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Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35–100 ppm [corresponding to (3–9) × 1018 cm−3] in doped AlxGa1–xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1–xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1–xN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost-effective and time-saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.

III-Nitride bandgap and refractive index data are of direct relevance for the design of (In, Ga, Al)N-based photonic and electronic devices. The bandgaps and bandgap bowing parameters of III-nitrides across the full composition range are reviewed with a special emphasis on InxAl1−xN, where less consensus was reached in the literature previously. Considering the available InAlN data, including those recently reported for low indium contents, empirical formulae for InAlN bandgap and bandgap bowing parameter are proposed. Applying the generalised bandgap data, the refractive index dispersion data available in the literature for III-N alloys is fitted using the Adachi model. For this purpose, a formalism involving a parabolic dependence of the Adachi parameters on the dimensionless bandgap $${\xi }_{{E}_{\mathrm{g}}}=\left({E}_{\mathrm{g}, {\mathrm{A}}_{x}{\mathrm{B}}_{1-x}\mathrm{N}}-{E}_{\mathrm{g},\mathrm{ BN}}\right)/\left({E}_{\mathrm{g}, \mathrm{AN}}-{E}_{\mathrm{g},\mathrm{ BN}}\right)$$ ξ E g = E g , A x B 1 - x N - E g , BN / E g , AN - E g , BN of the corresponding ternary alloys is used rather than one directly invoking the alloy composition.

Zinc oxide (ZnO) and Al-doped ZnO are important optoelectronic materials. ZnO in particular has a wide band gap (Eg ~ 3.3 eV at 300 K), large exciton binding energy (~66 meV) and especially for the variety of methods by which it can be processed. Moreover, ZnO is readily able to alloy with other metals in the oxide form and has a lattice that can facilitate interstitial doping using several metals, including Al and Sn. This gives ZnO a key role in optoelectronics, metal oxide thin films and thin film transistor (TFT) technologies. We demonstrate that crystalline, epitaxial-like and highly ordered ZnO and Al:ZnO (AZO) quasisuperlattice thin films can be achieved from a precursor liquid at relatively low temperature via spin-coating, which crystallize with near epitaxy with a pronounced c-plane texture. We also show the visible and near-infra red (VIS-NIR) light spectroscopy of ZnO and Al:ZnO multi-layered thin film structures grown on oxidized silicon substrates. We show measurements demonstrating sub-band optical pumping of AZO QSLs that enable significant photoconductivity. In AZO QSLs, conduction band electrons from valence band pumping at incident irradiation > Eg, are not required to initiate or enhance photoconductivity, and PL data indicate a donor level that enable enhance sub-band photoconductivity even up to bias voltages of 200 V. Photoexcitation with a 532 nm source efficiently pumps the AZO conduction band without increasing the density of electron-holes in the valence band, minimizing field-driven recombination and enabling significant photoconductance over wide voltage ranges. Density functional theory analysis on ZnO systems with Al-doping configurations of ~2% indicate that interstitial doping is required to generate a density of states that corroborates the proposed donor level to enable the photoconductive properties seen in this work. References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono, Nature 432, 488 (2004). [2] T. Kamiya, K. Nomura and H. Hosono, Sci. Tech. Adv. Mater. 11, 044305 (2010). [3] R. A. Street, Adv. Mater. 21, 2007 (2009). [4] H. Lingling, H. Dedong, C. Zhuofa, C. Yingying, W. Jing, Z. Nannan, D. Junchen, Z. Feilong, L. Lifeng, Z. Shengdong, Z. Xing and W. Yi, Jpn. J. Appl. Phys 54, 04DJ07 (2015). [5] C. Glynn and C. O'Dwyer, Adv. Mater. Interfaces 4, 1600610 (2017). [6] D. Buckley, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, J. Vac. Sci. Technol. A, 35, 061517 (2017). [7] D. Buckley, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, ECS Trans., 77, 99 (2017). [8] D. Buckley, R. McCormack and C. O'Dwyer, J. Phys. D: Appl. Phys., 50, 16TL01 (2017). [9] D. Buckley, R. McCormack, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, ECS Trans., 77, 75 (2017).

