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Perspectives about growth of bulk gallium nitride crystals, fabricating high structural quality gallium nitride wafers and the market demand for them are presented. Three basic crystal growth technologies, halide vapor phase epitaxy, sodium flux, and ammonothermal, are described. Their advantages and disadvantages, recent development, and possibilities are discussed. The main difficulty with crystallization of thick GaN is determined. Some new solutions for bulk growth are proposed. It is shown that only crystallization on high structural quality native seeds will ensure proper progress. New ideas for fabricating gallium nitride crystals and wafers with a better control of their structural properties and point defect concentration are proposed. [ABSTRACT FROM AUTHOR]

Unscreened (ungated) plasmons in large-area grating-gate AlGaN/GaN heterostructures were studied experimentally by Fourier-transform spectroscopy. Special attention was paid to the recently discovered THz plasmonic crystal modes observed at totally depleted gated regions when plasma oscillations were localized only in ungated parts of the grating-gate structures. The frequency of these modes is still gate voltage-dependent in the limited range due to the depletion of the ungated parts located close to the gate edges. Double gate structures with an additional bottom gate were fabricated and studied to improve the gate voltage tunability of the unscreened plasmons. Since this gate is located deep below the channel, the plasmons behaved as ungated ones, but their frequency still could be tuned by this bottom gate. We show that the combined effect of the top and bottom gates allows the efficient tuning of terahertz frequencies of unscreened modes in the grating-gate AlGaN/GaN plasmonic crystals. [ABSTRACT FROM AUTHOR]

Abstract Recent progress in ammonothermal technology of bulk GaN growth in basic environment is presented and discussed in this paper. This method enables growth of two-inch in diameter crystals of outstanding structural properties, with radius of curvature above tens of meters and low threading dislocation density of the order of 5 × 104 cm−2. Crystals with different types of conductivity, n-type with free electron concentration up to 1019 cm−3, p-type with free hole concentration of 1016 cm−3, and semi-insulating with resistivity exceeding 1011 Ω cm, can be obtained. Ammonothermal GaN of various electrical properties is described in terms of point defects present in the material. Potential applications of high-quality GaN substrates are also briefly shown. [ABSTRACT FROM AUTHOR]

Although gene duplication is seen as the main path to evolution of new functions, molecular mechanisms by which selection favours the gain versus loss of newly duplicated genes and minimizes the fixation of pseudo-genes are not well understood. Here, we investigate in detail a duplicate honeybee gene obp11 belonging to a fast evolving insect gene family encoding odorant binding proteins (OBPs). We report that obp11 is expressed only in female bees in rare antennal sensilla basiconica in contrast to its tandem partner obp10 that is expressed in the brain in both females and males (drones). Unlike all other obp genes in the honeybee, obp11 is methylated suggesting that functional diversification of obp11 and obp10 may have been driven by an epigenetic mechanism. We also show that increased methylation in drones near one donor splice site that correlates with higher abundance of a transcript variant encoding a truncated OBP11 protein is one way of controlling its contrasting expression. Our data suggest that like in mammals and plants, DNA methylation in insects may contribute to functional diversification of proteins produced from duplicated genes, in particular to their subfunctionalization by generating complementary patterns of expression. [ABSTRACT FROM AUTHOR]

Results of gallium nitride crystallization by ammonothermal method are presented. GaN crystals grown earlier by a HVPE method on an ammonothermal GaN substrate and an MOCVD-GaN/sapphire template were used as seeds. Structural and optical properties of the obtained materials are studied and compared. Large radius of curvature (>100 m) and low dislocation density (7×10 4 cm −2 ) can be reproduced in the ammonothermal method using an HVPE-GaN seed grown before on ammonothermal GaN. This proves that the use of HVPE-GaN grown on ammonothermal seeds allows to reproduce high crystallinity in a subsequent ammonothermal growth. It also demonstrates that a much more effective multiplication process of high quality GaN can be launched using a combination of the ammonothermal and HVPE methods. [ABSTRACT FROM AUTHOR]

