Physics Procedia 36 ( 2012 ) 508 – 513

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi: 10.1016/j.phpro.2012.06.075

Superconductivity Centennial Conference

Systematic modification of electrical and superconducting properties of YBCO and nano-patterning of high-Tc superconducting thin films by light-ion irradiation

J.D. Pedarnig a, *, M.A. Bodea a, B. Steiger b, W. Markowitsch b, W. Lang b a Institute of Applied Physics, Johannes Kepler University, A-4040 Linz, Austria

b Faculty of Physics, University of Vienna, A-1090 Vienna, Austria

Abstract

Irradiation of high-temperature superconducting (HTS) YBa2Cu3O7-d (YBCO) thin films with 75 keV He+ ions leads to a quasi-exponential increase of the in-plane ( ab) and the out-of-plane ( c) resistivity in the normal state and to a non-linear decrease of the critical temperature Tc with ion dose. In situ electrical measurements at room temperature reveal an irradiation-induced reduction of resistivity anisotropy c / ab and a slight relaxation of film resistivity after the ion irradiation is stopped. Ex situ measurements show a stretched-exponential relaxation of Tc and normal state resistivity that continues for several weeks after the ion irradiation. Irradiation of YBCO thin films by low-energy He+ ions through stencil masks results in local modification of the electrical and superconducting properties of the HTS material. We demonstrate that masked ion-beam lithography enables to produce structures smaller than 100 nm in size that have potential for applications in future superconducting nano-electronics. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and Peter Kes. PACS: 68.55.-a; 74.78.-w; 81.15.Fg; 81.65.Ps; 81.16.Nd Keywords: High-temperature superconductors ; YBa2Cu3O7-d ; thin films, ion irradiation ; point defects ; masked ion-beam lithography ; nano-patterning ;

1. Introduction

Superconducting materials provide substantial advantage for future electronic circuits as the dissipation of electro-magnetic energy is strongly reduced at temperatures below the critical temperature Tc. The

* Corresponding author. E-mail address: Johannes.Pedarnig@jku.at.

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© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

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cuprate high-temperature superconductors (HTS, Tc > 77 K) can be easily cooled to T < Tc making such materials attractive for technical applications at low operating temperature. Another advantage of superconductors is connected to the specific materials properties in the superconducting state. Novel functionalities can be exploited with superconducting devices that cannot be realized with other materials. With classical low-Tc superconductors (LTS) various applications have been successfully demonstrated [1]. Novel functionalities based on the manipulation of magnetic flux vortices in micro-patterned and nano-patterned materials have been demonstrated with LTS [2] and HTS [3] recently. The patterning of HTS on the nanometer-scale is complicated as well-established lithography technologies fail to produce reliable sub-μm structures. In addition, most of the currently explored patterning methods are based on the removal of material, so that the mechanical and environmental stability of the remaining HTS structures is problematic.

A different technique for patterning of HTS is based on ion irradiation. Focused ion beam patterning produces HTS nano-structures by abrasion of sample material from within the scanned ion beam spot [4]. Another approach is masked-ion beam lithography (MIBL) where collimated ion beams and stencil masks are used [5]. The mask shapes the beam profile and samples are patterned by irradiation with ion beamlets corresponding to the mask structures. In this paper, we explore the modification of the electrical and superconducting properties of c-axis oriented and vicinal YBa2Cu3O7-d (YBCO) thin films by irradiation with light ions of low energy. We employ MIBL technique to pattern YBCO thin films on the nanometer scale in a single-step resist-less process without removal of HTS material.

2. Experimental

YBCO thin films are grown by pulsed-laser deposition (PLD) technique on single crystal substrates [6]. Epitaxial films with c-axis orientation are deposited on (001) MgO substrates and vicinal films are grown on (001) SrTiO3 substrates that are miscut by an angle S = 10° [7]. For the irradiation of YBCO films with He+ ions of 75 keV energy an ion implanter (High Voltage Engineering Europa B.V.) with large exposure field (3×3 cm2) is used. For masked-ion beam nano-patterning of YBCO films commercial silicon stencil masks are used (mask thickness 2 μm).

