Temperature dependent microstructural modification in ion-irradiated Tl-type high temperature superconductors

PHYSICA ELSEVIER

Physica C 267 (1996) 243-253

Temperature dependent microstructural modification in ion-irradiated Tl-type high temperature superconductors P.P. Newcomer a,*, J.C. Barbour a, L.M. Wan~ b, E.L. Ven~rini a, J.F. Kwak a, R.C. Ewing b, M.L. Miller , B. Morosin a Sandia National Laboratories, Albuquerque, NM 87185, USA b University of New Mexico, Albuquerque, NM 87131, USA

Received 27 October 1995; revised manuscript received 26 April 1996

Abstract Ion irradiation damage creation and recovery were examined in Tl-based high temperature superconductors, HTSC, using TEM, resistivity, and magnetic measurements for irradiation temperatures of 20 to 650 K. During 1.5 MeV Kr + and Xe + ion irradiations of single-crystal T1-1212 and T1-2212 T I - B a - C a - C u - O HTSC, microstructural modification was observed in situ by electron diffraction and shows a remarkable temperature dependence. At selected sample temperatures, irradiations continued until a critical fluence, D c, was reached where the original structure disappeared. The temperature dependence of Dc shows a minimum near the superconducting transition temperature, Te, and is correlated with the temperature dependence of the thermal conductivity, which has a maximum near T~. At an irradiation temperature near this maximum in thermal conductivity, a minimum amount of damage recovery occurs because heat can be dissipated away from the displacement cascade. Ion irradiation suppresses the To. The rate of decrease in the T~ as a function of damage (measured in displacements per atom, dpa) was found to be the same for various incident ions (He +, O 2+, Au 5÷ which shows that the damage accumulation is a result of atomic collisions. Further, the rate of decrease in Tc was found to be the same for both transport and magnetization measurements, indicating that the displacements effect the bulk of the samples through point defect creation. An activation energy of 0.4 eV for ion irradiation damage recovery over the temperature range from 100 to 650 K was determined from normal state resistance versus time immediately after irradiation.

1. Introduction The high temperature superconducting, HTSC, T 1 - B a - C a - C u - O system is important because critical transition temperatures, T~, can be as high as 125 K [1]. Ion irradiation can be used to control the transport and magnetic properties o f a superconducting device without removing material. Basic investi-

* Corresponding author. Fax: + 1 505 844 4045; e-mail: [email protected].

gations of ion irradiation effects on superconducting properties are thus important for developing HTSC device technology. After a low-fluence (dose) ion irradiation in Tltype HTSC, the critical current density, Jc, can be increased with little change in To; however, higher fluences decrease T~ and increase normal state resistivity, p [2]. Ion irradiation damage can pin magnetic vortices, but large enhancement o f the flux pinning in a polycrystalline thin film requires pinning centers with energy barriers greater than those introduced by

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P.P. Newcomer et a l . / Physica C 267 (1996) 243-253

point defects alone [3]. Flux pinning will not be discussed in this paper. Several studies [4-7] on ion-beam irradiation of HTSC YBa2Cu3OT_~,, Y123, thin films showed that the Tc can be controllably reduced. Reports on the irradiation of Y-123 films have also shown that displacements by elastic collisions, rather than ionization, are predominantly responsible for the irradiation-induced decrease in the room temperature conductivity [4]. Ion-irradiation-induced damage accumulation can result in amorphization of the crystal structure. The ion irradiation energies and fluences discussed in this paper produce damage which results in point defects, clustered point defects, and amorphization. The ionirradiation-induced damage accumulation process is somewhat similar to metamictization in minerals which have accumulated alpha recoil damage; however, those produce extended defects (tracks) [8,9]. In this paper, the effects of 500 keV He ÷, 2 MeV He +, 740 keV O 2+, 1.5 MeV Kr ÷, and 1.5 MeV Xe ÷, and 20 MeV Au 5+ ion irradiations on Tl-based HTSC single crystals and thin films will be examined as a function of irradiation temperature. The ion energies were chosen to traverse the entire film or crystal and produce accumulated point defects (displacement cascades), as opposed to columnar defects. The irradiation effects are characterized in order to determine the dominant mechanisms causing irradiation-induced damage, temperature dependence of the irradiation damage, thermal recovery of damage, degradation of the superconducting properties, temperature dependence of irradiation-induced increases in p, and the activation energy for the damage recovery. We will examine the irradiation of single crystal TIBa2CaCu20 x, T1-1212, and Tl2Ba2CaCu2Oy, TI-2212, phases and polycrystalline films of the TI2Ba2Ca2Cu30 z, T1-2223, phase. Previous work [2,3,10] has examined and compared ion irradiation of single crystal and polycrystalline T1-2223. The total ion irradiation fluence at which amorphization is complete, or the original structure no longer exits, is the critical fluence, D~. In order to determine the D~, we examine the higher damage production regime where the magnetization and transport behavior of thin films and single crystalline material have shown that superconductivity disappears. In this regime we will report the measured

