Doping silver into YBa 2 Cu 3 O 7−δ films by 800 keV Ag + implantation at roomtemperature and elevated temperaturesYupu Li, J. R. Liu, Xingtian Cui, J. Z. Qu, Q. Y. Chen, and Wei-Kan Chu Citation: Applied Physics Letters 70, 3029 (1997); doi: 10.1063/1.118739 View online: http://dx.doi.org/10.1063/1.118739 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/70/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microwave impedance of YBa 2 Cu 3 O 7−δ high-temperature superconductor films in a magnetic field Low Temp. Phys. 31, 254 (2005); 10.1063/1.1884427 Spatially resolved studies of chemical composition, critical temperature, and critical current density of a YBa 2 Cu3 O 7−δ thin film J. Appl. Phys. 84, 5089 (1998); 10.1063/1.368758 Superconductor–normal–superconductor Josephson junctions fabricated by oxygen implantation into YBa 2 Cu 3O 7−δ Appl. Phys. Lett. 73, 2354 (1998); 10.1063/1.122459 Basal plane oxygen vapor pressure of Co-doped YBa 2 Cu 3 O 7−δ Appl. Phys. Lett. 72, 3512 (1998); 10.1063/1.121644 A study of oxygen diffusion in and out of YBa 2 Cu 3 O 7−δ thin films J. Appl. Phys. 83, 7736 (1998); 10.1063/1.367947

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Doping silver into YBa 2Cu3O72d films by 800 keV Ag 1 implantationat room temperature and elevated temperatures

Yupu Li,a) J. R. Liu, Xingtian Cui, J. Z. Qu, Q. Y. Chen, and Wei-Kan ChuTexas Center for Superconductivity, University of Houston, Houston, Texas 77204-5932

~Received 17 December 1996; accepted for publication 2 April 1997!

Thin films ~;0.43 and;0.95mm thick! of YBa2Cu3O72d on ~100! LaAlO3 substrates have beenimplanted with 800 keV Ag1 to a dose of 531014/cm2, at room temperature~i.e., the total range'0.4mm and the damage level'3.1 displacements per atom! and at elevated temperatures~450,650, and 780 °C!, followed by anin situ annealing schedule in flowing oxygen ambient. We havefound that the implantation at room temperature amorphizes the implanted layer. In such a case, theimplanted layer cannot regrow to the superconducting phase if there is no crystal seed remaining inthe bottom of the film, whereas implantation at elevated temperatures plus anin situ annealingschedule, including a step at 870 °C in flowing oxygen ambient, can maintain the crystal structureand superconductivity of the films. For the thicker film, we have found that after the implantationat 450 or 650 °C and thein situ annealing, the total volume of the film has recovered to thesuperconducting 123 phase with aTc589 K. © 1997 American Institute of Physics.@S0003-6951~97!00322-7#

Ion implantation~keV–MeV! of YBa2Cu3O72d ~YBCO!thin films has contributed to progress in highTc supercon-ductivity research in three ways:~1! performing fundamentalirradiation effects studies;1–9 ~2! modifying properties forspecific applications~for example, patterning the YBCOfilms!;1–9 and ~3! implanting 18O1 or impurities ~such asAu1! into YBCO films as a tracer in order to study oxygenor impurity diffusion in YBCO films.10,11 It has been foundthat electrical properties of YBCO films are very sensitive toirradiation damage.1–9 Implantation with light elements~such as H1 and He1! ~Refs. 2 and 4! at certain dosages orother elements5,6 at very low dosages can induce point de-fects and defect clusters and further makes YBCO filmsmore granular~i.e., weak links!. With increasing dose, im-plantation causes YBCO films to lose their superconductiv-ity, to change from metals to semiconductors, and, eventu-ally, to become insulators.1–3,7–9The insulating phase, whichis amorphous, is completed at a dose of approximately 0.1–0.2 dpa~displacements per atom!.1,2,8 Although recovery ofsuperconductivity and recrystallization of such amorphouslayers by postirradiation annealing have been reported,7–9

room-temperature implantation producing highly damaged oramorphous targets may not be the preferred doping route forYBCO samples. Instead, high-temperature implantation~where damage continually anneals! may provide an alterna-tive method for doping impurities into superconductingfilms.