Thermal annealing of top−down fabricated GaN nanocolumns (NCs) was investigated over a wide range of temperatures for ammonia-rich atmospheres of both nitrogen and hydrogen. It was found that in contrast to the annealing of planar GaN layers, where surface morphology change is governed purely by material decomposition, reshaping of GaN NCs is strongly affected by competition between different crystallographic facets, which in turn depends on ambient atmosphere and temperature. A qualitative mechanism explaining the observed behavior has been proposed. On the basis of the analysis of these annealing results, growth conditions suitable for either predominantly lateral expansion of the NCs turning their sidewalls into six well-defined vertical m-plane facets, or, vice versa, their infilling from the base regions between the NCs were determined. GaN NC arrays of increased filling factors as compared to the as top−down fabricated ones have been demonstrated using these optimized growth conditions.

Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35-100 ppm [corresponding to (3-9) × 1018 cm-3] in doped AlxGa1-xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1-xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1-xN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost-effective and time-saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.

Porous GaN epitaxial layers were prepared using single-step chemical vapor deposition (CVD) through the direct reaction of ammonia with gallium. The degree of porosity and pore diameters in the resulting GaN were analyzed by means of SEM and AFM and were found to depend on the GaN deposition time. Furthermore, the evolution of the contact angle of a droplet of water located on the surface of these GaN epitaxial layers with the deposition time was investigated. We observe a transition from the hydrophilic regime to the hydrophobic regime for deposition times longer than 15 min. The observed dependence of GaN hydrophobicity on its degree of porosity is discussed and explained in the framework of the Cassie-Baxter model.

With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11–22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x ≈ 0.57–0.85) and dopant concentration (3 × 1018–1 × 1019 cm−3) in various series of Al x Ga1-x N layers grown on (0001) and (11–22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0–5.4 eV as well as various deep impurity transition peaks in the range 2.7–4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments., publishedVersion

In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.

III-Nitride bandgap and refractive index data are of direct relevance for the design of (In, Ga, Al)N-based photonic and electronic devices. The bandgaps and bandgap bowing parameters of III-nitrides across the full composition range are reviewed with a special emphasis on In

Reduction of threading dislocation density in top-down fabricated GaN nanocolumns (NCs) via their successive lateral shrinkage by anisotropic wet etch and lateral overgrowth by metalorganic chemical vapor deposition is studied by transmission electron microscopy. The fabrication process involves a combination of dry and wet etches to produce NC arrays of a low fill factor (

Heteroepitaxial growth of GaN buffer layers on 3C-SiC/(001) Si substrates (4°-miscut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown Al x Ga 1 - x N/AlN interlayers were employed to control GaN surface orientations. Semipolar GaN layers with (10 1 ¯ 1), (20 2 ¯ 3) and (10 1 ¯ 2) surface orientations were achieved, as confirmed by X-ray diffraction. Due to the substrate miscut, the growth of (10 1 ¯ 1) layers was twinned along [ 1 1 ¯ 0 ] 3 C - SiC / Si and [ 1 ¯ 10 ] 3 C - SiC / Si while the growth of (20 2 ¯ 3) and (10 1 ¯ 2) layers was only along [ 110 ] 3 C - SiC / Si . The (10 1 ¯ 1) layers have rough surface morphology while the (20 2 ¯ 3) and (10 1 ¯ 2) layers have mirror-like smooth surface. For all samples with various surface orientations, different photoluminescence peak emission energies were observed at ∼ 3.45 eV, 3.78 eV and 3.27 eV at 10 K. These emissions are attributed to the near-band edge of hexagonal GaN, basal-plane stacking faults and partial dislocations, respectively. The dominant luminescence intensity of stacking faults indicates their high density in the GaN layers.

The limited modulation bandwidth of commercial light-emitting diodes (LEDs) is one of the critical bottlenecks for visible light communications. Possible approaches to increase the bandwidth include the use of micron sized LEDs, which can withstand higher current densities, as well as the use of LED structures that are grown on different crystal planes to the conventional polar c-plane. We compare c-plane InGaN/GaN LEDs with semipolar ( $11\overline {2}2$ ) LEDs containing a 4- and 8-nm single quantum well. The modulation bandwidth of semipolar LEDs with active areas varying from $200\times 200$ to $30\times 30\,\,\mu \text{m}^{2}$ is shown to be governed by both current density and size. A small signal bandwidth of over 800 MHz for a relatively low applied current density of 385 A/cm2 is reported for $30\times 30 \,\,\mu \text{m}^{2}$ LEDs with 8-nm thick quantum well. An optical link using an easy non-return-to-zero ON–OFF keying modulation scheme with a data rate of 1.5 Gb/s is demonstrated.

