Environmental Engineering Reference
In-Depth Information
6.2.1 n-Type Oxide (ZnO)
The native defect structure of such oxides involves excess cations at interstitial
positions of the oxide lattice with excess electrons in its conduction band. As
discussed earlier, this may be represented by the following defect formation equi-
librium:
1
2
O
2
(g)
ZnO s Zn
••
i
2e
′
(6.1)
for which one can write the following equilibrium constant at a constant tempera-
ture,
T
:
K
(
T
)
[Zn
•
i
]
n
2
P
1/2
(6.2)
O
2
where [Zn
•
i
] and
n
represent the concentrations of interstitial zinc ions and excess
electrons per m
3
of ZnO lattice.
To represent the addition of higher valency cations into the ZnO lattice, one
may choose Cr
2
O
3
as the impurity oxide. The solid solution of Cr
2
O
3
in ZnO
will bring about changes in defect concentration of the host lattice in the follow-
ing ways:
1.
Two Cr
3
ions may occupy the two normal lattice sites of Zn
2
ion. Because
only two corresponding anion lattice sites are available for the three oxygen
ions, one of them must be discharged, releasing oxygen to the gas phase and
putting two electrons in the conduction band as per the following defect
equilibrium:
1
2
O
2
(g)
Cr
2
O
3
s 2Cr
•
Zn
2e
′
2O
x
O
(6.3)
2.
The introduction of extra electrons to the conduction band will upset the
equilibrium between them and interstitial Zn
2
ions as per Eq. (6.2) at a
constant temperature. Thus, Cr
2
O
3
may also dissolve in a manner whereby
some interstitial Zn
2
ions are eliminated according to the following equilib-
rium:
Cr
2
O
3
Zn
••
i
s 2Cr
•
Zn
Zn
Zn
3O
x
O
(6.4)
Therefore, the net result of doping ZnO with Cr
2
O
3
is to reduce the concentra-
tion of interstitial Zn ions as well as to enhance the concentration of excess elec-
trons in the conduction band, thus reducing the cationic conductivity (
σ
i
) and
increasing the electronic conductivity (
σ
e′
). Since ZnO film growth is governed
by its cationic conductivity, effect of doping with Cr
2
O
3
would decrease the oxi-
dation rate of an alloy of Zn containing a small amount of Cr.
