Rutherford backscattering study of Ni(Pt/Pd)Ge formation on Ge (100)

(1) Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
(2) Ghent University, Krijgslaan 281 (S1), 9000 Ghent, Belgium
(3) Department of Physics, University of Cape Town, Rondebosch 7700, South Africa

Ge is a top candidate to replace Si in high performance metal-oxide-semiconductor (MOS) devices due to its high carrier mobility and compatibility with current silicon processing technology. However, a suitable contact material is needed for the successful realization of devices [1]. Based on the beneficial properties of NiSi contacts on Si, NiGe (which can be easily formed via solid-state reaction of a Ni thin film with the Ge substrate) has been identified as one of the most promising contact materials to Ge: it has a low resistivity, it can be formed at relatively low temperature and persists in a wide temperature range [2,3]. This thermal stability can even be further enhanced via co-depositing Pd or Pt with the Ni [4,5]. Moreover, for device integrity it is important to identify the dominant diffusing species (DDS) during the formation of the germanides [1,3]: If Ge is the DDS, overgrowth and bridging between gate and source regions in the device will occur [1]. Therefore, we investigated the phase formation, phase stabilization and diffusing species during solid-phase reaction of ternary Ni-Pd/Pt-Ge.  The diffusion and elemental redistribution during NiGe formation in Ni/Pd-Pt/Ge(100) were monitored via RBS analysis, using a variety of interlayer thicknesses (from 3 Å to 80 Å). Additionally, the use of a very thin extra interlayer of an inert element such as W [3] allows the identification of the DDS. Finally, the stability of the germanide phases is evaluated by sheet resistivity measurements. Despite the chemical similarity of Pd and Pt, a very different behavior is observed for the two types of interlayers: while Pd is mostly found near the surface after annealing at 300°C, the Pt interlayer has not moved at this temperature. After annealing at 600°C all elements have redistributed throughout the film. However, whereas most Pt is found near the surface, regardless of interlayer thickness, the Pd is primarily found near the surface for the thicker interlayer but diffuses towards the Ge-NiGe interface for thinner interlayers. The room temperature sheet resistivity decreases with increasing annealing temperature for all samples and, except for the thinnest Pt interlayer, increases with interlayer thickness. The measured values, in the order of 10-5 Ω/sq, are very similar for Pd or Pt interlayers of the same thickness. In this work, the dominant diffusing species during Ni-germanide formation is determined. The elemental diffusion during the NiGe formation is found to depend on the interlayer element, i.e. Pd or Pt. At 300°C Pt is an efficient diffusion barrier while Pd has already out-diffused. At 600°C Pt is found primarily near the surface for all interlayer thicknesses while the redistribution of Pd is found to depend on the thickness of this interlayer.  

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[3] C.M. Comrie et al., Thin Solid Films 526, 261–268 (2012)
[4] M.-H. Kang et al., IEEE Transactions on Nanotechnology, vol. 11, no. 4, 769 (2012)
[5] Y.-Y. Zhang et al., Junction Technology, IWJT '08. Extended Abstracts, (2008)