Gamma-phase lithium aluminate (LiAlO2) single crystal is grown by Czochralski pulling method and a-plane LiAlO2(LAO) is chosen as the substrate for subsequent gallium nitride (GaN) epitaxial growth by plasma-assisted molecular beam epitaxy (PAMBE). The lattice mismatch between the nitride and the substrate is greatly reduced due to small lattice mismatch of~0.3% between [0001]GaN and [010]LAO and of~1.7% between [11-20]GaN and [001]LAO in the plane of the substrate LAO(100). Pure hexagonal [10-10]GaN is successfully grown directly on the LAO substrate without buffer layer. Crystal quality and properties are analyzed through a series of measurements, including reflection high-energy electron diffraction (RHEED), field-emission electron microscopy (FESEM), electron backscatter diffraction (EBSD), x-ray diffraction and cathodo luminescence (CL).

The thesis mainly probes into the effects that the structure below multi quantum layer as regards the efficiency of luminescence of blue light LED in the different number layers and make further comparison. In the article, the students separately make analysis and comparison to the single quantum well , five multi quantum wells , ten multi quantum wells and thirty multi quantum wells. And discovered that different number layers of quantum well will occur different Phase Separation and Strain in the film. So the article mainly focuses on : (1.)Phase Separation in various of quantum well, it occurs different In-rich reaction and (2.)Different Strain levels which occurs different dislocation reaction. The two mechanisms will be discussed in detail with the effects of luminescence reaction of LED. According to the results of experiment, We found that it is easier to form V-shape defects and dislocation with the increasing indium content. Under the high indium content, the density of In-rich will increase obviously and spread to the GaN barrier, then the original structure of quantum well will be destroyed and descend the efficacy of luminescence. In the thicker GaInN quantum well, it will induce larger energy of strain inside the film, So the defect density will increase due to release the strain energy. It was also discovered the intensity of luminescence descend after measuring by PL. When grow different number layers , it was discovered that higher quantum layer will produce the roughness surfaces when using AFM . So the higher quantum layers will make greater influence in the efficacy of luminescence. By experiment, we found that the five to ten quantim wells will have the better photo characteristic.

In this study, the characteristics of low-pressure chemical vapor deposition deposited silicon nitride films on n-GaN substrate were investigated. The physical and chemical properties were measured and surveyed. And an Al/LPCVD-Si3N4/n-GaN MOS structure was used for the electrical characterizations. For the electrical property improvements, we investigated the low-pressure chemical vapor deposition deposited silicon nitride films by (NH4)2Sx treatment. Furthermore, the silicon nitride films were passivated by fluorine ions to improve the electrical characterizations that came from the liquid phase deposited SiO2 stacks. After the (NH4)2Sx treatment and fluorine ions passivation, the dielectric constant of low-pressure chemical vapor deposition deposited silicon nitride films were maintained and the leakage current density were improved. The highest dielectric constant is 12.13, and lowest leakage current density are 1.73¡Ñ10-10 A/cm2 at 1 MV/cm and 3.81¡Ñ10-10 A/cm2 at 1 MV/cm for the LPCVD-Si3N4 film after fluorine passivation and (NH4)2Sx treatment.

No description available.

abstract: Owing to their special characteristics, group III-Nitride semiconductors have attracted special attention for their application in a wide range of optoelectronic devices. Of particular interest are their direct and wide band gaps that span from ultraviolet to the infrared wavelengths. In addition, their stronger bonds relative to the other compound semiconductors makes them thermally more stable, which provides devices with longer life time. However, the lattice mismatch between these semiconductors and their substrates cause the as-grown films to have high dislocation densities, reducing the life time of devices that contain these materials. One possible solution for this problem is to substitute single crystal semiconductor nanowires for epitaxial films. Due to their dimensionality, semiconductor nanowires typically have stress-free surfaces and better physical properties. In order to employ semiconductor nanowires as building blocks for nanoscale devices, a precise control of the nanowires' crystallinity, morphology, and chemistry is necessary. This control can be achieved by first developing a deeper understanding of the processes involved in the synthesis of nanowires, and then by determining the effects of temperature and pressure on their growth. This dissertation focuses on understanding of the growth processes involved in the formation of GaN nanowires. Nucleation and growth events were observed in situ and controlled in real-time using an environmental transmission electron microscope. These observations provide a satisfactory elucidation of the underlying growth mechanism during the formation of GaN nanowires. Nucleation of these nanowires appears to follow the vapor-liquid-solid mechanism. However, nanowire growth is found to follow both the vapor-liquid-solid and vapor-solid-solid mechanisms. Direct evidence of the effects of III/V ratio on nanowire growth is also reported, which provides important information for tailoring the synthesis of GaN nanowires. These findings suggest in situ electron microscopy is a powerful tool to understand the growth of GaN nanowires and also that these experimental approach can be extended to study other binary semiconductor compound such as GaP, GaAs, and InP, or even ternary compounds such as InGaN. However, further experimental work is required to fully elucidate the kinetic effects on the growth process. A better control of the growth parameters is also recommended. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2010