Social environments are notoriously multifactorial, yet studies in rodents have suggested that single variables such as maternal care can in fact be disentangled and correlated with specific DNA methylation changes. This study assesses whether non-detrimental social environmental variation in a highly plastic social insect is correlated with epigenomic modifications at the DNA methylation level. Honey bee workers perform tasks such as nursing and foraging in response to the social environment in the hive, in an age-linked but not age-dependent manner. In this study, the methylation levels of 83 cytosine-phosphate-guanosine dinucleotides over eight genomic regions were compared between the brains of age-matched bees performing nursing or foraging tasks. The results reveal more changes correlated with task than with chronological age, and also hive-associated methylation at some sites. One methylation site from a gene encoding Protein Kinase C binding protein 1 was consistently more methylated in foragers than nurses, which is suggested to lead to production of task-specific protein isoforms via alternative splicing. This study illustrates the ability of the neural epigenome to dynamically respond to complex social stimuli. [ABSTRACT FROM AUTHOR]

In this paper we review the developments of producing non-polar (i.e. m-plane and a-plane) and semi-polar (i.e. (20.1)-plane) wafers by ammonothermal method. The growth method and polishing results are described. We succeeded in producing 26 mm × 26 mm non- and semi-polar wafers. These wafers possess outstanding structural and optical properties, with threading dislocation density of the order of 104 cm-3. Detailed studies of homoepitaxial layers, as well as AlGaN heterostructures are also presented, showing the potential of studied ammonothermal substrates in the fabrication of optoelectronic devices. [ABSTRACT FROM AUTHOR]

A defining characteristic of eusocial animals is their division of labour into reproductive and nonreproductive specialists. Here, we used a microarray study to identify genes associated with functional sterility in the worker honey bee Apis mellifera. We contrasted gene expression in workers from a functionally sterile wild-type strain with that in a mutant (anarchist) strain selected for high rates of ovary activation. We identified a small set of genes from the brain ( n = 7) and from the abdomen ( n = 5) that are correlated in their expression with early stages of ovary activation. Sterile wild-type workers up-regulated two unknown genes and a homologue of Drosophila CG6004. By contrast, reproductive anarchist workers up-regulated genes for the yolk protein vitellogenin, venom peptides and a member of the AdoHycase superfamily, among others. The differentially expressed genes identified are likely to be involved in early differentiation into sterile and reproductive worker phenotypes and may therefore form part of the gene networks associated with the regulation of honey bee worker sterility. Our study may have lacked sufficient power to detect all but a minority of biologically relevant changes taking place; however, the differential expression of vitellogenin and a putative AdoHycase suggests that our screen has captured core reproductive genes and that ovary activation may involve an epigenetic mechanism. [ABSTRACT FROM AUTHOR]

Fertile queens and sterile workers are alternative forms of the adult female honeybee that develop from genetically identical larvae following differential feeding with royal jelly. We show that silencing the expression of DNA methyltransferase Dnmt3, a key driver of epigenetic global reprogramming, in newly hatched larvae led to a royal jelly-like effect on the larval developmental trajectory; the majority of Dnmt3 small interfering RNA-treated individuals emerged as queens with fully developed ovaries. Our results suggest that DNA methylation in Apis is used for storing epigenetic information, that the use of that information can be differentially altered by nutritional input, and that the flexibility of epigenetic modifications underpins, profound shifts in developmental fates, with massive implications for reproductive and behavioral status. [ABSTRACT FROM AUTHOR]

Abstract: With the completion of the honey bee genome project, a transition is now occurring from the acquisition of gene sequence to understanding the role and context of gene products within the genome. Here we annotated and characterised a cluster of three genes in a GC-rich 11kb genomic region on the linkage group 4 encoding highly hydrophobic polypeptides (named apidermins; APD 1–3) containing both sequence motifs characteristic of cuticular proteins and distinctly novel features. Five amino acids, Ala, Gly, Leu, Pro and Val, account for 74–86% of their respective sequences with Ala being the most abundant residue (at least 30% of each peptide). A conserved tetra-peptide AAPA/V is found in all three proteins, but none has the ′R and R′ signature implicated in chitin binding. Two proteins, APD-1 and APD-2, contain an arginine-rich motif RERR in short non-hydrophobic stretches near the N-terminal of mature proteins and in both proteins tryptophan is the C-terminal residue. All three genes are spliced and highly expressed in a defined spatio-temporal pattern. apd-1 is expressed in the exoskeletal epidermis, but only during a restricted period of a few days of late pupal and early adult life when the cuticle becomes dark. APD2 appears to be a protein of “internal” cuticles and is expressed in the tracheas, oesophagus and stomach, and also in the embryo. The expression of apd-3 partly overlaps with both apd-1 and apd-3, but apd-3 also is uniquely associated with non-pigmented cuticles such as the eye cover and external cuticle of white pupae. This study expands the collection of genes encoding cuticular proteins by three novel and well characterised members. [Copyright &y& Elsevier]

