Cubic zincblende gallium nitride for green-wavelength light-emitting diodes

Gallium nitride (GaN)-based light-emitting diodes (LEDs) are highly energy efficient and their widespread usage in lighting can induce significant worldwide electricity savings. To achieve white and colour-tuneable lighting, the mixing of light from red-, blue- and green-wavelength LEDs is desired. At present, the efficiency of green-wavelength LEDs is only about half of that of red- and blue-wavelength LEDs, which is also known as the ‘green gap’ problem. Cubic zincblende GaN has the potential to bridge the ‘green gap’ due to the theoretical absence of internal electric fields that plague the commonly used c-plane hexagonal wurtzite structure. This review first looks at the various methods of achieving metastable zincblende GaN, and examines the crystal defects present. Then the different components towards a full zincblende GaN LED structure, including p- and n-type doping, zincblende InGaN and AlGaN on GaN heterostructures, together with the problems that need to be overcome to achieve green-wavelength emissions are reviewed.

This review was submitted as part of the 2017 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged     

Selective removal of gallium (III) from aqueous solutions containing zinc or aluminum using sodium di-(n-octyl) phosphinate

Gallium was removed selectively from aqueous solutions containing zinc or aluminum using sodium di-(n-octyl) phosphinate as a ligand (NaL). At low pH or low mole ratios, the gallium was removed by complexation with the ligand as GaL(3(S)), while the zinc or the aluminum remained in the solution. Nearly complete separation of gallium was obtained. By increasing the amount of ligand or by increasing the pH, the zinc or aluminum remaining in the solution was then removed as a solid complex: ZnL(2(S)) or AlL(3(S)), respectively. At a pH between 1.5 and 2 and a mole ratio ligand to total metals of 0.75 for zinc solutions and 1.0 for aluminum solutions, more than 98% of the gallium was selectively removed with a high molar selectivity, alpha(Ga/Zn) and alpha(Ga/Al), respectively. Over 95% of gallium was recovered from the solid GaL(3(S)) complex by treatment of the complex with a 3M NaOH solution and diethyl ether. The gallium was concentrated in the aqueous solution to 4 times its initial concentration and the ligand was extracted into the ether phase. After evaporation of the ether, 95% of the ligand was regenerated in its sodium form as a solid.     

Real-time pixel based early apple bruise detection using short wave infrared hyperspectral imaging in combination with calibration and glare correction techniques

High speed data processing for online food quality inspection using hyperspectral imaging (HSI) is challenging as over hundred spectral images have to be analyzed simultaneously. In this study, a real-time pixel based early apple bruise detection system based on HSI in the shortwave infrared (SWIR) range has been developed. This systems consists of a novel, homogeneous SWIR illumination unit and a line scan camera. The system performance was tested on Jonagold apples bruised less than two hours before scanning. Partial least squares-discriminant analysis was used to discriminate bruised pixel spectra from sound pixel spectra. As the glossiness of many fruit and vegetables limits the accuracy in the detection of defects, several reflectance calibrations and pre-processing techniques were compared for glare correction and maximizing the signal to noise ratio. With the best combination of first derivative and mean centering, followed by image post-processing, this system was able to detect fresh bruises in thirty apples with 98% accuracy at the pixel level with a processing time per apple below 200 ms.     

Influence of annealing porous templates in an ammonia atmosphere on gallium nitride growth behaviors in hydride vapor phase epitaxy

Porous templates were fabricated by hydrogen-etching metal organic chemical vapor deposited gallium nitride (GaN); these templates were used as substrates for the growth of GaN via hydride vapor phase epitaxy. The influence of annealing porous templates on GaN growth behavior was investigated. GaN epitaxied on the unannealing porous template followed the Volmer–Weber mode with the void preserved at the growth plane, whereas the GaN film on the annealed porous templates exhibited a layer-by-layer growth and filled the porous material. The GaN crystal quality was characterized by high-resolution XRD and CL, the results indicated that GaN grown with pores preserved at the template interface had a lower dislocation density than that grown with pores filled, and the best GaN film had a TD density of 10⁴ cm⁻².     

