Hexagonal SiGe alloys offer a group IV direct bandgap for integrated photonics, addressing the limitations of traditional silicon-based electronics. We have synthesized coaxial nanowire shells comprising direct bandgap hex-Ge/SiGe and hex-SiGe/SiGe Quantum Wells (QWs) around a wurtzite GaAs core. Time-resolved photoluminescence measurements demonstrate a 1 nanosecond radiative lifetime, proving direct bandgap emission. Photoluminescence spectra show the QW emission in between the emission of the well and the barrier material, indicating type-I band alignment. Measurements as a function of QW thickness demonstrate clear quantum confinement with emission up to room temperature for thick QWs. By changing the QW-thickness and the well composition, the emission could be tuned between 2000-3400 nm. The experimentally observed direct bandgap SiGe QWs with type-I band alignment are expected to be pivotal for the development of novel low-dimensional devices based on hex-Ge and hex SiGe.
Hexagonal SiGe has been theoretically shown to feature a tunable direct bandgap in the range 0.4-0.8eV. We study arrays of site-selectively grown Si_(1-x)-Ge_x nanowires (NWs) grown using the crystal transfer method in which wurtzite GaP core NWs are used as template for SiGe growth. Our approach opens up routes towards photonic band-edge lasers using group-IV NWs. Low-temperature µPL studies of arrays of SiGe NW-arrays reveal strong emission at 0.395eV and linear power dependence for weak excitation levels (P_ex~0.01-1kW/cm^2). For P_ex>4kW/cm^2, a new peak emerges at 0.37eV with an intensity that increases according to ~(P_ex)^5, indicative of stimulated emission close to the photonic band-edge.
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