Meiya Li

Influences of substrates and substrate temperatures on characteristics of epitaxial La0.5Sr0.5CoO3 thin films

Abstract Epitaxial growth of La 0.5 Sr 0.5 CoO 3 (LSCO) film has been achieved by pulsed laser deposition. Widely used substrates, MgO, SrTiO 3 and LaAlO 3 were chosen to examine the effect of the substrates on the growth of LSCO films. The influence of the substrate temperatures ( T s ) on the characteristics of the LSCO films was studied in a temperature range of 500–850°C. The LSCO films deposited at substrate temperatures of T s =800–850°C show good epitaxial features, better than that of the films deposited at lower T s . The experimental results reveal that the lattice mismatch and the difference of the expansion coefficients between the substrates and the LSCO films are important in the growth process of the LSCO films.

Structural characteristics and the control of crystallographic orientation of CeO2 thin films prepared by laser ablation

Abstract CeO2 thin films were grown on (1xa00xa00) silicon substrates by pulsed laser deposition. X-ray diffraction results indicated that fully (1xa01xa01) oriented and highly (1xa00xa00) oriented CeO2 films on Si (1xa00xa00) were achieved, respectively. Oxygen pressure was found to be important to control the crystallographic orientation. A possible mechanism was proposed to explain these results. Auger electron spectroscopy and Rutherford backscattering spectrometry analysis showed an uniform concentration distribution of Ce and O throughout the epitaxial layer independent of the oxygen pressure. The electrical properties of the films were characterized by capacitance–voltage measurements.

Epitaxial growth characteristics of oxide thin films prepared by pulsed laser deposition

Abstract Some unique characteristics of epitaxial growth of oxide thin films prepared by pulsed laser deposition have been observed in this study. The study demonstrates that deposition temperature is one of the most important parameters in controlling orientation and epitaxial growth of oxide thin films. Higher temperature is usually imperative for the epitaxial growth. The threshold temperature for epitaxial growth of oxide film depends on the substrate materials, which relate mainly to the lattice mismatch and component similarity between the film and the substrate. A small lattice mismatch and a high component similarity are favorable to epitaxial growth and may decrease the epitaxial threshold temperature significantly. In addition, the orientation of CeO 2 film, grown on Si substrate, possess a unique dependence on ambient oxygen pressure, which demonstrates the significant influence of oxygen pressure to the epitaxial orientation of oxide thin films.

Growth of ferroelectric (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 thin films by fulsed laser deposition

Abstract Growth of (K 0.5 Na 0.5 ) 0.2 (Sr 0.75 Ba 0.25 ) 0.9 Nb 2 O 6 (KNSBN) films on (0 0 1) MgO substrates and on (0 0 1) oriented La 0.5 Sr 0.5 CoO 3 (LSCO) layers by pulsed laser deposition has been studied. X-ray diffraction θ–2θ scans indicated that the KNSBN films were of c -axis orientation. X-ray Phi scans on the (2 2 1) reflection revealed that the films grown on the MgO substrates were epitaxial growth with two [1 0 0]KNSBN in-plane orientation rotating about ±18° with respect to [1 0 0]MgO. Sandwich structures of KNSBN films with LSCO as top and bottom electrodes have been performed on (0 0 1)LaAlO 3 (LAO) substrates. The measurements of the ferroelectricity of the as-grown film gave hysterisis loop with the remnant polarization of P r ∼ 8 μ C cm −2 and the coercive field of E c ∼ 55 KV cm −1 .

Epitaxial growth and electrical transport properties of La0.5Sr0.5Co03 thin films prepared by pulsed laser deposition

Epitaxial growth of the La0.5Sr0.53(LO) thin films has been realized on Lin3, SrTiC3 and MgO substrates by pulsed laser deposition. The epitaxial growth behavior and the electrical transport properties of these films were studied systematically. The temperature dependencies of the resistivity of the film have been determined. Studies indicate that close dependencies exist between the crystal structures and the electrical transport properties of the epitaxial LSCO films, and that the epitaxial thin films are of low resistivity and metallic conductive features. The epitaxial films deposited on the LaA103 substrates at about 700 °C possess the optimal properties compared with the others. Discussions of the dependencies and the mechanisms of the epitaxial structures on the electrical transport properties of the LSCO films have been made.

