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Behavior of the droplets ejecting from the YBCO target during the pulsed laser deposition
PIIYSlCA
Physica C 235-240 (1994)571-572 North-Holland
Behavior o f the droplets ejecting from the Y B C O target during the pulsed laser deposition Jeong Dae Suh and Gun Yong Sung Research Department, Electronics and Telecommunications Research Institute, Yusong P.O. Box 106, Taejon, 305-600 REP. OF KOREA The droplets density of the YBa2Cu307-x thin film was increased considerably at a certain laser shot number. Until 100 laser shots of cumulative irradiation, the films have the droplets density of -103 ram-2 and the YBCO target surface smoothness was maintained. However, after 100 laser shots, the density was increased more than 10 times and the target surface morphology modified to conical structure. This result has explained by the change of the droplets ejecting behavior with the development of the conical structure at the target surface.
1. INTRODUCTION In spite of the merit of stoichiometric YBCO thin film deposition, it is generally known that YBCO thin film which was deposited by pulsed laser deposition process have spherical particles on it's film surface. These particles (droplets) are detrimental to the film surface smoothness. The density of droplets are found to be a strong function of the laser wavelength[l] and the density of the target[2]. In this study, we have investigated the effect of the laser shot number on the droplets density of pulsed laser deposited YBCO thin films and the laser irradiated surface morphology of the YBCO target.
2. EXPERIMENTAL PROCEDURE In our experimental setup a XeCI excimer laser (Lumonics Inc.) was focused onto a 1 inch diameter YBCO pellet. Rotation speed of the target was 10 r. p.m. The laser is operated at a repetition rate of 10 Hz and energy density of 2 J/cm2. The number of laser shots exposed on the same site of the target were varied in the range of 1 shot to 450 shots. During the deposition process, (100) LaA103 substrate was maintained at 720 °C and oxygen pressure of 200 mTorr. When the deposition was completed the oxygen pressure was raised to 500 Torr and the substrate temperature was cooled naturally. Surface morphologies of the deposited films and target surfaces were observed by the
scanning electron microscope. The density of droplets (the number of droplets per mm2) on the film surface was obtained by counting the droplets in the micrographs. 3. RESULTS AND DISCUSSION
3-1. Density of the droplets of YBCO thin film surface with the laser shot number Variation of the droplets density on the YBCO film surface deposited at various laser shot number was investigated. The density of droplets deposited below 100 laser shot number was constant at a value of 5x103 mm-2. We interpret this result as follows. The fractured particles which were formed during the YBCO target polishing were weakly bonded to the YBCO target surface. When one pulse laser irradiated on the polished target surface the weakly bonded particles are almost ejected. Therefore, until 100 laser shots the density of droplets was unchanged. Meanwhile this result indicates that the pre-ablation of YBCO target surface before the YBCO film deposition is an important process to prevent the deposition of the weakly bonded particles. When the laser shot number increased more than 100 shots the density of droplets increased considerably . The density of droplets was 4x104 mm-2 at 250 laser shots.
3-2. YBCO target surface modification with the laser shot number
0921-4534/94/$07.00 © 1994- ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)00847-7
572
.LD.
Suh, G.Y Sung/Physica (' 235 240 (1994) 57] 572
Surface morphologies of the YBCO targets with the number of laser shots were observed by SEM. At a single shot per site, the surface morphology was similar to that of the as-polished surface except that surface melting was found on the laser irradiated site. As the laser shots increased to 100 shots per site, significant selective erosion of the target surface were observed. However, it's surface continuity was maintained until 100 laser shots. As the laser shots were increased more than 100, the target surface morphology was changed to conical structure[3] and surface continuity was no longer existed. It was generally known that the droplets are originated from the explosive nature of the lasertarget interaction. The possible processes that were
Excimerlaserpulse
(a)
i i i
Vaporization Liquiddroplet Lquidexpulsio~ ~ L l" .Target ............
