Showing papers by "Stefan E. Schulz published in 2019"

Electroforming-free resistive switching in yttrium manganite thin films by cationic substitution

Abstract: We report unipolar resistive switching in polycrystalline, hexagonal yttrium manganite thin films grown on unpatterned Pt metal coated SiO2/Si substrates with circular Al top electrodes. Electroforming-free or electroforming-based resistive switching is observed, depending on the chemical composition (Y1Mn1O3, Y0.95Mn1.05O3, Y1Mn0.99Ti0.01O3, and Y0.94Mn1.05Ti0.01O3). The number of loading cycles measured at room temperature for samples with Y1Mn1O3 and Y0.95Mn1.05O3 composition is larger than 103. The dominant conduction mechanism of the metal–insulator–metal structures between 295 K and 373 K in the high resistance state is space charge limited conduction and in the low resistance state is ohmic conduction. Activation energies in Ohm's law region in the high resistance state are calculated from the Arrhenius equation and are evaluated to be 0.39 ± 0.01 eV (Y1Mn1O3), 0.43 ± 0.01 eV (Y0.95Mn1.05O3), 0.34 ± 0.01 eV (Y1Mn0.99Ti0.01O3), and 0.38 ± 0.02 eV (Y0.94Mn1.05Ti0.01O3).

Electroforming-free BiFeO 3 switches for neuromorphic computing: Spike-timing dependent plasticity (STDP) and cycle-number dependent plasticity (CNDP)

Abstract: Memristor technology will strongly influence the architecture of computer systems in the near future. Its potential in several application domains, e.g. in-memory information processing, neuromorphic computing, hardware cryptography, and machine learning makes it more than ever necessary to understand the underlying resistive switching mechanisms and to look for electroforming-free memristors. We have developed an electroforming-free bipolar memristor, namely BiFeO 3 , which emulates spike-timing dependent plasticity. Neuromorphic engineering takes advantage of artificial neurons and artificial synapses to mimic the most complicated human attributes, learning and unlearning. Here we discuss how BiFeO 3 memristors as artificial synapse and artificial neurons are used to implement both spike-timing dependent plasticity and cycle number dependent plasticity.

Synthesis of Mg and Zn diolates and their use in metal oxide deposition

Abstract: The synthesis of complexes [M(OCHMeCH2NMeCH2)2] (5, M = Mg; 7, M = Zn) is described. Treatment of MeHNCH2CH2NMeH (1) with 2-methyloxirane (2) gave diol (HOCHMeCH2NMeCH2)2 (3), which upon reaction with equimolar amounts of MR2 (4, M = Mg, R = Bu; 6, M = Zn, R = Et) gave 5 and 7. The thermal behavior and vapor pressure of 5 and 7 were investigated to show whether they are suited as CVD (= chemical vapor deposition) and/or spin-coating precursors for MgO or ZnO layer formation. Thermogravimetric (TG) studies revealed that 5 and 7 decompose between 80–530 °C forming MgO and ZnO as evidenced by PXRD studies. In addition, TG-MS-coupled experiments were carried out with 7 proving that decomposition occurs by M–O, C–O, C–N and C–C bond cleavages, as evidenced from the detection of fragments such as CH4N+, C2H4N+, C2H5N+, CH2O+, C2H2O+ and C2H3O+. The vapor pressure of 7 was measured at 10.4 mbar at 160 °C, while 5 is non-volatile. The layers obtained by CVD are dense and conformal with a somewhat granulated surface morphology as evidenced by SEM studies. In addition, spin–coating experiments using 5 and 7 as precursors were applied. The corresponding MO layer thicknesses are between 7–140 nm (CVD) or 80 nm and 65 nm (5, 7; spin-coating). EDX and XPS measurements confirm the formation of MgO and ZnO films, however, containing 12–24 mol% (CVD) or 5–9 mol% (spin-coating) carbon. GIXRD studies verify the crystalline character of the deposited layers obtained by CVD and the spin-coating processes.

Magnetic Tunnel Junctions: Laser Annealing Versus Oven Annealing

Abstract: The performance of CoFeB/MgO tunnel junctions is strongly dependent on an annealing step at the end of the fabrication process to first, set a reference magnetization through the exchange bias effect and second, to crystallize the MgO/CoFe layers while boron diffuses out, in order to maximize the magnetoresistance ratio. In this regard, a laser-induced annealing process presents several advantages against traditional oven annealing techniques, providing a scalable approach with no limitations regarding the defined reference magnetization that can be aligned in different directions on a wafer. This paper concentrates on the mechanisms and dependences of laser annealing on the magnetic properties in comparison to the standard vacuum oven annealing, providing a first insight for the applicability of laser annealing for CoFeB-/MgO-based magnetic tunnel junctions with all concomitant needs.

A β-ketoiminato palladium(II) complex for palladium deposition

Abstract: The ¦-ketoiminato complex [Pd(OAc)L] (3) can be synthesized by the reaction of bis(benzoylacetone)diethylenetriamine (1, = LH) with [Pd(OAc)₂] (2). The structure of 3 in the solid state has been determined by single X-ray diffraction analysis. Complex 3 crystallizes as a dimer (3₂), which is formed by hydrogen bonds between NH and OOAc functionalities of two adjacent ligands. Each of the Pd atoms is complexed by one ON₂ donor unit of the polydentate ligand L⁻ and an acetate group. Pd–Pd interactions and hydrogen bond formation between a NH and the C=O acetate moiety lead to a [4 + 2] coordination at Pd. The non-coordinated part of L exists in its ¦-keto-enamine form. The thermal decomposition behavior of 3₂ was studied by TG (thermogravimetry) and TG-MS showing that 3₂ decomposes between 200 and 500°C independent of the applied atmosphere. Under oxygen PdO is produced, while under argon Pd is formed as confirmed by PXRD measurements. Complex 3₂ was applied as a spin-coating precursor (conc. 0.1 mol L⁻¹, volume 1.5 mL, 3000 rpm, deposition time 6 min, heating rate 50 K min⁻¹, holding time 60 min (Ar) and 120 min (air) at T = 800°C). The as-obtained samples are characterized by granulated particles of Pd/PdO on the substrate surface. EDX (energy-dispersive X-ray spectroscopy) and XPS (X-ray photoelectron spectroscopy) measurements confirmed the formation of Pd (Ar) or PdO (O₂) with up to 12 mol% C impurity.