Abstract:Herein, AlN growth by metalorganic vapor‐phase epitaxy on hole‐type nanopatterned sapphire substrates is investigated. Cracking occurs for an unexpectedly thin‐layer thickness, which is associated to altered nucleation conditions caused by the sapphire pattern. To overcome the obstacle of cracking and at the same time to decrease the threading dislocation density by an order of magnitude, high‐temperature annealing (HTA) of a 300 nm‐thick AlN starting layer is successfully introduced. By this method, 800 nm‐th… Show more
“…The applied TDD calculation procedure provides reliable results, since the estimated TDDs correlate well with TDDs determined by defect etching with a KOH solution (analyzed area of 10 µm x 10 µm) and dark spot densities determined by cathodoluminescence (CL) spectroscopy. [ 20,21 ]…”
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.
“…The applied TDD calculation procedure provides reliable results, since the estimated TDDs correlate well with TDDs determined by defect etching with a KOH solution (analyzed area of 10 µm x 10 µm) and dark spot densities determined by cathodoluminescence (CL) spectroscopy. [ 20,21 ]…”
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.
“…On the other hand, the hexagonal phase of AlGaN presents the advantage over the cubic phase of requiring a lower concentration of Al to reach higher E g ; nonetheless, when working with h-AlGaN, it is very important to consider that built-in internal electric fields cause a large shift to lessenergetic emission [80], as will be discussed in the next sections. In general, due to the fact that the hexagonal structure is the stable phase of III-nitrides, the state of art unfolds studies that support sapphire as the main substrate in UVC LED growth [74,[81][82][83][84][85][86][87].…”
Crisis in coronavirus times requires understanding the effects on society and establishing efficient mechanisms to prevent infections. The disinfection of personal protection equipment by UVC light remains a key opportunity area. Therefore, this letter presents the main drawbacks and challenges on the fabrication of deep ultraviolet LEDs based on III-nitrides, such as the substrate selection, dislocation reduction, the increase of external quantum efficiency, enhancement of the radiative recombination in the active region, the complications to reach high Al content in AlGaN-based UVC LED avoiding the reduction of the p-doping, replacing the p-GaN contact layer by p-AlGaN without hindering the deposition of ohmic contacts. Furthermore, the cubic phase is suggested as a promising candidate for AlGaN UVC-LEDs applications as is discussed in this work.
“…A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. In 2020, Hagedorn et al reported an 800 nm-thick, fully coalesced, and crack-free AlN grown on two-inch hole-type nanopatterned sapphire wafers by high-temperature annealing (1680 °C) method 86 . Fig.…”
As demonstrated during the COVID-19 pandemic, advanced deep ultraviolet (DUV) light sources (200–280 nm), such as AlGaN-based light-emitting diodes (LEDs) show excellence in preventing virus transmission, which further reveals their wide applications from biological, environmental, industrial to medical. However, the relatively low external quantum efficiencies (mostly lower than 10%) strongly restrict their wider or even potential applications, which have been known related to the intrinsic properties of high Al-content AlGaN semiconductor materials and especially their quantum structures. Here, we review recent progress in the development of novel concepts and techniques in AlGaN-based LEDs and summarize the multiple physical fields as a toolkit for effectively controlling and tailoring the crucial properties of nitride quantum structures. In addition, we describe the key challenges for further increasing the efficiency of DUV LEDs and provide an outlook for future developments.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