This letter describes the use of ion implantation plus anin situ annealing schedule to dope Ag into YBCO films.Ag1 is chosen for the experiment as silver has been shown tohave some beneficial effects on the transport properties andthe long term stability of YBCO films under the condition ofhigh humidity.12 In addition, silver and gold appear to be theonly two metals that do not lowerTc in the YBCOsystem.13,14

C-axis oriented YBCO films on~100! LaAlO3 singlecrystals were prepared by inverted cylindrical magnetron

sputtering of a stoichiometric YBa2Cu3O72d target. Twofilms of different thicknesses were chosen for the implanta-tion: e.g.,~1! ;430 nm thick and~2! ;950 nm thick. Piecesdivided from the two films were irradiated with 800 keVAg1 ~both 107Ag1 and109Ag1! to a dose of 531014/cm2, atroom temperature and at elevated temperatures~450, 650,and 780 °C!, using a 231.7 MV accelerator. After implan-tation, the samples werein situ annealed in flowing O2 am-bient, following a schedule shown in Fig. 1. We recommendthis schedule due to the following reasons:~i! the annealingschedule induces no degradation toTc and the x-ray diffrac-tion ~XRA! pattern of the as-deposited YBCO films;~ii ! wehave found that without irradiation even heating the films at450 °C for 50 min~i.e., the time needed for each implanta-tion on a scanning area of 2.4 cm2!, in vacuum (;131025 Torr), destroyed superconductivity and changed thenormal state resistance of the films to the insulating level.This is due to disordering of the oxygen sublattice and oxy-gen loss from the films. However, in such a case, if theinsitu annealing schedule is carried out,Tc is completely re-covered. Previous studies by other groups indicated that oxy-gen in the CuO-chain sites@O~1!# has the lowest bindingenergy, with larger binding energy in the edge-sharing apicalsites @O~4!#, and still stronger binding energy in the

a!Electronic mail: ypli@uh.edu FIG. 1. The optimizedin situ annealing schedule.

3029Appl. Phys. Lett. 70 (22), 2 June 1997 0003-6951/97/70(22)/3029/3/$10.00 © 1997 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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CuO2-plane sites@O~2!, O~3!#.15,16Therefore, we believe thatdifferent activating temperatures are necessary in order todrive oxygen to different sites. Simply, the highest tempera-ture step at 870 °C could ensure that the films revert to thetetragonal phase where@O~2!, O~3!# sites, most of the O~4!sites, and some of the O~1! sites fill, with oxygen vacanciesbeing distributed equally on the O~1! and O~5! sites@also ona few O~4! sites#.16,17 The 650 °C step could ensure that thefilms change to the orthorhombic phase where a few unoc-cupied O~4! sites and the most of the O~1! sites fill, and moreO~5! sites empty, i.e., the vacancies and oxygen ions onthese planes order.16,17The 450 °C step is for slowing downthe cooling process and ensuring that all O~1! sites are fulland stable, and the O~5! sites completely empty.16,17 Indeed,our experimental results showed that such an idea worked forannealing of the implanted YBCO films. In addition, itshould be noted that we have chosen 870 °C as the highesttemperature step in the annealing schedule since we havefound that 780 °C sometimes is not high enough for com-pletely recoveringTc , and 900 °C sometimes makes the sur-face of the film rougher.

However, it has also been found from this study that,after vacuum heating, recovering the oxygen sublattice byfurther annealing in flowing oxygen ambient is more difficultthan damaging it by vacuum heating. The higher the tem-perature, or the longer the annealing time, or the thinner thefilm, the more oxygen loss and, thus, the more difficult tocompletely recoverTc . For example, after the higher tem-perature~T5650 and 780 °C! vacuum heating for 50 min,for the thinner film we have found that the recommendedannealing schedule can only result in a partial recovery ofTc . However, for the thicker film,Tc can completely recoverafter the 650 °C vacuum heating, but can only partially re-cover after the 780 °C vacuum heating. We think that toomany oxygen losses from the CuO2-plane sites may inducesome irreversible damage~such as partial collapse of the 123phase!. It should be noted here that YBCO films usually havea less ‘‘perfect’’ crystal structure than the YBCO single crys-tals, and thus, vacuum heating at higher temperature mayinduce more serious damage to the YBCO films.

Figure 2 shows the simulated108Ag concentration, va-cancy, and dpa depth profiles for the Ag implantation intothe YBCO film calculated withTRIM-96 for random media.18

Lines 2 and 3 in Fig. 2 show that the irradiation damage isessentially concentrated at about 400 nm from the surface~dpa level>0.2!. It has been reported1,2,8 that a damage levelof 0.1–0.2 dpa will result in amorphizing of YBCO films.For this Ag1 implantation, the simulated damage levels areabout 1.5 dpa at the surface and about 3.1 dpa at the damagepeak. The dose used in this work is about 15 times higherthan that for amorphizition of the 123 phase. Therefore, itcan be expected that the thinner film will contain no or littlecrystal seed in the bottom of the film after the Ag1 implan-tation, whereas for the thick film the bottom half of the filmis free of irradiation damage.

