Cathodoluminescence and TEM investigations of structural and optical properties of AlGaN on epitaxial laterally overgrown AlN/sapphire templates

Surface steps as high as 15 nm on up to 10 μm thick AlN layers grown on patterned AlN/sapphire templates play a major role for the structural and optical properties of AlxGa1−xN layers with x ≥ 0.5 grown subsequently by metalorganic vapour phase epitaxy. The higher the Ga content in these layers is, the stronger is the influence of the surface morphology on their properties. For x = 0.5 not only periodic inhomogeneities in the Al content due to growth of Ga-rich facets are observed by cathodoluminescence, but these facets give rise to additional dislocation formation as discovered by annular dark-field scanning transmission electron microscopy. For AlxGa1−xN layers with x = 0.8 the difference in Al content between facets and surrounding material is much smaller. Therefore, the threading dislocation density (TDD) is only defined by the TDD in the underlying epitaxially laterally overgrown (ELO) AlN layer. This way high quality Al0.8Ga0.2N with a thickness up to 1.5 μm and a TDD ≤ 5x108 cm−2 was obtained

Origin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates

a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N[0001]Al2O3 at the ridge sidewalls. Faster growth of the top (11bar 20)(Al,Ga)N facet in the vertical direction leads to the overgrowth of c-plane (Al,Ga)N nucleated on the horizontal ridge and trench surfaces. Phase separation into binary GaN and AlN takes place during the first growth stages. However, this fades out and does not influence the composition of the final thick a-plane (Al,Ga)N layer

Abnormal Retinal Reflectivity To Short-Wavelength Light In Type 2 Idiopathic Macular Telangiectasia

PURPOSE: Macular telangiectasia Type 2 (MacTel) is a bilateral, progressive, potentially blinding retinal disease characterized by vascular and neurodegenerative signs, including an increased parafoveal reflectivity to blue light. Our aim was to investigate the relationship of this sign with other signs of macular telangiectasia Type 2 in multiple imaging modalities. METHODS: Participants were selected from the MacTel Type 2 study, based on a confirmed diagnosis and the availability of images. The extent of signs in blue-light reflectance, fluorescein angiographic, optical coherence tomographic, and single- and dual-wavelength autofluorescence images were analyzed. RESULTS: A well-defined abnormality of the perifovea is demonstrated by dual-wavelength autofluorescence and blue-light reflectance in early disease. The agreement in area size of the abnormalities in dual-wavelength autofluorescence and in blue-light reflectance images was excellent: for right eyes: ρ = 0.917 (P < 0.0001, 95% confidence interval 0.855-0.954, n = 46) and for left eyes: ρ = 0.952 (P < 0.0001, 95% confidence interval 0.916-0.973, n = 49). Other changes are less extensive initially and expand later to occupy that area and do not extend beyond it. CONCLUSION: Our findings indicate that abnormal metabolic handling of luteal pigment and physical changes giving rise to increased reflectance are widespread in the macula throughout the natural history of the disease, precede other changes, and are relevant to early diagnosis

GaAs-Based Superluminescent Light-Emitting Diodes with 290-nm Emission Bandwidth by Using Hybrid Quantum Well/Quantum Dot Structures

A high-performance superluminescent light-emitting diode (SLD) based upon a hybrid quantum well (QW)/quantum dot (QD) active element is reported and is assessed with regard to the resolution obtainable in an optical coherence tomography system. We report on the appearance of strong emission from higher order optical transition from the QW in a hybrid QW/QD structure. This additional emission broadening method contributes significantly to obtaining a 3-dB linewidth of 290 nm centered at 1200 nm, with 2.4 mW at room temperature

The 2020 UV emitter roadmap

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