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Chapter 6
6.4 Discussion
In this discussion session, first the observations and discussions for both the
experiments are summarised, while a general degradation mechanism for ZnO:Al
degradation is also introduced.
6.4.1 Damp heat treatment
When considering the observed results, the following conclusions on physical and
chemical changes in ZnO:Al due to damp heat treatment can be drawn:
1. The three dimensional structure of the samples stayed intact. The grain sizes
and the thickness of the layers did not change.
2. The main visual change was the appearance of white spots, containing
elements which migrated from the glass, like calcium and silicon, and reacted
with elements from the environment, including oxygen and carbon. These
spots have a small influence on the absorption of light in the IR region, but do
not have an influence the electrical properties or the overall transmission.
3. The resistivity increases, mainly driven by a large drop in the Hall mobility, while
the carrier concentration showed fluctuations, but did not change significantly.
This implies the formation of a potential barrier in the grain boundaries.
4. Chlorine, sulphur and carbon are present near the surface of degraded ZnO,
while hydroxide is present in the volume of the whole samples.
6.4.1.1 Physics of grain boundary degradation
When the chemical mechanisms for the degradation of ZnO:Al are considered, the
relevant degradation reactions are most likely to occur at the grain boundaries. These
grain boundaries thus have a changed composition, but the volume of the degraded
material is very small, so techniques like XRD and UV-VIS do not show direct evidence
for material change. The nature of the changes in the material and the relationship with
the mobility drop (Figure 6.8) can be found in the structure of the grain boundaries:
these are complex structures, usually consisting of a few atomic layers of disordered
atoms, representing a transitional region between two neighbouring crystallites.
The diffusion of environmental gases (e.g. oxygen, carbon dioxide and/or water
vapour) through these grain boundaries in doped ZnO films can lead to a reaction
between the atoms at grain boundaries and these species. This can result in the
accumulation of negatively charged trap states at the crystallite interface. This local
increase in the density of these states leads to a higher potential barrier, which hinders
the mobility of the carriers among the grains [12]. It is proposed that the carriers flow
across the grain boundaries via thermionic emission and tunneling increases with the
200
6.4 Discussion
In this discussion session, first the observations and discussions for both the
experiments are summarised, while a general degradation mechanism for ZnO:Al
degradation is also introduced.
6.4.1 Damp heat treatment
When considering the observed results, the following conclusions on physical and
chemical changes in ZnO:Al due to damp heat treatment can be drawn:
1. The three dimensional structure of the samples stayed intact. The grain sizes
and the thickness of the layers did not change.
2. The main visual change was the appearance of white spots, containing
elements which migrated from the glass, like calcium and silicon, and reacted
with elements from the environment, including oxygen and carbon. These
spots have a small influence on the absorption of light in the IR region, but do
not have an influence the electrical properties or the overall transmission.
3. The resistivity increases, mainly driven by a large drop in the Hall mobility, while
the carrier concentration showed fluctuations, but did not change significantly.
This implies the formation of a potential barrier in the grain boundaries.
4. Chlorine, sulphur and carbon are present near the surface of degraded ZnO,
while hydroxide is present in the volume of the whole samples.
6.4.1.1 Physics of grain boundary degradation
When the chemical mechanisms for the degradation of ZnO:Al are considered, the
relevant degradation reactions are most likely to occur at the grain boundaries. These
grain boundaries thus have a changed composition, but the volume of the degraded
material is very small, so techniques like XRD and UV-VIS do not show direct evidence
for material change. The nature of the changes in the material and the relationship with
the mobility drop (Figure 6.8) can be found in the structure of the grain boundaries:
these are complex structures, usually consisting of a few atomic layers of disordered
atoms, representing a transitional region between two neighbouring crystallites.
The diffusion of environmental gases (e.g. oxygen, carbon dioxide and/or water
vapour) through these grain boundaries in doped ZnO films can lead to a reaction
between the atoms at grain boundaries and these species. This can result in the
accumulation of negatively charged trap states at the crystallite interface. This local
increase in the density of these states leads to a higher potential barrier, which hinders
the mobility of the carriers among the grains [12]. It is proposed that the carriers flow
across the grain boundaries via thermionic emission and tunneling increases with the
200
