The realization of n-type conduction in directly bottom-up grown Si-doped GaAs nanowires (NWs) by molecular beam epitaxy has remained a long-standing challenge. Unlike the commonly employed vapor–liquid−solid growth, where the amphoteric nature of Si dopants induces p-type conduction, we report a completely catalyst-free, selective area molecular beam epitaxial growth that establishes n-type behavior under Si doping. The vapor–solid selective area growth on prepatterned Si (111) substrates is enabled by an important in situ substrate pretreatment to create an As-terminated 1 × 1-Si(111) substrate necessary for the growth of [111]-oriented GaAs:Si NWs with a large aspect ratio and high yield. Correlated resonant Raman scattering and single-NW micro-photoluminescence (μPL) experiments confirm the n-type nature of the Si-doped GaAs NWs evidenced by a dominant SiGa local vibrational Raman mode, a distinct band filling effect (up to > 10 meV) along with increased PL peak broadening upon increased Si concentration. Excessive Si doping is further found to induce some auto-compensation evidenced by red-shifted PL and the appearance of minor SiAs and SiGa–SiAs pair-like local vibrational Raman modes. Employing excitation power dependent μPL, we further discern signatures in below-gap defect luminescence (∼1.3–1.45 eV) arising from structural defects and Si dopant-point defect complexes.

Silicon dioxide films were deposited on crystalline silicon substrates by electron cyclotron resonant (ECR) microwave plasma-enhanced chemical vapor deposition (PECVD). Films were grown on Si〈100〉 substrates at temperatures of 140–600 °C, flow rates of 0.5–10 sccm SiH4, 10–30 sccm O2, and at a pressure of 10−3 Torr. Infrared absorption spectroscopy of the samples indicated no detectable SiH, OH, or SiOH groups. Neither an afterglow chemistry nor He dilution was required to eliminate H impurities as was previously reported for silicon oxide films deposited from rf plasmas. This suggests that significant differences exist between rf and ECR microwave plasma chemistries. We have found that the stoichiometry and index of refraction was not sensitive to oxidant ratio for a wide range of conditions in contrast to other studies. Stoichiometric SiO2 films, with good physical properties, were grown for a much wider range of oxidant ratios relative to those which are characteristic of the rf PECVD technique. In addition, films grown under optimal conditions had infrared absorption spectra nearly identical to those of thermally grown oxides and index of refraction of 1.456, as measured by ellipsometry. We concluded that by using an ECR microwave plasma, SiO2 films with optical and bonding properties comparable to oxides thermally grown at 1000 °C in dry oxygen can be deposited at a low temperature (350 °C) and a low pressure (10−3 Torr) in a O2/SiH4 reactant gas mixture without the need for a carrier gas.

No abstract

No abstract