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Crystal regrowth of YBCO thin films by ion implantation and rapid thermal annealing
PHYSICA
Physica C 235-240 (1994)569-570 North-Holland
Crystal regrowth of YBCO thin films by Ion Implantation and Rapid Thermal Annealing. T.J.Tate, M.J.Lee,.Yupu Li, J.A.Kilner, Y.H.Li, C.A.Leach, D.Lacey, A.D.Caplin, R.E.Somekh a, P.Przyslupski a, and P.G.Quincey b Centre for High Temperature Superconductivity, Imperial College, London SW7 2AZ, UK aDepartment of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK bNational Physical Laboratory, Middlesex TW11 0LW, UK
The overall aim of this work is to engineer YBCO films with well-characterised crystal structures, by selective patterning, amorphisation, and annealing. Thin films (80 - 400 nm) of YBa2CU3OT_~, deposited by sputtering onto LaAiO 3 and MgO substrates, have been implanted at 200 keV with either 2°Ne+ or 180+ ions, to a dose of 5x1014 ions cm "2. Both implantations amorphise the film, in some cases totally. Oxygen implantation produces less damage than the inert gas. Rapid Thermal Annealing (20 seconds, about 870°C, in oxygen) can induce recrystallisation, leading to a recovery of the superconducting 123 phase under certain conditions. Results from Tc, Jo, XRD, and TEM investigations are presented.
1. Introduction
Thin films grown by thermal deposition or sputtering typically have crystal grain sizes of the order of the film thickness. The position of grain boundaries is random and uncontrolled. We aim to prepare films with grains at known locations, and defined crystal size. The method involves amorphising the film in all but some small areas, as defined by a mask, using ion implantation, followed by regrowth from the small area seed crystals. This paper reports on the initial work, demonstrating successful amorphisation and regrowth using Rapid Thermal Annealing (RTA). 2. Experimental
YBCO films were grown on LaAIO3 and MgO substrates, to a nominal thickness of 300 nm, by sputtering. Tc was typically 87K. Films were principally c-axis orientated, with some second ~hase inclusions. The films were implanted with Ne +, and with 180+ (for SIMS analysis[I]) at 200 keV, to a dose of 5X1014 ions cm 2. These ions have very similar ranges[2] (fig. 1), but differ in their damage effects[3]. RTA was performed in flowing oxygen, using an AG Heatpulse 610 furnace. This cold wall technique uses fast heating ramps, minimising possible cross-diffusion from the substrate into the film. Samples were 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSD1 0921-4534(94)00846-9
placed on an oxidised silicon wafer for processing, and so quoted RTA temperatures are nominal, being the wafer temperature as measured by the system pyrometer. To was measured as the real part of the AC susceptibility, and Jc using the VSM described elsewhere[4]. Displacements / atom YBCO
Atoms/co SUBSTRATE
0.8
1 E19
0.6 1 E18 0.4 1 E17 0.2 0.0
1 E16 0
200
40O
6O0 Depth, nm
Figure 1. Simulated darnage[3] =and implantation[2] profiles for 200 keV, 5x10 ]4 ¢m "z implants in YBCO.
3. Results and Discussion
TRIM[3] Monte Carlo simulations were performed to find the number of displacements per atom (dpa)[5]. This may be seen as a displacement probability, and a dose of 5x1014 ions cm "2 will cause about 0.1 dpa for a 300 nm film (fig. 1) -
5? )
I',L 7"c1[c ~'f ill.
I~tn',~i~'~l ( ' 2.';_'T 24~ (/U~,LI~ ~(,~,' ~"/~
substantially less than that reported by Matsui et al[6]. Some thicker films were not fully amorphised, and oxygen ions tend to produce less damage than neon (an effect well documented in oxides[7]). Following implantation, the films did not show any Tc, and XRD revealed loss of crystal structure. In the case of oxygen implants, the loss was not complete, and TEM showed a region near the substrate interface which retained some degree of crystalline structure. This was also evident on a thicker (400 nm) film implanted with neon. In addition, the substrates showed damage bands to a depth of typically 50 nm, which were not affected by subsequent annealing. Samples were annealed at a nominal temperature of 870°C, for 20 seconds. AC Suscept b i l i t / A r b i t r a r y units
4. Conclusions
Thin YBCO films, amorphised by oxygen or neon ion implantation, but containing some seed information, can be grain grown using RTA It is possible to damage the top of the substrate during implantation, preventing epitaxial regrowth. This work has potential applications in engineering surfaces with preferred crystal grain orientations and size, with grain boundaries in reproducible positions, defined by lithography.