We show that differences in the reproductive development of honey bee workers are associated with locus-specific changes to abundance of messenger RNA. Using a cross-fostering field experiment to control for differences related to age and environment, we compared the gene expression profiles of functionally sterile workers (wild-type) and those from a mutant strain in which workers are reproductively active (anarchist). Among the set of three genes that are significantly differentially expressed are two major royal jelly proteins that are up-regulated in wild-type heads. This discovery is consistent with sterile workers synthesizing royal jelly as food for developing brood. Likewise, the relative underexpression of these two royal jellies in anarchist workers is consistent with these workers’ characteristic avoidance of alloparental behaviour, in favour of selfish egg-laying. Overall, there is a trend for the most differentially expressed genes to be up-regulated in wild-type workers. This pattern suggests that functional sterility in honey bee workers may generally involve the expression of a suite of genes that effectively ‘switch’ ovaries off, and that selfish reproduction in honey bee workers, though rare, is the default developmental pathway that results when ovary activation is not suppressed. [ABSTRACT FROM AUTHOR]

Highly metal-polluted (Pb, Cd, Zn) soil from a non-ferrous mine and smelter site in southern Poland, further referred to as “Warynski” soil, was used to test indigenous plant species for stabilization effectiveness of heavy metals in soils. Results of pilot investigations with commercially available cultivars of plant species showed that these cultivars could not grow on this highly polluted soil even with the application of soil amendments to stabilize the heavy metals. Based on these results, mesocosm and field experiments with an indigenous, metal-tolerant ecotype ofDeschampsia cespitosafrom the Waryñski site were carried out. The mesocosm experiment showed that applications of calcium phosphate (3.8% w/w) as a heavy metal-stabilizing amendment decreased Cd and Zn concentrations 2 and 3-fold respectively in leachates, whereas lead content was not significantly changed. This decrease in the concentration of heavy metals in leachates was correlated with a lower accumulation of Pb, Cd and Zn in the roots and shoots ofD. cespitosa, ecotype Waryñski. In the field experiment, lower accumulations of Cd in roots and shoots and Zn in shoots in the amendment added plot were observed during the second year of investigations. In the first growing season,D. cespitosaplant cover in the amendment enriched mesocosms ranged from 95 to 100%, compared to 10% in mesocosms without calcium phosphate. In the second year of the experiment, in non-amendment enriched mesocosmsD. cespitosawas substituted withCardaminopsis arenosa(95% cover).C. arenosais an undesirable species for phytostabilization, as it accumulates high amounts of zinc and cadmium in its shoots, even thought it provided better growth cover in not amended soils. However, in amended mesocosms, soil surface cover byD. cespitosawas still very high (90%). Similar results were obtained in field experiments. Addition of calcium phosphate to the soil also resulted in excellentD. cespitosaroot system development when compared to soils without amendment. In amended mesocosms, high plant cover and root system development significantly decreased the volume of leachates and improved water retention. These results indicate that the use ofD. cespitosa, ecotype Warynski in combination with calcium phosphate as a heavy metals immobilizing agent is sufficient to restore a dense vegetative cover to highly heavy metal-polluted soil. [ABSTRACT FROM AUTHOR]

In seeking genetic factors that may control the extended behavioural maturation of adult honeybees we found that inositol 1,4,5-trisphosphate (IP3) 3-kinase, a key enzyme in the IP3-mediated signalling cascade, is differentially expressed in brains of naive, newly emerged bees and experienced foragers. DNA sequencing yielded a contig of 21.5 kb spanning the honeybee IP3K locus and a 3′ flanking gene similar to a transcription factor NFR-κ-B. The IP3K locus gives rise to three differentially expressed major transcripts produced by alternative splicing that encode proteins with identical, highly conserved C-termini and distinct, non-conserved N-terminal domains. The type A transcript is dominant in the adult brain and its level of expression increases threefold during the first 4 days of adult development. The type B message is expressed in brains of naive bees, but is also found in the thorax and abdomen, whereas transcript C is expressed largely in non-neural tissues and in the antenna. In contrast to type A message, the brain levels of transcript B decrease during the first 4 days of adult life. Our data are evaluated in the context of the contrasting behavioural phenotypes of immature and experienced worker honeybees. [Copyright &y& Elsevier]

