Defects in Amorphous Oxide Semiconductors
Dr. Julia E. Medvedeva
Professor of Physics, Missouri University of Science and Technology, USA
E-mail:
Despite a tremendous technological appeal of amorphous oxide semiconductors
(AOSs) and a large body of experimental and theoretical publications in the area, the
structure-property relationships in AOSs have not been fully understood. The experimental
characterization, such as extended x-ray absorption fine structure (EXAFS) measurements,
combined with theoretical models derived from molecular dynamics (MD) simulations, it has
been established that the first-shell characteristics – the average Metal-Oxygen distances and
coordination – remain nearly intact upon the transition to the amorphous phase. This suggests
that, upon amorphization, both the optical band gap and the electron effective mass governed
by the metal-oxygen interactions, should deviate insignificantly from the crystalline values.
Hence, the key features of the electronic band structure of a transparent conducting oxide
host are preserved under the crystalline-amorphous transition.
However, the electrical conductivity in AOSs varies by orders of magnitude and
strongly depends on the preparation conditions. In marked contrast to the crystalline
transparent conducting oxides where the electron mobility is governed primarily by the
scattering on the ionized or neutral impurities, phonons, and grain boundaries, the transport
properties of amorphous oxides are more complex. Although amorphous oxides lack grain
boundaries, the structural long-range disorder will give rise to additional contributions to the
overall relaxation time. In multi-cation In-based amorphous oxides the electron scattering is
expected to occur due to (i) the size and spatial distribution of the nanocrystalline In
2
O
3
inclusions; (ii) piezoelectric effects associated with internal lattice strains; (iii) spatial
distribution and clustering of incorporated cations, and (iv) local point defects. Clearly,
chemical composition, oxygen environment, as well as deposition temperatures will have a
strong effect on the above processes.
In this work, ab-initio molecular dynamics and accurate density-functional approach are
employed to understand how the electronic, optical, transport, and mechanical properties of
amorphous ternary and quaternary oxides depend on quench rates, cation compositions, and
oxygen stoichiometry. To understand the origins of carrier generation and carrier transport in
AOSs – the topic that is still under active debate in the community – a new approach is
developed that allows one to study the long-range structural reconstruction as well as the
defect dynamics during the quench process. This novel approach provides a more realistic
model of conducting amorphous oxides with a complete defect picture since it involves the
formation of both shallow extended defects that produce carriers and localized deep defects
that significantly limit carrier mobility via electron trapping or scattering. In-depth
understanding of the complex defect formation in AOSs helps predict ways to remove those
defects (by passivation, doping, irradiation, etc) and, thus, to theoretically predict optimal
growth conditions that will lead to high electrical conductivity (carrier concentration and
mobility) while maintaining desired optical transparency (low short/long wavelength
absorption, low plasma frequency).
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