Nanocrystalline Group IV Alloy Semiconductors: Synthesis and Characterization of Ge_1–xSn_x Quantum Dots for Tunable Bandgaps

Ge 1– x Sn x 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 Ge 1– x Sn x 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 Ge 1– x Sn x nanoalloys and the effects of quantum confinement on structure and optical properties. Herein, we report the synthesis of phase pure Ge 1– x Sn x 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 Ge 1– x Sn x 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 Ge 1– x Sn x nanoalloys exhibit high thermal stability and moderate resistance against sintering up to 400–500 °C and are devoid of crystalline and amorphous Sn impurities.