Nanocrystalline Group IV Alloy Semiconductors: Synthesis and Characterization of Ge1–xSnx Quantum Dots for Tunable Bandgaps

Abstract

Ge1–xSnx alloys are among a small class of benign semiconductors with composition tunable bandgaps in the near-infrared (NIR) spectrum. As the amount of Sn is increased, the band energy decreases and a transition from indirect to direct band structure occurs. Hence, they are prime candidates for fabrication of Si-compatible electronic and photonic devices, field effect transistors, and novel charge storage device applications. Success has been achieved with the growth of Ge1–xSnx thin film alloys with Sn compositions up to 34%. However, the synthesis of nanocrystalline alloys has proven difficult, because of larger discrepancies (∼14%) in lattice constants. Moreover, little is known about the chemical factors that govern the growth of Ge1–xSnx nanoalloys and the effects of quantum confinement on structure and optical properties. Herein, we report the synthesis of phase pure Ge1–xSnx nanoalloys with sizes in the range of 15–23 and 3.4–4.6 nm and Sn compositions from x = 0.000–0.279, including the factors that have led to the elimination of undesired metallic impurities. The compositional dependence on lattice parameters has been studied using powder X-ray diffraction and Raman spectroscopy, which indicates a nonlinear expansion of the cubic Ge lattice with increasing Sn composition. Furthermore, the quantum size effects have resulted in bandgaps significantly blue-shifted from bulk Ge, for smaller Ge1–xSnx nanoalloys (3.4–4.6 nm) with indirect energy gaps from 1.31 eV to 0.75 eV and direct energy gaps from 1.47 eV to 0.95 eV for x = 0.000–0.116 compositions. Remarkably, as-synthesized Ge1–xSnx nanoalloys exhibit high thermal stability and moderate resistance against sintering up to 400–500 °C and are devoid of crystalline and amorphous Sn impurities.