Abstract
Geometric phase analysis has been applied to high resolution aberration corrected (scanning) transmission electron microscopy images of InAs/GaAs quantum dot (QD) materials. We show quantitatively how the lattice mismatch induced strain varies on the atomic scale and tetragonally distorts the lattice in a wide region that extends several nm into the GaAs spacer layer below and above the QDs. Finally, we show how V-shaped dislocations originating at the QD/GaAs interface efficiently remove most of the lattice mismatch induced tetragonal distortions in and around the QD.
Highlights
Quantum dots (QDs) are nano-objects exhibiting 3D quantum confinement of charge carriers[1]
With the development of aberration-corrected transmission electron microscopes, the resolution has reached the sub Å level, and this improved resolution opens the possibility for direct measurements of lattice strain on the atomic scale. These measurements are based on the assumption that there is a constant relationship between the intensity maxima in the transmission electron microscopy ((S)TEM) images and the location of the atomic columns
Geometric phase analysis (GPA)[15] has proven to be a reliable method to determine the strain in atomic resolution images and from QD systems in particular[16,17,18]
Summary
The samples were grown in a Varian Gen II Modular MBE system with a dual- filament Ga source, a SUMO In source, and a valved cracker As source from Veeco. 2.77 monolayers (ML) (AlAs capped sample) or 2.35 ML (GaAs capped sample) InAs were deposited in cycles. The AlAs capped sample consists of three QD layers separated by 85 and 50 nm GaAs spacers. The first 10 nm of the spacers were grown at 500 °C (AlAs capped sample) or 510 °C (GaAs capped sample) and the final part of the spacers at 586 °C. In both samples, the last QD layer was left uncapped on the surface, and the substrate temperature was ramped down towards room temperature immediately after the last QD layer was grown. All TEM images were taken with the electron beam parallel to the crystallographic [110] direction
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