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- ItemStrain induced power enhancement of far-UVC LEDs on high temperature annealed AlN templates(Melville, NY : American Inst. of Physics, 2023) Knauer, A.; Kolbe, T.; Hagedorn, S.; Hoepfner, J.; Guttmann, M.; Cho, H.K.; Rass, J.; Ruschel, J.; Einfeldt, S.; Kneissl, M.; Weyers, M.High temperature annealed AlN/sapphire templates exhibit a reduced in-plane lattice constant compared to conventional non-annealed AlN/sapphire grown by metalorganic vapor phase epitaxy (MOVPE). This leads to additional lattice mismatch between the template and the AlGaN-based ultraviolet-C light emitting diode (UVC LED) heterostructure grown on these templates. This mismatch introduces additional compressive strain in AlGaN quantum wells resulting in enhanced transverse electric polarization of the quantum well emission at wavelengths below 235 nm compared to layer structures deposited on conventional MOVPE-grown AlN templates, which exhibit mainly transverse magnetic polarized emission. In addition, high temperature annealed AlN/sapphire templates also feature reduced defect densities leading to reduced non-radiative recombination. Based on these two factors, i.e., better outcoupling efficiency of the transverse electric polarized light and an enhanced internal quantum efficiency, the performance characteristic of far-UVC LEDs emitting at 231 nm was further improved with a cw optical output power of 3.5 mW at 150 mA.
- ItemOptimized diamond inverted nanocones for enhanced color center to fiber coupling(Melville, NY : American Inst. of Physics, 2021) Torun, Cem GĂ¼ney; Schneider, Philipp-Immanuel; Hammerschmidt, Martin; Burger, Sven; Munns, Joseph H. D.; Schröder, TimNanostructures can be used for boosting the light outcoupling of color centers in diamond; however, the fiber coupling performance of these nanostructures is rarely investigated. Here, we use a finite element method for computing the emission from color centers in inverted nanocones and the overlap of this emission with the propagation mode in a single-mode fiber. Using different figures of merit, the inverted nanocone parameters are optimized to obtain maximal fiber coupling efficiency, free-space collection efficiency, or rate enhancement. The optimized inverted nanocone designs show promising results with 66% fiber coupling or 83% free-space coupling efficiency at the tin-vacancy center zero-phonon line wavelength of 619 nm. Moreover, when evaluated for broadband performance, the optimized designs show 55% and 76% for fiber coupling and free-space efficiencies, respectively, for collecting the full tin-vacancy emission spectrum at room temperature. An analysis of fabrication insensitivity indicates that these nanostructures are robust against imperfections. For maximum emission rate into a fiber mode, a design with a Purcell factor of 2.34 is identified. Finally, possible improvements offered by a hybrid inverted nanocone, formed by patterning into two different materials, are investigated and increase the achievable fiber coupling efficiency to 71%. © 2021 Author(s).
- ItemEnhanced thermal stability of yttrium oxide-based RRAM devices with inhomogeneous Schottky-barrier(Melville, NY : American Inst. of Physics, 2020) Piros, Eszter; Petzold, Stefan; Zintler, Alexander; Kaiser, Nico; Vogel, Tobias; Eilhardt, Robert; Wenger, Christian; Molina-Luna, Leopoldo; Alff, LambertThis work addresses the thermal stability of bipolar resistive switching in yttrium oxide-based resistive random access memory revealed through the temperature dependence of the DC switching behavior. The operation voltages, current levels, and charge transport mechanisms are investigated at 25 °C, 85 °C, and 125 °C, and show overall good temperature immunity. The set and reset voltages, as well as the device resistance in both the high and low resistive states, are found to scale inversely with increasing temperatures. The Schottky-barrier height was observed to increase from approximately 1.02 eV at 25 °C to approximately 1.35 eV at 125 °C, an uncommon behavior explained by interface phenomena. © 2020 Author(s).