Low-temperature ECR-PEMOCVD deposition of high-quality crystalline gallium nitride films: A comparative study of intermediate layers for growth on amorphous glass substrates

A low temperature growth method based on an electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system (ECR-PEMOCVD) was proposed for the growth of GaN (Gallium nitride) films on ordinary amorphous soda-lime glass substrates. To alleviate the large lattice mismatch between GaN film and glass substrate and improve the heat dissipation performance for potential optoelctrical device application, five intermediate layers (Cu, Ni, Ti, Ag, and ITO) were deposited on the glass substrate before the growth of GaN. A comparative study was performed through structural analysis of the as-grown GaN films with various intermediate layers investigated by means of in-situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), and atomic force microscopy (AFM). The results indicate that the Ti intermediate layer has a great advantage over other intermediate layers in view of crystalline quality and smooth surface, therefore is more suitable and preferred for the potential application in optoelectronic devices.     

GaN crystals prepared through solid-state metathesis reaction from NaGaO2 and BN under high pressure and high temperature

GaN crystals are successfully obtained through solid-state metathesis (SSM) reaction between sodium gallium oxide (NaGaO₂) and boron nitride (BN) under high pressure and high temperature. X-ray diffraction (XRD) pattern indicates that the attained GaN crystals possess a hexagonal wurtzite-type structure. Scanning electron microscopy (SEM) is used to estimate the size and morphology of GaN crystals, and results show that GaN grains with the size over 100 μm can be prepared at 5 GPa and 1600 °C. Moreover, pressure-temperature (P-T) formation region of GaN has been discussed. Our results suggest a promising novel route for synthesizing GaN crystals from SSM reactions under high pressure.     

离子交换法回收镓工艺中螯合树脂的研究

研究了工业酰胺肟树脂ES-346从拜耳法溶液中提取镓。这种树脂对镓的交换能力,选择性及动力学颇为适宜,其长期使用的关键问题在于酰胺基在酸性介质中的退化及钒的积累,文中讨论了解决这些问题的可能途径。     

广西某铝母液中镓钒铝的偕胺肟螯合树脂吸附与解吸试验

广西某铝土矿浸出循环母液中富含镓、钒,为给镓的分离、回收工艺研究提供依据,研究了偕胺肟螯合树脂吸附镓、钒、铝的规律和盐酸解吸的规律。结果表明:①偕胺肟螯合树脂对镓具有较高选择性,对铝的吸附率非常低;吸附温度对树脂吸附镓、铝的影响较小,对吸附钒影响较大,钒吸附率随温度升高而增大;树脂对镓的吸附速率比钒快很多;该吸附过程符合准二级动力学模型和颗粒扩散模型;镓的最优吸附条件为吸附温度25℃,吸附时间60 min。②在25℃情况下较低浓度的盐酸就能高效、快速解吸偕胺肟螯合树脂吸附的镓、铝,且受解吸温度的影响较小,钒难以解吸;提高盐酸浓度和解吸温度,盐酸对钒的解吸率明显上升;镓的最优解吸条件为盐酸浓度1.0 mol/L,解吸温度25℃,解吸时间5 min。③由于偕胺肟螯合树脂对铝土矿浸出循环母液中镓和钒的吸附率较高,且钒解吸困难,影响树脂的再生和循环利用。为了高效、低成本分离、回收镓,需在树脂吸附镓之前先采用氢氧化钠沉淀并回收原液中的钒,以尽量降低溶液的钒含量。     

N、0、Ga、Mg和Si离子注入n型GaN样品的光致发光谱中蓝光发射带的研究

采用光致发光手段研究了几种不同注入离子N、O、Mg、Si和Ga对n型GaN蓝光发射带的影响。其中离子的注入剂量分别为10^13、10^14、10^15和10^16cm^-2，注入温度为室温。注入后的样品在900℃流动氮气环境下进行10min的热退火。通过对实验测得的光致发光谱的分析，给出了不同注入离子对n型GaN蓝光发射带的影响随注入剂量的变化关系以及该影响的相对强弱，进而确定蓝光发射起源于注入离子引入的间位缺陷。     

Normally-off characteristics of LiNbO3/AlGaN/GaN ferroelectric field-effect transistor

With the help of MgO mask layer, LiNbO₃ (LN) ferroelectric films were etched effectively using wet etching method and LN/AlGaN/GaN ferroelectric field-effect transistors (FFETs) were fabricated. The electrical properties of the FFETs were studied. Due to the ferroelectric polarization nature of LN films, normally-off characteristics with a turn-on voltage of about + 1.0 V were exhibited in the device. The operation mechanisms of the LN/AlGaN/GaN FFET devices were proposed by the numerical calculations of the electronic band structure and charge distribution.