Optical determination of the oxygen content of YBa2Cu3O6+x thin films by IR reflectance and transmittance measurements

Recent studies of the optical properties of YBa2Cu3O6+x (YBCO) single cryst als by several authors have shown that the optical conductivity in the infr ared spectral region is a sensitive function of the oxygen content of the s amples. Infrared spectroscopy thus offers a possibility for oxygen concentr ation 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 ellipsome try at an electronic transition band centered around 4.1 eV whose strength decreases with increasing oxygen content. We explore the applicability of t he IR optical method for the case of YBCO thin films of about 300 nm thickn ess which are epitaxially grown on SrTiO3 substrates. We perform normal-inc idence transmittance and reflectance measurements between 2000 and 3000 mn to determine the refractive index and the absorption coefficient of the fil ms. Due to the large penetration depth of the IR light it is essential to p ay regard to the interferometric aspects of light propagation in the film a nd to take care of ambiguities in the mathematical inversion. The latter pr oblem is solved by using the asymmetry of the reflectance measured on the t wo different sides of the sample. It is shown that the accuracy of the resu lts depends on the quality of the films and on the accuracy with which thei r thickness can be determined by an independent measurement. Despite these difficulties the method provides a simple means for monitoring the oxygen c ontent of YBCO films in the whole doping regime (0 less than or equal to x less than or equal to 1). (C) 2001 Elsevier Science B.V. All rights reserve d.