Mid-IR GeSn alloys with narrow band gaps beyond 8 μm*

GeSn alloys with dilute Sn concentrations around the near-infrared indirect to direct transition have been thoroughly investigated in recent years. However, the GeSn alloy system, in much the same way as the HgCdTe alloy, is expected to have a continuum of band gaps that reach a value of zero. Covering this IR spectral range requires much higher Sn concentrations, which are not obviously attainable given the thermodynamic metastability of the Ge-Sn system. Sporadic reports of such alloys have appeared in the literature, but in most cases the samples were unsuitable for band gap determinations.

In this presentation we discuss recent work¹ characterizing the band gap and electronic structure of GeSn alloys which Sn concentrations as high as 33%. These materials have direct band gaps approaching 0.15 eV (~ 8 mm), thus reaching well into the mid-IR spectral range.
The high-quality GeSn films required for optical characterization were grown directly on Si substrates by CVD reactions of polygermanes and stannanes at temperatures between 240-290°C. The structural properties of the films are similar to those of their dilute counterparts, in spite of the fact that the fraction of Sn-Sn bonds is expected to be significant. For example, we find that the lattice parameter of the alloys still follows Vegard’s law, as reported for lower Sn concentrations.

Ellipsometric measurements of the complex dielectric function make it possible to determine the band gap by modeling the absorption edge. Our model includes excitonic, band filling (Burstein-Moss), and non-parabolicity effects. We find that the compositional dependence of the band gap cannot be described by a simple quadratic polynomial, as is the case in many alloy systems. The implications of this finding for the ability of the GeSn system to cover the 8-12 μm mid-IR window will be discussed in detail.

¹ C. Xu et al, Appl. Phys. Lett. 114 212104 (2019).