Transparent Conductive Oxides

Part of the Springer Handbooks book series (SPRINGERHAND)

Abstract

Transparent conducting oxides (TCO s) such as doped ZnO, In2O3 and SnO2 play important roles as transparent electrodes in commercial applications such as display and lighting devices. Although transparency and electrical conductivity are inherently conflicting, TCOs possess both properties simultaneously. To understand the fundamentals of TCOs, the essetials of the transparency and electrical conductivity are reviewed simply. Comprehension of the essentials enables us to develop novel TCOs following the principles of the materials design that carrier conduction paths should be constructed in wide-gap oxides. Because the electronic structure of oxides is considerably different between the conduction band and the valence band, procedures to form the conduction paths for electrons are in contrast to those for holes. In n-type TCOs, only isotropically spread ns0 orbitals of heavy cations such as Zn2+, In3+ and Sn4+ are necessary to form conduction paths for electrons at the bottom of the conduction band. In p-type TCOs, however, hybridization of orbitals between oxygen 2p6 orbitals and other orbitals such as Cu 3d10, Sn 5s2 and S 3p6 orbitals is essential to shape conduction paths for holes at the top of the valence band.

58.1 Overview

Transparent conducting oxides (TCOs) are known as materials that show both transparent and conducting properties, and their films are utilized in flat panel displays, solar cells and electroluminescent devices as transparent electrodes as reviewed in [58.1, 58.2, 58.3, 58.4, 58.5, 58.6, 58.7]. The transparent properties mean TCOs show high optical transmission in the visible range, which requires an energy gap larger than 3.3 eV. The conducting properties signify that TCOs show high electrical conductivity in the range from 1 to 104 S cm−1.

Although TCOs are common in industry nowadays, transparency and electrical conductivity are basically incompatible. Transparent materials are usually insulators such as SiO2 glasses. If they are used in powder or polycrystalline bodies, they look white. On the other hand, highly conductive materials usually show metallic luster or deep dark color. For instance, Ag and Al metals show metallic color with high reflectivity, which are utilized in mirrors. Oxide high-temperature superconductors such as YBa2Cu3O7 show black color. Therefore, to obtain TCOs, it is necessary to convert transparent insulators to highly conducting semiconductors by increasing electrical conductivity without coloration.

58.1.1 Electrical Conduction

Electrical current I in materials is expressed by Ohm’s law as \(I=V/R\), where V and R are potential difference and resistance respectively. When the equation is normalized by the sizes of materials, Ohm’s law can be expressed as J = σE, where J, σ, E are current density, conductivity , and electric field strength. The conductivity σ is generally dependent on materials themselves and a useful parameter, which varies by a factor of over 1020, to categorize materials into metals, semiconductors and insulators. Figure 58.1 shows the electrical conductivities of some materials. Metals such as Ag and Al possess high electrical conductivities of the order of 105 S cm−1. Semiconductors such as Si and Ge have medium conductivities. Although pure semiconductors are almost insulating, impurity doping markedly increases the conductivities up to the order of 102 S cm−1. The typical conductivities of TCOs are located in the middle of metals and doped semiconductors. This would be natural because TCOs are originally doped transparent oxide semiconductors (TOS s).
Fig. 58.1

