Two methods of electro-optically characterizing alternating-current thin-film electroluminescent (ACTFEL) devices are investigated: photo-induced transferred charge (PIQ) and luminescence (PIL), and subthreshold voltage-induced transferred charge (VIQ) techniques. Both techniques provide information related to traps within the phosphor layer. PIQ/PIL experiments monitor the transport of electrons and holes across the phosphor layer which are photo-injected by a UV laser pulse. VIQ experiments monitor the optical reset of traps ionized by bipolar subthreshold voltage pulses. PIQ/PIL experiments are performed on three different SrS ACTFEL devices: ALE-deposited SrS:Ce, sputter-deposited SrS:Cu, and undoped MOCVD-deposited SrS. From the PIQ/PIL experiments, two distinct electron thresholds in the luminescent impurity doped samples at ~0.8 (weak threshold) and ~1.2 MV/cm (strong threshold) are observed. These thresholds are independent of the phosphor thickness, indicating that they arise from a bulk property of the phosphor. The ~0.8 MV/cm weak threshold is attributed to field emission of relatively shallow (~0.6 eV) electron-emitting bulk traps (e.g. cerium or oxygen for SrS:Ce; a sulfur vacancy or oxygen for SrS:Cu). The ~1.2 MV/cm strong threshold is ascribed to the onset of trap-to-band impact ionization. In contrast to electron transport, PIQ/PIL studies reveal no hole transport in SrS doped with luminescent impurities, although hole transport is observed for an undoped SrS ACTFEL device. The lack of hole transport is attributed to the efficiency of hole capture in SrS doped with luminescent impurities. VIQ experiments are performed on the same SrS ACTFEL devices. VIQ trap energy depths are estimated as ~0.1 eV for SrS:Ce; ~0.9 eV for SrS:Cu (with a capture cross-section of ,~10�������cm��), and ~0.6 eV for undoped SrS. Tenative atomic identification of traps responsible for these VIQ trends are: chlorine or a Ce shallow donor state for SrS:Ce, a sulfur vacancy for SrS:Cu, and a sulfur vacancy or an oxygen isoelectronic trap for undoped SrS. / Graduation date: 2000

Graduation date: 1996

Graduation date: 1998

Graduation date: 1995

Graduation date: 1995

Graduation date: 1993

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Robert G. Hunsperger, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by Nan M. Jokerst. / Vita. Includes bibliographical references (leaves 132-140).

The irreversible trend of replacing the conventional incandescence light bulbs and fluorescent tubes with white light emitting diodes (LEDs) aims to use less energy for lighting. Plenty of the commercially available white LEDs are made from blue LED chips with few-micron-thick gallium nitride (GaN) grown on several hundred micron thick transparent sapphire substrates, followed by coating of yellow phosphor powder on top of the chips for converting the emitted blue light to white light. Not only does such approach give the white LEDs a high colour temperature, but also introduces conversion loss from the phosphor powder. The former issue makes users feel unpleasant for living while the latter wastes energy. Therefore, a new version of phosphor-free multilayered vertically-stacked colour-tunable LED structure is proposed in this thesis such that it allows users to regulate the colour temperature of light source according to their preference. Simultaneously, the device replaces light conversion agents with direct light generation. The fabrication of the proposed device involved the use of backside laser micromachining of trenches on the substrates of the upper layers of basic colour LED chips at a size just enough to fit the wire-bonded wire of lower layer LED chips inside. With equal-sized basic colour LED chips tightly packed together, colour homogeneity of the proposed device is enhanced and thus provides the proposed device the capability to substitute the conventional RGB LED devices with basic colour LED chips separately aligned. To improve the internal quantum efficiency and light extraction of nitride-based LEDs, thin film photonic crystal LED is proposed. Light and heat trapping sapphire substrate is removed by laser lift-off (LLO), forming GaN thin film on an electrically conductive opaque substrate with better heat conductivity than sapphire. By proper etching, N-dopped GaN layer can be exposed, resulting in the formation of vertical LED. Compared with conventional lateral LEDs with sapphire substrate, carrier path of vertical LED is greatly reduced and hence achieving lower internal resistance. To further boost light extraction, the device top surface is patterned with nanopillars by nanosphere lithography. A monolayer of closely-packed silica nanospheres is patterned on the N-GaN surface by spin coating. It acts as a mask for etching the nanopillars which bandfold lights from diffracted modes to radiative modes located above the light line for extraction. A typical laser LLO process results in thin films with undopped gallium nitride (U-GaN) surface or N-GaN (after etching) faces up. If P-side up is necessary, the GaN layers are first required to attach to a temporary substrate for LLO and then the LLO exposed surface is adhered to the real substrate before temporary substrate is detached. This method is proposed to relieve the issue of light channeling inside the sapphire substrate of full colour LED micro-display panel fabricated on a single GaN on Sapphire wafer. With the elimination of sapphire, “parasitic” blue emissions from the area surrounding pixels are reduced which in turns improved the observable effects from the microspheres jet-printed on the top surface of the panel. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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