values of D c for the T1-1212 and T1-2212 phases, as a function of irradiation temperature. During irradiation with 1.5 MeV Kr ÷ and 1.5 MeV Xe ÷ microstructural modification of T1-2212 and T-1212 single crystals was observed by in situ TEM until D c was reached. Unexpectedly, a minimum in D c was found near Tc. For temperatures away from Tc, the value of D c is higher and damage annealing is observed. Diffusion resulting in damage recovery a n d / o r nucleation of a second phase can be induced in displacement cascades during the thermal spike [11]. In order to consider such a diffusion/nucleation model, the thermal conductivity, K(T), of the material must be considered. A common behavior has been found for K(T) of HTSC. As temperature decreases, K(T) also decreases, as exhibited by most Debye solids; at T~ ( ~ 100 K) an increase in K(T) results in a significant peak, before resumption of the decrease toward 20 K [12]. The effects of low- and high-fluence ion irradiation on the Meissner signal and resistivity behavior in single crystal and polycrystalline films of T1-2223 have been investigated [3]. At high fluences, the damage mechanism in both films and single crystals is predominantly due to nuclear collisions which cause a decrease in T~. The quantitative agreement between the decrease in T~ measured from the transport and magnetization measurements suggest that the dominant effect of irradiation is a decrease in the superconducting order parameter. The rate of decrease in Tc is 1 K per 0.0002 dpa for both the polycrystalline thin films and the single crystals. Measurement of damage recovery in T1-2223 is determined using in situ resistivity versus time measurements immediately after irradiation. Thin films of the T1-2223 phase are examined to measure the damage recovery process through resistivity changes and then determine an activation energy for this recovery process. Although the T1-2223 phase has some structural difference with the T1-2212 and T11212, it is expected that the general irradiation and annealing behaviors are similar.

2. Experimental methods In situ TEM irradiations were performed on high quality single crystal T1-1212 and T1-2212, grown

P.P. Newcomer et a l . / Physica C 267 (1996) 243-253

from melts of metal oxides [13]. These specimen were selected based on their sharp X-ray diffraction maxima and on Meissner data showing sharp T~ transitions. Sample preparation for in situ ion irradiation and HRTEM focused on observation along the (100) direction. Submillimeter crystal plates were mounted in slots cut into LaAIO 3 and were polished down to approximately 80 ~ m , after which specimens were dimpled to 10-20 /xm. The specimens were ion milled with 4.5 keV Ar + employing an angle below 10 deg to minimize ion mill damage. Ion irradiations were done with 1.5 MeV Kr + and Xe ÷ at temperatures from 22 to 673 K and observed in situ using the HVEM Tandem Facility at Argonne National Laboratory [14]. During ion irradiation, selected area electron diffraction, SAED, was monitored employing a spread electron beam to avoid electron beam damage. Specimens were ion-irradiated until the original structure was amorphized or completely modified as evidenced by the disappearance of the original SAED pattern. Other specimens were irradiated to a lower fluence, 8.5 × 1012 i o n s / c m 2 (0.026 displacements per atom, dpa, for T1-2212 and T1-1212 with 1.5 MeV Kr ÷ which is large enough to suppress Tc). These were irradiated over the same temperature range and were subsequently studied with high resolution transmission electron microscopy, HRTEM, to observe isolated damage domains and damage recovery. The JEOL 2010 at the University of New Mexico was used to obtain the HRTEM data, which were collected using the Gatan 694 slow-scan camera coupled with Gatan's Digital Micrograph software package. In addition, nano-beam energy dispersive spectroscopy, EDS, data were obtained. The resistive transition temperature, Tc ( p - - 0 ) , was determined for thin films from low frequency ac four-point probe measurements and was defined as the temperature at which p = 0 to within the resolution of the instrument. This definition corresponds well with the Meissner effect diamagnetic onset temperature, T~ (M onset). Magnetic field expulsion (magnetization, M) due to the Meissner effect was determined by cooling the sample in an applied field of 2.5 mT. The Tc (M onset) was defined as the temperature at which the Meissner signal dominated the normal state paramagnetism. The effects of ion bombardment on T1-2223 films

245

were studied by irradiating T1-2223 samples up to a damage level which caused a loss of superconductivity. The resistivity, p, and the magnetization were measured at each increment of fluence as a function of temperature. In order to determine the damage mechanism, the incident ion species were varied from He + to 0 2+ to Au 5+, in order to bracket the irradiation conditions produced by the Kr ÷ and Xe + irradiations stated above. A sample was irradiated with 20 MeV Au 5+ ions and annealed immediately after irradiation for 15 min at 600°C, in order to measure the recovery of original resistance versus temperature characteristics. In order to measure an activation energy for damage recovery, T1-2223 thin films were irradiated with 500 keV He ÷ and 740 keV O 2÷ ions. Normal state p was measured in situ with respect to time after irradiation and an Arrhenius plot of the time constants versus irradiation temperature was generated (for the He +-irradiated samples). The ion energies for each irradiation and were chosen to put a uniform damage profile throughout the sample, averaged over many ion strikes.

