The history of development for gallium-nitride-based light-emitting diodes (LEDs) is reviewed. We identify two broad developments in GaN-based LED technology: first, the key breakthroughs that enabled the development of GaN-based devices on foreign substrates like sapphire (first-generation LEDs), and, second, a new wave of devices benefiting from developments in GaN substrate manufacturing, which has led to native bulk-GaN-based LEDs with unprecedented performance characteristics that portend a disruptive shift in LED output power density and the corresponding cost of generating light.

History of Gallium–Nitride-Based Light-Emitting Diodes for Illumination

Thick high-purity β-Ga2O3 layers of high crystalline quality were grown homoepitaxially by halide vapor phase epitaxy (HVPE) using gaseous GaCl and O2 on (001) β-Ga2O3 substrates prepared by edge-defined film-fed growth. The surface morphology and structural quality of the grown layer improved with increasing growth temperature. X-ray diffraction ω-rocking curves for the (002) and (400) reflections for the layer grown at 1000 °C had small full widths at half maximum. Secondary ion mass spectrometry and electrical characteristics revealed that the growth of high-purity β-Ga2O3 layers with low effective donor concentration (Nd − Na < 1013 cm−3) is possible by HVPE.

https://iopscience.iop.org/article/10.7567/APEX.8.015503/pdf

Homoepitaxial growth of β-Ga2O3 layers by halide vapor phase epitaxy

Abstract Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter=24.26 mm, thickness=1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Non-linear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

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A Review on Piezoelectric Energy Harvesting: Materials, Methods, and Circuits

Piezoelectric energy harvesting has attracted wide attention from researchers especially in the last decade due to its advantages such as high power density, architectural simplicity, and scalability. As a result, the number of studies on piezoelectric energy harvesting published in the last 5 years is more than twice the sum of publications on its electromagnetic and electrostatic counterparts. This paper presents a comprehensive review on the history and current state-of-the art of piezoelectric energy harvesting. A brief theory section presents the basic principles of piezoelectric energy conversion and introduces the most commonly used mechanical architectures. The theory section is followed by a literature survey on piezoelectric energy harvesters, which are classified into three groups: (i) macro- and mesoscale, (ii) MEMS scale, and (iii) nanoscale. The size of a piezoelectric energy harvester affects a variety of parameters such as its weight, fabrication method, achievable power output level, and potential application areas. Consequently, size-based classification provides a reliable and effective basis to study various piezoelectric energy harvesters. The literature survey on each scale group is concluded with a summary, potential application areas, and future directions. In a separate section, the most prominent challenges in piezoelectric energy harvesting and the studies focusing on these challenges are discussed. The conclusion part summarizes the current standing of piezoelectric energy harvesters as possible candidates for various applications and discusses the issues that need to be addressed for realization of practical piezoelectric energy harvesting devices.

Piezoelectric energy harvesting: State-of-the-art and challenges

Wide-bandgap semiconductors are expected to be applied to solid-state lighting and power devices, supporting a future energy-saving society. While GaN-based white LEDs have rapidly become widespread in the lighting industry, SiC- and GaN-based power devices have not yet achieved their popular use, like GaN-based white LEDs for lighting, despite having reached the practical phase. What are the issues to be addressed for such power devices? In addition, other wide-bandgap semiconductors such as diamond and oxides are attracting focusing interest due to their promising functions especially for power-device applications. There, however, should be many unknown phenomena and problems in their defect, surface, and interface properties, which must be addressed to fully exploit their functions. In this review, issues of wide-bandgap semiconductors to be addressed in their basic properties are examined toward their ?full bloom?.

/pdf/wide-bandgap-semiconductor-materials-for-their-full-bloom-7wjoaf5zpd.pdf

Wide-bandgap semiconductor materials: For their full bloom†

Fabrication of 3-in GaN substrates by hydride vapor phase epitaxy using void-assisted separation method

The fundamental material parameters associated with GaN, which are important for the design of devices such as light-emitting diodes and laser diodes, were investigated using large high-quality GaN single crystals fabricated through hydride vapor phase epitaxy using the void-assisted separation method. The thermal-expansion coefficients (298–573K) along the C[0001], A[112¯0], and M[101¯0] axes (αC, αA, and αM) were measured. Thermal expansion in each direction, approximately proportional to the temperature, was observed throughout the measured temperature range. Although the thermal-expansion coefficients in the high-temperature range, i.e., αC(573K)=7.2±0.02×10−6∕K, αA(573K)=5.7±0.2×10−6∕K, and αM(573K)=5.8±0.2×10−6∕K,were relatively close to the reported values, the thermal-expansion coefficients along the C axis in the low-temperature range, i.e., αC(298K)=5.3±0.02×10−6∕K, was significantly larger than the reported values. Thermal conductivities parallel and perpendicular to the C axis were almost the ...

Thermal and optical properties of bulk GaN crystals fabricated through hydride vapor phase epitaxy with void-assisted separation

Disclosed are a piezoelectric thin film element and a piezoelectric thin film device which have improved piezoelectric properties and high performance and can be produced in improved yields The piezoelectric thin film element ( 1 ) comprises: a substrate ( 10 ), and a piezoelectric thin film ( 40 ) which is arranged on the substrate ( 10 ), has at least one crystal structure represented by general formula (Na x K y Li z )NbO 3 (0≦x≦1, 0≦y≦1, 0≦z≦02, x+y+z=1) and selected from the group consisting of pseudo-cubic crystal, a hexagonal crystal, and an orthorhombic crystal, and contains an inert gas element at a ratio of 80 ppm or less by mass

Piezoelectric thin-film element and piezoelectric thin-film device

We demonstrate the ultrahigh-speed growth of thick GaN layers by hydride vapor phase epitaxy (HVPE) and present an investigation of the properties of the resulting GaN crystals. A series of GaN layers with a thickness of around 700 μm were grown homoepitaxially on freestanding GaN substrates at various growth rates ranging from 107–1870 μm/h, using a conventional atmospheric HVPE reactor. The growth rate increased almost linearly with the feed rate of source materials, reaching 1870 μm/h when the HCl flow rate is 650 sccm. The surfaces of all the homoepitaxial layers were specular, and no cracks or pits were observed. The X-ray rocking curve measurement showed that the full width at half maximum values did not depend on the growth rate, and had small values of 23–30 arcsec for (0002) reflections and 39–58 arcsec for (30-32) reflections. Plan-view cathodoluminescence imaging revealed that the dislocation density had virtually no dependence on the growth rate and had small values of around 3 × 106 cm-2 even at the highest growth rate (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrahigh‐speed growth of GaN by hydride vapor phase epitaxy

(K,Na)NbO3 (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na/(K+ Na) ratio of the films. Annealing in air at 750 °C led to a decrease in the residual strain in the KNN crystal and the disappearance of openings at the grain boundary, thereby improving the transverse piezoelectric coefficient and leakage current properties. We also investigated the transverse piezoelectric coefficient and dielectric constant as a function of the Na/(K+ Na) ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e31* ranged between -10.0 and -14.4 C/m2; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O3 films.

https://iopscience.iop.org/article/10.1143/JJAP.50.041503/pdf

Kazutoshi Watanabe

Papers

Fabrication of 3-in GaN substrates by hydride vapor phase epitaxy using void-assisted separation method

Thermal and optical properties of bulk GaN crystals fabricated through hydride vapor phase epitaxy with void-assisted separation

Piezoelectric thin-film element and piezoelectric thin-film device

Ultrahigh‐speed growth of GaN by hydride vapor phase epitaxy

Improvement of Piezoelectric Properties of (K,Na)NbO3 Films Deposited by Sputtering