This PDF file contains the front matter associated with SPIE Proceedings Volume 10754, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

GaN is an attractive material for high-power electronics due to its wide bandgap and large breakdown field. Verticalgeometry devices are of interest due to their high blocking voltage and small form factor. One challenge for realizing complex vertical devices is the regrowth of low-leakage-current p-n junctions within selectively defined regions of the wafer. Presently, regrown p-n junctions exhibit higher leakage current than continuously grown p-n junctions, possibly due to impurity incorporation at the regrowth interfaces, which consist of c-plane and non-basal planes. Here, we study the interfacial impurity incorporation induced by various growth interruptions and regrowth conditions on m-plane p-n junctions on free-standing GaN substrates. The following interruption types were investigated: (1) sample in the main MOCVD chamber for 10 min, (2) sample in the MOCVD load lock for 10 min, (3) sample outside the MOCVD for 10 min, and (4) sample outside the MOCVD for one week. Regrowth after the interruptions was performed on two different samples under n-GaN and p-GaN growth conditions, respectively. Secondary ion mass spectrometry (SIMS) analysis indicated interfacial silicon spikes with concentrations ranging from 5e16 cm^-3 to 2e18 cm^-3 for the n-GaN growth conditions and 2e16 cm^-3 to 5e18 cm^-3 for the p-GaN growth conditions. Oxygen spikes with concentrations ~1e17 cm^-3 were observed at the regrowth interfaces. Carbon impurity levels did not spike at the regrowth interfaces under either set of growth conditions. We have correlated the effects of these interfacial impurities with the reverse leakage current and breakdown voltage of regrown m-plane p-n junctions.

Recent advances in the ammonothermal growth of bulk, single crystal GaN will be presented. Building on demonstrated improvements in growth rate and purity of basic ammonothermal GaN crystals using capsule systems, new experimental setups were developed enabling in situ temperature measurements of the supercritical fluid simultaneously in the growth and dissolution zone during crystal growth. New insight which has been gained leveraging these probes will be presented, with a particular focus on impurity incorporation trends over a range of process conditions and fundamental insight into the growth kinetics. The continued pursuit of improved growth rate and reduced defect density material has led to the development of novel autoclave systems which may permit exploration of novel chemistries to enable growth of bulk III-N crystals using the ammonothermal method.

Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability up to 1400 C. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic β-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ɛ). The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make β-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal β-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Different from the molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) growth techniques, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality β-Ga2O3 thin films on both native Ga2O3 and c-sapphire substrates with controllable doping and fast growth rates up to 10 µm/hr. In this talk, we present the growth, material characterization and device demonstration of β-Ga2O3 thin films grown via LPCVD. The β-Ga2O3 thin films were grown on native β-Ga2O3 (010), (001) and (-201) substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. Fundamental material properties including temperature dependent Hall measurements will be discussed.

The epitaxial growth of group III-nitride structures on silicon by metalorganic chemical vapor deposition requires careful management of thermal stress. Cracking due to thermal mismatch in these structures can be mitigated using aluminum nitride (AlN) buffer layers which induce compressive strain in overgrown gallium nitride. However, lattice mismatch between AlN and silicon and reactions between ammonia, aluminum, and silicon during the initial stages of growth require careful process control during film nucleation. In this paper, transmission electron microscopy studies of dislocation structures at the interface between silicon and aluminum nitride buffer layers will be presented. The atomic structure at the interface differed when either aluminum or ammonia were introduced into the chamber first. These observations are generally consistent with what others have found and raise interesting questions about the formation mechanisms of dislocations during AlN growth.

The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate, the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (10-10), and semipolar (20-2-1) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. Record modulation bandwidths above 1 GHz are achieved for the nonpolar and semipolar orientations. We also present a small-signal method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection.

Wide Bandgap (WBG) semiconductor devices are becoming the simpler and cheaper option as compared to the limited capabilities of Si devices because of their better blocking voltages, switching frequencies, thermal conductivities and operating temperatures. WBG semiconductors like Gallium Nitride (GaN) have better materials properties specifically suited for high power and high frequency electronics and they are slowly being favored for such applications. GaN High Electron Mobility Transistors (HEMTs) have demonstrated superior performance characteristics as compared to Si Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) in terms of switching characteristics and switching losses. One particular GaN HEMT module investigated by the authors has been calculated to have less than five times the switching losses as compared to a similar Si MOSFET module under the same operating conditions. The use of the GaN module instead of Si module in an inverter application has also shown reduction of power losses and heatsink volume by 60% and 30% respectively for the GaN module. This paper investigates the effect of different heatsink materials (Aluminum, Copper, AlSiC and E-Material) on the overall temperature profile of the GaN module. The heatsink structure used for the simulations were obtained from commercially available straight-fin heatsink designs. Comparisons among these heatsink materials were done for the same operating and ambient conditions by simulating the combined HEMT-Heatsink structure in the Finite Element Analysis (FEA) software COMSOL Multiphysics. The simulation results indicated Copper to be the best heatsink material among the four materials tested.