Liquid v
Solid
=======================
sooo . . . . . . i:iiiiiiiiiiiii: . . . . . . . ..:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:,:.: • .....+:.:.:.:.:.:.:.:.
Fig. 1. Schematics of the droplet ejection behavior with the laser shot numbers ; (a) 5 shots, (b) 60 shots, (c) 100 shots, and (d) 250 shots.
considered to the ejection of droplets are recoil pressure model[4] and subsurface explosion model [5]. These models seems to be applicable to our results. We proposed an ejection behavior of the droplets with the laser shot number as shown in Fig. 1. As far as the target surface continuity was preserved, the droplets could pushed into the boundary of the laser irradiated area, and ejected along the inclined direction to the target surface normal.(Fig. 1 (b)). However, as the laser shot number increased, the target surface morphology changed to conical structure and it's surface continuity was broken. In this case, the droplets were trapped at the valleys between the cones as shown in Fig. 1 (d). Therefore the droplets could be ejected along the direction parallel to the target surface normal. It is thought that this ejection behavior causes an increase in the density of droplets on the film surface. 4. SUMMARY During the pulsed laser deposition, the ablated YBCO target surface morphology was changed from mirror-like polished structure to conical structure. After texturing of the target surface the density of droplets was increased considerably. Therefore, in order to minimize the number of droplets on the film surface, the YBCO film should be prepared by using the laser shot number before the development of conical microstructure.
REFERENCES 1. G. Koren, A. Gupta, R. J. Baseman, M. I. Lutwyche, and R. B. Laibowitz, Appl. Phys. Lett., 55 (1989) 2450. 2. R. K. Singh, D. Bhattacharya, and J. Narayan, Appl. Phys. Lett., 61 (1992) 483. 3. J. D. Suh, G. Y. Sung, and K. Y. Kang, Mat. Res. Soc. Symp. Proc. Vol.275, 329 (1992). 4. C. L. Chan and J. M~umder, J. Appl. Phys., 62 (1987) 4579. 5. R. K. Singh, O. W. Holland, and J. Narayan, J. Appl. Phys., 68 (1990) 233.
Physica C 235-240 (1994)571-572 North-Holland
Behavior o f the droplets ejecting from the Y B C O target during the pulsed laser deposition Jeong Dae Suh and Gun Yong Sung Research Department, Electronics and Telecommunications Research Institute, Yusong P.O. Box 106, Taejon, 305-600 REP. OF KOREA The droplets density of the YBa2Cu307-x thin film was increased considerably at a certain laser shot number. Until 100 laser shots of cumulative irradiation, the films have the droplets density of -103 ram-2 and the YBCO target surface smoothness was maintained. However, after 100 laser shots, the density was increased more than 10 times and the target surface morphology modified to conical structure. This result has explained by the change of the droplets ejecting behavior with the development of the conical structure at the target surface.
1. INTRODUCTION In spite of the merit of stoichiometric YBCO thin film deposition, it is generally known that YBCO thin film which was deposited by pulsed laser deposition process have spherical particles on it's film surface. These particles (droplets) are detrimental to the film surface smoothness. The density of droplets are found to be a strong function of the laser wavelength[l] and the density of the target[2]. In this study, we have investigated the effect of the laser shot number on the droplets density of pulsed laser deposited YBCO thin films and the laser irradiated surface morphology of the YBCO target.
