Non-equilibrium induction of tin in germanium: towards direct bandgap Ge(1-x)Sn(x) nanowires.

Subhajit Biswas,Jessica Doherty,Dipanwita Majumdar,Tomasz Ochalski,Dzianis Saladukha,Achintya Singha,Moneesh Upmanyu,Michael A Morris,Quentin Ramasse,Justin D Holmes

doi:10.1038/ncomms11405

Abstract

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1−xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour–liquid–solid growth. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 °C) during cool-down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth.

Highlights

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor
The choice of Au and AuAg catalysts and the growth temperature was driven by the Au-Ge and Au-Ag-Ge phase diagrams[30], where a faster growth rate of Ge nanowires is expected using AuAg catalysts[27]
At our growth temperature (440 °C) Au-Sn or Ag-Sn phase diagrams predict the formation of eutectic liquid alloys (Au-Sn-Ge or AuAg-Sn-Ge) with enormous Sn intakes in the catalyst, without any window for the precipitation of Sn layers[32]

Summary

Introduction

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. We describe the fabrication of uniform diameter, direct bandgap Ge1 À xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Direct bandgap semiconductor materials are needed for new device architectures such as band-to-band tunnelling tunnel FETs1, optical interconnects[2] and for the development of group IV photonics[3,4] because these technological modules are based on the direct transition of carriers between energy bands. Fundamental challenges (low solubility, metallic Sn segregation, lattice mismatch and so on) restrict the growth of Sn-based Si and Ge alloys with a high Sn content (48 at.%) in any nanoform, for example, thin film, nanowire and so on[14,15]. Using bottom-up growth paradigms, Ge and GeSn nanowires were synthesized by utilizing low-melting point Sn metal catalysts, but these techniques produced nanowires either with insufficient Sn incorporation[22] or low quality (bending and kinking) crystals with non-significant luminescence[23]

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Conclusion