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- ItemHigh‐Temperature Annealing and Patterned AlN/Sapphire Interfaces(Weinheim : Wiley-VCH, 2021) Hagedorn, Sylvia; Mogilatenko, Anna; Walde, Sebastian; Pacak, Daniel; Weinrich, Jonas; Hartmann, Carsten; Weyers, MarkusUsing 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.
- ItemTemperature-Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells(Weinheim : Wiley-VCH, 2020) Netzel, Carsten; Hoffmann, Veit; Tomm, Jens W.; Mahler, Felix; Einfeldt, Sven; Weyers, MarkusTemperature-dependent transport of photoexcited charge carriers through a nominally undoped, c-plane GaN layer toward buried InGaN quantum wells is investigated by continuous-wave and time-resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in [0001⎯⎯] direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low-temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN-based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers.
- ItemTemperature Dependence of Dark Spot Diameters in GaN and AlGaN(Weinheim : Wiley-VCH, 2021) Netzel, Carsten; Knauer, Arne; Brunner, Frank; Mogilatenko, Anna; Weyers, MarkusThreading 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.
- ItemHigh-Temperature Annealing of AlGaN(Weinheim : Wiley-VCH, 2020) Hagedorn, Sylvia; Khan, Taimoor; Netzel, Carsten; Hartmann, Carsten; Walde, Sebastian; Weyers, MarkusIn 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.
- ItemImproving 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, MarkusHerein, 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
- ItemImproved 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, MarkusThe 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
- ItemStatus 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, MarkusHerein, 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
- ItemWedged Nd:YVO4 crystal for wavelength tuning of monolithic passively Q-switched picosecond microchip lasers(Washington, DC : Soc., 2021) Marianovich, André; Spiekermann, Stefan; Brendel, Moritz; Wessels, Peter; Neumann, Jörg; Weyers, Markus; Kracht, DietmarWe present a monolithic integrated passively Q-switched sub-150 ps microchip laser at 1064 nm with a wedged Nd:YVO4 crystal operating up to a repetition rate of 1 MHz. The wedge enables to change the cavity length by a small amount to fine tune the spectral cavity mode position over the full gain bandwidth of Nd:YVO4 and hence to optimize the output power. This additional degree of freedom may be a suitable approach to increase the wafer scale mass production yield or also to simplify frequency tuning of CW single-frequency microchip lasers.
- ItemThe 2020 UV emitter roadmap(Bristol : IOP Publ., 2020) Amano, Hiroshi; Collazo, Ramón; De Santi, Carlo; Einfeldt, Sven; Funato, Mitsuru; Glaab, Johannes; Hagedorn, Sylvia; Hirano, Akira; Hirayama, Hideki; Ishii, Ryota; Kashima, Yukio; Kawakami, Yoichi; Kirste, Ronny; Kneissl, Michael; Martin, Robert; Mehnke, Frank; Meneghini, Matteo; Ougazzaden, Abdallah; Parbrook, Peter J.; Rajan, Siddharth; Reddy, Pramod; Römer, Friedhard; Friedhard, Jan; Sarkar, Biplab; Scholz, Ferdinand; Schowalter, Leo J; Shields, Philip; Sitar, Zlatko; Sulmoni, Luca; Wang, Tao; Wernicke, Tim; Weyers, Markus; Witzigmann, Bernd; Wu, Yuh-Renn; Wunderer, Thomas; Zhang, YueweiSolid 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. © 2020 IOP Publishing Ltd.
- ItemStructural and optical properties of (112̅2) InGaN quantum wells compared to (0001) and (112̅0)(Milton Park : Taylor & Francis, 2016) Pristovsek, Markus; Han, Yisong; Zhu, Tongtong; Oehler, Fabrice; Tang, Fengzai; Oliver, Rachel A.; Humphreys, Colin J.; Tytko, Darius; Choi, Pyuck-Pa; Raabe, Dierk; Brunner, Frank; Weyers, MarkusWe benchmarked growth, microstructure and photo luminescence (PL) of (112-2) InGaN quantum wells (QWs) against (0001) and (112-0). In incorporation, growth rate and the critical thickness of (112-2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112-0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112-0). The slight deviation of (112-2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112-2) and (0001) while (112-0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 lower on (112-2) and more than 10 lower on (112-0).