Localisation of Excitation in InGaN Epilayers and Quantum wells
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Localisation of Excitation in InGaN Epilayers and Quantum wells Kevin Peter O'Donnell Physics, Strathclyde University, John Anderson Building, 107 Rottenrow East, Glasgow, G4 0NG, United Kingdom ABSTRACT Using a synchrotron source, extended X-Ray Absorption Fine Structure (EXAFS) measurements were performed at the Ga and In K-edges of a range of (In,Ga)N samples, grown either by MBE or MOCVD. The resulting determinations of local alloy structure, complemented in selected cases by asymmetric x-ray diffraction reciprocal space mapping (XRD-RSM), show an inequality in the “mixed cation” separations, Ga-In and In-Ga, for samples with InN content less than about 50%. This asymmetry, which increases with decreasing InN content of the layers, is related to the high luminescence efficiency of the materials through a combination of percolation and localization of excitation on the GaN and InN sub-lattices, respectively.
INTRODUCTION Spatial localization of excitation by “In-rich regions” is routinely invoked to explain the high efficiency of (In,Ga)N luminescent devices in the face of large defect populations. The most direct evidence for such localization is the characteristic “spotty” luminescence texture, with a length scale of order 100 nm, seen in PL and CL micrographs of InGaN epilayers [1]. The nature of the localization mechanism in InGaN is however a matter of continuing dispute. Self-formed or ‘accidental’ InN quantum dots, In-rich (properly, InN-rich) InGaN clusters, InN ‘crowdions’ are all candidate centers for exciton localization. However, the discovery of the narrow band gap of InN [2], recently downsized to less than 1 eV, makes it seem unlikely that pure InN quantum dots can be solely responsible for InGaN luminescence, since quantum confinement would have to be very strong to upshift the emission energy by ~2 eV from the InN band edge to the visible spectral region: pure InN dots with emission near 3 eV necessarily contain only a few In atoms. Despite nearly 10 years of experimental effort, no uncontested evidence of a relation between InN-GaN phase segregation and enhanced luminescence efficiency of InGaN has been obtained. But phase segregation should lead to a characteristic non-randomness in the cation distributions in a common-anion pseudobinary alloy, and we can exploit experimental techniques that reveal this. Extended X-ray Absorption Fine Structure (EXAFS) analysis provides a unique structural tool to determine the environment of specific atoms in a solid, the so-called local (crystal) structure [3]. EXAFS is a modulation of the absorption coefficient above a characteristic X-ray absorption edge of the “target” atomic species. It results from the scattering of ejected photoelectrons by atoms in the neighborhood; these interfere with the outgoing photoelectron wave and hence modulate the absorption coefficient. EXAFS analysis reveals the chemistry and coordination number of surrounding atoms and their radial separation from the central absorber. 1
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