Zinc oxide is an important optoelectronic material, of particular interest due to its wide band gap (Eg ~ 3.3 eV at 300 K), large exciton binding energy (~66 meV) and especially for the variety of methods by which it can be processed. Moreover, ZnO is readily able to alloy with other metals in the oxide form and has a lattice that can facilitate interstitial doping. This gives ZnO a key role in the area of optoelectronics, metal oxide thin films and thin film transistor (TFT) technologies. We demonstrate that crystalline, epitaxial-like and highly ordered ZnO thin films can be achieved from a precursor liquid at relatively low temperature via spin-coating. The synthesised films are smooth, stoichiometric ZnO with controllable thickness. An iterative layer-by-layer coating schematic is employed to demonstrate the effects of film thickness on structure, morphology as well as the surface and internal defects. Characterisation of the crystallinity, morphology, O-vacancy formation, stoichiometry, surface roughness and thickness variation was determined through X-ray diffraction, scanning and transmission electron and atomic force microscopy, X-ray photoelectron and photoluminescence spectroscopy, and the data of multi-layered ZnO correlated to defect formation and electrical conductivity. It is imperative that high crystal quality epitaxial-like thin films can be formed from solution processing to compete with physical deposition methods. We demonstrate that iterative spin-coating of deposited ZnO films results in a transition in crystal texture with increasing thickness (number of layers) from the [] m-plane to the [] c-plane. The films attain a c-axis preferential orientation, with no other crystalline peaks present. Results show that the film’s surface morphology was very smooth, with average rms roughness

The effect of silicon doping on the growth and properties of ∼ 1.0 μ m - thick Al x Ga 1 - x N ( 0.50 ≤ x ≤ 0.55 ) layers grown on semipolar ( 11 2 ¯ 2 ) AlN templates by metalorganic vapour phase epitaxy was studied. The layers were grown with different disilane/group-III precursors ratios that varied from 2.8×10 −5 to 3.4×10 −4 . The surface morphology of the Si-doped ( 11 2 ¯ 2 ) AlGaN layers showed undulations along [ 1 1 ¯ 00 ] AlGaN , AlN with a root-mean square roughness of about 4.0 nm within a scan range of 20 × 20 μ m 2 . Different photoluminescence peaks have been linked to negatively charged cation vacancies ( V III 3 − ) and their complexes with impurities such as V III 3 − - 3 O N 1 + , (V III complex) 1− , and (V III complex) 2− . The optimised AlGaN:Si layer exhibited a carrier concentration of ∼1.2×10 19 cm −3 , a carrier mobility of 30.7 cm 2 /V s, and a resistivity of 0.018 Ω cm , as determined by Hall-effect measurements at room temperature. A correlation between the resistivity and luminescence emission intensities of AlGaN near-band-edge and impurity-related complexes was found.

Two-dimensional temperature distributions of thin-film edge-emitting gallium arsenide (GaAs) 3 μm wide ridge lasers transfer-bonded to substrates with different thermal conductivities, k , were simulated in COMSOL. The thermal resistance, R th , is compared with a simplified steady-state analytic expression. The effects of laser cavity length, thickness of dielectric passivation layer, contact metal layer thickness and submount material are investigated in order to reduce the thermal resistance of the laser when referenced to lasers with the native GaAs substrate. The simulations show the importance in reducing the GaAs substrate thickness especially for short cavity lengths. The R th of a 200 μm long laser with the substrate fully removed is 37.5 K/W, compared with 230 K/W for the laser with a 100 μm thick GaAs substrate. Increased p-contact metal thickness and reduced dielectric layer thickness further reduce R th . If alumina ( k = 1.35 W/mK) is used as a submount, its thickness above a perfect heat sink should be minimised to decrease the junction temperature. A 10 μm thick silicon submount ( k = 150 W/mK) above a perfect heat sink provides an acceptably low R th .

