Laser deposition of smooth high-T_c superconducting films using velocity filtration of plasma particles

Abstract

We report on pulsed laser deposition of smooth high-T_c superconducting films using velocity filtration of plasma particles. We have removed droplets and other macro particles from laser-induced plasma using a shutter technique. We have applied the technique to preparation of high quality YBa₂Cu₃O₇ films on (100)-oriented SrTiO₃, MgO, Y₂O₃-stabilized ZrO₂ (YSZ) substrates and, furthermore, on (102)-oriented sapphire covered with (100) sublayers of Si and (100) YSZ buffer layers.

PACS No.: 74.75.+t Superconducting films
74.70.Vy Superconducting perovskites and related structures

In this Letter we report on a study showing that a density of macro particles on YBa₂Cu₃O₇ thin films prepared by pulsed laser deposition can be reduced remarkably, namely up to a factor of 10⁵, by using a mechanical shutter synchronized with the laser pulses.

The principle of the velocity filtration of plasma particles is shown in Fig.1. Particles leaving the target with different velocities are separated by time of flight and only the fast particles reach the substrate while slow particles are retained by the shutter. We will show that the average velocity of droplets (macro particles) V_d is much smaller than the initial velocity V of molecular fragments and that it is possible, by using a rotating disk-chopper as a shutter.

We deposited YBCO films and also buffer layers by use of excimer laser radiation (wavelength 308 nm, pulse duration 15 ns, repetition rate 8-25 Hz, pulse energy 0.15 J) focused on rotating disk-shaped targets to a pulse energy density of 10-20 J/cm². The HTS films were prepared by ablation of Y-Ba-Cu-O ceramic targets, and 20-100 nm thick buffer layers were predeposited in situ using an yttria stabilized ZrO₂ (YSZ) target of pressed oxide powder annealed in oxygen at 1500^oC. The YBCO films were grown on (100)-oriented single-crystal substrates of SrTiO₃, YSZ, and MgO as well as on (102) cut Al₂O₃ substrates with a 0.5 m thick epitaxial (100)-oriented sublayers of Si covered with (100) YSZ buffer layers. The buffer layers were prepared by a two-step method described in [7]. The YBCO films were deposited in oxygen atmosphere at a pressure of 0.3 mbar. The substrates of 11 cm² size were placed 4.5 cm from the targets and had a temperature of 740-760^oC. The deposition rate was 0.025 nm/pulse. Surfaces of the films were investigated by use of a ZEISS digital scanning electron microscope DSM 950. We measured the intensities of radiation emitted by the laser-induced plasma in the wave length range of 0.5-1 m with a time resolution of 2 ns using a photo-sensitive diode.

We have inserted in a laser deposition apparatus, between the target and substrate, a disk-chopper with an opening of 2.5 cm diameter performed 6 cm from the disk center. The chopper was rotated by a motor with a revolution speed up to 500 Hz. The laser pulses were triggered by a phase-adjustable electronic device which synchronized the laser pulses with the disk rotation. The chopper made from 10 to 50 revolutions between two subsequent laser beam pulses.

For obtaining information about velocities of the molecular fragments we measured intensity of the plasma radiation focusing radiation from different points between target and substrate on a photo diode. In the signal observed from different positions (Fig.2a) we found three characteristic peaks. At a time t = 0.1 s after the laser pulse stray irradiation occurred immediately from a range of 2mm size near the target. Its intensity decreased exponentially indicating that the process in this range was related to irradiation of the originally excited target material. A second peak (s) reached its maximum in the range of 0.5-2 cm distanced from the target may be related to recombination of oxygen excited most effectively at this distance by the emitted particles. While the two first peaks were observed almost at the same time (stray radiation), there was a third peak of lower intensity which became to be noticeable beyond 1.5 cm from the target and moved in time with distance. We attribute this peak movement with propagation of the plasma front and estimate speed of the front of about 510⁵ cm/s. We conclude from our result that the plasma front extends over the whole range between target and substrate (taking account of decay of the plasma radiation) at a time of about 10 s.

However, the process of the laser-induced stream motion was not finishing with the plum extinction (in about 10 s). The oxygen we needed for accomplishing the in-situ YBCO film preparation process caused a very effective deceleration (thermalization) of the fast molecular fragments. To estimate the time stretch in which the thermalized stream covered the distance from the targets to the substrates, we placed the chopper close to the substrates and adjusted the laser shooting earlier than the disc openings passed the target-substrate line. We measured the thicknesses of the films produced with the different phase delays of the disc openings. By zero delay the disc rotated with the revolution speed of 400 Hz (and with the window edge linear velocity of 1.410⁴ cm/s) shut the substrate in 80-100 s and decreased the YBCO film mean deposition rate about three times. Reducing the rotation speed to 200 Hz we found that the deposition rate did not increase much. We suggested that a significant part of the YBCO material from the thermalized cloud near the substrate was deposited on the disk surface during comparably long time. Changing the delay it was possible to obtain YBCO films with high superconducting properties depositing only the thermalized fragments with reduced amount of droplets. The deposition rate R change with the the disc opening time delay was approximated (for the disc rotation speed of 400 Hz) by the formula , where t₀ = 230 s. This formula characterizes feed of the deposition region by the slowest thermalized molecular fragments and allows to estimate the life time of the cloud. About the same estimation was obtained with use of a polished disc as the chopper. We found that after deposition of a YBCO film in the normal synchronized regime (i.e. without the delay) with the disc revolution rate of 400 Hz the disc separated from the target by 4 cm was covered on more than 1/4 of a turn with both the film and droplets. These experiments showed that the deposition process lasted about 0.5-1 ms after the laser pulses and the macro particles came to the substrate nearly at the same time.