IT1-2212 1.5MeV X e ~

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~

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only

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Irradiation T e m p e r a t u r e , K Fig. 1. The critical fluence, De, is the maximum dose of irradiation where, by in situ electron diffraction, the original structure no

longer exists. Dc is plotted with respect to the temperatureof the sample during irradiation. The legend shows the HTSC sample type and the ion irradiation energy. The open symbols represent complete amorphization, the hatched symbols represent partial amorphization and crystallization, and the solid symbolsrepresent complete crystallization.

246

P.P. Newcomer et aL / Physica C 267 (1996) 243-253

3. Results and discussion

The D c for amorphization or complete modification during Kr ÷ and Xe ÷ ion irradiation of T1-2212 and T1-1212 was found to be temperature dependent. The temperature dependence behavior for D c, shown in Fig. 1, is in marked contrast to other complex ceramics [14-17]. The studies of temperature dependence of Dc in ZrSiO 4 and other complex ceramics [15,16], reveal De decreases consistently towards zero K. During 1.5 MeV Kr ÷ and Xe ÷ ion beam irradiations, in situ observation showed both the TI-2212 and T1-1212 initially undergo an irradiation-induced crystalline to amorphous transformation as indicated by the development of a diffraction halo. The D~ value and HRTEM observations of isolated damage (low-fluence irradiation) were compared at the different irradiation temperatures in order to attain a better understanding of the irradiation-induced behavior of these materials. In low-fluence irradiation cases, the damage profile may vary with depth and Fourier transforms of TEM images may reveal periodic structure, although at some depth through the observed thickness an amor-

phous region may exist. This is consistent with a model by Miller [18] who has shown 30% of a structure through the observed thickness can be periodic or amorphous and yet the HRTEM image does not reveal the damage. Complete amorphization occurred at irradiation temperatures of less than 470 down to 90 K. Unexpectedly, the minimum D c was at 90 K, where complete amorphization occurred rapidly. Further, at an irradiation temperature of 22 K amorphization and nucleation of cascade-size crystallites occurred, where complete amorphization was expected; and at 470 K amorphization and nucleation of approximately 10 nm crystallites occurred. These results are consistent with the temperature and cooling rate of a displacement cascade using a one dimensional classical heat transfer calculation [19]. Above 500 K, ion irradiation did not result in amorphization. Diffraction rings developed in the SAED pattern and no halo was observed, indicating a polycrystalline matrix with no amorphous component. HRTEM shows 10 to 20 nm thallous oxide, TI20 crystallites well dispersed through the polycrystalline matrix. The Tl-type HTSC phase is poly-

Fig. 2. After ion irradiation at 22 K, HRTEM images of microstructurc looking down (001) are compared after low fluence (8.5 x 10 ~2 ions/era 2 0.026 dpa) and D c (and 3.2 × l012 ions/era 2 0.92 dpa) on Tl-1212 with 1.5 MeV Kr +. (A) After low flucnce at 22 K, localized amorphous domains and moire fringes indicate damage takes place in the displacement cascade. (B) After D c at 22 K, nano-crystaUites arc found embedded in an amorphous ma~'ix.

P.P. Newcomer et a l . / Physica C 267 (1996) 243-253

crystalline with random orientations and grain sizes of about 100 nm. In contrast, after irradiation at 470 K, HRTEM revealed 10 to 20 nm thallous oxide crystallites in an amorphous matrix. After ion irradiation at 300 K, HRTEM of lowfluence specimens (8.5 X 1012 ions/cm 2 0.026 dpa for T1-2212 and T1-1212 with 1.5 MeV Kr ÷, and revealed amorphization occurred in localized damage domains. In addition, D c for T1-1212 was 8.5 X 1013 ions/cm 2 (0.26 dpa) and for TI-2212 a value of 1.30 x 1014 (0.40 dpa). It took longer to amorphize the more thallium-rich phase, a phase which has been found to be more stable in phase equilibria studies [20], but the dpa was greater indicating more damage recovery occurred. Further, D c with 1.5 MeV Xe ÷ was lower than for Kr ÷, 7.48 × 1013 ions/cm 2 (0.53 dpa), and the dpa was greater indicating the larger ion damaged the structure more easily. Ion irradiations at 90 K (near HTSC T~) resulted in the minimum D c (4.25 X 1013 ions/cm 2, 0.33 dpa for T1-2212 with 1.5 MeV Xe÷; 8.5 x 1013 ions/cm 2, 0.26 dpa for T1-2212 and 6.8 x 1013 ions/cm 2, 0.21 dpa for T1-1212 with 1.5 MeV Kr ÷)

