GeSn alloys with mid-IR direct band gaps

GeSn alloys represent the only group IV system with an expected direct band gap over a broad range of compositions. Since the indirect to direct transition takes place at rather modest (~8%) Sn concentrations, research interest has focused on this dilute alloy regime with band gaps in the near-IR. However, the expected electronic structure of GeSn is similar to that of HgCdTe, featuring a continuum of direct band gaps that reach a value of zero. GeSn systems with direct band gaps in the mid-IR and beyond require much higher Sn concentrations, which are not obviously attainable due to the thermodynamic metastability of the alloy. Fortunately, this fundamental limitation doesn’t appear to be unsurmountable, as several reports on high-Sn alloys have already appeared in the literature, although the materials were not always suitable for optical measurements. In this presentation I discuss recent optical characterization work on GeSn alloys with Sn concentrations as high as 36%. These materials were synthesized directly on Si substrates using custom CVD methods that combine stannane with polygermanes, making it possible to tune the Sn concentration over a much broader range than previously possible. These materials are found to have direct band gaps approaching 0.15 eV (~ 8 μm), well within the mid-IR spectral range.