The effect of V/III ratio on the growth and properties of AlGaN layers grown on (112¯2) AlN templates grown on (101¯0) sapphire by metalorganic vapour phase epitaxy was studied. The surface morphology of the (112¯2) AlGaN layers and the (112¯2) AlN templates showed an undulation along [ 1 1 ¯ 00 ] AlGaN , AlN . The Al-content and thickness of the layers increased with decreasing V/III ratio due to a reduction in the parasitic reactions of the precursors. The Al-content of the (112¯2) layers was found to be in the range of 29.5−47.9%, which is lower than the composition of the simultaneously grown (0001) reference layers (30.4−58.0%). This was attributed to a higher density of cation (nitrogen) dangling bonds on the (112¯2) surface. Low temperature photoluminescence measurements of the (112¯2) layers showed an emission wavelength that shifts gradually from 273 nm to 306 nm with increasing V/III ratio. A decreased PL intensity of the layers with decreasing V/III ratio was attributed to an increase in cation vacancies. The Stokes-shift of the (112¯2) layers was estimated to be about 60−194 meV, and this shift increases with increasing Al-content (decreasing V/III ratio) correlated to an increased exciton localization.

International audience; Relaxation of tensile strain in AlGaN heterostructures grown on GaN template can lead to the formation ofcracks. These extended defects locally degrade the crystal quality resulting in a local increase of non-radiativerecombinations. The effect of such cracks on the optical and structural properties of core-shell AlGaN/AlGaNmultiple quantum wells grown on GaN microwires are comprehensively characterized by means of spectrallyand time-correlated cathodoluminescence (CL). We observe that the CL blueshifts near a crack. By performing6x6 k.p simulations in combination with transmission electron microscopy analysis, we ascribe this shift tothe strain relaxation by the free surface near cracks. By simultaneously recording the variations of both theCL lifetime and the CL intensity across the crack, we directly assess the carrier dynamics around the defectat T = 5 K. We observe that the CL lifetime is reduced typically from 500 ps to less than 300 ps and the CLintensity increases by about 40% near the crack. The effect of the crack on the optical properties is thereforeof two natures. First, the presence of this defect locally increases non-radiative recombinations while at thesame time, it locally improves the extraction efficiency. These findings emphasize the need for time-resolvedexperiments to avoid experimental artifacts related to local changes of light collection

The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.

InxGa1−xN/GaN multiple quantum well structures (x = 0.13 and 0.18) embedded into p–i–n diodes on ( 20 2 ¯ 1 ¯) and ( 20 2 ¯ 1) oriented GaN substrates were investigated by electroreflectance, photocurrent, and electroluminescence. Transition energies in absorption and emission experiments were measured as a function of the polarization orientation of light and applied bias voltage. The results were analyzed by a perturbation theoretical model to determine polarization fields. For the ( 20 2 ¯ 1 ¯) sample (x = 0.18), the flatband voltage is found at + 1 V corresponding to a polarization field of − 458 kV / cm. For the ( 20 2 ¯ 1 ) sample (x = 0.13), the polarization field is estimated to be ≈ + 330 kV / cm at flatband voltage higher than turn-on voltage of this light emitting diode.InxGa1−xN/GaN multiple quantum well structures (x = 0.13 and 0.18) embedded into p–i–n diodes on ( 20 2 ¯ 1 ¯) and ( 20 2 ¯ 1) oriented GaN substrates were investigated by electroreflectance, photocurrent, and electroluminescence. Transition energies in absorption and emission experiments were measured as a function of the polarization orientation of light and applied bias voltage. The results were analyzed by a perturbation theoretical model to determine polarization fields. For the ( 20 2 ¯ 1 ¯) sample (x = 0.18), the flatband voltage is found at + 1 V corresponding to a polarization field of − 458 kV / cm. For the ( 20 2 ¯ 1 ) sample (x = 0.13), the polarization field is estimated to be ≈ + 330 kV / cm at flatband voltage higher than turn-on voltage of this light emitting diode.

A comprehensive description of a procedure to form dense locally ordered 2D arrays of vertically aligned hexagonal in section GaN nanocolumns (NCs) without height deviations will be presented. Particular focus will be given for the preparation of silica nanosphere hard masks, dry etching to form GaN NCs, wet etching to modify NC shape, thermal annealing and regrowth to recover non-polar m-plane facets, improve NC crystal quality and array fill factor.

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8–34 × 10−5 and 1.3–6.6 × 10−5, respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al0.6Ga0.4N series. In the doping range explored, impurity-band transport was not only dominant at low temperature, but also significant at room-temperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al0.6Ga0.4N material reaches its maximum at about 1.6 × 1018 cm−3, well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1 × 1018 cm−3 up to 4.3 × 1018 cm−3. For the Al0.85Ga0.15N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material.

