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Optical properties of nanocrystalline ZnO thin films grown using pulsed laser deposition

McGlynn, Enda orcid logoORCID: 0000-0002-3412-9035, Fryar, James, Tobin, Gerard, Roy, C., Byrne, S., Mosnier, Jean-Paul, de Posada, Eduardo, O'Mahony, Donagh, Lunney, James G. and Henry, Martin O. (2006) Optical properties of nanocrystalline ZnO thin films grown using pulsed laser deposition. In: Chang, John V., (ed.) New Developments in Condensed Matter Physics. Nova Science Publishers, New York, USA, pp. 105-128. ISBN 1-59454-822-6

Abstract
Raman spectroscopy, x-ray diffractometry, atomic force microscopy, photoluminescence spectroscopy and reflectance spectroscopy have been used to characterize ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. Raman results show enhancement and broadening of certain Raman features which correlate with changes in the widths of the x-ray diffraction peaks for samples with varying grain size in the 50-100 nm range. These data suggest that electric fields, arising from charge trapping at grain boundaries, in conjunction with localised and surface phonon modes, are the cause of the intensity enhancement and asymmetry of the Raman features. Band-edge photoluminescence and reflectance spectra also altered considerably with increases in grain size, showing clearly observable excitonic structure in the reflectance spectra. An analysis using a deformation potential Hamiltonian demonstrates that the experimental exciton energies are not explicable solely in terms of sample strain and give additional evidence for electric fields in the samples due to charge trapping at grain boundaries. This is supported by theoretical estimates of the exciton energy perturbation due to electric fields and also by the behaviour of the green band in the samples. Detailed studies show that reflectance spectra in nanocrystalline ZnO differ substantially from bulk material. Interaction of excitons, damped by strong electric field effects, with photons leads to exciton-polaritons with substantial damping, eliminating the normal Fabry-Perot structure seen in thin films. Good qualitative agreement is achieved between the model and data and the conclusions are also in good agreement with the photoluminescence and Raman data. Finally, high intensity optical pumping data of these samples again shows a dependence on grain size. All samples show evidence of high excitation effects and the sample with the largest grain size displays random lasing at room temperature. All our results indicate the very strong influence of electric fields due to charge trapping at grain boundaries on the optical properties of nanocrystalline ZnO.
Metadata
Item Type:Book Section
Refereed:Yes
Uncontrolled Keywords:ZnO; thin films; photoluminescence; x-ray diffraction; Raman spectroscopy; exciton; stimulated emission
Subjects:Engineering > Materials
Physical Sciences > Nanotechnology
Physical Sciences > Semiconductors
Physical Sciences > Spectrum analysis
Physical Sciences > Thin films
DCU Faculties and Centres:DCU Faculties and Schools > Faculty of Science and Health > School of Physical Sciences
Research Initiatives and Centres > National Centre for Plasma Science and Technology (NCPST)
Publisher:Nova Science Publishers
Copyright Information:© 2006 Nova Science Publishers, Inc.
ID Code:23275
Deposited On:09 May 2019 12:27 by Enda Mcglynn . Last Modified 29 Apr 2021 09:16
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