Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47

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Nuclear Instruments and Methods in Physics Research B

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Effect of doping on the radiation response of conductive Nb–SrTiO3

Weixing Li a,⇑, Matias D. Rodriguez b, Patrick Kluth b, Maik Lang a, Nikita Medvedev c, Michael Sorokin d,Jiaming Zhang a, Boshra Afra b, Markus Bender e, Daniel Severin e, Christina Trautmann e,Rodney C. Ewing a,f,g

a Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USAb Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australiac Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germanyd National Research Centre ‘Kurchatov Institute’, Kurchatov Sq. 1, 123182 Moscow, Russiae GSI Helmholtz Centre for Heavy Ion Research, Planckstr. 1, 64291 Darmstadt, Germanyf Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USAg Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 January 2013Received in revised form 6 March 2013Available online 23 March 2013

Keywords:Track formationAmorphizationSrTiO3

Non-thermal processThermal spikeConductivity

0168-583X/$ - see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.nimb.2013.03.010

⇑ Corresponding author. Tel.: +1 734 615 2048; faxE-mail address: wxli@umich.edu (W. Li).

Based on the Coulomb spike model, track formation depends strongly on the electrical resistivity of amaterial, and ion tracks form only in insulating materials. However, there are no systemic studies ofthe effect of resistivity on the track formation in materials, such as SrTiO3 (STO), where with the additionof low concentrations of Nb, the resistivity dramatically decreases covering the entire electronic regimefrom an insulating to conducting material. In this study, high energy (8.6 MeV/u) ion-induced track for-mation in STO was characterized by transmission electron microscopy (TEM) and small-angle X-ray scat-tering (SAXS) techniques as a function of Nb-doping concentrations. Contrary to the Coulomb spikemodel’s predictions, the Nb-doping had no evident influence on track formation, as confirmed by bothTEM and SAXS. This may be the result of the low electron density in the bulk material or the minor effectof the Nb-doping on the bonding in the material. In situ TEM studies of low energy (1 MeV Kr2+) ion irra-diations show that the low concentration doping has a minor influence on the crystalline-to-amorphoustransformation as a result of subtle structural variations of incorporated impurity atoms.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The response of materials to ion irradiation is of significant inter-est in nuclear engineering [1–3], particle damage in microelectron-ics [4], nanoengineering [5–7], and fission track thermochronology[8,9]. The slowing down of ions in matter is characterized by twoindependent processes: electronic energy loss, �(dE/dx)e = Se, dueto the electronic excitation and ionization, dominated at high-energy regime (MeV–GeV), and nuclear energy loss, �(dE/dx)n = Sn,due to elastic collisions, dominated at low-energy regime (keV–MeV). The formation of tracks created by high-energy heavy ionsdepends, in part, on the electrical resistivity of the target material[10,11]. According to the Coulomb spike model, tracks do not formin conducting materials because the electrical conduction promptlyneutralizes the positive core [10,11]. In contrast to this prediction,tracks have been observed in a few metallic materials [12–14],although the energy loss threshold for track formation is usuallymuch higher as compared with insulators. However, there are very

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: +1 734 647 5706.

few studies of the effect of electrical resistivity on track formation.In order to conduct such a study, the major difficulty is to separatethe electron density from other physical properties, as tracks areformed in different materials, where many physical properties otherthan electron density can vary.

Strontium titanate perovskite, SrTiO3 (STO), is an importantmaterial because of its wide variety of applications in electronics[15–17], optics [18,19] and nuclear materials [7,20,21], and in clearenergy research [22]. At room-temperature, STO is an insulator(band gap: �3.2 eV), with a simple cubic perovskite structure(Pm �3 m). However, both the growth of SrTiO3�d in an oxygen defi-cient atmosphere and the chemical substitution (e.g., La3+ for Sr2+,or Nb5+ for Ti4+) can make the originally insulating STO semicon-ducting [16] or even metallic [18,19], due to the upward shift ofthe Fermi level into the conduction band [23,24]. Therefore, theinsulator-to-metal transition in STO provides a unique opportunityto study the details of how the electronic properties affect radia-tion response.

