Evolution of the m-plane quantum well morphology and composition within a GaN/InGaN core–shell structure

Coulon, Pierre-Marie and Vajargah, Shahrzad hosseini and Bao, An and Edwards, Paul R. and Le Boulbar, Emmanuel D. and Girgel, Ionut and Martin, Robert W. and Humphreys, Colin J. and Oliver, Rachel A. and Allsopp, Duncan W. E. and Shields, Philip A. (2017) Evolution of the m-plane quantum well morphology and composition within a GaN/InGaN core–shell structure. Crystal Growth and Design, 17 (2). pp. 474-482. ISSN 1528-7483 (https://doi.org/10.1021/acs.cgd.6b01281)

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GaN/InGaN core-shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume and strain relaxation at the free surfaces. The core-shell geometry allows the growth of thicker InGaN shell layers, which would benefit the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two dimensional (2D) growth mode to three dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the <10-10> directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN core-shell interface were observed to occur with a similar frequency as quasi-periodic indium fluctuations along [0001] observed within the 2D layer, to provide evidence that the resulting local strain relief at the steps acts as the trigger for a change of growth mode by elastic relaxation. This study demonstrates that misfit dislocation generation during the growth of wider InGaN shell layers can be avoided by using pre-etched GaN nanorods. Significantly, this enables the growth of absorption-based devices and light-emitting diodes with emissive layers wide enough to mitigate efficiency droop.