• Round shape of seeds allowed to crystallize GaN without cracks at the edges. • 60 µm/day growth rate in ammonothermal crystallization is presented. • Etch pit density in laterally-grown GaN was of the order of 102 cm−2. • Stress Induced Polarization Effect (SIPE) was visible in Am-GaN crystals. Progress in basic ammonothermal gallium nitride crystallization is reported. In order to prevent the appearance of cracks in growing GaN a new shape of native seeds is proposed. Crystallization results on irregular (old) and round (new) shapes are compared in terms of growth rate and structural quality. The new type of seeds allowed to grow crystals twice as fast without losing the very high structural quality. Etch pit density in the new-grown material is examined and compared to that in the seeds. Material deposited in vertical <000–1> and lateral <11–20> directions is analyzed. In the latter case the etch pit density is as low as 102 cm−2. An influence of growth process parameters and crystal growth configuration on the structural quality of crystallized GaN is discussed. [ABSTRACT FROM AUTHOR]

In this letter, the authors demonstrate large size m-plane GaN substrates grown by ammonothermal method. These substrates have excellent structural quality. The concentration of threading dislocation density is below 5×104 cm-2 and the full width at half maximum for the symmetrical and asymmetrical peaks equals 16 and 19 arc sec, respectively. Also good optical quality, the energy gap-related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving truly nonpolar character of the material. [ABSTRACT FROM AUTHOR]

Zinc is often regarded as an alternative to magnesium p-type dopants in gallium nitride. However, besides many theoretical predictions, at present, there are poor data on experimentally revealed p-type conductivity and evaluation of Zn activation energy by means of electrical transport measurements. In this paper, ammonothermal crystallization of bulk GaN:Zn monocrystals is reported. Despite a high doping level of Zn (up to 2 × 1020 cm−3), p-type conductivity with hole concentration as low as of 4 × 1015 cm−3 at room temperature and mobility about 3 cm2/V s was observed. A deep nature of the Zn acceptor was proved, as the ionization energy can exceed 260 meV. In addition, conduction in the impurity band appeared at temperatures as high as room temperature. [ABSTRACT FROM AUTHOR]

The 2011 highly publicised Nature paper by Kamakura on honeybee phenotypic dimorphism, (also using Drosophila as an experimental surrogate), claims that a single protein in royal jelly, Royalactin, essentially acts as a master 'on-off' switch in development via the epidermal growth factor receptor (AmEGFR), to seal the fate of queen or worker. One mechanism proposed in that study as important for the action of Royalactin is differential amegfr methylation in alternate organismal outcomes. According to the author differential methylation of amegfr was experimentally confirmed and shown in a supportive figure. Here we have conducted an extensive analysis of the honeybee egfr locus and show that this gene is never methylated. We discuss several lines of evidence casting serious doubts on the amegfr methylation result in the 2011 paper and consider possible origins of the author's statement. In a broader context, we discuss the implication of our findings for contrasting context-dependent regulation of EGFR in three insect species, Apis mellifera, D. melanogaster and the carpenter ant, Camponotus floridanus, and argue that more adequate methylation data scrutiny measures are needed to avoid unwarranted conclusions. [ABSTRACT FROM AUTHOR]

In this work we present structural, optical and electrical properties of GaN epilayers and AlGaN/GaN MOCVD heterostructures obtained on polar and non-polar 10x10 mm2, 1 and 1.5 inch bulk ammonothermal GaN substrates. The results were obtained based not only on a single-point measurement but also on mapping across the sample area. In contactless electroreflectance (CER) spectra CER resonances related to optical transitions between excited states were clearly observed for GaN/AlGaN QWs grown on c -plane GaN substrates, whereas such features were not noted in the case of GaN/AlGaN QWs grown on m -plane GaN substrates. This experimental result clearly shows that the polarization-related electric field leads to the quantum confinement of some extra states in the polar QW system. To check the advantage of homo-epitaxy using ammonothermal GaN, high electron mobility transistors (HEMTs), and light emitting diodes (LEDs) structures were grown on (0001) Ga-polarity, semi-insulating and conductive GaN substrates respectively (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Carbohydrate-metabolizing enzymes may have particularly interesting roles in the honey bee, Apis mellifera, because this social insect has an extremely carbohydrate-rich diet, and nutrition plays important roles in caste determination and socially mediated behavioural plasticity. We annotated a total of 174 genes encoding carbohydrate-metabolizing enzymes and 28 genes encoding lipid-metabolizing enzymes, based on orthology to their counterparts in the fly, Drosophila melanogaster, and the mosquito, Anopheles gambiae. We found that the number of genes for carbohydrate metabolism appears to be more evolutionarily labile than for lipid metabolism. In particular, we identified striking changes in gene number or genomic organization for genes encoding glycolytic enzymes, cellulase, glucose oxidase and glucose dehydrogenases, glucose-methanol-choline (GMC) oxidoreductases, fucosyltransferases, and lysozymes. [ABSTRACT FROM AUTHOR]