Because amorphous tracks have been observed in insulating STOirradiated by 92 MeV Xe ions [21,25], this paper describes the firstsystematic investigation of the effects of low concentrations of

W. Li et al. / Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47 41

impurities on track formation in a material, e.g., STO, that trans-forms being an insulator to being a conductor [16,23]. Sampleswere characterized by TEM and SAXS measurements after irradia-tion by swift heavy ions (2.0 GeV U or 1.7 GeV Au). Due to its com-plex behavior during low-energy ion irradiations, perovskite,ideally ABO3, has been of interest because of its potential use as anuclear waste form [1,20,26–28]. Although the radiation damageis generally attributed to atomic collisions for low-energy ion irra-diations, a notable ionization effect has been suggested for damagein SrTiO3 [28]. We have therefore extended the investigation to in-clude the influence of electronic conductivities, as well as ionizationeffect on the low-energy ion irradiations (1 MeV Kr2+) that induce acrystalline-to-amorphous transformation. The experimental resultsfor both energy loss regimes are discussed with respect to the elec-tronic conductivity, the bond-type, atomic structure and defectmigration.

Fig. 1. The electronic energy loss, �(dE/dx)e = Se, and nuclear energy loss, �(dE/dx)n = Sn, as a function of depth, x, for both (a) high- and (b) low-energy ionirradiations in STO. There are no obvious variations in energy deposition betweenthe pure STO and the doped samples.

Table 1

2. Experimental

High-quality single crystals of Nb-doped STO were grown bythe conventional Verneuil method (MTI Corporation). Sampleswith Nb concentrations of 0, 0.1, and 1 wt.% were obtained withcorresponding resistivities of �104, 8 � 10�2 and 3.5 � 10�3 X cm,respectively, at room temperature [19,29]. This resistivity rangecovers the separation between track formation and non-track-forming materials �2 � 103–2 � 104 X cm, as defined by Fleischeret al. [10,11,30]. A doping concentration higher than 1 wt.% is notcommercially available for high-quality single-crystal samples.Irradiation experiments were completed with two types of ion pro-jectiles: (1) high-energy (8.6 MeV/u) ions, i.e., 2.0 GeV U ions of flu-ence 7.5 � 1010 ions/cm2 or 1.7 GeV Au ions of fluence5 � 1010 ions/cm2, and (2) low-energy ions (1 MeV Kr2+). In a pre-vious study of Ca-Y3Fe5O12, the amorphization cross-section wasfound to be insensitive to the resistivity of the pure phase [31].However, the resistivity range (104–109 X cm) in this materialwas limited to the insulating regime.

Irradiation parameters (ion species and irradiation temperature), and track radius, R,deduced by the fitting of the SAXS patterns. Radius polydispersity, rr, is a fittingparameter.

Nb (wt.%) Temp. Ions R(Å) rr(Å)

0 RT 2.0 GeV U 31.3(0.1) 4.1(0.1)0.1 RT 2.0 GeV U 30.7(0.2) 3.1(0.1)1 RT 2.0 GeV U 31.2(0.2) 3.8(0.2)0 RT 1.7 GeV Au 22.8(0.5) 3.4(0.1)0.1 RT 1.7 GeV Au 23.6(0.5) 3.6(0.2)1 RT 1.7 GeV Au N.A. N.A.0 24 K 1.7 GeV Au 22.8(0.2) 3.4(0.2)0.1 24 K 1.7 GeV Au 19.8(0.3) 3.2(0.2)1 24 K 1.7 GeV Au N.A. N.A.

