CRITICAL CURRENT DENSITY MEASUREMENTS OF THIN FILMS OF YBaCuO

Thin films of the YBaCuO superconductor have been deposited by evaporation on SrTiO3 substrates. After a post-anneal, the films were patterned into a four-probe geometry, and critical current density measurements made using a transport technique. Compositions both on and off the stoichiometric Y1 Ba2Cu307., were studied, and measurements made as a function of temperature and magnetic field. At 4.2 K and in ambient field, a stoichiometric film has a Jc greater than 106 Ncm2 which falls off slowly out to =55 K and then follows a power law dependence (1 t)33, where t is the reduced temperature. The magnetic field dependence of Jc at 4.2 K is weak, and for applied fields of 90 kOe, Jc has dropped only a factor of 10 from the ambient field. Off-stoichiometric films have lower Jc's which fall off faster and at lower temperatures.


INTRODUCTION
The discovery of high-Tc superconducting transition temperatures in metallic oxides with a perovskite crystal structure has generated much interest in device applications. The critical current density is an important figure of merit for applications such as magnet technology, AC power line transmission, and high frequency interconnects. Since devices for these applications will function in many different environmental conditions, a thorough study of Jc as a function of temperature and field is needed.
The films were deposited on (100) single crystal SrTiO3 substrates in an UHV system (background pressure of 10-8 mbar with all sources on). Yttrium and copper were evaporated from electron beam guns and barium fluoride from a Knudsen cell. The substrates' temperature was raised to 50 "C and molecular oxygen was introduced at the substrates for the film growth. When removed from the chamber, the films were amorphous and insulating and were then annealed in flowing wet oxygen to get the correct crystal structure. A more detailed description of the film preparation is given by Broussard, et al. in these proceedings.' In this paper we present results for two films: one which has essentially the stoichiometric composition, Y1Ba2Cu307-,, referred to as "1-2-3", and an offstoichiometric film which is barium-rich and copper-poor, Y1 Ba3Cu207-,, referred to as "1 -3-2". The compositions were determined using an "elastic" Rutherford backscattering spectroscopy technique, which uses higher energy alpha particles than is usual. X-ray diffraction was used to analyze the crystalline phases present in our films.
The  and observing how the light reflects. Grains with orthorhombic structures such as the superconducting phase of YBaCuO will produce a maximum intensity reflection when rotated 90" from an orientation that produces a minimum reflection. The entire surface of our film went from light to dark with a 90" rotation, suggesting that the orthorhombic crystal structure of the film has in-plane order on the length scale of at least a few millimeters (the size of the microscope's field of view). A resistive technique was used to measure the room temperature resistivity (p), resistance ratio (RR) between 298 K and 100 K, and the superconducting transition temperature (Tc) of the films.
The annealed films were patterned for the critical current density measurement into a four-probe geometry which was 80 pm wide with 940 pm between the voltage leads. Standard photolithographic techniques were used to apply the pattern and a dilute mixture of hydrochloric acid with water (1 :9) used to etch the films. Measurements of p before and after the patterning showed no change for the films, indicating that the procedure did not damage the materials. (For off-stoichiometric compositions, evidence that suggests otherwise is presented in the next section.] Contact to the films was made by first sputtering a 1000 A  Critical current density as a function of temperature at ambient field for two YBaCuO films. The "1 -2-3" film has the stoichiometric composition, and the "1-3-2" film is barium-rich and copper-poor. layer of gold onto the contact pads of the pattern, and then layering a freshly-cut piece of indium, a gold wire, followed by another piece of indium on top of that. Contact resistances of better than 1 Cl were achieved with this technique, which resulted in less than a 50 mK rise in temperature for the largest current (0.5 A) we used.

