August 26, 2011 -- Imec performed boron-doped selective epitaxial growth via chemical vapor deposition (CVD) of germanium tin (GeSn) in a production-like environment, using commercially available Ge and Sn precursors. The technique could introduce uniaxial compressive stress in a Ge channel to attain high mobility values in MOSFETs.

The GeSn layers with 8% Sn were defect free, fully strained and thermally stable for temperatures up to 500°C for 10 minutes.

Figure 1. Cross-section TEM and 224 XRD-RSM of fully strained defect-free GeSn layers grown on a Ge substrate.

Germanium could potentially replace Si in pMOSFETs in future semiconductors. Imec's epitaxial growth of GeSn CVD in embedded source/drain areas was demonstrated on a 200mm wafer, and should transfer to 300mm fab environments, the researchers assert.

Imec was able to grow GeSn in a CVD environment using the stable SnCl₄ and Ge₂H₆ as Sn and Ge precursors, respectively. The precursor stability enabled a 40nm GeSn layer without defects (inspected via transmission electron microscopy) on a Ge substrate. X-ray diffraction reciprocal space mapping (XRD-RSM) measurements show that the layer is fully strained.

Figure 2. C-V characteristic of a GeSn capacitor realized by 4nm Al2O3 ALD on GeSn.

In addition, in-situ boron (B) doped GeSn CVD growth was investigated by using a combination of Ge₂H₆, SnCl₄ and B₂H₆ precursors. B was found to be 100% electrically active in GeSn:B layers grown with a B concentration of 1.7^e19cm^-3.

The CVD-grown GeSn layer can also be used as a high-mobility channel material on Ge, a possible enabler of Ge-based MOSFETs. First working GeSn capacitors were realized by depositing Al₂O₃ on the CVD grown GeSn layers. Other application areas include photonics (having indirect-to-direct-bandgap transition expected for about 10% Sn incorporated in monocrystalline GeSn alloys) and photovoltaics (ternary SiGeSn alloys).

Results were published in B. Vincent et al., Microelec. Eng. 88 (2011) 342 and will be available inB. Vincent et al., Electrochem. Soc. Trans. 2011 (accepted for publication).

Imec also recently developed a new method to preferentially deposit silicon germanium (SiGe) via CVD for trench narrowing and via filling through deposition. This technique eliminates lithography/etch and subsequent chemical mechanical polishing (CMP).

Learn more about imec at www.imec.be.

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