X. Z. Liu

Factors Affecting the Growth of SiC Nano-whiskers

Silicon carbide (SiC) is a IV-IV compound semiconductor material with a wide band gap. Semiconductor electronic devices and circuits made from SiC are presently developed for high-temperature, high-power, and high-radiation conditions, in which conventional semiconductors cannot be adequately performed. In this paper, SiH 4 and C 2 H 2 were used to synthesize SiC nano-whiskers. Metal Ni was the catalyst. SiC nano-whiskers were grown by vapor-liquid-solid mechanism. The effects of the H 2 flow rate, growth temperature, catalyst thickness and growth pressure to grow SiC nano-whiskers were studied. 3C-SiC thin film and nano-tips can be synthesized by controlling the growth conditions.

The effect of deposition rate on the microstructure of YBCO thin films prepared by inverted cylindrical magnetron sputtering

The dependence of YBCO thin film properties on the deposition rate was studied in this report. The thermodynamic growth parameters (substrate temperature, total pressure and oxygen partial pressure) were optimized at a fixed deposition rate first. Then keeping the thermodynamic growth parameters optimized values, YBCO thin films were deposited at different deposition rates. The effect of the deposition rate on the superconducting transition properties, c-axis lattice constant, concentration of lattice disorder and surface smoothness of YBCO thin films was studied. The measurements show that the optimized thermodynamic growth parameters are closely correlated to the deposition rate. At a fixed deposition rate of 0.5 A/s and its optimal thermodynamic growth parameters, YBCO thin films with superior superconducting transition temperature, the lowest concentration of lattice disorder, and the lowest surface roughness were prepared. By using the same thermodynamic growth parameters, changes in deposition rate were shown to degrade the superconducting transition temperature, increase the lattice disorder, and increase the roughness of the YBCO thin films.

Resistive switching properties of HfxZr1−xO2 thin films for flexible memory applications

The flexible unipolar resistive switching characteristics of Au/Ni/HZO/Au devices on Polyethylene terephthalate substrates have been investigated for the RRAM applications. The devices demonstrated a bistable and reproducible unipolar RS behavior with a high OFF/ON ratio about 103, and the memory widow could be maintained in repetitive programming/erase at least 1000 cycles. The retention property has no degradation at 6.3 × 104 seconds. The current–voltage characteristics of the HZO samples show that the Ohmic contact and space charge limited current are suggested to response for the low resistance state and high resistance state, respectively. Combined with the conductance mechanism, the resistive switching behaviors can be explained by the conductive filaments model. The RS mechanism is attributed to restore and rupture caused by the joule heating and the redox reaction induced by the external electron injection. The memory devices also show good mechanical flexibility. It is believed that the HZO-based memory device has great potential to be used in high performance, flexible memory applications.

The preparation of two inch double-sided YBCO thin films

The preparation of two inch double-sided YBCO thin films by simultaneous sputtering from a single target is reported. The lateral homogeneity of microwave surface resistance of the YBCO thin films, on both sides of the two inch wafer, is characterized by using a Fabry?Perot resonator at 145 GHz and 75 K. Values of microwave surface resistance Rs (75 K, 145 GHz, 0 T) below 55 m? were reached over the whole area of YBCO thin films on two inch LaAlO3 wafers. The majority of the wafer area has Rs (75 K, 145 GHz, 0 T) values in the range of 15 m? to 40 m?. The uniformity of Rs values in the whole two inch wafer is excellent and the properties of YBCO thin films were found to be very similar on both sides of the wafer.

The preparation of double-sided YBCO thin films by simultaneous sputtering from single target

Abstract The preparation of 1-inch double-sided YBCO thin films by simultaneous sputtering from a single target is reported. By rotating the substrate around the rod of the substrate holder and the normal of the substrate, YBCO thin films were simultaneously deposited on both sides of 1-inch (100)LaAlO 3 substrate from a single inverted cylindrical sputter gun. The T C0 values of the YBCO thin films on both sides of the wafer were 90.3 and 90.4 K, respectively. The transition width was 0.8 K. The microwave surface resistance, R s (77 K, 10 GHz), of the YBCO thin films on both sides of the wafer were 330 and 400 μΩ, respectively. The uniformity of R s values over the whole 1-inch wafer is good and the properties of YBCO thin films were found to be very similar on both sides of the wafer.

