Giant Positive Magnetoresistance in Magnetic Multilayer Film Prepared by Ion—Beam Sputtering

The magnetic multilayers Ni78Co22//Cu//Ni78Co22//Ni78Co22O//Ta were fabricated by ion-beam sputtering through applied magnetic field and treatment under high vacuum. Resistance against applied magnetic field was measured by the standard four-point probe method at room temperature. The giant positive magnetore-sistance has been observed. A maximum positive magnetoresistance at room temperature was obtained to be 280%.     

Influence of LiB3O5 Structure on Microstructure and Optical Properties of ZrO2 Thin Films Prepared by Electron Beam Evaporation

ZrO2 thin films were deposited by using an electron beam evaporation technique on three kinds of lithium triborate (LIB3O5 or LBO) substrates with the surfaces at specified crystalline orientations. The influences of the LBO structure on the structural and optical properties of ZrO2 thin films are studied by spectrophotometer and x-ray diffraction. The results indicate that the substrate structure has obvious effects on the structural and optical properties of the film: namely, the ZrO2 thin film deposited on the X-LBO, Y-LBO and Z-LBO orients to m(-212), rn(021) and o(130) directions. It is also found that the ZrO2 thin film with m(021) has the highest refractive index and the least lattice misfit.     

Fabrication and Magnetic Properties of Co-Doped TiO2 Powders Studied by Nuclear Magnetic Resonance

Co0.04 Ti0.96O2 powders are fabricated by sol-gel method. The structure and magnetic properties are investigated under different annealing conditions systematically with emphasis on the influence of oxygen pressure. Pure anatase structure was acquired for all the samples annealed at 450℃ for one hour. The samples annealed in air exhibit evident room-temperature ferromagnetism (RTFM) with a small magnetic moment of 0.029μB per Co atom and coercivity Hc of 26 Oe, while the samples annealed in vacuum have strong RTFM with a larger magnetic moment of 1.18μB per Co atom and Hc of 430 Oe. The zero-field spin echo nuclear magnetic resonance spectrum of ^59Co is obtained to prove the existence of Co dusters in the latter samples, implying that the Co dusters are responsible for the strong RTFM in the samples annealed in vacuum. No Co cluster could be observed using both XPS and NMR techniques in the samples annealed in air, implying that the RTFM found in these sample sis intrinsic.     

Mechanical and Magnetic Properties of Rh and RhH： First-Principles Calculations

Mechanical and magnetic properties of Rh in bcc, fcc structures and RhH in cubic structure are investigated by using first-principles calculations. Theoretical strengths of these structures are given for the first time. The results show that the NaCl-type cubic RhH has a lower bulk modulus and a theoretical strength larger than those of Rh in bcc and fcc structures. A strong magneto-volume effect of a transition from low magnetic moment-low cell volume＇＇ to high magnetic moment-large cell volume＇＇ is also found for both the bcc and fcc Rh structures as well as RhH.     

Monte Carlo Simulation of Kerr Effect and Giant Magnetoresistance in CoxAg（1-x） Granular Films

Based on the Monte Carlo simulation, we present the theoretical calculation of the Kerr loops and giant magnetoresistance (GMR) curves in CoxAg（1-x) granular films. The calculated results of both the Kerr loops and the GMR curves are in good agreement, with the experimental ones. The anisotropy energy and the dipolar interaction evidently influence the Kerr loops and the GMR effect and it is found that with the increasing Co content X, the value of anisotropy constant K decreases and the value of dipolar interaction constant D increases.The values of Kt for the CoxAg(1-x) granular films are calculated, which plays an important role in explaining the GMR effect of the granular films. Here Kt is a parameter related to the resistivity at zero field and to the scattering parameter constant.     

optical, Structural and Laser-Induced Damage Threshold Properties of HfO2 Thin Films Prepared by Electron Beam Evaporation

We prepare HfO2 thin films by electron beam evaporation technology. The samples are annealed in air after deposition. With increasing annealing temperature, it is found that the absorption of the samples decreases firstly and then increases. Also, the laser-induced damage threshold (LIDT) increases firstly and then decreases.When annealing temperature is 473 K, the sample has the highest LID T of 2.17 J/cm^2, and the lowest absorption of 18ppm. By investigating the optical and structural characteristics and their relations to LIDT, it is shown that the principal factor dominating the LIDT is absorption.     

