Optical properties and carrier dynamics of ensembles of InP nanowires grown on nonsingle-crystal platforms
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Optical properties and carrier dynamics of ensembles of InP nanowires grown on nonsingle-crystal platforms Takehiro Onishi1,2 , Andrew J. Lohn1,2,3, and Nobuhiko P. Kobayashi1,2,3 1 Baskin School of Engineering at University of California Santa Cruz, Santa Cruz, CA 95064 U.S.A. 2 Nanostructured Energy Conversion Technology and Research (NECTAR) at Advanced Studies Laboratories of University of California Santa Cruz NASA Ames Research Center, Moffett Field, CA 94035 U.S.A. 3 Information and Quantum Systems Laboratory at Hewlett-Packard Laboratories, Palo Alto, CA 94034 U.S.A. ABSTRACT Optically active InP nanowires were grown on a quartz substrate covered with a layer (100 nm) of hydrogenated microcrystalline silicon (mc-Si:H) by metalorganic chemical vapor deposition (MOCVD), demonstrating that single-crystal semiconductor nanowires can be formed on non-single-crystal surfaces. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, cathodoluminescence (CL), and photoluminescence (PL) were used to characterize the structural and optical properties of the nanowires. The nanowires on a-Si:H grew in random directions with high density. The XRD suggests that nanowires having either hexagonal-close-packed or face-centered cubic lattices co-exist. The Raman spectrum shows peaks associated with transverse optical (TO) and longitudinal optical (LO) branches of InP. The CL intensity does not vary significantly along the growth direction and appears to be originated from the entire structure of the nanowire when probed at various positions. The CL data suggests that recombination is slow enough to allow the carriers to diffuse the complete length of the nanowires (~2 um in length) before recombining. The PL spectrum suggested the nanowire tip which has a size close to Bohr radius 8.52 nm contributed to the observed blue shift while the root part had nearly bulk feature in their structure. INTRODUCTION Recently nanowires have acquired growing interests due to their fascinating properties and applications superior to their bulk counterparts [1].The Vapor-Liquid-Solid (VLS) growth process proposed in 1964 [2] has been one of the most popular methods for growing crystalline nanowires. The VLS mechanism works in the existence of metal nanoparticle catalyst. In the VLS growth process, Nanowires grow with a neck that lies underneath the nanoparticle. It has been postulated that the nanowires grown by the VLS highly reflect the crystallographic structure provided by a starting substrate. Therefore, it is common that semiconductor nanowires are grown on a single crystal substrate to ensure that a specific crystallographic structure is available. However, this is a necessary condition. A few decade nanometer scale local ordering of the substrate at the root of the growing nanowire is sufficient for growth [3]. The nanowires studied in this paper were grown on microcrystalline silicon layers deposited on a variety of non-single-crystalline substrates including quartz and stainless steel. The microc
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