2.1. Swift ion tracks

The swift ion irradiations were performed at the UNILAC accel-erator of the GSI Helmholtz Centre for Heavy Ion Research Darms-tadt, Germany, by exposing a (100) surface of a single crystallinesample to a beam at perpendicular incidence. The samples werepre-polished to a thickness of about 40 lm to ensure that theGeV projectiles completely penetrated the specimens, inducingan approximately uniform electronic energy loss over their entiretrajectory, as shown in Fig. 1a (SRIM-2012 code [32]). Thus, thetrack radii are similar throughout the entire sample thickness,which is of advantage for the applied analytical techniques and re-lated sample preparation. The contribution of nuclear energy lossto track formation is negligibly small as compared with electronicenergy loss for most of the target depths of high-energy ions, espe-cially the regions near the irradiated surface (Fig. 1a). For bothhigh- and low-energy ion irradiations (Fig. 1), there are no obviousvariations in energy loss (both Se and Sn) between the pure STO andthe Nb-doped samples, because of the very low concentrations ofNb. The applied ion fluences (�5 � 1010 ions/cm2) were sufficientlylow to yield well separated aligned tracks. Table 1 summarizes allirradiation conditions: ion type and energy, irradiation tempera-ture, and Nb concentration for each sample. The damage morphol-ogy of the GeV ions induced tracks was investigated by a JEOL2010F TEM (Fig. 2). TEM samples were prepared by dispersingcrushed fine powders of the irradiated samples onto a holey carbongrid.

SAXS is a very useful tool for studying ion-track morphologyand dimensions [14,33–36]. Without further preparation, sampleswere investigated after irradiation using transmission SAXS at theSAXS/WAXS beamline at the Australian Synchrotron in Melbourne,Australia, with an X-ray energy of 12 keV and a camera length ofapproximately 1600 mm. The samples were mounted on a three-axis goniometer for precision alignment. Measurements were col-lected by a Pilatus 1 M detector with the ion tracks tilted between0� and 10� with respect to the X-ray beam. Additionally, scatteringwas measured from an unirradiated sample for background re-moval. Scattering images of 0.1 wt.% Nb-STO irradiated 1.7 GeVAu ions are shown in Fig. 3. The radially symmetrical image inFig. 3(a) is consistent with tracks aligned normal to the surfaceof the sample (or with tracks parallel to the X-ray beam). As shownin Fig. 3(b), tracks are tilted by 10� with respect to the X-ray beam

Fig. 2. Bright-field TEM images showing the morphologies of tracks created by 2.0 GeV U ions at room temperature in (a) undoped STO and (b) 1 wt.% Nb-doped STO, and by1.7 GeV Au ions at 24 K in (c) undoped STO and (d) 1 wt.% Nb-doped STO.

Fig. 3. SAXS images for 0.1 wt.% Nb-STO irradiated by 1.7 GeV Au ions at room temperature with tracks aligned at 0� (a) and 10� (b) with respect to the X-ray beam.

42 W. Li et al. / Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47

resulting in the highly anisotropic scattering apparent from the im-age. The slightly curved streaks in Fig. 3(b) with tilted tracks con-firm the high aspect ratio of the ion tracks that are only a fewnanometres wide and extend through the entire sample thickness.For analysis of the SAXS data, the scattering intensities correspond-ing to the tracks were extracted from the isotropic images of thealigned tracks (Fig. 3(a)), and by masking the streaks correspond-ing to the scattering of the tracks when tilted with respect to theX-ray beam (Fig. 3(b)). Both methods yield identical results.

At the given low fluences, the tracks are sufficiently separatedsuch that overlap effects are negligible. We can estimate theamount of track overlap using a simple overlap model:d ¼ 1� expð�pR2UÞ , where d is the area of the modified material,R is the track radius and U is the ion fluence [33]. With a fluence of<1 � 1011 ions/cm2 the amount of track overlap is less than 1% for

all track radii extracted from the SAXS measurements, confirmingisolated tracks. Given the random distribution of the tracks, thescattering amplitude thus only contains a form factor, i.e. no struc-ture factor that describes scattering related to ordered specialarrangement or overlap of the tracks need be considered. For acylindrical geometry, the form factor can be written as:

f ðqÞ ¼ 2pLZ 1

0qðrÞ � r � J0ðrqÞdr ð1Þ

where L is the track length (equivalent to the sample thickness),q(r) the track radial density distribution, q is the radial componentof the scattering vector q, and J0 the Bessel function of zero order.

