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The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB-LED Structures
2020, Knauer, Arne, Mogilatenko, Anna, Weinrich, Jonas, Hagedorn, Sylvia, Walde, Sebastian, Kolbe, Tim, Cancellara, Leonardo, Weyers, Markus
Strain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB-LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB-LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology.
Improved Efficiency of Ultraviolet B Light-Emitting Diodes with Optimized p-Side
2020, Kolbe, Tim, Knauer, Arne, Rass, Jens, Cho, Hyun Kyong, Mogilatenko, Anna, Hagedorn, Sylvia, Lobo Ploch, Neysha, Einfeldt, Sven, Weyers, Markus
The effects of design and thicknesses of different optically transparent p-current spreading layers [short-period superlattice, superlattice (SL), and bulk p- (Formula presented.)] as well as the type and thickness of the p-GaN cap layer on the electrical and optical characteristics of 310 nm ultraviolet light-emitting diodes (LEDs) are investigated. Scanning transmission electron microscopy measurements display self-organized composition variations in the nonpseudomorphically grown SLs, reducing the effect of increased hole injection efficiency of a SL. In addition, the effect leads to an increased operation voltage. In contrast, the bulk p-AlGaN layer has a uniform composition and the corresponding LEDs show only a slightly lower output power along with a lower operating voltage. If the thickness of the p-AlGaN bulk layer in the LED is reduced from 150 nm to 50 nm, the output power increases and the operating voltage decreases. Finally, LEDs with a nonuniform (Formula presented.) -GaN cap layer from a 3D island-like growth mode feature the highest output power and operating voltage. In contrast, the output power and operating voltage of LEDs with a smooth and closed cap depend on the thickness of (Formula presented.) -GaN. The highest output power and lowest operating voltage are achieved for LEDs with the thinnest (Formula presented.) -GaN cap. © 2020 The Authors. Published by Wiley-VCH GmbH
High‐Temperature Annealing and Patterned AlN/Sapphire Interfaces
2021, Hagedorn, Sylvia, Mogilatenko, Anna, Walde, Sebastian, Pacak, Daniel, Weinrich, Jonas, Hartmann, Carsten, Weyers, Markus
Using the example of epitaxial lateral overgrowth of AlN on trench-patterned AlN/sapphire templates, the impact of introducing a high-temperature annealing step into the process chain is investigated. Covering the open surfaces of sapphire trench sidewalls with a thin layer of AlN is found to be necessary to preserve the trench shape during annealing. Both the influence of annealing temperature and annealing duration are investigated. To avoid the deformation of the AlN/sapphire interface during annealing, the annealing duration or annealing temperature must be low enough. Annealing for 1 h at 1730 °C is found to allow for the lowest threading dislocation density of 3.5 × 108 cm−2 in the subsequently grown AlN, while maintaining an uncracked smooth surface over the entire 2 in. wafer. Transmission electron microscopy study confirms the defect reduction by high-temperature annealing and reveals an additional strain relaxation mechanism by accumulation of horizontal dislocation lines at the interface between annealed and nonannealed AlN. By applying a second annealing step, the dislocation density can be further reduced to 2.5 × 108 cm−2.
Status and Prospects of AlN Templates on Sapphire for Ultraviolet Light-Emitting Diodes
2020, Hagedorn, Sylvia, Walde, Sebastian, Knauer, Arne, Susilo, Norman, Pacak, Daniel, Cancellara, Leonardo, Netzel, Carsten, Mogilatenko, Anna, Hartmann, Carsten, Wernicke, Tim, Kneissl, Michael, Weyers, Markus
Herein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light-emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high-temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Temperature Dependence of Dark Spot Diameters in GaN and AlGaN
2021, Netzel, Carsten, Knauer, Arne, Brunner, Frank, Mogilatenko, Anna, Weyers, Markus
Threading dislocations in c-plane (Al,Ga)N layers are surrounded by areas with reduced light generation efficiency, called “dark spots.” These areas are observable in luminescence measurements with spatial resolution in the submicrometer range. Dark spots reduce the internal quantum efficiency in single layers and light-emitting devices. In cathodoluminescence measurements, the diameter of dark spots (full width at half maximum [FWHM]) is observed to be 200–250 nm for GaN. It decreases by 30–60% for AlxGa1−xN with x ≈ 0.5. Furthermore, the dark spot diameter increases with increasing temperature from 83 to 300 K in AlGaN, whereas it decreases in GaN. Emission energy mappings around dark spots become less smooth and show sharper features on submicrometer scales at low temperature for AlGaN and, on the contrary, at high temperature for GaN. It is concluded that charge carrier localization dominates the temperature dependence of dark spot diameters and of the emission energy distribution around threading dislocations in AlGaN, whereas the temperature-dependent excitation volume in cathodoluminescence and charge carrier diffusion limited by phonon scattering are the dominant effects in GaN. Consequently, with increasing temperature, nonradiative recombination related to threading dislocations extends to wider regions in AlGaN, whereas it becomes spatially limited in GaN.
Stability of ZnSe-Passivated Laser Facets Cleaved in Air and in Ultra-High Vacuum
2022, Boschker, Jos E., Spengler, Uwe, Ressel, Peter, Schmidbauer, Martin, Mogilatenko, Anna, Knigge, Andrea
Catastrophic optical mirror damage (COMD) is one of the main failure mechanisms limiting the reliability of GaAs based laser diodes. Here, we compare the facet stability of ZnSe-passivated ridge-waveguide lasers (RWLs) that are cleaved in air and subsequently cleaned using atomic hydrogen with RWLs that are cleaved in ultra-high vacuum. RWLs cleaved in ultra-high vacuum show a superior performance and reach power densities up to 58 MW/cm 2 under extended continuous wave operation at 1064 nm. This is attributed to the reduction of defects at the interface between ZnSe and the cleaved facet as evidenced by transmission electron microscopy and X-ray diffraction.