The objective of this research was to design semiconductor material structures for a number of different devices, including GaN metal-semiconductor-metal (MSM), InGaAs/InAlAs MSM, and InAs/GaAs quantum dot photodetectors, and to study the growth conditions for epitaxial material using molecular beam epitaxy (MBE) augmented with an rf-plasma nitrogen source. GaN was grown on a LiGaO2 substrate, which has multiple advantages over the most commonly used substrates for III-nitride growth. LiGaO2 substrates have a small lattice mismatch of approximately 1% with GaN, which leads to high-quality epitaxy film by optimization of the growth condition. The combination of nitridation, buffer, super lattice, and Ga-rich condition is the key to improving the quality of GaN film grown by MBE on LiGaO2. The first GaN MSM grown on LiGaO2 was reported, which has the dark current in the range of 10-12A. The device was then lift off and bonded on SiO2/Si wafer. The performance of the device did not degrade after integration. However, the orthorhombic crystal structure of LiGaO2 results in the unusual asymmetric strains within GaN, causing changes in the microstructure of GaN and making integration difficult. The strains within GaN grown on LiGaO2 were investigated using high resolution x-ray. It was found that the critical thickness of GaN on LGO was around 10nm, and the strains relieve with film thickness increasing. The dislocation densities were also calculated and confirmed by AFM, which can be as low as 2䥱07/cm2. Through studying of the strains with the insertion of AlGaN/GaN superlattice buffer, it was found that the strain of the epitaxial layer is dependent on the thickness and critical thickness of both epitaxy and buffer layer. For thin GaN films, Al0.12GaN superlattice buffer would relieve the strain most, and for the thicker layers, the Al0.44GaN superlattice buffer relieves the strain most. The dislocation density measurement shows that an insertion of buffer decreases dislocations significantly. Insertion of superlattice buffers does not decrease dislocation density further. The material structure of InGaAs/InAlAs was studied. The device quality was improved by optimizing the material structure. Depends on the application as optical interconnects, the optimized material structure should insure the device with high speed but reasonable responsivity. Finally, devices were fabricated that achieve speeds as high as 50-70 GHz, comparable with the commercial ultra-fast MSM. The research of quantum dots was focused on modification of the size, strains, and structures of quantum dots by annealing the spacer between quantum dot layers using As4 and P2, respectively, at different temperatures. It was found that the annealing under P2 results in surface exchange, and the annealing under As4 mostly changes dots?zes, causing the changes of energy level.

Si QDs embedded in SiOx or SiNx thin films, which could emit light in the entire visible range from 440 nm to 840 nm by controlling their size and/or their matrix, were synthesized by evaporation or plasma enhanced chemical vapor deposition techniques. Various shades of white could be obtained from multi-layered SiNx film structures by controlling the size of Si QDs and layer thickness. It was shown that the combination of these films can produce white emission spectra with superior color rendering properties compared to conventional fluorescent tubes. Such Si-based QDs can be used as down-converting phosphors to coat a blue/UV LED to generate white light, providing a less expensive fabrication process to obtain advanced solid state lighting devices. As a supplement, free CdTe QDs with emission colors spanning 520~700 nm and quantum efficiency up to 54%, were synthesized using a colloidal chemical method for white LED applications. White PL and a range of emission colors were obtained from mixed CdTe QD samples excited by a 420 nm blue LED. Another part of this research was to develop a new x-ray powder phosphor, ZnTe:O, for biological imaging applications used in CCD-based synchrotron x-ray detectors. A unique dry synthesis process, including gaseous dry doping and etching procedures, was developed to synthesize ZnTe:O phosphors. The excellent x-ray luminescence results of oxygen doped ZnTe, including high efficiency, high resolution, fast decay, low afterglow and an improved spectral match to the CCD detector, indicated that ZnTe:O is a promising phosphor candidate for x-ray imaging applications.

Using the method of breaking circular symmetry and the subsequent symmetry restoration via projection techniques, we calculate the ground-state energies and excitation spectra of N-electrons confined in parabolic quantum dots in strong magnetic fields in the medium-size range 10<=N <=30. The physical picture is that of finite rotating electron molecules (REMs) comprising multiple rings, with the rings rotating independently of each other. A derived analytic expression for the energetics is applicable to arbitrary sizes given the corresponding ring configuration of classical point charges. Also by exact diagonalization (EXD) method, we show the spectrum and structure of few electrons, 2<=N<=3, confined in elliptical dots at finite magnetic field. The results suggest the formation of a state of Wigner-molecular properties with spin associated, which has great instructions for the development of quantum register in quantum computing.

In this thesis, we used the E-Beam lithography to fabricate a device of coupled microdisk-ring laser on the sample which was grown by molecular beam epitaxy (MBE), and analyzed the coupled effect of the device. The active layer was composed of six compressively strained InGaAs quantum dots (QDs) that were designed to support gain at 1200nm. Under the active layer, we replaced sacrificial layer by distributed bragg reflector (DBR). The purpose of the DBR was used like a mirror to reflect the particular wavelength which located at DBR¡¦s stop band, so the energy would be confined in the active layer. The device was composed of a microdisk and a ring. The diameter of the microdisk was 3£gm, and the width of the ring is 250nm. The microdisk was placed in the ring, and the gap of both was 100nm. After design, we simulated whether the device could generate coupled modes by Finite-Difference Time-Domain (FDTD). In experiment, we used the E-Beam lithography to define negative pattern on the sample which is spread with the PMMA. We also used the thermal evaporation to evaporate the metal, and lift the metal to form our pattern. Finally, we used the dry etching to transform the pattern to the epitaxial layer, and then the device was completed. In measurement, we used the micro-PL to measure our device, and got a successful result. The result showed our device generated eight resonant modes. The measured result matched the simulation result. Through simulation, the device generated three coupled modes, 1173.8nm, 1206nm, and 1214nm. We expect that the device will be used to generate terahertz source in the future.