A simple cylinder of radius R with sharp boundaries and con-stant density (different from that of the matrix material) best de-scribes the observed scattering. The track radial density

W. Li et al. / Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47 43

distribution for this model takes the form of a step function repre-sented by the Heaviside function H:

qðrÞ ¼ q0 �HðR� rÞ; ð2Þ

where q0 is the density difference between track and matrix mate-rial. The form factor then equates to,

f ðqÞ ¼ ð2pLRq0=qÞJ1ðR:qÞ; ð3Þ

where J1 is the Bessel function of first order. The scattering intensityis expressed as

IðqÞ / jf ðqÞj2: ð4Þ

A narrow Gaussian distribution of width rr for the track radiuswas implemented to account for deviations from perfectly aligned,identical tracks [36]. Therefore, the radius polydispersity rr entersas a fitting parameter in the scattering intensity as [14]

IðqÞ /Z

1rr

e� r�Rð2rr Þ2 jf ðqÞj2dr: ð5Þ

Fig. 4. Scattering intensities of tracks as a function of the scattering vector q andcorresponding fits with the hard cylinder model (solid lines). The irradiationconditions (irradiation temperature, Nb concentration and ion type) are marked foreach curve.

2.2. Low-energy ion irradiations

Irradiation-induced amorphization with low-energy ions(1 MeV Kr2+) was observed by in situ TEM using the IVEM TandemFacility at the Argonne National Laboratory over a temperaturerange from 100 to 373 K. The flux was approximately6.25 � 1011 ion/cm2/s. Samples were prepared for irradiationexperiments by dispersing crushed fine powders of Nb-STO on aholey carbon grid. Using the SRIM-2012 code [32], the ion rangewas calculated to be �350 nm for 1 MeV Kr2+ ions (Fig. 1b), whichis greater than the typical thickness of electron transparent sam-ples suitable for TEM observation (a few tens nm to �150 nm).The electronic to nuclear stopping power ratio (ENSP) at an averagedepth (�100 nm) is around 60% (Fig. 1b). Ion-induced amorphiza-tion was monitored for the different Nb-doped samples by selectedarea electron diffraction (SAED) patterns as a function of irradia-tion fluence. The critical amorphization dose is determined by apoint where all the diffraction maxima are no longer evident.

3. Results

3.1. Track formation

Parallel tracks have been observed by TEM after irradiation withGeV ions in the doped and undoped STO samples (Fig. 2). They areevident by the dark contrast of their damage trails with respect tothe unirradiated matrix for different ion projectiles (2.0 GeV U and1.7 GeV Au), and different irradiation temperatures (RT and 24 K).Most of tracks are continuous, but some show contrast fluctuationswith gaps along the ion trajectory, indicating a discontinuous trackcharacter. While tracks in apatite and zircon show a sharp bound-ary between the track core and surrounding matrix, [9,37] the con-trast of the tracks in both STO and the Nb-doped STO are ratherweak. The track radius as shown in TEM images is similar (�20–30 Å) for all STO samples, independent of the Nb-doping level.However, the weak contrast prevents a precise determination ofthe track radius by TEM as would be required to detect the effectsof variations in resistivity due to Nb-doping.

In contrast to the limited number of tracks in a localized area asobserved by TEM, the strong scattering oscillations from a verylarge number (�106) of well aligned, identical tracks in a bulk sam-ple, as detected by SAXS, provide a very reliable means for deter-mining the mean track radius. The scattering intensities of tracksand corresponding analytical fits to the hard cylinder model areshown in Fig. 4. As summarized in Table 1, the Nb-doping has no

significant influence on the mean track radius, R, which is in agree-ment within the experimental error for all samples. For example,2.0 GeV U ions at room temperature induce tracks in STO of radius31.3 ± 0.1 Å (undoped), 30.7 ± 0.2 Å (0.1 wt.% Nb), and 31.2 ± 0.2 Å(1 wt.% Nb). The errors correspond to the uncertainty of the fitting.The lack of correlation between track morphology and doping-levelwas also confirmed by the 1.7 GeV Au ion irradiation. However, asexpected, the larger Se (47 keV/nm) in the case of 2.0 GeV U ions re-sulted in a larger mean track radius �31 Å, as compared with the�23 Å track radius for 38 keV/nm energy deposition of the1.7 GeV Au ions. These values, as obtained by SAXS, are consistentwith the value (�20–30 Å) as observed by TEM. In addition, theirradiation temperature had no obvious influence on the track for-mation process in STO. SAXS measurements evidenced a very sim-ilar track size for the 1.7 GeV Au ion irradiations at 24 K and roomtemperature, respectively (Table 1).

