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An accurate knowledge of lattice parameters of Sc xAl1− xN is essential for understanding the elastic and piezoelectric properties of this compound as well as for the ability to engineer its strain state in heterostructures. Using high-resolution x-ray diffractometry, we determine the lattice parameters of 100-nm-thick undoped Sc xAl1− xN layers grown on GaN(0001) templates by plasma-assisted molecular beam epitaxy. The Sc content x of the layers is measured independently by both x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy, and it ranges from 0 to 0.25. The in-plane lattice parameter of the layers linearly increases with increasing x, while their out-of-plane lattice parameter remains constant. Layers with [Formula: see text] are found to be lattice matched to GaN, resulting in a smooth surface and a structural perfection equivalent to that of the GaN underlayer. In addition, a two-dimensional electron gas is induced at the Sc xAl1− xN/GaN heterointerface, with the highest sheet electron density and mobility observed for lattice-matched conditions.

SummaryBackgroundNanocovax is a recombinant severe acute respiratory syndrome coronavirus 2 subunit vaccine composed of full-length prefusion stabilized recombinant SARS-CoV-2 spike glycoproteins (S-2P) and aluminum hydroxide adjuvant. In a Phase 1 and 2 studies, (NCT04683484) the vaccine was found to be safe and induce a robust immune response in healthy adult participants.MethodsWe conducted a multicenter, randomized, double-blind, placebo-controlled study to evaluate the safety, immunogenicity, and protective efficacy of the Nanocovax vaccine against Covid-19 in approximately 13,007 volunteers aged 18 years and over. The immunogenicity was assessed based on Anti-S IgG antibody response, surrogate virus neutralization, wild-type SARS-CoV-2 neutralization and the types of helper T-cell response by intracellular staining (ICS) for interferon gamma (IFNg) and interleukin-4 (IL-4). The vaccine efficacy (VE) was calculated basing on serologically confirmed cases of Covid-19.FindingsUp to day 180, incidences of solicited and unsolicited adverse events (AE) were similar between vaccine and placebo groups. 100 serious adverse events (SAE) were observed in both vaccine and placebo groups (out of total 13007 participants). 96 out of these 100 SAEs were determined to be unrelated to the investigational products. 4 SAEs were possibly related, as determined by the Data and Safety Monitoring Board (DSMB) and investigators. Reactogenicity was absent or mild in the majority of participants and of short duration. These findings highlight the excellent safety profile of Nanocovax.Regarding immunogenicity, Nanocovax induced robust IgG and neutralizing antibody responses. Importantly, Anti S-IgG levels and neutralizing antibody titers on day 42 were higher than those of natural infected cases. Nanocovax was found to induce Th2 polarization rather than Th1.Post-hoc analysis showed that the VE against symptomatic disease was 51.5% (95% confidence interval [CI] was [34.4%-64.1%]. VE against severe illness and death were 93.3% [62.2-98.1]. Notably, the dominant strain during the period of this study was Delta variant.InterpretationNanocovax 25 microgram (mcg) was found to be safe with the efficacy against symptomatic infection of Delta variant of 51.5%.FundingResearch was funded by Nanogen Pharmaceutical Biotechnology JSC., and the Ministry of Science and Technology of Vietnam; ClinicalTrials.gov number, NCT04922788.

The impacts of pulsed-flow growth on aluminium incorporation in polar (0001), semipolar (103) and (112), as well as nonpolar (100) AlGaN layers have been investigated. The layers were grown simultaneously on differently oriented AlN/sapphire templates by metal–organic vapour phase epitaxy. The AlN mole fraction (0 < xAlN ≤ 0.85) of the layers was varied by simply changing the supply time of the aluminium precursor while keeping nitrogen and gallium precursors constant. Phase separation has been observed for the (0001) and (112) layers by X-ray diffraction, and is attributed to their different surface reconstructions during growth. In contrast, no phase separation has been observed for the (100) and (103) layers, attributed to their stable surfaces during growth. The AlN mole fraction of the differently oriented layers generally follows the order: (112) < (0001) < (103) ≤ (100), attributed to their different surface dangling-bond densities. By means of room-temperature photoluminescence measurements, high carbon-incorporation has quantitatively been found in all the layers.