In recent years, the Surface Plasmon Polariton effect has played an important role for entering the Nano-world. When the metallic materials reach the nanometer level, many special characteristics show up. As the progress of advanced technology development, the equipments which can be operated in nano grade level are more stabilized. Many special surface Plasmonic properties have been discovered through the measurements. This research is to focus on using the Surface Plasmon coupling to excite colloidal quantum dots and observing the emissive behavior of quantum dots. The experiments of changing the distance between the quantum dots and the metal film were performed. The blinking effect disappeared when the quantum dots are very close to the metal film. It showed that some other mechanism is competing with Auger recombination in the quantum dots. The lifetime modification and emission intensity were measured when one quantum dot was placed near a silver cube. The coupling between the surface Plasmon polariton and the quantum dot was discussed.

Invented in the 1950s, gel electrophoresis has now become a routine analytical method to verify the size of nucleic acids and proteins in molecular biology labs. Conventional gel electrophoresis can successfully separate DNA fragments from several base pairs to a few tens of kilo base pairs, beyond which a point is reached that DNA molecules cannot be resolved due to the size independent mobility. In this case, pulsed field gel electrophoresis (PFGE) was introduced to extend the range of DNA fragment sizes that can be effectively separated. But despite the incredible success of PFGE techniques, some important drawbacks remain. First, separation time is extremely long, ranging from several hours to a few days. Second, detection methods still rely on staining the gel after the run. Real time observation and study of band migration behavior is impossible due to the large size of the PFGE device. Finally, many commercial PFGE instruments are relatively expensive, a factor that can limit their accessibility both for routine analytical and preparative use as well as for performing fundamental studies. In this research, a miniaturized PFGE device was constructed with dimension 2cm x 2.6cm, capable of separating DNA fragments ranging from 2.5kb to 32kb within three hours using low voltage. The separation process can be observed in real time under a fluorescence microscope mounted with a cooled CCD camera. Resolution and mobility of the sample were measured to test the efficiency of the device. We also explored manipulating DNA fragments by end labeling DNA molecules with quantum dot nanocrystals. The quantum dot-DNA conjugates can be further modified through binding interactions with biotinylated single-stranded DNA primers. Single molecule visualization was performed during gel electrophoresis and the extension length, entanglement probability and reorientation time of different conjugates were measured to study their effect on DNA migration through the gel. Finally, electrophoresis of DNA conjugates was performed in the miniaturized PFGE device, and shaper bands were observed compared with the non end-labeled sample. Furthermore, by end-labeling DNA with quantum dots, the migration distance of shorter fragments is reduced, providing the possibility of separating a wider range of DNA fragment sizes on the same gel to achieve further device miniaturization.

This thesis presents a novel method for the preparation of quantum dot-thermoresponsive polymer nanocomposite hydrogels. The quantum dots (QD’s) were synthesized in a microwave reactor using a high temperature organometallic synthesis procedure. The initial hydrophobic surface layer on the QD’s was coated with an amphiphilic polymer to enable phase transfer from non-polar solvent to water followed by physical immobilization of the QD’s in the thermoresponsive polymer hydrogel by photopolymerization. Their temperature dependent emission properties were investigated as a function of concentration of the incorporated QD’s. The resultant temperature dependent changes in the position of the peak emission wavelength of the QD-polymer nanocomposite hydrogels were found to be due to the change in the physical environment causing increased interaction between the embedded amphiphilic polymer coated QD’s and/or due to aggregation of QD’s. This change in peak emission position was found to be reversible in the temperature range from 29 to 37 °C.

The carrier capture and relaxation of Type II ZnTe/ZnSe quantum dots(QDs) were investigated with ultrafast photoluminescence upconversion. We found that carrier relaxation of QDs under Volmer-Weber(VW) growth mode exhibits faster decay and rise than that of QDs under Stranski-Krastanow(SK) growth mode due to the wetting layer in SK growth mode provides as a pathway for carriers to diffuse and migrate from large(small) to small (larger) QDs. The wetting layer level was found by analyze the decay time of PL with different wavelength and temperature. The PL of VW mode and SK mode by using 532nm Nd-YAG laser also prove the existence of wetting layer. We interpret our results of VW mode in terms of Auger process with large carrier density.

Fluorescence resonance energy transfer (FRET)-based biosensors have been designed to fluorometrically detect everything from proteolytic activity to receptor-ligand interactions and structural changes in proteins. While a wide variety of fluorophores have demonstrated effectiveness in FRET probes, several potential sensor components are particularly notable. Semiconductor quantum dots (QDs) are attractive FRET donors because they are rather bright, exhibit high quantum yields, and their nanoparticulate structure enables the attachment of multiple acceptor molecules. Fluorescent proteins (FPs) are also of particular interest for fluorescent biosensors because design elements necessary for signal transduction, probe assembly, and device delivery and localization for intracellular applications can all be genetically incorporated into the FP polypeptide. The studies described in this thesis elucidate the important parameters for concerted QD-FP FRET probe design. Experimental results clarify issues of FRET pair selection, probe assembly, and donor-acceptor distance for the multivalent systems. Various analysis approaches are compared and guidelines asserted based on the results. To demonstrate the effectiveness of the QD-FP FRET probe platform, a ratiometric pH sensor is presented. The sensor, which uses the intrinsic pH-sensitivity of the FP mOrange to modulate the FP/QD emission ratio, exhibits a 20-fold change in its ratiometric measurement over a physiologically interesting pH range, making it a prime candidate for intracellular imaging applications.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 89-95). Available also in a digital version from Dissertation Abstracts.