abstract: Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion. In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters. In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

The InGaN system provides the opportunity to fabricate light emitting devices over the whole visible and ultraviolet spectrum due to band-gap energies E[subscript g] varying between 3.42 eV for GaN and 1.89 eV for InN. However, high In content in InGaN layers will result in a significant degradation of the crystalline quality of the epitaxial layers. In addition, unlike other III-V compound semiconductors, the ratio of gallium to indium incorporated in InGaN is in general not a simple function of the metal atomic flux ratio, f[subscript Ga]/f[subscript In]. Instead, In incorporation is complicated by the tendency of gallium to incorporate preferentially and excess In to form metallic droplets on the growth surface. This phenomenon can definitely affect the In distribution in the InGaN system. Scanning electron microscopy, room temperature photoluminescence, and X-ray diffraction techniques have been used to characterize InGaN layer grown on InN and InGaN buffers. The growth was done on c-plane sapphire by MOCVD. Results showed that green emission was obtained which indicates a relatively high In incorporation. / Singapore-MIT Alliance (SMA)

Since the initial breakthroughs in structural quality and p-type conductivity in GaN during the late 1980s, the group-III nitride material system has attracted an enormous amount of interest because of its properties and applications in both electronics and optoelectronics. Although blue light-emitting diodes have been commercialized based on this success, much less progress has been made in ultraviolet emitters, green emitters, and photovoltaics. This lack of development has been attributed to insufficient structural and electrical material quality, which is directly linked to the growth of the material. The objective of this work is to expand the understanding of III-nitride growth towards the improvement of current device capabilities and the facilitation of novel device designs. Group-III nitride thin films are grown by molecular-beam epitaxy in a pulsed, metal-rich environment. The growths of nitride binaries and ternaries are observed in situ by transient reflection high-energy electron diffraction (RHEED) intensities, which respond to the behavior of atoms on the growing surface. By analyzing and interpreting these RHEED signatures, a comprehensive understanding of nitride thin film growth is obtained. The growth kinetics of unintentionally doped GaN by metal-rich MBE are elucidated, and a novel method of in situ growth rate measurement is discovered. This technique is expanded to InN, highlighting the similarity in molecular-beam epitaxy growth kinetics between III-nitride binaries. The growth of Mg-doped GaN is then explored to increase Mg incorporation and electrical activation. The growth of InxGa1-xN alloys are investigated with the goal of eliminating phase separation, which enables single-phase material for use in photovoltaics. Finally, the growth of unintentionally doped and Mg-doped AlGaN is investigated towards higher efficiency light emitting diodes. These advancements in the understanding of III-nitride growth will address several critical problems and enable devices relying on consistent growth in production, single-phase material, and practical hole concentrations in materials with high carrier activation energies.

This work presents the development of a GaN-based dilute magnetic semiconductor (DMS) by metal organic chemical vapor deposition (MOCVD) that is ferromagnetic at room temperature (RT), electrically conductive, and possesses magnetic properties that can be tuned by n- and p-doping. The transition metal series (TM: Cr, Mn, and Fe) along with the rare earth (RE) element, Gd, was investigated in this work as the magnetic ion source for the DMS. Single- phase and strain-free GaTMN films were obtained. Optical measurements revealed that Mn is a deep acceptor in GaN, while Hall measurements showed that these GaTMN films were semi-insulating, making carrier mediated exchange unlikely. Hysteresis curves were obtained for all the GaTMN films, and by analyzing the effect of n- and p-dopants on the magnetic properties of these films it was determined that the magnetization is due to magnetic clusters. These findings are supported by the investigation of the effect of TM dopants in GaN nanostructures which reveal that TMs enhance nucleation resulting in superparamagnetic nanostructures. Additionally, this work presents the first report on the development of GaGdN by MOCVD providing an alternate route to developing a RT DMS. Room temperature magnetization results revealed that the magnetization strength increases with Gd concentration and can be enhanced by n- and p-doping, with holes being more efficient at stabilizing the ferromagnetic signal. The GaGdN films obtained in this work are single-phase, unstrained, and conductive making them suitable for the development of multifunctional devices that integrate electrical, optical, and magnetic properties.