3.2. Amorphization by low energy ions

In situ TEM studies of 1 MeV Kr2+ induced amorphization showthat at a given irradiation temperature, the difference in thecritical amorphization dose for STO samples with different Nb-concentrations is small (Fig. 5). The 1 MeV Kr2+ ion irradiation atroom temperature induces a complete amorphization in the un-doped STO sample at 1.75 � 1015 ions/cm2, or 3.6 displacementper atom (dpa) (Fig. 5(a–e)), which is slightly greater than the crit-ical amorphization dose of 1.25 � 1015 ions/cm2 or 2.5 dpa for the0.1 wt.% Nb-STO (Fig. 5(f–j)). The temperature dependence of thecritical amorphization dose for the Nb-doped STO samples irradi-ated with 1 MeV Kr2+ ions is shown in Fig. 5(k). The critical amor-phization dose increases with increasing temperature because ofdynamic annealing effects [26]. Above a critical amorphizationtemperature, Tc, the critical amorphization dose increases to infin-ity, and complete amorphization does not occur. The temperaturedependence of the amorphization dose, Dc, can be fitted by [38],

Dc ¼D0

1� Exp½Ea=kBTc � Ea=kBT� ; ð6Þ

Fig. 5. Amorphization of Nb-doped STO under 1 MeV Kr2+ ion irradiation. (a–e) SAED patterns for the undoped STO at room temperature as a function of fluence at: (a) 0, (b)4 � 1014, (c) 8 � 1014, (d) 1.6 � 1015, (e) 1.75 � 1015 ions/cm2. (f–j) SAED patterns for the 0.1 wt.% Nb-doped STO at room temperature as a function of fluence at: (f) 0, (g)4 � 1014, (h) 8 � 1014, (i) 1 � 1015, (j) 1.25 � 1015 ions/cm2. (k) Temperature dependence of the critical amorphization fluence for the Nb-doped STO.

44 W. Li et al. / Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47

where D0 is the amorphization dose extrapolated to 0 K, Ea the acti-vation energy for the dynamic annealing process during irradiation,kB the Boltzmann constant and Tc the critical amorphization tem-perature above which the specimen remains crystalline. For un-doped STO, the critical amorphization temperature is 426 K(Table 2), which agrees with previous studies [20,26,39]. Althoughafter doping, the Tc value drops slightly to 398 K (0.1 wt.% Nb)and 392 K (1 wt.% Nb), respectively, and the three amorphizationcurves (for Nb: 0, 0.1 and 1 wt.%) actually almost overlap(Fig. 5(k)). This suggests that in general, the doping of Nb intoSTO has very limited influence on its radiation response. For the un-doped sample, the activation energy obtained from Eq. (6) is only0.025 eV. After doping, it increases to 0.080 eV (0.1 wt.% Nb) and0.085 eV (1 wt.% Nb), respectively. As reported in previous studies,these activation energies obtained from Eq. 6 are often very low[20], as compared with the known defect migration energies, whichare on the order of �1 eV. The Ea values, as determined by Eq. (6),provide only limited information on the kinetics of the amorphiza-tion and recovery processes.

Table 2Amorphization dose Dc, activation energy Ea and critical amorphization temperatureTc obtained from Eq. (6), and activation energies for the irradiation assisted recoveryEirr and for the thermal assisted recovery Eth obtained from Eqs. (7) and (8).