Jr RESULTS
A standard transport technique was used to measure the critical currents. At fixed temperature and field, a ~ constant current was passed through the film while we monitored the voltage drop between the voltage leads. We used a one microvolt criteria to determine the critical current. Using a profilemeter to measure the films' thicknesses gave 5900 A for the "1 -2-3" film and 1.06 pm for the "1-3-2" film, and allowed us to calculate Jc. Fig. 1 is a plot of the temperature dependence of Jc in ambient field for both the films. At 4.2 K, the critical current density of the stoichiometric film is 1.04 x 106 A/cm2. As temperature increases, Jc slowly decreases until above 55 K where it falls off more rapidly. By 77 K, Jc has dropped to 3.5 x 103 A/cm2. The "1 -3-2" film's Jc is lower, 1.64 x 105 A/cm2, and also rolls over in temperature at =15 K. Also in Fig. 1, the Jc of the "1 -2 -3 film approaches zero close to the 88 K which was the Tc obtained with a resistance measurement. If the "1-3-2" curve has the same shape as the "1 -2 -3 curve, then Tc as determined by Jc going to zero is much lower than the 79 K measured resistively. This may indicate that off-stoichiometric material is more sensitive to an etching process than stoichiometric material. greater than 50 K follow a power law dependence For the "1-2-3 film, the Jc data for temperatures J c x ( 1 -t ) " as shown in Fig. 2. A least squares fit of log Jc to log (1 -t) is illustrated by the straight line in Fig. 2 and yields a value for n of 3.27 with a standard deviation of 0.06 and a linear correlation coefficient of 0.9988. Furthermore, if we leave Tc as an adjustable parameter, the best fit to the data (lowest correlation coefficient) is obtained when Tc is 88K f 1 K, which is the same value obtained with a resistance measurement. Recently, Deutscher and Muller have proposed that Josephson tunneling that occurs at the twin boundaries inside the grains of high-Tc oxides will dominate the transport process.3 They predict a power law dependence with n = 2.0. Our results do not support their prediction, but we find no theories in the literature that propose a closer value to ours.
To obtain the field dependence of Jc, the films were placed in a Bitter magnet and oriented so the field was perpendicular to the film surface. Fig. 3 shows the results forthe "1-2-3" film. At 4.2 K, Jc was not strongly affected by field, and for fields as high as 90 kOe was reduced less than a factor of 10 from the ambient field value. By 77 K, however, only a few kOe was needed to sharply diminish

JC.
The pinning force is related to Jc by -1 and Fig. 4 is a plot of field dependence of the pinning force at 4.2 K. As indicated, the pinning forces are very high, = l o 9 dyn/cm3, which is comparable to the values obtained for commercially used materials such as NbgSn4 and NbTi alloyss. Up to 90 kOe, F is still increasing and has not reached the maximum wEich is characteristic of Fp(H) curves. Assuming that YBaCuO has a similar Hc2 as EuBaCuO for which a lower limit of Hc2 has been To close, we note that the "1 -2-3 film was "burned out" when a transient voltage spike was applied, creating a crack across the current path. Interestingly, when SEM was used to inspect the crack, "charging" was observed in the region around it, which is an indication of semiconducting or insulating material. In the YBaCuO system, non-metallic transport properties have been correlated with a tetragonal crystal structure that is oxygen deficient compared to the superconducting phase. A portion of this region was identified as a potential weak spot before the Jc measurement and may already have been tetragonal, but the fatal pulse appears to have enlarged the tetragonal region and in the process produced the crack.

CONCLUSIONS
We have prepared thin films of the YBaCuO superconductors by evaporation on SrTiO3 substrates and measured their critical current densities as a function of temperature and field. We find that a film with the stoichiometric "1 -2-3 composition has a higher Jc (1 .O x 106 Ncm2 at 4.2 K) which decreases more slowly with increasing temperature than a film with an off-stoichiometric composition (Jc = 1.6 x IO5 A/cm2 at 4.2 K). We can fit the high temperature data of the "1-2-3 film to a power law dependence, but our results do not agree with current theoretical predictions. At 4.2 K, the Jc of a stoichiometric film is reduced to IO5 Ncm2 by a perpendicular magnetic field of 90 kOe, and this corresponds to a pinning force of =lo9 dynlcms.