Chemistry Reference
In-Depth Information
0.01
Doped SnO
2
Doped ZnO
1E-3
1E-4
Doped In
2
O
3
1E-5
1970
1980
1990
2000
Ye a r
FIGURE 8.55
Reported resistivity of impurity-doped binary compound TCO films, 1972 to
present. (From Minami, T.,
Semicond. Sci. Tech.
, 20, S35, 2005.)
major material for TCO at present. However, it has the problem of limited resources.
Indium metal is the by-product of zinc metal. At present, ZnO is a suitable material for
production of large-sized devices such as flat panel displays and solar cells because
the abundant sources and the low costs of Zn metal are major advantages in industrial
production.
For manufacturing technology, further demands have yet to be established. So,
growth methods of low costs, high throughput, and low-energy consumption are
required.
8.2.4.2.3.3 Plasma-Enhanced TCO Film Deposition Techniques
Plasma methods are characterized by a set of free parameters to optimize the deposi-
tion process. Plasma power, working pressure, oxygen partial pressure, and substrate
bias can influence the deposition rate and so also the film thickness [386], the film
stoichiometry, the defect concentration, and, therefore, the optoelectronic properties.
Target materials are either oxide ceramics or metals and metallic alloys, respectively.
A further advantage of plasma-enhanced deposition techniques is the low thermal
load for the substrates as mentioned before; so TCO films can be deposited on
organic foils too.
AnattractivetechniqueforthedepositionofTCOmaterialistheso-calledreactive
sputtering.[387,388],wheremetallicorceramictargetsaresputteredinanatmosphere
of argon admixed with the reactive gas oxygen. The gas phase chemically active
species are produced by dissoziation of the feed gas (O
2
O) introduced
into the discharge. The sputtering deposition of ZnO:Al explains the advantages of
reactive sputter method. After ZnO:Al target physical sputtering, Zn, Al, and O atoms
approach the substrate. Zn and Al atoms stick to the substrate surface, while O atoms
partially form molecular oxygen, leaving the deposited film with a deficiency of
oxygen. To restore the ZnO:Al stoichiometry of the deposited film, O
2
is added as
a feed gas to the discharge, which leads to the additional formation of O atoms and
→
O
+
