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

PHYSICA

rhysica C 185-189 (1991) 2105-2106 North-Holland

Observation of oxygen in YBCO thin films during the cooling down process using lsO tracer K.Tsuda, T.Suzuki, T.Matsui, H.Kimura, M.Nagano and ICMukae Fuji Electric Corporate Research and Development, Ltd., 2-2-1, Nagasaka, Yokosuka, Kanagawa, 240-01 Japan We studied the behavior of the oxygen in the YBa2Cu3Ox films using ~aO as a tracer. Pure ~eO2 gas was •!ntroduced "intoa chamber during" depos~ion: Then it was. changed,efor 1602 gas in the subsequent cool=ng oown process, t nough YBCO films aosolutely contained only O atoms in their oxygen sites during growth orocess, almost all leO atoms were replaced with ~sO atoms while the films were cooled down; 95% of ~80 atoms were substituted using if-plasma, and 85% of ~eO atoms without the plasma. It shows that rf-plasma strongly affected the oxygenation of the YBCO films even in the cooling process. 1. Introduction An oxygen plasma is generally used to make the superconducting YBa2Cu3Ox (YBCO) in situ films by coevaporation 1. Few papers, however, showed the role of the plasma during fabrication process of the films, except C.B.Eom et al.'s work 2. Thus, we started to study the behavior of the oxygen in the YBCO films using lad as a tracer. 2. Experiment The films were fabricated on (100)SrTiO3 substrates kept at 600°C Dy coevaporation with the oxygen gas pressure of 1.7x102pa. To study the behavior of oxygen, pure laO2 (>98.7%) gas was introduced into a chamber during deposition, and theq was changed for 1aO2gas immediately before the films were cooled. The oxygen gas pressure during the cooling down was also kept at 1.7xl0aPa. Being cooled below 150°C, the films were removed from the chamber. To obtain the activated oxygen gas, if-plasma was introduced between metal sources and the substrates through the fabrication process. The film was also prepared without the plasma in the cooling down process, so as to clarify the effect of the plasma. Depth profiles of 180 and 160 concentration in the films were studied by secondary ion mass spectroscopy (SIMS). Resistivity vs. temperature characteristics were tested using the conventional four probe method. Structural information was given by X-ray

diffraction (XRD). The surface morphology was observed by scanning electron microscope (SE:.,).

3. Results and Discussion Figure 1 shows that the resistivity vs. temperature characteristics of the films depend on the presence of the oxygen plasma during the cooling down. Tc of the film without the plasma was 37.6K, whereas the Tc was raised up to 83.3K by the plasma. The films were perfectly c-axis orientation with grain sizes of 300500nm. And, the c-axis lattice constant varied from

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K Tsuda et al. / Observation of oxygen in YBCO thin films

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Thickness of the YBCO Film (nm) FIGURE 2 Depth profiles of relative SIMS intensity, 161//(161 + 181) 1.179nm to 1.170nm owing to the presence of the plasma. Next, we will discuss SIMS analyses of 160 and 1sO in the films. A direct comparison of SIMS intensity among specimens gives us incorrect results because of different sputtering rates during SIMS analyses. Thus, we compared the relative SIMS intensity 161/(161 + lSl) as shown in Fig.2, where 161and 181indicate the SIMS intensi~ of 160 and 1sO. We had expected that only 180 atoms located in the chain sites could move in the cooling down process. Contrary to our expectation, almost all 180 atoms were replaced with 160 atoms, though the YBCO films absolutely contained only 180 atoms in theP oxygen sites during the deposition process; 95% of 180 atoms were substituted by using the plasma, and 85% of 180 atoms without the plasma. To comfirrn our experiment, we estimated a relative 160 concentration, c(x)/cs, as given by Eq. (1). c(x)/cs= 1- erf(x/2/D-~) (1) where c(x) is an 160 concentration at a position in the Yeco film, cs is that on the surface of the film, err(z) means error function, x is the distance from the surface, D is the diffusion coefficient, and t is a diffusion time. Figure 3 show£ c(x)/cs ratios using 3x10 -~° cm2/s as D at 595°C 3, and demonstrates that the

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Thickness of the YBCO Film (nm) FIGURE 3 Depth profiles of relative 160 concentration, c(x)/cs experimental results agree well with the calculated value within a thickness of 100nm, then are far from the theoretical value over a distance of 100nm from the surface. We infer that these deviations from the theory were caused by the oxygen diffusion along the grain boundaries because of polycrystalline films. We consider that the oxygen plasma increased the effective 1602gas pressure in the chamber, so that the film which was cooled in the if-plasma contained 160 atoms more than that without the plasma. This fact caused the c-axis lattice constant to decrease and the Tc to increase. 4. Conclusions The if-plasma caused the c-axis lattice constant to decrease and the Tc to increase, and accelerated the replacement of !80 atoms with 160 atoms in the films. These facts show that the rf-piasma strongly affected the oxygenation process of the films even in the cooling process. References 1. T.Terashima et aL, Jpn.J.Appl.Phys. 27 (1988) L91. 2. C.B.Eom et al., Physica C 171 (1990) 354. 3. S.J.Rothman et ag., Phys.Rev.S 40 (1989) 8852.