Surface chemistry of carbon removal from indium tin oxide by base and plasma treatment, with implications on hydroxyl termination

The surface chemistry of carbon removal from indium tin oxide (ITO) has been investigated with Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and high-resolution energy loss spectroscopy (HR-ELS). A vibrating Kelvin probe (KP) was used to monitor the work function (Φ) of ITO after cleaning, either by base-cleaning with alcoholic-KOH or by O2 plasma-cleaning. Base-cleaning lowered ΦITO as seen in the KP analysis, whereas plasma-cleaning slightly increased ΦITO by an oxidative process. The degree of ΦITO depression by base-cleaning was seen to depend on the initial surface, but the Φ depression itself was nonreductive to the ITO, as seen in the In-MNN AES lineshape. The nonreductive depression of ΦITO by base-cleaning was further supported by a constant charge carrier density, as estimated from the HR-ELS. Base-cleaning was slightly more effective than the oxygen plasma in removing carbon from the ITO surface. However, base-cleaning preferentially removed graphitic carbon while leaving significant hydrocarbon contaminants, as determined by vibrational analysis with HREELS. All other ITO surfaces retained a significant carbon and hydrocarbon contamination as evidenced by AES and HREELS. There was little evidence of the formation of surface hydroxyl species, as expected for such an inherently contaminated surface as ITO.