Electric Properties of Grain Boundaries in Transparent Oxide Conductors
Andreas Klein
Technische Universität Darmstadt, Jovanka-Bontschits-Straße 2, 64287 Darmstadt, Germany
E-Mail:
Oxides are mostly used in polycrystalline form, for example as sintered ceramics or as differently
processed thick or thin films. Grain sizes in these materials vary from a few nanometers up to several
micrometers. The grain boundaries can determine the properties of the materials. The electric and
dielectric properties of grain boundaries are directly related to the charges, which are trapped at grain
boundaries and which result in space layers in the adjacent grains. The space charge layers do not only
affect electronic charge transport, they can also be the origin of giant dielectric permittivity. Moreover,
as defect concentrations depend on the electrostatic potential, they are modified in space layers. This
can result in an enhanced or a reduced ionic conductivity along or perpendicular to grain boundaries.
The ionic space layers can also lead to dopant segregation at grain boundaries, when the dopants are
mobile at the processing temperature. This is particularly the case for doped In
2
O
3
, where Sn
segregation results in an increase of grain boundary barrier height and to a reduction of a carrier
mobility almost up to the highest possible carrier concentrations.
The presentation will give an introduction into the electrical properties of grain boundaries and a brief
summary of the related knowledge and open questions. Then our experimental approach based on
semiconducting oxide thin films and the obtained results on the nature and influence of grain
boundaries, the identification of electronic states and segregation of dopants, will be presented.
Differently doped In
2
O
3
and Cu
x
O will be used to demonstrate the various effects of grain boundaries.
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