(Invited) Strained Germanium Nanowire MOSFET with Low-Parasitic Resistance Metal Source/Drain

Introduction

  High mobility channel materials are extensively investigated to replace strained-Si in MOSFET devices to reduce power consumption by lowering supply voltages, V_dd, without degrading circuit performance. Among the various high mobility channel materials, strained-Ge[1-4] is recognized as the most promising option for p-channel FETs due to its significantly high hole-mobility and compatibility with Si-CMOS process. This paper presents metal S/D nanowire pFETs having a high-mobility strained Ge channel formed by doping-free processes for channels and S/D. The large compressive strain as high as -3.9% resulted in a record high hole mobility (μ_eff = 1992 cm²/Vs @ N_s = 1.7x10¹² cm^-2) and a low contact resistivity (ρ_c ~ 3.8x10^-8 Ω cm²).  

Device fabrication

 Uniaxially strained-Ge nanowire channel was formed by the two-step Ge condensation technique, which induced uniaxial stress along the channel direction. As a gate insulator, GeO_x interlayer and 3.2 nm- Al₂O₃ were grown by using MMT-plasma[4,5] and ALD, respectively. NiGe-metal S/D structures were formed by SALICIDE-like process on the unintentionally doped Ge nanowire. Detailed device fabrication process was shown in [3,4]. Figure 1(a) shows a XTEM of the narrowest channel with a wire width (W_wire) of 15 nm. The Relationship between the W_wire and strain applied to the channel was measured by a Raman spectroscopy. The dependence of strain along the channel direction ε_xx on W_wire is shown in Fig. 1(b). Extremely high (over -3.9%) strain, which is almost identical with the misfit strain between Ge and Si, was evaluated for sub W_wire=40 nm devices.

Device characterization

 Hole mobility was extracted by a split CV method for long-channel and multi-wire devices. Figure 2(a) shows hole-mobility of the fabricated multi-wire MOSFETs. The W_wire = 40nm device, which has 95% Ge channel, showed a peak hole mobility of 1922 cm²/Vs at N_s = 1.7x10¹²cm^-2, which exceeds that of -3.8% strained-Ge nanowire MOSFET with  Al₂O₃ gate stack[3]. Moreover, the obtained mobility at N_s = 5x10¹²cm^-2 is corresponding to 5.8 times of the counterpart of a state-of-the-art strained-Si pMOSFETs[1]. I_on / I_off ~ 10⁵ has also been attained for the strained-Ge nanowire pMOSFET with a gate length (L_g) of 40 nm (Fig. 2(b)).

 A metal S/D MOSFET is an attractive candidate because of its low extension sheet resistance properties without a heavy doping in extension regions. However, parasitic resistance (R_para) which is dominated by contact resistance (R_c) between the extension metal and the inversion layer in this device, have been still an issue because of the decreasing contact area. Here, we demonstrate low R_c characteristic due to low Schottky barrier height (SBH) for NiGe/ strained-Ge interface. The W_wire dependence of parasitic resistance (R_SD) were evaluated as shown in Fig. 3. Reduced R_SD as low as 490 Ωμm for W_wire = 54 nm device is achieved by using undoped Ge nanowire formed by two-step Ge condensation. The specific contact resistivity, ρ_c of NiGe / strained-Ge interface was estimated by using previously proposed model[3]. Extremely low ρ_c, that is as low as ~ 3.8x10^-8 Ω cm² has been achieved in spite of doping-free Schottky contact presumably due to the strong Fermi-level pinning to the valence band edge of the strained-Ge and reduction of SBH at Ni(Si)Ge/(Si)Ge interface by rising valence subband top energy for higher Ge concentration and strain.

Conclusion

 Record high hole mobility (μ_eff = 1922 cm²/Vs) and extremely low S/D contact resistivity (ρ_c ~ 3.8x10^-8 Ω cm²) were achieved for the highly strained-Ge nanowire pMOSFET. These results indicate that strained-Ge channels have a potential to serve as pFET channel of ultimately-scaled future CMOS.

References

[1] R. Pillarisetty et al., IEDM p.150 (2010). [2] W. Chern et al., IEDM p.387 (2012).  [3] K. Ikeda et al., VLSI Symp. p.165 (2012). [4] K. Ikeda et al., VLSI Symp. p.T31 (2013). [5] Y. Kamimuta et al., ISTDM. p.34(2012).

Acknowledgements

This work was granted by JSPS through FIRST Program initiated by the Council for Science and Technology Policy (CSTP).