D. N. Zheng

Frequency and temperature dependence of tunable dielectric properties of Ba(Zr0.2Ti0.8)O3 thin films grown on (001) MgO

High quality Ba(Zr0.2Ti0.8)O3 (BZT) thin films were epitaxially grown on MgO single-crystal substrates. No structural phase transition took place in the temperature range of 80–300K, and the films displayed a typical behavior of relaxor ferroelectrics, which was demonstrated by micro-Raman spectroscopy. Frequency and temperature dependence of dielectric properties was systematically investigated for the BZT thin films. It was found that enhanced dielectric properties could be obtained over a wide frequency and temperature range. This is of great importance for practical microelectronic device applications. At room temperature, a high dielectric tunability of nearly 50% was achieved over the whole measurement frequency range of 10kHz–1MHz by applying an electric field with intensity as low as 40kV∕cm; meanwhile, a relatively low dielectric loss was obtained and the value could be further reduced significantly with the applied dc electric field, indicating an excellent figure of merit and promising applicat...

Magnetotransport and rectifying properties in La0.67Ca0.33MnO3/yttrium-stabilized zirconia/Si heterojunction

A heterojunction has been fabricated by growing a La0.67Ca0.33MnO3 film on silicon with a buffer layer of yttrium-stabilized zirconia (YSZ). The current-voltage measurement shows that it is a diode with a good rectifying property. At low positive bias voltage, temperature dependence of the junction resistance shows a peak at a certain temperature, which shifts to low temperatures when the voltage is increased from 0.3Vto0.7V. This behavior is quite different from the previous reports on p-n junctions composed of manganites and Nb-doped SrTiO3. The heterojunction shows remarkable magnetoresistance for both positive and negative biases. The results were discussed by considering the depletion layers in both La0.67Ca0.33MnO3 and Si, and the tunneling through YSZ. This work shows the potential application of integrating manganite-based devices and semiconductor circuits.

Resistive switching properties and low resistance state relaxation in Al/Pr0.7Ca0.3MnO3/Pt junctions

Metal/insulator/metal structures composed of active Al top electrodes (TEs) and oxygen-deficient Pr0.7Ca0.3MnO3 (PCMO) insulator layers are prepared on platinized silicon substrates. The junction resistance exhibits an obvious negative differential resistance region in the first bias sweep and an irreversible increase from 2 to 100 MΩ in repeated ±4 V sweeps. The pulse duration needed to fully switch the junctions is found to be on the order of milliseconds. When 100–500 µs negative pulses are used, the junctions show an incomplete switch to the low resistance state (LRS) which exhibits fluctuating resistances. The fluctuation in the LRS is suppressed and the high-to-low resistance ratio increases gradually when the negative pulse duration is increased from 100 to 500 µs. For relaxed junctions, pulse switching experiments reveal that the LRS undergoes a dynamically stable process at the beginning and then reaches a lower and metastable resistance value. Resistance retention tests also indicate that the high resistance state is very stable, while the metastable LRS gradually relaxes to higher resistance values. The experimental results are discussed with the formation and dissociation of an interfacial AlOx layer at the interface between Al TEs and PCMO layers.

Quantum phase diffusion in a small underdamped Josephson junction.

Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (∼0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.

Superconducting properties and c-axis superstructure of Mg1xAlxB2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with a significant change in interboron layer distance as reported previously by Slusky Moreover, transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is probably related to the structural transition. The modifications of the superconducting transition temperature T-c, the normal-state resistivity, and the upper critical field B-c2(0) by Al doping are discussed in terms of Al-substitution-induced changes in the electronic structure at the Fermi energy.

Influence of oxygen pressure on the structural and dielectric properties of laser-ablated Ba0.5Sr0.5TiO3 thin films epitaxially grown on (001) MgO for microwave phase shifters