Nb (wt.%) Dc (0 K) (dpa) Ea (eV) Tc (K) Eirr (eV) Eth (eV)

0 0.92 0.025 426 0.33 1.000.1 1.44 0.080 398 0.32 0.951 1.65 0.085 392 0.31 0.94

4. Discussion

4.1. Track formation

There are vigorous debates over the details of the physics oftrack formation [10,11,40–44]. For example, the roles of the carrierdensity, as well as bond type, in track formation are still unclear.However, it is generally believed that there are basic processesassociated with the ion–solid interactions where Se� Sn. The firststage occurring in less than 10�16 s is electronic excitations andionizations by which the moving energetic ion removes electronsfrom atoms near the ion path, leaving behind a positively chargedchain of atoms within the inner track region [10,44]. This stage isperhaps followed by a ‘‘Coulomb explosion’’ by which vacantatomic sites are produced by the mutual repulsion of the ionizedatoms into adjacent interstitial positions [10,11]. However, it hasbeen argued that the neutralization time (less than 10�15 s) is fartoo short for the Coulomb force to replace atoms because very littleenergy is likely transferred by this force [44,45]. According to thethermal spike model, the electrons deposit their energy to the tar-get atoms by electron–lattice interactions [40–43]. Depending onthe target material, local thermalization in the electronic systemis complete in about �10�14 s, and heat transfer from the elec-tronic to the atomic subsystem becomes substantial between10�14 and 10�12 s [43,44,46]. Due to intense electron–phonon cou-pling, the region subjected to ion irradiations may become fluid,leading to anisotropic plastic deformation by viscous flow [46–48]. Contrary to theoretical predictions [10], our results show thatthe Nb-doping has no evident influence on track formation in STO,despite the significant decrease in electrical resistivity over the

W. Li et al. / Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47 45

insulating-to-conducting transition. There are several possibilitiesthat explain the experimental results.

Firstly, the electron density of a material should play an impor-tant role in the track formation process [10,13,44,49], becauseswift ion tracks form more efficiently in materials with a higherelectronic resistivity. To ensure track formation, hot electronsshould be confined within the track region, and should not becooled down by interaction with free electrons before the transferof the electron energy to the lattice [50]. Due to the lack of freeelectrons in the bulk of the insulating material, the energy leakagefrom the track region mainly occurs by spreading of the hot elec-trons themselves, a process which is suppressed by the electricalfield of remaining positively ions. The hot electrons propagate bythe impact ionization of atoms at the periphery of the excited re-gion [41]. This relaxation process occurs within a time-frame com-parable to the time required for the transfer of the electron energyto the lattice (greater than �10�13 s) [41,50], which is thereforefavorable for track formation in insulators. In contrast, tracks aredifficult to form in metals, as the higher electronic conductivity al-lows rapid energy diffusion in the subsystem of free electrons in avery short time (less than �10�14 s) [30,41]. In insulating STO(q �104 X cm), there are practically no free electrons in the bulk(ðn0

e Þ�1015 cm�3) [29]. For Nb-STO (1 wt.%), ðn0e Þ increases to

�3.7�1020 cm�3 [29]. According to Fleisher et al. [11], an increasein the bulk electron density would crucially decrease the time ofthe charging screening, possibly suppressing track formation.However, this electron density is still two orders of magnitudelower than the density of hot electrons ðneÞ in an excited region[41,43], which is on an order of �1022 cm�3. Therefore, the bulkelectron density may be still too low to influence the cooling ofelectronic subsystem. The majority of hot electrons in conductingNb-STO transfer their energy to the bound electrons, leading tovery similar energy dissipation and track formation as comparedwith the insulating STO. Currently, the thermal spike model hasbeen widely used to simulate track formation in many differentmaterials [42,43,45]. Unfortunately, there are no models that havedirectly included the electron density in the simulation.

Secondly, rather strong effects of the non-thermal process [51]also can be expected for semiconductors and insulating materials,apart from the effects of temperature and Coulomb repulsion. Thenon-thermal melting process implies that the lattice can be disor-dered due to a change of the interatomic potential by direct exci-tation of the electron system in very short times (�10�14 s) whilethe lattice modes remain vibrationally cold [51]. Such a conceptis well known in laser physics, and recently was brought to theattention of the swift ion track community [45,52,53]. Since thenon-thermal mechanism relates to the bond rupture for the disor-dering of materials induced by femtosecond laser pulses [53], thedifference in the type of bonding between metals and dielectricsmay be important for track formation by swift ions as well. Ioniza-tion might not change metallic bonds very much, while covalent orionic bonds actually can be broken. However, the low concentra-tion doping in this study does not change the bonding, just theelectronic conductivity, and therefore has no significant effect ontrack formation.