We report on metalorganic vapor phase epitaxial growth of InAlN layers on (0001) and (112) AlN templates at different temperatures (725–800 °C). The indium content (8.0–15.2%) of the (112) InAlN layers was found to be lower compared to that for the (0001) layers (9.0–23.0%). The higher indium content of the (0001) layers was attributed to a high density of small hillocks and a larger degree of relaxation. The small hillocks on the (0001) layer surface acted as quantum-dot-like structures that caused a stronger emission at longer wavelength compared to weaker emission at shorter wavelength from a “bulk” layer underneath. A large Stokes-shift of about 300 and 480 meV was observed for the (0001) and () layers, respectively. The larger shift of the (112) layers was attributed to a higher degree of localization. The larger degree of localization and higher unintentional oxygen incorporation enhanced the luminescence intensity of the (112) layers compared to that for the (0001) layers.

authoren InGaN multiple quantum well light-emitting diodes (LEDs) were grown on chemically–mechanically polished (112―2) GaN templates (up to 100 mm diameter wafers) by metalorganic vapour phase epitaxy. Initial GaN overgrowth on the polished templates in nitrogen ambient maintained the polished surface. The peak emission wavelength of the LEDs varied from 445 to 550 nm. In contrast to the simultaneously grown LEDs on as-grown templates, the LEDs on polished templates have very smooth surface morphology, uniform luminescence, and higher output power. Fluorescence images of 445 nm LEDs grown on (a) as-grown and (b) polished (112―2) GaN templates.

Heteroepitaxial growth of AlN buffer layers directly on (101¯0) sapphire substrates by metalorganic vapour phase epitaxy has been investigated. A single-step growth procedure without a sapphire nitridation was employed resulting in mirror-like crack free ≈ 1.1 – 1.6 μ m thick AlN layers of single phase (112¯2) orientation. Trimethylaluminum pre-dose time and reactor pressure were optimized for surface roughness and crystal quality. The crystal quality was found to degrade with increasing pre-dose time and also reactor pressure. The smallest full width at half maximum value for on-axis X-ray rocking curve of the (112¯2) AlN layers was about 610 arcsec and 1480 arcsec along [1¯1¯23]AlN and [11¯00]AlN, respectively. The surface roughness, measured by atomic force microscopy using a 10 × 10 μ m 2 area, was in the range 2.6–3.5 nm. A basal stacking fault density of (7±1)×105 cm−1 was estimated by transmission electron microscopy.

We report an inexpensive nanoscale patterning process for epitaxial lateral overgrowth (ELOG) in AlN layers grown by metal organic vapour phase epitaxy (MOVPE) on sapphire. The pattern was produced by an inductively coupled plasma etch using a self-assembled monolayer of silica spheres on AlN as the lithographic mask. The resulting uniform 1 [small mu ]m length rod structure across a wafer showed a massive reduction in threading dislocations (TDs) when annealed at 1100 [degree]C. Overgrowing homoepitaxial AlN on top of the nanorods, at a temperature of 1100 [degree]C, produced a crack free coalesced film with approximately 4 [small mu ]m of growth, which is formed at a much lower temperature compared to that typically required for microscale ELOG. The improved crystal quality, in terms of TD reduction, of the AlN above the rods was determined by detailed weak beam (WB) electron microscopy studies and showed that the threading dislocation density (TDD) was greatly reduced, by approximately two orders of magnitude in the case for edge-type dislocations. In situ reflectance measurements during the overgrowth allowed for thickness coalescence to be estimated along with wafer curvature changes. The in situ measurements also confirmed that tensile strain built up at a much slower rate in the ELOG AlN layer compared to that of AlN prepared directly on sapphire.

In this study, aluminium oxide films were deposited on BK7 glass substrates using radio frequency mag- netron sputtering. The purposes of this study are to clarify the influence of O2 flow as reactive partial gas, which is necessary to form Al2O3 films, and then the influence of substrate temperature on structure and rigidity of coat- ings towards water injection. The fabricated metal oxide films were characterized using techniques such as atomic force microscopy (AFM), X-ray diffraction (XRD), spectrophotometry, ellipsometry and Rutherford backscatter- ing (RBS) analysis. Modifications of the partial gas percentage influences the optical properties and composition of the deposited aluminium oxide, the best samples being those deposited with 5% and 8% oxygen. The substrate temperature affects the structure and crystallization of the films. Nanocrystalline γ -Al2O3 has been observed at temperatures above 300 ◦ C with the grain size of 25 nm. After water injection, there was a large diversity in the surface roughness of samples with different substrate temperature. Experiments have shown that the best resistance against water injection occurs for the sample deposited at 350 ◦ C with 5% partial gas. We conclude that the rigidity of nanocrystalline γ -Al2O3 coatings can be explained by both Hall-Petch and Coble creep mechanism. In this case, there is an optimum grain size of around 42 nm against water spray.

