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- ItemCharge carrier density, mobility, and Seebeck coefficient of melt-grown bulk ZnGa2O4 single crystals(New York, NY : American Inst. of Physics, 2020) Boy, Johannes; Handwerg, Martin; Mitdank, Rüdiger; Galazka, Zbigniew; Fischer, Saskia F.The temperature dependence of the charge carrier density, mobility, and Seebeck coefficient of melt-grown, bulk ZnGa2O4 single crystals was measured between 10 K and 310 K. The electrical conductivity at room temperature is about σ = 286 S/cm due to a high electron concentration of n = 3.26 × 1019 cm−3 caused by unintentional doping. The mobility at room temperature is μ = 55 cm2/V s, whereas the scattering on ionized impurities limits the mobility to μ = 62 cm2/Vs for temperatures lower than 180 K. The Seebeck coefficient relative to aluminum at room temperature is SZnGa2O4−Al = (−125 ± 2) μV/K and shows a temperature dependence as expected for degenerate semiconductors. At low temperatures, around 60 K, we observed the maximum Seebeck coefficient due to the phonon drag effect. © 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
- ItemAnalysis of electronic properties frommagnetotransport measurements on Ba(Fe1-xNix)2As2 thin films(Basel : MDPI AG, 2020) Shipulin, I.; Richter, S.; Thomas, A.A.; Nielsch, K.; Hühne, R.; Martovitsky, V.We performed a detailed structural, magnetotransport, and superconducting analysis of thin epitaxial Ba(Fe1-xNix)2As2 films with Ni doping of x = 0.05 and 0.08, as prepared by pulsed laser deposition. X-ray diffraction studies demonstrate the high crystalline perfection of the films, which have a similar quality to single crystals. Furthermore, magnetotransport measurements of the films were performed in magnetic fields up to 9 T. The results we used to estimate the density of electronic states at the Fermi level, the coefficient of electronic heat capacity, and other electronic parameters for this compound, in their dependence on the dopant concentration within the framework of the Ginzburg-Landau-Abrikosov-Gorkov theory. The comparison of the determined parameters with measurement data on comparable Ba(Fe1-xNix)2As2 single crystals shows good agreement, which confirms the high quality of the obtained films.
- ItemDiffusion and interface effects during preparation of all-solid microstructured fibers(Basel : MDPI AG, 2014) Kobelke, J.; Bierlich, J.; Wondraczek, K.; Aichele, C.; Pan, Z.; Unger, S.; Schuster, K.; Bartelt, H.All-solid microstructured optical fibers (MOF) allow the realization of very flexible optical waveguide designs. They are prepared by stacking of doped silica rods or canes in complex arrangements. Typical dopants in silica matrices are germanium and phosphorus to increase the refractive index (RI), or boron and fluorine to decrease the RI. However, the direct interface contact of stacking elements often causes interrelated chemical reactions or evaporation during thermal processing. The obtained fiber structures after the final drawing step thus tend to deviate from the targeted structure risking degrading their favored optical functionality. Dopant profiles and design parameters(e.g., the RI homogeneity of the cladding) are controlled by the combination of diffusion and equilibrium conditions of evaporation reactions. We show simulation results of diffusion and thermal dissociation in germanium and fluorine doped silica rod arrangements according to the monitored geometrical disturbances in stretched canes or drawn fibers. The paper indicates geometrical limits of dopant structures in sub-μm-level depending on the dopant concentration and the thermal conditions during the drawing process. The presented results thus enable an optimized planning of the preform parameters avoiding unwanted alterations in dopant concentration profiles or in design parameters encountered during the drawing process.
- ItemGlow discharge optical emission spectrometry for quantitative depth profiling of CIGS thin-films(Cambridge : Royal Society of Chemistry, 2019) Kodalle, T.; Greiner, D.; Brackmann, V.; Prietzel, K.; Scheu, A.; Bertram, T.; Reyes-Figueroa, P.; Unold, T.; Abou-Ras, D.; Schlatmann, R.; Kaufmann, C.A.; Hoffmann, V.Determining elemental distributions dependent on the thickness of a sample is of utmost importance for process optimization in different fields e.g. from quality control in the steel industry to controlling doping profiles in semiconductor labs. Glow discharge optical emission spectrometry (GD-OES) is a widely used tool for fast measurements of depth profiles. In order to be able to draw profound conclusions from GD-OES profiles, one has to optimize the measurement conditions for the given application as well as to ensure the suitability of the used emission lines. Furthermore a quantification algorithm has to be implemented to convert the measured properties (intensity of the emission lines versus sputtering time) to more useful parameters, e.g. the molar fractions versus sample depth (depth profiles). In this contribution a typical optimization procedure of the sputtering parameters is adapted to the case of polycrystalline Cu(In,Ga)(S,Se)2 thin films, which are used as absorber layers in solar cell devices, for the first time. All emission lines used are shown to be suitable for the quantification of the depth profiles and a quantification routine based on the assumption of constant emission yield is used. The accuracy of this quantification method is demonstrated on the basis of several examples. The bandgap energy profile of the compound semiconductor, as determined by the elemental distributions, is compared to optical measurements. The depth profiles of Na-the main dopant in these compounds-are correlated with measurements of the open-circuit voltage of the corresponding devices, and the quantification of the sample depth is validated by comparison with profilometry and X-ray fluorescence measurements.