III-nitride optoelectronic devices have become ubiquitous due to their ability to emit light efficiently in the blue and green spectral ranges. Specifically, III-nitride light emitting diodes (LEDs) have become widespread due to their high brightness and efficiency. However, III-nitride devices such as single photon sources are also the subject of research and are promising for various applications. In order to improve design efficient devices and improve current ones, the relationship between the structure of the constituent materials and their optical properties must be studied. The optical properties of materials are often examined by photoluminescence or cathodoluminescence, whilst traditional microscopy techniques such a transmission electron microscopy and scanning electron microscopy are used to elucidate their structure and composition. This thesis describes the use of a dual-beam focussed ion beam/scanning electron microscope (FIB/SEM) in bridging the gap between these two types of techniques and providing a platform on which to perform correlative studies between the optical and structural properties of III-nitride materials. The heteroepitaxial growth of III-nitrides has been known to produce high defect densities, which can harm device performance. We used this correlative approach to identify hexagonal defects as the source of inhomogeneous electroluminescence (EL) in LEDs. Hyperspectral EL mapping was used to show the local changes in the emission induced by the defects. Following this the FIB/SEM was used to prepare TEM samples from the apex of the defects, revealing the presence of p-doped material in the active region caused by the defect. APSYS simulations confirmed that the presence of p-doped material can enhance local EL. The deleterious effects of defects on the photoelectrochemical etching of cavities were also studied. We performed TEM analysis of an edge-defect contained in unetched material on the underside of a microdisk using FIB/SEM sample preparation methods. The roughness and morphology of microdisk and nanobeam cavities was studied using FIB-tomography (FIBT), demonstrating how the dual-beam instrument may be used to access the 3D morphology of cavities down to the resolution of the SEM and the slicing thickness of the FIB. This tomography approach was further extended with electron tomography studies of the nanobeam cavities, a technique which provided fewer issues in terms of image series alignment but also the presence of reconstruction artefacts which must be taken into account when quantitatively analysing the data. The use of correlative techniques was also used to establish the link between high Si content in an interlayer running along the length of microrods with changes in the optical emission of these rods. The combination of CL, FIB/SEM and TEM-based techniques has made it possible to gain a thorough understanding of the link between the structural and optical properties in a wide variety of III-nitride materials and devices.

abstract: High electron mobility transistors (HEMTs) based on Group III-nitride heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and electrical measurements, as well as other techniques. The dissertation was organized primarily into three topical areas: (1) characterization of near-gate defects in electrically stressed AlGaN/GaN HEMTs, (2) microstructural and chemical analysis of the gate/buffer interface of AlN/GaN HEMTs, and (3) studies of the impact of laser-liftoff processing on AlGaN/GaN HEMTs. The electrical performance of stressed AlGaN/GaN HEMTs was measured and the devices binned accordingly. Source- and drain-side degraded, undegraded, and unstressed devices were then prepared via focused-ion-beam milling for examination. Defects in the near-gate region were identified and their correlation to electrical measurements analyzed. Increased gate leakage after electrical stressing is typically attributed to "V"-shaped defects at the gate edge. However, strong evidence was found for gate metal diffusion into the barrier layer as another contributing factor. AlN/GaN HEMTs grown on sapphire substrates were found to have high electrical performance which is attributed to the AlN barrier layer, and robust ohmic and gate contact processes. TEM analysis identified oxidation at the gate metal/AlN buffer layer interface. This thin a-oxide gate insulator was further characterized by energy-dispersive x-ray spectroscopy and energy-filtered TEM. Attributed to this previously unidentified layer, high reverse gate bias up to −30 V was demonstrated and drain-induced gate leakage was suppressed to values of less than 10−6 A/mm. In addition, extrinsic gm and ft * LG were improved to the highest reported values for AlN/GaN HEMTs fabricated on sapphire substrates. Laser-liftoff (LLO) processing was used to separate the active layers from sapphire substrates for several GaN-based HEMT devices, including AlGaN/GaN and InAlN/GaN heterostructures. Warpage of the LLO samples resulted from relaxation of the as-grown strain and strain arising from dielectric and metal depositions, and this strain was quantified by both Newton's rings and Raman spectroscopy methods. TEM analysis demonstrated that the LLO processing produced no detrimental effects on the quality of the epitaxial layers. TEM micrographs showed no evidence of either damage to the ~2 μm GaN epilayer generated threading defects. / Dissertation/Thesis / Ph.D. Physics 2012

