Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: a comparison

We report a comparison of the work functions of thin films of indium tin ox ide (ITO), carried out by means of ultraviolet photoelectron spectroscopy ( UPS) and by measurements of the contact potential difference with respect t o a gold reference electrode (Kelvin probe (KP) method). We investigated co mmercially available ITOs both "as-received", and after certain surface tre atments, such as oxygen plasma. First, we find measurable discrepancies bet ween KP values measured with three different instruments, and between the K P and the UPS values. Secondly, and unexpectedly, we find that the KP, alth ough more sensitive than UPS, does not detect certain differences between I TOs with different surface treatments. We discuss the results in view of th e different environments in which the measurements are carried out (UHV for the UPS and air/Ar for the Kelvin method), of the effects which may be ind uced by the high-energy photon irradiation in the UPS measurement, and of t he stability of the gold probe work function in gas ambient. We conclude th at UPS is better-suited for absolute work function determination, although KP remains a convenient and inexpensive tool for fast screening of contact potential differences. (C) 2000 Elsevier Science S.A. All rights reserved.