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Title: A TEM study of ohmic contact microstructures for GaN-based electronic devices
Author: Bright, A.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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The GaN system has been of growing interest in the past decade for its potential as a blue-light emitter, and has rapidly achieved commercial importance, but the system also has very attractive properties for high power microwave-frequency transistors, notably formation of a two-dimensional electron gas at the AlGaN/GaN heterointerface, high electron velocity and a large breakdown electric field. The commercialization of such devices is now imminent. Ohmic contacts to GaN and AlGaN/GaN heterostructures are an essential part of the production of all optical and electronic devices from the GaN system, but the requirements are particularly stringent for high power microwave devices, and the contacts are still not well understood or optimized. To understand such contacts, and hence establish the major factors influencing electrical behaviour, studies of the microstructure of various contacts are required. These can then be linked with observed variations in contact resistance and morphology. The many techniques of transmission electron microscopy (ТЕМ) are the most powerful way to characterize contact microstructures, and here ТЕМ studies of several different Al/Ti-based metal contact structures to n-GaN and AlGaN/GaN wafers as a function of annealing temperature are reported. ТЕМ techniques include bright field and dark field imaging of metal microstructures, selected area diffraction identifications of metal and interfacial grains, chemical mapping to establish phase distributions using energy-filtered ТЕМ and energy-dispersive X-ray mapping, and high resolution lattice imaging to establish the identity and morphology of reaction phases at the metal/nitride interface. Links between contact structure or processing conditions and the resulting electrical behaviour are thus proposed. Thin, patchy nitride phases (such as α-AIN and TiN) were observed at the metal/AlGaN interface in good contacts, and a thick (8nm) uniform AlTi2N interfacial layer was found to cause poor contact behaviour.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available