Tuning Ultra-Narrow Direct Bandgap in alpha-Sn Nanocrystals: A CMOS-Compatible Approach for THz Applications

TL;DR

This paper demonstrates a CMOS-compatible microwave synthesis method to produce alpha-Sn nanocrystals with tunable ultra-narrow direct bandgaps suitable for THz applications, overcoming stability challenges of alpha-Sn.

Contribution

It introduces a novel microwave-assisted process for stabilizing alpha-Sn nanocrystals on silicon with controlled size and bandgap tuning for THz device integration.

Findings

Nanoparticle size can be controlled via microwave-induced dewetting and coalescence.

Alpha-Sn nanocrystals are stabilized within a SnO2 shell confirmed by XRD.

Bandgap tuning from 64 to 137 meV was achieved by size control.

Abstract

alpha-Sn has recently been attracting significant interest due to its unique electronic properties. However, this allotrope of Sn is stable only below 13 {\deg}C and alternative options to the conventional stabilization by epitaxial growth on InSb are still a challenge. In this work, nanoparticles with inner alpha-Sn nanocrystals were synthesized on a Silicon substrate via a CMOS-compatible process through microwave irradiation. The nanoparticle morphology was characterized by Scanning Electron Microscopy and Atomic Force Microscopy, demonstrating the ability to control the nanoparticle size by a dewetting process combined with a coalescence process induced by the microwaves. Grazing Incidence X-Ray Diffraction analyses confirmed the stabilization of the alpha-Sn phase within a SnO2 shell, while X-Ray Photoemission Spectroscopy measurements revealed the presence of a bandgap. Infrared transmission spectroscopy combined with a Tauc-plot extrapolation led to an estimate of the gap in the range from 64 to 137 meV range. Furthermore, the possibility to tune the bandgap by controlling the nanoparticle size, possibly leveraging weak quantum confinement effects, was demonstrated, unveiling the potential of alpha-Sn nanoparticles on Si for the development of CMOS-compatible THz devices.