3. Results and discussion

3.1. Ion modification of YBCO electrical transport properties

Ion irradiation is commonly used to create various kinds of defects in HTS. Depending primarily on the ion’s mass and energy, point-like defects, columnar defect tracks, and splayed columnar defects are produced. Irradiation of HTS with light ions of low energy, e.g. 75 keV He+ ions, is attractive for systematic modification of thin films. Point defects are efficiently created by collisions of projectile ions and target ions and the stopping range of ions is sufficiently large to penetrate YBCO films that are several 100 nm thick [8].

Figure 1 shows the modification of in-plane resistivity ab and critical temperature Tc,mid (at midpoint of transition) for a c-axis oriented YBCO thin film on (001) MgO substrate that is repeatedly irradiated by He+ ions of 75 keV energy and measured after each ion exposure. With increasing ion fluence D the ab shows a quasi-exponential increase and Tc,mid decreases non-linearly [8]. A dose of 3 × 1015 ions/cm2 marks a crossover from superconducting material with metallic-like temperature dependence to a semiconductor-like temperature behavior of ab(T) and the disappearance of superconductivity is observed. The irradiation-induced increase of resistivity is tentatively described by a resistor network model [8]. The reduction of Tc may be explained by in-plane scattering at irradiation defects due to the

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particular gap symmetry of HTS like YBCO. Related studies of ion-beam induced disorder in HTS have also found a substantial decrease of Tc [9].

Fig. 1. Modification of in-plane resistivity ab at T = 100 K and 300 K and of Tc, mid of c-axis oriented YBCO thin film on (001) MgO single crystal substrate with accumulated ion irradiation dose (He+, 75 keV)

Figure 2 shows the modification of anisotropic resistivities ortho and para at room temperature for a

vicinal YBCO thin film on SrTiO3 substrate. Resistivity is measured parallel and orthogonal to the vicinal film steps and is denoted by subscripts “para” and “ortho”, respectively. Measurements are performed in situ during irradiation. A dose of 3.5 × 1015 ions/cm2 is accumulated within approx. 93 min of exposure.

Fig. 2. Increase of anisotropic resistivities ortho and para of vicinal YBCO thin film measured in situ during irradiation

Fig. 3. Reduction of resistivity anisotropy c / ab of vicinal YBCO film by He+ ion irradiation

The initial anisotropic film resistivity is ortho = 0.64 m cm and para = 0.23 m cm and by ion

irradiation it increases 17× and 23×, respectively. The resistivity anisotropy c / ab is calculated from measured resistivities ortho and para ( c denotes the out-of-plane resistivity of YBCO) [7]. The anisotropy decreases exponentially with He+ ion dose (Fig. 3). From electrical transport measurements we conclude that He+ ion irradiation results in enhanced scattering and reduced mobility of charge carriers in YBCO films. The density of mobile charge remains constant up to 1.5 × 1015 ions/cm2 [8]. This conclusion is supported by the observed irradiation-induced reduction of resistivity anisotropy.

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Figure 4 shows details of electrical in situ measurements for a c-axis oriented YBCO film that is irradiated repeatedly with dose Di = 1.0 × 1014 ions/cm2 (i = 1-5). The ion beam is deflected from the sample as dose Di is reached and the subsequent irradiation is started after some 100 seconds delay. The film resistance strongly increases as soon as the sample is exposed to the ion beam. During the beam-off time the sample resistance shows slight relaxation from its irradiation-induced increased resistance value.

Fig. 4. Resistance Rab(300K) of c-axis YBCO / MgO thin film measured in situ for repeated ion irradiation

Fig. 5. Relaxation of anisotropic resistivities ortho and para of vicinal YBCO / SrTiO3 film after ion irradiation

Similar relaxation behavior is observed also for the vicinal YBCO / SrTiO3 thin film (Fig. 5). The

increased anisotropic resistivities ortho(300 K) and para(300 K) slightly relax after irradiation is stopped (ion beam current density JB = 0.1 μA/cm2).

