Optical determination of the oxygen content of YBa 2Cu 3O 6+ x thin films by IR reflectance and transmittance measurements

Abstract

Recent studies of the optical properties of YBa 2Cu 3O 6+ x (YBCO) single crystals by several authors have shown that the optical conductivity in the infrared spectral region is a sensitive function of the oxygen content of the samples. Infrared spectroscopy thus offers a possibility for oxygen concentration analysis and is an alternative to other methods such as (i) the X-ray determination of the length of the c-axis and (ii) spectroscopic ellipsometry at an electronic transition band centered around 4.1 eV whose strength decreases with increasing oxygen content. We explore the applicability of the IR optical method for the case of YBCO thin films of about 300 nm thickness which are epitaxially grown on SrTiO 3 substrates. We perform normal-incidence transmittance and reflectance measurements between 2000 and 3000 nm to determine the refractive index and the absorption coefficient of the films. Due to the large penetration depth of the IR light it is essential to pay regard to the interferometric aspects of light propagation in the film and to take care of ambiguities in the mathematical inversion. The latter problem is solved by using the asymmetry of the reflectance measured on the two different sides of the sample. It is shown that the accuracy of the results depends on the quality of the films and on the accuracy with which their thickness can be determined by an independent measurement. Despite these difficulties the method provides a simple means for monitoring the oxygen content of YBCO films in the whole doping regime (0⩽ x⩽1).