Ni(SiGeSn) Metal Contact Formation on Low Bandgap Strained (Si)Ge(Sn) Semiconductors

GeSn has gained a lot of attention as alternative high mobility and low bandgap material to boost the performance of MOSFETs and Tunnel-FETs.  Self-aligned contacts with low sheet resistance and good thermal stability are desired for GeSn FETs. Preliminary works on Ni-GeSn contacts indicate a low thermal stability. However, to date only GeSn with low Sn concentrations below 6 % have been reported. Here we investigate the formation of metallic contacts for high mobility Sn-based group IV semiconductors. The formation of ternary NiGeSn alloys for a large range of Sn compositions up to 12 at.% and quaternary NiSiGeSn alloys, formed on SiGeSn ternaries with Si/Sn compositions ranging from 20%/3% to 3%/12% is presented.

(Si)GeSn layers with thicknesses of about 30-60 nm were pseudomorphically grown on high quality Ge virtual substrates (VS) on Si(100) by RP-CVD. The growth temperature was varied between 350°C and 475°C to adjust the Sn and Si concentration within the epilayers.For the layers under investigation Sn and Si concentrations between 3 – 12 at.% and 3 – 18 at.% have been obtained. All GeSn layers exhibit high compressive strain while for SiGeSn ternaries biaxial compressive and biaxial tensile strain is observed according to the Si/Sn ratio. For the metallization studies 10 nm Ni were evaporated followed by a forming gas annealing step for 10 or 30 s. Here, the temperature was varied between 250°C and 400°C.

Specular XRD, taken in off-orientation relative to the Si substrate was performed for phase identification of differently annealed (275°C – 350°C) Ni-Ge_0.91Sn_0.09 samples (Fig. 1a). The temperature dependence of the stanogermanide phase formation shows that the Ni₅X₃ phase (x = Ge_0.88Sn_0.12, Ge_0.91Sn_0.09 Ge_0.94Sn_0.06) appears already at 275°C. The epitaxial Ni₅X₃ phase transforms into a [Ni₁X₁ + Ni₅X₃] multi-phase at 300°C while at 325°C only poly crystalline grains of the Ni₁X₁ phase are observed. The morphology of the stanogermanides was investigated by TEM. For all investigated Sn concentrations at a formation temperature of 300°C, the formed metallic layers are continuous with a mean thickness of about 25 nm. High resolution TEM images taken on NiGe_0.94Sn_0.06 and NiGe_0.88Sn_0.12 are shown in Fig. 1b,c. The rough Ni₁X₁/GeSn interface is typical for poly-crystalline layers containing large and non-substrate oriented grains. The TEM micrograph also proves that the grains are crystalline with well-defined grain boundaries and an unconsumed single crystalline GeSn layer. The well-oriented Ni₅X₃ phase possess a flat Ni₅X₃/GeSn interface (not shown here).

Compared to the stanogermanides, we obtained an extended thermal stability window for the Ni₅(SiGeSn)₃ phase. This epitaxial phase, demonstrated by pole figures, is observed for all Si/Sn ratios, from high Si content (20% Si and 3% Sn) to high Sn content (3% Si and 12% Sn) alloys. The sheet resistance of NiSiGeSn alloys versus temperature is shown in Fig. 1d for different compositions. For Sn concentrations up to 8 at.% low sheet resistances (< 10 W/sq) are obtained for the complete temperature range. The thermal stability for 12 at.% Sn is very limited.