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Chapter 2
boundaries’. These are localised layers of small grains within the ZnO:Al layer that
have non-perpendicular crystallographic axes and a higher percentage area of grain
boundaries as compared to the rest of the ZnO:Al layer, which consists of compact
grains with grain boundaries in parallel. These extended grain boundaries are even
more unstable than standard grain boundaries. These are local perturbations of the
ZnO:Al morphology, which also allow fast diffusion of environmental species into the
material. Owen et al. [76] has also shown that etched, structured ZnO:Al degraded
quicker than the same ZnO:Al in an unetched state.
This important role for the grain boundaries in the electrical properties of zinc oxide
is also confirmed by Beyer et al. [83]. It was reported that neon cannot effuse out of
o
the bulk material of sputtered zinc oxides for temperatures as high as 1000C. How-
ever, transport through the grain boundaries was possible. Therefore, the transport
of molecules like oxygen, carbon dioxide and water is also only possible through the
grain boundaries.
Arzel et al. [84] also compared the changes induced by damp heat in optical (intra-
grain) and Hall (intergrain) resistivity of four types of sputtered ZnO:Al. They conclud-
ed that a large change in resistivity for less stable ZnO:Al was caused by grain bound-
ary degradation, while the stable ZnO:Al samples degraded only slightly and mostly
within the grains.
Based on these references, it can be concluded that in general, extended grain bound-
aries are the most sensitive to damp heat exposure. If these structures are either sta-
ble or not present, than the degradation of normal grain boundaries will become
dominant. If this does not happen, ingrain degradation will have the most important
influence on the degradation of the sample.
2.3.4.1.2 Degrading species in ZnO:Al
Many references (e.g. [71,72,76]) describe that the deterioration of the electrical prop-
erties and the change of the composition of the ZnO:Al film and its grain boundaries
was accompanied by migration of atmospheric species, like water, within the ZnO:Al
samples. Due to the square root shape of the evolution of the resistivity as a function
of degradation time, it was concluded in reference [71] that the degradation of ZnO:Al
is a diffusion process. Many studies included in Figure 2.8 for zinc oxide degradation
also show a shape that could be fitted with a square root function. However, a large
number of data points is required in order to find this relationship, which are not avail -
able in all studies.
54
boundaries’. These are localised layers of small grains within the ZnO:Al layer that
have non-perpendicular crystallographic axes and a higher percentage area of grain
boundaries as compared to the rest of the ZnO:Al layer, which consists of compact
grains with grain boundaries in parallel. These extended grain boundaries are even
more unstable than standard grain boundaries. These are local perturbations of the
ZnO:Al morphology, which also allow fast diffusion of environmental species into the
material. Owen et al. [76] has also shown that etched, structured ZnO:Al degraded
quicker than the same ZnO:Al in an unetched state.
This important role for the grain boundaries in the electrical properties of zinc oxide
is also confirmed by Beyer et al. [83]. It was reported that neon cannot effuse out of
o
the bulk material of sputtered zinc oxides for temperatures as high as 1000C. How-
ever, transport through the grain boundaries was possible. Therefore, the transport
of molecules like oxygen, carbon dioxide and water is also only possible through the
grain boundaries.
Arzel et al. [84] also compared the changes induced by damp heat in optical (intra-
grain) and Hall (intergrain) resistivity of four types of sputtered ZnO:Al. They conclud-
ed that a large change in resistivity for less stable ZnO:Al was caused by grain bound-
ary degradation, while the stable ZnO:Al samples degraded only slightly and mostly
within the grains.
Based on these references, it can be concluded that in general, extended grain bound-
aries are the most sensitive to damp heat exposure. If these structures are either sta-
ble or not present, than the degradation of normal grain boundaries will become
dominant. If this does not happen, ingrain degradation will have the most important
influence on the degradation of the sample.
2.3.4.1.2 Degrading species in ZnO:Al
Many references (e.g. [71,72,76]) describe that the deterioration of the electrical prop-
erties and the change of the composition of the ZnO:Al film and its grain boundaries
was accompanied by migration of atmospheric species, like water, within the ZnO:Al
samples. Due to the square root shape of the evolution of the resistivity as a function
of degradation time, it was concluded in reference [71] that the degradation of ZnO:Al
is a diffusion process. Many studies included in Figure 2.8 for zinc oxide degradation
also show a shape that could be fitted with a square root function. However, a large
number of data points is required in order to find this relationship, which are not avail -
able in all studies.
54