BackgroundNanocovax is a recombinant severe acute respiratory syndrome coronavirus 2 subunit vaccine composed of full-length prefusion stabilized recombinant SARS-CoV-2 spike glycoproteins (S-2P) and aluminum hydroxide adjuvant.MethodsWe conducted a dose-escalation, open label trial (phase 1) and a randomized, double-blind, placebo-controlled trial (phase 2) to evaluate the safety and immunogenicity of the Nanocovax vaccine (in 25 microgram (mcg), 50 mcg, and 75 mcg doses, aluminum hydroxide adjuvanted). In phase 1, 60 participants received two intramuscular injection of the vaccine following dose-escalation procedure. The primary outcomes were reactogenicity and laboratory tests to evaluate the vaccine safety. In phase 2 which involved in 560 healthy adults, the primary outcomes are vaccine safety; and anti-S IgG antibody response. Secondary outcomes were surrogate virus neutralization, wild-type SARS-CoV-2 neutralization, and T-cell responses by intracellular staining (ICS) for interferon gamma (IFNg). We performed primary analyses at day 35 and day 42.ResultsFor phase 1 study, no serious adverse events (SAE) were observed for all 60 participants. Most adverse events (AE) were grade 1 and disappeared shortly after injection. For phase 2 study, after randomization, 480 participants were assigned to receive the vaccine with adjuvant, and 80 participants were assigned to receive placebo. Reactogenicity was absent or mild in the majority of participants and of short duration (mean, ≤3 days). Unsolicited adverse events were mild in most participants. There were no serious adverse events related to Nanocovax. Regarding the immunogenicity, Nanocovax induced robust anti-S antibody responses. There was no statistical difference in antibody responses among dose strengths on Day 42, in terms of anti S-IgG level and neutralizing antibody titer.ConclusionsUp to 42 days, Nanocovax vaccine was safe, well tolerated and induced robust immune responses. We propose using Nanocovax 25 mcg for Phase 3 to evaluate the vaccine efficacy. (Research funded by Nanogen Pharmaceutical Biotechnology JSC., and the Ministry of Science and Technology of Vietnam;ClinicalTrials.govnumber,NCT04683484.)

Heteroepitaxial growth of single-phase nonpolar (10 1 ¯ 0) m -plane Al x Ga 1 - x N layers on m -plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different Al/Ga gas phase ratios were used to adjust the AlN mole fraction over the entire range of composition. All m -plane AlGaN layers show an orthorhombic distortion in the wurtzite unit cell due to anisotropic in-plane strain. This distortion decreases with increasing AlN mole fraction due to a decreased anisotropic biaxial strain. The AlN mole fraction of m -plane layers and c -plane co-loaded layers estimated by X-ray diffraction is comparable. This is consistent with their comparable energy bandgaps, which were estimated from room-temperature dielectric functions. The dependence of the energy bandgap on composition indicates a bowing parameter of about 0.9 eV. The crystal-field splitting energy is found for the m -plane layers with a bowing parameter b cf of about −0.15 eV.

Heteroepitaxial growth of AlN layers on (10 1 ¯ 0) m -plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different nucleation temperatures were used to achieve single phase (10 1 ¯ 0) AlN layers. The crystallinity of the layers further increased with increasing annealing temperature and annealing time. The full-width at half maximum of the symmetric (10 1 ¯ 0) X-ray rocking curves decreased from about 2900/1940 arcsec to 1030/800 arcsec along [0001]/[11 2 ¯ 0] AlN . The density of basal stacking faults of the annealed layers was found to decrease from ∼ 2.8 × 10 5 to ∼ 1.5 × 10 4 cm −1 . The annealed layers had a larger effective optical bandgap energy of ∼ 6.15 eV than ∼ 6.04 eV of the as-grown layers due to their better crystallinity and structural order.

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.

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.

Thermal annealing at high temperatures of nonpolar (1 0 1 ¯ 0) m-plane AlN layers directly sputtered on m-plane sapphire was investigated. The crystallinity of the layers increased with increasing annealing temperature. The full-width at half maximum of the symmetric (1 0 1 ¯ 0) X-ray rocking curves along [0 0 0 1]/[1 1 2 ¯ 0 ]AlN decreased from about 3.5/2.0 ° to 0.24/0.19 ° . The density of basal stacking faults of the annealed layers was found to decrease from ∼ 1 × 105 to ∼ 5 × 103 cm−1. The annealed layers had a larger optical bandgap energy than the as-sputtered layers due to their better crystallinity and structural order.

