The calculation of the thermal conductivity of condensed matter has posed a significant challenge to engineers and scientists for almost a century. Thermal conductivity models have been successfully applied to many materials however many challenges still remain. One serious challenge is the inability of current thermal conductivity models to calculate the thermal conductivity of highly complex materials. Another challenge is managing error introduced by using an effective interatomic potential, for many materials this problem is exacerbated because their effective potentials have not been extensively used or characterized. Recent interest in nanostructures has initiated a new set of challenges and unanswered questions. This work addresses different aspects of the aforementioned challenges by using zeolite MFI and gallium nitride as case studies.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 209-217).

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

Silicon based semiconductor devices are rapidly approaching the theoretical limit of operation and are becoming unsuitable for future military requirements. The scope of semiconductor devices has been expanded by wide bandgap devices such as gallium nitride (GaN) to include the possibility for high power and high frequency operation. A new generation of high speed â high frequency devices is required to meet current and future military needs. The Gallium Nitride High Electron Mobility Transistor (HEMT) is showing great promise as the enabling technology in the development of military radar systems, electronic surveillance systems, communications systems and high voltage power systems. Typically, sapphire or silicon carbide is utilized as the substrate material in most HEMT designs. This thesis explores the possibility of utilizing a diamond substrate to increase the power handling capability of the AlGaN/GaN HEMT. Diamond offers increased thermal property parameters that can be simulated in the commercially available Silvaco software package. A complete electrical and thermal analysis of the model was conducted and compared to actual device characteristics. The results of the software simulation and measurements on the test devices indicate diamond substrates will enable the HEMT to be operated at a higher power than traditional sapphire substrate HEMTS.

The objective of this dissertation is to understand the structural and optoelectronic properties of group III-nitride materials grown by High-Pressure Metal Organic Chemical Vapor Deposition (HP-MOCVD) and Migration Enhanced Plasma Assisted MOCVD by FTIR reflectance spectroscopy, Raman spectroscopy, X-ray diffraction, and Atomic Force Microscopy. The influence of the substrates/templates (Sapphire, AlN, Ga-polar GaN, N-polar GaN, n-GaN, and p-GaN) on the free carrier concentration, carrier mobility, short-range crystalline ordering, and surface morphology of the InN layers grown on HP-MOCVD were investigated using those techniques. The lowest carrier concentration of 7.1×1018 cm-3 with mobility of 660 cm2V-1s-1 was found in the InN film on AlN template, by FTIR reflectance spectra analysis. Furthermore, in addition to the bulk layer, an intermediate InN layers with different optoelectronic properties were identified in these samples. The best local crystalline order was observed in the InN/AlN/Sapphire by the Raman E2 high analysis. The smoothest InN surface was observed on the InN film on p-GaN template. The influence of reactor pressures (2.5–18.5 bar) on the long-range crystalline order, in plane structural quality, local crystalline order, free carrier concentration, and carrier mobility of the InN epilayers deposited on GaN/sapphire by HP-MOCVD has also been studied using those methods. Within the studied process parameter space, the best material properties were achieved at a reactor pressure of 12.5 bar and a group-V/III ratio of 2500 with a free carrier concentration of 1.5x1018 cm-3, a mobility in the bulk InN layer of 270 cm2 V-1s-1 and the Raman (E2 high) FWHM of 10.3 cm-1. The crystalline properties, probed by XRD 2θ–ω scans have shown an improvement with the increasing reactor pressure. The effect of an AlN buffer layer on the free carrier concentration, carrier mobility, local crystalline order, and surface morphology of InN layers grown by Migration-Enhanced Plasma Assisted MOCVD were also investigated. Here, the AlN nucleation layer was varied to assess the physical properties of the InN layers. This study was focused on optimization of the AlN nucleation layer (e.g. temporal precursor exposure, nitrogen plasma exposure, and plasma power) and its effect on the InN layer properties.