2.6
, _
,
'50
These investigations support the hypothesis that, if there is a seed region, regrowth of YBCO thin films is possible following ion beam amorphisation. In the case of the fully amorphised film, regrowth is highly polycrystalline. In the case of the partially amorphous films, there are many, randomly orientated seed regions, which lead to regrowth of only partially ordered material.
1 0 4 1 . 8 kNz
2.7
2.5
regrown as large grains, with disordered polycrystalline layers near the surface. The critical current Jc was degraded (at best to about 10% of its former value). This may be due to the defective region above the regrown thin film, or it may be that the mean grain size had decreased - this would not necessarily be evident from TEM because of the small sample dimensions.
~
~
-
'60'
-
,
~
70
--
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Un~reated
'90
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Tempercfure
~'
Figure 2. Typical plots showing Tc recovery. These data are for a 40Onto film implanted with neon. None of the neon - implanted samples with film thickness ~ 350 nm recovered T c, and although XRD showed crystalline recovery, this was not to a superconducting phase, and TEM revealed that the films were highly polycrystalline. However, the thicker neon - implanted sample, and the oxygen - implanted samples, all recovered T c, although the transition was spread over 10-15 K and the value varied from 72K to 82K (fig.2). There is some evidence that further annealing (870 ° C, 120 seconds) may improve Tc recovery. XRD showed recovery of YBCO 123 structure, and TEM revealed that a large proportion of the films had
REFERENCES
1. Li Y e t al. Paper on SIMS of implanted YBCO to be published. 2. PROFILE CODE Implant Sciences Co. Mass. USA 3. TRIM 92 Ziegler & al IBM research, NY USA 4. Radcliffe JW, Cohen LF, Perkins GK, Caplin AD, Tate T J, Lee M J, Saba FM, Quincey P, Somekh RE, Przyslupski P. J Alloys Compounds 195 467 (1993) 5. Meyer O, Egner B, Xiong GC, Xi XX, Linker G, Geerk J. Nucl Instr Meth Phys Res B39 628 (1989) 6. Matsui S, Matsutera H, Yoshitake T, Fujita J, Satoh T. Nucl Instr Meth Phys Res B39 635 (1989) 7. Kelly R, Bertoti I, Miotello A. Nucl Instr Meth Phys Res B80/81 1154 (1993)
Physica C 235-240 (1994)569-570 North-Holland
Crystal regrowth of YBCO thin films by Ion Implantation and Rapid Thermal Annealing. T.J.Tate, M.J.Lee,.Yupu Li, J.A.Kilner, Y.H.Li, C.A.Leach, D.Lacey, A.D.Caplin, R.E.Somekh a, P.Przyslupski a, and P.G.Quincey b Centre for High Temperature Superconductivity, Imperial College, London SW7 2AZ, UK aDepartment of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK bNational Physical Laboratory, Middlesex TW11 0LW, UK
The overall aim of this work is to engineer YBCO films with well-characterised crystal structures, by selective patterning, amorphisation, and annealing. Thin films (80 - 400 nm) of YBa2CU3OT_~, deposited by sputtering onto LaAiO 3 and MgO substrates, have been implanted at 200 keV with either 2°Ne+ or 180+ ions, to a dose of 5x1014 ions cm "2. Both implantations amorphise the film, in some cases totally. Oxygen implantation produces less damage than the inert gas. Rapid Thermal Annealing (20 seconds, about 870°C, in oxygen) can induce recrystallisation, leading to a recovery of the superconducting 123 phase under certain conditions. Results from Tc, Jo, XRD, and TEM investigations are presented.
1. Introduction
Thin films grown by thermal deposition or sputtering typically have crystal grain sizes of the order of the film thickness. The position of grain boundaries is random and uncontrolled. We aim to prepare films with grains at known locations, and defined crystal size. The method involves amorphising the film in all but some small areas, as defined by a mask, using ion implantation, followed by regrowth from the small area seed crystals. This paper reports on the initial work, demonstrating successful amorphisation and regrowth using Rapid Thermal Annealing (RTA). 2. Experimental
YBCO films were grown on LaAIO3 and MgO substrates, to a nominal thickness of 300 nm, by sputtering. Tc was typically 87K. Films were principally c-axis orientated, with some second ~hase inclusions. The films were implanted with Ne +, and with 180+ (for SIMS analysis[I]) at 200 keV, to a dose of 5X1014 ions cm 2. These ions have very similar ranges[2] (fig. 1), but differ in their damage effects[3]. RTA was performed in flowing oxygen, using an AG Heatpulse 610 furnace. This cold wall technique uses fast heating ramps, minimising possible cross-diffusion from the substrate into the film. Samples were 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSD1 0921-4534(94)00846-9
placed on an oxidised silicon wafer for processing, and so quoted RTA temperatures are nominal, being the wafer temperature as measured by the system pyrometer. To was measured as the real part of the AC susceptibility, and Jc using the VSM described elsewhere[4]. Displacements / atom YBCO
Atoms/co SUBSTRATE
0.8
1 E19
0.6 1 E18 0.4 1 E17 0.2 0.0
1 E16 0
200
40O
6O0 Depth, nm
Figure 1. Simulated darnage[3] =and implantation[2] profiles for 200 keV, 5x10 ]4 ¢m "z implants in YBCO.
