GeSn-based alloys present a novel pathway towards the monolithic integration of light sources in mid infrared CMOS-compatible Si photonic circuitry [1]. Semiconductor optoelectronic devices need efficient metallic contacts to receive and deliver power and signal. Ni-stanogermanides (Ni-GeSn) have been proposed as a promising candidate material to reach both low specific contact (Rc) and sheet resistance (Rsh) [2,3] to contact efficiently GeSn-based photonic or electronic devices. However, a low thermal stability of these layers has been evidenced, together with compound agglomeration and segregation [4]. As a solution to these problems Wang et al. showed that the co-sputtering of Ni and Pt extended the thermal stability of the stanogermanides to higher temperatures. Thus leading to a broader window for stanogermanidation and suppressing, to some extent, agglomeration and degradation [4].

In this work, we propose a comparative and comprehensive study of the solid-state reaction of Ni and Ni_0.9Pt_0.1 films with compressively-strained, 60 nm-thick Ge_0.9Sn_0.1 layers, epitaxially grown on Ge-buffered Si (100) substrates. Prior to metallization, surface preparation was performed with a 1 min dip in 1% diluted HF sequentially rinsed in deionized water and dried with a N₂ gun. Then, 10 nm of Ni or Ni_0.9Pt_0.1 were deposited by physical vapor deposition and capped with 7 nm of TiN. The Ni and Ni_0.9Pt_0.1-Ge_0.9Sn_0.1 phase sequence and crystalline evolution were monitored by in-situ x-ray diffraction (XRD) using an Empyrean PANalytical X-ray diffractometer equipped with a copper (Cu Kα) source, a linear detector (PIXcel^1D) and a HTK 1200 Anton Paar furnace working under secondary vacuum. Profiles were acquired from 50 to 600 °C, with 5 °C steps and a scanning time of 7 min per temperature step. The phase analysis was complemented by ex-situ in-plane reciprocal space map with a Rigaku SmartLab X-ray diffractometer. Additionally, samples metallized with the same procedure were ex-situ annealed for 30 s at temperatures ranging from 150 up to 550 °C (with 50 °C steps), in a rapid thermal annealing (RTA) treatment in a N₂ environment. The evolution of the surface morphology and the electrical properties were measured by atomic force microscopy (AFM) in Tapping mode with a Bruker FastScan equipment and sheet resistance (Rsh) with a four-point probe meter.

AFM and R_sh evidenced that Pt addition improves both morphological and electrical properties of the layers at high temperature. The addition of Pt prevents the surface Root-mean-square (RMS) roughness from increasing with temperature (Figure 1) and postpones morphological surface degradation. Concerning the electrical properties (Figure 2), a plateau where sheet resistance remains stable and low is obtained with Pt addition between 350 and 450 °C. Meanwhile, a sudden increase of R_sh values for Ni-system is observed, this immediately after obtaining the minimum values (9.1 Ω/sq at 350 °C).

XRD results for the Ni-Ge_0.9Sn_0.1 system (Figure 3) reveal a sequential growth in which the first phase appearing corresponds to a Ni-rich phase, i.e. Ni₅(Ge_0.9Sn_0.1)₃. Then, at 245 °C, the mono-stanogermanide phase Ni(Ge_0.9Sn_0.1) is observed; this latter is stable up to 600 °C. The addition of 10 at.% of Pt in Ni thin films (Figure 4) modifies the reaction kinetics, evidenced by the delay in Ni consumption together with a snowplow phenomenon. Additionally, it strongly affects the phase formation sequence promoting PtSn_x compound formation and β-Sn segregation as well as a preferential out-of-plane orientation of the mono-stanogermanide phase. Further, the Ni_0.9Pt_0.1 mono-stanogermanide is unstable and a destabilization of the system is observed to the benefit of the Ni mono-stanogermanide phase, around the 345 – 355 °C temperature range. A summary of the phase formation sequence for both systems is given in Figure 5.

The potential impact on device integration of these observations will be discussed. Even though the addition of Pt complicates the solid-state reaction, it was observed that its beneficial impact on surface morphology has an important role for maintaining the integrity of the electrical properties.

References:

[1] S. Wirths et al. Nat. Photonics 9, 88 – 92 (2015).

[2] S. Wirths et al. ECS Trans. 64, 107 – 112 (2014).

[3] T. Nishimura et al. Solid-State Electronics 60, 46 - 52 (2011).

[4] L. Wang et al. Solid-State Lett. 15, 179 - 181 (2012).