(Invited) Achievement of Ultralow Contact Resistivity of Metal/n+-Ge Contacts with Zr-N-Ge Amorphous Interlayer
Authors: Nakashima Hiroshi , Okamoto Hayato , Yamamoto Keisuke , Wang Dong 
Published: February 2020

1) Introduction

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 (ρ
C) of 1.4×10−7 Ω·cm2 on epitaxial n+-Ge (2.5×1019 cm−3) using a thin ZnO interlayer capped with Ti [1]. Chen et al. have reported a low ρ
C of 3×10−7 Ω·cm2 on epitaxial n+-GeSn (1019 cm−3) with the O2 plasma treatment [2].We also reported a low ρ
C 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 [3].
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 ρ
C.

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
ρ
C of ZrN contact on n+-Ge (3.9×1019 cm-3)　was as high as 6.3×10-4 Ω·cm2[4], 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 ρ
C contact on the n+-Ge using this chemical feature. For ρ
c 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 (N
D) 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 ρ
c (>10-5 Ω·cm2). On the other hand, contacts with a-IL showed lower ρ
c. It was found that Ag and Ti contacts on n+-Ge represent extremely low ρ
c (4.4 – 7.2×10-7 Ω·cm2). In particular, the ρ
c 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.

Acknowledgement

This work was supported by (JSPS) KAKENHI (grant No. 26289090) and was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

References

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