Anomalous Resistance Behavior Observed Across the Interface of Superconducting YBa2Cu3O7 and Ferromagnetic La0.7Ca0.3MnO3

Abstract Temperature dependencies of the resistance across the interface of superconducting YBa2Cu3O7 and ferromagnetic La0.7Ca0.3MnO3 were investigated in La0.7Ca0.3MnO3/YBa2Cu3O7 double-layer and cross-strip samples. Anomalous resistance behavior, a relatively large resistance value with a superconducting-like transition, was observed in both samples. In the cross-strip sample the voltage signal changed to extremely large negative values in the temperature region of 170–90xa0K, which was corresponding to the resistance peak in the LCMO strip. A possible explanation is discussed.

Large low-field magnetoresistance observed in heterostructure multilayers

The behaviour of resistivity versus temperature in a series of (LCMO)/(GCMO) multilayers has been studied. The resistivity peak temperature of the multilayers decreases with decreasing thickness of the LCMO layers. Large low-field magnetoresistance (LFMR) of % is obtained at 78.5 K in heterostructure LCMO/GCMO multilayers when a magnetic field is swept within Oe. This result suggests that the interface strain between LCMO and GCMO layers may influence the magnetic transport properties and a suitable film structure could be used to increase the LFMR.

EFFECTS OF MEV SI ION IRRADIATION ON THE PROPERTIES OF SHALLOW P + N JUNCTIONS

Abstract The effects of post-ion implantation on the properties of Pxa0+xa0N junction formed by BF 2 ion implantation in SiO 2 /Sixa0(1xa00xa00) have been studied in this paper. As for the post-irradiation using MeV Si ions, there is a critical dose for Si ions. When the dose is less than the critical dose, post MeV Si ion irradiation induces reduction of the anomalous diffusion of B atoms. However, when the dose is higher than the critical dose, MeV Si ion irradiation induces further enhancement of the anomalous diffusion of B atoms. As for the electrical properties of the Pxa0+xa0N junction, post-ion irradiation using 1.0 MeV 5xa0×xa010 14 Si/cm 2 reduced the density of the leakage currents by three orders of magnitude. The post-Si ion irradiation either by MeV ions or 70 keV ions leads to a reduction of the breakdown voltage of the junction.

Growth dependencies of epitaxial oxide thin films prepared by pulsed laser deposition

Abstract Based on the achievement of epitaxial growth in superconducting YBa 2 Cu 3 O 7−δ , ferromagnetic Nd 0.7 Sr 0.3 MnO 3 and conductive La 0.5 Sr 0.5 CoO 3 films, we discuss influences of substrate temperature ( T s ) and substrate material in pulsed laser deposition. Influences of T s on growth orientation and oxygen content of as-grown films, and thus on transport properties were observed. Lattice mismatch between substrates and films has a direct effect on the epitaxial growth. Surface morphology of substrates should be considered in preparation of superlattices and multilayer devices, since high-resolution-transmission-electron-microscopy revealed that a one step defect on a SrTiO 3 substrate surface developed into dislocations in a YBCO film. Additionally, undesired damage caused by differences in thermal expansion between substrates and films can be avoided by lowering cooling rates.

Large Low-Field Magnetoresistance Observed in Polycrystalline La0.7Ca0.3MnO3 Films

Large low-field magnetoresistance (LFMR) of Δρ/ρH = 40% is obtained at 163 K within ±3000 Oe in a polycrystalline La0.7Ca0.3MnO3 (LCMO) films prepared by pulsed laser deposition. The ratio Δρ/ρH increases almost linearly with H in the measuring magnetic field range and the peak of LFMR ratios is observed near the resistivity peak temperature, which is different from the observation of spin-polarized intergrain tunnels. This experiment suggests that grain boundaries have large influences on the LFMR. The results indicate that a suitable film process can be used to increase the LFMR of LCMO films.