Investigation of the Shape of InGaAs/GaAs Quantum Dots
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Investigation of the Shape of InGaAs/GaAs Quantum Dots Susan Y. Lehman, Alexana Roshko, Richard P. Mirin, and John E. Bonevich1 National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305 1 National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899 ABSTRACT Three samples of self-assembled In0.44Ga0.56As quantum dots (QDs) grown on (001) GaAs by molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM) and high-resolution transmission electron microscopy (TEM) in order to characterize the height, faceting, and densities of the QDs. The cross-sectional TEM images show both pyramidal dots and dots with multiple side facets. Multiple faceting has been observed only in dots more than 8.5 nm in height and allows increased dot volume without a substantial increase in base area. Addition of a GaAs capping layer is found to increase the diameter of the QDs from roughly 40 nm to as much as 200 nm. The areal QD density is found to vary up to 50 % over the central 2 cm x 2 cm section of wafer and by as much as 23 % on a length scale of micrometers. INTRODUCTION Self-assembled quantum dots continue to grow in importance in the optoelectronics marketplace. Although they are already in use in lasers and other devices, there are many aspects of QD growth that are not well understood. In particular, dot nucleation, dot size and shape, and the effect of capping the dots have not been satisfactorily explained. The optical and electrical properties of the QDs and resulting devices depend strongly on the shape and composition. Control over nucleation sites would enable production of regular arrays of QDs. In addition, the uniformity of dot density, which is important for device performance, has not been investigated. QDs in devices are invariably capped, while studies of size and shape are more easily and commonly performed on uncapped dots. In this article we describe the results of AFM and TEM studies of InGaAs quantum dots grown on (001) GaAs substrates by MBE. We have examined the size and shape of the dots as well as the uniformity of their distribution on the GaAs wafer and the effect of depositing a cap layer over the QDs. EXPERIMENTAL DETAILS AND RESULTS Three QD samples were studied; samples B and C form a set and were grown within a few hours of each other. All samples consist of In0.44Ga0.56As deposited on (001) GaAs substrates. The sample growth conditions are summarized in Table I. The first 5 nm of the GaAs cap layer on sample C were grown at 530 ˚C; the remaining 15 nm were grown as the temperature was increased to 580 ˚C. QD shape and size From AFM analysis of the samples, we measured the average dot densities and heights reported in Table I. For sample B, the distribution of dot heights was bimodal, with one maximum at a dot height of ≈ 5 nm and another at ≈ 8.5 nm. Sample C was also scanned by
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Table I Growth conditions and dot characteristics for the InGaAs QD samples Average Growth Portion of Dot Monolayers Sample Cap temperatu
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