These time-of-flight measurements indicated that the proper placing of the chopper between the targets and the substrates was very important. As the average velocity of the molecular fragments was decreasing with distance passed from the target but that of the macro particles was not much changing by passing through the gas, we chose the position of the chopper 2 cm from the target, i.e. at the region of the most intensive deceleration of the plasma stream. In this configuration the main part of the molecular stream passed the chopper window ballistically in a very short time (of the order of 10 s) and the most effective velocity separation between the macro particles and the molecular fragments was obtained. The chopper placed in this position did not change noticeably the plasma plum and the film deposition rate even at the highest rotation speed but effectively screened the droplets.

To estimate quantitatively efficiency of the method we measured the density of macro particles arriving at the substrates; in this case we used as substrates highly polished silicon plates and, to exclude completely formation of precipitates on the film surfaces, reduced the deposition temperature to 500^oC. For different chopper rotation frequencies we counted densities of the macro particles deposited for 8000 pulses, and determined, taking into account the geometric dimensions, the shutter opening times . We found (Fig.2b) a sharp onset of the particle density N for 50 s and a saturation of N for >150 s. From analysis we found that the droplets arrived with an average velocity V 210⁴ cm/s. The average velocity was not much different for the droplet diameters in a range of 0.1-1 m. The experiment demonstrates that it is well possible to reduce droplet density by orders of magnitude for an appropriate rotational frequency. In our arrangement about half of the macro particles arrived at the substrates at a rotational frequency of 250 Hz and strong reduction was observed at 350-500 Hz. At 460-500 Hz reduction of the macro particle density by the velocity filter reached about 10⁵ times. Fast (higher than 430 Hz) rotation of the chopper with two symmetrical windows resulted in appearance of the macro particles on the substrates again because the slowest particles passed throw the second window after a half-turn of the disc. The chopper with one window was free from this disadvantage.

The method of pulsed laser deposition of films using velocity filtration allowed us to produce high-quality YBCO films with smooth surfaces. Samples of different films studied by electron microscopy are shown in Fig.3. YBCO films on MgO showed a large amount of droplets in case of no velocity filtration (Fig.3a). The number of droplets was very low if filtration was used (Fig.3b). Similar results were found for YBCO on other substrates. The number of droplets usually was not more than 10³ cm^-2 (and of precipitates between 10³ and 10⁵ cm^-2).

Making small corrections of the Y-Ba-Cu-O target composition we found that formation of Cu-rich precipitates on the film surface could be suppressed with the target compositions YBa₂Cu_3-xO_7- ,where x = 0.15-0.2. If the shining plum containing the fast particles did not touch the growing film surface, the films produced with these targets (both with and without the chopper) had practically no or a very small amount of precipitates and better superconducting properties (for instance, the critical temperature was about 1K higher) than that prepared with the stoichiometrical targets. It showed that the film stoichiometry was improved by these small corrections.

Fig.4 shows sharp alternating-field screening curves of the films deposited on different substrates with the velocity filtration of the laser-induced streams and with the corrected composition of the Y-Ba-Cu-O target. Zero resistance in these films was reached at the temperatures (R=0) = 90.3K (1), 91K (2), 91.4K (3), and 92K (4). As an example, the curve 1 in Fig.4 belongs to a 112 nm thick film which for the first time was grown on a sapphire substrate with a Si sublayer and an YSZ buffer layer on the top. This film has the resistivity of 60 cm at 100K and the critical current density (measured in a 42 m wide bridge using criteria 10V/mm) J_c = 310⁶ A/cm² at 77K that is nearly the same as ones of the best films on Si with buffer layers [3]. X-ray diffraction measurements showed that all the YBCO films were highly (00l) oriented and phasepure.

In conclusion, we have investigated the time of flight of both fast and thermalized components of laser-induced plasma, as well as of droplets and other macro particles ejected from the targets. Our experiments have shown that use of a fast shutter for velocity filtration of a laser-induced particle stream is an effective and convenient method of protecting a substrate to be hit by macro particles. We have also found that a small reduction of Cu in the target composition from the stoichiometry allows to suppress creation of precipitates on the film surfaces and improves superconducting properties of the YBCO films. Using the technique developed smooth YBCO films with (R=0) = 90.3K and a critical current density of 310⁶ A/cm² at 77K on sapphire substrates with Si sublayers and YSZ buffer layers as well as high-quality YBCO films on other substrates have been grown.

The work was supported by the Deutscher Akademischer Austauschdienst (DAAD) and the Bayerische Forschungsstiftung through the Bayerische Forschungsverbund Hochtemperatur-Supraleitung (FORSUPRA).

References

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