D. Wang, and H. Nakashima, JAP 118, 115701 (2015).