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Chapter 6
Abstract
Sputtered thin ZnO:Al films on borosilicate glass were exposed to 85°C/85% RH and
to various combinations of atmospheric gases, which led to changes in the optical and
especially electrical properties of the films. For the damp heat exposed samples, it was
observed that the bulk structure did not change, which was confirmed by the stable carrier
concentration. However, the Hall mobility and thus the overall resistivity decreased, which
implied the increase of the height of the potential barriers, which are likely located at the
grain boundaries. Furthermore, white spots appeared, containing elements that might
have migrated from the glass, like silicon and calcium, which did react with elements from
the environment, including oxygen, carbon and chloride.
Treatment with atmospheric species helped the identification of the species most
detrimental to ZnO:Al. It was shown that the driving force behind ZnO:Al degradation
is the combined presence of HO and CO. Individually, gaseous CO did not impact the
2
2
2
degradation at all during the tested period, while the individual impact of H O is minor: the
2
latter leads to slow diffusion of water down the grain boundaries, where it reacts, possibly
resulting in the formation of Zn(OH), which led to a decrease in mobility. However, in the
2
presence of CO , the electrical and optical properties changed very quickly. Depth profiling
2
showed that the concentration of hydroxide increased with factors as high as 20 in the
bulk due to H O and CO exposure, while carbon, hydrogen, chlorine and sulphur were also
2
2
observed. Exposure to H O and CO also led to local dissolution of the ZnO:Al at the ZnO:Al/
2
2
glass interface. Additionally, it was observed that the influence of oxygen and nitrogen on
ZnO:Al stability was very small.
Together, these experiments showed that exposure of ZnO:Al the atmospheric conditions
mostly led to a decrease in mobility, likely caused by an increased potential barrier at
the grain boundaries. This effect on the mobility is limited when only HO/OH-diffuses
2
down, which can lead to the formation of Zn(OH) or adsorption of OH. In the presence
-
2
of CO , the effect on the mobility is larger, probably due to the formation of a Zn(OH)
x
2
(CO ) compound, like Zn (OH) (CO ) in the grain boundaries. Furthermore, chlorine and
y 2 z 5 6 3 2
sulphide were found in the top layer of the degraded samples, so equivalent molecules
containing these elements might also have formed.
176
Abstract
Sputtered thin ZnO:Al films on borosilicate glass were exposed to 85°C/85% RH and
to various combinations of atmospheric gases, which led to changes in the optical and
especially electrical properties of the films. For the damp heat exposed samples, it was
observed that the bulk structure did not change, which was confirmed by the stable carrier
concentration. However, the Hall mobility and thus the overall resistivity decreased, which
implied the increase of the height of the potential barriers, which are likely located at the
grain boundaries. Furthermore, white spots appeared, containing elements that might
have migrated from the glass, like silicon and calcium, which did react with elements from
the environment, including oxygen, carbon and chloride.
Treatment with atmospheric species helped the identification of the species most
detrimental to ZnO:Al. It was shown that the driving force behind ZnO:Al degradation
is the combined presence of HO and CO. Individually, gaseous CO did not impact the
2
2
2
degradation at all during the tested period, while the individual impact of H O is minor: the
2
latter leads to slow diffusion of water down the grain boundaries, where it reacts, possibly
resulting in the formation of Zn(OH), which led to a decrease in mobility. However, in the
2
presence of CO , the electrical and optical properties changed very quickly. Depth profiling
2
showed that the concentration of hydroxide increased with factors as high as 20 in the
bulk due to H O and CO exposure, while carbon, hydrogen, chlorine and sulphur were also
2
2
observed. Exposure to H O and CO also led to local dissolution of the ZnO:Al at the ZnO:Al/
2
2
glass interface. Additionally, it was observed that the influence of oxygen and nitrogen on
ZnO:Al stability was very small.
Together, these experiments showed that exposure of ZnO:Al the atmospheric conditions
mostly led to a decrease in mobility, likely caused by an increased potential barrier at
the grain boundaries. This effect on the mobility is limited when only HO/OH-diffuses
2
down, which can lead to the formation of Zn(OH) or adsorption of OH. In the presence
-
2
of CO , the effect on the mobility is larger, probably due to the formation of a Zn(OH)
x
2
(CO ) compound, like Zn (OH) (CO ) in the grain boundaries. Furthermore, chlorine and
y 2 z 5 6 3 2
sulphide were found in the top layer of the degraded samples, so equivalent molecules
containing these elements might also have formed.
176
