Oxygen content and inhomogeneity effects on the electrical properties of YBa 2 Cu 3 O y thin films
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Oxygen content and inhomogeneity effects on the electrical properties of YBa2 Cu3 Oy thin films R. Aguiar, F. S´anchez, and M. Varela Departament de F´ısica Aplicada i Electr`onica, Universitat de Barcelona, Av. Diagonal 647, E-08028 Barcelona, Spain (Received 2 January 1995; accepted 16 August 1996)
Oxygen content is a very important factor influencing the electrical properties of YBa2 Cu3 Oy . In this work the electrical properties of laser deposited YBa2 Cu3 Oy thin films on LaAlO3 (100), in the whole range 6 < y < 7, are studied. An electrical network model, which randomly assigns oxygen contents and RsTd characteristics to the different elements in the circuit according to an arbitrary distribution, is used to analyze several features in the measured RsTd characteristics as a function of oxygen homogeneity. The model takes into account both short-range and long-range oxygen inhomogeneities. Good agreement between estimated oxygen contents from x-ray diffraction data in our samples and the average oxygen contents used to reproduce their RsTd characteristics is found. The model points out that oxygen homogeneity is very important in order to get the best and reproducible properties, and for conduction and superconductivity analysis through the shape or derivatives of RsTd characteristics.
I. INTRODUCTION
The strong dependence of the properties of YBa2 Cu3 Oy on the carrier concentration has been widely evidenced. This concentration can be controlled by doping the compound with oxygen atoms. YBa2 Cu3 O6 is a tetragonal semiconducting and nonsuperconducting material, but its properties can be modified by adding oxygen atoms to the Cu–O chains. That is why the usual formula YBa2 Cu3 Oy s6 < y < 7d is used. The extra oxygen atoms introduce holes in the structure, which are responsible for the increase in the conductivity1 and the transition to a metallic behavior in the normal state. This behavior is not exactly linear and a roundup of the RsTd characteristics is found in some cases. This fact is still under discussion by many authors and has sometimes been related to some specific transport properties in the high temperature superconducting cuprates.1 When the oxygen content of YBa2 Cu3 Oy is below 6.3 –6.4 the material is still nonsuperconducting. When 6.4 < y < 6.7 the crystal structure becomes orthorhombic (so-called orthorhombic II phase), and the material is already metallic in the normal state and superconducting with Tc about 60 K. If y goes up, the critical temperature suddenly increases and remains about 90 K for the highest oxygen contents (orthorhombic I phase). Though this is the most common behavior, the two plateaus about 60 K and 90 K may disappear, depending on the sample preparation, due to the influence of oxygen ordering inside the crystal structure on the electrical behavior.2,3 Most previous work on the relationships between electrical properties and oxygen content in YBa2 Cu3 Oy has been carried out on bulk material3–5 and J. Mater. Res., Vol. 12
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