The dielectric properties of pulsed laser deposited SrTiO 3 thin films

Dielectric thin films of SrTiO3 (STO) were deposited on Y1Ba2Cu3O7-x (YBCO) HTSC bottom layer by PLD method. X-ray diffraction results show that thin films were well crystallized and highly oriented in the (100) direction. Parallel capacitors were fabricated to investigate the low frequency dielectric properties of STO films. The current-voltage (I-V) characteristics indicated that a Shottcky barrier exist at the interface between STO films and YBCO films. Dielectric dissipation of STO capacitor showed large frequency dispersion under high electric fields. It was explained by the space charge polarization near the interface.

The crystal perfection improvement of Y1Ba2Cu3O7-x thin films by afterglow plasma process

In this article, we report the growth of purely (001) oriented single crystal Y1Ba2Cu3O7−x thin films with excellent crystal perfection by afterglow plasma sputtering. Because of little energetic ion and electron bombardment, the crystal perfection of Y1Ba2Cu3O7−x thin films was significantly improved. The full width at half-maximum value of the rocking curve around the (005) diffraction peak of the films is 0.12°. The experiments demonstrate that the afterglow plasma process is very appropriate for the preparation of high-quality Y1Ba2Cu3O7−x thin films, which are very sensitive to ion bombardment effects.

Photovoltaic effect of ferroelectric Pb(Zr0.52,Ti0.48)O3 deposited on SrTiO3 buffered n-GaAs by laser molecular beam epitaxy

Ferroelectric Pb(Zr0.52,Ti0.48)O3(PZT) thin film was grown on n-type GaAs (001) substrate with SrTiO3 (STO) buffer layer by laser molecular beam epitaxy (L-MBE). The epitaxial process of the STO was in situ monitored by reflection high-energy electron diffraction (RHEED). The crystallographical growth orientation relationship was revealed to be (002) 〈100〉 PZT//(002) 〈100〉 STO//(001) 〈110〉 GaAs by RHEED and X-ray diffraction (XRD). It was found that a small lattice mismatch between PZT and GaAs with a 45∘ in-plane rotation relationship can be formed by inserting of a buffer layer STO. Besides, the enhanced electrical properties of the heterostructure were obtained with the short-circuit photocurrent increased to 52mA/cm2 and the better power conversation efficiency increased by 20% under AM1.5G (100mW/cm2) illumination. The work could provide a way for the application of this kind of heterostructure with high photocurrent response in optoelectronic thin film devices.

Disk Resonators as Nonlinear Surface Impedance Measurement Tools

The edge-current–free rotational symmetric disk resonator mode, TM010, was used as characterization tool for nonlinear surface impedance measurement of Y1Ba2Cu3O7−x (YBCO) thin films. The microwave surface resistance of high quality epitaxial YBCO thin films as a function of frequencies and rf surface magnetic field Hrf was measured using the circular disk resonator technique. Using this technique the properties degradation of the resonators by edge current crowding effect was avoided. The measured Rs values are divided into three regions. In the low field region (Hrf ≤ Hc1J), Rs is independent of Hrf. In the intermediate Hrf region, Rs can be well fitted to the formula of Rs(Hrf) = a(f) + b(f) Hrf2, and b(f) is proportional to f2. This form of microwave surface resistance quantitatively agrees with the modified coupled-grain model with the idea that the materials are composed of identical superconducting grains coupled together by identical intergranular regions that function as Josephson junctions. The experimental results show that the loss mechanisms are extrinsic properties resulting from defects in the films.

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y1Ba2Cu3O7 − x (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a Tconset of 85 K and a Δ Tc of 5 K. The resistance measurements gave a Tconset of 90 K and a Δ Tc of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.