During the last five years, Ge_1-xSn_x has gained widespread attention as being the only group IV semiconductor material that has a direct band gap. The realization of this material will have an enormous impact on optoelectronics applications because the material is fully compatible with current silicon technology. In theory this possibility has been predicted since the 1980s [1,2], in which the lowest concentration of Sn required for the direct bandgap transition was 26%. More recent theoretical work [3,4] has suggested a more accessible Sn concentration from 6.5 to 11%. Experimentally, the partial demonstration of a direct band Ge_1-xSn_x was first reported in 1997 [5], then again in 2014 [6], but only conclusively verified recently [7] where the authors demonstrated a laser operating at the wavelength of 2250 nm.

In most of the theoretical and experimental reports, the required concentration of Sn for the indirect-to-direct transition ranges from 6.5 to 11%. This is a challenging requirement as the equilibrium solubility of Sn in Ge is around 0.52% at ambient temperature [8]. As a result of this limitation, attempts at achieving Sn supersaturated Ge require techniques that provide conditions far from thermodynamic equilibrium. In fact, the majority of publications on the topic use non-equilibrium growth techniques such as molecular beam epitaxy [5,6,9], sputter deposition [10] or chemical vapour deposition [7]. Another technique that can also provide highly non-equilibrium conditions is ion beam synthesis, which is a combination of ion implantation and rapid thermal annealing (RTA) or pulsed laser melting (PLM).

In this contribution, it is the first time that ion beam synthesis has proven to be a promising method for producing a highly Sn concentrated Ge_1-xSn_x material with high crystal quality. Up to 6.2% substitutional Sn in Ge has been achieved by this technique. This value is 12 times higher than the equilibrium value and 6 times higher than results in previous reports using a similar concept [11,12]. Besides RBS, XRD and Raman spectroscopy to characterise the Sn content and substitutionality, crystal quality, strain distribution and other microscopic properties of the material, TEM study elucidates the origin of defects in the samples and more importantly the reason that limits an even higher Sn concentration in the lattice. To investigate the effect of Sn incorporation on the band structure of the compound, spectroscopic ellipsometry has been used to characterise the optical transitions in the crystalline material. Ellipsometric data show that there is a reduction of excitonic effect, represented by a lower transition activity at the critical points E₁ and E₁+Δ₁ as well as the shrinkage of the band gap at these critical points [13,14]. At the E₀ of the direct band valley Γ, there is a splitting between light and heavy holes valence band. The splitting is thought to be beneficial in term of hole mobility since the scattering of holes between the subbands reduces the carrier mobility. The ellipsometric data also present a significant enhancement of the optical transition in the vicinity of the Γ valley.

Finally, more recent results suggest a solution to achieve higher Sn content that is sufficient for the realization of a direct bandgap alloy.