We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104 cm-2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures. [ABSTRACT FROM AUTHOR]

Room temperature contactless electroreflectance (CER) has been applied to study optical transitions and the distribution of the built-in electric field in AlGaN/GaN heterostructures grown on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. It has been clearly shown that polarization effects in the AlGaN/GaN heterostructures grown on the c-plane lead to a strong built-in electric field in the AlGaN layer. The aforementioned field was determined to be ∼0.43 MV/cm from the period of Franz-Keldysh oscillations (FKOs). In addition, polarization effects lead to the formation of a two dimensional electron gas at the AlGaN/GaN interface, which screens the band bending modulation in the GaN buffer layer, and, therefore, GaN-related excitonic transitions are not observed for this heterostructure. Such features/effects are also not observed in the AlGaN/GaN heterostructures grown on nonpolar and semipolar GaN substrates because any strong polarization effects are not expected in this case. For these heterostructures, very strong and sharp GaN excitonic resonances are clearly visible in CER spectra. The resonances are very similar to the excitonic transitions observed for the GaN epilayers deposited on nonpolar and semipolar substrates. Moreover, there is a very weak AlGaN-resonance without FKO for nonpolar and semipolar heterostructures instead of the strong AlGaN-related FKO, which is typical of polar AlGaN/GaN heterostructures. [ABSTRACT FROM AUTHOR]

Study on the sources of stress in HVPE-GaN layer crystallized on 1-in. ammonothermally grown GaN seed is presented in this paper. Characterization by means of X-ray diffraction and transmission electron microscopy is performed. HVPE-GaN samples of high quality and those with visible quality deterioration are investigated on c-plane and m-plane cross-sections. Special attention is paid to HVPE material growing in semi-polar and non-polar directions on the edges of the seed and the growing layer. It is shown that this material generates significant stress leading to a structural deterioration of HVPE-GaN growing in the c -direction. [ABSTRACT FROM AUTHOR]

The phenomenon of high on-acetylsalicylic acid (ASA) treatment platelet (PLT) reactivity – HATPR – and its clinical implications have not been fully understood. Little data is available on assessing PLT activity based on the severity of intra- and postoperative bleeding in a population of orthopedic patients with normal closure time (CT) measured by a PLT function analyzer PFA-100®, despite being given long-term ASA therapy. The aim is to assess PLT function using PFA-100® in patients with ASA therapy and qualified for trauma and orthopedic surgery procedures. The retrospective analysis covered 384 patients whose PLT reactivity was assessed using PFA-100®. Out of those, 198 had been taking ASA with a 75 mg dose until hospital admission. In addition, a group of 70 patients with a proximal femoral fracture surgically treated using the dynamic hip screw (DHS) was selected, in whom severity of bleeding was assessed by HIP ASA (+). The reference group comprised 52 patients (without ASA therapy) who were operated on due to the same indications. Normal CT was found in 37% of ASA-receiving patients. Patients with normal CT, despite ASA therapy, exhibited significantly more intense bleeding after DHS surgery. A similar number of patients required red blood cells (RBCs) transfusion in HIP ASA (+) and HIP ASA (-). Increased risk of complications in HIP ASA (+) group was not found. Conclusions: Normal PLT function assessed using PFA-100® is a common phenomenon in patients with long-term ASA treatment and who are qualified for trauma and orthopedic surgery procedures. In many cases, it seems that inadequate response to ASA is only a laboratory phenomenon. [ABSTRACT FROM PUBLISHER]