Heterogeneous multilayered oxide channel materials have enabled low temperature, high mobility thin film transistor technology by solution processing. The authors report the growth and characterization of solution-based, highly uniform and c-axis orientated zinc oxide (ZnO) single and multilayered thin films. Quasisuperlattice (QSL) metal oxide thin films are deposited by spin-coating and the structural, morphological, optical, electronic, and crystallographic properties are investigated. In this work, the authors show that uniform, coherent multilayers of ZnO can be produced from liquid precursors using an iterative coating-drying technique that shows epitaxial-like growth on SiO2, at a maximum temperature of 300 °C in air. As QSL films are grown with a greater number of constituent layers, the crystal growth direction changes from m-plane to c-plane, confirmed by x-ray and electron diffraction. The film surface is smooth for all QSLs with root mean square roughness

The structural and luminescent properties of InxAl1-xN/Al0.53Ga0.47N multiple quantum wells (MQW) grown on an Al0.5Ga0.5N buffer partially relaxed with respect to an underlying AlN-template are reported. A significant redshift and improvement of ultraviolet (UV) photoluminescence (PL) intensity is found for InAlN MQWs grown on AlGaN buffers with higher relaxation degree. This is attributed to a higher QW indium incorporation as confirmed also by X-ray diffraction (XRD). The nature of room temperature time resolved PL is studied and discussed from the point of view of the possibility of a type I-type II band lineup transition in the InAlN-AlGaN system.

Producción Científica, In this paper we study the optical properties of nanoporous gallium nitride (GaN) epitaxial layers grown by chemical vapour deposition on non-porous GaN substrates, using photoluminescence, cathodoluminescence, and resonant Raman scattering, and correlate them with the structural characteristic of these films. We pay special attention to the analysis of the residual strain of the layers and the influence of the porosity in the light extraction. The nanoporous GaN epitaxial layers are under tensile strain, although the strain is progressively reduced as the deposition time and the thickness of the porous layer increases, becoming nearly strain free for a thickness of 1.7 μm. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain., Ministerio de Economía, Industria y Competitividad (Projects No. TEC2014-55948-R and MAT2016-75716-C2-1-R (AEI/FEDER, UE), Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Projects VA293U13 and VA081U16), Comisión Interministerial de Ciencia y Tecnología (Proyect CICYT MAT2010-20441-C02), Ministerio de Economía, Industria y Competitividad (Projects No.ENE2014-56069-C4-4-R)

Design considerations for vertical Gallium Nitride (GaN) nanowire Schottky barrier diodes (NWSBDs) for high voltage applications is discussed in this paper. Preliminary quasi-vertical NWSBDs fabricated on a Sapphire substrate show rectifying properties with breakdown voltage of 100 V. The principle of dielectric Reduced SURface Field (RESURF) which is naturally compatible with the NW structure, is utilized to block high voltages (> 600 V) within the fabrication constraints of nano-pillar height and drift doping concentration. Design considerations for the NWSBD is explored through 3D TCAD simulations. TCAD simulations show the NWSBDs can block voltages upward of 700 V with very low on-resistance with optimal design. The measured and simulated results are compared with state of the art GaN devices to provide an understanding of the true potential of the GaN NW architecture as power devices offering high breakdown voltages and low on-state resistance and a reliable device operation, all on a vertical architecture and a non-native substrate.

