This report covers the resistive switching characteristics of cross-bar type semi-transparent (or see-through) resistive random access memory (RRAM) devices based on ZnO. In order to evaluate the transmittance of the devices, we designed the memory array with various electrode sizes and spaces between the electrodes. To prevent read disturbance problems due to sneak currents, we employed a metal oxide based p-NiO/n-ZnO diode structure, which exhibited good rectifying characteristics and high forward current density. Based on these results, we found that the combined metal oxide diode/RRAM device could be promising candidate with suppressed read disturbances of cross-bar type ZnO RRAM device.

Further improvements of integrated circuits depend on the continuous downscaling of MOSFET´s, well beyond the limits for which direct tunneling currents are acceptable. These leakage currents affect both the stand-by power dissipation and the formation of the inversion layer ate the semiconductor surface, i.e., the channel formation. The most promising strategy to overcome this problem is the use of high-κ insulator in substitution of or as an additional layer on the traditional silicon dioxide. The aim of this work is using a recently developed theory to describe tunneling from inversion layers for high-κ insulators or stacks and analyze the effects of tunneling current on the thermal equilibrium in these cases.

Vanadium oxides are strongly correlated electron systems that are interesting both from a fundamental scientific point of view and for possible future applications including memory and sensors. In this contribution, we report on the epitaxial growth of V2O3 thin films on c-Al2O3 (0001) with molecular beam epitaxy and atomic oxygen. We studied the influence of deposition rate and substrate temperature on the structural properties and the metal-insulator transition.

Multiferroics, the study of materials which possess ferromagnetic and ferroelectric ordering in a single phase, has become an area of prominent research. Moreover, this behavior has been extensively studied in materials which possess a perovskite crystal structure such as BiFeO3 and YMnO3. Due to their weak saturation magnetic moment, many rare-earth orthoferrites are currently of extreme interest. Utilizing a solid-state reaction between Y2O3 and Fe2O3 we have developed the rare-earth orthoferrite YFeO3 and conducted a bulk material study to determine this material’s availability for thin film multiferroic research. The absence of Y2O3 and Fe2O3 impurities was confirmed using Copper-Kα XRD. Examination of the dependence of the magnetization M on the temperature T was conducted to determine the reliability of multiferroic behavior across varying temperatures in conjunction with the investigation of the dependence of M on the electric field strength H. Results clearly display ferromagnetic behavior in our bulk material, providing ample evidence that our bulk material is an excellent candidate for thin film studies. Future studies on multiferroic YFeO3 thin films grown via pulsed laser deposition on Lanthanum Aluminate substrates will be conducted. Detailed data will be provided via XRD and SQUID to confirm magnetic properties while impurities are non-existent in our thin films.

We report heteroepitaxial growth of VO2 thin film on c-plane sapphire by pulsed DC magnetron sputtering. X-ray diffraction experiment indicates that the 150 nm thick film is in triple-domain (020)-epitaxial structure with six-fold rotational symmetry in the basal plane; in particular, off-axis Φ scans from (011) and (220) show twin and triple peaks in each group of the diffraction profiles due to angle β mismatch and V4+-V4+ dimerization, respectively. The epitaxial relationship between VO2 and c-plane sapphire can be concluded as be , with the in-plane lattice mismatch of 2.66% (tensile) along and the out-of-plane lattice mismatch of -2.19% (compressive). Temperature dependence of resistivity in van der Pauw method shows that the resistivity changes by ~5 orders of magnitude through the metal-insulator transition, and a narrow hysteresis window of ~3 K is obtained between cooling and heating cycles with respect to phase-transition temperatures at 347.1 and 350.1 K.

BiFeO3 (BFO) thin films have been deposited on SrRuO3/SrTiO3 (001) substrate by using ion beam sputtering process. At low oxygen partial pressure of 11 m Pa, rhombohedral and large c/a mixed phase thin film have been obtained in spite of rhombohedral BFO single phase formation at high oxygen partial pressure of 73 mPa. From wide area 2θ-Ψ mappings, diffraction peaks from large c/a phase BFO thin film were obtained with the same extinction rule as those of rhombohedral BFO. Reciprocal space mappings around BFO (003) and BFO (103) spots indicate that lattice parameters of large c/a phase BFO were a = 0.381 nm and c = 0.461 nm (c/a =1.22), respectively. Moreover ferroelectric domain switching could be observed in both of rhombohedral BFO and mixed phase BFO thin films.

We have studied the magnetic properties of 100 nm thick ZnO thin films prepared by magnetron sputtering in different oxygen partial pressures (ratio of oxygen pressure to total pressure in deposition chamber, POxy). Only the films fabricated at POxy below ~ 0.5 show room temperature ferromagnetism. The saturation magnetization at room temperature is initially found to increase as POxy increases and reaches maximum value of ~ 5 emu/gm at POxy ~ 0.3 and then starts to decrease and becomes diamagnetic for POxy > 0.5. From small angle XRD study of structural properties of the films, we find that the lattice stress developed in the film along c-axis also exhibits a similar behavior with the variation of POxy. Thus, both the room temperature ferromagnetism and lattice stress appear to originate from the intrinsic defects present in the sample.

