Characterization of Red Emission in Nominally Undoped Hydride Vapor Phase Epitaxy GaN
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Internet Journal Nitride Semiconductor Research
Characterization of Red Emission in Nominally Undoped Hydride Vapor Phase Epitaxy GaN E.M.Goldys1, M. Godlewski1, T. Paskova2, G. Pozina2 and B. Monemar2 1Division
of Information and Communication Sciences, Macquarie University, of Physics and Measurement Technology, Linköping University,
2Department
(Received Friday, November 10, 2000; accepted Thursday, January 18, 2001)
We report characterization of the red emission band in hydride vapor phase epitaxial GaN using cathodoluminescence spectroscopy and imaging and time-resolved photoluminescence. The observed properties of the emission are consistent with recombination of excitons bound at close donor-acceptor pairs. The time evolution of the emission signal during electron beam irradiation supports the association of the red emission with charged centres.
1
Introduction
Light emission from GaN films is a sensitive probe of defects and impurities present in the material and the associated radiative and nonradiative processes. In addition to the band edge emission, deep emission bands frequently observed in this material continue to be a subject of intensive studies. The yellow band with the peak at about 2.2 eV, common in wurtzite GaN, has attracted a significant research effort as it was considered to compete with the technologically desirable edge emission [1]. Its red counterpart in cubic GaN was examined as well [2]. In this work we concentrate on the red emission band present in wurtzite GaN films grown by hydride vapor phase epitaxy (HVPE). The HVPE technique often yields films with a broad emission band similar in appearance to the yellow band, but with the maximum at about 1.8-1.9 eV. The purpose of this paper is to examine its properties so that a framework can be established for the microscopic picture of the relevant defects. Relative to the band edge emission, the red band is weaker than the yellow band. The strength of the red band varies from sample to sample; this study was carried out on films selected for the high intensity of the red band. Measurements reported by other authors on HVPE grown films indicate a sporadic coexistence of the yellow and red bands [3] [4]. In our films, however, the yellow band was absent. The red emission band discussed here should be distinguished from other bands also referred to as "red", these include the red emission in cubic GaN examined by the present authors [2], and the red band in GaN codoped with Mg and Si [3].
The red emission is observed both in nominally undoped HVPE GaN films grown on a- and c-oriented sapphire substrate without buffers (for details of growth procedures see Ref [5]), and in HVPE films grown on GaN templates prepared by metalorganic chemical vapor deposition [6]. The latter layers show a much higher degree of structural quality than the films simply deposited on sapphire, thus suggesting inadvertent defect incorporation in the growth process. 2
Experiments
The GaN films were grown on a-sapphire without any buffers, but using pre-nit
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