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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/).
- ItemTwo-dimensional electron gas of the In2O3 surface: Enhanced thermopower, electrical transport properties, and reduction by adsorbates or compensating acceptor doping(Woodbury, NY : Inst., 2020) Papadogianni, Alexandra; Rombach, Julius; Berthold, Theresa; Polyakov, Vladimir; Krischok, Stefan; Himmerlich, Marcel; Bierwagen, OliverIn2O3 is an n-type transparent semiconducting oxide possessing a surface electron accumulation layer (SEAL) like several other relevant semiconductors, such as InAs, InN, SnO2, and ZnO. Even though the SEAL is within the core of the application of In2O3 in conductometric gas sensors, a consistent set of transport properties of this two-dimensional electron gas (2DEG) is missing in the present literature. To this end, we investigate high-quality single-crystalline as well as textured doped and undoped In2O3(111) films grown by plasma-assisted molecular beam epitaxy to extract transport properties of the SEAL by means of Hall effect measurements at room temperature while controlling the oxygen adsorbate coverage via illumination. The resulting sheet electron concentration and mobility of the SEAL are ≈1.5×1013cm−2 and ≈150cm2/Vs, respectively, both of which are strongly reduced by oxygen-related surface adsorbates from the ambient air. Our transport measurements further demonstrate a systematic reduction of the SEAL by doping In2O3 with the deep compensating bulk acceptors Ni or Mg. This finding is supported by x-ray photoelectron spectroscopy (XPS) measurements of the surface band bending and SEAL electron emission. Quantitative analyses of these XPS results using self-consistent, coupled Schrödinger-Poisson calculations indicate the simultaneous formation of compensating bulk donor defects (likely oxygen vacancies), which almost completely compensate the bulk acceptors. Finally, an enhancement of the thermopower by reduced dimensionality is demonstrated in In2O3: Seebeck coefficient measurements of the surface 2DEG with partially reduced sheet electron concentrations between 3×1012 and 7×1012cm−2 (corresponding average volume electron concentration between 1×1019 and 2.3×1019cm−3) indicate a value enhanced by ≈80% compared to that of bulk Sn-doped In2O3 with comparable volume electron concentration.
- ItemChirality flip of Weyl nodes and its manifestation in strained MoTe2(College Park, MD : APS, 2021) Könye, Viktor; Bouhon, Adrien; Fulga, Ion Cosma; Slager, Robert-Jan; van den Brink, Jeroen; Facio, Jorge I.Due to their topological charge, or chirality, the Weyl cones present in topological semimetals are considered robust against arbitrary perturbations. One well-understood exception to this robustness is the pairwise creation or annihilation of Weyl cones, which involves the overlap of two oppositely charged nodes in energy and momentum. Here we show that their topological charge can in fact change sign, in a process that involves the merging of not two, but three Weyl nodes. This is facilitated by the presence of rotation and time-reversal symmetries, which constrain the relative positions of Weyl cones in momentum space. We analyze the chirality flip process, showing that transport properties distinguish it from the conventional, double Weyl merging. Moreover, we predict that the chirality flip occurs in MoTe$_2$, where experimentally accessible strain leads to the merging of three Weyl cones close to the Fermi level. Our work sets the stage to further investigate and observe such chirality flipping processes in different topological materials.
- ItemOptical, electrical and chemical properties of PEO:I2 complex composite films(Heidelberg [u.a.] : Springer, 2022) Telfah, Ahmad; Al-Bataineh, Qais M.; Tolstik, Elen; Ahmad, Ahmad A.; Alsaad, Ahmad M.; Ababneh, Riad; Tavares, Carlos J.; Hergenröder, RolandSynthesized PEO:I2 complex composite films with different I2 concentrations were deposited onto fused silica substrates using a dip-coating method. Incorporation of PEO films with I2 increases the electrical conductivity of the composite, reaching a maximum of 46 mS/cm for 7 wt% I2. The optical and optoelectronic properties of the complex composite films were studied using the transmittance and reflectance spectra in the UV-Vis region. The transmittance of PEO decreases with increasing I2 content. From this study, the optical bandgap energy decreases from 4.42 to 3.28 eV as I2 content increases from 0 to 7 wt%. In addition, the refractive index for PEO films are in the range of 1.66 and 2.00.1H NMR spectra of pure PEO film shows two major peaks at 3.224 ppm and 1.038 ppm, with different widths assigned to the mobile polymer chains in the amorphous phase, whereas the broad component is assigned to the more rigid molecules in the crystalline phase, respectively. By adding I2 to the PEO, both peaks (amorphous and crystal) are shifted to lower NMR frequencies indicating that I2 is acting as a Lewis acid, and PEO is acting as Lewis base. Hence, molecular iodine reacts favorably with PEO molecules through a charge transfer mechanism, and the formation of triiodide (I3-), the iodite (IO2-) anion, I 2· · · PEO and I2+···PEO complexes. PEO:I2 complex composite films are expected to be suitable for optical, electrical, and optoelectronic applications.
- ItemHidden impurities in transparent conducting oxides: study of vacancies-related defects and impurities in (Cu–Ni) co-doped ZnO films(Heidelberg [u.a.] : Springer, 2022) Al-Bataineh, Qais M.; Ahmad, Ahmad A.; Aljarrah, Ihsan A.; Alsaad, Ahmad M.; Telfah, AhmadThe effect of hydrogen and nitrogen impurities on the physical properties of transparent conductive oxides is investigated in this study. Therefore, 5 wt.% of copper and 5 wt.% of nickel co-doped zinc oxide ((Cu–Ni)/ZnO) films were prepared using the sol–gel method. The (Cu–Ni)/ZnO films were annealed in an oven at 500 °C for 2 h under air, vacuum, nitrogen, and argon atmospheres. The synthesized zinc hydroxide film was transformed to zinc oxide film during the annealing by evaporating H 2O. Films annealed under the mentioned atmosphere including as-prepared one were characterized by analyzing with UV–Vis and FTIR spectra in addition to the 2D mapping electrical conductivity of the surface measured by the 4-point probe. The annealed films under air, vacuum, and argon atmospheres led to generate H-related impurities bounded to the oxygen vacancy (H O) which they act as shallow donor defects resulting in forming (Cu–Ni)/ZnO films into n-type materials. Whereas, the film annealed under a nitrogen atmosphere has N-related defects bounding to the zinc vacancy (N Zn) which they act as shallow acceptor defects resulting in transforming the film from n-type to p-type. These defects affect the optical, electrical, and optoelectronic properties of the (Cu–Ni)/ZnO films.
- ItemExploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films(College Park, MD : APS, 2020) Held, R.; Mairoser, T.; Melville, A.; Mundy, J.A.; Holtz, M.E.; Hodash, D.; Wang, Z.; Heron, J.T.; Dacek, S.T.; Holländer, B.; Muller, D.A.; Schlom, D.G.Raising the Curie temperature TC of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of TC with free carrier density n and the surprisingly low dopant activation p, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether TC could be considerably enhanced by increasing p. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation (p) values of up to 67%, effectively more than doubling p as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating n, p, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the TC of these samples allows us to identify several different mechanisms influencing TC and causing an experimental maximum in TC. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.