B. H. Park

Effects of very thin strain layers on dielectric properties of epitaxial Ba0.6Sr0.4TiO3 films

We have epitaxially grown Ba0.6Sr0.4TiO3 (BST-0.4) thin films on MgO(001) substrates. By inserting a very thin Ba1−xSrxTiO3 (x=0.1–0.7) interlayer between the MgO substrate and the main layer of BST-0.4, we are able to manipulate the degree of the stress in BST-0.4 films. We have controlled the stress states, i.e., the lattice distortion ratio (D=in-plane lattice constant/out-of-plane lattice constant) of the BST-0.4 films by varying the chemical composition of the interlayers. We have found that small variations of D value can result in significantly large changes of dielectric properties. A BST-0.4 film under small tensile stress, which has a D value of 1.0023, shows the largest dielectric permittivity and tunability.

Microstructure and dielectric properties of Ba1−xSrxTiO3 films grown on LaAlO3 substrates

We report a systematic study of the microstructure and dielectric properties of barium strontium titanate, Ba1−xSrxTiO3, films grown by laser ablation on LaAlO3 substrates, where x=0.1–0.9 at an interval of 0.1. X-ray diffraction analysis shows that when x 0.4, compared with the peak temperatures of the bulk Ba1−xSrxTiO3. At room temperature, the dielectric constant and tunability are relatively high when x⩽0.4 but start to decrease rapidly as x increases.

High nonlinearity of Ba0.6Sr0.4TiO3 films heteroepitaxially grown on MgO substrates

We have heteroepitaxially deposited Ba0.6Sr0.4TiO3 (BST) thin films on (001)-oriented MgO substrates using pulsed-laser deposition. By optimizing the deposition temperature and adjusting the film thickness, we have successfully increased the dielectric nonlinearity and decreased the dielectric loss of BST films. BST thin films grown at 750 °C with a thickness of 1.1 μm showed a dielectric constant tunability of greater than 65% and a tunability/loss of 43 at a surface electric field of 80 kV/cm at room temperature. X-ray diffraction and transmission electron microscopy analyses indicated that the tunability and dielectric loss were closely related to the crystallinity of the BST films.

Photovoltaic response and dielectric properties of epitaxial anatase-TiO2 films grown on conductive La0.5Sr0.5CoO3 electrodes

We have grown epitaxial anatase-TiO2 (001) films on La0.5Sr0.5CoO3 (001) bottom electrodes using pulsed-laser deposition. The small lattice mismatch (0.5%) between the anatase-TiO2 and the La0.5Sr0.5CoO3 makes it possible to grow anatase-TiO2 films with excellent crystallinity on conductive metal oxides. The photovoltaic properties of the epitaxial anatase-TiO2 on the La0.5Sr0.5CoO3 were characterized using a Kelvin probe. The optical band-gap energy was found to be 3.05 eV. The dielectric properties of the epitaxial anatase-TiO2 films were characterized using a capacitor structure of Au/anatase-TiO2/La0.5Sr0.5CoO3 on a LaAlO3 substrate. The dielectric dispersion exhibited a power-law dependence, and the dielectric constant measured at room temperature and 1 MHz was 38.

Role of atomic arrangements at interfaces on the phase control of epitaxial TiO2 films

Epitaxial rutile-TiO2 and anatase-TiO2 films were grown at 800 °C on Al2O3(1102) and LaAlO3(001), respectively, using pulsed laser deposition. Both films showed high crystalline quality, evidenced by x-ray diffraction and high-resolution electron microscopy. The formation of different phases on different substrates could be qualitatively explained by the atomic arrangements at the interfaces. We also deposited epitaxial rutile-TiO2 and anatase-TiO2 films on conductive RuO2 and La0.5Sr0.5CoO3 electrodes, respectively. Using a Kelvin probe, we measured the photovoltaic properties of these multilayer structures. A rutile-TiO2 film grown on RuO2 showed a very broad peak in the visible light region. An epitaxial anatase-TiO2 film grown on La0.5Sr0.5CoO3 showed a strong peak with a threshold energy of 3.05 eV.Epitaxial rutile-TiO2 and anatase-TiO2 films were grown at 800 °C on Al2O3(1102) and LaAlO3(001), respectively, using pulsed laser deposition. Both films showed high crystalline quality, evidenced by x-ray diffraction and high-resolution electron microscopy. The formation of different phases on different substrates could be qualitatively explained by the atomic arrangements at the interfaces. We also deposited epitaxial rutile-TiO2 and anatase-TiO2 films on conductive RuO2 and La0.5Sr0.5CoO3 electrodes, respectively. Using a Kelvin probe, we measured the photovoltaic properties of these multilayer structures. A rutile-TiO2 film grown on RuO2 showed a very broad peak in the visible light region. An epitaxial anatase-TiO2 film grown on La0.5Sr0.5CoO3 showed a strong peak with a threshold energy of 3.05 eV.

