Visible Luminescence from ZnO Nanostructures
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Visible Luminescence from ZnO Nanostructures An-jen Cheng1, Dake Wang2, Hee Won Seo2, Minseo Park2, and Yonhua Tzeng1 1 Alabama Micro/Nano Science and Technology Center, Department of Electrical and Computer Engineering, Auburn Univeristy, Auburn, AL, 36849 2 Department of Physics and Laboratory for Nanophotonics, Auburn Univeristy, Auburn, AL, 36849 Abstract Room temperature photoluminescence (PL) spectra from zinc oxide (ZnO) nanostructures were studied. ZnO samples were produced via thermal chemical vapor deposition (thermal-CVD) and a variety of ZnO nanostructures were synthesized by adjusting the oxygen content during the growth process. All samples exhibit a sharp and strong ultra-violet near-band-edge (NBE) emission at about 3.18 eV. The visible emission from the samples deposited under an oxygen-deficient condition were dominated by bluegreen band emission at 2.34 eV. The intensity of the blue-green band was greatly reduced (so-called green band free) for the ZnO deposited at the center of the wafer while strong violet-blue emission bands and broad bands at yellow-orange-red range were collected from the ZnO grown along the edge of the wafer. We believe that the spatial inhomogeniety was caused by turbulent gas flow in the reaction chamber, which resulted in different local oxygen concentration. Origin of visible luminescence from ZnO nanostructures will be discussed and a model to explain the observed visible luminescence process will be presented.
Introduction Functional metal oxide semiconductor nanostructures with low dimensionality have received dramatically increasing attention over the past few years. Among the group of functional metal oxides, ZnO shows its remarkable electrical and optical properties. ZnO is a wide band gap semiconductor with a large band gap of 3.37 eV and an exciton binding energy of 60 meV, which make it a promising material for optoelectronic applications in UV spectrum. In addition, 1-dimensional (1D) ZnO exhibits novel properties due to the quantum confinement effect and/or its very large surface-to-volume ratio. Photoluminescence (PL) behavior of ZnO exhibits a band edge UV emission peak and a broad visible emission band due to the deep level intrinsic and/or extrinsic defects1. The mechanism of the broad-band visible emission, especially the blue-green emission, still remains controversial. Among the various hypotheses intended to explain the visible emission, oxygen vacancies (VO) seems to be the most accepted mechanism although no consensus has been reached regarding the charge state of the oxygen vacancy; i.e., singly ionized2 or doubly ionized oxygen vacancy3. In this work, different PL spectra were observed by varying the concentration ratio of oxygen to zinc vapor by means of varying the corresponding gas flow rates. It is confirmed that oxygen is the most important factor that causes the broad visible emission. Furthermore, the spectrum with strong violet-blue emission bands and broad bands at yellow-orange-red range is first discovered and
discussed.
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