published_or_final_version / Physics / Master / Master of Science

Thesis (M. Sc.)--University of Hong Kong, 1972. / Offset from typescript.

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Thesis (M.S.)--Marshall University, 2004. / Title from document title page. Document formatted into pages; contains viii, 54 p. including illustrations. Includes bibliographical references (p. 47-49).

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This thesis reports the photophysical and electroluminescence studies undertaken on two types of material: polymeric and dendritic. The dendritic architecture is a recent concept adopted to develop new materials for light-emitting diodes. The dendritic structure offers a combination of properties of both polymers and small organic molecules whilst having their own interesting characteristic of optimising processibility, charge transport, and optical properties independently. The dendritic structure consists of functional surface groups, conjugated dendrons and a conjugated core. Initial optical (absorption and photoluminescence) studies revealed that the dendrimer emission originates from the core and is independent of excitation wavelength. This was investigated further in distyrylbenzene based dendrimers where the effect of dendrimer generation number on photoluminescence and electroluminescence properties was studied. All dendrimers emit blue electroluminescence with, in some cases, reasonable electroluminescence quantum efficiency in the range of 0.09 % and brightness up to 150 Cd m(^-2). Having established that the furmel effect, where excitation is successfully transferred to the dendrimer core in both PL and EL, different chromophores were incorporated in the dendrimer structure. Colour control was thus demonstrated in EL devices of the different dendrimers, showing the possibility of using a large number of chromophores in a processible form for EL applications. Conjugated polymers were also studied to investigate the nature of the emitting species (poly(p-pyridine)) and the effect of side- chains (poly(p-phenylenevinylene)). In poly(p-pyridine) the emission was found to be strongly dependent on pyridyl ring rotation affecting the emission and its quantum yield while the side-chains in the poly(p-phenylenevinylene) derivatives were found to affect polymer properties such as degree of conversion of non-conjugated to conjugated polymer. The PL quantum yield system was set-up and proved useful in assessing synthesis of new materials.

The goal of this thesis is the identification and synthesis of high-luminance, primary color alternating-current thin-film electroluminescent (ACTFEL) devices. Special attention is paid to the synthesis of primary color green ACTFEL devices because of the lack of an adequate primary green ACTFEL phosphor and the dominance of green wavelengths in the response of the human eye. The sulfide materials family is the focus of this investigation of ACTFEL phosphor hosts due to its importance in the field of electroluminescence, although oxide and selenide phosphor hosts are also considered. Particular attention is paid to the coactivation of the SrS:Cu ACTFEL phosphor because of the ability to control the emission spectrum with coactivators. Green ACTFEL devices exhibiting a high-luminance, high-efficiency, saturated green color are demonstrated via the coactivation of SrS:Cu with alkali metals. In addition, Zn���GeO���:Mn is demonstrated as a potentially bright and stable ACTFEL phosphor. Finally, the "phosphor sandwich" technique of ACTFEL fabrication is presented as a means of depositing highly-stable, high-luminance ACTFEL phosphors. The fabrication techniques presented in this thesis are subsequently employed to deposit custom ACTFEL devices for characterization studies aimed at determining fundamental physical properties of ACTFEL phosphors. A method for establishing the relative hot electron distributions inside operating ACTFEL phosphor hosts is presented. In addition, the fabrication of SrS:Cu ACTFEL devices for characterization has allowed new insight into the density and physical basis of phosphor space charge. This insight is gained by monitoring the electrical and optical characteristics of SrS:Cu ACTFEL devices coactivated with various non-isovalent impurities to assess the influence of native defects and extrinsic impurities on space charge related behavior. It is found that native sulfur vacancies are not the physical basis for dynamic space charge in SrS:Cu ACTFEL devices. / Graduation date: 2000

Graduation date: 1999

The thesis focuses on the study of asymmetric multiple quantum wells (AMQWs). There are two main sources of the samples. One is from our laboratory. We used molecular-beam epitaxy to grow the InGaAs/InGaAlAs AMQWs of different well widths and different position arrangement of well width. And we also designed the AMQWs with p-type modulation doping at the different barrier region. The other is from Land Mark Optoelectronics Corporation. They used Metalorganic Chemical Vapor Deposition (MOCVD) to grow the InGaAsP AMQWs of different well widths and different position arrangement of well width. There are five experiments in my thesis. First of all, we use electroluminescence (EL) measurement to discuss the EL spectra of the samples. The EL spectra can show the shape, intensity and full width half maximum. Second, we use the photoreflectance (PR) measurement, which uses laser beam to modulate the dielectric constant of samples, to discuss the transition energies by using simulation and curve fitting. And we got build-in electric field from FKO data and simulation. In the last three experiments, we analyzed photocurrent spectra, photoluminescence spectra and electro-absorption spectra individually and then compared the three to all experiments in the thesis. In these experiments, we discovered that the sample C092 exhibits a broad and flat EL spectrum and 2500Å covering from 1.38~1.63£gm. Besides, we also found that the emission of wells is dominated by the arrangement of quantum wells. Moreover, the arrangement of quantum wells is relative to material. Therefore, even if we use the same way of arrangement, the different materials will lead to different results. Finally, we expect that we can use our AMQWs samples to produce broadly tunable laser and broad-band semiconductor optical amplifier (SOA) in the future.

As the great studies have been made in light emitting diodes, the application becomes more variety for different field. High bright light emitting diodes apply a new light source for fluorescence lifetime measurements. We use confocal scanning microscopy and a high frequency sensitive lock-in amplifier to obtain the electroluminescence image at ten of micrometer. We drive the light emitting diodes by radio frequency signal and observe it. We parallel connection an inductance to match the parasitic capacity in high frequency to obtain better light emitted brightness.