3. Results and Discussion
TRIM[3] Monte Carlo simulations were performed to find the number of displacements per atom (dpa)[5]. This may be seen as a displacement probability, and a dose of 5x1014 ions cm "2 will cause about 0.1 dpa for a 300 nm film (fig. 1) -
5? )
I',L 7"c1[c ~'f ill.
I~tn',~i~'~l ( ' 2.';_'T 24~ (/U~,LI~ ~(,~,' ~"/~
substantially less than that reported by Matsui et al[6]. Some thicker films were not fully amorphised, and oxygen ions tend to produce less damage than neon (an effect well documented in oxides[7]). Following implantation, the films did not show any Tc, and XRD revealed loss of crystal structure. In the case of oxygen implants, the loss was not complete, and TEM showed a region near the substrate interface which retained some degree of crystalline structure. This was also evident on a thicker (400 nm) film implanted with neon. In addition, the substrates showed damage bands to a depth of typically 50 nm, which were not affected by subsequent annealing. Samples were annealed at a nominal temperature of 870°C, for 20 seconds. AC Suscept b i l i t / A r b i t r a r y units
4. Conclusions
Thin YBCO films, amorphised by oxygen or neon ion implantation, but containing some seed information, can be grain grown using RTA It is possible to damage the top of the substrate during implantation, preventing epitaxial regrowth. This work has potential applications in engineering surfaces with preferred crystal grain orientations and size, with grain boundaries in reproducible positions, defined by lithography.
2.6
, _
,
'50
These investigations support the hypothesis that, if there is a seed region, regrowth of YBCO thin films is possible following ion beam amorphisation. In the case of the fully amorphised film, regrowth is highly polycrystalline. In the case of the partially amorphous films, there are many, randomly orientated seed regions, which lead to regrowth of only partially ordered material.
1 0 4 1 . 8 kNz
2.7
2.5
regrown as large grains, with disordered polycrystalline layers near the surface. The critical current Jc was degraded (at best to about 10% of its former value). This may be due to the defective region above the regrown thin film, or it may be that the mean grain size had decreased - this would not necessarily be evident from TEM because of the small sample dimensions.
~
~
-
'60'
-
,
~
70
--
~0'
Un~reated
'90
i0o
Tempercfure
~'
Figure 2. Typical plots showing Tc recovery. These data are for a 40Onto film implanted with neon. None of the neon - implanted samples with film thickness ~ 350 nm recovered T c, and although XRD showed crystalline recovery, this was not to a superconducting phase, and TEM revealed that the films were highly polycrystalline. However, the thicker neon - implanted sample, and the oxygen - implanted samples, all recovered T c, although the transition was spread over 10-15 K and the value varied from 72K to 82K (fig.2). There is some evidence that further annealing (870 ° C, 120 seconds) may improve Tc recovery. XRD showed recovery of YBCO 123 structure, and TEM revealed that a large proportion of the films had
REFERENCES
1. Li Y e t al. Paper on SIMS of implanted YBCO to be published. 2. PROFILE CODE Implant Sciences Co. Mass. USA 3. TRIM 92 Ziegler & al IBM research, NY USA 4. Radcliffe JW, Cohen LF, Perkins GK, Caplin AD, Tate T J, Lee M J, Saba FM, Quincey P, Somekh RE, Przyslupski P. J Alloys Compounds 195 467 (1993) 5. Meyer O, Egner B, Xiong GC, Xi XX, Linker G, Geerk J. Nucl Instr Meth Phys Res B39 628 (1989) 6. Matsui S, Matsutera H, Yoshitake T, Fujita J, Satoh T. Nucl Instr Meth Phys Res B39 635 (1989) 7. Kelly R, Bertoti I, Miotello A. Nucl Instr Meth Phys Res B80/81 1154 (1993)