5.27 Chemical Message
The oxygen vacancies dominate the total of defects in ZnO, and they are responsible for increase or decrease of conductivity as their amount increases or decreases.
For the first time, the defects in semiconductors, though ionized, became directly
visible and could be associated with the physical properties of the oxide.
5.28 The Case of Functional Defects in Semiconductor
Oxides: SnO 2
As well as ZnO, SnO 2 is one of the most common commercially materials used in
gas sensor devices [17]. Although SnO 2 -based gas sensors are widely used, their
improvement in sensitivity and selectivity passes through a better knowledge of the
mechanism by which they function.
The conductivity, whose variation is at the origin of the SnO 2 -based gas sensors,
has been postulated to be controlled by the Schottky barriers, through interaction
with surface chemisorbed species. Neither interaction with lattice centers nor the
consequent formation of donor centers has been postulated at the origin of the
conductivity variations.
The scientific aim of this study is to assess whether SnO 2 conductance is controlled by Schottky barrier height and whether new donor centers become active.
5.29 Discussion of the Case
5.29.1 Air Interaction with SnO 2
When SnO 2 was pretreated in dry air at 673 K, no signals have been observed in the
ESR spectrum. If the air is moist, strong resonance lines appear at g = 1.89
(Fig. 5.19). The resonances could be attributed to the creation of oxygen vacancies
following the reaction
H 2 O þ 2Sn Sn þ O 0 $ 2 HOÀSn Sn
ð
ÞþV 0
V 0 $ V 0
þ
þ e
À where the paramagnetic species is V 0
þ
Sn Sn and O 0 represent Sn
4+ and O
2− in regular sites and V 0 a neutral oxygen
vacancy.
5.27 Chemical Message
111
The oxygen vacancies dominate the total of defects in ZnO, and they are responsible for increase or decrease of conductivity as their amount increases or decreases.
For the first time, the defects in semiconductors, though ionized, became directly
visible and could be associated with the physical properties of the oxide.
5.28 The Case of Functional Defects in Semiconductor
Oxides: SnO 2
As well as ZnO, SnO 2 is one of the most common commercially materials used in
gas sensor devices [17]. Although SnO 2 -based gas sensors are widely used, their
improvement in sensitivity and selectivity passes through a better knowledge of the
mechanism by which they function.
The conductivity, whose variation is at the origin of the SnO 2 -based gas sensors,
has been postulated to be controlled by the Schottky barriers, through interaction
with surface chemisorbed species. Neither interaction with lattice centers nor the
consequent formation of donor centers has been postulated at the origin of the
conductivity variations.
The scientific aim of this study is to assess whether SnO 2 conductance is controlled by Schottky barrier height and whether new donor centers become active.
5.29 Discussion of the Case
5.29.1 Air Interaction with SnO 2
When SnO 2 was pretreated in dry air at 673 K, no signals have been observed in the
ESR spectrum. If the air is moist, strong resonance lines appear at g = 1.89
(Fig. 5.19). The resonances could be attributed to the creation of oxygen vacancies
following the reaction
H 2 O þ 2Sn Sn þ O 0 $ 2 HOÀSn Sn
ð
ÞþV 0
V 0 $ V 0
þ
þ e
À where the paramagnetic species is V 0
þ
Sn Sn and O 0 represent Sn
4+ and O
2− in regular sites and V 0 a neutral oxygen
vacancy.
5.27 Chemical Message
111