Towards a direct band gap group IV Ge-based material
A silicon-compatible laser source is of utmost importance for a successful photonic integrated circuit. The conventional solution using direct band gap III-V materials adds significant complexity into the fabrication process because the active materials have to be bonded or grown on a largely mismatched silicon substrate. Recently, germanium has been considered a promising material for silicon photonic applications due to its interesting electronic band structure. Several concepts to realise a direct band gap Ge-based material will be reviewed in this paper, such as: tensile strain combined with high n-type doping, high tensile strain created by micromachining, synthesis of Ge-Sn alloys by chemical vapour deposition and, in particular, synthesis of Ge-Sn alloys by ion implantation followed by pulsed laser melting (PLM). Besides providing a very high level of reproducibility and purity in conventional device fabrication, ion implantation followed by PLM is shown to have much potential for realising an intrinsically direct band gap material of high quality. Producing a 10 at.% Sn alloy is now possible and a highly strain-relaxed layer can also be realised by this technique.