Chemistry Reference
In-Depth Information
polycrystalline material, at (ionized) impurities, and at phonons. The influence of
grain boundary scattering decreases with increasing grain size due to the lower grain
boundary density. Phonon scattering is often negligible compared to (ionized) impu-
rity scattering [380]. Ionized impurity scattering is the dominant effect limiting the
conductivity of the best highly doped TCOs reported in the literature [380]. Although
the obtainable electrical properties are strongly dependent on the deposition method
as well as the deposition conditions, TCO films with the low resistivity of about 10
−
4
cm can only be prepared with impurity-doped binary compounds. This resistivity
is still remarkably higher than the values for typical conductive materials like Ag or
Cu, which is about 2
·
10
−
6
cm.
8.2.4.2.3.2 TCO Materials
Classical TCO layer systems are based on semiconducting oxides like indium-oxide
(In
2
O
3
), tin-oxide (SnO
2
), zinc-oxide (ZnO), and their compounds. These compounds
exhibit high transmittance in the visible spectral range and high reflectance in the
infrared. Moreover, they are convenient for a high degenerated n-type doping due
to their electronic structure [381,382]. High carrier concentrations of the order of
n
e
10
20
-10
21
cm
−
3
can be obtained by incorporation of higher or lower valent
cations or anions into the host lattice of the basic oxides. In ZnO, Zn
2
+
-ions can be
substituted by Al
3
+
-orGa
3
+
-ions. One electron of the three valent cation contributes
to the conduction band. Feed materials and deposition conditions should be carefully
coordinated to prevent segregation between host material and the as much as possible
high concentrated doping material. The oxidation process of the doping material is
thermodynamically in favor in comparison to the insertion by substitution. In case
of the ZnO:Al system for Al
2
O
3
, the free enthalpy (
=
R
G
=−
1688 kJ/mol) is much
R
G
=−
lower than the free enthalpy for ZnO (
363 kJ/mol) for instance.
This means that high Al concentrations as well as high partial pressures of the
reaction gas oxygen cause the oxidation of Al to Al
2
O
3
and form insulating material
in the deposited films and must be prevented. On the other hand small reactive gas
concentrations form substoichiometric film materials that act as absorbent medium.
Besides the n-doping by cation substitution, an n-doping by anion substitution is
possible. O
2
−
ions can be replaced by F
−
ions, for instance, and therefore an addi-
tionally unbounded electron per exchanged ion is released into the conducting band.
Nevertheless, TCOs with the best optical and electrical properties were manufac-
tured by cation substitution because substoichiometric films have the tendency to
form preferable dopant oxides, and halogenoid-substituted films have lower carrier
concentrations.
Apartfromthedopantsolubilityinthehostlatticeandthefreeenthalpy,thedoping
materialshouldcauseonlysmalllatticedistortionandthedepositionprocedureshould
form TCO films with only small amounts of imperfections (interstices, dislocations,
grain boundaries) [383,384].
Figure 8.55 shows the change in minimum resistivity of typical impurity-doped
basic binary compound TCO films reported in recent years [385].
The best values will be reached by doped In
2
O
3
and ZnO. The obtained minimum
resistivity of doped ZnO films is still decreasing, whereas those of doped SnO
2
and
In
2
O
3
films have essentially remained unchanged for more than 20 years. ITO is the
