Surface electronic structure of plasma-treated indium tin oxides

Abstract

X-ray photoelectron spectroscopy (XPS) has been used to study the electronic structures of indium tin oxide (ITO) surfaces treated by ${\mathrm{O}}^{+},$ ${\mathrm{Ar}}^{+},$ and ${\mathrm{NH}}_x^{+}$ plasmas. The XPS data show that there is a significant change in core level energies $(\mathrm{In}{\text{\hspace{0.17em}3d}}_{\text{5/2}}$ O $\text{1s,}$ and Sn ${\text{3d}}_{\text{5/2}}\mathrm{),}$ in donor concentration $({\mathrm{Sn}}^{\mathrm{4+}}\mathrm{),}$ in valence band maximums (VBM), and in work functions on ITO surfaces being treated by ${\mathrm{O}}^{\mathrm{+}}$ and ${\mathrm{NH}}_x^{+}$ plasmas, compared with that of virgin and ${\mathrm{Ar}}^{+}$ 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.