Lattice Defects in SrRuO 3 Thin Films and Their Contribution to film Resistivity
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INTRODUCTION SrRuO 3 (SRO) is one of the ternary transition metal oxides having a high electrical conductivity, with a room temperature resistivity in the order of 10-4 Qcm. This property permits the application of SRO films as oxide electrodes for ferroelectric devices and as non-superconducting barriers for Josephson junctions involving high T, superconducting oxide films. The effect of deposition parameters such as substrate temperature and oxygen pressure on structure and electrical properties of SRO films grown on different substrates were investigated previously [1-10]. However, there is a lack of information concerning the correlations between defect structure and electrical properties. In this paper lattice defects and resistivity of thin SrRuO 3 films grown on SrTiO 3 (001) substrates by pulsed laser deposition at different substrate temperatures are investigated. EXPERIMENT Epitaxial SrRuO 3 films were grown on SrTiO 3 (001) substrates by pulsed laser deposition (PLD) employing a KrF excimer laser (k = 248 nm). Each of these films was grown with 1000 laser pulses. The films were deposited in an oxygen ambient of 200 mTorr with a pulse energy density of 2 J/cm 2 at a pulse rate of 5Hz and at different substrate temperatures between 700 TC and 850 'C. After deposition they were cooled to 450 'C and annealed for 30 minutes. The electrical resistivity was measured down to 75K by a four point probe method. Phase composition and structure of the films were investigated by high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX). 113
Mat. Res. Soc. Symp. Proc. Vol. 574 1999 Materials Research Society
RESULTS 1. Crystal structure of SrRu03 SrRuO 3 is orthorhombic and belongs to the space group Pbnm with the lattice parameters ao= 0.557 nm, b o= 0.553 nm and c0 = 0.785 nm [11], but it can be treated as slightly distorted tetragonal, with the tetragonal unit cell parameters at = 0.393 nm, ct = co = 0.785 nm and the angle y= 89.6' between the [100], and [010], axes (Fig.1). 2. Conductivity The electrical conductivity of the films grown at different substrate temperatures exhibits a typical metallic behaviour with a paramagnetic-to-ferromagnetic transition at about 155K. The best conductivity was obtained for the films grown at 775°C (Fig.2). The other films, grown at g00 [0101,
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700
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050
900
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Fig.l Schematic of the pseudotetragonal unit Fig.2 Variation of room temperature cell of SrRuO3. The angle between the [010], resistivity as a function of substrate and [ 100], axes is y--89.6'. temperature. higher or lower temperatures, clearly had a lower conductivity, with the lowest values measured for films grown at 700 TC and 850 TC. The paramagnetic-to-ferromagnetic transition is less pronounced and shifted to lower temperatures in the films grown at 700°C and 850'C as compared with the film grown at 775°C. To evaluate the origin of this behavior, morphology and structure of three films grown at 7000C, 775 0 C
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