GaN/AlGaN quantum wells (QWs) were grown by metal–organic vapor phase epitaxy (MOVPE) on c -, a -, m -, and (20.1)-plane GaN substrates obtained by the ammonothermal method in the same MOVPE process, i.e. a process with growth parameters optimized for c -plane GaN templates. The structural properties of GaN/AlGaN QWs were carefully investigated by high angle annular dark field scanning transmission electron microscopy. Sharp GaN/AlGaN interfaces were seen for QWs grown on the c -, a -, and m -plane GaN substrates, but very rough interfaces with {1-100} and {1-101} facets were observed on the (20.1)-plane GaN substrate. In addition, the Al-rich region of AlGaN and GaN transition was identified for each of the GaN/AlGaN QW samples deposited in this process. The thickness and composition of this region varied with the crystallographic orientation of GaN substrates. [ABSTRACT FROM AUTHOR]

One of the main advantages of the hydride vapor phase epitaxy (HVPE) method for crystallizing bulk gallium nitride is the crystallization of GaN with a relatively high growth rate (>100 μm/h). In this paper various growth rates in the c -direction during crystallization of GaN by HVPE on ammonothermally grown GaN crystals are determined and examined. The influence of the highest (380 μm/h) and the lowest (40 μm/h) growth rate on the structural quality and purity of the HVPE-GaN crystals is analyzed. The optimal macroscopically stable growth rate (without cracks and pits) and the way of achieving it are presented and discussed. [ABSTRACT FROM AUTHOR]

The results of the Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal GaN seeds (Am-GaN) with various misorientations: 0.3, 0.5 and 1 degree to the [1 0 −1 0] and [1 1 −2 0] directions are presented. Growth rate and structural quality of the HVPE-GaN layers are analyzed. Morphology of the crystal growing surface at the beginning of the crystallization process (after one and two hours of growth) and at the end of it (after eight hours) is presented as a function of the Am-GaN seed misorientation. Based on these results a rough growth model is proposed and discussed in detail. [ABSTRACT FROM AUTHOR]

We demonstrate graded-index-separate-confinement-heterostructure InGaN laser diodes (GRINSCH) grown by metal organic vapor phase epitaxy. In this type of structure, the optical mode is confined close to the active region by cladding layers with linearly changing Al content. The virtue of this structure lies in simultaneous improvement of injection efficiency through the funneling effect and mode confinement. After a careful modeling we demonstrate that GRINSCH InGaN laser indeed possess very encouraging properties. Low threshold current density of around 3.5 kA/cm2 accompanied by high differential gain (dG/dJ = 14.93 cm kA-1) make these devices an interesting alternative for the classical step-index structure. [ABSTRACT FROM AUTHOR]

We demonstrate ultra-violet laser diodes emitting at 388 nm grown by plasma-assisted molecular beam epitaxy on semipolar [formula]GaN substrates under metal-rich conditions. The threshold current density and voltage of 13.2 kA/cm2 and 10.8 V were measured at room temperature for devices with the laser ridge waveguide oriented along the [formula] direction. We show smooth, atomically flat surface morphology after growth. The excellent structural quality of the laser heterostructure was corroborated by transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Multi-quantum well (MQW) structures and light emitting diodes (LEDs) were grown on semipolar [formula] and polar (0001) GaN substrates by plasma-assisted molecular beam epitaxy. The In incorporation efficiency was found to be significantly lower for the semipolar plane as compared to the polar one. The semipolar MQWs exhibit a smooth surface morphology, abrupt interfaces, and a high photoluminescence intensity. The electroluminescence of semipolar [formula] and polar (0001) LEDs fabricated in the same growth run peaks at 387 and 462 nm, respectively. Semipolar LEDs with additional (Al,Ga)N cladding layers exhibit a higher optical output power but simultaneously a higher turn-on voltage. [ABSTRACT FROM AUTHOR]

