Optimized NdBa2Cu3Oy thin films deposited by eclipsed pulsed laser ablation

Optimized NdBa2Cu3Oy thin films deposited by eclipsed pulsed laser ablation

Physica C 356 (2001) 205±211 www.elsevier.com/locate/physc Optimized NdBa2Cu3Oy thin ®lms deposited by eclipsed pulsed laser ablation C. Cai a,b,*, ...

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Physica C 356 (2001) 205±211

www.elsevier.com/locate/physc

Optimized NdBa2Cu3Oy thin ®lms deposited by eclipsed pulsed laser ablation C. Cai a,b,*, M. Tachiki a, T. Fujii a, T. Kobayashi a, H.K. Liu b, S.X. Dou b a

Division of Advanced Electronics and Optical Science, Department of Physical Science, Faculty of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan b Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia Received 15 November 1999; received in revised form 14 February 2001; accepted 15 February 2001

Abstract NdBa2 Cu3 Oy (Nd123) thin ®lms are deposited on (1 0 0)MgO, (1 0 0)Y-stablized ZrO (YSZ) and (1 0 0)SrTiO3 (STO) at the low temperature of 650°C by pulsed laser ablation with a shadow mask (eclipsed pulsed laser deposition (PLD)). X-ray di€raction and re¯ection high-energy electron di€raction patterns show that all thin ®lms are oriented in (0 0 1) direction. Compared with the conventional PLD samples, the eclipsed PLD samples exhibit a narrower superconducting transition width (DTc ). The suppression of DTc is believed to result from the elimination of droplets and the stoichiometric deviation of Nd specie. The ®lm on STO shows better superconducting properties than those on YSZ and MgO. Its onset Tc , DTc and Jc reaches 93.5 K, 3.5 K and 6  106 A/cm2 , respectively. A Nd123/MgO thin ®lm stepedge junction is prepared as well. A typical RSJ-like (Josephson) current±voltage curve is observed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.76 Bz; 81.15 Fg; 74.62-c; 74.62 Dh Keywords: Nd123 thin ®lm; Eclipsed PLD; Superconducting transition; Nd±Ba substitution

1. Introduction Although several oxide superconducting materials, such as the Bi-based and Hg-based, exhibit higher critical superconducting temperature (Tc ), REBa2 Cu3 Oy (RE123, RE: rare earth element) remains very attractive for practical applications, especially for microelectronic devices operating at 77 K [1±7]. Among RE123 systems, NdBa2 Cu3 Oy * Corresponding author. Present address: The School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Tel.: +44-121-4145170. E-mail address: [email protected] (C. Cai).

(Nd123) has the highest Tc (up to 96 K). Meltprocessed bulk samples of Nd123 show superior magnetization irreversibilities (to that of Y123) in high magnetic ®eld [8±10]. Moreover, high quality Nd123 thin ®lms demonstrate extraordinary properties unprecedented in other RE123 ®lms. These include ease of oxidation, very high crystallography and surface stability [1,3]. Therefore, Nd123 is widely considered as a good material for a superconducting device. However, there exists the substitution of Ba for Nd ion in Nd±Ba±Cu±O system. Solid solutions, commonly described as RE1‡x Ba2 x Cu3 Oy , easily form in the compound with a nominal composition of 1:2:3. Zero-resistance superconducting