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.

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.

Growth of AlxGa1−xN layers (0 ≤ x ≤ 1) simultaneously on polar (0001), semipolar ($$10\bar{{\rm{1}}}$$ 10 1 ¯ 3) and ($$11\bar{{\rm{2}}}2$$ 11 2 ¯ 2 ), as well as nonpolar ($$10\bar{{\rm{1}}}0$$ 10 1 ¯ 0 ) and ($$11\bar{{\rm{2}}}0$$ 11 2 ¯ 0 ) AlN templates, which were grown on planar sapphire substrates, has been investigated by metal-organic vapour phase epitaxy. By taking into account anisotropic in-plane strain of semi- and non-polar layers, their aluminium incorporation has been determined by x-ray diffraction analysis. Optical emission energy of the layers was obtained from room-temperature photoluminescence spectra, and their effective bandgap energy was estimated from room-temperature pseudo-dielectric functions. Both x-ray diffraction and optical data consistently show that aluminium incorporation is comparable on the polar, semi- and non-polar planes.

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.

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.

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.

Indium incorporation and the optical properties of Inx Al1−x N layers (0 ≤ x ≤ 0.45) grown by metal-organic vapour phase epitaxy have been investigated simultaneously on polar (0001), untwinned semipolar (10overline 13) and nonpolar (10overline 10) AlN templates, which were prepared on planar sapphire substrates. The InN mole fraction XInN of the layers was tuned by changing growth temperature from 660°C to 860°C. XInN determined by x-ray diffraction was found to be comparable for the polar, semipolar and nonpolar surface orientations. This is consistent with comparable effective bandgap energy of the layers obtained from optical transmission measurements at room temperature. The bandgap bowing parameter was found to be strongly composition-dependent. Room-temperature photoluminescence measurements showed impurity transitions for the layers with XInN ≤ 0.2, while InAlN near-band-edge luminescence was observed for the layers with higher XInN., ファイル公開：2021-01-29

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.

InGaN layers were grown simultaneously on polar (0001), and on semipolar (112) and (201) GaN templates with different In/Ga ratios by metalorganic vapour phase epitaxy with a thickness of about 50 nm. The indium content of the layers behaved as follows: (0001) (112)(201). The relaxed (0001) layers showed the highest indium content (In 17.4–48.7%) with an island-like surface morphology, while the relaxed (112) layers (In 14.0–40.0%) had nearly unchanged morphology compared to the semipolar GaN templates. The (201) layers exhibited a three-dimensional morphology that was attributed to differences in growth on the (101) and (100) surfaces. The degree of relaxation also reduced from 100% for the (0001) layers to less than 15% for the (201) layers that was attributed to the lowest indium content of the (201) layers. Room-temperature photoluminescence (PL) measurements of the semipolar layers showed a peak in emission wavelength that shifts gradually from 450 to 820 nm with increasing indium content. The stronger PL intensity of the semipolar layers was attributed to background impurity and band filling effects in the 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.

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.

Comprehensive analysis of the surface and crystal properties has been performed at clean c-plane sapphire substrates, sapphire layers after nitridation, and subsequently grown InN layers deposited by metal–organic vapor phase epitaxy. The (1 × 1) surface of clean sapphire reconstructs into a ( 31 × 31 ) R ± 9° structure after annealing at 1050 °C, which was performed prior to the nitridation process. The formation of crystalline AlN was observed for nitridation above 800 °C. X-ray photoelectron spectroscopy performed on the nitridated layers shows that N Al chemical bonds dominate this structure, while the number of N O bonds is negligibly small. Amorphous AlNxOy layers form during nitridation below 800 °C, where N O bonds dominate. All layers formed by nitridation show defects associated with N bonds. The morphology of the nitridated layers affects the surface and crystal quality of the subsequently grown polar InN layers. N-polar InN layers with a smooth surface and single crystalline structure were grown on the AlN nitridated layers, while In-polar InN layers with a rough surface and a polycrystalline structure were grown on the amorphous nitridated layers.