- ItemEpitaxial stannate pyrochlore thin films: Limitations of cation stoichiometry and electron doping(Melville, NY : AIP Publishing, 2021) Hensling, Felix V. E.; Dahliah, Diana; Dulal, Prabin; Singleton, Patrick; Sun, Jiaxin; Schubert, Jürgen; Paik, Hanjong; Subedi, Indra; Subedi, Biwas; Rignanese, Gian-Marco; Podraza, Nikolas J.; Hautier, Geoffroy; Schlom, Darrell G.We have studied the growth of epitaxial films of stannate pyrochlores with a general formula A2Sn2O7 (A = La and Y) and find that it is possible to incorporate ∼25% excess of the A-site constituent; in contrast, any tin excess is expelled. We unravel the defect chemistry, allowing for the incorporation of excess A-site species and the mechanism behind the tin expulsion. An A-site surplus is manifested by a shift in the film diffraction peaks, and the expulsion of tin is apparent from the surface morphology of the film. In an attempt to increase La2Sn2O7 conductivity through n-type doping, substantial quantities of tin have been substituted by antimony while maintaining good film quality. The sample remained insulating as explained by first-principles computations, showing that both the oxygen vacancy and antimony-on-tin substitutional defects are deep. Similar conclusions are drawn on Y2Sn2O7. An alternative n-type dopant, fluorine on oxygen, is shallow according to computations and more likely to lead to electrical conductivity. The bandgaps of stoichiometric La2Sn2O7 and Y2Sn2O7 films were determined by spectroscopic ellipsometry to be 4.2 eV and 4.48 eV, respectively. © 2021 Author(s).
- ItemConversion of p–n conduction type by spinodal decomposition in Zn-Sb-Bi phase-change alloys([London] : Macmillan Publishers Limited, part of Springer Nature Tokyo, 2020) Wang, Guoxiang; Shi, Haizhou; Lotnyk, Andriy; Shi, Daotian; Wang, RongpingPhase-change films with multiple resistance levels are promising for increasing the storage density in phase-change memory technology. Diffusion-dominated Zn2Sb3 films undergo transitions across three states, from high through intermediate to low resistance, upon annealing. The properties of the Zn2Sb3 material can be further optimized by doping with Bi. Based on scanning transmission electron microscopy combined with electrical transport measurements, at a particular Bi concentration, the conduction of Zn-Sb-Bi compounds changes from p- to n-type, originating from spinodal decomposition. Simultaneously, the change in the temperature coefficient of resistivity shows a metal-to-insulator transition. Further analysis of microstructure characteristics reveals that the distribution of the Bi-Sb phase may be the origin of the driving force for the p–n conduction and metal-to-insulator transitions and therefore may provide us with another way to improve multilevel data storage. Moreover, the Bi doping promotes the thermoelectric properties of the studied alloys, leading to higher values of the power factor compared to known reported structures. The present study sheds valuable light on the spinodal decomposition process caused by Bi doping, which can also occur in a wide variety of chalcogenide-based phase-change materials. In addition, the study provides a new strategy for realizing novel p–n heterostructures for multilevel data storage and thermoelectric applications.
- ItemHighly efficient modulation doping: A path toward superior organic thermoelectric devices(Washington, DC [u.a.] : Assoc., 2022) Wang, Shu-Jen; Panhans, Michel; Lashkov, Ilia; Kleemann, Hans; Caglieris, Federico; Becker-Koch, David; Vahland, Jörn; Guo, Erjuan; Huang, Shiyu; Krupskaya, Yulia; Vaynzof, Yana; Büchner, Bernd; Ortmann, Frank; Leo, KarlWe investigate the charge and thermoelectric transport in modulation-doped large-area rubrene thin-film crystals with different crystal phases. We show that modulation doping allows achieving superior doping efficiencies even for high doping densities, when conventional bulk doping runs into the reserve regime. Modulation-doped orthorhombic rubrene achieves much improved thermoelectric power factors, exceeding 20 μW m−1 K−2 at 80°C. Theoretical studies give insight into the energy landscape of the heterostructures and its influence on qualitative trends of the Seebeck coefficient. Our results show that modulation doping together with high-mobility crystalline organic semiconductor films is a previosly unexplored strategy for achieving high-performance organic thermoelectrics.