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Telecommunication of the next generation demands filters that can operate in the 10 GHz range with sufficient bandwidths. For surface-acoustic-wave (SAW) devices this prerequisite translates into high sound velocities and high piezoelectric couplings. Wurtzite AlN on diamond, which exploits the strong piezoelectricity of AlN with the very high SAW velocity of diamond, has been considered a promising platform. A significant boost (up to a factor of 4) of the piezoelectric response can be obtained by alloying AlN with Sc. Here, the main challenge lies in the synthesis of highly-oriented thin (Sc,Al)N films on diamond. In this work, we aim at establishing a platform for SAW devices using plasma-assisted molecular beam epitaxy for the deposition of Sc0.2Al0.8N on diamond. We investigate the structural properties related to SAW generation gearing towards applications at high frequencies. To this end, we prepare (Sc,Al)N thin films on polished polycrystalline diamond wafers and demonstrate the efficient generation of SAW modes with frequencies up to 8 GHz. Systematic studies of the dependence of the SAW velocity and electromechanical coupling coefficient on the Sc0.2Al0.8N film thickness is presented for various SAW modes. Our result demonstrates the potential of this material combination for future application that requires large bandwidth in the ultra-high frequency range. [ABSTRACT FROM AUTHOR]

Accurate knowledge of optical constants (refractive index n and extinction coefficient k) of ScN is crucial for understanding optical properties of this binary nitride semiconductor as well as for its use in optoelectronic applications. Using spectroscopic ellipsometry in a spectral range from far infrared to far ultraviolet (0.045–8.5 eV), we determine n and k of ScN layers grown on Al2O3(0001) substrates by plasma-assisted molecular beam epitaxy. Fits of ellipsometry data return the energies of four oscillators representing critical points in the band structure of ScN, namely, 2.03, 3.89, 5.33, and 6.95 eV. As the infrared range is dominated by free carriers, the vibrational properties of the layers are examined by Raman spectroscopy. Despite the rock salt structure of ScN, several first-order phonon modes are observed, suggesting a high density of point defects consistent with the high electron density deduced from Hall measurements. Finally, photoluminescence measurements reveal an emission band slightly above the lowest direct bandgap. We attribute the redshift of the peak emission energy from 2.3 to 2.2 eV with increasing layer thickness to a reduction in the O concentration in the layers. [ABSTRACT FROM AUTHOR]

An accurate knowledge of lattice parameters of ScxAl1−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 ScxAl1−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 x ≈ 0.09 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 ScxAl1−xN/GaN heterointerface, with the highest sheet electron density and mobility observed for lattice-matched conditions. [ABSTRACT FROM AUTHOR]

We compare the optical properties and device performance of unpackaged InGaN/GaN multiplequantum-well light-emitting diodes (LEDs) emitting at ~430 nm grown simultaneously on a high-cost small-size bulk semipolar (112 2) GaN substrate (Bulk-GaN) and a low-cost large-size (1122) GaN template created on patterned (1012) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ~105 cm-2-106cm-2 and basal-plane stacking fault (BSF) density of 0 cm-1, while the PSS-GaN substrate has the TDD of ~2 x 108 cm-2 and BSF density of ~1 x 10³ cm-1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density. [ABSTRACT FROM AUTHOR]

The exciton localization in semipolar (11⁻22) 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. [ABSTRACT FROM AUTHOR]

InGaN layers were grown simultaneously on ... GaN and (0001) GaN templates by metalorganic vapour phase epitaxy. At higher growth temperature (≥750 °C), the indium content (<15%) of the ... and (0001) InGaN layers was similar. However, for temperatures less than 750 C, the indium content of the ... 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 ... and (0001) InGaN layers. Room temperature photoluminescence measurements of the ... 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 ... InGaN layers with an indium content of more than 10% blue-shifted a constant value of ≈(50-60) nm when using higher excitation power densities. This blue-shift was attributed to band filling effects in the layers. [ABSTRACT FROM AUTHOR]

Highlights • (10 1 ¯ 0) m -plane AlGaN grown on m-plane sapphire over the entire range of composition. • The structural, morphological and optical properties were investigated. • The AlN mole fraction for m -plane layers is comparable with c -plane co-loaded layers. • The crystal-field splitting energy has a bowing parameter of about −0.15 eV. Abstract 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. [ABSTRACT FROM AUTHOR]

Highlights • Ga-polar untwinned (1 0 −1 3) GaN was obtained by directional sputtering. • FWHM values are less than 550 aresec. • Al nucleation layer in directional sputtering affects the GaN layer much. Abstract Directional sputtering of Al and AlN on (1 0 −1 0) sapphire was used to obtain metal-polar (1 0 −1 3) templates. After overgrowth with AlN and GaN using metal-organic vapor phase epitaxy, we obtained untwinned (1 0 −1 3) GaN layers. Full width at half maximum of the X-ray rocking curve of symmetric (1 0 −1 3) GaN is less than 550 arcsec along both [3 0 −3 −2] GaN and [1 −2 1 0] GaN directions. Ga-polarity of the layers was confirmed by high-resolution scanning transmission electron microscopy. Careful optimization of time and temperature of the initial Al sputtered layer was a key parameter to achieve high quality GaN templates. [ABSTRACT FROM AUTHOR]

