Correlation Between Microstructure and Optical Properties of ZnO Based Nanostructures Grown by MOCVD
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1074-I07-08
Correlation Between Microstructure and Optical Properties of ZnO Based Nanostructures Grown by MOCVD Farid Falyouni1, Julien Barjon1, Vincent Sallet1, Alain Lusson1, Guy Garry2, and Pierre Galtier1 1 Groupe d'Etude de la Matière Condensée-GEMaC, Université de Versailles Saint QuentinCentre National de la Recherche Scientifique, 45, avenue des Etats-Unis, Versailles, 78035, France 2 Thales Research & Technology France, RD 128, Palaiseau, 91767, France ABSTRACT The correlation between structural properties of ZnO sharp conical needles grown by Metallorganic Chemical Vapor Deposition (MOCVD) on sapphire substrate and their optical signature measured by low temperature cathodoluminescence (CL) is investigated. Transmission Electron Microscopy (TEM) shows the excellent structural properties of these needles from their base up to the end of the tip. In order to probe the emission of the needles along their length, UV CL mapping has been performed at low temperature on a single needle previously characterized by TEM. A clear blue shift of 25meV is observed for the excitonic emission close to the needle tip. This shift is too high to be fully attributed to quantum confinement. Although, it qualitatively agrees with previous observations which assigned it to a surface contribution becoming dominant upon size shrinking, the effect is less pronounced. The results are discussed in term of surface quality and other possible contributions associated to a decrease of the n-dopant concentration and to quantum confinement effect close to the top of the needle. INTRODUCTION There has been a considerable interest in the realization of semiconductor based nanostructures thanks to the expected modification of their physical properties with size shrinking. Furthermore, the reduced size and high surface to volume ratio of such nanostructures make possible the growth of strongly mismatched, dislocation free, heterostructures and quantum wells in rods or wires that are not possible with classical 2D epitaxial growth. Among various materials, ZnO is particularly interesting due to its wide band gap (3.37eV), large exciton binding energy (60 meV), great optical transparency combined with a high electrical conductivity and shows great promise for applications to UV laser, lighting and biotechnology. Thus the development of bandgap engineering at the nanometer scale requires monitoring the structural integrity and functional properties down to the nanometer scale. In this context, the spectroscopic characterization of ZnO nanostructures is of particular interest. Although, as stressed above, their fabrication should allow achieving new type of objects with high structural quality and enhanced optical, properties, recent results suggest that their real structural properties should be addressed in order to ensure that they are of device-grade quality. For example, it has been shown that ZnO nanorods exhibit a blue shift in their emission spectra with size beyond the quantum confinement regime [1,2]. On the other hand, other study sho
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