Transparency of truly bulk GaN substrates obtained by ammonothermal method was measured in the mid infrared spectral range by Fourier spectroscopy. The same measurements were performed for GaN templates grown by meatlorganic vapour phase epitaxy on sapphire and SiC substrates. It has been clearly observed that truly bulk GaN substrates are transparent up to ∼7 µm whereas GaN templates grown on sapphire and SiC are transparent only up to ∼6.5 and ∼5.5 µm, respectively, due to non-transparency of sapphire and SiC in this spectral range. It has been shown that the transparency 'cut off' at ∼7 µm for GaN crystals results from the absorption of light by the second harmonic of optical phonons, which is very significant due to the strong electron-phonon coupling in this material. Also it has been clearly presented that the absorption 'cut-off' in the infrared spectral region can be easily tuned by increasing of free carrier concentration in GaN. It was observed that the infrared transparency can be shifted from ∼7 to ∼2 µm or even shorter wavelengths when the electron concentration is increased up to ∼1019 cm-3. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: GaN epilayers were grown by metalorganic chemical vapor deposition (MOCVD) on c-, m-, a-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. The influence of (i) the surface preparation of substrates, (ii) MOCVD growth parameters, and (iii) the crystallographic orientation of substrates on the structural and optical properties of GaN epilayers was investigated and carefully analyzed. It was observed that the polishing of substrates and their misorientation have strong impact on the quality of GaN epilayers grown on these substrates. The MOCVD growth process was optimized for epilayers grown on m-plane GaN substrates. The best structural and optical properties were achieved for epilayers deposited at 1075°C and the total reactor pressure of 50mbar. These conditions were applied to grow GaN epilayers on substrates with other (c-, a-, and (20.1)-plane) crystallographic orientations in the same MOCVD process. Particularly good optical properties were obtained for GaN epilayers deposited on polar and non-polar (m- and a-plane) substrates, whereas slightly worse optical properties were observed for epilayers deposited on the semi-polar substrate. It therefore means that MOCVD growth conditions optimized for a given crystallographic direction (m-plane direction in this case) work rather well also for other crystallographic directions. [Copyright &y& Elsevier]

Abstract: In this paper we present progress made recently in the development of the growth of truly bulk GaN crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at half maximum equals about 15arcsec). GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges like homogenous insulating properties or high p-type conductivity have been also accomplished by means of ammonothermal method. Semi-insulating crystals of resistivity up to 1011 Ωcm and p-type conductivity within hole concentration up to 1018 cm−3 are already available in diameters up to 1.5-in. [ABSTRACT FROM AUTHOR]

GaN-based structures are promising for production of radiation detectors and high-voltage high-frequency devices. Particle detectors made of GaN are beneficial as devices simultaneously generating of the optical and electrical signals. Photon-electron coupling cross-section is a parameter which relates radiation absorption and emission characteristics. On the other hand, photon-electron coupling cross-section together with photo-ionization energy are fingerprints of deep centres in material. In this work, the wafer fragments of the GaN grown by ammonothermal (AT) technology are studied to reveal the dominant defects introduced by growth procedures and reactor neutron irradiations in a wide range, 1012-1016 cm−2, of fluences. Several defects in the as-grown and irradiated material have been revealed by using the pulsed photo-ionization spectroscopy (PPIS) technique. The PPIS measurements were performed by combining femtosecond (40 fs) and nanosecond (4 ns) laser pulses emitted by optical parametric oscillators (OPO) to clarify the role of electron-phonon coupling. Variations of the operational characteristics of the tentative sensors, made of the AT GaN doped with Mg and Mn, under radiation damage by reactor neutrons have been considered. [ABSTRACT FROM AUTHOR]

The work shows a successful fabrication of AlGaN/GaN high electron mobility transistor (HEMT) structures on the bulk GaN substrate grown by ammonothermal method providing an ultralow dislocation density of 104 cm−2 and wafers of size up to 2 inches in diameter. The AlGaN layers grown by metalorganic chemical vapor phase epitaxy method demonstrate atomically smooth surface, flat interfaces with reproduced low dislocation density as in the substrate. The test electronic devices—Schottky diodes and transistors—were designed without surface passivation and were successfully fabricated using mask-less laser-based photolithography procedures. The Schottky barrier devices demonstrate exceptionally low reverse currents smaller by a few orders of magnitude in comparison to the Schottky diodes made of AlGaN/GaN HEMT on sapphire substrate. [ABSTRACT FROM AUTHOR]

We demonstrate InGaN laser diodes with substantially reduced thickness of their bottom AlGaN cladding grown on plasmonic GaN substrate. The electron concentration in plasmonic substrate grown by ammonothermal method was of the order of 1020 cm-3, which corresponds to the refractive index reduction by 0.7% when comparing to undoped GaN. We were able to reduce the thickness of AlGaN bottom claddings from the initial 800 nm down to 400 nm without any worsening of laser threshold current or near-field patterns. The results are in agreement with the optical modeling of the InGaN laser diode structure fabricated on plasmonic GaN substrate. [ABSTRACT FROM AUTHOR]