Germanium-tin alloy nanowires hold promise as silicon-compatible optoelectronic elements with the potential to achieve a direct band gap transition required for efficient light emission. In contrast to GeSn epitaxial thin films, free-standing nanowires deposited on misfitting germanium or silicon substrates can avoid compressive, elastic strains that inhibit formation of a direct gap. We demonstrate strong room temperature photoluminescence, consistent with band edge emission from both Ge core nanowires, elastically strained in tension, and the almost unstrained GeSn shells grown around them. Low-temperature chemical vapor deposition of these core-shell structures was achieved using standard precursors, resulting in Sn incorporation that significantly exceeds the bulk solubility limit in germanium.

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The full exploration of Si-based photonic integrated circuits is limited by the lack of an efficient light source that is compatible with the complementary metal− oxide−semiconductor process. Highly strained germanium (Ge) is a promising solution, as its band structure can be fundamentally altered by introducing tensile strain. However, the main challenge lies in the incorporation of an electrical structure while maintaining high strain with uniform distribution in the active region. Here we present highly strained Ge LEDs driven by lateral p−i−n junctions and report the strain-induced enhancement of electroluminescence (EL) from Ge. Raman characterization shows that 1.76% strain along the ⟨100⟩ direction with relatively uniform strain distribution is achieved. The observed strain-induced red-shifts of EL spectra agree well with the theoretical prediction, revealing that the direct band gap of Ge can be tuned in the range of 0.785 eV (1580 nm) to 0.658 eV (1885 nm). This work offers a pathway toward a strained Ge laser with low threshold current, as well as opens possibilities for new types of optoelectronics devices based on strain engineering.

We demonstrate tensile-strained pseudomorphic Ge0.934Sn0.066/Ge quantum wells in a microdisk resonator using silicon nitride stressor layers. The hydrostatic and biaxial strain distributions are studied through finite element modeling, while confocal Raman spectroscopy shows local biaxial strain transfers as high as 1.1% at freestanding microdisk edges. These strains are sufficient to overcome the original compressive strain in Ge0.934Sn0.066 epitaxy and reach a direct band gap according to deformation potential theory. A red-shift in microdisk photoluminescence confirms the reduced band gap energies in response to tensile strain and suggests an average biaxial strain transfer of 0.55%.

We study the effect of surface passivation on pseudomorphic multiple-quantum-well Ge0.97Sn0.03/Ge p-i-n photodetectors. A combination of ozone oxidation to form GeOx and GeSnOx on the surface of the diodes followed by atomic layer deposition of Al2O3 for protection of these native oxides provides reduced dark current. With a temperature-dependent investigation of dark current, we calculate the activation energy to be 0.26 eV at a bias of −0.1 V and 0.05 eV at −1 V for the sample passivated by this ozone method. Based on these activation energy results, we find that the current is less dominated by bulk tunneling at lower reverse bias values; hence, the effect of surface passivation is more noticeable with nearly an order-of-magnitude improvement in dark current for the ozone-passivated sample compared to control devices without the ozone treatment at a voltage of −0.1 V. Passivation also results in a significant enhancement of the responsivity, particularly for shorter wavelengths, with 26% higher responsivity at 1100 nm and 16% higher performance at 1300 nm.