Effects of low-temperature capping on the optical properties of GaAs/AlGaAs quantum wells
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NANO EXPRESS
Open Access
Effects of low-temperature capping on the optical properties of GaAs/AlGaAs quantum wells Masafumi Jo*, Guotao Duan, Takaaki Mano, Kazuaki Sakoda
Abstract We study the effects of low-temperature capping (200-450°C) on the optical properties of GaAs/AlGaAs quantum wells. Photoluminescence measurements clearly show the formation of abundant nonradiative recombination centers in an AlGaAs capping layer grown at 200°C, while there is a slight degradation of the optical quality in AlGaAs capping layers grown at temperatures above 350°C compared to that of a high-temperature capping layer. In addition, the optical quality can be restored by post-growth annealing without any structural change, except for the 200°C-capped sample. Introduction Self-assembled semiconductor nanostructures have attracted tremendous interest due to their excellent electronic and optical properties. Since the properties of nanostructures strongly depend on their size, shape, and composition, it is important to reduce the morphological change of nanostructures during the capping process. In this context, much research has recently focused on low-temperature capping with less atomic intermixing, although it is commonly believed that the crystalline quality of the capping layer deteriorates quickly with decreasing temperature. Droplet epitaxy is a self-assembled growth technique based on the formation of metallic droplets followed by crystallization into semiconductor quantum dots (QDs) [1-13]. Droplet epitaxy allows the self-assembly of QDs in lattice-matched systems such as GaAs/AlGaAs, which is unattainable in a conventional Stranski-Krastanow growth mode. In the growth of GaAs/AlGaAs QDs, various quantum structures such as monomodal dots [3], single/multiple rings [4,8,9], and nanoholes [10-13] have been derived by controlling the As pressure and temperature during the crystallization of Ga droplets. However, in droplet epitaxy, low-temperature processes at around 200°C are required for the formation of droplets and their crystallization, which often causes degradation of the crystalline and optical qualities of the QDs and subsequent AlGaAs capping layer. Uncapped * Correspondence: [email protected] National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 3050047, Japan
annealing of QDs is, therefore, used as an effective way to improve the quality of the QDs [14]. This annealing step, however, can also cause significant morphological changes in the QD. For example, GaAs QDs grown on GaAs(001) substrates elongate in the [-110] direction when annealed at temperatures higher than 400°C [15], and so a capping temperature below 400°C is necessary for embedding QDs with their original morphology maintained. However, such a low temperature is challenging for the growth of high-grade AlGaAs, and indeed, the effects of a low-temperature AlGaAs capping layer on the optical properties of adjacent GaAs quantum structures have not yet been clarified. We studied the optical qualities of GaAs nanostructures
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