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- ItemSimultaneous Effect of Ultraviolet Radiation and Surface Modification on the Work Function and Hole Injection Properties of ZnO Thin Films(Weinheim : Wiley-VCH, 2020) Raoufi, Meysam; Hörmann, Ulrich; Ligorio, Giovanni; Hildebrandt, Jana; Pätzel, Michael; Schultz, Thorsten; Perdigon, Lorena; Koch, Norbert; List-Kratochvil, Emil; Hecht, Stefan; Neher, DieterThe combined effect of ultraviolet (UV) light soaking and self-assembled monolayer deposition on the work function (WF) of thin ZnO layers and on the efficiency of hole injection into the prototypical conjugated polymer poly(3-hexylthiophen-2,5-diyl) (P3HT) is systematically investigated. It is shown that the WF and injection efficiency depend strongly on the history of UV light exposure. Proper treatment of the ZnO layer enables ohmic hole injection into P3HT, demonstrating ZnO as a potential anode material for organic optoelectronic devices. The results also suggest that valid conclusions on the energy-level alignment at the ZnO/organic interfaces may only be drawn if the illumination history is precisely known and controlled. This is inherently problematic when comparing electronic data from ultraviolet photoelectron spectroscopy (UPS) measurements carried out under different or ill-defined illumination conditions. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- 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.
- ItemUsing Active Surface Plasmons in a Multibit Optical Storage Device to Emulate Long-Term Synaptic Plasticity(Weinheim : Wiley-VCH, 2020) Rhim, Seon-Young; Ligorio, Giovanni; Hermerschmidt, Felix; Hildebrandt, Jana; Pätzel, Michael; Hecht, Stefan; List-Kratochvil, Emil J.W.Artificial intelligence takes inspiration from the functionalities and structure of the brain to solve complex tasks and allow learning. Yet, hardware realization that simulates the synaptic activities realized with electrical devices still lags behind computer software implementation, which has improved significantly during the past decade. Herein, the capability to emulate synaptic functionalities by exploiting surface plasmon polaritons (SPPs) is shown. By depositing photochromic switching molecules (diarylethene) on a thin film of gold, it is possible to reliably control the electronic configuration of the molecules upon illumination cycles with UV and visible light. These reversible changes modulate the dielectric function of the photochromic film and thus enable the effective control of the SPP dispersion relation at the molecule/gold interface. The plasmonic device displays fundamental functions of a synapse such as potentiation, depression, and long-term plasticity. The integration of such plasmonic devices in an artificial neural network is deployed in plasmonic neuroinspired circuits for optical computing and data transmission. © 2020 The Authors. Published by Wiley-VCH GmbH
- ItemBiaxially Textured Titanium Thin Films by Oblique Angle Deposition: Conditions and Growth Mechanisms(Weinheim : Wiley-VCH, 2020) Liedtke-Grüner, Susann; Grüner, Christoph; Lotnyk, Andriy; Gerlach, Juergen W.; Rauschenbach, BerndGrowing highly crystalline nanowires over large substrate areas remains an ambiguous task nowadays. Herein, a time-efficient and easy-to-handle bottom-up approach is demonstrated that enables the self-assembled growth of biaxially textured Ti thin films composed of single-crystalline nanowires in a single-deposition step. Ti thin films are deposited under highly oblique incidence angles by electron beam evaporation on amorphous substrates. Substrate temperature, angle of the incoming particle flux, and working pressure are varied to optimize the crystallinity in those films. Height-resolved structure information of individual nanowires is provided by a transmission electron microscopy (TEM) nanobeam, high-resolution TEM, and electron diffraction. Ti nanowires are polycrystalline at 77 K, whereas for ≥300 K, single-crystalline nanowires are tendentially found. The Ti crystals grow along the thermodynamically favored c-direction, but the nanowires’ tilt angle is determined by shadowing. Biaxially textured Ti thin films require a certain temperature range combined with highly oblique deposition angles, which is proved by X-ray in-plane pole figures. A general correlation between average activation energy for surface self-diffusion and melting point of metals is given to estimate the significant influence of surface self-diffusion on the evolution of obliquely deposited metal thin films.
- ItemWeak electron irradiation suppresses the anomalous magnetization of N-doped diamond crystals(Weinheim : Wiley-VCH, 2021) Setzer, Annette; Esquinazi, Pablo D.; Daikos, Olesya; Scherzer, Tom; Pöppl, Andreas; Staacke, Robert; Lühmann, Tobias; Pezzagna, Sebastien; Knolle, Wolfgang; Buga, Sergei; Abel, Bernd; Meijer, JanSeveral diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so-called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered.
