20.3 Properties
513
Fig. 20.3 The bixbyite crystal structure of indium oxide (In 32 O 48 ) showing one unit cell where the indium and oxygen
atoms are represented by the full and empty circles, respectively. Adapted from [1671]
Fig. 20.4 Comparison of
the band structure of In 2 O 3
(left) and
(Sn 0.065 In 0.935 ) 2 O 3 (right).
The position of the Fermi
level is for both cases at
E = 0. Adapted
from [1672]
(doping level dependent) ionized impurity scattering (Sect. 8.3.3) as shown in Fig. 20.5 for various
films. A detailed discussion of ionized impurity scattering in doped ZnO films can be found in [1674].
Also the mobility is correlated (Fig. 20.6) with scattering at structural defects such as grain boundaries
[1675] (cf. Fig. 8.6). We note that the carrier mobility in a (polycrystalline) TCO is not very different
from that in highly doped (crystalline) silicon (Fig. 8.9).
Conductivity and transparency are, however, linked. The high doping of the TCO leads to shifts
in the band gap (renormalization and Burstein-Moss shift), band tails and the like that can introduce
absorption in the visible spectral region. Also the infrared transparency is related to the conductivity
by free carrier absorption and the plasma edge (Sect. 9.9.1). With increasing carrier density, the plasma
edge shifts into the visible spectral range (Fig. 20.7a), limiting the possible maximum carrier density
to several 10
21 cm
−3 , the exact value depending on the carrier mass.
In Fig. 20.7b the transparency spectra of two SnO 2 films with different conductivity are compared.
The higher conductivity due to larger carrier concentration leads to reduced IR transparency. Generally,
the transmission of a TCO is limited by the band edge on the high energy side of the spectrum and the
plasma edge on the low energy side of the spectrum. The limited carrier concentration due to the onset
of free carrier absorption in the visible spectral range (∼ 3 × 10
21 cm
−3 ) and the limited mobility in
the presence of such high impurity concentration (max. 50 cm
2 /Vs) restrict the minimum resistivity of
a TCO (or any transparent conductor) to about 4 × 10
−5
cm [1677].
513
Fig. 20.3 The bixbyite crystal structure of indium oxide (In 32 O 48 ) showing one unit cell where the indium and oxygen
atoms are represented by the full and empty circles, respectively. Adapted from [1671]
Fig. 20.4 Comparison of
the band structure of In 2 O 3
(left) and
(Sn 0.065 In 0.935 ) 2 O 3 (right).
The position of the Fermi
level is for both cases at
E = 0. Adapted
from [1672]
(doping level dependent) ionized impurity scattering (Sect. 8.3.3) as shown in Fig. 20.5 for various
films. A detailed discussion of ionized impurity scattering in doped ZnO films can be found in [1674].
Also the mobility is correlated (Fig. 20.6) with scattering at structural defects such as grain boundaries
[1675] (cf. Fig. 8.6). We note that the carrier mobility in a (polycrystalline) TCO is not very different
from that in highly doped (crystalline) silicon (Fig. 8.9).
Conductivity and transparency are, however, linked. The high doping of the TCO leads to shifts
in the band gap (renormalization and Burstein-Moss shift), band tails and the like that can introduce
absorption in the visible spectral region. Also the infrared transparency is related to the conductivity
by free carrier absorption and the plasma edge (Sect. 9.9.1). With increasing carrier density, the plasma
edge shifts into the visible spectral range (Fig. 20.7a), limiting the possible maximum carrier density
to several 10
21 cm
−3 , the exact value depending on the carrier mass.
In Fig. 20.7b the transparency spectra of two SnO 2 films with different conductivity are compared.
The higher conductivity due to larger carrier concentration leads to reduced IR transparency. Generally,
the transmission of a TCO is limited by the band edge on the high energy side of the spectrum and the
plasma edge on the low energy side of the spectrum. The limited carrier concentration due to the onset
of free carrier absorption in the visible spectral range (∼ 3 × 10
21 cm
−3 ) and the limited mobility in
the presence of such high impurity concentration (max. 50 cm
2 /Vs) restrict the minimum resistivity of
a TCO (or any transparent conductor) to about 4 × 10
−5
cm [1677].