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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]

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]

The impacts of pulsed-flow growth on aluminium incorporation in polar (0001), semipolar (101¯3) and (112¯2), as well as nonpolar (101¯0) 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¯2) 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 (101¯0) and (101¯3) layers, attributed to their stable surfaces during growth. The AlN mole fraction of the differently oriented layers generally follows the order: (112¯2) < (0001) < (101¯3) ≤ (101¯0), 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. [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]

[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]

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]

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]

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. [ABSTRACT FROM AUTHOR]

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.