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Structural and optical properties of InGaN/GaN layers close to the critical layer thickness

PEREIRA, SERGIO MANUEL DE SOUSA and Correia, M.R. and Pereira, Eduarda and Trager-Cowan, Carol and Sweeney, Francis and O'Donnell, Kevin and Alves, E. and Franco, N. and Sequeira, A.D. (2002) Structural and optical properties of InGaN/GaN layers close to the critical layer thickness. Applied Physics Letters, 81 (7). p. 1207.

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In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1−xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1−xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at ∼2.50 and ∼2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectroscopy. DSR leads directly to the emergence of the second, lower-energy, peak. Based on this experimental evidence, it is argued that the appearance of luminescence doublets in InGaN is not evidence of “quantum dotlike In-rich” or “phase separated” regions, as commonly proposed.