Does In form In-rich clusters in InGaN quantum wells?

Abstract

The reason the InGaN/GaN quantum well system emits intense light even though the dislocation density is high is assessed. First, the evidence from electron microscopy for nanometre-scale In-rich clusters in InGaN quantum wells is presented. Such clusters would localize the excitons away from the dislocations and hence the dislocations would not quench the light emission, consistent with observations. However, it is then shown that InGaN damages extremely rapidly under an electron beam, and that the damage causes In-rich clusters to form. No evidence is found of gross indium clustering at low beam currents and short exposure times in the electron microscope. However, at such low electron doses the image is noisy, and low-level indium clustering in InGaN quantum wells could possibly exist, but be masked by the noise. A different technique, 3-dimensional atom probe field ion microscopy, has therefore been used to image the InGaN quantum wells. This reveals that InGaN is a random alloy, with the local statistical fluctuations in indium content expected in a random alloy, but with no indium clustering. Since In-rich clusters are clearly not necessary for bright light emission from InGaN quantum wells, another mechanism must be responsible for the exciton localization observed. It is shown that thickness fluctuations in the InGaN quantum wells of only one monolayer, observed in electron microscopy, result in an exciton confinement energy of 58 meV, sufficient to localize the carriers at room temperature. An alternative localization mechanism due to randomly formed In-N-In chains proposed by others is discussed. It is concluded that In-rich clusters in InGaN quantum wells do not exist in the specimens we have studied, and in any case they are not necessary to localize the excitons and for bright light emission.

Acknowledgements

The author is grateful to Dr T M Smeeton, M J Galtrey, and Dr M J Kappers for stimulating discussions, and to the EPSRC, the EU PARSEM grant, and Thomas Swan Scientific Equipment Limited for financial support.