Julia Medvedeva1

1, Missouri University of Science and Technology, Rolla, Missouri, United States

Tunable electrical conductivity – the ability to change carrier concentration over a wide range of useful values while maintaining superior mobility – is arguably the central technological advantage of an Amorphous Oxide Semiconductor (AOS) such as ternary or quaternary oxides of post-transition metals, for example, In-Sn-O, Zn-Sn-O, or In-Ga-Zn-O. Compared to the crystalline counterparts, where the electron mobility is governed primarily by scattering on ionized impurities, phonons, and grain boundaries, the nature of and the relationship between the carrier generation and transport in AOSs are more complex. Although amorphous materials lack grain boundaries, the strong local distortions in the Metal-Oxygen (M-O) polyhedra associated with a weak ionic M-O bonding as well as long-range structural correlations in the disordered system give rise to entangled transport phenomena at different length scales [1-3]. Given the many degrees of freedom in the amorphous structure, the long-range structural characteristics and the electronic properties of the donor defects in AOSs differ fundamentally from those in the crystalline transparent conducting oxides. Therefore, defects in AOSs must be considered along with the structural evolution of the disordered system.

In this work, we report the results of computationally intensive ab-initio molecular dynamics (MD) simulations combined with accurate density functional electronic structure calculations for amorphous In-based and Sn-based oxide semiconductors. Novel approaches for non-stoichiometric-melt cooling and time-dependent statistical analysis not only show a significant improvement over the standard oxygen vacancy models but also allow us to simultaneously address the structural morphology, evolution, and the dynamics of defect formation, thereby providing the necessary integral framework to comprehensively understand the fundamental materials properties of AOSs. We demonstrate that the approach provides a statistically complete defect picture of conducting amorphous oxides by capturing the formation of both shallow defects that produce carriers and localized deep defects that limit carrier mobility via electron trapping or scattering. The scheme also allows us to study the long-range structural reconstruction in AOSs and the defect dynamics during quenching or annealing processes providing the necessary information for optimizing the electronic and optical properties of AOSs toward their application in optoelectronic technologies.

[1] D. Buchholz, Q. Ma, D. Alducin, A. Ponce, M. Jose-Yacaman, R. Khanal, J.E. Medvedeva, and R.P.H. Chang,
Chemistry of Materials, 26, 5401-5411 (2014).
[2] R. Khanal, D.B. Buchholz, R.P.H. Chang, and J.E. Medvedeva, Physical Review B, 91, 205203 (2015).
[3] J.E. Medvedeva, D.B. Buchholz, and R.P.H. Chang, Advanced Electronic Materials, 3, 1700082 (2017).