247

to completely amorphize the original structure. Damage accumulation was rapid. After irradiation, HRTEM of low fluence, 8.5 x 1012 ions/cm 2 (0.026 dpa), revealed amorphous domains were not localized, probably due to rapid overlap of displacement cascades, as shown in Fig. 2. Thermal recovery was negligible. Dark contrast regions, 20 to 100 nm in size were observed and were found to be amorphous. D c at 22 K, was much higher than expected (3.13 X l012 ions/cm 2, 2.33 dpa for T1-2212 with 1.5 MeV Xe÷; 4.0 x 1012 ions/cm 2, 1.25 dpa for T1-2212 and 3.2 X l012 ions/cm 2, 0.92 dpa for T11212 with 1.5 MeV Kr+). HRTEM revealed circular, misoriented, well-dispersed, cascade-size crystallites (4-6 nm) in an amorphous matrix. After a low fluence of 8.5 x 1012 (0.026 dpa) at 22 K, HRTEM revealed crescent-shaped amorphous domains. In addition, moire fringes were observed which indicate nucleation of a structurally mismatched phase. These observations are shown in Fig. 3. In addition, the crystallites in the Xe+-irradiated specimens at very low temperature are larger (6-10 nm) than those in the Kr+-irradiated specimens (2-5 nm). Nano-beam EDS revealed that these crystallites are thallium-rich.

Fig. 3. After ion irradiation at 90 K, HRTEMimages of microstructurelooking down (001) are comparedafter low fluence (8.5 × 1012 ions/cm2 0.026 dpa) and Dc (6.8 X 1013ions/era2, 0.21 dpa) for T1-1212with 1.5 MeV Kr+. (A) After low fluenceat 90 K, the extensive damage indicatesdisplacementcascades rapidly overlappedand negligiblerecoveryoccurred.(B) At the minimum De, at 90 K, complete amorphization occurredrapidly.

248

P.P. Newcomer er al. / Physica C 267 (1996) 243-253 i

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Fig. 4. (a) The Meissnersignal for a T1-2223 film as a function of temperature for successive He+ fluences. Tc (M onse0 was defined as the temperatureat which the diamagnetic signal dominates the paramagneticsignal. (b) At higher fluences, the Meissher signal at 5 K decreases rapidly. A fluence of 1.45X 1016 He+/cm~ (0.020 dpa) suppresses superconductivitybelow 5 K.

The reduced thallium phase, thallous oxide, has been identified by SAED. The Meissner signal for the He +-irradiated T12223 films is shown in Fig. 4a, Fig. 4b as a function of temperature for different He + fluences. The unitradiated T1-2223 film had a Tc (M onset) of 102 K which corresponded well with the Tc ( p = 0) value of 104 K. The diamagnetic shielding signal ( - M ) for a fixed temperature (e.g., 60 K in Fig. 4a or 20 K in Fig. 4b) was observed to decrease as the damage level increased beyond 0.0014 dpa. This decrease may result from either a decrease in the amount of superconducting material or an increase in the flux

pinning; however, the curves in Fig. 4a all tend to the same magnetization value at low temperatures, indicating little change in the flux expulsion with increasing fluence which suggests a negligible change in the flux pinning. A fluence of 1 x 1012 H e + / c m 2 (0.000014 dpa) had no effect on T~ (M onset) or the pinning barrier. In fact, the transition did not begin to broaden until a fluence of 1 x 1014 H e + / c m 2 (0.0014 dpa), but the transition continued to broaden with increasing fluence. At 1 x 1015 H e + / c m 2, T~ (M onset) decreased by 7.8% to 94 K. Further implantation to a fluence of 2 x l015 H e + / c m 2 caused To to decrease to 83 K. At 4 x 1015 H e + / c m 2 and higher fluences, the diamagnetic shielding decreased rapidly (even at 5 K). A fluence of 1.45 X l016 H e + / c m 2 (0.020 dpa) caused complete suppression of superconductivity as measured down to 5 K. Each value of T~ (M onset) for the irradiated samples corresponded well with the value of T~ ( p - - 0 ) for the same fluence, and both measures of Tc decreased at the same rate as a function of deposited energy. Therefore, the T c / T° (ratio of the T~ of the irradiated to the unirradiated film) for the He + irradiations is also representative of the decrease in the transition temperature determined from the Meissner signal. Decreases in the T~ (M onset), Meissner signal, temperature determined from Meissner measurements showed the suppression of Te occurs throughout the film indicating superconductivity is suppressed in the bulk of the material. The rate of decrease in the transition temperature (T~) was determined to be a material property. A damage level of 0.020 dpa suppressed superconductivity below 5 K in these films. In order to compare the decrease in T~ ( p = 0) for the different irradiations on a linear plot, the decrease in T~ normalized by T° was plotted as a function of dpa created by collisional energy deposition (Fig. 5). T J T ° decreases approximately linearly as a function of collisional damage although heavier ion irradiations cause a slightly more negative deviation from linearity at the higher damage levels. One possibility for this deviation may be in the different size of damage cascades for these ions and the amount of thermal recovery of the damage for each size cascade. The resistivity, p, as a function of temperature for successive damage levels are shown in Fig. 6a-Fig.

