Ensemble Effects on the Optical Properties of Indium Phosphide Nanowires at Various Temperatures
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Ensemble Effects on the Optical Properties of Indium Phosphide Nanowires at Various Temperatures Andrew J. Lohn, Milo Holt, Noel Dawson, Nobuhiko P. Kobayashi Baskin School of Engineering, University of California Santa Cruz, Santa Cruz, CA 95064 USA and Nanostructured Energy Conversion Technology and Research (NECTAR), Advanced Studies Laboratories, Univ. of California Santa Cruz – NASA Ames Research Center, Moffett Field, CA 94035
Abstract Ensembles of indium phosphide nanowires were grown on amorphous quartz substrates and their optical properties were examined at various cryogenic temperatures. Complex dynamics result from the large areal densities, random orientation, combination of both zincblende and wurtzite phases, and the geometries of the nanowires. Those complex dynamics are discussed in relation to their effect on the temperature dependence of photoluminescence and Raman spectroscopy. Five peaks are found to exist in the photoluminescence spectra at low temperatures which are attributed to radiative recombinations associated with quantum confined zinc blende, quantum confined excitons in zinc blende, quantum confined wurtzite, excitons in bulk zinc blende and impurity states. An energy transfer mechanism between two types of radiative recombinations among the five is proposed to explain intensity variations and the temperature dependence of the PL peaks is discussed. The Raman spectra is observed to have peaks created by a combination of zinc blende and wurtzite vibrational modes which is explained by folding the phonon dispersion.
Introduction Optical devices such as photodetectors and solar cells utilizing nanowires as the optically active material are becoming interesting alternatives to traditional thin-film technologies. The most popular method for growing single-crystal nanowires is through the vapor-liquid-solid (VLS)1 mechanism whereby a gold nanoparticle is used to catalyze nanowire growth on a crystalline substrate. The single-crystal substrates provide epitaxial information for the nanowire but represent a significant cost if the technology is to be scaled up. By using microcrystalline surfaces on amorphous substrates it is possible to retain the epitaxial information at length scales relevant to nanowire growth while significantly reducing the cost of nanowire-based optical devices. Many of these devices will need to employ ensembles of nanowires but the majority of reports on optical characterization still focus on nanowires in isolation2, 3, 4 despite the fact that the effects inherent to ensembles of nanowires, particularly those prepared on amorphous
substrates, are of our interests due to the large collective volume of ensembles of nanowires. These optical devices are of particular interest because some of the fundamental restrictions of epitaxial growth are lightened allowing for integration of group III-V compound semiconductors in the form of nanowires onto various material platforms such as silicon and even amorphous materials5. Epitaxial growth of single crystal II
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