Nature of germanium nanowire heteroepitaxy on silicon substrates

Systematic studies of the heteroepitaxial growth of germanium nanowires on silicon substrates were performed. These studies included the effect of sample preparation, substrate orientation, preanneal, growth temperature, and germane partial pressure on the growth of epitaxial germanium nanowires. Scanning electron microscopy and transmission electron microscopy were used to analyze the resulting nanowire growth. Germanium nanowires grew predominantly along the ⟨ 111 ⟩ crystallographic direction, with a minority of wires growing along the ⟨ 110 ⟩ direction, irrespective of the underlying silicon substrate orientation [silicon (111), (110), and (100)]. Decreasing the partial pressure of germane increased the number of ⟨ 111 ⟩ nanowires normal to the silicon (111) surface, compared to the other three available ⟨ 111 ⟩ directions. The growth rate of nanowires increased with the partial pressure of germane and to a lesser degree with temperature. The nucleation density of nanowire growth and the degree of epitaxy both increased with temperature. However, increasing the growth temperature also increased the rate of sidewall deposition, thereby resulting in tapered nanowires. A two-step temperature process was used to initiate nanowire nucleation and epitaxy at a high temperature, followed by nontapered nanowire growth at a lower temperature. Preannealing gold films in hydrogen or argon before nanowire growth reduced the yield of nanowires grown on silicon samples, especially on silicon (111) substrates, but not on silicon oxide. Gold annealing studies performed to investigate this preanneal effect showed greater gold agglomeration on the silicon samples compared to silicon oxide. The results and conclusions obtained from these studies give a better understanding of the complex interdependencies of the parameters involved in the controlled heteroepitaxial growth of vapor-liquid-solid grown germanium nanowires.