Radial Epitaxy of Silicon for Optoelectronic Applications
We show that Si epitaxial growth at the nanoscale becomes size dependent at dimensions significantly larger than the onset of thermodynamic limits, and that this previously unknown behavior within the mesoscopic size regime for one of the best known crystal growth systems is a general phenomenon. A monotonic reduction in homoepitaxial growth rate with size is observed for facet widths below certain scale and the results modeled by an area-dependent precursor incorporation rate. The presence of n and p type dopants results in even greater reductions in low temperature radial growth rates, and a critical thicknesses for Si single crystal radial epitaxy is found for phosphorus. The results provide new insights on the nature of CVD crystal growth at small dimensions and have significant implications for 3D nanoscale fabrication of technologically important Si electronic device architectures.
Si radial p-i-n junction nanowire structure is a suitable platform to integrate fundamental understanding and applications since the structure provides an approach for realizing concurrent maximization of light absorption and photogenerated carrier separation in photovoltaic (PV) devices. Additionally, NW arrays embedding radial p-(i)-n junctions enhance light absorption significantly in wide range of wavelengths. Si radial p-i-n junctions consisted of core Si NWs and Si shells. Core Si NWs were prepared by top-down consisting of lithographic technique and deep reactive ion etching. After formation of Si NWs, the color of Si substrate in NW regions turned to be black from shiny metallic surface. The change of color indicates enhanced light absorption in visible wavelengths. Electrically doped Si radial shell was prepared by epitaxial growth on the surfaces of Si NWs via low-pressure chemical vapor deposition. PV characteristics of crystalline Si radial p-i-n junction arrays prepared by top-down approach and epitaxial shell growth were investigated by current–voltage measurements under dark and Air Mass 1.5G conditions. Though the unoptimal dimensions of NWs and top electrode configuration, the open circuit voltage, short circuit current density, and photoconversion efficiency of the device were 0.46 V, 40 mA/cm2, and 10%, respectively. The high short circuit current density comparable to that of commercialized Si PV cell indicates that efficient photogenerated generation/collection is occurred in Si radial p-i-n junction.