Electrical conductivities of some conductive materials

To understand the nature of the conductivities of TCOs, it is necessary to compare carrier concentration and mobility between some materials. The electrical conductivity is a product of carrier concentration n, elementary electric charge e, and mobility μ as shown in Fig. 58.2 and in (58.1).
$$\sigma=ne\mu$$
(58.1)
These parameters straightforwardly characterize conducting materials of metal, semiconductor and TCO. The parameters of typical materials are listed in Table 58.1 [58.17, 58.18]. Ag metal is found to show high electrical conductivity because of high carrier concentration in the order of approximately 1023 cm−3. The mobility of Ag metal is not markedly high among these conductors. P-doped Si shows the lowest electrical conductivity among them because the carrier concentration is fairly low. It can become an electrical conductor simply because the carrier mobility of Si is much higher than that of Ag metal. Even if Si is heavily doped with the carrier concentration up to 1019 cm−3, the mobility decreases by carrier scattering and the conductivity does not increase as high as Ag metal. Sn-doped In2O3 (ITO) shows the conductivity that is approxmately one-tenth of the Ag value. Because the carrier mobility of ITO is not largely different from that of Ag metal, the carrier concentration of ITO, which is one-tenth of the Ag value, is responsible for its smaller conductivity. These features can be extended to other conducting materials and their carrier concentrations and mobilities are plotted in Fig. 58.3. Metals are located in the top-left region in Fig. 58.3, which indicates the metals have extremely high carrier concentration but their mobilities are not so high. In contrast, doped semiconductors are located in the bottom-right region because of low carrier concentration and high mobility. Figure 58.3 clearly illustrates TCOs are located between metals and doped semiconductors. Because the mobilities of TCOs are not largely different from those of typical metals such as Ag, each TCO can be regarded as a metal with small carrier concentration, which is called as a degenerate semiconductor. The schematic electronic structure of a degenerate semiconductor is shown in Fig. 58.4. Although the thermal activation energy is necessary for semiconductors to generate carriers, it is unnecessary for metals and degenerate semiconductors. This feature is experimentally observed as absence of thermal activation energy as shown in Fig. 58.5. Figure 58.5 shows the carrier concentration n as a function of reciprocal temperatures [58.19]. At increasing doping concentration, namely carrier concentration, the slope of the lines, which is derived from the activation energy for thermal ionization of dopant, becomes shallow and the conductivity finally becomes almost independent of temperatures.
Table 58.1

Electrical properties of Ag, Si:P and ITO

Materials

σ a

(S cm−1)

n b

(cm−3)

μ c

(\(\mathrm{cm^{2}{\,}V^{-1}{\,}s^{-1}}\))

Ag

6.80 × 105

5.76 × 1022

72

Si:P n

10−2

1014

1500

Si:P n++

102

1019

100

ITO

104

1020–1021

20–80

a Conductivity, b Carrier concentration, c Mobility

Fig. 58.2

Schematic illustration for Ohm’s law

Fig. 58.3

Carrier density and mobility for some metals, semiconductors and TCOs. (After [58.10, 58.11, 58.12, 58.13, 58.14, 58.15, 58.16, 58.8, 58.9])

Fig. 58.4a–c

Electronic structures of (a) metal; (b) semiconductors; (c) degenerate semiconductors

Fig. 58.5

Carrier concentration as a function of inversed temperature in ITO

58.1.2 Transparency

The main group metal oxides such as MgO and Al2O3 are electrically insulating but optically transparent in the wavelength range from 380 to 780 nm, namely the visible region. Therefore, oxide single crystals or glasses are frequently used as windows in optical applications. Figure 58.6 shows optical transparent ranges of several materials [58.20]. Typical oxides are transparent in the range approximately from 300 to 10 μm including the visible region. Typical fluorides such as LiF and CaF2 show a much wider window range than oxides, especially in deep ultraviolet or vacuum ultraviolet regions [58.21]. On the other hand, conventional semiconductors such as Si and Ge have a window range only in the infrared region. Therefore, it is not surprising that TCOs are transparent in the visible region. Figure 58.7 shows a typical transmission spectrum of an oxide single crystal. As shown in Fig. 58.7, the absorption edge is controlled by the lattice vibration in the infrared region, and by the fundamental absorption caused by the electron transition from the valence band to the conduction band in the ultraviolet region.
Fig. 58.6