GaN-based light emitting diodes (LEDs) face several challenges if the technology is to make a significant impact on the solid state lighting market. The two most pressing of these challenges are cost and efficiency. The development of alternative substrate technologies shows promise toward addressing both of these challenges, as both GaN-based device technology and the associated metalorganic chemical vapor deposition (MOCVD) technology are already relatively mature. Zinc oxide (ZnO) and silicon (Si) are among the most promising alternative substrates for GaN epitaxy. This work focuses on the development of MOCVD growth processes to yield high quality GaN-based materials and devices on ZnO and Si. ZnO, because of its similar lattice constant and thermal expansion coefficient, is a promising substrate for growth of low defect-density GaN. The major hurdles for GaN growth on ZnO are the instability of ZnO in a hydrogen atmosphere and out-diffusion of zinc and oxygen from the substrate. A process was developed for the MOCVD growth of wurtzite GaN and InxGa1-xN on ZnO, and the structural and optical properties of these films were studied. High zinc and oxygen concentrations remained an issue, however, and the diffusion of zinc and oxygen into the subsequent GaN layer was studied more closely. Silicon is the most promising material for the development of an inexpensive, large-area substrate technology. The challenge in GaN growth on Si is the tensile strain induced by the lattice and thermal mismatch between GaN and Si. A thin atomic layer deposition (ALD)-grown Al2O3 interlayer was employed to relieve strain while also simplifying the growth process. While some strain was still observed, the oxide interlayer leads to an improvement in thin film quality and a reduction in both crack density and screw dislocation density in the GaN films. A comparison of GaN-based LEDs grown on sapphire and Al2O3/Si shows similar performance characteristics for both devices. IQE of the devices on silicon is ~32%, compared to ~37% on sapphire. These results show great promise toward an inexpensive, large-area, silicon-based substrate technology for MOCVD growth of GaN-based optoelectronic devices.

This work targeted the growth of gadolinium (Gd)-doped gallium nitride (GaN) thin films (Ga₁₋ₓGdₓN) by metal organic chemical vapor deposition (MOCVD). Characterization and evaluation of these Ga₁₋ₓGdₓN thin films for application in spintronics/optoelectronics devices also formed part of this work. This work presents: (1) the first report of stable, reproducible n- and p-type Ga₁₋ₓGdₓN thin films by MOCVD; (2) the first Ga₁₋ₓGdₓN p-n diode structure; and (3) the first report of a room temperature spin-polarized LED using a Ga₁₋ₓGdₓN spin injection layer. The Ga₁₋ₓGdₓN thin films grown in this work were electrically conductive, and co-doping them with Silicon (Si) or Magnesium (Mg) resulted in n-type and p-type materials, respectively. All the materials and structures grown in this work, including the Ga₁₋ₓGdₓN-based p-n diode and spin polarized LED, were characterized for their structural, optical, electrical and magnetic properties. The spin-polarized LED gave spin polarization ratio of 22% and systematic variation of this ratio at room temperature with external magnetic field was observed.

Possessing a wide band gap and large break down field, gallium nitride (GaN) is of interest for a host of high power, high frequency applications including next generation cellular base stations, advanced military radar, and WiMAX networks. Much of this interest stems from the continued development of the AlGaN/GaN high electron mobility transistor (HEMT) that is capable of operating at sizable power densities and switching speeds. The same fields responsible for this performance, however, also elicit acute device heating and elastic loads. These induced thermomechanical loads limit both performance and reliability thus necessitating continued improvement in the management and characterization of the coupled environments. In response, this study establishes a new implementation of Raman spectroscopy capable of simultaneously measuring the operational temperature and stress in a HEMT using only the Stokes response. First, the linewidth (FWHM) of the Stokes signal is utilized to quantify the operating temperature of a HEMT independent to the influences of stress. Second, a new method, incorporating the use of the linewidth and peak position in tandem, is developed to estimate the biaxial thermoelastic stress that arises during device operation. With this capability, the HEMT's resultant load is assessed, highlighting the large role of the residual stress on the total mechanical state of the device. Subsequently, this same linewidth is leveraged to identify the distinct effect that electrical carriers have on the thermally relevant decay of longitudinal optical phonon modes. Further investigation of the lattice transport then concludes the study by way of an analytical treatment describing the significant influence of interfacial disorder on the energy transport at GaN/substrate boundaries.

Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4 - 2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. In the present work, various challenges in the novel III-nitride technology are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. Due to the unavailability of a suitable modeling program for InGaN solar cells, PC1D is modified up to a source-code level to incorporate spontaneous and piezoelectric polarization in order to accurately model III-nitride solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 1e18 cm-3. A robust fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.

Thesis (Ph.D.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Co-Chair: Nair, Sankar; Committee Member: Grover - Gallivan, Martha; Committee Member: McDowell, David; Committee Member: Schelling, Patrick; Committee Member: Zhang, Zhuomin

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 191-206).