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Title: Ultraviolet light emission from tunnelling metal-insulator-semiconductor devices
Author: Lin, Chen-Sheng
ISNI:       0000 0001 2442 894X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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High efficient light emitting diodes (LEDs) have been successfully applied to the visible lighting market for many years. However, the development of ultraviolet (UV) LEDs is still hindered, where the efficiency of UV-LEDs is significantly lower than that of visible LEDs. The challenges come from the difficulty of obtaining efficient p-type doped AlxGa1-xN (especially for the high-aluminium-content materials), light absorption by the p-type layer, and the complexity of device structures. These problems have motivated the investigation for the feasibility of non p-doped UV emitters. This thesis presents a study of UV emissions from simple metal-insulator-semiconductor (MIS) tunnel light emitting diodes, with simple structures and without p-doped layers. Firstly, epitaxial thin film were deposited and characterized layer by layer, where n-type AlxGa1-xN film was used as the active layer and AlN film was used as the insulator layer followed by a semi-transparent metal thin layer for current spreading. Device with different thicknesses of AlN were compared, and its thickness of 10 nm resulted in optimized diode behaviour and maximum UV emissions. More importantly, a region of clear negative differential resistance was observed in the I-V characteristic, indicating free holes in the valence band of n-type active layer are most likely generated by resonant tunnelling. A decrease in emitted intensity at high injection currents was observed, which is attributed to a reduction in the density of conduction band electrons via Fowler-Nordheim tunnelling occurring in parallel. A defect band in the AlN barrier layer is suggested as the origin of the quantum states via which the resonant tunnelling occurs. Finally, from the measurements of electroluminescence on both the n-GaN based and the n-AlxGa1-xN (x = 0.14) based MIS diodes, near band-edge UV emissions of 365 nm and 335 nm were achieved respectively. Through the study of MIS light emitting diodes and their achievement of UV light emissions, this research provides a potential solution to solve the problems of current low-efficiency p-n junction based UVLEDs.
Supervisor: Moram, Michelle ; Blackman, Bamber Sponsor: Leverhulme Trust ; European Research Council ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral