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The progress in optoelectronics and power electronics devices and their industrial applications has been remarkable since the beginning of the 21st century. Gallium nitride (GaN) based devices have shown high efficiency and good reliability, but their full potential is hindered by the shortage of native substrates. The Na-flux technique has the ability to produce high quality and low cost bulk GaN. At UCSB, a novel Na-flux system was designed by Von Dollen et al. to monitor GaN growth in-situ.‎ In addition, they demonstrated the fastest growth rate (>50 μm/h) reported for this technique. However, the main challenges such as low structural quality, high impurity, and opaqueness were usually noticed in the grown GaN. Furthermore, the poor control of growth and the low reproducibility of experiments limited the reliability of the system. The focus of this dissertation is on understanding the reasons behind these challenges, finding possible solutions, and testing the capability of the system to monitor and control the growth in-situ.Higher structural quality and lower impurity transparent GaN growth was noticed on some regions of the seed related to its position within the system. First, higher structural quality was linked to the switch from 3D to 2D growth mode as was evident from the optical microscope images. For 2D growth, the full width half maximum (FWHM) of the (0002) and (112 ̅0) rocking curves were as low as 89 and 44 arcseconds respectively. This switch – from 3D to 2D – occurred at a late growth stage and the reason for the observed crystal quality improvement is speculated to be decreased thermodynamic driving force as growth progressed. Second, in optically transparent regions, concentrations of oxygen, sodium, and carbon were as low as 7×1016, 2×1016, and 5×1016 atoms/cm3, respectively, while the concentration of molybdenum was below the detection limit. In addition, the area, the thickness, and the reproducibility of transparent regions were increased by controlling the seed position and omitting the addition of carbon (a common additive in the Na-flux method). Furthermore, the opaqueness was found to correlate with oxygen impurity as it was established by secondary ion mass spectrometry (SIMS) analysis. GaN growths which contain oxygen concentrations above 1019 atoms/cm3 were highly absorbing regardless of the presence of other impurities. Thus, the effect of three oxygen getters – i.e. magnesium, aluminum, and titanium – on the GaN transparency was investigated but was not significant. However, polycrystalline GaN was found to preferentially form on titanium sheets; and the reproducibility of growth conditions was enhanced with titanium sheets in the Na flux reactor. The effective solution to grow transparent GaN was by depleting oxygen impurity in the system via extended GaN growths. Extended growth typically consisted of two consecutive single growth experiments; and it was found that the all second experiments were thermodynamically limited in contrast with many of the first or single experiments. Third, the second experiments of extended growths were used to inspect the thermodynamics of GaN growth in-situ using the UCSB system. The ability of the system to determine equilibrium pressure and temperature for a specific sodium-gallium melt composition was demonstrated. Furthermore, positive deviation of the melt from ideality was noticed and the gallium activity and activity coefficient were calculated based on experimental data. Finally, some changes and improvements to the system – based on the outcomes of this dissertation – were suggested for better monitoring and controlling of GaN crystal growth.

With the growing complexity of high-power converters, it has never been more important to have accurate sensors for optimal control and stability. With significantly better power handling than silicon materials, bulk gallium nitride–based devices can withstand high voltages and current while simultaneously emitting a unique light spectrum, the characteristics of which are a function of the current and temperature of the device. Using bulk GaN PN junctions, preliminary measurements of the emitted light showed a distinct multi-peak spectrum corresponding to excitonic and impurity-assisted optical processes. Optical filters and photodiodes are utilized to measure the light intensity of each spectral peak for up to 5 A applied current at a temperature range of 20 °C – 100 °C. Linear polynomial models fitted to the filter output show a strong relationship between the light intensity, current, and temperature, as well as a significant energy shift in light emitted through exciton annihilation. The experimental data is then used to fit a linear polynomial model estimating current from temperature and filter output. Mean errors of 2.5%, 2.7%, and 3.7% are achieved for three of the four current models. Recommendations are made for future work in utilizing the sensor data for real-time current and temperature measurements. Ensign, United States Navy Approved for public release; distribution is unlimited.

