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Title: Measurement of III-V quantum dot composition in three dimensions using transmission electron microscopy
Author: Wahra, Assma
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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Three-dimensional compositional models of individual InAs/GaAs quantum dots (QDs) have been generated using compositionally-sensitive dark field 002 cross sectional transmission electron microscope (TEM) images. Recovery of a sample's structure from transmission electron microscope image data is an inverse problem, which is solved here by the use of simplifying assumptions. The reconstruction of the 3-D concentration is made possible using image analysis and fitting of the image data with a series of Gaussian functions, under an assumption of cylindrical symmetry. A further complication in the analysis is the existence of double-valued solutions for the composition, which required further information in order to find the right solution. For high quality TEM specimens the model can determine the location and concentration of indium and gallium in different regions inside QDs and reveals differences in composition between samples, for example due to differing thermal treatments. Knowledge of the TEM specimen thickness is essential for quantitative analysis, as shown by comparison between simulated and experimental images. A new technique for measurement of specimen thickness using convergent beam electron diffraction techniques, to a small fraction of the extinction distance, is made possible using digital combination of several individual diffraction patterns. Our measurements of composition give insight into the growth process, as well as quantitative measurements which can be used for modelling of QD behaviour. The model is applied to different InxGa1-xAs QD structures, i.e. an InAs QD laser, D-WELL structure, QD infrared photodetector and double QD structure. The thesis starts with an introduction in an epitaxial growth of III-V quantum dots, some of their applications and currently available 3D characterization techniques. Chapter 2 presents a general introduction to transmission electron microscopy, sample preparation and contrast mechanisms with particular attention paid to dark field 002 imaging of III-V quantum dots. The principles used throughout the analysis, and the approach taken to implement them, are given in chapter3. It presents basic concepts, data processing algorithms and experimental imaging conditions. A data processing program, taking experimental transmission electron microscope images of QDs to produce a 3-D composition model; and an image simulation program, using dynamical diffraction theory to produce a TEM image from a 3-D composition model, were written using MatLab. Chapter 4 focuses mainly on the measurement of specimen thickness including digital large angle convergent beam electron diffraction (D-LACBED) and the 113-DF imaging technique. Experimental results are given in chapter 5. Chapter 6 is given to discussion and some conclusions and suggestions for further work given in chapter 7.
Supervisor: Not available Sponsor: Government of Libya
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics