Defect characterization in 4% cadmium zinc telluride semiconductors
Date
2018-06-27
Authors
Penkova, Silvia I.
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Abstract
This thesis presents a characterization study of Cadmium Zinc Telluride with a 4% Zinc molar concentration using five different techniques. Three of the characterization methods – X-ray diffraction (XRD), transmission electron microscopy (TEM) and wet chemical etching – were used to study the atomic arrangement of the lattice and the type of defects in the material. On the other hand, the other two methods – photoluminescence (PL) and the Hall Effect – provided information about the electronic properties of the material and identified the electronic signatures of the defects present. This thesis study applied these five methods to 29 samples with the [111] and [211] crystal orientations.
XRD was used to calculate the Zn concentration and the atomic spacing of the samples. TEM allowed to map the crystal structure using Kikuchi diffraction and to calculate the variation of the local lattice constant. This showed that the sample was under strain. A CBED imaging condition producing HOLZ lines allowed for qualitatively imaging the strain field in the crystal and contrast phase imaging showed stacking faults and dislocations present in the lattice. Next, wet chemical etching - using Nakagawa and Everson solutions – revealed several different types of etch pits were present on the material’s surface and provided a value for the etch pit density (EPD). PL at room temperature was used to calculate the Zn concentration and to produce contour maps of the samples showing the variation in Zn. Low temperature PL was used to conduct intensity and thermal studies of the material providing information about the type of emission (donor-acceptor pair, free exciton or excitonic) and the activating energies. This information was used to assign the peaks seen in the PL spectrum at 8K. Lastly, the Hall Effect experiment was used to calculate the resistivity and the mobility of carriers, i.e., electrons and holes, of the samples.
The last stage of the project was to seek correlations between the data obtained during the five characterization techniques. Correlations were noted between the XRD FWHM broadening and the EPD, the disturbances in the atomic lattice and the defect band broadening and a summary of all the values calculated during the project was provided. Overall this project provided a very thorough study of 4% CZT.
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Keywords
CZT, semiconductors, x-ray diffraction, photoluminescence, etching, Everson, Nakagawa, Hall Effect, TEM, SEM