Non radiative recombination centers in ZnO nanorods
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Non radiative recombination centers in ZnO nanorods 1
D. Montenegro , V. Hortelano2, O. Martínez2, M. C. Martínez-Tomas 1, V. Sallet3, V. Muñoz1 and J. Jiménez2 1
Departamento de Física Aplicada y Electromagnetismo, Universitat de Valencia, Dr. Moliner 50, 46100 Burjassot, Spain 2 GdS-Optronlab, Departamento Física Materia Condensada, Edificio I+D, Universidad de Valladolid, Paseo de Belén 1, 47011, Valladolid, Spain 3 Groupe d'Etude de la Matière Condensée (GEMAC), CNRS-Université de Versailles StQuentin, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France ABSTRACT Nowadays, the nature of the non radiative recombination centres in ZnO is a matter of controversy; they have been related to extended defects, zinc vacancy complexes, and surface defects, among other possible candidates. We present herein the optical characterization of catalyst free ZnO nanorods grown by atmospheric MOCVD by microRaman and cathodoluminescence spectroscopies. The correlation between the defect related Raman modes and the cathodoluminescence emission along the nanorods permits to establish a relation between the non radiative recombination centers and the defects responsible for the local Raman modes, which have been related to Zn interstitial complexes. INTRODUCTION One dimensional ZnO nanostructures have attracted a great deal of attention because of its potential application in UV light emitting devices [1, 2]. The properties of ZnO, as the direct band-gap energy (3.37 eV at room temperature), and the large free exciton binding energy (60 meV), make from it a candidate for highly efficient UV lasers. However, the attractive properties of ZnO are hindered by the difficulty of stable p-doping, and the insufficient knowledge about the electro-optic role played by native defects and their complexes [3,4]. The quantum efficiency of the UV emission is reduced by the presence of deep levels, which are responsible for a broad luminescence band in the yellow-orange spectral window, and the non radiative recombination centers (NRRCs). While the origins of the deep level emission (DLE) have received a great deal of attention, captured in a huge literature, little is known about the NRRCs. It is usually assumed that a high UV/DLE emission ratio means a good crystalline quality; however, this criterion does not take account of NRRCs, which reduce the internal quantum efficiency of both UV and visible emissions. There are non negligible differences in the luminescence emission between samples prepared under different growth methods, giving similar UV/DLE emission ratio, but with very different overall emission levels. It is usually assumed that the non radiative recombination is concerned with extended defects and/ or surface defects. However, good quality hydrothermal crystals are almost dislocation free; also, nanorods present a good crystalline quality, in spite of a few stacking faults. Chichibu el al. [5] argued that the recombination lifetime was governed by the occurrence
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of defect complexes associated with Zn vacancies, VZn, wh
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