Blue light-emitting diodes based on III-nitride semiconductors are nowadays widely used for solid-state lighting. They exhibit impressive figures of merit like an internal quantum efficiency close to 100%. This value is intriguing when considering the high dislocation density running throughout the InGaN/GaN quantum well (QW) active region. This striking feature is currently ascribed to carrier localization occurring in the InGaN alloy, which hinders their diffusion toward dislocations. However, it was recently reported that another source of defects, disconnected from dislocations, dramatically decreases the radiative efficiency of InGaN/GaN QWs. Those defects, present at the surface, are usually trapped in an InGaN underlayer (UL), which is grown before the QW active region. To get insight into the trapping mechanism, we varied the UL thickness, In content, and materials system (InGaN or InAlN) and studied the photoluminescence decay time at 300 K of a single InGaN/GaN QW. Our data demonstrate that defects are incorporated proportionally to the indium content in the UL. In addition, we show that those defects are created during the high-temperature growth of GaN and that they segregate at the surface even at low-temperature. Eventually, we propose an intrinsic origin for these surface defects. (C) 2018 Author(s).

This paper presents an investigation of flow and heat transfer in a large diameter (6.25 in) cylindrical enclosure heated laterally and containing a porous block that simulates the basket of nutrients used in a crystal growth reactor. The numerical model entails the use of a commercially available computational engine provided by ANSYS FLUENT, and based on a two-dimensional (2D) axisymmetric Reynolds-averaged Navier Stokes (RANS) equations. The porous medium is simulated using the Brinkman-extended model accounting for the Darcy and Forchheimer induced pressure drops. The porous 'plug' effects are analyzed as both its permeability/inertial resistance and locations in the reactor are changed on a parametric basis, while the Rayleigh number (Ra = g beta Delta TL3/v alpha) is kept constant at 1.98 x 10(9). Additionally, the effect of different ratios of the hot to the cold zone lengths are investigated as a part of the current effort. For all cases, the velocity and temperature distributions in the reactor are analyzed together with the flow patterns in, and around the porous block. A comprehensive discussion is provided with regard to the effects of the position of the porous block and its permeability on both the immediately adjacent, and far flows. The consequences on the temperature distribution in the enclosure, when the ratio of the length of the hot-to-cold zones is changed, are also analyzed. (C) 2017 Elsevier B.V. All rights reserved.

In this work, the electrical characteristics of bulk gallium nitride doped n have been investigated by the current–voltage (I–V) and capacitance-voltage (C-V) at 300 K temperature. Using the thermionic emission theory, the saturation current, IS (2.45×10-7 A), the ideality factor n (1.13), the barrier height ϕbn (0.65) and the serial resistance RS (670) are determined for our structure (Hg/n-GaN). It has also been calculated the barrier height from C–V experimental data. The determined value is higher (1.24 eV) compared with the value obtained from I–V (0.65 eV) characteristics, it was found the following electrical parameters, doping concentration (ND = 1.68×1016 cm-3, diffusion voltage (Vd = 1.11 V) and density of interface states (Nss).

Native bulk gallium nitride (GaN) has emerged as an alternative for sapphire and silicon as a substrate material for III-N devices. While quasi-bulk GaN substrates are currently commercially available, single crystal GaN substrates are considered essential for future high performance light emitters and power devices. The ammonothermal method is currently considered one of the most feasible methods to grow large truly bulk crystals of GaN at low cost and high structural quality. High crystalline quality GaN substrates sliced from ammonothermally grown crystals have been demonstrated and utilized in homoepitaxy for III-N devices. However, despite the high crystalline quality the properties of as-grown ammonothermal GaN crystals and substrates are affected by the presence of impurities and other defects that hinder their use for device applications. Here, the main developments of ammonothermal growth of GaN and the effects of impurities, native point defects, and dislocations on the material properties are summarized. Additionally, measurement techniques that enable the evaluation of point defect concentration and low dislocation density distribution over a large area on bulk GaN substrates are reviewed.

Low-aluminum composition AlGaN/GaN double-barrier resonant tunneling structures were grown by plasma-assisted molecular-beam-epitaxy on free-standing c-plane GaN substrates grown by hydride-vapor phase epitaxy. Clear, exactly reproducible, negative-differential resistance signatures were observed from 4 x 4 mu m(2) devices at 1.5V and 1.7V at 77K. The relatively small value of the maximum peak-to-valley ratio (1.03) and the area dependence of the electrical characteristics suggest that charge transport is affected by leakage paths through dislocations. However, the reproducibility of the data indicates that electrical traps play no significant role in the charge transport in resonant tunneling diodes grown by molecular-beam-epitaxy under Ga-rich conditions on free-standing GaN substrates. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729819]

International audience; Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm−2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K theQWemission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described.