We report on metal–organic vapor phase epitaxy (MOVPE) of (0001)InN layers simultaneously grown on a-plane (112¯0) and c-plane (0001) sapphire substrates. The substrates were nitridated at temperatures from 500 °C to 1050 °C prior to the growth of c-plane InN layers. Nitridation determined the polarity, the crystallinity, and the surface morphology of the InN layers. Nitridation temperatures above 800 °C lead to N-polar InN layers, while nitridation temperatures from 700 °C to 750 °C produce mixed-polar InN layers, and nitridation temperatures from 500 °C to 650 °C produce In-polar InN layers. The roughness and crystallinity of the InN layers are correlated with the changes of polarity. The incorporation of nitrogen into the nitridation layers at different nitridation temperatures was measured. A strong N–Al bond signal after nitridation is correlated with N-polarity layers after overgrowth.

We report on blue and green light-emitting-diodes (LEDs) grown on (11-22)-GaN templates. The templates were created by overgrowth on structured r-plane sapphire substrates. Low defect density, 100 mm diameter GaN templates were obtained by metal organic vapour phase epitaxy (VPE) and hydride VPE techniques. Chemical-mechanical polishing was used to obtain smooth surfaces for the subsequent growth of LED structures. Ohmic contacts to the p-type GaN were obtained despite the lower activated acceptor levels. The LEDs show excellent output power and fast carrier dynamics. Freestanding LEDs have been obtained by use of laser-lift-off. The work is the result of collaboration under the European Union funded ALIGHT project.

Conventional c-plane LEDs have limited electro-optic bandwidths especially in the green spectral region. By forming the LED on the (11-22) plane we demonstrate a bandwidth of 500 MHz at 490 nm. The small signal response shows resonant features which are believed to be associated with the strong carrier localization observed in these LEDs. Thus, LEDs on semipolar planes are promising for application in visible light communications.

We compare InN layers grown directly on c -plane (0001) sapphire and on (0001) GaN templates using metal-organic vapor phase epitaxy. InN grown on nitridated c -plane sapphire showed N-polarity, while InN grown on c -plane GaN templates showed In-polarity. N-polar and In-polar InN layers showed different surface morphology and crystallinity. N-polar InN was smoothest when grown at higher growth temperatures, while In-polar InN was smoothest at intermediate growth temperatures. Growth mode of the N-polar InN on sapphire is Frank-Van der Merwe, while growth mode of the In-polar InN on GaN templates is Volmer-Weber. Electrical properties were similar for both layers. Lowtemperature photoluminescence of In-polar InN layers is shifted to higher energies most likely due to biaxial compressive stresses. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A gallium nitride (GaN) epitaxial layer was grown by metal-organic chemical vapor deposition (MOCVD) on Si (111) substrates with aluminum nitride (AlN) buffer layers at various thicknesses. The AlN buffer layers were deposited by two methods: radio frequency (RF) magnetron sputtering and MOCVD. The effect of the AlN deposition method and layer thickness on the morphological, structural and optical properties of the GaN layers was investigated. Field emission scanning electron microscopy showed that GaN did not coalesce on the sputtered AlN buffer layer. On the other hand, it coalesced with a single domain on the MOCVD-grown AIN buffer layer. Structural and optical analyses indicated that GaN on the MOCVD-grown AlN buffer layer had fewer defects and a better aligned lattice to the a- and c-axes than GaN on the sputtered AlN buffer layer.

The synthesis of hexagonal wurzite one-dimensional (1D) GaN nanostructures on c-Al"2O"3 substrates was investigated using a thermal chemical vapor deposition (CVD) process. The diameter of the GaN nanostructures was controlled by varying the growth time using a mixture of GaN powder and Ga metal with the ammonia gas reaction. The morphologies of the GaN nanowires and nanorods were confirmed by field emission scanning electron microscopy. The micro-Raman spectroscopy and X-ray scattering measurements indicated that the GaN nanostructures had a hexagonal wurzite structure without any oxide phases. We investigated the difference in the structural properties between the GaN nanowires and nanorods. Deep-level emission bands were not observed in cathodoluminescence measurements from either the GaN nanowires or nanorods, indicating the incorporation of low-level impurities into our 1D GaN nanostructures.