In fact, it has been found that the thinner film, afterroom-temperature implantation became x-ray amorphous andinsulating. The postirradiation annealing following the870 °C schedule changed the film to polycrystalline material,with no superconductivity observed in resistance measure-

ments@line 2 in Fig. 3~a!#. For the same initial film@line 1 inFig. 3~a!#, after the high-temperature implantation~450 °C!plus in situ annealing by following the 870 °C schedule,XRD measurement showed that the film still remained asc-axis oriented; the resistance versus temperature for thisfilm is plotted in Fig. 3~a! ~line 3!. It is clear that the crystalstructure and superconductivity are preserved in this case,althoughTc was found to decrease to 67 K, accompanied byany increase of the normal state resistance.

In films implanted at higher temperatures of 650 or780 °C, and annealed by following the schedule, the super-conductivity of the thinner film was maintained, however,the absolute values ofTc shown in Fig. 3~a! ~lines 4 and 5!are obviously lower than that for the initial film@line 1 inFig. 3~a!#, and also lower than that for the 450 °C implantedthinner film @line 2 in Fig. 3~a!#. This is probably due to thecombined effects of the remaining irradiation damage andthe higher temperature vacuum heating damage. By increas-ing the implantation temperature, the dynamic annealing ef-fects at this temperature maintain the crystal structure andthe normal-state resistance of the film. However,Tc is alsocontrolled by the oxygen content and the oxygen order-

FIG. 2. The simulated108Ag concentration~line 1!, vacancy~line 2!, anddpa~displacements per atom! ~line 3! distributions. For calculating the dam-age distribution, a threshold displacement energy,Ed of 20 eV for all ele-ments is assumed.

FIG. 3. ~a! Resistance vs temperature plots of a thinner YBCO film beforeand after the Ag implantation at different temperatures plus thein situ an-nealing.~b! Resistance vs temperature plots of a thicker YBCO film beforeand after the Ag implantation at different temperatures plus thein situ an-nealing.

3030 Appl. Phys. Lett., Vol. 70, No. 22, 2 June 1997 Li et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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ing, and with increasing the implantation temperature, thevacuum heating has a more negative effect on both of them.

It has also been found that, for the thicker film, afterroom-temperature implantation plus the postirradiation an-nealing schedule,Tc remained as 87 K, just 1 K lower thanthat before implantation, as shown in Fig. 3~b! ~lines 1 and2!. However, after the high-temperature implantations at 450or 650 °C plus thein situ annealing schedule,Tc became 89K, as shown in Fig. 3~b! ~lines 3 and 4!. A slight Tc increasewas observed in both cases. It should be noted that any highTc path in the film, including the seed layer, may shorten therest of the material, thus, we used XRD patterns~measuredbefore and after treatment of the same piece of film! to de-termine if the total volume had recovered to the supercon-ducting 123 phase; an example is shown in Fig. 4~for the650 °C case!. It can be seen from Fig. 4, that the line inten-sities and breadths of (00l ) peaks for the 123 phase do notshow an obvious change. After the treatment, the line posi-tions slightly shift to the higher angle direction@see the inset~005! peaks#, which may be due to a slight reduction ofoxygen deficiency in the film after annealing at 870 °C.Similar observation has been made in the 450 °C case.Therefore, we conclude that in both cases the total volume ofthe film has recovered to the superconducting 123 phase.This illuminates that the remaining crystal seed in the bottomof the film and the dynamic annealing effect can improve the

regrowth of the Ag-implanted layer. This result also con-firmed that silver is a dopant that does not lowerTc inYBCO films. In addition, by increasing the implantationtemperature to 780 °C, the implantation plus thein situ an-nealing was less beneficial toTc @see line 5 in Fig. 3~b!#,probably due to the effect of oxygen loss from theCuO2-plane sites during vacuum heating.

In summary, these results indicate that by designing theenergy and the dose of the ion and the thickness of the film,Ag1 implantation at higher temperature~450 and 650 °C!can provide an alternative method for doping Ag into YBCOfilms.

This work was supported by the State of Texas throughthe Texas Center For Superconductivity at the University ofHouston.

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FIG. 4. XRD patterns for the thick film before~a! and after treatment~b!~650 °C implantation plus the 870 °C annealing!. The inset figures are theextended~005! peaks.

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