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

Abstract

Ge1–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 Ge1–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 Ge1–x Sn x nanoalloys and the effects of quantum confinement on structure and optical properties. Herein, we report the synthesis of phase pure Ge1–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 Ge1–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 Ge1–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.

SnxGe1–x Alloy Nanocrystals: A First Step toward Solution-Processed Group IV Photovoltaics

“…Ge and Sn in higher oxidations states (II, and IV in particular) can also be indicative of post-synthesis oxidation, and indeed a relatively high overall fraction of XPS signal from these species has been directly related to formation of oxides, most probably at the surface of alloy NCs. 28 The large relative contribution from Ge (0) and Sn(0) in our measurements suggest very little, if any, oxide formation, which is consistent with our high-resolution TEM/EDX data.…”

“…Finally, we note that the absorption spectra in Ref. 28, which were extracted from diffuse reflectance measurements, have noticeably different shapes than our directly measured absorption spectra, as well as those reported elsewhere for pure Ge NCs 38 . Because its correct application requires reliable data over a significant spectral region, a Tauc analysis of converted reflectance data may produce quite different results, further diminishing confidence in band gap values derived in this manner.…”

“…As this manuscript was under review, the synthesis of SnGe alloy NCs with tin content up to 27% and with absorption onsets also in the IR, was reported. 28 Our analysis of the reaction mechanism suggests that among other differences, our use of lower reaction temperatures allows us to achieve better control over size and size-distribution, more regular particle shape and higher Sn concentrations. Moreover, a more rigorous exclusion of air during physical and spectroscopic characterization of our NCs largely precluded the previously observed surface oxidation, removing a potential source of uncertainty in our conclusions regarding crystal structure and optical properties.…”

Microwave-assisted solution–liquid–solid growth of Ge1−xSnxnanowires with high tin content

“…2b,4 Recent theoretical models for calculating the band structure of the Ge 1Àx Sn x alloy predict a transition from an indirect to a direct band gap in range between 5.3-11.0% Sn incorporation in the Ge lattice. 10 In the here presented study we investigated the synthesis of Ge 1Àx Sn x nanowires in a microwave supported solvent-based growth process. 6 The low tin content observed in such thermodynamically driven processes already suggests that successful incorporation of higher tin concentrations in the Ge lattice should be achieved under non-equilibrium conditions.…”

“…10,24 Therefore, further methods are employed to determine the tin composition in the Ge 1Àx Sn x nanowires. 10,24 Therefore, further methods are employed to determine the tin composition in the Ge 1Àx Sn x nanowires.…”