Philipp Komissinskiy

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.

Effect of composition and strain on the electrical properties of LaNiO3 thin films

The Ni content of LaNi1−xO3 epitaxial thin films grown by pulsed laser deposition has been varied by ablation from targets with different composition. While tensile strain and Ni substoichiometry reduce the conductivity, nearly stoichiometric and unstrained films show reproducibly resistivities below 100 μΩ × cm. Since the thermodynamic instability of the Ni 3+ state drives defect formation, Ni defect engineering is the key to obtain highly conducting LaNiO3 thin films.

Magnetism and spin-orbit coupling in Ir-based double perovskites La_{2−x}Sr_{x}CoIrO_{6}

We have studied Ir spin and orbital magnetic moments in the double perovskites La2−xSrxCoIrO6 by x-ray magnetic circular dichroism. In La2CoIrO6, Ir4+ couples antiferromagnetically to the weak ferromagnetic moment of the canted Co2+ sublattice and shows an unusually large negative total magnetic moment (−0.38 μB/f.u.) combined with strong spin-orbit interaction. In contrast, in Sr2CoIrO6, Ir5+ has a paramagnetic moment with almost no orbital contribution. A simple kinetic-energy-driven mechanism including spin-orbit coupling explains why Ir is susceptible to the induction of substantial magnetic moments in the double perovskite structure.

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.

Increased magnetic moment induced by lattice expansion from α-Fe to α′-Fe8N

Buffer-free and epitaxial α-Fe and α′-Fe8Nx thin films have been grown by RF magnetron sputtering onto MgO (100) substrates. The film thicknesses were determined with high accuracy by evaluating the Kiessig fringes of X-ray reflectometry measurements allowing a precise volume estimation. A gradual increase of the nitrogen content in the plasma led to an expansion of the iron bcc unit cell along the [001] direction resulting finally in a tetragonal distortion of about 10% corresponding to the formation of α′-Fe8N. The α-Fe lattice expansion was accompanied by an increase in magnetic moment to 2.61 ± 0.06μB per Fe atom and a considerable increase in anisotropy. These experiments show that—without requiring any additional ordering of the nitrogen atoms—the lattice expansion of α-Fe itself is the origin of the increased magnetic moment in α′-Fe8N.

The role of cationic and anionic point defects in pulsed laser deposition of perovskites

Pulsed laser-deposited thin films often show properties that are less optimized compared to single crystalline materials, or thin films grown by molecular beam epitaxy. The main reasons include the high particle energies and the dynamics of the laser plasma. Furthermore, point defect engineering is difficult to achieve experimentally since the adjustable parameters show a complex interplay. Using the examples of the simple perovskite SrMoO3 and the double perovskite Sr2CrWO6 we discuss the role of cationic and anionic point defects, and how far they can be engineered during pulsed laser deposition.

Joint effect of composition and strain on the anomalous transport properties of LaNiO3 films

In the present work, epitaxial LaNi1−xO3 films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates by pulsed laser deposition from targets with different Ni stoichiometry. Effect of Ni content on the transport properties of LaNiO3 films was investigated under different strain states. It is found that under tensile strain the carriers in the LaNi0.98O3 films behave according to the Fermi-liquid theory, while under compressive strain an anomalous T1.5 dependence of the resistivity which does not agree with the Fermi-liquid theory is observed in the films. On the other hand, only T1.5 dependence is observed in the LaNi1.20O3 films irrespective of strain states. The results show that strain has a profound influence on the transport properties of LNO films with Ni deficiency and the effect of strain is eliminated in the LNO films with excess Ni. Combined with the XRD results, it is proposed that the change in transport behavior is closely related to the stoichiometry of the LNO films. The mechanism behind is investigated based on the intrinsic and extrinsic factors of the films under different strains.

Highly conducting SrMoO3 thin films for microwave applications

We have measured the microwave resistance of highly conducting perovskite oxide SrMoO3 thin film coplanar waveguides. The epitaxial SrMoO3 thin films were grown by pulsed laser deposition and showed low mosaicity and smooth surfaces with a root mean square roughness below 0.3 nm. Layer-by-layer growth could be achieved for film thicknesses up to 400 nm as monitored by reflection high-energy electron diffraction and confirmed by X-ray diffraction. We obtained a constant microwave resistivity of 29 μΩ·cm between 0.1 and 20 GHz by refining the frequency dependence of the transmission coefficients. Our result shows that SrMoO3 is a viable candidate as a highly conducting electrode material for all-oxide microwave electronic devices.

Origin of superstructures in (double) perovskite thin films

We have investigated the origin of superstructure peaks as observed by X-ray diffraction of multiferroic Bi(Fe0.5Cr0.5)O3 thin films grown by pulsed laser deposition on single crystal SrTiO3 substrates. The photon energy dependence of the contrast between the atomic scattering factors of Fe and Cr is used to rule out a chemically ordered double perovskite Bi2FeCrO6 (BFCO). Structural calculations suggest that the experimentally observed superstructure occurs due to unequal cation displacements along the pseudo-cubic [111] direction that mimic the unit cell of the chemically ordered compound. This result helps to clarify discrepancies in the correlations of structural and magnetic order reported for Bi2FeCrO6. The observation of a superstructure in itself is not a sufficient proof of chemical order in double perovskites.

Polarization investigation of a tunable high-speed short-wavelength bulk-micromachined MEMS-VCSEL

We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser (VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more than 35 dB during the tuning.