Optically transparent regions in some materials

Fig. 58.7

Transmission spectrum of a typical oxide

When carriers are introduced into the parent material (undoped) of TCOs, optical absorption and reflection by the carriers newly appears. The collective motion of the conducting carriers behave as a kind of plasma in conductors. When light, namely an electromagnetic wave, is irradiated to the conductors, the carriers oscillate at the frequency of the light, which is called plasma oscillation. This results in the reflection of the light at the surface of the conductors. However, against light with higher frequency, the carriers cannot catch up with the fast electric field oscillation of the light. The light is transmitted through the conductors without causing the plasma oscillation or reflection at the surface. The maximum threshold frequency that enables the plasma oscillation is called the plasma frequency ωp and can be expressed by (58.2) [58.17]. The plasma frequency ωp can be converted to the wavelength λp as shown in
$$\omega_{\mathrm{p}} =\frac{n\mathrm{e}^{2}}{\varepsilon_{0}m}\;,$$
(58.2)
$$\lambda_{\mathrm{p}} =\frac{2\uppi\mathrm{c}}{e}\sqrt{\frac{\varepsilon_{0}m}{n}}\;,$$
(58.3)
where n, m, ε0 and c denote carrier concentration, electron rest mass, permittivity and speed of light in vacuum respectively. In Al metal with high carrier concentration, over 1022 cm−3, the threshold λp is located in the vacuum ultraviolet region as shown in Fig. 58.8, and visible light is totally reflected at the surface [58.22]. This is the origin of the metallic luster of the Al metal, and why the wide-ranged reflection of Al is often used as a mirror. On the other hand, since ITO has a carrier concentration one order of magnitude smaller than Al metal, the λp moves to the infrared region, which results in the transparency in the visible region because ITO has a sufficient bandgap (> 3.5 eV) to transmit visible light. Consequently, the maximum carrier concentration of TCOs should be less than 1021 cm−3 because the carriers make the optical windows of oxides narrower in the infrared region.
Fig. 58.8

Reflection spectra of Al and ITO

The carriers influence the optical windows of oxides not only in the infrared side but also in the ultraviolet side. Energy gaps are usually estimated by the ( hνα)2E plot, where h, ν and α are Planck constant, frequency and absorption coefficient respectively, as shown in Fig. 58.9. As the carrier concentration increases, absorption edges move to the higher energy side, enlarging apparent energy gaps and making optical windows wider in the ultraviolet region. This variation is attributed to the occupation of the conduction band by carrier electrons as shown in the inset in Fig. 58.9. In n-type TCOs, because carrier electrons occupy the bottom of the conduction band, the optical bandgap energy Eg needed for the optical transition from the valence band to the unoccupied conduction band becomes larger than the original bandgap Eg0. This energy shift is called band filling or Bernstein moss (BM) shift.
Fig. 58.9

Blue shift of fundamental absorption by band filling

58.2 Materials Design for TCOs

The essential factor of materials design for TCOs is determining how to dope carriers into oxides, because the metal oxides (except transition metals with an open shell electronic configuration) are basically transparent due to the large bandgap. As shown in (58.1 ), to increase the electrical conductivity is to increase carrier concentration and mobility. Because the carrier concentration can be controlled by users depending on purposes, the carrier concentration is generally independent from materials themselves. Therefore, the mobility is the material-dependent parameter to be designed. The mobility is a velocity of carriers under unit electric field strength and inversely proportional to the effective mass m* of carriers as in (58.4).
$$\mu=\frac{v}{E}=\frac{e\tau}{m^{*}}$$
(58.4)
The effective mass m* is theoretically deduced so that m* is inversely proportional to the curvature of an energy band in the energy diagram as a function of wavenumber k as in (58.5)
$$\frac{1}{m^{*}}=\left(\frac{2\uppi}{h}\right)^{2}\frac{\partial^{2}E}{\partial k^{2}}$$
(58.5)
In a simple one-dimensional crystal with lattice constant a, a simple energy band is formed and composed of a one-dimensionally aligned atomic orbital φ m as shown in Fig. 58.10a,b [58.23]. The energy E of the band is expressed in (58.6) and (58.7) by using the orbital interaction or resonant integral H mn between neighboring orbitals.
$$H_{\textit{mn}} =\int\phi_{m}^{*}\hat{H}\phi_{n}\mathrm{d}x\;,$$
(58.6)
$$\begin{aligned}\displaystyle E&\displaystyle=H_{nn}+2H_{\textit{mn}}\cos(ka)\\ \displaystyle&\displaystyle\approx H_{nn}+2H_{\textit{mn}}-H_{\textit{mn}}(ka)^{2}\quad(x\approx 0)\end{aligned}$$
(58.7)
Equations (58.4)–(58.7) lead to a simple conclusion that the mobility is proportional to the orbital interaction H mn
$$\mu\propto\frac{\partial^{2}\mathrm{E}}{\partial k^{2}}=-2H_{\textit{mn}}a^{2}\;.$$
(58.8)
The value of H mn is simply estimated so that H mn becomes large when the overlap of atomic orbitals between φ m and φ n is large, because H mn is calculated from a product of the atomic orbitals as seen in (58.6). Because making large overlap of the orbitals is just equal to increasing a probability of carrier transport or forming a conduction path in the crystal, carriers move between the orbitals through their overlapped areas. Therefore, a smooth conduction path for carriers must be constructed in wide-gap oxides by making sufficient overlap of component orbitals to provide them with large carrier mobility.
Fig. 58.10a,b

