Megumi Nagano

Sputter deposition of YBa2Cu3O7-x thin films with low gas pressure

Thin films of YBa2Cu3O7-x have been prepared by rf-magnetron sputtering using a single oxide target with a low operating gas pressure of 0.5 Pa at which almost all the sputtered atoms reach the substrate without colliding with the gas. In spite of the low oxygen partial pressure of 0.25 Pa, sufficient oxygen was supplied to the film during the deposition to form the superconducting phase on a MgO(100) substrate. The Tc of the as-grown film rose with the increase of the substrate temperature and reached 83 K at 650°C. Thanks to the high reactivity and high kinetic energy of the depositing atoms, superconducting films with excellent crystallinity were obtained over a wide range of the substrate temperature between 525 and 650°C despite the high deposition rate of 2 A/s, and the 123phase was formed even at 500°C.

Low temperature synthesis of YBa sub 2 Cu sub 3 O sub 7 minus x thin films

Thin films of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} (YBCO) have been prepared on MgO substrates at 650{degree}C by rf-planar magnetron sputtering using a single oxide target. Deposition at two different substrate positions were examined: (A) above the inner boundary of the erosion area of a target (position A), (B) right above the center of a target (position B). The films deposited at position A suffered a serious deficiency in the contents of Ba and Cu, especially in the low gas pressure region due to the bombardment of the growing films by high-energy particles. Though we could obtain almost stoichiometric films by raising the gas pressure to 15 Pa to reduce the bombardment, high temperature annealing at 950{degree}C was required to improve their crystallinity and to achieve high critical temperatures ({ital T}{sub {ital c}}). On the other hand, those deposited at position B were more excellent in crystallinity than those at position A showed high {ital T}{sub {ital c}} above 80 K without any annealing. Critical current density ({ital J}{sub {ital c}}) at 77 K was improved from about 10{sup 3} A/cm{sup 2} to about 10{sup 4}/cm{sup 2} by annealing at low temperature of 500{degree}C. It is concluded that the depositionmorexa0» with reduced bombardment by high-energy particles enables the fabrication of high {ital T}{sub {ital c}} YBCO films without a high temperature process.«xa0lessThin films of YBa 2 Cu 3 O 7− x (YBCO) have been prepared on MgO substrates at 650 °C by rf-planar magnetron sputtering using a single oxide target. Depositions at two different substrate positions were examined: (A) above the inner boundary of the erosion area of a target (position A), (B) right above the center of a target (position B). The films deposited at position A suffered a serious deficiency in the contents of Ba and Cu, especially in the low gas pressure region due to the bombardment of the growing films by high-energy particles. Though we could obtain almost stoichiometric films by raising the gas pressure to 15 Pa to reduce the bombardment, high temperature annealing at 950 °C was required to improve their crystallinity and to achieve high critical temperatures ( T c ). On the other hand, those deposited at position B were more excellent in crystallinity than those at position A and showed high T c above 80 K without any annealing. Critical current density C/r) at 77 K was improved from about 10 3 A/cm 2 to about 10 4 A/cm 2 by annealing at low temperature of 500 °C. It is concluded that the deposition with reduced bombardment by high-energy particles enables the fabrication of high T c YBCO films without a high temperature process.

Morphology of barrier layer formed on YBCO

The morphology of barrier layers, such as LaSrGaO/sub 4/ (LSGO) and SrTiO/sub 3/ (STO), was investigated using an atomic force microscope (AFM) and tunnel junctions with an Au/barrier/YBCO layered structure. The barrier morphology depended strongly on both the surface roughness of YBCO films and the deposition temperature of the barrier materials. It was improved by reducing the surface roughness of the YBCO films and keeping the deposition temperature lower than 400 degrees C during the barrier formation. The experiments performed are described, and the results are presented and discussed.<<ETX>>

