Abstract
Transparent conducting oxides (TCOs) are ubiquitous in modern consumer electronics. SnO2 is an earth abundant, cheaper alternative to In2O3 as a TCO. However, its performance in terms of mobilities and conductivities lags behind that of In2O3. On the basis of the recent discovery of mobility and conductivity enhancements in In2O3 from resonant dopants, we use a combination of state-of-the-art hybrid density functional theory calculations, high resolution photoelectron spectroscopy, and semiconductor statistics modeling to understand what is the optimal dopant to maximize performance of SnO2-based TCOs. We demonstrate that Ta is the optimal dopant for high performance SnO2, as it is a resonant dopant which is readily incorporated into SnO2 with the Ta 5d states sitting ∼1.4 eV above the conduction band minimum. Experimentally, the band edge electron effective mass of Ta doped SnO2 was shown to be 0.23m0, compared to 0.29m0 seen with conventional Sb doping, explaining its ability to yield higher mobilities and conductivities.
Highlights
Transparent conducting oxides (TCOs) are materials which possess the generally mutually exclusive properties of high electrical conductivity and optical transparency
We have demonstrated that Mo-doped In2O3 (IMO) outperforms ITO as Mo behaves very differently from Sn in In2O3.28,29 In ITO, Sn is in the (IV) oxidation state,[30] and the Sn 5s orbitals mix with the In 5s states at the bottom of the conduction band of In2O3
SnO2 as predicted by the calculations, Ta and Sb doped SnO2 films were deposited by aerosol assisted chemical vapor deposition (AACVD)
Summary
Transparent conducting oxides (TCOs) are materials which possess the generally mutually exclusive properties of high electrical conductivity and optical transparency These properties are achieved through degenerate doping of wide band gap semiconductors (Eg > 3.1 eV) giving rise to applications in a variety of crucial modern technologies such as touch screen displays, solar cells, low emissivity windows, and gas sensors.[1−6]. Traditional wisdom dictates that the best choice of dopant is the element positioned directly to the right of the host element in the periodic table This represented a logical choice as they should possess both the correct oxidation state and similar ionic radii, minimizing lattice distortion which can lower solubility.[4] Sb (on the Sn-site) or F (on the O site) have typically been the dopants of choice. Both F-doped SnO2 (FTO) and Sb-doped SnO2 (ATO) thin films have displayed resistivities of ∼ 5 × 10−4 Ω cm[4,12] and have been successfully deposited with a wide range of techniques including pulsed laser deposition,[13,14] spray pyrolysis,[15,16] sol−gel,[17,18] and sputtering.[19]
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