The Structural and Optical Properties of Self-assembled InGaN/GaN Quantum Dots Grown by Molecular Beam Epitaxy
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The Structural and Optical Properties of Self-assembled InGaN/GaN Quantum Dots Grown by Molecular Beam Epitaxy Tim M Smeeton, Mathieu Sénès, Katherine L Smith, Stewart E Hooper, and Jon Heffernan Sharp Laboratories of Europe, Edmund Halley Road, Oxford Science Park, Oxford, OX4 4GB, United Kingdom
ABSTRACT The structural and optical properties of InGaN quantum dots grown by plasma-assisted molecular beam epitaxy (MBE) have been characterised using atomic force microscopy, high-resolution transmission electron microscopy (TEM), Z-contrast scanning TEM, micro-photoluminescence (PL), temperature dependent PL and time-resolved PL. The uncapped InGaN nano-islands have densities of ~1.5 × 1011 cm-2, heights of (1.7 ± 1.0) nm and diameters of (10 ± 4) nm. These parameters are not substantially changed during overgrowth of a GaN cap and the resulting quantum dots have a composition of In0.15Ga0.85N. The observation of narrow luminescence peaks in micro-PL measurements proves light emission from discrete energy states and the optical properties indicate strong confinement of carriers in the quantum dots and an unusually weak impact of piezoelectric field effects. INTRODUCTION Significant improvements in the performance of InGaN-based optoelectronic devices are predicted if quantum dot active regions are used instead of quantum wells [1]. The low threshold currents and high temperature stabilities expected for laser diodes with active regions containing homogeneous quantum dot ensembles with narrow distributions of discrete and widely spaced energy levels are particularly attractive [1,2]. Although it has been widely reported that many InGaN quantum wells exhibit exciton localisation effects similar to those expected in quantum dot systems [e.g. 3,4], the precise origin and significance of these effects continues to be debated [e.g. 3,5]. Irrespective of this debate, a promising route towards exploiting the advantages of InGaN quantum dot active regions is to prepare them using the controlled self-organised epitaxial growth of InGaN nano-islands – an approach analogous to the one used to develop In(Ga)As quantum dot lasers [e.g. 2]. The growth of InGaN nano-islands has been reported using molecular beam epitaxy (MBE) [6,7,8] and metalorganic vapour phase epitaxy (MOVPE) [9]. High densities (~1010-1011 cm-2) of nanoislands were formed for MBE growth using either ammonia [6] or a nitrogen plasma [7,8] as the nitrogen source. After overgrowth with a GaN cap some of these samples [6,7] were shown to exhibit optical properties unlike those of InGaN quantum wells and consistent with confinement of carriers in quantum dots. However, conclusive evidence uniquely attributable to quantum dot behaviour was not given and the nanostructure of the InGaN after cap growth was not reported. MOVPE samples were shown to exhibit luminescence from discrete energy levels characteristic of quantum dot emission [10] but again no detail of the structure of these quantum dots in the capped samples was provided. The recent demonstration o
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