Characterization of YBa₂Cu₃O_x using core- and valence-level XPS

Abstract

Hundreds of papers have been published involving the photoelectron spectroscopy of the high-Tc superconducting oxides since 1987. The early work, originally on bulk-sintered material, sputtered films, and later on "single crystals", concentrated on finding "unusual" features in valence- or core-level spectra to relate to electronic structure effects which might explain the superconducting mechanism. The majority of this work has not adequately taken into account the facts that (a) photoemission probes only the top few monolayers of material, and (b) in many cases the top few layers are completely unrepresentative of the bulk material. This is particularly true for YBa2Cu3Ox, where the surface is extremely reactive, unstable, and prone to contaminating phases, even when prepared under UHV conditions. This has led to a flood of misinformation concerning the true characteristic spectra of this material and their interpretation. In this paper, we present core- and valence-level XPS for YBa2Cu3Ox single-crystal, bulk-sintered, and thin-film samples, and show that, when artifacts are eliminated, the characteristic spectra are the same, to first order, and easily allow distinction of surfaces consisting of the genuine orthorhombic phase (x > 6.4) from those with the non-superconducting tetragonal phase (x<6.4) or contaminant or reaction-product phases. With this information, it is possible to eliminate much of the previous literature discussion and also to follow the material changes occuring, for instance, during annealing, adsorption and reaction. We then discuss some detailed interpretations, including the DOS observed at and near EF, the explanations for the ∼1.5 eV chemical shift in Ba core-level BE between orthorhombic and tetragonal forms, and the implications of the very low O(1s) BE of the orthorhombic form. © 1993.