2. EXPERIMENTAL PROCEDURE In our experimental setup a XeCI excimer laser (Lumonics Inc.) was focused onto a 1 inch diameter YBCO pellet. Rotation speed of the target was 10 r. p.m. The laser is operated at a repetition rate of 10 Hz and energy density of 2 J/cm2. The number of laser shots exposed on the same site of the target were varied in the range of 1 shot to 450 shots. During the deposition process, (100) LaA103 substrate was maintained at 720 °C and oxygen pressure of 200 mTorr. When the deposition was completed the oxygen pressure was raised to 500 Torr and the substrate temperature was cooled naturally. Surface morphologies of the deposited films and target surfaces were observed by the
scanning electron microscope. The density of droplets (the number of droplets per mm2) on the film surface was obtained by counting the droplets in the micrographs. 3. RESULTS AND DISCUSSION
3-1. Density of the droplets of YBCO thin film surface with the laser shot number Variation of the droplets density on the YBCO film surface deposited at various laser shot number was investigated. The density of droplets deposited below 100 laser shot number was constant at a value of 5x103 mm-2. We interpret this result as follows. The fractured particles which were formed during the YBCO target polishing were weakly bonded to the YBCO target surface. When one pulse laser irradiated on the polished target surface the weakly bonded particles are almost ejected. Therefore, until 100 laser shots the density of droplets was unchanged. Meanwhile this result indicates that the pre-ablation of YBCO target surface before the YBCO film deposition is an important process to prevent the deposition of the weakly bonded particles. When the laser shot number increased more than 100 shots the density of droplets increased considerably . The density of droplets was 4x104 mm-2 at 250 laser shots.
3-2. YBCO target surface modification with the laser shot number
0921-4534/94/$07.00 © 1994- ElsevierScience B.V. All rights reserved. SSDI 0921-4534(94)00847-7
572
.LD.
Suh, G.Y Sung/Physica (' 235 240 (1994) 57] 572
Surface morphologies of the YBCO targets with the number of laser shots were observed by SEM. At a single shot per site, the surface morphology was similar to that of the as-polished surface except that surface melting was found on the laser irradiated site. As the laser shots increased to 100 shots per site, significant selective erosion of the target surface were observed. However, it's surface continuity was maintained until 100 laser shots. As the laser shots were increased more than 100, the target surface morphology was changed to conical structure[3] and surface continuity was no longer existed. It was generally known that the droplets are originated from the explosive nature of the lasertarget interaction. The possible processes that were
Excimerlaserpulse
(a)
i i i
Vaporization Liquiddroplet Lquidexpulsio~ ~ L l" .Target ............
Liquid v
Solid
=======================
sooo . . . . . . i:iiiiiiiiiiiii: . . . . . . . ..:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:,:.: • .....+:.:.:.:.:.:.:.:.
Fig. 1. Schematics of the droplet ejection behavior with the laser shot numbers ; (a) 5 shots, (b) 60 shots, (c) 100 shots, and (d) 250 shots.
considered to the ejection of droplets are recoil pressure model[4] and subsurface explosion model [5]. These models seems to be applicable to our results. We proposed an ejection behavior of the droplets with the laser shot number as shown in Fig. 1. As far as the target surface continuity was preserved, the droplets could pushed into the boundary of the laser irradiated area, and ejected along the inclined direction to the target surface normal.(Fig. 1 (b)). However, as the laser shot number increased, the target surface morphology changed to conical structure and it's surface continuity was broken. In this case, the droplets were trapped at the valleys between the cones as shown in Fig. 1 (d). Therefore the droplets could be ejected along the direction parallel to the target surface normal. It is thought that this ejection behavior causes an increase in the density of droplets on the film surface. 4. SUMMARY During the pulsed laser deposition, the ablated YBCO target surface morphology was changed from mirror-like polished structure to conical structure. After texturing of the target surface the density of droplets was increased considerably. Therefore, in order to minimize the number of droplets on the film surface, the YBCO film should be prepared by using the laser shot number before the development of conical microstructure.
REFERENCES 1. G. Koren, A. Gupta, R. J. Baseman, M. I. Lutwyche, and R. B. Laibowitz, Appl. Phys. Lett., 55 (1989) 2450. 2. R. K. Singh, D. Bhattacharya, and J. Narayan, Appl. Phys. Lett., 61 (1992) 483. 3. J. D. Suh, G. Y. Sung, and K. Y. Kang, Mat. Res. Soc. Symp. Proc. Vol.275, 329 (1992). 4. C. L. Chan and J. M~umder, J. Appl. Phys., 62 (1987) 4579. 5. R. K. Singh, O. W. Holland, and J. Narayan, J. Appl. Phys., 68 (1990) 233.