Lastly, the high-velocity ions near the Bragg-peak may not beideal for demonstrating the doping effect on the track size. The Se

values (38 and 47 keV/nm) for the GeV ions in the Nb-STO aremuch greater than that (�20 kev/nm) of the 92 MeV Xe ions-in-duced tracks in pure STO [25]. However, the track radius in thecase of 92 MeV Xe ions in STO, as was determined by both TEMand XRD, is approximately 25–30 Å [25], which is not so differentfrom those (20–30 Å) created in the case of GeV ions in Nb-STO.This small difference in track radius for ions with different energiescan be caused by the velocity effect: at a given Se the damage crosssection is higher for low-velocity ions than for high-velocity ions

[54]. As mentioned, high-velocity ions were used in this study toproduce tracks with similar radii throughout the entire samplethickness. However, an effect of doping on the track size mightbe more pronounced for beam energies near the threshold Se ratherthan high energies near the Bragg-peak.

4.2. Amorphization by low energy ions

During low-energy ion irradiations, the kinetic energy depos-ited through nuclear collisions results in radiation damage. Thestructural and chemical properties of the target control the produc-tion and recovery of radiation damage [55]. The doping in STO notonly changes the electrical resistivity but also the local structure ofthis material due to impurity atoms. In the nuclear collision dom-inated regime, the significant decrease of resistivity upon dopingmay change charge build-up and dissipation. As previously dis-cussed for BaTiO3 [26], these changes may not be able to alterthe radiation damage response of the target because of the verysmall amount of energy carried by electrons. However, this needsto be confirmed for other materials. In this study, the almost over-lapping amorphization curves for STO samples with different levelsof doping (Nb: 0, 0.1 and 1 wt.%) suggest that the doping has verylimited influence on the low energy ion-induced crystalline-to-amorphous transformation in STO. This result demonstrates thatdramatic changes in resistivity do not affect the damage in the nu-clear collision regime. Further, this result is also consistent withthe subtle structural modifications caused by the doping with Nb(up to 1 wt.%) [56]. First principle calculations show that the Nb-substitution is energetically more favorable at the Ti site with adopant formation-energy of 1.24 eV, instead of the Sr-site(8.04 eV) [23]. The expected lattice expansion due to doping is verysmall, i.e., lattice constant: 0.391 nm for 0% Nb vs. 0.392 nm for�1 wt.% Nb [56]. This is because of the very small amount of Tisubstituted by Nb, and very close ionic radii for Nb5+(0.064 nm)and Ti4+ (0.061 nm) [56].

As mentioned before, the Ea values as determined by Eq. (6) areunreasonably low as compared with the known defect migrationenergies. Actually, the damage recovery can be induced by bothirradiation-enhanced and thermal annealing processes. If irradia-tion-assisted recovery dominates, the activation energy for theirradiation assisted recovery, Eirr, can be estimated by the expres-sion [38],

Eirr ¼ Tc½kB lnðmirr=ra/Þ�; ð7Þ

where mirr is the effective jump frequency for the irradiation recov-ery process, ra the amorphization cross section ra ¼ 1=D0, and /the ion flux (so ra/ the damage rate). The activation energy forthe thermal recovery, Eth, can be estimated by a similar expression,

Eth ¼ Tc½kB lnðmth=ra/Þ� ð8Þ

where mth is the effective jump frequency for the thermal recoveryprocess. In this study, the Eirr and Eth values for each compositionwere calculated using Eqs. (7) and (8) by assuming values of 10and 109 s�1 for mirr and mth, respectively, as have been used in thecalculations for STO [20,27]. The D0 and Tc values were taken fromthe fitted results based on Eq. (6). For the undoped STO, the calcu-lated Eth value (1.0 eV) is consistent with the measured activationenergy of 0.98 eV by the oxygen diffusion study in single crystalof STO [57], while the calculated Eirr value (0.33 eV) is consistentwith the measured activation energy of 0.29 eV as obtained bythe 200 keV Ar ion irradiated method [58]. The values of activationenergy (Eirr and Eth) in this study for the undoped STO are also veryclose to those as calculated by Smith et al. [20]. The activation ener-gies as determined by Eqs. (7) and (8) are able to reflect the defectrecovery process because they are consistent with the known defectmigration energies. Interestingly, within the experimental errors,

