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Title: Low-loss electron energy loss spectroscopy in a scanning transmission electron microscope of GaInNAs
Author: Gass, Mhairi Hope
ISNI:       0000 0001 3492 0874
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2004
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This thesis presents new methodologies for obtaining the composition and distribution of elements, and determining electronic properties such as the effective electron mass within GalnNAs quantum wells (QWs) using electron energy loss spectroscopy. The aim of this investigation was to explore new techniques in a scanning transmission electron microscope for studying the elemental composition and elemental distribution of dilute nitride semiconductors. To improve the understanding of the dilute nitride materials such as GaInNAs QWs, knowledge of the distribution and composition of nitrogen is very important. Howeverit has not been possible to determine nitrogen distribution using conventional methods. The determination of composition of a GaInNAs QW using techniques such as photoluminescence relies on a reference GalnAs QW, assuming the same In concentration in both wells. The composition of GaInNAs QWshas previously been investigated using energy dispersive X-ray analysis in a scanning transmission electron microscope. However, as nitrogen is a light element the X-rays produced are generally too weak to be detected, and unlessit is present in high concentrations there is no possibility of mapping the N signal. The current work exploits the high energy resolution and fast acquisition times associated with modern low-loss electron energy loss spectroscopy, combined with the spectrum imaging capabilities on a scanning transmission electron microscope. Using this technique, high spatial and energy resolution elemental maps from valence and semi-core states can be produced. Further, by applying the KramersKronig transformations, the imaginary part of the dielectric function can be obtained, which represents the absorption of the material and does not exhibit the bulk plasmon. As the bulk plasmon is the major feature in the low-loss it obscures many of the weakertransitions. By analysing the imaginary part of the dielectric function instead of the loss spectrum the quantification of the semi-core states is improved. This technique produces high spatial resolution maps that for certain elements are quantifiable. The effect of composition on plasmon energy has been investigated and it is shown that through the effective numberof electrons obtained from the Kramers-Kronig transformations and the plasmon energy, the effective electron mass can be determined. High spatial resolution maps can be produced, and the change in effective electron mass can be directly compared with the composition. By combining the techniques of elemental composition mapping and effective electron mass mapping, a greater understanding of the distribution of nitrogen on the electronic properties has been achieved.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral