Jose Kurian

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.

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.

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.

Rare-Earth Barium Niobates:* A New Class of Potential Substrates for YBa2Cu3O7-δ Superconductor

A class of complex perovskites REBa2NbO6 (where RE=Pr, Nd, Sm and Eu) have been sintered as single phase materials having a high sintered density and stability, for their use as substrates for YBa2Cu3O7-δ superconductors. The structure of these materials was studied by X-ray diffraction technique and all of them were found to be isostructural having a cubic perovskite structure. These newly developed materials do not react with YBa2Cu3O7-δ superconductors even after annealing 1:1 volume mixture at 950° C for 15 h. The presence of REBa2NbO6 upto 20 vol% in the YBa2Cu3O7-δ–REBa2NbO6 composite did not show any deterimental effect on the superconducting transition temperature of YBa2Cu3O7-δ. Dielectric constant and loss factor of REBa2NbO6 were found to be in the range suitable for their use as substrates for microwave applications. Superconducting YBa2Cu3O7-δ thick films screen-printed on these new substrates gave a zero transition temperature T c0≈92 K and current density≈2×105 A/cm2 at 77 K.

Third-order intermodulation measurements of microstrip bandpass filters based on high-temperature superconductors

The insufficient power-handling capability of high-temperature superconducting microwave filters has been a serious barrier to their application. To clarify the key parameters for improving the power-handling capability of RF filters based on high-temperature superconductors (HTSs) with microstrip structures, we synthesized bandpass filters with different layouts using several kinds of HTS thin films and performed third-order intermodulation measurements on them. The experimental results indicate that increasing the film thickness and utilizing molecular-beam-epitaxy-grown films of NdBa/sub 2/Cu/sub 3/O/sub 7/ (NBCO) are effective in obtaining microstrip filters with high power-handling capability. For NBCO filters, we also investigated the effects of oxygen annealing, passband width, and the frequencies of the input signals on the power-handling capability.

Growth of superconducting epitaxial LaNixBi2pnictide thin films with a Bi square net layer by reactive molecular beam epitaxy

We have grown superconducting epitaxial thin films of LaNixBi2 which crystallize in the ZrCuSiAs structure with a peculiar Bi square net layer. This material represents an additional class of pnictide superconductors since superconductivity is assumed to occur in the Bi square net layer. Optimized thin films grown by molecular beam epitaxy have a superconducting transition temperature of around 4 K and are stable in ambient air.

Impact of oxygen stoichiometry on electroforming and multiple switching modes in TiN/TaOx/Pt based ReRAM

We have investigated the material and electrical properties of tantalum oxide thin films (TaOx) with engineered oxygen contents grown by RF-plasma assisted molecular beam epitaxy. The optical bandgap and the density of the TaOx films change consistently with oxygen contents in the range of 3.63 to 4.66 eV and 12.4 to 9.0 g/cm3, respectively. When exposed to atmosphere, an oxidized Ta2O5-y surface layer forms with a maximal thickness of 1.2 nm depending on the initial oxygen deficiency of the film. X-ray photoelectron spectroscopy studies show that multiple sub-stoichiometric compositions occur in oxygen deficient TaOx thin films, where all valence states of Ta including metallic Ta are possible. Devices of the form Pt/Ta2O5-y/TaOx/TiN exhibit highly tunable forming voltages of 10.5 V to 1.5 V with decreasing oxygen contents in TaOx. While a stable bipolar resistive switching (BRS) occurs in all devices irrespective of oxygen content, unipolar switching was found to coexist with BRS only at higher oxygen contents, which transforms to a threshold switching behaviour in the devices grown under highest oxidation.

Femtosecond laser ablation-based mass spectrometry: An ideal tool for stoichiometric analysis of thin films

An accurate and routinely available method for stoichiometric analysis of thin films is a desideratum of modern materials science where a material’s properties depend sensitively on elemental composition. We thoroughly investigated femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs-LA-ICP-MS) as an analytical technique for determination of the stoichiometry of thin films down to the nanometer scale. The use of femtosecond laser ablation allows for precise removal of material with high spatial and depth resolution that can be coupled to an ICP-MS to obtain elemental and isotopic information. We used molecular beam epitaxy-grown thin films of LaPd(x)Sb2 and T′-La2CuO4 to demonstrate the capacity of fs-LA-ICP-MS for stoichiometric analysis and the spatial and depth resolution of the technique. Here we demonstrate that the stoichiometric information of thin films with a thickness of ~10 nm or lower can be determined. Furthermore, our results indicate that fs-LA-ICP-MS provides precise information on the thin film-substrate interface and is able to detect the interdiffusion of cations.