X-ray Photoelectron Spectroscopy Study of Indium Tin Oxide Films Deposited at Various Oxygen Partial Pressures

Here, a systematic experimental study on indium tin oxide (ITO) films is presented to investigate the effects of oxygen partial pressure on the film's electrical properties. The results of Hall measurements show that adding more oxygen in the sputtering gas has negative influences on the electrical conductivity of ITO films. As O₂/(O₂ + Ar)% in the sputtering gas is increased from 0 to 6.98%, the resistivity of ITO film rises almost exponentially from 7.9 × 10⁻⁴ to 4.1 × 10⁻² Ω cm, with the carrier density decreasing from 4.8 × 10²⁰ to 5.4 × 10¹⁸ cm⁻³. The origins of these negative effects are discussed with focuses on the concentration of ionized impurities and the scattering of grain barriers. Extensive x-ray photoelectron spectroscopy (XPS) analyses were employed to gain insight into the concentration of ionized impurities, demonstrating a strong correlation between the oxygen vacancy concentration and the carrier density in ITO films as a function of sputtering O₂ partial pressure. Other microstructural characterization techniques including x-ray diffraction (XRD), high-magnification scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) analyses were used to evaluate the average grain size of ITO films. For ITO films that have carrier density above 10¹⁹ cm⁻³, scattering on grain boundaries and other crystallographic defects show negligible effects on the carrier transport. The results point to the oxygen vacancy concentration that dictates the carrier density and, thus, the resistivity of magnetron-sputtered ITO films.