249

P.P. Newcomer et a l . / Physica C 267 (1996) 243-253 i ....

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Fig. 5. The transition temperature (Tc) normalized to the unirradiated state (T°) as a function of damage from atomic collisions in T1-2223. An extrapolation of the data shows that T~/ T° goes to zero at approximately 0.020 dpa. The fact that the He +, O 2+ , and Au5+ irradiations all cause approximately the same rate of decrease in Tc / T ° indicates that collisional damage dominates ionization effects.

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6c. T~ ( p = 0) for the samples in the unirradiated state (T° ) were: 104 K for the He + irradiation sample, 99 K for the 0 2+ irradiation sample, and 97 K for the Au 5+ irradiation sample. The room temperature resistivities, PaT, for the unirradiated samples were: 0.8 m t I - c m for the He + sample, 1.4 m f l . cm for the 0 2+ sample, and 1.7 m ~ . cm for the Au 5+ sample. The p - T data are similar for the He +, O 2+, and Au 5+ irradiations in that Tc ( p = 0) decreases rapidly with increasing ion fluence, but over three orders o f magnitude more He + fluence is needed to cause a decrease in T~ ( p = 0) similar to that decrease caused by Au 5+ irradiation. Also, each sample showed an increase in the normal state resistivity as the level of damage increased, but again differing by orders o f magnitude in the fluence needed for comparable increases in PaT" Fig. 7 shows the change in the room temperature resistivity compared to the unirradiated state as a function of dpa for each set o f ion irradiations. ( A p = p R r ( i r r a d i a t e d ) - PRT (unirradiated).) Initially, A p increases approximately linearly with deposited energy, but for higher damage levels the resistivity increase is superlinear. The normal-state resistivity was found to recover faster for the He+-irradiated sample than for the O 2+ or AuS+-irradiated samples [2].

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Temperature (K) Fig. 6. (a) The resistivityversus temperature in TI-2223 as a function of 2 M c V He + ion irradiation:(a) unirradiatcd;(b) I Xl015 He+/cm2; (c) 2 × I015 Hc+/cm2; (d) 4XI015 He+/cm2; (e) 7 × I015 He+/cm2; (f) ll.5× 1015 H e + / c m 2. (b) The resistivityversus temperature in TI-2223 as a functionof 740 kcV 0 2+ ion irradiation:(a)unirradiated;(b) 7× 1012 O2+/cm2; (c) 2.1X 1013 O2+/cm2; (d) 4.2X 1013 O2+/cm2; (e) 1 X 1014

0 2+/cm 2. (c) The resistivity versus temperature in T1-2223 as a function of 20 MeV Au5+ ion irradiation: (a) unirradiated; (b) 1.45x 1012 AuS+/cm2; (c) 1.92X 1012 AuS+/cm2; (d) 2.38× 1012 AuS+/cm2; (e) 2.96× 10t2 AuS+/cm 2.

250

P.P. Newcomer et a l . / Physica C 267 (1996) 243-253

In order to better understand the rate and mechanisms of damage recovery, a T1-2223 sample was irradiated with the Au 5+ ion beam to a damage level of 0.010 dpa and was annealed for 15 min at 600°C. The sample nearly recovered its original p--T characteristics. In order to get a measure of the mobility of defects at room temperature, a more detailed examination of the recovery of PRT was done. A sample was irradiated at room temperature with 740 keV 0 2+ ions to 0.006 dpa and allowed to come to an equilibrium level of damage. The sample was then irradiated at room temperature an additional 0.0014 dpa and the recovery of PRT as a function of time was monitored in situ immediately after the irradiating beam was stopped. Further, T1-2223 samples were irradiated with 500 keV He + [2] ions at temperatures from 100 to 375 K to a damage level of 0.001 dpa. The rate of recovery in p was monitored in situ immediately after the irradiation was stopped in order to determine the time constant for thermal recovery. These data are shown in Fig. 8. The rate of damage recovery below 250 K was prohibitively long and above 375 K the oxygen loss to the vacuum became significant. This demonstrates that for temperatures below 250 K the rate of diffusion is too low for damage recovery and therefore the D c is expected to be lower at these temperatures as is found and shown in Fig. 1.

