Tailoring of carrier concentration and engineering of band gap for Sn-doped In₂O₃ films by postirradiation of negatively charged oxygen ions

Abstract

We demonstrate that the state-of-the-art postirradiation technology for negatively charged oxygen (O−) ions is effective for tailoring carrier concentration (n e ), electrical resistivity (ρ), and optical band gap (E g) in a wide range for polycrystalline 50-nm-thick Sn-doped In2O3 (ITO) films on glass substrates by reactive plasma deposition with direct-current arc discharge. As-deposited ITO films showed n e of 9.2 × 1020 cm−3, ρ of 1.5 × 10−4 Ω cm, and E g of 3.50 eV. The postirradiation of O− ions for 180 min at 250 °C decreased n e to 2.4 × 1018 cm−3. This resulted in a significant increase in ρ to 3.5 × 10−1 Ω cm while retaining the bixbyite crystal structure and the spatial distribution of Sn dopant atoms. The postirradiation of O− ions led to the continuous decrease in the optical E g ranging from 3.50 to 3.02 eV, which is smaller than that of undoped In2O3. For degenerate ITO films, conventional theories about the broadening and narrowing of the optical E g explain the experimental results well. On the other hand, for nondegenerate ITO films, the optical E g shrinkage would be mainly caused by an upward energy shift attributable to the generation of the anti-bonding π* states between O 2p and In 4d orbitals within the topmost valence band owing to the lattice disorder associated with incorporated interstitial oxygen atoms that fill structural vacancy sites. On the basis of the Ioffe–Regel criterion utilizing the electron mean free path, Fermi momentum, and their product, we determined the critical n e at which degenerate ITO films transform to nondegenerate ones.