Cathodoluminescence study of defects created by Vickers indentation in hydrothermal ZnO crystals
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. Avella and J. Jiméneza) Física de la Materia Condensada, ETSII, 47011 Valladolid, Spain
M. Callahan and D. Bliss Air Force Research Laboratory, Sensors Directorate, Hanscom Air Force Base, Massachusetts 01731
Buguo Wang Solid State Scientific Corporation, Hollis, New Hampshire 03049 (Received 23 July 2007; accepted 12 September 2007)
Vickers indentations of ZnO crystals grown by the hydrothermal method were studied by cathodoluminescence. The defects induced by indentation influenced the luminescence spectrum, indicating the generation of non radiative recombination centers and a band close to the first phonon replica of the free exciton, in the surrounding area near the indentation. The possible nature of the defects responsible for such band is discussed. A comparison with polishing induced damage is also presented.
I. INTRODUCTION
ZnO is a wide band gap semiconductor with many potential applications for ultraviolet (UV) optoelectronics. It exhibits several properties that make it a candidate of choice for high-power, high-efficiency UV light emitting devices, i.e., large band gap (3.4 eV), large free exciton binding energy (60 meV), and low-voltage threshold.1 However, the internal quantum efficiency of ZnO excitonic emission is limited by competitive recombination paths, mostly related to point defects and extended defects. To improve the suitability of ZnO for high-efficiency UV optoelectronic devices, a better comprehension of these defects is needed. The defects play a pivotal role in creating luminescence bands, the origin of which is still a matter of controversy.2,3 Therefore, a fundamental understanding is needed to boost the development of ZnO-based devices. It is our inability to control the defects that not only reduce the efficiency of the excitonic emission, but also compensates p-doping, which makes it difficult to achieve stable p-type material. On the other hand, large ZnO crystals are available, which should allow the homoepitaxial growth of ZnO films.4 However, improvements in the crystal growth and surface treatments are required to grow optical grade
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Address all corresponding to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0444 3526
http://journals.cambridge.org
J. Mater. Res., Vol. 22, No. 12, Dec 2007 Downloaded: 13 Mar 2015
epitaxial layers on these substrates. The surface finish has been proven to have a significant influence on the luminescence spectrum,5 which presents marked differences from sample to sample, in both the overall efficiency and the spectral shape.6 This variability is the consequence of both the existence of point defects due to the growth process, and the defects created by polishing. Indentation is a suitable method to induce mechanical damage in crystals. Some recent articles concerning the effects of indentation on the luminescence efficiency of ZnO are available.7–9 Lattice disorder induced by mechanical damage leads to a decrease of the luminescence efficiency; nevertheless, some reports have claimed that radiative recombination cente
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