0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 1 6 3 - 0

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temperature (Tc…Rˆ0† ) decreases and superconducting transition width (DTc ˆ Tc…onset† Tc…Rˆ0† ) broadens as the substitution content (x) increases. For the purpose of high Tc and narrow DTc , melt-processed Nd123 bulks have been widely investigated. It has been established that Nd±Ba substitution and DTc in the bulks are e€ectively suppressed by reducing oxygen partial pressure or post-annealing in argon gas [8±10]. Compared with the bulks, Nd123 thin ®lms are less extensively studied. In 1995, Badaye et al. [1] ®rst prepared Nd123 thin ®lm on (1 0 0)SrTiO3 (STO) substrate (Nd123/STO) by pulsed laser deposition (PLD). They obtained the c-axis oriented ®lms at a growth temperature above 780°C, but with a low Tc…Rˆ0† and a broad DTc . In 1997, Moon and Oh [2] achieved c-axis Nd123/STO thin ®lms by using a mixed gas of oxygen and argon. Tc…Rˆ0† reached 93 K. Recently, several groups have studied the e€ect of target composition on the superconducting properties of Nd123 thin ®lm. Salluzzo et al. [4] reported that as the substrate temperature decreased to 685°C from 735°C, o€stoichiometric-target ®lms exhibited a increasing Tc , while the stoichiometric target ®lms showed a decreasing Tc . Badaye et al. [3] prepared the Nd123/MgO thin ®lms by three Ba-rich targets (NdBa2:05 Cu3 Oy , NdBa2:08 Cu3 Oy and NdBa2:15 Cu3 Oy ). The target with 2.08 Ba per formula produced a small Nd±Ba substitution and a high Tc . To seek more practical methods for the fabrication of high quality Nd123 thin ®lms, this work applies a newly developed PLD technique, i.e. eclipsed PLD. So-called eclipsed PLD was lately proposed to solve the problem of droplets commonly existing in the conventional PLD thin ®lm [11,12]. Compared with previous methods regarding the droplet problems, such as o€-axis deposition and crossed ¯uxes techniques, the eclipsed PLD is more convenient and e€ective. In particular, the method causes a novel deposition mechanism due to the introduction of a shadow mask. In the process, the atomic species are scattered by the background gas and transported through di€usion in the ambient. They may deposit on the substrate at a di€erent relative ratio from that of the initial target, i.e. the Nd±Ba relative ratio may be adjusted during the deposition. Eclipsed PLD thus

has great promise for the fabrication of Nd123 thin ®lms In the previous reports about eclipsed PLD, major attention is paid to the deposition mechanism and droplet problem, merely with respect to Y±Ba±Cu±O material. The present work is the ®rst attempt to apply the eclipsed PLD to the Nd±Ba±Cu±O system, which is expected to develop a new method to achieve high quality Nd123 thin ®lms.

2. Experimental As shown in Fig. 1, eclipsed PLD was carried out by inserting a shadow mask between the target and substrate in the conventional PLD con®guration. The target±mask and mask±substrate distances were 30 and 20 mm, respectively. The shadow mask dimension was about 10 mm  10 mm. Sintered stoichiometric Nd123 was used as the target. The wavelength, repetition rate, and ¯uence of the applied ArF excimer laser beam were 193 nm, 10 Hz and 120 mJ/cm2 , respectively. The substrate was heated to a stable temperature of 650°C (hereinafter called deposition temperature). The total ambient pressure of a mixed gas (O2 :Ar ˆ 1:1) was kept at 0.2 Torr. Under these conditions, Nd123 thin ®lms were deposited on

Fig. 1. Schematic illustration of eclipsed PLD and particle movements.

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(1 0 0)MgO, (1 0 0)Y-stablized ZrO (YSZ) and (1 0 0)STO single crystals, respectively. After 45min deposition, the chamber was ®lled with pure oxygen until to the ambient pressure of 750 Torr. Then, the substrate heater was powered o€, and the sample was cooled to room temperature. No extra oxygenation treatment was performed. For a comparison, Nd123 ®lms were also prepared by the conventional PLD with the same conditions except for a shorted deposition time of approximately 20 min. The thickness of either ®lm was about 40 nm. The crystal orientation was checked by CuKa X-ray di€raction (XRD) and re¯ection highenergy electron di€raction (RHEED). The superconducting transition and critical current were measured by the standard four-probe method in a microbridge pattern prepared by H3 PO4 wetchemical etching technique. The four electrodes were gilded. The criterion for critical current was 1 lV/cm. For the preparation of Nd123 thin ®lm junction, a step edge, 200 nm high, was created on (0 0 1)MgO single crystal by ion-beam-etching method. A bridge-pattern junction was fabricated 23 lm wide and 86 lm long.

