Y. S. Gou

Preparation and characterization of some tin oxide films

Tin oxide films were successfully prepared by the reactive evaporation method with a d.c. glow discharge of oxygen. The structure and composition of the films were characterized by Mossbauer spectroscopy, X-ray diffraction and scanning electron microscopy. The valency of tin atoms in the films determined by Mossbauer spectroscopy is mostly divalent in the samples prepared without discharge, but mostly tetravalent in the samples prepared with discharge. Measurements of electrical conductivity and Hall mobility were also carried out and the data were found to correlate with the concentration ratio [Sn4+]/[Sn2+] determined from Mossbauer spectroscopy.

Temperature dependence of transport properties of evaporated indium tin oxide films

Abstract Indium tin oxide films with high conductivity and transparency were successfully prepared by evaporating an alloy of indium and tin covered with In 2 O 3 powder. At room temperature the films have high carrier mobilities of about 60 cm 2 V -1 s -1 . Conductivities as high as 5 × 10 3 Ω -1 cm -1 and transmittance values of greater than 90% in the visible region of light were obtained. The carrier mobility was found to be inversely proportional to temperature in the high temperature range and independent of temperature in the low temperature range. The temperature dependence of the carrier conductivity indicated that the carrier excitation energy was less than 8.6 × 10 -6 eV. Mossbauer spectroscopy showed that the tin in all the high quality films was tetravalent.

Monophasic TiO2 films deposited on SrTiO3(100) by pulsed laser ablation

Single phase TiO2 thin films, of either rutile or anatase structure, have been prepared on SrTiO3(STO)(100) substrates by in situ pulsed laser deposition (PLD). Thermodynamically unfavorable, for films deposited on STO(100) substrate directly, pure anatase TiO2(00l) films were formed even when a rutile TiO2(110) substrate was used as a target. On the other hand, pure rutile TiO2(110) films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700u200a°C. The optimized deposition conditions for preparing TiN and TiO2 films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed.

Enhancement of critical current density in direct‐current‐sputtered TlBa2Ca2Cu3O9±δ superconducting thin films

A multistep postannealing scheme has been developed for preparing nearly single phased, c‐axis oriented TlBa2Ca2Cu3Ox (Tl‐1223) superconducting thin films fabricated by the direct‐current‐sputtering process. Films obtained by the present process have shown, for the first time in this system, a critical current density (Jc) above 106 A/cm2 at 77 K with a zero‐resistance transition temperature Tc0≊110 K. The order of magnitude enhancement in Jc is attributed to the improvement of film morphology which, in turn, removed most of the weak links encountered previously.

Optimization of depositing Y1Ba2Cu3O7-δ superconducting thin films by excimer laser ablation with Co2 laser-heated substrates

Abstract The optimum deposition conditions for growing Y 1 Ba 2 Cu 3 O 7 -δ superconducting thin films in situ by using xenon, Nd:YAG, and KrF excimer lasers with CO 2 laser-heated substrates were studied.The dependences of the T co and microstructure of the films on the deposition parameters such as substrate temperature, oxygen partial pressure, laser energy density and repetition rates have been investigated systematically. It was found that both the surface mobility on the substrate and the reaction temperatures for the particulates arriving at the substrates are the key factors for obtaining high-quality films. High-quality Y 1 Ba 2 Cu 3 O 7 -δ thin films with mirror-like surface morphology, T co ≈90K, and nearly perfect c -axis oriented normal to the film surface were routinely grown in situ on SrTiO 3 substrates with the following deposition conditions: substrate temperature at 670°C, oxyegen partial pressure of 0.1 torr, laser energy density of 4J/cm 2 per pulse, and repition rate of 10hz. More significantly, it too only about 3 min to grow a 0.3μm thick film and less than 50 s to down the film to room temperature after deposition by the present process. Subsequent processes, such as slow cooling in oxygen atmosphere or any further heat treatments, were unnecessary for the present process. The unique advantage of the present process further offered us good oppurtunities to delineate the effects of each deposition parameter on the detailed film growth mechanisms without having to deal with the complications introduced by prolonged post-deposition treatments. The details of all the interesting features observed will be presented and their physical implications will be discussed.