We have prepared a CrO2 thin film by chemical vapor deposition from a Cr8O21 precursor and studied the bulk and surface physical properties. The CrO2 thin film is grown on TiO2(100) substrate by heating precursor and TiO2 (100) substrate together in a sealed quartz tube. The prepared film is found from x-ray diffraction analysis to be an (100)-oriented single phase. The magnetization and resistivity measurements indicate that the film is a ferromagnetic metal with a Curie temperature of about 400 K. Cr 3s core-level and valence band photoelectron spectroscopy spectra reveal the presence of a metallic CrO2 in the surface region of the film. Our work indicates that preparation from a Cr8O21 precursor in a closed system is promising for obtaining a CrO2 thin film with the metallic surface.

We report on ‘in-situ’ solution processed homogeneous (200) oriented MgO ~85nm thin films deposited on Si substrates by inkjet printing. These films are found to show ferromagnetic order beyond room temperature with a saturation magnetization MS as high as ~0.63 emu/g. X-ray photoelectron spectroscopy investigations show the absence of any possible contamination effects, while the Mg 2p, and O 1s spectra indicate that the role of defect structure at the Mg site is important in the observed magnetism. By controlling the pH values of the precursors the concentration of the defects can be varied and hence tune the magnetization at room temperature. The origin of magnetism in these MgO thin films appears to arise from the cation vacancies.

Nanocrystalline Zn1-xCoxO powders exhibiting a preferential crystal growth along the (002) plane have been synthesized in the atomic fraction, ‘x’, range of 0.0-0.0625 by a wet chemical method. The effect of the dopant concentration on the corresponding structural, optical and magnetic properties was also evaluated. XRD analyses evidenced the development of single-phase wurtzite with no traces of any impurity for all the dopant levels. The higher intensity of the (002) peak, when compared to the XRD peaks in bulk ZnO, indicates the preferential crystal growth along the c-axis in hexagonal wurtzite cell. The linear dependence of cell parameters a and c with ‘x’ suggests the actual replacement of Zn by Co ions in the host oxide lattice. Micro Raman spectroscopy measurements showed a band centered at 535cm-1, which can be assigned to a local vibrational mode related to Co species in addition to the normal modes associated with wurtzite. The relative broadening of this band at 535cm-1 was enhanced by increasing ‘x’. The other characteristic bands of ZnO corresponding to A1 (E2, E1) and E2High modes were red shifted for all Co contents. UV-vis measurements showed that the energy band gap of as-synthesized nanopowders decreased with increasing Co2+ content up to x = 0.03 and increased for higher contents. Room-temperature magnetization measurements revealed the paramagnetic behavior of the Co-doped ZnO nanopowders.

Optimum processing conditions for fabricating SnO2 thin films were investigated to detect low ppm levels of ethylene gas for future on-field gas sensor applications. Different argon-to-oxygen ratios during R.F. sputtering were attempted to find the optimum gas ratio in depositing SnO2 thin film. Post-annealing was performed at 650°C to investigate the influence of film property change on ethylene sensing property of sensor. As-deposited and post-annealed films prepared under four different argon-to-oxygen ratios were studied by SEM, XRD, and sensitivity measurement. It was found that the stoichiometry and crystallinity of SnO2 films determined by post annealing was more influential than those by the argon to oxygen ratio during R.F sputtering on ethylene gas detection. An ethylene gas-sensing mechanism on R.F. sputtered SnO2 thin films for the design of processing conditions is proposed.

The properties of radio frequency, rf magnetron sputtered Barium Strontium Titanate (Ba1-xSrxTiO3), BST, thin films were investigated and compared with BST thin films deposited by sol-gel method with the aim of determining relationships between the oxide deposition parameters, the film structure, and the electric field dependence. This work presents noncontact electrical characterization of BST films using Corona Kelvin metrology (C-KM) which has been employed earlier only in the silicon industry. The films were structurally characterized using thickness profilometer, X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. The use of sol-gel technique to fabricate small area metal-insulator-metal (MIM) structures is found to be beneficial from the point of saving fabrication time and production costs.

Using Scanning Spreading Resistance Microscopy and direct current-voltage measurements, a long-relaxation transport current in polycrystalline PZT films is shown to depend on the polarization direction and voltage rise rate, the latter is typical for a capacitive current. The clockwise current hysteresis is observed at any polarization of the film. We suppose that the long current relaxation is due to recharge of traps, which participate in screening of polarization charges on PZT grain boundaries. The polarization charges response to applied bias for a short time, whereas the traps response to variation of the polarization charges takes much longer time.

Polycrystalline samples of the single-layered cobaltate La2-xCaxCoO4 were prepared in a wide doping range of 0 ≤ x ≤ 1.5. Structural properties were characterized at room temperature. The orthorhombic distorted structure of the mother compound La2CoO4 changes to a tetragonal structure for x = 0.5 and then becomes orthorhombic again for x > 0.5. The magnetic properties were investigated in the temperature range from 5 K ≤ T ≤ 300 K. With increasing hole-doping a successive decrease of antiferromagnetic exchange is observed for x ≤ 0.5 whereas an increase of ferromagnetic exchange evolves for x ≥ 0.5.