Dielectric response and structural properties of TiO2-doped Ba0.6Sr0.4TiO3 films

Dielectric Ba0.6Sr0.4TiO3 films doped with different weight percentages of TiO2 were deposited by pulsed-laser deposition. The dielectric constant, dielectric loss, and the dielectric tunability of the films were found to be a strong function of the weight ratio of TiO2/Ba0.6Sr0.4TiO3. Compared to the pure Ba0.6Sr0.4TiO3, the TiO2-doped Ba0.6Sr0.4TiO3 exhibits lower dielectric loss while maintaining a significantly adjustable dielectric constant and desirable capacitance tunability. The change of the dielectric properties of TiO2-doped Ba0.6Sr0.4TiO3 films is closely related to the change in microstructure of the films as revealed by transmission electron microscopy.

Imaging transport current distribution in high temperature superconductors using room temperature scanning laser microscope

We report the feasibility of room temperature scanning laser microscopy (RTSLM) for the study of high temperature superconducting films. RTSLM images from SmBa2Cu3O7 and YBa2Cu3O7 thin films show that the ac voltage response exists only in the section of the bridge where the transport current produces a voltage drop. A photolithographically defined 60 μm×60 μm void in a 300-μm-wide bridge was clearly visible in a RTSLM image giving the spatial resolution smaller than 60 μm. In addition, the void disturbs the transport current distribution beyond itself generating an elongated shape void of 64 μm×85 μm with the longer side along the direction of current flow in the RTSLM image. Our results indicate that the RTSLM is a useful tool for investigating the transport current distribution in high temperature superconductors.

Structure, processing, and property relationships in tunable rf and microwave devices

Abstract Our work on tunable rf and microwave devices based on 90° off-axis magnetron sputtered Ba0.6Sr0.4TiO3 epitaxial thin films is reviewed. In particular, we have studied the effects of energetic bombardment during deposition on the structural and electrical properties of these films. The fundamental result of changing the energetic bombardment conditions during deposition is to affect the stress/strain state of the films. We show that high bombardment conditions result in films with significantly extended out-of-plane lattice constants and depressed dielectric constants and tunabilities.

Spatial Distribution Analyses of Superconducting Transition Temperature in Epitaxial YBa 2 Cu 3 O 7 Film Using Variable Temperature Scanning Laser Microscopy

The spatial distribution of superconducting properties using variable temperature scanning laser microscope (VTSLM) has been investigated. The superconducting thin film used in this study is an epitaxial YBa 2 Cu 3 O 7 film photolithographically patterned to a 300 μm-wide bridge. Since the voltage response, δV(x,y) is proportional to dR/dT(x,y), the spatial distribution of superconducting transition can be obtained in VTSLM images. In the resistive transition region, there is a strong correlation between the VTSLM images and the resistance of the sample. With decreasing resistance, the area with large δV(x,y) shifts toward the ends of the bridge. This indicates that the resistive transition is not uniform and both ends of the bridge have lower transition temperature, Tc. Different currents or different output power of lasers do not affect the images. VTSLM technique is a powerful tool to image the local superconducting properties and to identify the weaker superconducting areas.

Dielectric properties of Ba0.6Sr0.4TiO3 thin films with various strain states

Abstract We deposited epitaxial Ba0.6Sr0.4TiO3(BST) films having thickness of 400 nm on MgO(001) substrates, where a 10 nm thick Ba1−xSr x TiO3 (x = 0.1 – 0.7) interlayer was inserted between BST and MgO to manipulate the stress of the BST films. Since the main difference of those epitaxial BST films was the lattice constant of the interlayers, we were very successful in controlling the stress of the BST films. BST films under small tensile stress showed larger dielectric constant than that without stress as well as those under compressive stress. Stress relaxation was investigated using epitaxial BST films with various thicknesses grown on different interlayers. For BST films grown on Ba0.7Sr0.3TiO3 interlayers, the critical thickness was about 600 nm. On the other hand, the critical thickness of single-layer BST film was less than 100 nm.