Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Garimella, Srinivas; Committee Member: Kippelen, Bernard; Committee Member: Melkote, Shreyes; Committee Member: Ready, Jud. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Joshi, Yogendra; Committee Member: Kumar, Satish. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M.S.)--Youngstown State University, 2009. / Includes bibliographical references (leaves 44-45). Also available via the World Wide Web in PDF format.

This thesis reports on the design, fabrication, and testing of state-of-the-art, high-performance inverted top-emitting organic light-emitting diodes (OLEDs). The vast majority of research reports focuses on a device architecture referred to as a conventional OLED which has its anode on the bottom of the device and its cathode on the top. Moreover, most conventional OLEDs are bottom-emitting such that light exits the structure through both a semitransparent bottom electrode of indium-tin oxide and a glass substrate. The particular device architecture developed in this thesis is one in which the devices are inverted (i.e. their cathode is on the bottom as opposed to on top) and top-emitting. Despite the advantages that inverted top-emitting OLEDs possess over conventional bottom-emitting OLEDs, their development has been relatively slow. This is because inverted OLEDs have traditionally been hampered by the difficulty of injecting electrons effectively into the device. In this work, a novel method of injecting electrons from bottom cathodes into inverted OLEDs is discovered. In several previous reports, bottom Al/LiF cathodes had been used with the electron-transport material Alq3 to produce inverted OLEDs, but the resulting inverted OLEDs exhibited inferior performance to conventional OLEDs with top cathodes of Al/LiF. A new route for the development of highly efficient inverted OLEDs is shown through the use of electron-transport materials with high electron mobility values and large electron affinities. After systematic device optimization, inverted top-emitting OLEDs are demonstrated that currently define the state-of-the-art in terms of device efficiency. Optimized green and blue inverted top-emitting OLEDs are demonstrated that have a current efficacies of 92.5 cd/A and 32.0 cd/A, respectively, at luminance values exceeding 1,000 cd/m2. Finally, this discovery has enabled the development of the first stacked inverted top-emitting OLEDs ever made, combining all of the advantages offered by an inverted architecture, a top-emissive design, and a stacked structure. These OLEDs have a current efficacy of 200 cd/A at a luminance of 1011 cd/m2, attaining a maximum current efficacy of 205 cd/A at luminance of 103 cd/m2.

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Monte Carlo modelling is a common technique in numerous fields, and is widely used in semiconductor device simulation. This thesis describes the application of Monte Carlo modelling to the simulation of Gunn diode devices, focusing on devices composed of Gallium Arsenide (GaAs) and Gallium Nitride (GaN). Gunn diodes are simple structures that take advantage of negative differential resistance to act as a source of high frequency radiation, from 10 GHz to over 100 GHz in GaAs devices. It has been theorised that GaN should exhibit negative differential resistance and a GaN Gunn diode could produce radiation of even higher frequency, within the terahertz band. Gunn diodes have the advantage of being cheap and portable, and so are worth exploring as such a source. Unfortunately, GaN devices have a high electron density and so they tend to generate heat quickly. It therefore becomes important to include modelling of heat generation and flow in simulations of these devices. This is uncommon in Monte Carlo models of Gunn diodes, as in less highly doped devices thermal effects can usually be assumed to result in the device reaching an equilibrium temperature of about 100 K above the ambient. This thesis describes the creation of a model to track the generation and distribution of heat during operation of a GaN device. Simulations found that thermal effects within the device were significant. Heat generation occurred to the extent that the device could only be operated in pulsed mode, with on pulses of 2 ns requiring 50 ns of cooling for sustainable operation. The increased temperature within the device also lead to deleterious changes in the Gunn diode's operating frequency. In the simulated device, a 150 K change in temperature lead to a decrease in operating frequency of 40 GHz, from an initial frequency of 280 GHz. At the end of 2 ns of operation, the mean temperature within the device had increased by 120 K. The high accidental doping level in GaN also means the use of a doping notch to act as a nucleation point for dipoles within a Gunn diode, a common technique in other materials, becomes less feasible. As an alternative to a notch, a device was simulated incorporating a doping spike to nucleate the dipole. The use of a doping spike is not novel, however its use in GaN has not been previously explored. Simulations found that a fully-depleted p-type doping notch of length 2.1 nm, doped at 1x1024 m-3 would act as a nucleation point for dipole operation. The device was compared to a simulated device incorporating a doping notch of width 0.25 µm doped at 0.5x1023 m-3 and found to operate at a similar frequency and RF efficiency, making it a viable substitute. One limitation of Gunn diodes is that when operated in transit-time mode, the operating frequency is determined by the length of the diode's transit region and so is well-defined and fixed. This means that traditional Gunn diodes are not as useful a source of radiation for spectroscopic applications as might be desirable. Recent experimental results for planar devices have shown a broadening in operation frequency and even multiple frequencies. This thesis explores the hypothesis that such a broadening might be achieved in a vertical structure via the incorporation of an additional notch into the Gunn diode's transit region, effectively incorporating two transit regions into the device. Results showed that this novel device structure did show multiple modes of operation. Under a DC applied voltage, the device showed spontaneous switching behaviour, oscillating between dipole and accumulation layer operation from the second notch. Changes in the frequency of an applied RF voltage would shift the device from operating from the first or second notch, in dipole and accumulation layer mode respectively.

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

published_or_final_version / Chemistry / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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