Highlights • MOVPE-growth and high-temperature annealing of m -plane AlN on m -plane sapphire. • (10 1 ¯ 0) XRC FWHM along [0001]/[11 2 ¯ 0] decreased from 2900/1940 to 1030/800 arcsec. • The density of basal stacking faults decreased from ~ 2.8 × 10 5 to ~ 1. 5 × 10 4 cm−1. • Larger effective bandgap after annealing (6.15 eV) than as-grown layers (6.04 eV). Abstract 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 × 105 to ∼ 1.5 × 104 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. [ABSTRACT FROM AUTHOR]

Highlights • Single phase (10 1 ¯ 1), (10 1 ¯ 2) and (20 2 ¯ 3) GaN grown on 4°-miscut 3C-SiC/Si. • Different Al x Ga 1−x N interlayers used to control GaN surface orientations. • Substrate miscut caused twins in (10 1 ¯ 1) GaN. • (10 1 ¯ 2) and (20 2 ¯ 3) GaN only grown along the miscut direction. Abstract 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. [ABSTRACT FROM AUTHOR]

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 × 10 5 to ∼ 5 × 10 3  cm −1 . The annealed layers had a larger optical bandgap energy than the as-sputtered layers due to their better crystallinity and structural order. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on metalorganic vapor phase epitaxial growth of InAlN layers on (0001) and (11 [ABSTRACT FROM AUTHOR]

InGaN layers were grown simultaneously on polar (0001), and on semipolar (11 [ABSTRACT FROM AUTHOR]

Abstractauthoren InGaN multiple quantum well light-emitting diodes (LEDs) were grown on chemically-mechanically polished (11 [ABSTRACT FROM AUTHOR]

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)×10 5 cm −1 was estimated by transmission electron microscopy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Gallium nitride (GaN) nanowires, grown on gold-coated n-type Si(100) substrates, were synthesized using the vapor-phase epitaxy method. The grown GaN nanowires, with diameters in the range 20–60nm 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(100) are of good crystalline quality. The electron field emission properties of the GaN nanowires (at room temperature) showed a low turn-on field of ∼3.96V/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. [Copyright &y& Elsevier]

We report on metal-organic vapor phase epitaxial growth of ([formula]) InN on ([formula]) GaN templates on m-plane ([formula]) sapphire substrates. The in-plane relationship of the ([formula]) InN samples is [[formula]]InN||[0001]sapphire and [[formula]]InN||[[formula]]sapphire, replicating the in-plane relationship of the ([formula]) GaN templates. The surface of the ([formula]) InN samples and the ([formula]) GaN templates shows an undulation along [[formula]]InN,GaN, which is attributed to anisotropic diffusion of indium/gallium atoms on the ([formula]) surfaces. The growth rate of the ([formula]) InN layers was 3-4 times lower compared to c-plane (0001) InN. High resolution transmission electron microscopy showed a relaxed interface between the ([formula]) InN layers and the ([formula]) GaN templates, consistent with x-ray diffraction results. Basal plane stacking faults were found in the ([formula]) GaN templates but they were terminated at the InN/([formula]) GaN interface due to the presence of misfit dislocations along the entire InN/GaN interface. The misfit dislocations were contributed to the fully relaxation and the tilts of the ([formula]) InN layers. X-ray photoelectron spectroscopy was used to determine the polarity of the grown ([formula]) InN sample, indicating an In-polar ([formula]) InN. The valence band maximum was determined to be at (1.7 ± 0.1) eV for the ([formula]) InN sample, comparable to In-polar c-plane InN. [ABSTRACT FROM AUTHOR]

Growth of AlxGa1−xN layers (0 ≤ x ≤ 1) simultaneously on polar (0001), semipolar (10 1 ¯ 3) and (11 2 ¯ 2 ), as well as nonpolar (10 1 ¯ 0 ) and (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. [ABSTRACT FROM AUTHOR]

[formula] GaN layers were grown by metal-organic vapor phase epitaxy on [formula] bulk GaN substrates and [formula] 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 [formula] and one period along [formula]. 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 [formula] and [formula]. [ABSTRACT FROM AUTHOR]

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 IPL(300 K)/IPL(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. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis of hexagonal wurzite one-dimensional (1D) GaN nanostructures on c-Al2O3 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. [Copyright &y& Elsevier]

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 AlxGa1-xN alloys are extensively compared. This study has been designed to determine how the different AlxGa1-xN 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 AlxGa1-xN 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]