Formation of Hexagonal Ge Stripes on the Side Facets of AlGaAs Nanowires: Implications for Near-Infrared Detectors

Obtaining hexagonal Ge is a great challenge and has previously been achieved by transferring the wurtzite phase of GaAs nanowires, where Ge forms a conformal shell around a nanowire. Here, we demonstrate hexagonal Ge stripes of 6 nm height and 17 nm width, which decorate 25 nm-wide side facets of wurtzite AlGaAs nanowires. Ge shells are grown by molecular beam epitaxy at a low temperature of 320 °C. The hexagonal structure of Ge is revealed by Raman spectroscopy and high-resolution transmission electron microscopy (TEM), the latter shows the pure 2H phase. The formation of Ge stripes on the side facets of AlGaAs nanowires is demonstrated by scanning TEM in the angular annular dark-field mode and explained within an energetic model. These results show a possibility of forming 2H wire-like Ge structures with tunable geometry using AlGaAs nanowire templates. Such structures can be exploited for direct band gap engineering and quantum confinement effects and used in near-infrared optoelectronics based on nanostructured Ge.