Three-dimensional nanoporous silicon (PSi), with inherently large surface areas, tunable pore sizes, film thicknesses, and effective refractive indices, has been utilized as a platform for the detection of biomolecules and high-dose radiation. A brief overview of the fabrication and characterization of the nanoporous framework is presented for novel applications that benefit from such sponge-like, high surface area devices. For many of these applications, it is necessary to ensure that the PSi surfaces are well-passivated and stabilized for subsequent conjugation with linker molecules and for emitters to maintain their emissive properties post-integration with the porous matrix. We present a detailed analysis of the influence that varied levels of interfacial oxide (SiOx) growth has on the optical properties of quantum dots (QDs) immobilized within the PSi thin-films. Reflectance spectroscopy, continuous wave photoluminescence (CWPL) and time-resolved photoluminescence (TRPL) studies provide a comprehensive understanding of the complex QD exciton dynamics at the PSi/SiOx-QD interfaces. The gradual conversion of PSi thin-films into fully-oxidized porous silicon oxide (PSiO2) thin-films is shown to significantly suppress non-radiative recombination pathways of photogenerated QD excitons and achieve almost a five-fold increase in QD exciton lifetimes. This conversion of PSi into PSiO2, a wide bandgap nanoporous material, also circumvents loss of QD emission due to absorption by PSi based devices. Future avenues of research into PSi based devices will be presented based on analyzing the optical scattering response of nanoscale PSi annular rings fabricated over PSi Bragg mirrors via dark field microscopy.

Gallium oxide (Ga2O3) is a promising wide bandgap semiconductor for power electronic applications. Investigation into the conduction mechanism of Ga2O3 Schottky diodes is important for improving the device performance. In this study, the forward-biased temperature dependent current-voltage (I-V-T) characteristics of Ni/(-201) β-Ga2O3 Schottky diodes have been investigated in the temperature range of 298-473 K. The apparent barrier height (ϕ_ap) increased while the ideality factor (n) decreased with the increase in temperature. Such a temperature dependent behavior of ϕ_ap and n was explained by the inhomogeneity of ϕ_ap, which obeyed Gaussian distribution with mean barrier height of 1.8 eV and standard deviation of 201 mV. Subsequently, zero-bias barrier height (¯ϕ_B0) and Richardson constant (A*) were obtained from the slope and intercept of the modified Richardson plot as 1.18 e V and 94.04 A·cm-2·K-2, respectively. The ¯ϕ_B0 obtained from the modified Richardson plot was in good agreement with the theoretical value calculated from the work function of Ni and electron affinity of β-Ga2O3. The I-V-T characteristics of Ni/-Ga2O3 Schottky diodes can be successfully explained by the thermionic emission theory with a single Gaussian distribution of the barrier height.

This paper studies the method to theoretically calculate the power loss, maximum switching frequency, and thermal resistance for a grid-connected multilevel converter. Especially, the commercial IGBT modules were selected to demonstrate the calculation procedures, which provides a useful tool for the selection of devices only based on the data sheet to satisfy the design requirement for a multilevel converter.

The main goal of this paper is to detail the challenges and factors taken into consideration when implementing a hyperspectral imaging system in an agricultural environment for blend grade verification of cured tobacco. Cultivated tobacco plants are harvested and cured before using in the manufacture of tobacco products. Processed tobacco leaves are graded by Subject Matter Experts (graders). Hyperspectral imaging can be used to optimize the grading process in an efficient, cost-effective and commercially applicable manner. VNIR (Visible- Near Infrared) range of 400 -1000 nm was used for imaging the samples and developing the algorithm. Development of the grade verification system consisted of multiple years of data collection, along with iterative design changes. This paper details the challenges in both developing the initial control method for grading cured tobacco as well as adapting to the changing needs of those who ultimately utilize the technology.

DC microgrids are gaining significant attention for smart distributed power systems, particularly in commer- cial and residential sectors, because of their increased energy efficiency, improved power quality, and reduced generation cost. In DC microgrids, distributed renewable energy sources, such as wind turbines, photovoltaic (PV) arrays, and fuel cells, along with energy storage systems–batteries and ultracapacitors–are increasingly implemented as a method of sustainable and clean power generation. Power electronic converters, especially bidirectional buck/boost topologies, play a major role in interfacing these renewable energy sources and energy storage systems with the utility network. However, most existing bidirectional converters face serious conduction and switching losses caused by conventional silicon (Si) devices, which are reaching their theoretical and oper- ational limits. Wide bandgap (WBG) semiconductor devices, such as silicon carbide (SiC) and gallium nitride (GaN), are not only exceed the current Si devices’ limitations but also provide great potential for improving power converters. This paper presents the impact of cascode GaN power devices on a bidirectional DC–DC buck/boost converter in DC microgrids. The results reveal that cascode GaN power devices considerably im- prove the converter performance and efficiency at various switching frequencies, junction temperatures, and output power levels.

