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Does electron microscopy produce In clustering in InGaN?
|Colin J. Humphreys 1, Tim M. Smeeton 2, Mark J. Galtrey , Jonathan S. Barnard , Menno J. Kappers
1. University of Cambridge, Department of Materials Science and Nanoscience Centre, Pembroke Street, Cambridge CB2 3QZ, United Kingdom
High resolution electron micrographs (HREM) of InGaN/GaN quantum wells from many research groups appear to demonstrate very indium-rich clusters on a nanometer scale and PL results suggest that the excitons in InGaN quantum wells have a localisation of about 2 nm. However, we have studied bright blue and green LEDs grown by our Thomas Swan MOCVD reactor and from elsewhere, and our HREM results using a very low intensity electron beam show no evidence for such strong indium clustering. However, nanometre-scale strain inhomogeneities are introduced into the quantum wells by radiation damage very soon after exposing the material to the electron beam. The image features caused by the radiation damage are similar to those expected to be caused by indium-rich regions so we therefore suggest that reports of indium clustering may be influenced by electron beam damage artefacts. We find that in each quantum well the lower InGaN/GaN interface is atomically smooth, but that the upper interface has atomic height steps, and the lateral scale of these steps is on a nanometer scale. Because of the strong piezoelectric effect in GaN, an atomic size step in an InGaN quantum well is sufficient to cause exciton localisation. It is suggested that this may be the mechanism, or a contributory mechanism, to the exciton localisation observed in InGaN/GaN quantum wells.
Presentation: invited oral at E-MRS Fall Meeting 2005, Symposium A, by Colin J. Humphreys
See On-line Journal of E-MRS Fall Meeting 2005
Submitted: 2005-05-27 15:40 Revised: 2009-06-07 00:44