There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

Major developments, as well as remaining challenges and the associated research opportunities, are evaluated for three technologically distinct approaches to solar energy utilization: solar electricity, solar thermal, and solar fuels technologies. Much progress has been made, but research opportunities are still present for all approaches. Both evolutionary and revolutionary technology development, involving foundational research, applied research, learning by doing, demonstration projects, and deployment at scale will be needed to continue this technology-innovation ecosystem. Most of the approaches still offer the potential to provide much higher efficiencies, much lower costs, improved scalability, and new functionality, relative to the embodiments of solar energy-conversion systems that have been developed to date.

http://science.sciencemag.org/content/sci/351/6271/aad1920.full.pdf

Research opportunities to advance solar energy utilization

Recent research results pertaining to InN, GaN and AlN are reviewed, focusing on the different growth techniques of Group III-nitride crystals and epitaxial films, heterostructures and devices. The chemical and thermal stability of epitaxial nitride films is discussed in relation to the problems of deposition processes and the advantages for applications in high-power and high-temperature devices. The development of growth methods like metalorganic chemical vapour deposition and plasma-induced molecular beam epitaxy has resulted in remarkable improvements in the structural, optical and electrical properties. New developments in precursor chemistry, plasma-based nitrogen sources, substrates, the growth of nucleation layers and selective growth are covered. Deposition conditions and methods used to grow alloys for optical bandgap and lattice engineering are introduced. The review is concluded with a description of recent Group III-nitride semiconductor devices such as bright blue and white light-emitting diodes, the first blue-emitting laser, high-power transistors, and a discussion of further applications in surface acoustic wave devices and sensors.

https://iopscience.iop.org/article/10.1088/0022-3727/31/20/001/pdf

Growth and applications of Group III-nitrides

Photovoltaic (PV) customers need to have confidence in the PV modules they purchase. Currently, no test can quantify a module's lifetime with confidence, but stress tests are routinely used to differentiate PV product designs. We suggest that the industry would be strengthened by using the wisdom of the community to develop a single set of tests that will help customers quantify confidence in PV products. This paper evaluates the need for quality assurance (QA) standards and suggests a path for creating these. Two types of standards are needed: 1) QA of the module design and 2) QA of the manufacturing process.

/pdf/ensuring-quality-of-pv-modules-4d3l4vqd2f.pdf

Ensuring quality of PV modules

Potential-induced degradation (PID) is investigated in small-size Cu(In,Ga)Se2 (CIGS) submodules of individual cells. Samples with conventional soda-lime glass (SLG) and low-alkaline glass (borosilicate glass, BSG) were analyzed applying positive and negative 1000 V bias at 85 °C and low relative humidity after a heat-soaking phase. During the heat-soaking phase, BSG samples showed degradation, with variations in their dark current–voltage ( I–V ) curve parameters compared with the stability of SLG samples. During the PID stress phase, back-grounded SLG samples with −1000 V applied to the cell experienced fast degradation, with complete failure in 25 h, whereas, BSG samples only showed changes because of the dark heat environment. When grounded on their front face, SLG samples remained stable during more than 150 h of PID testing, beginning degradation after longer applications of stress. The analysis of the dark I–V curves of SLG samples through the single-diode model showed that an increase in saturation current J 0 occurs first, attributed to Na+ ions migrating into the CdS/ZnO layer; later, a decrease in shunt resistance occurs. The application of the model in BSG samples showed an increase in series resistance evaluated at 25 °C. When the bias is reversed, SLG samples gradually recover their performance.

Analysis of Potential-Induced Degradation in Soda-Lime Glass and Borosilicate-Glass Cu(In,Ga)Se 2 Samples

Use of indentation to study the degradation of photovoltaic backsheets

Characterization of mid-gap states in HVPE and MOVPE-grown n-type GaN

We report on an investigation of the optical gain in undoped and Si-doped metal-organic vapor phase epitaxially grown GaN. For carrier densities around the Mott transition (at approximately 8×1017 cm−3), the optical gain was observed to increase from around 102 cm−1 up to about 103 cm−1, suggesting a much larger optical gain for an electron-hole plasma than for excitons. A maximum optical gain of 2700 cm−1 at an excitation density of 9.2×1018 cm−3 was found for undoped GaN. This value is the highest achieved for epitaxially grown GaN, verifying the high potential for low-threshold lasing in this material. The optical gain was observed to decrease with increasing doping density. On the contrary, the luminescence efficiency was found to increase up to a doping density of approximately 1018 cm−2. Together these effects create an optimum doping density of around 5×1017 cm−2 for stimulated emission in bulk GaN.

Peter Hacke

Papers

Ensuring quality of PV modules

Analysis of Potential-Induced Degradation in Soda-Lime Glass and Borosilicate-Glass Cu(In,Ga)Se 2 Samples

Use of indentation to study the degradation of photovoltaic backsheets

Characterization of mid-gap states in HVPE and MOVPE-grown n-type GaN

Doping-dependent optical gain in GaN