Ni(SiGeSn) Metal Contact Formation on Low Bandgap Strained (Si)Ge(Sn) Semiconductors
Authors: Wirths Stephan , Troitsch Rene , Mussler Gregor , Zaumseil Peter , Hartmann Jean-Michel , Schroeder Thomas , Mantl Siegfried , Buca Dan 
Published: February 2020

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-Ge0.91Sn0.09 samples (Fig. 1a). The temperature dependence of the stanogermanide phase formation shows that the Ni5X3 phase (x = Ge0.88Sn0.12, Ge0.91Sn0.09 Ge0.94Sn0.06) 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 NiGe0.94Sn0.06 and NiGe0.88Sn0.12 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.