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Assuming p increases proportionally to damage, the rate of decrease in p as a function of time after irradiation gives a measure of the annealing of the damage. If the kinetics of the recovery process are controlled by atomic diffusion, then the fraction, f, of material which remains in the defective state at time, t, is given by [21] f = e x p [ - ( t / 7 ) ' ] , where ~is the time constant characteristic of the annealing process and n is the order of the kinetics. The value of f can be determined from the resistivity fraction: f = [ p(t) - p(0)]/[ p0 _ p(0)], where p(0) is p at time zero after irradiation and p0 is p for the sample before irradiation [2]. Simple thermal annealing of defects by random diffusion to a uniform distribution of sinks would follow first-order reaction kinetics, n = 1, and this behavior is followed by the data in Fig. 8. In contrast, the room temperature damage recovery from the 0 2+ irradiation exhibited a behavior more consistent with n = 0.5. This difference probably reflects the difference in the density of the damage cascade for He + and 0 2+ irradiation. The more dense 0 2+ cascade may show a greater dependence on recombining interstitials with vacancies of similar type atoms. The solution to this correlated vacancy-interstitial annihilation reaction was given by Waite [22] and can be approximated by n = 0.5. The curve for A p = [ p ( 0 ) p ° ] e x p [ - ( t / ~ ) l / 2 ] then gives a time constant, r, equal to 1.3 × 107. Annealing of defects at room

P.P. Newcomer et aL / Physica C 267 (1996) 243-253 10 s

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(K "~)

Fig. 9. The damage-recovery time constant (r) for annealing of irradiation damage in T1-2223 was determined from the time rate of change in p at temperature T immediately after the ion beam was stopped, as shown in Fig. 3. Q is the activation energy for the recovery process.

temperature has a large time constant, but as the temperature is increased r is expected to decrease exponentially. The He + data was then fit by exponential curves (n = l) I to determine ~- for each temperature. An activation energy was determined for the damage recovery process of the He+-irradiated sample, as shown in Fig. 9, for the temperature range from 273 to 373 K. The activation energy (Q) for the recovery process can be determined from the temperature behavior of r. From an Arrhenius plot of ~with respect to irradiation temperature, an activation energy for damage annealing was determined from the slope. The Arrhenius behavior of ~ = roexp(Q/kT), as shown in Fig. 9, was observed which is characteristic of a thermally activated process with a single activation energy of 0.4 eV. This small value for the activation is consistent with the idea that a defect formation energy is not needed under irradiation conditions, and it indicates that a relatively small energy is needed for atomic motion.

4. Summary and conclusions Temperature dependent damage recovery during irradiation was indicated by in situ D c measure-

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ments, by HRTEM observation of microstructural changes in the irradiated Tl-type HTSC, and by in situ measurements of the recovery of normal state resistance immediately after irradiation. It is believed that during ion irradiation of T1-1212 and T1-2212, amorphization occurs directly in the displacement cascades just as in most other complex ceramic materials [14] based on observation of isolated damage domains by HRTEM after low fluence (8.5 × 1012 ions/cm 2 0.026 dpa) 1.5 MeV Kr ÷ and Xe ÷. This was observed at irradiation temperatures below 470 K. After irradiation at 90 K (near the HTSC T~) large regions of damage and less discrete amorphous domains were observed with HRTEM after lowfluence irradiation, indicating rapid overlap of displacement cascades. This suggests damage accumulation takes place in the displacement cascades; but accumulation is very rapid because no thermal anhealing occurs. This is possibly due to the increase in thermal conductivity below superconducting T~. After irradiation at a temperature well below Tc, HRTEM revealed localized amorphous domains and moire fringes. In samples which received Dc at those low temperatures, HRTEM revealed nano-crystallites in an amorphous matrix. This indicates damage recovery processes play a role at very low temperature. The temperature dependence of the ion irradiation damage in T1-2212 and T1-1212, shows a significant relation to the remarkable change in thermal conductivity, K(T), as a function of temperature in the HTSC [12]. The structure is more easily amorphized at irradiation temperature near Tc, because K(T) has a sharp increase at Tc and maximizes at approximately 0.5 To, and heat generated by a thermal spike would easily dissipate. Therefore, displacement cascades overlap and negligible recovery takes place. The inability to completely amorphize the Tl-type HTSC single crystals well below T~, may be related to the drastic reduction in K(T) below approximately 0.5 Tc. Thermal annealing takes place at temperatures as low as 22 K, because the heat generated by the thermal spike during irradiation is not easily dissipated. Low thermal conductivity during the thermal spike could confine the thermal spike heating to a localized region. The nucleation of thallous oxide in this study raises several points. First the question of why the TI20 phase rather than TI20 3. This may be related