- ItemCurrent Status of Carbon‐Related Defect Luminescence in GaN(Weinheim : Wiley-VCH, 2021) Zimmermann, Friederike; Beyer, Jan; Röder, Christian; Beyer, Franziska C.; Richter, Eberhard; Irmscher, Klaus; Heitmann, JohannesHighly insulating layers are a prerequisite for gallium nitride (GaN)-based power electronic devices. For this purpose, carbon doping is one of the currently pursued approaches. However, its impact on the optical and electrical properties of GaN has been widely debated in the scientific community. For further improvement of device performance, a better understanding of the role of related defects is essential. To study optically active point defects, photoluminescence is one of the most frequently used experimental characterization techniques. Herein, the main recent advances in the attribution of carbon-related photoluminescence bands are reviewed, which were enabled by the interplay of a refinement of growth and characterization techniques and state-of-the-art first-principles calculations developed during the last decade. The predicted electronic structures of isolated carbon defects and selected carbon-impurity complexes are compared to experimental results. Taking into account both of these, a comprehensive overview on the present state of interpretation of carbon-related broad luminescence bands in bulk GaN is presented.
- ItemMolecular Beam Epitaxy Growth and Characterization of Germanium-Doped Cubic AlxGa1−xN(Weinheim : Wiley-VCH, 2020) Deppe, Michael; Henksmeier, Tobias; Gerlach, Jürgen W.; Reuter, Dirk; As, Donat J.In cubic (c-)GaN Ge has emerged as a promising alternative to Si for n-type doping, offering the advantage of slightly improved electrical properties. Herein, a study on Ge doping of the ternary alloy c-AlxGa1−xN is presented. Ge-doped c-AlxGa1−xN layers are grown by plasma-assisted molecular beam epitaxy. In two sample series, both the Al mole fraction x and the doping level are varied. The incorporation of Ge is verified by time-of-flight secondary ion mass spectrometry. Ge incorporation and donor concentrations rise exponentially with increasing Ge cell temperature. A maximum donor concentration of 1.4 × 1020 cm−3 is achieved. While the incorporation of Ge is almost independent of x, incorporation of O, which acts as an unintentional donor, increases for higher x. Dislocation densities start increasing when doping levels of around 3 × 1019 cm−3 are exceeded. Also photoluminescence intensities begin to drop at these high doping levels. Optical emission of layers with x > 0.25 is found to originate from a defect level 0.9 eV below the indirect bandgap, which is not related to Ge. In the investigated range 0 ≤ x ≤ 0.6, Ge is a suitable donor in c-AlxGa1−xN up to the low 1019 cm−3 range.
- 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
- ItemEvolution of Low-Frequency Vibrational Modes in Ultrathin GeSbTe Films(Weinheim : Wiley-VCH, 2021) Zallo, Eugenio; Dragoni, Daniele; Zaytseva, Yuliya; Cecchi, Stefano; Borgardt, Nikolai I.; Bernasconi, Marco; Calarco, RaffaellaGeSbTe (GST) phase-change alloys feature layered crystalline structures made of lamellae separated by van der Waals (vdW) gaps. This work sheds light on the dependence of interlamellae interactions at the vdW gap on film thickness of GST alloys as probed by vibrational spectroscopy. Molecular beam epitaxy is used for designing GST layers down to a single lamella. By combining density-functional theory and Raman spectroscopy, a direct and simple method is demonstrated to identify the thickness of the GST film. The shift of the vibrational modes is studied as a function of the layer size, and the low-frequency range opens up a new route to probe the number of lamellae for different GST compositions. Comparison between experimental and theoretical Raman spectra highlights the precision growth control obtained by the epitaxial technique.
- ItemIncreasing the Diversity and Understanding of Semiconductor Nanoplatelets by Colloidal Atomic Layer Deposition(Weinheim : Wiley-VCH, 2020) Reichhelm, Annett; Hübner, René; Damm, Christine; Nielsch, Kornelius; Eychmüller, AlexanderNanoplatelets (NPLs) are a remarkable class of quantum confined materials with size-dependent optical properties, which are determined by the defined thickness of the crystalline platelets. To increase the variety of species, the colloidal atomic layer deposition method is used for the preparation of increasingly thicker CdSe NPLs. By growing further crystalline layers onto the surfaces of 4 and 5 monolayers (MLs) thick NPLs, species from 6 to 13 MLs are achieved. While increasing the thickness, the heavy-hole absorption peak shifts from 513 to 652 nm, leading to a variety of NPLs for applications and further investigations. The thickness and number of MLs of the platelet species are determined by high-resolution transmission electron microscopy (HRTEM) measurements, allowing the interpretation of several contradictions present in the NPL literature. In recent years, different assumptions are published, leading to a lack of clarity in the fundamentals of this field. Regarding the ongoing scientific interest in NPLs, there is a certain need for clarification, which is provided in this study. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim