Developing Structure-Property Relationships in Amorphous Transparent Conducting
Oxides Through Local Structure Studies of the In-Ga-O System
Stephanie L. Moffitt, Qimin Zhu, Qing Ma, D. Bruce Buchholz, Robert P. H. Chang, Thomas
O. Mason, Tobin J. Marks, and Michael J. Bedzyk
Northwestern University, Evanston, IL 60208
Much of what is understood about transparent conducting oxides (TCOs) comes from
structure-property relationships that presuppose the presence of long-range order. Crystalline
TCOs helped revolutionize display technology, but their high deposition temperatures
prohibit the use of flexible plastic substrates. Required for the new frontier of display
technology, deposition on flexible substrates can be realized with the use of amorphous (a-)
TCOs created through low-temperature deposition. The consequence of low temperature is a
loss of long-range structural order, which negates the use of characterization techniques such
as X-ray diffraction and simulations with periodic boundary conditions. We employ local
structure, element-specific, X-ray measurements, performed at the Advanced Photon Source,
to gain an understanding of how cation composition affects the local structure in a-TCOs.
With this knowledge of structure, we develop structure-property relationships unique to a-
TCOs.
The a-TCO system of indium gallium oxide (a-IGO) is a model system for developing
structure-property relationships to investigate the influence of substitutional cations on the
performance of a-TCOs. We will present a systematic study of the evolution of thermal and
electrical properties, as a function of gallium content, in a series of a-IGO thin films. These
property studies were understood on a fundamental level by pairing them with local structural
studies, X-ray absorption spectroscopy (XAS) and anomalous X-ray scattering (AXS).
The onset of crystallization in pure a-IO reduces the mobility.
1
Our work shows that
this temperature threshold can be increased from 125 to 250°C with less than 5% substituted
gallium. We attribute this effect to gallium’s ability to remain 4-fold coordinate even when
surrounded by a higher coordinate indium oxide matrix and thus resists the transition into the
bixbyite crystalline phase. In terms of electrical performance, oxygen vacancies have been
shown as the source of carriers in indium-oxide-based a-TCOs.
2
Gallium is shown to
decrease the carrier concentration when added to a-IO. Our work shows that the decreased
carrier concentration can be understood through the comparison of metal-oxygen
coordination with resulting metal oxygen distances. We reveal that gallium does not produce
carriers itself and reduces carrier concentration by diluting oxygen-vacancy-producing
indium octahedra. From a deep understanding of gallium’s effect on a-IO, future
substitutional atoms can be considered for the modification of a-TCOs to continue their
expansion into flexible display technology and beyond.
This work is supported by the NSF MRSEC Program No. DMR1121262
[1]
D. B. Buchholz, Q. Ma, D. Alducin, A. Ponce, M. Jose-Yacaman, R. Khanal, J. E. Medvedeva, and R. P. H. Chang,
Chem. Mater.
, vol. 26, pp. 5401–5411, 2014.
[2]
S. L. Moffitt, A. U. Adler, T. Gennett, D. S. Ginley, J. D. Perkins, and T. O. Mason,
J. Am. Ceram. Soc.
, vol. 7, pp.
2099-2103, 2015.
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