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DDC: 530 × Publisher: Weinheim : Wiley-VCH × Subject: ultraviolet light-emitting diodes ×

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Now showing 1 - 4 of 4
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    Status and Prospects of AlN Templates on Sapphire for Ultraviolet Light-Emitting Diodes
    (Weinheim : Wiley-VCH, 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
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    High-Temperature Annealing of AlGaN
    (Weinheim : Wiley-VCH, 2020) Hagedorn, Sylvia; Khan, Taimoor; Netzel, Carsten; Hartmann, Carsten; Walde, Sebastian; Weyers, Markus
    In the past few years, high-temperature annealing of AlN has become a proven method for providing AlN layers with low dislocation densities. Herein, the example of Al0.77Ga0.23N is used to investigate whether annealing can also improve the material quality of the ternary alloy. A detailed analysis of the influence of annealing temperature on structural and optical material properties is presented. It is found that with increasing annealing temperature, the threading dislocation density can be lowered from an initial value of 6.0 × 109 down to 2.6 × 109 cm−2. Ga depletion at the AlGaN surface and Ga diffusion into the AlN buffer layer are observed. After annealing, the defect luminescence between 3 and 4 eV is increased, accompanied by an increase in the oxygen concentration by about two orders of magnitude. Furthermore, due to annealing optical absorption at 325 nm (3.8 eV) occurs, which increases with increasing annealing temperature. It is assumed that the reason for this decrease in ultraviolet (UV) transmittance is the increasing number of vacancies caused by the removal of group-III and N atoms from the AlGaN lattice during annealing.
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    Improving AlN Crystal Quality and Strain Management on Nanopatterned Sapphire Substrates by High-Temperature Annealing for UVC Light-Emitting Diodes
    (Weinheim : Wiley-VCH, 2020) Hagedorn, Sylvia; Walde, Sebastian; Susilo, Norman; Netzel, Carsten; Tillner, Nadine; Unger, Ralph-Stephan; Manley, Phillip; Ziffer, Eviathar; Wernicke, Tim; Becker, Christiane; Lugauer, Hans-Jürgen; Kneissl, Michael; Weyers, Markus
    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-thick, fully coalesced and crack-free AlN is grown on 2 in. nanopatterned sapphire wafers. The usability of such templates as basis for UVC light-emitting diodes (LEDs) is furthermore proved by subsequent growth of an UVC-LED heterostructure with single peak emission at 265 nm. Prerequisites for the enhancement of the light extraction efficiency by hole-type nanopatterned sapphire substrates are discussed. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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    Improved Efficiency of Ultraviolet B Light-Emitting Diodes with Optimized p-Side
    (Weinheim : Wiley-VCH, 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