Gallium nitride (GaN) nanowires, grown on gold-coated n-type Si(1 0 0) substrates, were synthesized using the vapor-phase epitaxy method. The grown GaN nanowires, with diameters in the range 20–60 nm and lengths of several micrometers, were uniformly distributed on Si substrates. The GaN nanowires were grown via a vapor–liquid–solid mechanism. The characteristics of the grown GaN were investigated using X-ray diffraction, micro-Raman, and transmission electron microscopy, which found that the GaN nanowires on the Si(1 0 0) are of good crystalline quality. The electron field emission properties of the GaN nanowires (at room temperature) showed a low turn-on field of ∼3.96 V/μm and a field enhancement factor of ∼1050. The sharp ends and rough surfaces of the GaN nanowires are responsible for their good field emission properties.

Vertically well-aligned ZnO nanowires were synthesized on c -Al 2 O 3 substrates at 950 °C by the vapor-phase epitaxy (VPE) method. An Au thin film with a thickness of 3 nm was used as a catalyst. The growth direction and length of the ZnO nanowires were successfully controlled by adjusting the ramping rate. Tilted nanowires with a shorter length were grown by increasing the ramping rate, while vertically well-aligned nanowires with a longer length were uniformly formed by decreasing the ramping rate. The X-ray diffraction (XRD) and high-resolution transmission electron microscope measurements showed that the vertically well-aligned ZnO nanowires on the c -Al 2 O 3 substrate have a single-crystalline hexagonal structure and preferred c -axis growth orientation.

InGaN layers were grown simultaneously on (11 (2) over bar2) GaN and (0001) GaN templates by metalorganic vapour phase epitaxy. At higher growth temperature (>= 750 degrees C), the indium content (2) and (0001) InGaN layers was similar. However, for temperatures less than 750 degrees C, the indium content of the (11 (2) over bar2) InGaN layers (15%-26%) were generally lower than those with (0001) orientation (15%-32%). The compositional deviation was attributed to the different strain relaxations between the (11 (2) over bar2) and (0001) InGaN layers. Room temperature photoluminescence measurements of the (11 (2) over bar2) InGaN layers showed an emission wavelength that shifts gradually from 380 nm to 580 nm with decreasing growth temperature (or increasing indium composition). The peak emission wavelength of the (11 (2) over bar2) InGaN layers with an indium content of more than 10% blue-shifted a constant value of approximate to(50-60) nm when using higher excitation power densities. This blue-shift was attributed to band filling effects in the layers. (C) 2014 AIP Publishing LLC.

We report on the electrical-to-optical modulation bandwidths of non-mesa-etched semipolar (112¯2) InGaN/GaN light-emitting diodes (LEDs) operating at 430–450 nm grown on high-quality (112¯2) GaN templates, which were prepared on patterned (101¯2) 𝑟-plane sapphire substrates. The measured frequency response at −3 dB of the LEDs was up to 1 GHz. A high back-to-back data transmission rate of above 2.4 Gbps is demonstrated using a non-return-to-zero on-off keying modulation scheme. This indicates that (112¯2) LEDs are suitable gigabit per second data transmission for use in visible-light communication applications.

We report on the growth of semi-polar InN directly on m -plane sapphires by metal-organic vapor phase epitaxy with different nitridation times from 45 s to 6 min. Surface morphology and crystalline orientation depended on the nitridation time. Semipolar InN with (10) dominant orientation was obtained at 2 min and 4 min nitridation, shorter or longer nitridation times showed additional orientations. The (10) InN has a roughness about 30 nm (scale size 5 × 5 µm2), likely due to twinning. Low temperature photoluminescence showed emission peaks between 0.72 eV to 0.75 eV, which is slightly below the energy observed for (0001) InN. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The exciton localization in semipolar (112⎯⎯2112¯2) InxGa1−xN (0.13 ≤ x ≤ 0.35) multiple-quantum-well (MQW) structures has been studied by excitation power density and temperature dependent photoluminescence. A strong exciton localization was found in the samples with a linear dependence with In-content and emission energy, consistent with the Stokes-shift values. This strong localization was found to cause a blue-shift of the MQW exciton emission energy at temperature above 100 K, which was found to linearly increase with increasing In-content.