The rapid growth in renewable energy based electric power generation continuously pushes for the need of high performance power conversion systems. This paper mainly focuses on the design and performance study of a DC– DC ZETA converter using wide bandgap power devices. The converter is designed based on a SiC MOSFET/SiC Schottky diode, and its performance is compared with a Si IGBT/SiC Schottky diode based converter. The switching characteristics of the SiC MOSFET and Si IGBT power devices within the converter are studied and compared. A comprehensive evaluation of the total power loss and overall efficiency of the converter is analyzed and reported. The results indicate that the converter with the SiC MOSFET/SiC Schottky diode has great potential to work efficiently under different operating conditions.

Pyroelectric materials show a change in their spontaneous polarization due to the temperature variations. This property makes these materials unique for sensing radiation in the infrared (IR) broad range. Here, we report the deposition and characterization of pyroelectric Calcium Lead Titanate (PCT) thin films for using them to fabricate pyroelectric detectors. PCT films were deposited on both silicon and Si/SiN/Ti/Au substrates at 13 mTorr pressure by 200W Radio Frequency (RF) sputtering in Ar+O₂ environment for four hours. Substrates were kept at variable temperatures starting from 550 ºC up to 800 ºC during the deposition. The PCT films were annealed at 550, 600, 650 and 700 ºC in O₂ environment for 15 minutes. X-ray diffraction (XRD) results confirm the polycrystalline nature of these films. Energy dispersive spectroscopy (EDS) function of scanning electron microscope (SEM) was done to determine the elemental composition of PCT films. Our EDS result reveals the presence of the elements such as Calcium, Lead, Titanium , and Oxygen in the thin films. Moreover, it shows that the films are stoichiometric (Ca0.43Pb0.57)TiO3 (Ca/Ti=0.5, Pb/Ti=0.66). The film thicknesses were measured using a Dektak model XT profilometer which ranges from ~ 250 to 400 nm. The surface morphology obtained from SEM and atomic force microscopy confirms the crack-free nature of our films as well as their smoothness and low surface roughness. Temperature dependence of capacitance, pyroelectric current, and pyroelectric coeeficient were investigated for different PCT films. Our results show that films deposited at 550ºC and 600 ºC demonstrate better quality and larger values of pyroelectric coefficient. On the other hand, the capacitance fabricated on the PCT films at 550 ºC showed the highest value of pyroelectric current and pyroelectric coefficient which are 14 pA and 50 μC/m2K respectively at higher temperature.

The negative environmental impacts of energy production from gas and fossil fuels are causing widespread concern to developed countries. However, electricity production from wind turbines and solar energy systems is evolving rapidly to meet the demand for clean and renewable energy. Integrating renewable energy sources with power conversion systems is an area of intense research. Among possible alternative energy resources, solar photovoltaic (PV) systems are increasingly used for electric power generation because they are eco-friendly, emission-free, and relatively cost-effective. High-gain converters are an essential component utilized mainly in low-voltage renewable energy sources and dc-distribution systems because they provide a high-voltage gain and are more efficient than other step-up converters. Interleaved high-gain dc-dc converters promise efficient energy conversion across a range of applications, including distributed generation and grid integration. This paper presents a performance analysis of an interleaved high-gain dc-dc converter for dc-distributed renewable energy systems with 650 V GaN HEMTs. The converter design with GaN power transistors and SiC Schottky diodes is discussed. The performance of the high-gain converter is examined at different input voltages and output power levels.

Two dimensional (2D) materials have become a growing subject in the last 15 years mainly due to the isolation of graphene, which created a completely different class of material based on its unique, monolayer design. Since then, various stable materials of few atoms thick are showing emerging capabilities in optical electronics and photonics. Semiconducting monolayers of transition metal dichalcogenides (TMDs) such as MoS₂, Mo_1-xWxS₂, and WS₂ exhibit direct electronic band gaps; bulk crystals display indirect band gaps. Interestingly, these 2D materials show significant light interaction over a broad bandwidth ranging from infrared to ultraviolet wavelengths. The materials allow photodetection in this bandwidth without the need of cooling, thus creating new potential for uncooled detection. In this review, we discuss various 2D materials and their interaction with light for photodetection applications.