- ItemA comprehensive study of charge transport in Au-contacted graphene on Ge/Si(001)(Melville, NY : American Inst. of Physics, 2020) Sinterhauf, Anna; Bode, Simeon; Auge, Manuel; Lukosius, Mindaugas; Lippert, Gunther; Hofsäss, Hans-Christian; Wenderoth, MartinWe investigate the electronic transport properties of Au-contacted graphene on Ge/Si(001). Kelvin probe force microscopy at room temperature with an additionally applied electric transport field is used to gain a comprehensive understanding of macroscopic transport measurements. In particular, we analyze the contact pads including the transition region, perform local transport measurements in pristine graphene/Germanium, and explore the role of the semiconducting Germanium substrate. We connect the results from these local scale measurements with the macroscopic performance of the device. We find that a graphene sheet on a 2 μm Ge film carries approximately 10% of the current flowing through the device. Moreover, we show that an electronic transition region forms directly adjacent to the contact pads. This transition region is characterized by a width of >100 μm and a strongly increased sheet resistance acting as the bottleneck for charge transport. Based on Rutherford backscattering of the contact pads, we suggest that the formation of this transition region is caused by diffusion. © 2020 Author(s).
- ItemInsights into surface modification and erosion of multi-element arc cathodes using a novel multilayer cathode design(Melville, NY : American Inst. of Physics, 2020) Golizadeh, Mehran; Anders, AndrĂ©; Martin, Francisca Mendez; KolozsvĂ¡ri, Szilard; Franz, RobertNowadays, multi-element cathodes are frequently employed to grow multi-element thin films and coatings using cathodic arc deposition processes. During cathode erosion, the cathode spot sequentially ignites on the cathode surface and imposes melting-solidification cycles that lead to material intermixing and the formation of a modified layer on the cathode surface. To allow us to study these surface modifications, a 10 µm thick Mo/Al multilayer coating was sputter-deposited onto a standard Ti arc cathode. This cathode was eroded by a dc steered arc discharge for a short duration enabling the observation of single craters formed by type 1 and 2 cathode spots. Furthermore, separated clusters of overlapping craters and a fully eroded surface caused by different stages of erosion were differentiated when scanning the erosion track in the lateral direction. Cross sections of single craters were prepared by focused ion beam techniques while metallographic methods were applied to obtain cross sections of overlapping craters and the modified layer. The layers of the multilayer coating acted as trace markers providing new insights into the material intermixing within craters, the material displacements during crater formation, the plasma pressure acting on the craters, and the temperature gradient (heat-affected zone) below the craters. The observations are discussed within the framework of established arc crater formation models. © 2020 Author(s).
- ItemSpin pumping at interfaces with ferro- and paramagnetic Fe60Al40films acting as spin source and spin sink(Melville, NY : American Inst. of Physics, 2022) Strusch, T.; Lenz, K.; Meckenstock, R.; Bali, R.; Ehrler, J.; Lindner, J.; Fassbender, J.; Farle, M.; Potzger, K.; Semisalova, A.We present a study of spin pumping efficiency and determine the spin mixing conductance and spin diffusion length in thin bilayer films based on 3d transition metal alloy Fe60Al40. Due to its magnetostructural phase transition, Fe60Al40 can be utilized as a ferromagnetic (FM) or paramagnetic (PM) material at the same temperature depending on its structural order; thus a thin Fe60Al40 film can act as a spin source or a spin sink when interfaced with a paramagnet or a ferromagnet, respectively. Ferromagnetic resonance measurements were performed in a frequency range of 5-35 GHz on bilayer films composed of FM-Fe60Al40/Pd and PM-Fe60Al40/Ni80Fe20 (permalloy). The increase in damping with the thickness of the paramagnetic layer was interpreted as a result of spin pumping into the paramagnet. We determine the spin mixing conductance g P d ↑↓ = (3.8 ± 0.5) × 10 18 m - 2 at the FM-Fe60Al40/Pd interface and the spin diffusion length λ P d = 9.1 ± 2.0 nm in Pd. For the PM-Fe60Al40/permalloy interface, we find a spin mixing conductance g F e A l ↑↓ = (2.1 ± 0.2) × 10 18 m - 2 and a spin diffusion length λ F e A l = 11.9 ± 0.2 nm for PM-Fe60Al40. The demonstrated bi-functionality of the Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications.
