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- ItemLocal electronic structure in AlN studied by single-crystal 27Al and 14N NMR and DFT calculations(Basel : MDPI, 2020) Zeman, Otto E.O.; Moudrakovski, Igor L.; Hartmann, Carsten; Indris, Sylvio; Bräuniger, ThomasBoth the chemical shift and quadrupole coupling tensors for 14N and 27Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of 14N (I = 1) and 27Al (I = 5/2), the quadrupolar coupling constants were determined to ?(14N) = (8.19 ± 0.02) kHz, and ?(27Al) = (1.914 ± 0.001) MHz. The chemical shift parameters obtained from the data fit were diso = -(292.6 ± 0.6) ppm and d? = -(1.9 ± 1.1) ppm for 14N, and (after correcting for the second-order quadrupolar shift) diso = (113.6 ± 0.3) ppm and d? = (12.7 ± 0.6) ppm for 27Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for 27Al with ?calc = (1.791 ± 0.003) MHz, but not for 14N with ?calc = -(19.5 ± 3.3) kHz. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
- 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.
- 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.
- 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
- ItemElectromechanical losses in carbon- and oxygen-containing bulk AlN single crystals(Amsterdam [u.a.] : Elsevier Science, 2019) Kogut, Iurii; Hartmann, Carsten; Gamov, Ivan; Suhak, Yuriy; Schulz, Michal; Schröder, Sebastian; Wollweber, Jürgen; Dittmar, Andrea; Irmscher, Klaus; Straubinger, Thomas; Bickermann, Matthias; Fritze, HolgerBulk single-crystalline aluminum nitride (AlN) is potentially a key component for low-loss high-temperature piezoelectric devices. However, the incorporation of electrically active impurities and defects during growth of AlN may adversely affect the performance of piezoelectric resonators especially at high temperatures. The electrical conductivity and electromechanical losses in bulk AlN single crystals are analyzed in the temperature range of 300–1200 K with respect to various contents of growth-related impurities in them. For AlN with [O]/[C] ≤ 1, an increase of electrical conductivity due to thermal activation of charge carriers in the temperature range of 850–1200 K has been observed and was determined to be a major contribution to electromechanical losses Q−1 rising up to maximum values of about 10−3 at 1200 K. As the oxygen content in AlN increased, the magnitude and the activation energy of high-temperature electrical conductivity increased. In oxygen-dominated AlN, two major thermally activated contributions to electromechanical losses were observed, namely, the anelastic relaxations of point defects at temperatures of 400–800 K and electrical conductivity at T > 800 K.