Characterization of Mn—Doped Ba1—xSrxTiO3 Thin Films Prepared by the Sol—Gel Method

Undoped and Mn-doped Ba1-xSrxTiO3(BST) thin films have been fabricated on Pt/Ti/SiO2/Si by an aqueous acetate sol-gel method.The BST stock solution can be easily mixed with an aqueous metal ion solution and is stable at room temperature.The annealing temperature of the doped and undoped films is between 650-750℃.The x-ray photoelectron spectra results show that the Mn2p3/2 valence state in the BST is the sampe as that of the original Mn(Ⅱ） dopant,The dielectric constant of the BST thin films can be increased to 800,and the loss tangent can be decreased to 0.01 due to the Mn(Ⅱ） doping.The leakage current of the BST films can also be greatly reduced.     

Preparation and Characterization of BPO Film as Electrode for Using of FeRAM

Conductive perovskite BaPb03 （BPO） films as a potential electrode material of PZT capacitors used in ferroelectric random access memory are prepared by rf magnetron sputtering. An x-ray diffractometer and standard four probe method are employed to investigate the dependence of growth conditions on crystal structure and conductivity of BPO films. It is found that BPO films with perovskite phase can be obtained at substrate temperatures above 425℃, and the sample with the lowest resistivity is obtained at 450℃ under pure argon atmosphere. Using this BPO film as electrode, ferroelectric properties of BPO/PZT/BPO and Pt/PZT/BPO sandwiched structures are evaluated. Their remanent polarization and coercive field are 36.6 ℃/cm^2 （81.3 k V/cm） and 36.9℃/cm^2 （89.1 kV/cm）, respectively. The coercive field of the former structure is lower than that of the latter, but remanent polarizations are almost the same. In addition, the results imply that BPO electrode is helpful to improve the fatigue resistance of PZT. The reasons are discussed.     

The effect of non-constant effective tunneling mass and asymmetry for resonant tunneling in double-barrier structures

We discuss the transmission coefficient τ_d in non-repetitive, one-dimensional, rectangular double-barrier structures without simplifications such as strongly attenuating barriers, strong localization, or overall constant effective tunneling mass of the electron. For resonance τ_d=1, we obtain two non-approximative conditions which require different resonance energies of the tunneling electron than previously reported in the literature. In fact, the resonance peaks are shifted to higher energy levels in the order of the width of the peaks due to the effect of non-constant tunneling mass. We investigate the dependence of the resonance condition and the shape of the resonance peaks in regard to perturbation of the electron energy, the gap width as well as the barrier width and height. Resonance is stable for variation of the barrier width but sensitive for variation of the barrier height and the gap width. Received: 9 December 1998 / Accepted: 5 January 1999 / Published online: 31 March 1999     

Magnetoresistance and interlayer exchange coupling in perovskite manganite junctions

We study magnetoresistance (MR) and interlayer exchange coupling (IEC) in perovskite manganite junctions. We show that in La_2/3Sr_1/3MnO₃/SrTiO₃/La_2/3Sr_1/3MnO₃ tunneling junctions, the MR ratio remains finite up to high temperatures near T_C of bulk manganites. In the case of La_2/3Ba_1/3MnO₃/LaNiO₃/La_2/3Ba_1/3MnO₃ metallic trilayers, we predict that the oscillation period of the IEC constant is dramatically changed by hole doping into the LaNiO₃ spacer, while the MR ratio is relatively unaffected.     