Relationship between Surface Roughness and Barrier Uniformity

The relationship between the surface roughness of YBa2Cu3Ox (YBCO) films and the uniformity of a barrier layer was studied using atomic force microscope (AFM) and tunnel junctions with an Au-LaSrGaO4 (LSGO)-YBCO layered structure. AFM images showed that the surface roughness of both (001)- and (013)-oriented films depended strongly on their thickness, and that roughness decreased with a reduction in thickness. When an LSGO barrier was formed on these YBCO films, barrier height increased from 24 mV to 1.3 V with reduction in the thickness of the YBCO film from 400 nm to 70 nm. This suggests that barrier uniformity was improved by reducing the roughness of the YBCO films.

Superconducting YBCO Thin Films with (013) and (103) Orientation

We studied the YBCO thin films without c-axis orientation. (013) oriented films were obtained by a sputtering at a low gas pressure. On the other hand, the film deposited by activated reactive deposition showed (103) orientation. The surface of sputtered film was smooth enough to fabricate a Josephson junction, but stripe-like cracks were often found in the films with thickness of more than 1500 A. We assumed these cracks were caused by the large change in a-axis lattice constant associated with the phase transition during cooling.

Fabrication of the superconducting YBa2Cu3Oy thin films

We studied two synthetic processes of YBa2Cu3Oy thin films using rf-planar magnetron sputtering. (a)deposition at a low temperature followed by a post-annealing at around 900°C. (b)deposition at 650°C. We obtained the high Tc superconducting thin films in both processes. Higher Jc was obtained by deposition at 650°C than that at a low temperature.

Properties of Junctions with Layered Structure

The relation between surface roughness of the YBCO films and uniformity of a barrier layer was studied using AFM and tunnel junctions with Au-LSGO-YBCO layered structure. AFM images showed that the surface roughness of both (001) and (013) oriented films had strong dependence on their thickness, and the roughness decreased with reducing the thickness. When the LSGO barrier was formed on these YBCO films, the barrier height increased from 24 mV to 1.3 V with reducing the thickness of the YBCO from 400 nm to 70 nm. It suggests that the uniformity of barrier layer was improved by reducing the roughness of the YBCO films.

Observation of oxygen in YBCO thin films during the cooling down process using 18O tracer

Abstract We studied the behavior of the oxygen in the YBa 2 Cu 3 O x films using 18 O as a tracer. Pure 18 O 2 gas was introduced into a chamber during deposition. Then it was changed for 16 O 2 gas in the subsequent cooling down process. Though YBCO films absolutely contained only 18 O atoms in their oxygen sites during growth process, almost all 18 O atoms were replaced with 16 O atoms while the films were cooled down; 95% of 18 O atoms were substituted using rf-plasma, and 85% of 18 O atoms without the plasma. It shows that rf-plasma strongly affected the oxygenation of the YBCO films even in the cooling process.

Properties of YBCO films prepared by pure argon sputtering

Abstract Behavior of oxygen during the deposition process was investigated by an rf magnetron sputtering method with pure Ar gas. Although pure Ar gas was used as a sputtering gas, Y1Ba2Cu3Ox(YBCO) films deposited in the pressure region from 0.2 to 2Pa showed superconductivity. It suggests that sufficient oxygen to form the superconducting structure at a low gas pressure around 1Pa was supplied from the target.

Properties of Sputter-Deposited YBa2Cu3O7−x Thin Films

Properties of sputter-deposited YBa2Cu3O7−x thin films are discussed in terms of the kinetic energy of the depositing atoms. When the sputtered atoms were thermalized before reaching the substrate, the superconducting phase was formed only at the substrate temperature above 650 °C. In contrast, when the sputtered atoms traveled straight to the substrate and arrived there with most of their initial energies, the superconducting phase was formed even at 500°C due to the enhanced migration of the deposited atoms on the substrate. The Tc of the as-deposited film increased with the increase of the substrate temperature and reached 83K at the substrate temperature of 650°C.