Cathodoluminescence and microRaman study of GaN ELO structures
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G7.17.1
Cathodoluminescence and microRaman study of GaN ELO structures O. Martínez1, M. Avella1, J. Jiménez1, B. Gérard2, S. Galloway3 Dpto. Física de la Materia Condensada, E.T.S.I.I., 47011, Valladolid, Spain 2 THALES Research and Technology, Domaine de Corbeville, 91404 Orsay, France 3 Gatan UK, Ferrymills 3, Osney Mead, Oxford, OX2 0ES, United Kingdom 1
ABSTRACT Epitaxial Lateral Overgrowth (ELO) is an effective method to reduce dislocations in heterostructures with large lattice mismatch. This method has been widely used to improve the quality of GaN layers. We present herein a study of the properties of ELO GaN layers grown by HVPE from an MOVPE GaN buffer layer. Cathodoluminescence shows a strong enhancement of the luminescence emission in the ELO regions, where TEM has proven the absence of dislocations. Local cathodoluminescence spectra show that this enhancement is mostly due to the yellow luminescence band. Besides Donor-Acceptor Pair recombination bands are observed in the near band gap spectral range. Raman data show that the ELO layers present a good crystalline quality. The Raman spectra did not reveal the presence of free carriers in concentration high enough to allow the presence of Longitudinal Optic Phonon Plasmon Coupled modes. The results are discussed in terms of the incorporation of impurities in the ELO layers together with the formation of compensating deep acceptors, probably VGa.
INTRODUCTION The luminescence spectrum of GaN presents a broad band centered at 2.25 eV, the so-called yellow luminescence (YL) band, which the whose origin is a matter of controversy [1]. The quality of GaN layers has been related to the relative intensity of the YL band to the near band gap (NBG) luminescence intensity [2]. It was demonstrated that at high injection levels, at which optoelectronic devices operate, the intensity of this band is negligible when compared to the NBG emission [3], however, to understand the origin of this band is essential for improving the quality of GaN layers. Its intensity can vary over a large range of values, and a clear correlation between the growth conditions and the YL intensity has not been established. It has been associated with the presence of extended crystal defects, e.g. dislocations and low angle grain boundaries [4]. This assertion was mostly based on the results reported by Ponce et al [4] and also on the enhancement of its intensity near the layer/substrate interface [5]; however, the observation of the YL even in single crystals where the dislocation density is reduced by several orders of magnitude in relation to the thin layers suggests that the YL is caused by point defects, which eventually could be attracted by the strain field of the dislocations. The absence of YL in Mg-doped (p-type) films suggested that the VGa native defect (deep acceptor) was involved in the center responsible for the YL [6], which was also pointed out by first principle calculations [7]. The possibility to grow layers of superior quality has been demonstrated by using the Epitaxia
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