Fluctuation-enhanced conductivity in YBa2Cu3Ox (6.5 ≤ x ≤ 6.95) and Tl2Ba2Ca2Cu3O10±δ superconducting thin films

Abstract The fluctuation-induced conductivity (Δ σ ( T )) near T c of a sole YBa 2 Cu 3 O x (YBCO) film with various precisely controlled oxygen contents (6.5≤ x ≤6.95) was studied and compared with those obtained from its Tl-based counterparts. For YBCO films with x >6.7, Δσ( T ) displays a distinct 3D to 2D crossover as the temperature approaches T c . On the other hand, as T → T c , Δ σ ( T ) of both Tl-based and YBCO film with x ≤6.7 exhibits a reversed 2D to 3D crossover. It is suggestive that the coupling between the CuO 2 layers may have changed significantly with reducing oxygen.

Preparation of large area and investigation of initial film growth of YBa2Cu3O7 by scanning pulsed laser deposition

A pulsed laser deposition system with a rotating reflector, which can generate an extended plume with a diameter of about 40 mm, has been proven to be viable in producing large area YBa2Cu3O7 (YBCO) thin films. Films with uniformity of thickness and electrical properties to within a few percents were obtained. By making a further modification on this laser scanning system, YBCO films with various thicknesses can be deposited in situ on several pieces of substrates under essentially identical deposition conditions, and particularly, in a continuous fashion. The atomic force microscopy (AFM) images reveal that films grown on NdGaO3 follow the Stranski–Krastanov (layer then island growth) mode, while films grown on as‐polished MgO and on annealed MgO follow the Volmer–Weber (islands without layer growth) mode and step‐flow mode, respectively.

In situ growth of Y1Ba2Cu3O7−x superconducting thin films using a pulsed neodymium:yttrium aluminum garnet laser with CO2 laser heated substrates

By replacing the resistive heater with a CO2 laser to heat the substrates together with concentered oxygen stream blowing onto the substrate during deposition, high quality Y1Ba2Cu3O7−x thin films with nearly perfect c‐axis orientation and Tc0=87 K were grown in situ without subsequent slow cooling in oxygen atmosphere or any further heat treatment. The rapid quenching, from typically 600u2009°C to room temperature in less than 50 s of the as‐deposited films, excludes the possibility of further oxygen incorporation during the cooling process and indicates that the in situ growth may have completed during deposition under proper deposition conditions.

Transport properties of CrO2 (110) films grown on TiO2 buffered Si substrates by chemical vapor deposition

Epitaxial CrO2 (110)-oriented films were fabricated on Si (100) substrates buffered by rutile TiO2 derived from oxidation of a pulsed-laser-deposited TiN layer. The epitaxial films of CrO2 were prepared by chemical vapor deposition in a two-zone furnace with oxygen flow from a CrO3 precursor. The transport measurements show that the CrO2 films are metallic with a Curie temperature of about 380 K. The temperature dependence of resistivity was best described by a phenomenological expression ρ(T)=ρ0+AT2e(−Δ/T) over the range of 5–350 K with Δ=94u2009K. The magnetic measurements show the in-plane coercive fields are about 30 and 60 Oe at 300 and 5 K, respectively. The temperature dependent spontaneous magnetization follows Bloch’s T3/2 law and the slope suggests a critical wavelength of λΔ∼30.6u2009A beyond which spin-flip scattering becomes important.

Metallic percolation in La0.67Ca0.33MnO3 thin films

Phase separation in La0.67Ca0.33MnO3 thin films was investigated by scanning tunneling microscopy. The correlation between the grain structure and the spatial distribution of the coexisting metallic and insulating phases was evidently established. At temperatures not far below the metal–insulator transition, the spatial variation of the coexisting metallic and insulating phases is susceptible to magnetic field in an irreversible manner. The irreversibility suggests that the metallic percolation paths can be affected randomly by magnetic field. However, the variation becomes insensitive to magnetic field at lower temperatures.