Pseudomorphic GeSn layers with Sn atomic percentages between 4.5% and 11.3% were grown by chemical vapor deposition using digermane and SnCl4 precursors on Ge virtual substrates grown on Si. The layers were characterized by x-ray diffraction rocking curves and reciprocal space maps. Photoconductive devices were fabricated, and the dark current was found to increase with Sn concentration. The responsivity of the photoconductors was measured at a wavelength of 1.55 μm using calibrated laser illumination at room temperature and a maximum value of 2.7 mA/W was measured for a 4.5% Sn device. Moreover, the responsivity for higher Sn concentration was found to increase with decreasing temperature. Spectral photoconductivity was measured using Fourier transform infrared spectroscopy. The photoconductive absorption edge continually increased in wavelength with increasing tin percentage, out to approximately 2.4 μm for an 11.3% Sn device. The direct band gap was extracted using Tauc plots and was fit to a bandgap model accounting for layer strain and Sn concentration. This direct bandgap was attributed to absorption from the heavy-hole band to the conduction band. Higher energy absorption was also observed, which was thought to be likely from absorption in the light-hole band. The band gaps for these alloys were plotted as a function of temperature. These experiments show the promise of GeSn alloys for CMOS compatible short wave infrared detectors.
Epitaxial layers of Ge1−xSnx with Sn compositions up to 18.5% were grown on Ge (100) substrates via solid-source molecular beam epitaxy. Crystallographic information was determined by high resolution x-ray diffraction, and composition was verified by Rutherford backscattering spectrometry. The surface roughness, measured via atomic force microscopy and variable angle spectroscopic ellipsometry, was found to scale with the layer thickness and the Sn concentration, but not to the extent of strain relaxation. In addition, x-ray rocking curve peak broadening was found not to trend with strain relaxation. The optical response of the Ge1−xSnx alloys was measured by spectroscopic ellipsometry. With increasing Sn content, the E1 and E1 + Δ1 critical points shifted to lower energies, and closely matched the deformation potential theory calculations for both pseudomorphic and relaxed Ge1−xSnx layers. The dielectric functions of the high Sn and strain relaxed material were similar to bulk germanium, but with slightly lower energies.
Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the g factor for a single confined electron can be tuned in situ by over 50% with a minimal change in applied voltage.
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