| About this Abstract |
| Meeting |
2011 Electronic Materials Conference
|
| Symposium
|
2011 Electronic Materials Conference
|
| Presentation Title |
HH1, Surface Donors Dominate the Conductivity of In2O3 Thin Films |
| Author(s) |
Stephan Lany, Andriy Zakytayev, Thomas O. Mason, John F. Wager, John D. Perkins, Joseph J. Berry, David S. Ginley, Alex Zunger |
| On-Site Speaker (Planned) |
Stephan Lany |
| Abstract Scope |
The underlying mechanism for conductivity in transparent conducting oxides (TCO) has so far been discussed mostly in terms of bulk-defect models, involving traditional doping with donor impurities, intrinsic defects, or unintentional impurities like hydrogen. In the technologically important thin-film form of pure (undoped) In<sub>2</sub>O<sub>3</sub> one observes often mysteriously high carrier densities, which can rival that achieved in bulk specimens only through heavy doping by Sn<sub>In</sub> donor impurities. Based on first-principles electronic structure calculations for bulk defects [1], we show that the formation of oxygen vacancies in thermal equilibrium can explain the experimentally observed carrier densities at elevated temperatures and the O-deficient non-stoichiometry in bulk In<sub>2</sub>O<sub>3</sub>. However, the thin-film conductivities, which under comparable conditions (T, <i>p</i>O<sub>2</sub>) exceed the bulk conductivities by several orders of magnitude, lie outside the thermodynamic bounds of a bulk-defect mechanism. Utilizing complementary experimental and theoretical approaches, we performed surface calculations and thickness-dependent Hall measurements in epitaxial In<sub>2</sub>O<sub>3</sub> films to unearth the puzzling cause of conductivity in undoped In<sub>2</sub>O<sub>3</sub> thin films. From the electronic structure calculations for the particularly stable (111) surface, we find that the formation energies of electron-producing intrinsic surface donors (surface O vacancies and In adatoms) are much reduced compared to their bulk counterparts, and the shallow donor levels of the surface defects can release electrons into both the surface and the bulk conduction band. Thus, intrinsic surface defects are a likely source of the recently observed electron accumulation in In<sub>2</sub>O<sub>3</sub> films [2]. The film thickness-dependent transport measurements allow decomposition of the total sheet carrier density of the film into a bulk-like component in the film interior and into a surface component. We find that the conductivity of the epitaxial films is dominated by the surface contribution up to a thickness of about 150 nm, and therefore conclude that the conductivity of In<sub>2</sub>O<sub>3</sub> thin-films is decisively controlled by surface defects. The generation of carriers via surface defects is therefore a viable route for the design of highly conductive transparent thin films. |
| Proceedings Inclusion? |
Undecided |