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to a thermal spike process under vacuum, to the availability of TI ~+, and to the lower melting temperature of that phase (573 K). During irradiation under vacuum, an equilibrium model allows for redistribution of any transferred electrons from the displaced thallium or oxygen atoms within the cascade to the neighboring undamaged matrix. Annealing studies of HTSC have established that oxygen balance has relatively wide tolerance. A model has been proposed previously, based on the small atomic displacements determined by X-ray diffraction, in single crystal T1-1223 annealed under oxygenating and reducing environments, that the HTSC contain both Wl3+ and T1 l+ in a complex equilibrium with respect to the amount of oxygen present at sites on the T1-O planes as well as to the density of hole carriers [23]. Because of the reducing environment (vacuum) during irradiation of the T1-1212 and T1-2212, the low melting point of T120, and the inability to readily dissipate heat from the thermal spike, nano-size well-dispersed globules of TI20 would crystallize within the cascade. The results of the magnetization and resistivity studies demonstrate that the degradation in the transport and magnetic superconducting properties is caused by the same mechanism in that both Tc (M onset) and Tc ( p = 0) show the same dependence on damage. Also, the decrease in T~ (M onset) as a function of damage characterizes a general intragranular decrease in superconductivity rather than simple disruption of transport paths at grain boundaries because the Meissner signal reflects superconductivity throughout the film rather than a percolation path for carrier transport across the film. Further, our studies on ion irradiation of 50 /~m thick T1-2223 single crystals [24] shows that the decrease in Tc (M onset) for single crystals follows the same damage dependence as given by the curves in Fig, 5. Therefore, these curves are representative of a material property of T1-2223 rather than a property which is dependent on differences in the sample microstructure. The He ÷ ions have a greater fraction of their deposited energy in the form of ionization than the 02÷ or Au 5+ ions, nonetheless the rate of decrease in T J T ° for the different irradiating species is nearly equal. Thus, although small deviations are present between the T J T ° curves for the different

irradiating ions, the predominant mechanism goveming the loss of superconductivity in T1-2223 is from atomic collisions. A collisional damage level of 0.020 dpa is needed in order to suppress T~ below 5 K. The predominant dependence on collisional damage was observed not only in a change in the superconducting properties (Tc) but also in a change in the normal state properties (Ap). The increases in the room temperature resistivities (Fig. 7) scale with the collisional damage level rather than the fluence level. These results suggest that the defects created by ion irradiation are created by collisional damage rather than ionization, and the accumulation of these defects causes the decrease in Tc and increase in A p. Further, the effects of room temperature annealing can also help to explain some of the differences in the rate of decrease of T J T ° and in the rate of increase of Ap for the He + and Au 5+ ion irradiations. If the amount of room temperature annealing were greater for the He +-irradiated sample than for the AuS+-irradiated sample, then at each level of calculated damage the He +-irradiated sample would show an increase in T c / T ° and a decrease in A p as compared to the AuS+-irradiated sample. Several factors can induce the amount of room temperature annealing which occurred in these samples. For example, different defect densities from a single cascade can lengthen the distance over which defects must diffuse in order to annihilate damage. A detailed three-dimensional analysis of the cascades created by 20 MeV Au 5+ and 2 MeV He + in T1-2223 yielded the following relative results. The cascades for the 2 MeV He + ions generally do not progress beyond one secondary event whereas the cascades for the 20 MeV Au 5+ ions do produce defects from higher order events. The lighter target atoms are recoiled farther than the heavier target atoms for both Au 5÷ and He + irradiation. The light target atoms recoiled by the He + ions come to rest at a distance of 1-1.5 nm from the point of vacancy production and the heavy target atoms come to rest at a distance of 0.2-0.4 nm from the point of vacancy production. In comparison, the light atoms recoiled by Au 5÷ ions travel from 2 nm to tens of nm before coming to rest and the heavier atoms recoiled by the Au 5+ ions can travel 1-8 nm before coming to rest. If the amount of overlap area between different vacancy-interstitial pairs is negligi-