We have studied luminescence properties of Mg-doped GaN grown by hydride vapor phase epitaxy. Steady state photoluminescence (PL) spectra have been analyzed. Exciton, ultraviolet luminescence (UVL) and blue luminescence (BL) bands are the dominant PL bands in the spectra. At low temperature, Exciton and UVL bands show almost no shift with excitation intensity, whereas the BL band blueshifts by almost 0.4 𝑒𝑉 with increasing excitation intensity by seven orders of magnitude. Such shifting nature of bands with excitation intensity is explained by assuming that the BL band is detected from the region of the sample where potential fluctuations are very large, but the UVL and exciton bands originate from the region of the sample where there are no potential fluctuations. After the careful analysis of potential fluctuations model and the donor-acceptor pair model, we conclude that the BL band in the studied GaN:Mg sample is not a separate band but the UVL band itself, which is significantly distorted by potential fluctuations. Now, we call this band the BL* band. Temperature dependence of the BL*, UVL and Exciton peak intensity is analyzed. We see abrupt and tunable thermal quenching of the BL* and Exciton bands. Temperature dependence of the BL* and UVL bands at fixed excitation intensities but at different environmental conditions is also investigated. Finally, giant redshift of the BL* band with increasing temperature is explained by a combination of potential fluctuations and abrupt quenching of the BL* band with temperature.

Modern wireless communication systems transmit complex modulated signals with high peak to average ratio in order to deliver high data rates. It demands wide bandwidth and rigorous efficiency performance for power amplifiers. Today’s conventional RF power amplifiers have relatively poor operating efficiency and require more power and area for operation. Therefore, more research on high efficiency power amplifier is crucial to the growth of the wireless industry. Until recent days, WiMAX systems are using technology processes such as Gallium Arsenide (GaAs) and Si LDMOSFET to obtain the performance. Although they are providing the required functional performance, they do not optimize cost and/or size. The primary focus of this thesis is to enhance the efficiency and output power of a compact microwave Power Amplifier suitable for a WiMAX base station. To achieve this goal, this thesis explores the highly efficient switched mode Class E microwave power amplifier using the Gallium Nitride on Silicon Carbide HFET (GaN-on-SiC) technology. The smallest gate length (0.15 µm) device recently released by NRC is used in this design. It provides higher performance at lower cost and area than the alternative Gallium Arsenide (GaAs) technology. Importance is given in designing the bias network of the device. The biasing network has a great impact on efficiency of power amplifiers. Many new techniques of Class E design have been presented to date, but there is not significant improvement related to the biasing network. A highly efficient Class E power amplifier for WiMAX base station transmitter was developed in this thesis for 2.5 GHz application. An improved bias network was introduced for biasing the active device. This successful design shows acceptable simulated performance with a gain of 10.12 dB, an output power of 34.12 dB, and a power added efficiency of 41.7 % at the peak output power.

Presented in this thesis are extensive theoretical investigations into the causes and effects of carrier localization in InGaN/GaN quantum wells. The results of the calculations agree well with experimental data, where it is available, and provide additional insights into the mechanisms that lead to some of the experimentally observed effects of localization. Firstly, the wave functions of the electrons and holes in InGaN/GaN quantum wells have been calculated by numerical solution of the effective-mass Schrödinger equation. In our calculations we have assumed a random distribution of indium atoms, as suggested by the results of atom probe tomography: this allows us to find the contributions to the carriers' potential energy that arise from band gap fluctuations, the deformation potential and the spontaneous and piezoelectric fields. We show that the fluctuations in alloy composition can be sufficient to localize the carriers; our results are in good agreement with the results of experiment and more detailed ab-initio calculations, but we also obtain information about the distribution of localized states which those methods cannot yet provide. We find that the holes are localized on a short scale in randomly-occurring regions of high indium content, whereas the electrons are localized on a longer length scale. We consider the effect of well width fluctuations and find that these contribute to electron localization, but not to hole localization. We also simulate the low-temperature photoluminescence spectrum and find good agreement with experiment for the energy, width and shape of the photoluminescence peak. Secondly, we have used first-order time-dependent perturbation theory to study the diffusion of the carriers between their localized states at non-zero temperatures. The rates for scattering via the interaction with acoustic phonons are calculated using the carrier wave functions, and the resulting master equation for the distribution of the carriers is solved by a Monte Carlo method. We find that, even towards room temperature, the carriers are localized to a small number of states, and that their diffusion lengths are proportional to a combination of the density of localized states and the localization length. The experimentally-observed `S-shape' of the photoluminescence peak energy as a function of temperature is reproduced in our results and is explained by the thermal redistribution of holes among the localized states. A reduction of the depth of this S-shape is found as the excitation power is increased, as has been observed experimentally, and which we attribute to the saturation of the localized states.