Microstructure of laser-ablated superconducting La 2 CuO 4 F x thin films on SrTiO 3
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P.P. Edwardsb) School of Chemistry and the School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
S.T. Lees, K.E. Gibbons, and I. Gameson School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
M. Aindow Department of Metallurgy and Materials Engineering, Institute of Materials Science, 97 North Eagleville Road, Unit 3136, University of Connecticut, Storrs, Connecticut 06269-3136 (Received 23 January 2001; accepted 6 September 2001)
Thin films of lanthanum cuprate were grown on SrTiO3 substrates by pulsed laser deposition and made superconducting (Tc ∼ 38 K) through the process of post-deposition fluorination using elemental fluorine. A microstructural analysis showed that the [1¯10] zone of the film grows parallel to the [100] zone of the SrTiO3 substrate, reducing the lattice mismatch from 37.5% to 2.4%. At the film–substrate interface there is an intermediate layer 3–4 nm thick and twin-related grains emanate from this region. Stacking faults are present in the bulk of the film, with misoriented subgrains present at the deposit surface.
I. INTRODUCTION
One of the most intensely studied materials in the field of high-Tc superconductivity has been lanthanum cuprate. Stoichiometric La2CuO4 (Fig. 1) is an antiferromagnetic semiconductor, which transforms continuously to delocalized character when the electronically active copper oxide sheets are partially oxidized (i.e., hole doping). On cooling, hole-doped La2CuO4 exhibits hightemperature superconductivity. The optimum concentration of holes in the valence band corresponds to an average copper oxidation state of approximately +2.17, at which the superconducting transition temperature (Tc) is maximized. Control of this carrier concentration has routinely been accomplished by partial substitution of the trivalent lanthanum cation with divalent alkali or alkaline-earth cations1–6 or the insertion of interstitial anions7–19. Tissue et al.19 first reported the conversion of undoped, polycrystalline La2CuO4 into a superconductor by an oxidative treatment in F2 gas. It has since been shown that under optimized conditions (1.3 bar of pure
a) b)
Equal weighing should be given to these authors. Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 11, Nov 2001
F2 gas, 200 °C, 20 h) fluorination of bulk prepared La2CuO4 yields a virtually monophasic product with a Tc of 40 K.17,18 The fluorination causes a significant enhancement of the c lattice parameter and neutron diffraction, and extended x-ray absorption fine structure (EXAFS) measurements at the La LIII x-ray absorption edge indicate that the fluorinated structure contains additional anions between consecutive LaO layers as indicated in Fig. 2.19,20 It has been established that pulsed laser ablation is one of the most versatile techniques for the fabrication of high critical transition temperature films and devices.21–25 We have previously reported that superconducting properties can be induced
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