46 W. Li et al. / Nuclear Instruments and Methods in Physics Research B 302 (2013) 40–47

the calculated values of Eirr (0.32 eV for 0.1 wt.% and 0.31 eV for1 wt.%) and Eth (0.95 eV for 0.1 wt.% and 0.94 eV for 1 wt.%) forthe doped SrTiO3 are very close to those (Eirr: 0.33 eV; Eth: 1.0 eV)for the undoped SrTiO3. This suggests that despite the significant in-crease in resistivity, the doping with Nb has no significant influenceon the energy barrier for the defect migration for both the thermalrecovery and irradiation annealing processes. This agrees with thevery limited lattice expansion upon doping [23,56]. In addition, de-spite the general overlap of the three amorphization curves(Fig. 5(k)), the increase of D0 (from 0.92 to 1.44 and 1.65 dpa) withincreasing Nb-content at the low temperatures is consistent withthe slight decrease of Tc (from 426 to 398 and 392 K) with increas-ing Nb-content, as they both suggest a slight improvement of thedamage resistance of doped samples. Besides the atomic collisionsduring ion irradiations, the energy transferred to electrons produceselectron–hole pairs that can result in localized electronic excitation,rupture of covalent or ionic bonds, as well as the formation of atom-ic-scale defects [59]. Because of the significant increase in electronicconductivities caused by the Nb-doping, there are less electron–hole pairs in the vicinity of the defects in the doped samples ascompared with the undoped STO. Therefore, the slight improve-ment of the damage resistance of doped samples may indicate thatthe localized electronic excitation due to the low-energy ion irradi-ations actually enhance the formation of atomic-scale defects inSTO. Another possible contribution to the slight improvement ofthe damage resistance of doped samples may be the increase inthe concentration of Sr vacancies caused by the Nb-doping. Substi-tution of Nb5+ for Ti4+ in the doped STO produces extra Sr vacancies[60,61]. These Sr vacancies can enhance the recovery of atomic-scale defects by defect recombination, resulting in increased dam-age resistance.

5. Conclusions

We have investigated Nb-doping effects on the radiation re-sponse of Nb-SrTiO3 (Nb: 0, 0.1 and 1 wt.%) in an effort to correlatethe effect of changes in electronic resistivity to track formation, aswell as the radiation-induced crystalline-to-amorphous transfor-mation. The radii of tracks induced by swift ion irradiation areinsensitive to the pristine resistivity of Nb-STO, despite the factthat the electrical resistivity has decreased by seven orders of mag-nitude. The density of the conduction electrons significantly in-creases up to �1020 cm�3 after Nb-doping. However, this numberis still low as compared with the number of excited electrons(�1022 cm�3) in the track core, possibly having no appreciable ef-fect on the cooling of the electronic subsystem. During the low-en-ergy (1 MeV Kr2+) ion irradiation, the doping has very limitedinfluence on the energy barriers for the irradiation-enhanced andthermal annealing in Nb-STO process because of the very small lat-tice expansion and low concentration of Nb. However, the extra Srvacancies caused by the substitution of Nb5+ for Ti4+ increase thedamage recovery rate.

Acknowledgements

The funding for this study was provided by the Office of BasicEnergy Sciences of the USDOE (DE-FG02-97ER45656). The authorsgratefully thank Marcel Toulemonde at CIMAP-GANIL (CEA-CNRS-ENSICAEN-Univ. Caen) for valuable discussions. We thank the staffat the IVEM-Tandem Facility at the Argonne National Laboratoryfor assistance with the 1 MeV Kr2+ ion irradiation experiments. Partof this research was undertaken on the SAXS/WAXS beam-line atthe Australian Synchrotron, Victoria, Australia. One of the authors,P.K., acknowledges the Australian Research Council for financialsupport.

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