P.P. Newcomer et al./Physica C 267 (1996) 243-253

ble, then the annihilation o f defects occurs by recombining an interstitial with the vacancy from which it was produced. (This assumption is reasonable for the low fluences used in these experiments and for the model given above that these samples recover to the complicated tetragonal perovskite structure without site substitutions, i.e., without c a t i o n - o x y g e n substitutions.) The diffusion distances for defect annihilation in the He +-irradiated samples is shorter than the diffusion distances in the AuS+-irradiated samples and thus the He +-irradiated samples show increased room temperature recovery from the ion irradiation effects. A comparison of He +, O 2+, and Au 5+ ion-irradiated polycrystalline T1-2223 films shows that irradiation-induced defects cause identical rates of degradation in magnetic and transport superconducting properties, and these defects are created predominantly by atomic collisions. The rate of increase in the normal state resistivity as a function of ion damage is also predominantly governed by atomic collision processes. A damage level o f 0.020 dpa is sufficient to suppress superconductivity below 5 K in T1-2223. The activation energy for damage recovery is 0.4 eV. Ion irradiation-induced amorphization in Tl-type H T S C occurs in the displacement cascades as a function of both damage recovery and damage accumulation. The recovery o f irradiation damage is diffusion controlled which is strongly dependent on temperature. The higher irradiation temperature corresponds to greater damage recovery and less tendency toward amorphization. Further, near Tc the thermal conductivity is increased; therefore, damage recovery would be less because of heat loss in the displacement cascade (quenching). Thus, D c is found to exhibit a remarkable temperature dependence consistent with atomic diffusion-controlled recovery.

Acknowledgements W e appreciate Ed Ryan, at the H V E M Tandem Facility at Argonne National Lab, for support with in situ ion beam irradiation and Dave Brice for help with TRIMRC. This work was supported by the Office of Basic Energy Sciences, US Department o f

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Energy under U N M ' s grant DE-FG03-93ER45498 and Sandia National L a b ' s DE-AC04-94AL85000.

References [1] E.L. Venturini, C.P. Tigges, R.J. Baughman, B. Morosin, J.C. Barbour and M.A. Mitchell, J. Crystal Growth 109 (1991)441. [2] J.C. Barbour, E.L. Venturini, D.S. Ginley and J.F. Kwak, Nucl. Instrum. Methods B 65 (1992) 531. [3] J.C. Barbour, E.L. Venturini and D.S. Ginley, Nucl. Instrum. Methods Phys. Res. B59/60 (1991) 1395. [4] G.J. Clark, A.D. Marwick, R.H. Koch and R.B. Laibowitz, Appl. Phys. Lett. 51 (1987) 139. [5] A.E. White, K.T. Short, D.C. Jacobson, J.M. Poate, R.C. Dynes, P.M. Mankiewich, W.J. Skocpol, R.E. Howard, M. Anzlowar, K,W. Baldwin, A.F.J. Levi, J.R. Kwo, T. Hsieh and M. Hong, Phys. Rev. B 37 (1988) 3755. [6] G.C. Xiong, H.C. Li, G. Linker and O. Meyer, Phys. Rev. B 38 (1988) 240. [7] G.P. Summers, E.A. Burke, D.B. Chrisey, M. Nastasi and J.R. Tesmer, Appl. Phys. Lett. 55 (1989) 1469. [8] R.C. Ewing, Nucl. Instrum. Methods Phys. Res. B 91 (1994) 22. [9] L.W. Hobbs, F.W. Clinard, S.J. Zinkle and R.C. Ewing, J. Nucl. Mater. 216 (1994) 291. [10] J.C. Barbour, J.F. Kwak, D.S. Ginley and P.S. Peercy, Appl. Phys. Lett. 55 (1989) 507. [11] R.S. Averback, Nucl. Instrum. Methods Phys. Res. B 15 (1986) 675. [12] C. Uher, J. Supercond. 3 (1990) 337. [13] B. Morosin, E.L. Venturini, R.G. Dunn, S.E. Schirber and P.P. Newcomer, Bull. Am. Phys. Soc. 39 (1994) 878. [14] L.M. Wang and R.C. Ewing, MRS Bull. 17 (1992) 38. [15] W.J. Weber, R.C. Ewing and L.M. Wang, J. Mater. Res. 9 (1994) 688. [16] L.M. Wang, W.L. Gong and R.C. Ewing, Mater. Res. Soc. Symp. Proc. 316 (1994) 247. [17] L.M. Wang, R.C. Birteher and R.C. Ewing, Nucl. lnstrum. Methods Phys. Res. B80/81 (1993) 1109. [18] M.L. Miller and R.C. Ewing, Ultramicroscopy 48 (1993) 203. [19] E. Krzyzig, Engineering Math., 7th edn. (Wiley, 1993). [20] T.L. Aselage, E.L. Venturini and S.B. Van Deusen, J. Appl. Phys. 75 (1994) 1023. [21] J.W. Christian, The Theory of Transformations in Metals and Alloys, Part I, 2nd. edn. (Pergamon, New York, 1975) p. 542. [22] T.R. Waite, Phys. Rev. 107 (1957) 471. [23] B. Morosin, E.L. Venturini and D.S. Ginley, Physica C 183 (1991) 90. [24] E.L. Venturini, J.C. Barbour, D.S. Ginley, R.J. Baughman and B. Morosin, Appl. Phys. Lett. 56 (1990) 2456.