Optical Properties of Zinc Oxide Quantum Dots Embedded Films by Metal Organic Chemical Vapor Deposition
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Optical Properties of Zinc Oxide Quantum Dots Embedded Films by Metal Organic Chemical Vapor Deposition S. T. Tan1,2, X. W. Sun1,*, X. H. Zhang2, B. J. Chen1, and S. J. Chua2 1 School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore. 2 Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore. * Author to whom correspondence should be addressed; electronic mail: [email protected] ABSTRACT Zinc oxide (ZnO) quantum dots (QDs) embedded films were fabricated on silicon substrates by metal organic chemical vapor deposition at 350oC. The QDs can be obtained in a matrix of amorphous ZnO films by introducing a large amount of precursors. The size of the QDs ranged from 3 to 12 nm, which was estimated by high-resolution transmission electron microscopy. The photoluminescence measured at 80 K showed that the emission of QDs embedded film ranged from 3.0 to 3.6 eV. The broad near-band-edge emission is due to the quantum confinement effect of the QDs. The quantum confinement effect of the QDs was quenched after the post-growth annealing due to the ripening of QDs. INTRODUCTION Zinc oxide (ZnO) is a wide-direct-bandgap semiconductor that has been intensively investigated over the decades due to its potential as a promising material for optoelectronic applications. It is well known that ZnO has a large exciton binding energy of 60 meV [1] that is much higher than the room temperature thermal energy (26 meV). This makes ZnO much more attractive, as the large exciton binding energy will result in more efficient ultraviolet (UV) emission even at room temperature [2, 3]. The nanostructures of the ZnO material have also been investigated intensively recently due to their unique and novel properties over the bulk materials. In fact, ZnO shows the richest nanostructural morphologies of all materials. Among these ZnO nanostructures, nanowire [4], nanorod [5], nanopin [6], nanoring [7], nanodisk [8], and quantum dots (QDs) [9, 10], have been fabricated and investigated. It is expected that the significance of the exciton effects will become more prominent in nanostructure materials especially in zero-dimensional nanostructures, i.e. QDs. Hence, QDs are expected to have many interesting and useful properties for opto-electronic applications, for example, QDs in semiconductor laser application, which was firstly proposed by Arakawa et al. [11] in 1982. However, most reports focus on obtaining single-crystal ZnO thin films, while there is little attention paid to QDs fabrication. In this paper, we shall report the optical properties of ZnO QDs embedded in a matrix of amorphous ZnO films and discuss the dependency of QDs bandgap on dot size prepared by metal organic chemical vapor deposition (MOCVD). EXPERIMENT The ZnO QDs used in this paper were embedded in a matrix of amorphous ZnO films grown on Si (100) by MOCVD. The growth temperature was set at 350oC. DimethylZinc (DMZn), N2 gas and high-purity O2 were used as the z
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