One of the most fascinating ideas of the semiconductors physics is the realization of a direct band gap material based fully on group IV elements. The metastable Ge1-xSnx alloy has the potential of a tuneable direct energy gap material with Sn composition. It had been probed that a direct band gap transition prevails for Sn concentrations below 0.15, with direct band gap transition between 0.35 and 0.79 eV [1-2][Fig 1]. Furthermore, very low effective electron masses are predicted for the alloy and hence high electron mobility [3-5].
The growth of this alloy is very difficult due to the lattice mismatch (14.7%). The solid solubility limit of Ge in diamond structure Sn is 1% and Sn tends to segregate to the surface. Bulk Sn transforms from α-phase (diamond cubic, gray tin) to β-phase (body-centred tetragonal, white tin) at 13.20C [6-8]. But in non equilibrium growth systems like MBE or Sputter Deposition it is possible to grown these alloys at temperatures between 100 and 2000C [1, 3, 4].