This dissertation details the study of band bending in n- and p-type GaN samples with a Kelvin probe utilizing different illumination geometries, ambients (air, oxygen, vacuum 10-6 mbar), and sample temperatures (77 – 650 K). The Kelvin probe, which is mounted inside an optical cryostat, is used to measure the surface potential. Illumination of the GaN surface with band-to-band light generates electron-hole pairs, which quickly separate in the depletion region due to a strong electric field caused by the near-surface band bending. The charge that is swept to the surface reduces the band bending and generates a surface photovoltage (SPV). Information about the band bending can be obtained by fitting the SPV measurements with a thermionic model based on the emission of charge carriers from bulk to surface and vice versa. The band bending in freestanding n-type GaN templates has been evaluated. The Ga-polar and N-polar surfaces exhibit upward band bending of about 0.74 and 0.57 eV, respectively. The surface treatment also plays a major role in the SPV behavior, where the SPV for mechanical polished surfaces restores faster than predicted by a thermionic model in dark. When measuring the photoluminescence (PL) signal, the PL from mechanically polished surfaces was about 4 orders of magnitude smaller than the PL from chemically mechanically polished surfaces. The PL and SPV behaviors were explained by the presence of a large density of defects near the surface, which quench PL and aid in the restoration of the SPV via electron hopping between defects. Temperature-dependent SPV studies have also been performed on doped n- and p-type GaN samples. In Si-doped n-type GaN, the estimated upward band bending was about 1 eV at temperatures between 295 and 500 K. However, in p-type GaN, the downward band bending appeared to increase with increasing temperature, where the magnitude of band bending increased from 0.8 eV to 2.1 eV as the temperature increased from 295 to 650 K. It appears that heating the p-type GaN samples allows for band bending values larger than 1 eV to fully restore. Pre-heating of samples was of paramount importance to measure the correct value of band bending in p-type GaN. The slope of the dependence of the SPV on excitation intensity at low temperatures was larger than expected; however, once the temperature exceeded 500 K, the slope began to reach values that are in agreement with a thermionic model.

The Micro-Raman scattering technique has been used for the study of GaN and ZnO. Capabilities of the Raman technique and existing literature on Raman spectroscopy in GaN and ZnO are reviewed. About 50 GaN and ZnO samples with a wide range of properties are studied. From the analysis of positions of the E2H and A1(LO) phonon modes, biaxial stress and plasmon coupling of the Al(LO) mode are observed and compared to a bulk GaN sample. The stress-related shift rate for the AI(LO) mode in hexagonal GaN is established to be 2.7 ± 0.4 cm-1/GPa through series of GaN with low free carrier concentration. Bulk ZnO and ZnO layers grown on sapphire have been studied, and no biaxial stress is found in ZnO layers. Doping and impurity modes resulted in disorder-activated scattering in ZnO. The choice of the laser for study of GaN and ZnO layers on sapphire substrate is discussed.

In the past decade, GaN-based nitrides have had a considerable impact in solid state lighting and high speed high power devices. InGaN-based LEDs have been widely used for all types of displays in TVs, computers, cell phones, etc. More and more high power LEDs have also been introduced in general lighting market. Once widely used, such LEDs could lead to the decrease of worldwide electrical consumption for lighting by more than 50% and reduce total electricity consumption by > 10%. However, there are still challenges for current state-of-the art InGaN-based LEDs, including ‘efficiency droop’ issues that cause output power quenching at high current injection levels (> 100 A/cm2). In this dissertation, approaches were investigated to address the major issues related to state-of-the-art nitride LEDs, in particular related to (1) efficiency droop investigations on m-plane and c-plane LEDs: enhanced matrix elements in m-plane LEDs and smaller hole effective mass favors the hole transport across the active region so that m-plane LEDs exhibit 30% higher quantum efficiency and negligible efficiency droop at high injection levels compared to c-plane counterparts; (2) engineering of InGaN active layers for achieving high quantum efficiency and minimal efficiency droop: lower and thinner InGaN barrier enhance hole transport as well as improves the quantum efficiencies at injection levels; (3) double-heterostructure (DH) active regions: various thicknesses were also investigated in order to understand the electron and hole recombination mechanism. We also present that using multi-thin DH active regions is a superior approach to enhance the quantum efficiency compared with simply increasing the single DH thickness or the number of quantum wells (QWs, 2 nm-thick) in multi-QW (MQW) LED structures due to the better material quality and higher density of states. Additionally, increased thickness of stair-case electron injectors (SEIs) has been demonstrated to greatly mitigate electron overflow without sacrificing material quality of the active regions. Finally, approaches to enhance light extraction efficiency including using Ga doped ZnO as the p-GaN contact layer to improve light extraction as well as current spreading was introduced.