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We investigate in-situ laser reflectometry for measuring the axial growth rate in chemical vapor deposition of assemblies of well-aligned vertical germanium nanowires grown epitaxially on single crystal substrates. Finite difference frequency domain optical simulations were performed in order to facilitate quantitative analysis and interpretation of the measured reflectivity data. The results show an insensitivity of the reflected intensity oscillation period to nanowire diameter and density within the range of experimental conditions investigated. Compared to previous quantitative in-situ measurements performed on III–V nanowire arrays, which showed two distinct rate regimes, we observe a constant, steady-state nanowire growth rate. Furthermore, we show that the measured reflectivity decay can be used to determine the germanium nanowire nucleation time with good precision. This technique provides an avenue to monitor growth of nanowires in a variety of materials systems and growth conditions. [ABSTRACT FROM AUTHOR]

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 Ge1-xSnx 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 Ge1-xSnx 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. [ABSTRACT FROM AUTHOR]