Lambert Alff

Hall effect, magnetization and conductivity of Fe3O4 epitaxial thin films

Magnetite epitaxial thin films have been prepared by pulsed laser deposition on MgO and Si substrates. The magnetic and electrical properties of these epitaxial films are close to those of single crystals. For 40-50 nm thick films, the saturation magnetization and electrical conductivity are ∼450emu∕cm3 and 225Ω−1cm−1 at room temperature, respectively. The Verwey transition temperature is 117K. The Hall effect data yield an electron concentration corresponding to 0.22 electrons per formula unit at room temperature. Both normal and anomalous Hall effect have been found to have negative sign.

Towards forming-free resistive switching in oxygen engineered HfO2−x

We have investigated the resistive switching behavior in stoichiometric HfO2 and oxygen-deficient HfO2− x thin films grown on TiN electrodes using reactive molecular beam epitaxy. Oxygen defect states were controlled by the flow of oxygen radicals during thin film growth. Hard X-ray photoelectron spectroscopy confirmed the presence of sub-stoichiometric hafnium oxide and defect states near the Fermi level. The oxygen deficient HfO2− x thin films show bipolar switching with an electroforming occurring at low voltages and low operating currents, paving the way for almost forming-free devices for low-power applications.

Controlled oxygen vacancy induced p-type conductivity in HfO2−x thin films

We have synthesized highly oxygen deficient HfO2−x thin films by controlled oxygen engineering using reactive molecular beam epitaxy. Above a threshold value of oxygen vacancies, p-type conductivity sets in with up to 6 times 1021 charge carriers per cm3. At the same time, the band-gap is reduced continuously by more than 1 eV. We suggest an oxygen vacancy induced p-type defect band as origin of the observed behavior.

Superconductivity phase diagrams for the electron-doped cuprates R2-xCexCuO4 (R=La, Pr, Nd, Sm, and Eu)

The superconductivity phase diagrams of electron doped cuprates of the form R(2-x)CexCuO4 (with R= La, Pr, Nd, Sm, and Eu) have been determined for cerium compositions 0 < x < 0.36 in a consistent series of epitaxial thin films grown by reactive molecular beam epitaxy (MBE). The use of epitaxial thin films allows the growth of materials away from thermodynamical equilibrium expanding the accessible phase space beyond the availability of bulk material. The superconducting phase space systematically increases with the rare earth ionic size. The doping concentration where the maximal transition temperature occurs in La(2-x)CexCuO4 is considerably shifted to lower doping (x ~ 0.09) compared to La(2-x)SrxCuO4 (x ~ 0.15). At the same time, the width of the superconducting region is broadened.

In-operando hard X-ray photoelectron spectroscopy study on the impact of current compliance and switching cycles on oxygen and carbon defects in resistive switching Ti/HfO2/TiN cells

In this study, direct experimental materials science evidence of the important theoretical prediction for resistive random access memory (RRAM) technologies that a critical amount of oxygen vacancies is needed to establish stable resistive switching in metal-oxide-metal samples is presented. In detail, a novel in-operando hard X-ray photoelectron spectroscopy technique is applied to non-destructively investigates the influence of the current compliance and direct current voltage sweep cycles on the Ti/HfO2 interface chemistry and physics of resistive switching Ti/HfO2/TiN cells. These studies indeed confirm that current compliance is a critical parameter to control the amount of oxygen vacancies in the conducting filaments in the oxide layer during the RRAM cell operation to achieve stable switching. Furthermore, clear carbon segregation towards the Ti/HfO2 interface under electrical stress is visible. Since carbon impurities impact the oxygen vacancy defect population under resistive switching, this dynamic carbon segregation to the Ti/HfO2 interface is suspected to negatively influence RRAM device endurance. Therefore, these results indicate that the RRAM materials engineering needs to include all impurities in the dielectric layer in order to achieve reliable device performance.

