Effects of free carriers on the optical properties of high mobility transition metal doped In₂O₃ transparent conductors

Transition metal doped In₂O₃ with high mobility can be used as a transparent conductor with enhanced transparency spectral window. In this work, we carried out a comprehensive study on the electrical and optical properties of In₂O₃ doped with several transition metal (TM) species (In₂O₃ : TM) including W, Zr, Mo, and Ti. Detailed optical properties obtained by spectroscopic ellipsometry (SE) are correlated with electrical properties obtained by Hall effect measurements. We find that the mobility of In₂O₃ : TM thin films lies in the range of 50-75 cm² V^-1 s^-1, much higher than the typical mobility of 30-40 cm² V^-1 s^-1 for conventional ITO. The complex dielectric functions of the thin films reveal remarkable carrier density dependent changes in the optical properties. SE analyses show that the electron effective mass of In₂O₃ : TM at the bottom of the conduction band m_o^∗ (0.11-0.14m_o) is much smaller than the reported m_o^∗∼0.18-0.30m_o for ITO, which directly results in their higher mobility. This low m_o^∗ is consistent with recent theoretical studies which proposed that 4d donor states of the TMs are resonance in the CB. For films with comparably low resistivity of 1-2 × 10^-4 Ω cm, we find that In₂O₃ : TM films have ∼4-10 times lower absorption coefficient at λ = 1300 nm due to free carrier absorption and have their plasma reflection edge extended to ∼1.7 μm compared to ∼1.2-1.4 μm for ITO. Hence, using TM doping we have achieved transparent conductors with conductivity comparable to ITO but with transmission extended to >1600 nm. These materials will be potentially important as transparent conductors for optoelectronic devices utilizing NIR photons.