Highly oriented Ba0.5Sr0.5TiO3 (BST) thin films were grown on MgO (001) single-crystal substrates by pulsed-laser deposition at 800 degrees C in oxygen pressure ranging from 1.2 x 10(-3) to 40 Pa. A strong correlation was observed between the growth process, structure and dielectric properties for the BST films. The dielectric properties in the low frequency range 10k-1MHz were measured in the interdigital capacitor configuration. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), surface morphology and dielectric response of the films are strongly dependent on the oxygen deposition pressure. With increase in oxygen pressure, the in-plane strain for the BST films changes from compressive to tensile. The BST film grown at 25 Pa exhibits the best overall dielectric properties. This corresponds to the film with a slight in-plane tensile strain (D = 1.0012). It is believed that a reasonable tensile strain, which increases the ionic displacement and thus promotes the in-plane polarization in the field direction, could enhance the tunability and dielectric constant. Based on the BST film grown at 25 Pa, distributed phase shifters were successfully fabricated with promising performance. The phase shifter shows a relatively low insertion loss of about 3.5 dB at zero bias and 10 GHz, a good return loss better than -15 dB for all phase states from dc to 16 GHz and a differential phase shift of about 43 degrees with 120V dc bias at 10 GHz.

The origin of the weak ferroelectric-like hysteresis effect in paraelectric Ba0.5Sr0.5TiO3 thin films grown epitaxially on LaAlO3

Perovskite Ba0.5Sr0.5TiO3 (BST) thin films, with thickness of about 350 nm, have been epitaxially grown on (001) LaAlO3 substrates by pulsed-laser deposition. The good crystallography and epitaxy characteristics were confirmed using x-ray diffraction and transmission electron microscopy (TEM). The dielectric properties of the BST thin films were measured with a planar capacitor configuration in the temperature range of 77-300 K. The capacitance temperature characteristics, measured with no and several different DC biases, reveal that the BST films are in the paraelectric state at room temperature, with a very good dielectric tunability. However, a butterfly-shaped curve, typical for a ferroelectric material, was obtained from room temperature capacitance-voltage (C-V) measurements, suggesting a faint ferroelectric-like effect for the BST thin films. Careful dielectric property characterizations showed significant temperature and frequency dependence, probably indicating a behaviour of Maxwell-Wagner-type dielectric relaxation. As a result, the weak C-V hysteresis effect in our paraelectric BST thin films is believed to be ascribable both to oxygen vacancies and to the presence of other space charges, trapped at the grain boundaries and/or at the substrate/dielectric film interface, which give rise to local polar regions in the thin-film samples. Furthermore, this explanation is supported by cross-sectional TEM and off-axis electron holographic observations.

Large low-field magnetoresistance observed in twinned La2∕3Ca1∕3MnO3 thin films epitaxially grown on yttria-stabilized zirconia-buffered silicon on insulator substrates

La2∕3Ca1∕3MnO3 thin films have been grown on yttria-stabilized zirconia (YSZ) buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved for the YSZ layer, the top manganite layer was multioriented in plane, with a coexistence of cube-on-cube and cube-on-diagonal epitaxial structures. Due to a combined influence from the magnetocrystalline, shape, and stain-induced magnetic anisotropy, in zero field and low temperatures the local spin orientation varies across the large-angle grain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunneling was observed. The film shows a resistance change of ∼20% in a magnetic field <1000Oe at 50K, which is promising for real applications.

The rectifying property and magnetoresistance of La0.67Ca0.33MnO3∕SiO2∕Si heterojunction

We have fabricated a heterojunction by depositing La0.67Ca0.33MnO3 film on electron doped silicon wafer with a buffer layer of natural SiO2. The current-voltage measurement shows that it is a diode with a good rectifying property in a wide temperature range. At high positive voltages, the current-voltage curve shows space charge limited (SCL) current behavior, manifesting itself as a SCL diode. At low positive and negative voltages, the resistance of the junction shows a peak at a certain temperature, which decreases with increasing positive voltage and keeps constant at negative voltages. Magnetoresistance of the junction shows a similar temperature dependence as the resistance of the junction. Calculations show that these phenomena can be attributed to the depletion layer of the La0.67Ca0.33MnO3 film. This work also demonstrates that SCL diode can be realized in La0.67Ca0.33MnO3∕SiO2∕Si with the presence of the SiO2 layer.

A cryogen-free dilution refrigerator based Josephson qubit measurement system

We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about whether the noise introduced by the coldhead can be made sufficiently low. In this work, we demonstrate some effective approaches of acoustic isolation to reduce the noise impact. The electronic circuit that includes the current, voltage, and microwave lines for qubit coherent state measurement is described. For the current and voltage lines designed to have a low pass of dc-100 kHz, we show that the measurements of Josephson junctions switching current distribution with a width down to 1 nA, and quantum coherent Rabi oscillation and Ramsey interference of the superconducting qubit can be successfully performed.