Oxygen Vacancy Induced Room Temperature Ferromagnetism in Pr-Doped CeO2 Thin Films on Silicon

Integration of functional oxides on Si substrates could open a pathway to integrate diverse devices on Si-based technology. Oxygen vacancies (Vo(··)) can strongly affect solid state properties of oxides, including the room temperature ferromagnetism (RTFM) in diluted magnetic oxides. Here, we report a systematical study on the RTFM of oxygen vacancy engineered (by Pr(3+) doping) CeO2 epitaxial thin films on Si substrates. High quality, mixed single crystalline Ce1-xPrxO2-δ (x = 0-1) solid solution films were obtained. The Ce ions in CeO2 with a fluorite structure show a Ce(4+)-dominant valence state in all films. The local crystal structures of the films were analyzed in detail. Pr doping creates both Vo(··) and PrO8-complex defects in CeO2 and their relative concentrations vary with the Pr-doping level. The RTFM properties of the films reveal a strong dependence on the relative Vo(··) concentration. The RTFM in the films initially increases with higher Pr-doping levels due to the increase of the F(+) center (Vo(··) with one occupied electron) concentration and completely disappears when x > 0.2, where the magnetic polaron concentration is considered to decline below the percolation threshold, thus long-range FM order can no longer be established. We thus demonstrate the possibility to directly grow RTFM Pr-doped CeO2 films on Si substrates, which can be an interesting candidate for potential magneto-optic or spintronic device applications.

Thickness independent reduced forming voltage in oxygen engineered HfO2 based resistive switching memories

The conducting filament forming voltage of stoichiometric hafnium oxide based resistive switching layers increases linearly with layer thickness. Using strongly reduced oxygen deficient hafnium oxide thin films grown on polycrystalline TiN/Si(001) substrates, the thickness dependence of the forming voltage is strongly suppressed. Instead, an almost constant forming voltage of about 3 V is observed up to 200 nm layer thickness. This effect suggests that filament formation and switching occurs for all samples in an oxidized HfO2 surface layer of a few nanometer thickness while the highly oxygen deficient thin film itself merely serves as a oxygen vacancy reservoir.

X-ray magnetic circular dichroism study of Re 5d magnetism in Sr2CrReO6

We have measured Re 5d spin and orbital magnetic moments in the ferrimagnetic double perovskite Sr2CrReO6 by x-ray magnetic circular dichroism at the L2,3 edges. In fair agreement with recent band structure calculations [G. Vaitheeswaran, V. Kanchana, and A. Delin, Appl. Phys. Lett. 86, 032513 (2005)], at the Re site a large 5d spin magnetic moment of −0.68μB and a considerable orbital moment of +0.25μB have been detected. We found that the Curie temperature of the double perovskites A2BB′O6 scales with the spin magnetic moment of the “nonmagnetic” B′ ion.

Epitaxial growth and transport properties of Sr2CrWO6 thin films

We report on the preparation and characterization of epitaxial thin films of the double-perovskite Sr2CrWO6 by pulsed laser deposition. On substrates with low lattice mismatch like SrTiO3, epitaxial Sr2CrWO6 films with high crystalline quality can be grown in a molecular layer-by-layer growth mode. Due to the similar ionic radii of Cr and W, these elements show no sublattice order. Nevertheless, the measured Curie temperature is well above 400 K. Due to the reducing growth atmosphere required for double perovskites, the SrTiO3 substrate surface undergoes an insulator–metal transition impeding the separation of thin film and substrate electric transport properties.

Physical properties and band structure of reactive molecular beam epitaxy grown oxygen engineered HfO2±x

We have conducted a detailed thin film growth structure of oxygen engineered monoclinic HfO2±x grown by reactive molecular beam epitaxy. The oxidation conditions induce a switching between (1¯11) and (002) texture of hafnium oxide. The band gap of oxygen deficient hafnia decreases with increasing amount of oxygen vacancies by more than 1 eV. For high oxygen vacancy concentrations, defect bands form inside the band gap that induce optical transitions and p-type conductivity. The resistivity changes by several orders of magnitude as a function of oxidation conditions. Oxygen vacancies do not give rise to ferromagnetic behavior.