A novel semiconductor alloy can be both light source and mid-infrared sensor according to work published in Materials Today. The germanium-tin alloy can be fabricated as nanowires and with a sufficiently high proportion of tin will display a direct band gap of almost 0.5 electronvolts. Unfortunately, achieving such high ratios of tin to germanium was difficult, until now. [Meng, A.C. et al., Mater Today (2020); DOI: 10.1016/j.mattod.2020.05.019]

Researchers from Stanford University and Massachusetts Institute of Technology (MIT), point out that relatively high tin concentrations have been achieved using various growth strategies, chemical vapor deposition (CVD) would be the approach of choice for many applications but understanding is quite lacking on how to exploit this approach in this context. As such, the team has carried out a systematic study of CVD approaches to make GeSn semiconductor alloys. They repeated the same synthesis and varied gas precursor partial pressures and shell growth temperatures to see whether they could glean any guiding principles for making these semiconductors and for revealing obstacles that might arise in attempting to make them with high tin incorporation.

Fundamentally, their systematic study has shed light on the specifics of the CVD mechanism for these GeSn alloys. They note hydrogen gas passivation effect whereby a higher ratio of hydrogen partial pressure to tin chloride precursor partial pressure leads to an increase in axial wire growth but a decrease in radial growth. They have also demonstrated that shell growth is mass transport limited, which has implications for optimizing the process. Finally, they found that low shell growth temperature and high shell growth rate lead to a higher proportion of tin present in the final product because of solute trapping due to the suppression of surface diffusion relative to the velocity of the advancing shell surface steps.

Ultimately, they have fabricated nanowires with the optimized composition Ge/Ge0.88Sn0.12, which gives rise to minimal residual strain in the shell, high crystalline quality with the requisite large tin incorporation needed for the desired optical properties. The same insights are not only applicable to nanowires made from this semiconductor alloy but apply to etched nanowires, nanosheets, and free-standing two-dimensional crystals.

The nanowires formed in the current work are free of dislocations and exhibit room temperature, single nanowire spectra consistent with direct gap emission from both the shell and the highly tensile-strained core, the team reports.