- ItemMagnetic field and angle-dependent photoluminescence of a fiber-coupled nitrogen vacancy rich diamond(Melville, NY : American Inst. of Physics, 2021) Wunderlich, Ralf; Staacke, Robert; Knolle, Wolfgang; Abel, Bernd; Meijer, JanHere, we investigate the magnetic field dependent photoluminescence (PL) of a fiber-coupled diamond single crystal with a high density of nitrogen vacancy (NV) centers. Angle-dependent magnetic field sweep measurements between 0 and 111 mT were performed using an oscillating illumination combined with lock-in techniques. Besides the expected superposed PL of differently oriented NV centers, a zoo of features in the PL are found. These features can be associated with level anti-crossings and cross relaxations. In particular, PL measurements allowed us to detect auto-cross relaxation between coupled NV centers. Moreover, the PL measurements at low magnetic fields show dips suggesting an interaction of NV centers with additional spin defects. The results presented here are not only a study for NV-based fiber-coupled sensors made of diamond, but also show a way to investigate with manageable effort and purely an optical multispin interaction with at least one NV center as a constituent.
- ItemInfluence of wavelength and accumulated fluence at picosecond laser-induced surface roughening of copper on secondary electron yield(Melville, NY : American Inst. of Physics, 2023) Bez, Elena; Himmerlich, Marcel; Lorenz, Pierre; Ehrhardt, Martin; Gunn, Aidan Graham; Pfeiffer, Stephan; Rimoldi, Martino; Taborelli, Mauro; Zimmer, Klaus; Chiggiato, Paolo; Anders, AndréUltrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu 2 O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (,1000 J/cm2 ), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10 -4 C/mm2 . Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between pro- cessing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders.
- ItemRobust nuclear hyperpolarization driven by strongly coupled nitrogen vacancy centers(Melville, NY : American Inst. of Physics, 2021) Wunderlich, Ralf; Staacke, Robert; Knolle, Wolfgang; Abel, Bernd; Haase, JĂ¼rgen; Meijer, JanNuclear magnetic resonance techniques are widely used in the natural sciences but they lack sensitivity. Therefore, large sample volumes or long measurement times are necessary. In this work, we investigate the polarization of bulk 13C nuclei in a diamond above the thermal equilibrium at room temperature. Previously studied mechanisms utilize direct coupling to nitrogen vacancy centers or the additional assistance of substitutional nitrogen impurities for this purpose. We exploit strongly coupled nitrogen vacancy centers as polarization sources. We study two approaches to transfer the optically induced polarization of the electron spins of the nitrogen vacancy centers to nearby nuclear spins. First, the electron-nuclear polarization transfer is achieved by energy matching conditions or, second, by magnetic field sweeps inducing Landau–Zener-like transitions. Simulations according to a quantum mechanical system consisting of two coupled nitrogen vacancy centers and a weakly coupled 13C spin show an excellent agreement with the experimental data. Both approaches allow a reduction of the measurement time by roughly three orders of magnitude.
- ItemReduce and refine: Plasma treated water vs conventional disinfectants for conveyor-belt cleaning in sustainable food-production lines(Melville, NY : American Inst. of Physics, 2021) Weihe, Thomas; Schnabel, Uta; Winter, Hauke; Möller, Timon; Stachowiak, Jörg; Neumann, Sabine; SchlĂ¼ter, Oliver; Ehlbeck, JörgSustainable and microbiologically secure foodstuff production lines are of increasing scientific interest and are in the focus of recent research programs. Additionally, they are of great importance for the production industry due to the prevention of food-borne illnesses caused by pathogens such as Salmonella sp., Listeria monocytogenes, or Escherichia coli. These pathogens are responsible for production losses, loss of customer acceptance, and severe food-borne illnesses. A pathogenic threat is frequently combated with sanitizing steps of the production lines. For conveyor band cleaning, this study compares the cleaning abilities of nitric acid (HNO3) and plasma treated water (PTW), which have been sprayed via a commercially available nozzle on two different polymeric surfaces (polysiloxane and polyurethane). Additionally, the cleaning agents HNO3 and PTW have been characterized through their pH and their conductivity. These findings have been underpinned by experiments that focus on a possible influence of nozzle abrasion, such as brass and stainless-steel nanoparticles, on the antimicrobial potential of PTW and HNO3. Adversely acting effects like an enhanced abrasion of conveyer band materials due to PTW or HNO3 treatment have been checked by using light microscopic micrographs and topographic scans in high-resolution mode. Based on the presented results of the experiments, the suitability of an in-place sanitation step in foodstuff production lines has been demonstrated on a laboratory scale.