YBa2Cu3O7−x (YBCO) superconducting thin films that show no x-ray diffraction peaks due to any other non-superconducting phases have been synthesized by annealing Y-Ba-Cu-O amorphous precursors at 750 °C. The Y-Ba-Cu-O precursors have been fabricated by oxidizing Y-Ba-Cu metallic precursors coevaporated from Y, Ba, and Cu metallic sources under ultrahigh vacuum conditions. Crystallization behavior from the Y-Ba-Cu-O precursor to YBCO films drastically depends on an oxidation temperature for the Y-Ba-Cu metallic precursor. YBCO thin film synthesized from the precursor oxidized at an optimum temperature shows a zero resistance temperature of over 80 K and a very smooth surface.

We have studied the effects of the kinetic energy of mass-separated O+ beams on the growth of CuO thin films deposited on unheated MgO(001) and at 510 °C, in energy ranging from 10 to 200 eV by x-ray photoemission spectroscopy, x-ray diffraction, reflection high-energy electron diffraction, and scanning electron microscopy. The films deposited at 510 °C show a full width at the half maximum (FWHM) of 0.06°for the rocking curve through the (111) peak, regardless of the kinetic energy of oxygen ions. CuO has been epitaxially grown on MgO(001) without heating it in a vacuum of 4 × 10−7 Pa. The x-ray diffraction intensity of the CuO(111) increases with an increase in the kinetic energy of O+, and its FWHM approaches that of the CuO film grown at 510 °C. The surface morphology is improved for the films deposited on unheated substrates.

Growth behavior of YBa2Cu3O7-x (YBCO) thin films from metallic Y–Ba–Cu precursors was investigated. The precursors were prepared by coevaporation of the constituent metallic sources under ultra-high vacuum conditions, and they were oxidized in a load lock chamber and a tube furnace. The grain size of YBCO and degree of crystallographic orientation of the films increased with decreasing oxygen partial pressure p(O2) during transformation of the precursor into the crystal close to the thermodynamic stability line of YBa2Cu3O6.0 in a p(O2) – 1/T phase diagram. The growth of YBCO crystal is controlled by the oxygen diffusion along the c-axis direction of the crystal. The diffusion coefficient of oxygen is determined as D = 1.94 × 1010 exp(−433 000/RT) (cm2/s).

Resistivity measurement of an a-axis-oriented YBa2Cu3O7-δ (YBCO) thin film was carried out in a stream of O2/Ar mixtures at 450–650 °C. Rapidly reversible change in the resistivity of the YBCO film was observed as a function of the oxygen partial pressure in the ambient atmosphere due to the oxygen in-/out-diffusion in the YBCO film. Resistivity measurement of YBCO thin films at higher temperatures of 450–650 °C was found to be quite effective to predict the influence of fabricating processes on the superconducting properties of YBCO films.

The growth of BaO and SrO on SrTiO3(100) substrates using mass-separated low-energy (50 eV) O+ beams has been studied using x-ray diffraction, reflection high-energy electron diffraction, and high-resolution transmission electron microscopy. It was found that the BaO and SrO films have been epitaxially grown with new structures different from those of corresponding bulk crystals: The BaO films have a cubic structure with a lattice constant of 4.0 Å, and the SrO films have a tetragonal structure with a lattice constant of a = 3.7 Å parallel to the substrate and with c = 4.0 Å normal to the substrate.

The interface between an a-axis oriented YBa2Cu3O7-x film and a NdGaO3(110) substrate has been investigated by cross-section transmission electron microscopy (TEM). The orientational relationship between the a-axis oriented film and substrate is YBa2Cu3O7-x[001] / NdGaO3[001]. This preferentially c-axis aligned direction of the YBa2Cu3O7-x film would be caused by a very small lattice mismatch (0.1%) between b(=a) lattice constant of YBa2Cu3O7-x and of the pseudo-cubic sub-lattices in NdGaO3 at a substrate temperature of 750°C. Two kinds of imperfections have been observed in the crystal lattice of YBa2Cu3O7-x near the interface; One is the deviation of YBa2Cu3O7-x [301] from NdGaO3 [111]. The other type is pair dislocations with a positive and negative Burgers vectors in the YBa2Cu3O7-x (103) planes. These two kinds of defects at the interface would be act to reduce the tensile stress within a distance of about l.lnm from the substrate interface.