On the interpretation of structural and light emitting properties of InGaN/GaN epitaxial layers grown above and below the critical layer thickness

Abstract

In this contribution, the optical and structural properties of InGaN/GaN layers grown by metal-organic chemical vapour deposition (MOCVD) are studied. The main focus of this investigation is on the difference between microstructural and luminescence characteristics, for layers grown below and above the critical layer thickness (CLT) for elastic strain relaxation. By comparing the photoluminescence properties of samples grown under the same nominal conditions, except the deposition time, it is shown that in InGaN films grown above the CLT ( x), an additional lower energy secondary luminescence component emerges. Specifically, for an InGaN layer with x∼0.1, the energy splitting between the two components is about 160 meV. The surface of samples with thicknesses larger than CLT( x), are found to be rough with pronounced islanding occurring, indicating that a Stranski–Krastanow 2D to 3D growth mode transition takes place after the CLT. A detailed structural characterization by high-resolution reciprocal space mapping reveals that the appearance of 3D islands is associated with elastic strain relaxation. Strong lateral and depth variations of the strain field associated to a peculiar 2D/3D growth mode can explain structural and optical properties, which are typically considered “anomalous” and frequently ascribed to phase segregation effects in InGaN. A simple calculation based on elastic strain relaxation, accounts for the observed energy splitting on the photoluminescence.