PbTiO3-covered ZnO nanorods were grown on Al2O3 by metalorganic chemical vapor deposition (MOCVD), and their crystalline orientation was investigated by x-ray diffraction (XRD). Structural analysis by scanning electron microscopy and XRD revealed that the hexagonal ZnO nanorods had -side facets. XRD analysis of PbTiO3 thin films on ZnO/Al2O3revealed that PbTiO3 was epitaxially grown on ZnO, showing 6 variants of crystallites with the c-axis tilted either 27o or 69o from the surface normal to the ZnO plane. Effective piezoelectric coefficients calculated for the 27o and 69o-crystallites using piezoresponse force microscopy confirm that deformation of nanorods and nanotubes contributed to the large electrically-induced strain along the radial direction.

The CaFeOX(CFO) and LaFeO3(LFO) thin films as well as superlattices were fabricated on SrTiO3(100) substrates by pulsed laser deposition (PLD) method. The tetragonal LFO film grew with layer-by-layer growth mode until approximately 40 layers. In the case of CFO, initial three layers showed layer-by-layer growth, and afterward the growth mode was transferred to two layers-by-two layers (TLTL) growth mode. The RHEED oscillation was observed until the end of the growth, approximately 50nm. Orthorhombic twin CaFeO2.5 (CFO2.5) structure was obtained. However, it is expected that the initial three CFO layers are CaFeO3 (CFO3) with the valence of Fe4+. The CFO and LFO superlattice showed a step-terraces surface, and the superlattice satellite peaks in a 2θ-θ and reciprocal space mapping (RSM) x-ray diffraction (XRD) measurements, indicating that the clear interfaces were fabricated.

We report synthesis of some binary and ternary metal oxide nanostructures using microwave irradiation-assisted chemical synthesis, either in the presence or absence of a surfactant/structure directing agent. The method is simple, inexpensive, and yields nanoparticles of desired metal oxides in minutes, and requires no conventional templating. Nanoparticles of some functionally advanced binary/ternary metal oxides (MnO2, ZnO, CuO, ZnMn2O4 etc) have been synthesized using metal acetylacetonates as the starting precursor material and microwave as the source of energy, in a process developed in detail in our laboratory. The nanoparticle size varies from 7-50 nm. Emphasis has been placed on the synthesis of ZnO nanostructures, particularly ZnO nanoshells, which do not require any surfactant/structure-directing agent for synthesis. There is a systematic variation in the morphology of the ZnO nanostructures with variation of process parameters, such as microwave power, microwave irradiation time, type of solvents, surfactants/structure-directing agents and its type and concentration. The as-prepared powder sample may either need a very brief exposure to heat to remove the surfactant or no post-synthesis processing, and is found to be well-crystallised. Determination of their crystallinity, actual shape, and orientation was made using X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Thin films of Ba1-xSrxTiO3 (BST) are being actively investigated for applications in dynamic random access memories (DRAM) because of their properties such as high dielectric constant, low leakage current, and high dielectric breakdown strength. Various approaches have been used to improve the dielectric properties of BST thin films such as doping, graded compositions, and multilayer structures. We have found that inserting a ZrO2 layer in between two BST layers results in a significant reduction in dielectric constant as well as dielectric loss. In this work the effect of Y2O3 doped ZrO2 on the dielectric properties of BST/ZrO2/BST trilayer structure is studied. The structure Ba0.8Sr0.2TiO3/(Y2O3)x(ZrO2)1-x/Ba0.8Sr0.2TiO3 is deposited by a sol-gel process on platinized Si substrate. The composition (x) of the middle layer is varied while keeping the total thickness of the trilayer film constant. The dielectric constant of the multilayer film decreases with the increase of Y2O3 amount in the film whereas there is a slight variation in dielectric loss. In Y2O3 doped multilayer thin films, the dielectric loss is lower in comparison to other films and also there is good frequency stability in the loss in the measured frequency range and hence very suitable for microwave device applications.

Exchange coupling observed recently in Fe/ Fe3O4 (001) junctions shows comparable intensity to that in Co/Ru/Co trilayers, and has potential applicability to spintronics devices. To clarify the mechanism of the exchange coupling, electronic and magnetic states of Fe/ Fe3O4 junctions are calculated in the first principles method by assuming four junction structures of bcc Fe and Fe3O4 layers. It is shown that the local moments of bcc Fe atoms at the interface increase, but those of Fe ions at the interface of Fe3O4 layer decrease. The total energy of the junctions is plotted as a function of distance between Fe and Fe3O4 layers. Calculated results of the coupling energy between Fe and Fe3O4 layers, however, are larger than experimental ones by two orders of magnitude, and they correspond to inter-atomic exchange interactions at the interface. In order to explain the experimental results, we propose a mechanism of exchange coupling mediated by impurity-like states of interfacial Fe atoms which possess reversed magnetic moments in bcc Fe layer. Frustration effects in exchange coupling between Fe and Fe3O4 layers are also discussed.