A detailed investigation of YBCO / MgO thin films reveals a stretched exponential relaxation of ab and Tc that continues for several weeks after the He+ ion irradiation [10]. The Tc relaxes with time t according to Tc(t) = Tc( ) – [Tc( ) - Tc(0)]×exp[-(t/ ) ]. For example, the critical temperature of a film sample relaxes from Tc(0) = 71.7 K as measured immediately after the ion irradiation to Tc( ) = 76 ± 2 K as obtained from the fit to the Tc(t) data. The stretched exponential behavior indicates that irradiation creates defects with relatively broad energy distribution enabling shallow defects to be annihilated at room temperature. YBCO films that are irradiated at T 120 K do not show resistivity relaxation as long as samples are kept at low temperature [11].

3.2. Masked-ion beam patterning of YBCO thin films

In a further study we have employed MIBL for direct patterning of YBCO thin films. For MIBL, a thin silicon stencil mask is placed close to the YBCO sample and the film is irradiated by He+ ions traversing the mask openings (Fig. 6). Irradiated zones of YBCO are transformed from superconducting to non-superconducting or insulating material depending on the ion dose employed. Figure 7 shows an optical micrograph (200×) of pre-patterned YBCO film (black) on MgO substrate (bright) with a stencil mask (blue arrows) that is aligned to a film track for ion beam patterning. Holes in the Si membrane (green arrows) are for mask alignment. A stencil mask with holes of diameter 170 nm is shown in Figure 8. The holes are arranged in a quadratic array with a pitch of 300 nm.

The scanning electron microscopy (SEM) image in Figure 9 shows an YBCO thin film that is nano-patterned by MIBL [12]. Regions irradiated by He+ ions appear darker (secondary electron detection). Different structures according to the mask features are patterned into the film in a single-step process. The size of smallest MIBL structures produced is around 70 nm.

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Fig. 6. Masked-ion beam lithography. Irradiated zones of YBCO film are transformed into insulating material

Fig. 7. Alignment of stencil mask on YBCO film for MIBL (opt. microscopy 200×, see text for description)

Numerical simulations of ion collision cascades in YBCO thin films (program code Stopping and

Range of Ions in Matter, SRIM) are in support of our results. Defects are created close to the ballistic He+ ion trajectory and structures as small as 20 nm in size should be achievable by MIBL [13].

Fig. 8. Stencil mask with array of holes (diameter 170 nm, pitch 300 nm). SEM image (courtesy ims-chips, Germany)

Fig. 9. Nano-patterned YBCO thin film (electron microscopy). (a) array of irradiated nano-dots; (b) right-angle stripe pattern; (c) comb-like pattern; (white lines are guides to the eye)

MIBL enables the fabrication of novel functional HTS nano-structures. Magneto-resistivity

measurements on a nano-patterned YBCO thin film in the superconducting state (quadratic array of dots, dot diameter 170 nm, pitch d = 300 nm) show resistance minima at fields of B 25 and 46 mT. These fields are very similar to the calculated matching fields Bm = n × 0 / d2 = n × 23 mT indicating vortex matching [11]. With FIB patterned HTS (d = 500 nm) matching fields of n × 8.3 mT are reported [14].

4. Conclusions and outlook

Low-energy light-ion irradiation of HTS thin films is an easy method for systematic modification of the electrical and superconducting materials properties. Irradiation of YBCO films by He+ ions of 75 keV

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energy introduces point defects, increases the in-plane and out-of-plane resistivities, and decreases the critical temperature of the HTS. Masked-ion beam lithography is an intriguing method for nano-patterning of HTS films. The fabrication of various functional HTS nano-structures of arbitrary shape and size by a single exposure step can be envisioned.

Acknowledgements

The authors acknowledge financial support from the Austrian Science Fund FWF and the European Science Foundation (Research Networking Program Nanoscience and Engineering in Superconductivity). The corresponding author acknowledges support by the Johannes Kepler University Linz.

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