Direct comparison of photoemission spectroscopy and in situ Kelvin probe work function measurements on indium tin oxide films

Abstract

The work function of commercially available indium tin oxide (ITO) films on glass substrates was measured using photoemission spectroscopy (PES) and ultra-high vacuum (UHV) Kelvin probe in direct comparison. Absolute Kelvin probe work function values were determined via calibration of the measured contact potential difference (CPD) using an in situ sputtered Au reference sample. The Kelvin probe data confirmed that ultraviolet photoemission spectroscopy (UPS) measurements change the work function of ITO surfaces previously exposed to ambient environment, when measured without in situ surface cleaning procedures. The results also demonstrate that both Kelvin probe and PES yield virtually identical work function values, as long as the Kelvin probe data are calibrated against a known standard. As a consequence, previously reported higher work function values determined with Kelvin probe as compared to values obtained with UPS on similar samples are likely related to a photochemically generated surface dipole during UPS measurements. Comparison between Kelvin probe and low intensity X-ray photoemission spectroscopy (LIXPS) work function measurements demonstrated that accurate work function measurements on ITO previously exposed to the ambient are possible with PES.