A new concept of vertical gallium nitride (GaN) Schottky barrier diode based on nanowire (NW) structures and the principle of dielectric REduced SURface Field (RESURF) is proposed in this paper. High-threading dislocation density in GaN epitaxy grown on foreign substrates has hindered the development and commercialization of vertical GaN power devices. The proposed NW structure, previously explored for LEDs offers an opportunity to reduce defect density and fabricate low cost vertical GaN power devices on silicon (Si) substrates. In this paper, we investigate the static characteristics of high-voltage GaN NW Schottky diodes using 3-D TCAD device simulation. The NW architecture theoretically achieves blocking voltages upward of 700 V with very low specific on-resistance. Two different methods of device fabrication are discussed. Preliminary experimental results are reported on device samples fabricated using one of the proposed methods. The fabricated Schottky diodes exhibit a breakdown voltage of around 100 V and no signs of current collapse. Although more work is needed to further explore the nano-GaN concept, the preliminary results indicate that superior tradeoff between the breakdown voltage and specific on-resistance can be achieved, all on a vertical architecture and a foreign substrate. The proposed NW approach has the potential to deliver low cost reliable GaN power devices, circumventing the limitations of today's high electron mobility transistors (HEMTs) technology and vertical GaN on GaN devices.

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer. (ET) efficiencies of up to similar to 25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d-4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Frster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to similar to 21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

InAlN/AlGaN multiple quantum wells (MQWs) emitting between 300 and 350 nm have been prepared by metalorganic chemical vapor deposition on planar AlN templates. To obtain strong room temperature luminescence from InAlN QWs a two temperature approach was required. The intensity decayed weakly as the temperature was increased to 300 K, with ratios I PL (300 K)/ I PL ( T ) max up to 70%. This high apparent internal quantum efficiency is attributed to the exceptionally strong carrier localization in this material, which is also manifested by a high Stokes shift (0.52 eV) of the luminescence. Based on these results InAlN is proposed as a robust alternative to AlGaN for ultraviolet emitting devices.

With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III–N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be “self-healed”. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.

In this review paper we will report the current state of research regarding the doping of III-nitride materials and their alloys. GaN is a mature material with both n -type and p -type doping relatively well understood, and while n -GaN is easily achieved, p -type doping requires much more care. There are significant efforts to extend the composition range that can be controllably doped for AlGaInN alloys. This would allow application in shorter and longer wavelength optoelectronics as well as extending power electronic devices. It is found that doping of AlGaN and InGaN alloys with low-gallium-content has particular challenges, especially for p -materials and these issues are described.

In this paper we describe the scanning electron microscopy techniques of electron backscatter diffraction, electron channeling contrast imaging, wavelength dispersive X-ray spectroscopy, and cathodoluminescence hyperspectral imaging. We present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping, and light emission from a range of UV-emitting nitride semiconductor structures. We aim to illustrate the developing capability of each of these techniques for understanding the properties of UV-emitting nitride semiconductors, and the benefits were appropriate, in combining the techniques.

Fully functional InAlN-based ultraviolet LEDs emitting at 340–350 nm were demonstrated for the first time; detailed electrical and optical characterization is presented and discussed. Results from the measurements at pulsed conditions are in agreement with the attribution of the dominant electroluminescence peak to near-band-edge emission. The composition of the AlGaN barriers was chosen to give the same internal polarization field as that of the InAlN wells. A simulation study of this polarization-matched heterostructure shows a significant increase in the electron-hole overlap integral if compared with a standard AlGaN/AlGaN active region having the same level of carrier confinement. Limitations and problems of these preliminary devices are also presented, and possible future work aimed at increasing their efficiency is discussed.

A range of seven different Ag plasmonic arrays formed using nanostructures of varying shape, size and gap were fabricated using helium-ion lithography (HIL) on an InGaN/GaN quantum well (QW) substrate. The influence of the array geometry on plasmon-enhanced F\"orster resonance energy transfer (FRET) from a single InGaN QW to a ~ 80 nm layer of CdSe/ZnS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix is investigated. It is shown that the energy transfer efficiency is strongly dependent on the array properties and an efficiency of ~ 51% is observed for a nanoring array. There were no signatures of FRET in the absence of the arrays. The QD acceptor layer emission is highly sensitive to the array geometry. A model was developed to confirm that the increase in the QD emission on the QW substrate compared with a GaN substrate can be attributed solely to plasmon-enhanced FRET. The individual contributions of direct enhancement of the QD layer emission by the array and the plasmon-enhanced FRET are separated out, with the QD emission described by the product of an array emission factor and an energy transfer factor. It is shown that while the nanoring geometry results in an energy transfer factor of ~ 1.7 the competing quenching by the array, with an array emission factor of ~ 0.7, results in only an overall gain of ~ 14% in the QD emission. The QD emission was enhanced by ~ 71% for a nanobox array, resulting from the combination of a more modest energy transfer factor of 1.2 coupled with an array emission factor of ~ 1.4., Comment: 19 pages, 5 figures