Mechanism and control of current transport in GaN and AlGaN Schottky barriers for chemical sensor applications

Strong interests are recently emerging for development of integrated high-performance chemical sensor chips. In this paper, the present status of understanding and controlling the current transport in the GaN and AlGaN Schottky diodes is discussed from the viewpoint of chemical sensor applications. For this purpose, a series of works recently carried out by our group are reviewed in addition to a general discussion. First, current transport in GaN and AlGaN Schottky barriers is discussed, introducing the thin surface barrier (TSB) model to explain the anomalously large leakage currents. Following this, attempts to reduce the leakage currents are presented and discussed. Then, as an example of gas-phase sensors using Schottky barriers, a Pd/AlGaN/GaN Schottky diode hydrogen sensor developed recently by our group is presented with a discussion on the sensing mechanism and related current transport. On the other hand, in liquid-phase sensors, contact is made between liquid and semiconductor which is regarded as a kind of Schottky barrier by electrochemists. As one of such liquid-phase sensors, open-gate AlGaN/GaN heterostructure field effect transistor (HFET) pH sensor developed recently by our group is presented. Finally, a brief summary is given together with some remarks for future research.     

Field-effect transistor based on a combination of nanometer film and undoped semiconductor

The metal oxide nanometer film semiconductor field-effect transistor (MONSFET) is reported. In this device, a combination of undoped semiconductor and nanometer film serves as the active layer. When a negative gate-source voltage is applied, electrons from the nanometer film enter into the semiconductor layer to form the conducting channel, and the drain current increases without saturation. This structure makes more materials available for the active layer, and thus suggests a new route to enrich the applications as well as to enhance the performances.     

Nonvolatile Memory Devices Based on Gold Nanoparticle and Poly （N-Vinylcarbazole） Composite

A rewritable polymer memory device based on gold nanoparticle doped poly （N-vinylcarbazole） （PVK）, which can be easily fabricated by simple spin coating, has been described. An electrical bistable phenomenon is observed in the current-voltage characteristics of this device, and it is found that the electrical bistability is repeatable by proper writing voltage and erasing voltage. The unique behaviour of the devices provides an interesting approach such that doping nanoparticles in polymer can be used to realize high performance nonvolatile polymer memory devices.     

UV-CVD deposited SiNH films on InP: Optimization of the physico-chemical and interface properties

High quality silicon nitride films are deposited at low temperature on InP substrates by direct photolysis at 185 nm of a NH₃-SiH₄ gas mixture. The composition of the films is measured by nuclear analysis. The thickness and refractive index are obtained by ellipsometry at 632.8 nm. As-deposited and post annealed samples are electrically characterized: quasi-static I(V) at 5×10^–4 Hz and C(V) characteristics at 1 MHz are performed on InP MIS diodes structures in order to optimize bulk and interface properties. At 250° C and 4 Torr, it is found that the highest critical field (measured for a leakage current density of 10^–9 A/cm²) is obtained for the injected ratio [SiH₄]/[NH₃]=2%. For these conditions, the film is stoichiometric, the critical field is 4 MV/cm and the resistivity is 6×10¹⁵ cm. The interface state density (N _ss) on InP is deduced from Terman analysis. The annealing conditions and the surface cleaning of InP have been optimized in order to reduce the N _ss which is, for our best conditions, as low as 2×10¹¹ eV^–1 cm^–2.     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Properties of Seamless W Sub-Micro Electrode Used for Phase Change Memory

In order to improve the reliability of C-RAM devices, a seamless sub-micro W heating electrode in diameter 260 nm is fabricated with standard 0.18 μm CMOS processing line. Then we successfully manufacture a chalcogenide random access memory device using this seamless sub-micro W heating electrode. The results show good electrical performance, e.g. the reset current of 1.3mA and the set/reset cycle up to 10^9 have been achieved.     

Influence of the broken symmetry of defect state distribution at the a-Si:H/c-Si interface on the performance of hetero-junction solar cells

Taking into account the fact that the distribution of defect states at the interface does not have strictly symmetrical shape, we present a simulation study of a-Si:H(n)/c-Si(p) and a-Si:H(p)/c-Si(n) structures with regard to the defect states at the interface, band offsets and doping concentration of the emitter. The presented results suggest for a-Si:H(n)/c-Si(p) solar cells a strong influence of the introduced broken symmetry between acceptor and donor defect states on the open-circuit voltage, whereas the a-Si:H(p)/c-Si(n) structure benefits from inherent favorable band alignment and remains unaffected.