Evidence of 2D-3D transition during the first stages of GaN growth on AlN
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Internet Journal o f
Nitride S emiconductor Research
Volume 2, Article 20
Evidence of 2D-3D transition during the first stages of GaN growth on AlN F. Widmann, B. Daudin, G. Feuillet, Y. Samson, M. Arlery, J. L. Rouviere CEA/Grenoble, Département de Recherche Fondamentale sur la Matière Condensée/SP2M This article was received on July 8, 1997 and accepted on August 21, 1997.
Abstract In order to identify the strain relaxation mechanism, Molecular Beam Epitaxy of wurtzite GaN on AlN was monitored in situ using Reflection High Energy Electron Diffraction (RHEED). In the substrate temperature range between 620°C and 720°C, a Stransky-Krastanov (SK) transition was evidenced, resulting in a 2D-3D transition after completion of 2 monolayers, with subsequent coalescence of 3D islands, eventually resulting in a smooth surface. Quantitative analysis of the RHEED pattern allowed us to determine that island formation is associated with elastic relaxation. After island coalescence, a progressive plastic relaxation is observed. The size and density of 3D islands was varied as a function of the growth parameters. AFM experiments revealed that the size of the GaN islands, about 8 nm large and 2 nm high, was small enough to expect quantum effects. It was found that capping of the islands by AlN resulted in a smooth surface after deposition of a few monolayers allowing us to grow a »superlattice» of islands by periodically repeating the process.
1. Introduction In order to grow high quality heterostructures of nitride semiconductors, such as GaN/AlN/GaN SIS structures, or GaN/AlN superlattices and superalloys, it is important to control the strain relaxation mechanism of GaN on AlN, and conversely of AlN on GaN, for which the mismatch is 2.5%. Until now studies dealing with this problem have been scarce. For example, Bykhovski et al. [1] calculated a critical thickness, but to the best of our knowledge, the mechanism of strain relaxation and its kinetics has never been studied in detail. In particular, the point of whether the relaxation process is totally plastic or also elastic needs to be clarified. Elastic relaxation through 3D islanding, or Stransky Krastanov (SK) growth mode, is commonly observed in a large variety of systems, the most widely studied being InAs/GaAs [2]. GaN islands formation has already been reported by Tanaka et. al. [3], but in this case the growth, that was undertaken by CVD, was not controlled by an intrinsic SK mode, and the size of the dots was rather large (typically 40 nm). Self-organized and controlled growth of quantum dots is of primary interest in the case of nitrides. In particular, the luminescence properties of laser diodes are believed to originate from InGaN quantum dots [4], and controlled formation of such structures would be a way to improve the properties of the devices. In this article, we study the strain relaxation process of GaN grown on AlN by Molecular Beam Epitaxy, using in situ RHEED analysis, and we demonstrate that the formation of GaN dots on AlN can be controlled by a ca
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