3. Results and discussion XRD patterns for various samples show similar (0 0 1) peaks in addition to those from the substrate materials. The (0 0 1) peaks correspond to a wellcrystallized single orthorhomic phase of Nd123. This means that the c-axis oriented Nd123 thin ®lms are feasible on (1 0 0)MgO, (1 0 0)YSZ, and (1 0 0)STO single crystals. Clear streaks can be observed in the RHEED patterns of all ®lms, showing the likelihood of Nd123 epitaxial growth on the three substrates. Typical XRD and RHEED patterns are showed in Fig. 2. Within the resolution limit, no signi®cant di€erences of XRD and RHEED patterns are found between the eclipsed PLD and conventional PLD ®lms. However, apparent disparities in Tc…Rˆ0† and DTc come up between the two types of ®lms. As shown in Fig. 3, the eclipsed PLD samples give a narrower superconducting transition. DTc (ˆ 3:5 K) of the eclipsed

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PLD Nd123/STO, for example, is only half the DTc (ˆ 7 K) of the conventional one, meaning that the eclipesed PLD is e€ective in improving superconducting properties. The suppression of DTc may be attributed to the e€ects of shadow mask on two aspects. First, the shadow mask probably results in the elimination of droplets and the improvement of composition homogeneity. It is well known that the temperature of laser-focussed part of target surface can exceed the boiling point in an instant and causes explosive spouting of the melt. In conventional PLD, droplets and even raw target materials have plentiful opportunities to deposit on the substrate surface directly. Whereas the droplets in the eclipsed PLD are almost captured by the shadow mask before reaching the substrate as they generally ¯y straight or di€use at small scattering angles resulting from their relatively heavy mass. Growth species reach substrate mainly through di€usion. Consequently, the ®lm hardly consists of large particles, and its composition and thickness are quite homogenous. This is likely one of reasons for the narrow DTc . Second, the shadow mask may lead to the adjustment of deposition composition against that of original target material. There are di€erences in propagating route and relative distribution among various growing species. The more volatile elements, Cu and Ba, are preferentially ablated [12]. Trajanovic et al. [13] quantitatively analyzed Y, Ba and Cu cationic distributions in the eclipsed PLD Y123 ®lms by Rutherford back-scattering spectrometry. It is found that Ba and Cu distributions follow the target composition of 2:3, nearly unaffected by ambient pressures and target-shadow distances. However, the Y distribution is an unstable factor, subject to ambient pressure and target-shadow distance. The reason is probably that Y has a strong tendency to form simple oxide particles, which increases both specie mass and scattering cross-section. In the case of Nd123, above phenomenon should be more apparent due to heavier Nd element (atomic weight ˆ 144) instead of Y element (atomic weight ˆ 89). Particles including Nd species are inclined to ¯y straight or di€use at smaller scattering angles in a similar way to droplets. As illustrated in Fig. 1, they are also

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Fig. 2. Typical XRD and RHEED patterns, indicating the c-axis orientation of various Nd123 thin ®lms (The sample presented here is a thin ®lm on MgO substrate prepared by eclipsed PLD). (a) XRD pattern; (b) RHEED pattern.

easily captured by shadow mask. An o€-stoichiometric composition such as Nd1 x Ba2 Cu3 Oy probably forms in the ®lm, with a lower Nd element ratio than the stoichiometric one of target. If it is referred that the Nd±Ba substitution and DTc are improved by using o€-stoichiometric target [3,5], the stoichiometric deviation of Nd specie is reasonably believed to play a signi®cant role in the suppression of DTc . Some crystalline and superconducting properties are summarized in Table 1. The resistivities of the ®lms are low down to 10 4 X cm at room temperature, which indicates their good metal conducting capacities in the normal state. The caxis lattice constant, c, is determined from (0 0 5)

peak of XRD. For all samples in the table, c ranges from 1.170 to 1.172 nm, close to that of Nd123 orthorhombic phase (c ˆ 1:165±1:170 nm) [2,10]. According to the studies on bulk Nd123 [10,14,15], it is assumed that the oxygen content in any ®lm is enough for orthorhombic±tetragonal phase transition although no special oxygenation treatment is performed. The Tc…onset† and Jc for all eclipsed PLD samples are more than 90 K and 8:5  105 A/cm2 , respectively. Nd123 ®lm on STO has Tc…onset† of 93.5 K and DTc of 3.5 K, showing superior superconducting properties over those on YSZ and MgO. Its Jc at 77 K reaches the highest value, 6  106 A/cm2 .

