Belgian research institute Imec describes, for the first time, the selective chemical vapor deposition (CVD) of germanium-tin (GeSn) in a production-like environment using commercially available Ge and Sn precursors. The resulting GeSn layers with 8% Sn are defect free, fully strained and thermally stable for temperatures up to 500°C. The technique is used to implement uniaxial compressive stress in a Ge channel, the key method for reaching very high mobility values in MOSFETs.
Germanium has attracted much interest for future CMOS applications where it could potentially replace silicon in pMOSFETs. High mobility values can be obtained by stress engineering in the germanium channel. Imec has developed an innovative concept for implementing uniaxial compressive stress in germanium channels by boron-doped selective epitaxial growth of germanium-tin (GeSn) CVD in embedded source/drain areas*. The technique has been demonstrated in a 200mm production-like environment, and is expected to be easily transferable to a 300mm environment.
The key to the technique is a new approach to growing GeSn in a CVD environment. CVD growth of GeSn has so far only been reported by using SnD4 as a tin gas precursor. However, the instability of this precursor restricts the applicability of the technique. Imec uses stable SnCl4 and Ge2H6 respectively as commercially available tin and germanium precursors. This permits the growth of GeSn layers on a germanium surface with tin content up to 8%. As shown by transmission electron microscopy (TEM), no defects are found in the 40nm GeSn layer and according to x-ray diffraction reciprocal space mapping (XRD-RSM) measurements, the layer is fully strained (Figure 1). The GeSn layer grown with this approach survives further thermal treatments at temperatures up 500°C (for 10 minutes). In addition, in-situ boron-doped GeSn CVD growth was investigated by using a combination of Ge2H6, SnCl4 and B2H6 precursors. Boron was found to be 100% electrically active in GeSn:B layers grown with a boron concentration of 1.7e19cm-3.
The CVD grown GeSn layer can also be used as a high-mobility channel material on germanium. A strained GeSn channel on germanium is a candidate for use in the device channel of future Ge-based MOSFETs. In this work, the first working GeSn capacitors were realized by depositing Al2O3 on the CVD grown GeSn layers**.
The CVD growth of GeSn with commercially available precursors will boost research into high-mobility MOSFETs. It also opens new routes for group-IV semiconductors research in other fields, such as photonics (having indirect-to-direct-bandgap transition expected for about 10% Sn incorporated in monocrystalline GeSn alloys) and photovoltaics (ternary SiGeSn alloys).
More detailed results can be found in *B. Vincent et al., Microelec. Eng. 88 (2011) 342 and in **B. Vincent et al., Electrochem. Soc. Trans. 2011 (accepted for publication).
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