X-ray photoelectron spectroscopy (XPS) has been used to study the electronic structures of indium tin oxide (ITO) surfaces treated by O~(+), Ar~(+), and NH_(x)~(+) plasmas. The XPS data show that there is a significant change in core level energies (In 3d_(5/2) O 1s, and Sn 3d_(5/2)), in donor concentration (Sn~(4+)), in valence band maximums (VBM), and in work functions on ITO surfaces being treated by O~(+) and NH_(x)~(+) plasmas, compared with that of virgin and Ar~(-) plasma treated surfaces. Based on these experimental data, a surface band-bending theory is proposed. The theory explains that when Fermi energy of the plasma-treated surface is shifted towards the middle of the band gap: core levels will shift their energies to lower binding energies. VBM will bend upward, and work function will increase, as observed.
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