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Table 1 Summary of the properties of Nd123 thin ®lms deposited on three substrates at 650°C by eclipsed PLD Substrate q…RT† (X cm) c (nm) Tc…onset† (K) DTc =DTc (K) Jc (A/cm2 )

MgO 3:6  10 1.171 91 5.5/13 2:1  106

YSZ 4

2:0  10 1.172 90 6/9 8:3  105

STO 4

2:4  10 1.170 93.5 3.5/7 6  106

4

q…RT† : Room temperature resistivity; c: c-axis lattice constant; Tc…onset† : Onset superconducting temperature; DTc : Superconducting transition width; DTc : Superconduting transition width for the conventional PLD samples; Jc : Critical current density at 77 K and zero magnetic ®eld.

Fig. 3. Resistance±temperature curves for Nd123 thin ®lms on MgO, YSZ and STO substrates, showing a suppressed superconducting transition width in the eclipsed PLD samples.

It is worth noting that the present Nd123 ®lms are prepared at the lower deposition temperature, 650°C, while their c-axis orientations and superconducting properties are as good as those prepared at a higher temperature [1,2,5]. Badaye et al.

[1] fabricated Nd123/STO thin ®lm at various deposition temperatures of 750±900°C. Below the threshold temperature of 780°C, the Nd123 ®lms were purely a-axis oriented. Moon and Oh [2] obtained high quality Nd123/MgO thin ®lms by low oxygen partial pressure technique. The deposition temperatures was as high as 760°C for a high Tc of 93 K. Hakuraku et al. [5] fabricated the c-axis Nd123/MgO thin ®lm at the deposition temperatures of 710±750°C. These works seem that a high deposition temperature should be required for c-axis formation of Nd123 thin ®lm, at least 50± 100°C higher that of Y123. However, the present work indicates the possibility to obtain c-axis Nd123 thin ®lm at the low deposition temperature of 650°C, almost that required by Y123. Although current experiments cannot o€er the explanation to above di€erence, it is de®nitely a good sign for thin ®lm device applications since a lower processing temperature is bene®cial to avoiding the impurity and deterioration. As a trial of device processing, we prepared a Nd123/MgO step-edge junction, using the lower deposition temperature of 650°C. The optical image of the junction is shown in Fig. 4. The current±voltage curve exhibits a typical resistively shunted junction (RSJ) characteristic, meaning that a Josephson junction is achieved. As seen in Fig. 5, its critical current (Ic ) is 15 lA. So far, few reports are known showing RSJ-type current±voltage characteristic for Nd123 step-edge junction at 77 K, although Nd123 is well known for many advantages over Y123.

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Fig. 4. Optical image of the Nd123/MgO thin ®lm step-edge junction.

narrower DTc . The DTc suppression is attributed to the elimination of droplets as well as the stoichiometric deviation of Nd specie. Nd123/STO ®lm exhibits better superconducting properties over those on YSZ and MgO. Its Tc…onset† , DTc and Jc reach 93.5 K, 3.5 K and 6  106 A/cm2 , respectively. A Nd123 thin ®lm step-edge junction is also fabricated by the eclipsed PLD at 650°C. The RSJ-like current±voltage curve implies that a Josephson junction has been achieved. References

Fig. 5. A typical RSJ-like current±voltage curve at 77 K for the Nd123/MgO thin ®lm step-edge junction.

4. Conclusions we have obtained high quality c-axis oriented Nd123 thin ®lm on various substrates at a relatively low deposition temperature (650°C) by using eclipsed PLD. In contrast to the conventional PLD ®lms, the eclipsed Nd123 ®lms show a

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