The electrodeposition of bulk and mesoporous lead telluride films onto gold and silicon substrates

Abstract

Lead telluride is a semi-conductor with a band gap in the infra-red region of the electromagnetic spectrum. This makes it a very useful material for infrared radiation detectors, thermal imaging and photovoltaic devices. Thin films of lead telluride are often produced by expensive and technically complex methods including chemical vapour deposition and molecular beam epitaxy. Electrochemical deposition provides a cheaper and easier way to produce these films. Recently the fabrication of electrodeposited films of metals and semiconductors with a mesoporous structure has been investigated. Due to their increased surface area and the dimensions of the mesoporous structure these materials are expected to exhibit enhanced properties compared to bulk films of the same materials. These include increased photovoltaic efficiency due to the fact the materials are almost entirely composed of surface atoms, they also can have greatly improved catalytic activity. This thesis describes the fabrication of bulk and mesoporous lead telluride films on silicon and gold-on-glass substrates. The electrochemistry of lead telluride deposition was investigated using cyclic voltammetry which demonstrated the under potential co-deposition process. Bulk films were deposited onto gold and silicon over a range of potentials. The film morphology and composition were investigated using SEM, EDXS and wide angle XRD. Most films were powdery and had poor adhesion however shorter durations of deposition gave better film quality. Mesoporous films were deposited by liquid crystal templating using the surfactant octaethylene glycol monohexadecyl ether in the hexagonal phase. Mesoporous films on silicon often peeled off during removal of the liquid crystal. To improve adhesion chemical modification of the silicon wafer using 3-mercaptopropyl-trimethoxysilane was investigated. The film adhesion dramatically improved and small angle XRD of the films suggested that a mesoporous structure was present. The film composition determined by EDXS however showed that the films were tellurium rich. A double pulse nucleation technique was also employed to improve film adhesion and morphology. Films deposited onto silicon using this technique did show some improvement in quality compared to films deposited without this technique or chemical modification. Small angle XRD of films deposited onto gold using the double pulse regime again indicated the possibility of a mesoporous structure being present. Atomic force microscopy (AFM) was also used to examine the surface morphology of substrates. deposited silicon etched some films and

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