80–120 nm thick In x Al1−x N epitaxial layers with 0 < x < 0.224 were grown by metalorganic vapour phase epitaxy on AlN/Al2O3-templates. The composition was varied through control of the growth temperature. The composition dependence of the band gap was estimated from the photoluminescence excitation absorption edge for 0 < x < 0.11 as the material with higher In content showed no luminescence under low excitation. A very rapid decrease in band gap was observed in this range, dropping down below 5.2 eV at x = 0.05, confirming previous theoretical work that used a band-anticrossing model to describe the strongly x-dependent bowing parameter, which in this case exceeds 25 eV in the x → 0 limit. A double absorption edge observed for InAlN with x < 0.01 was attributed to crystal-field splitting of the highest valence band states. Our results indicate also that the ordering of the valence bands is changed at much lower In contents than one would expect from linear interpolation of the valence band parameters. These findings on band gap bowing and valence band ordering are of direct relevance for the design of InAlN-containing optoelectronic devices.

Heteroepitaxial growth of GaN buffer layers on 3C–SiC/(001) Si templates (4°-offcut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown Al0.5Ga0.5N/AlN interlayers were employed to produce a single (203) GaN surface orientation. Specular crack-free GaN layers showed undulations along [10] with a root mean square roughness of about 13.5 nm (50 × 50 μm2). The orientation relationship determined by x-ray diffraction (XRD) was found to be [20]GaN ∥[10] and [034]GaN ∥[110]3C − SiC/Si . Low-temperature photoluminescence (PL) and XRD measurements showed the presence of basal-plane stacking faults in the layers. PL measurements of (203) multiple-quantum-well and light-emitting diode structures showed uniform luminescence at about 500 nm emission wavelength. A small peak shift of about 3 nm was observed in the electroluminescence when the current was increased from 5 to 50 mA (25–250 A cm−2).

We demonstrate that the polarity of polar (0001), (0001¯) and semipolar (112¯2) InN and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar (112¯2) InN and GaN layers can be determined by recording valence band ph...

We report on the evolution of N-polar wurtzite (0001̄) InGaN layers with high indium content grown directly on nitridated (0001) sapphire by metal–organic vapor phase epitaxy. In-situ ellipsometry showed an initial growth delay of 5–8 nm for layers with more than 45% indium content. Atomic force microscopy showed small dense islands which reached their final size after 5–8 nm deposition. The roughness saturated only after 15 nm, and the width of the X-ray diffraction (00.2) reflex showed a similar behavior. Therefore, N-polar InGaN grows via nucleation (d < 10 nm) followed by coalescence (d < 20 nm). With less than 40% indium the coalescence is much slower, very similar to Ga-polar (0001) GaN despite N-polarity of the InN layers. This indicates towards a surface transition between 40 and 50% indium content.

We report on metal-organic vapor phase epitaxial growth of (112¯2) InN on (112¯2) GaN templates on m-plane (101¯0) sapphire substrates. The in-plane relationship of the (112¯2) InN samples is [1¯1¯23]InN||[0001]sapphire and [11¯00]InN||[12¯10]sapphire, replicating the in-plane relationship of the (112¯2) GaN templates. The surface of the (112¯2) InN samples and the (112¯2) GaN templates shows an undulation along [11¯00]InN,GaN, which is attributed to anisotropic diffusion of indium/gallium atoms on the (112¯2) surfaces. The growth rate of the (112¯2) InN layers was 3-4 times lower compared to c-plane (0001) InN. High resolution transmission electron microscopy showed a relaxed interface between the (112¯2) InN layers and the (112¯2) GaN templates, consistent with x-ray diffraction results. Basal plane stacking faults were found in the (112¯2) GaN templates but they were terminated at the InN/(112¯2) GaN interface due to the presence of misfit dislocations along the entire InN/GaN interface. The misfit dis...

(112¯2) GaN layers were grown by metal-organic vapor phase epitaxy on (112¯2) bulk GaN substrates and (101¯0) sapphire substrates. The growth temperature was varied between 950 and 1050 °C and the total reactor pressure between 50 and 600 mbar. The growth conditions show a strong impact on the yellow band luminescence properties, while weak impact on the threading dislocation density was observed. The layer morphologies exhibit undulations with two periods along GaN [11¯00] and one period along [112¯3¯]. The different period lengths are connected to anisotropic adatom surface diffusion lengths. Arrow like features on the surface originate from the interference of the undulations along [112¯3¯] and [11¯00].