Band dispersion in k space (a) and overlap of orbitals in real space (b)

58.2.1 Electronic Structures of Oxides

Understanding of the electronic structures of simple oxides is inevitable for the materials design for TCOs. Figure 58.11 shows the typical electronic structure of metal oxide (MO) illustrated in the form of molecular orbitals and energy bands. The conduction band is composed of empty metal ns0 orbitals and the valence band is composed of occupied oxygen 2p orbitals. The conduction band has antibonding features, and the bottom of the valence band has bonding features as a counterpart. The top of the valence band has nonbonding features because nonbonding oxygen 2p orbitals remain without changing the energies.
Fig. 58.11

Schematic molecular orbital diagram and band structure of oxides

58.2.2 Materials Design for n-Type TCOs

For n-type TCOs, a conduction band consisting of empty orbitals becomes a conduction path for electrons. To obtain the sufficient overlap of orbitals for the conduction path, cations’ orbitals should be extended as widely as possible and intercation distances should be as short as possible. Therefore, the essentials for developing n-type TCOs are to choose proper cations for widely extended orbitals, and to choose a proper crystal structure for shorter intercation distances [58.24, 58.25, 58.26].

In the choice of cations, p-block atoms should be chosen for widely extended orbitals. As known in a simple hydrogen atomic model, radii of orbitals are proportional to principal quantum number n, namely, the period in the periodic table in Fig. 58.12. Although heavy atoms with large n are preferable, atoms with n = 6 such as Tl and Pb are inadequate because their orbitals are too widely extended to keep the wide bandgap for transparency. Transition metal ions with d n open shells are also improper due to the coloration. In addition to the radii of orbitals, the shape of orbitals is also important to maximize the orbital overlap. Isotropic s orbitals, which are free from bonding direction, are more preferable than p- or d orbitals. Therefore, cations with d10ns0 (n = 4,5) electronic configuration are favorable to form the conduction path for electrons in the conduction band. Actually, the selected cations includes cations such as Zn2+, In3+ and Sn4+ found in major TCOs.
Fig. 58.12

Key elements for TCOs in the periodic table

In the choice of crystal structures, the configuration of polyhedra consisting of a cation at the center and oxygen ions at the apexes should be examined. In the case of the cations with d10ns0 (n = 4,5) such as In3+ and Sn4+, they usually form oxygen octahedra due to the size of the ions. Various configurations such as isolated octahedra, corner-sharing octahedra, edge-sharing and face-sharing octahedra are seen in crystal structures. Among them, intercation distances are the shortest in face-sharing structures as shown in Fig. 58.13 . It is known, however, that such structures are usually unstable in oxides due to the intercation coulomb repulsion [58.27] and hardly seen in many crystal structures. Therefore, the edge-sharing structure substantially gives the shortest intercation distances. Accordingly, with widely extended orbitals, edge-sharing structures can form conduction paths in n-type TCOs.
Fig. 58.13

Configuration of octahedra in crystal structures

Typical n-Type TCOs and Their Electronic Structure

From the concept of the materials design for n-type TCOs, the oxides, which include the cations with d10ns0 (n = 4,5) configuration and the partial crystal structures with edge-sharing octahedra, can be leading candidates. Actually, such materials often show transparent n-type conductive properties and some typical n-type TCOs are shown in Fig. 58.14. SnO2 with a rutile structure composed of edge-sharing octahedra along the c-axis has been known as an n-type TCO for a long time. CdIn2O4 and MgIn2O4 with spinel structure have three-dimensionally connected edge-sharing octahedra [58.28, 58.29]. Although the effective ionic radii of In3+ and Sn4+ are 0.8 and 0.7 Å respectively, such ionic sizes neglect 5s0 orbitals. Interatomic distances are 3.2 Å in In metal and 3.0 Å in Sn metal, while intercationic distances are 3.3 Å in In2O3 and 3.2 Å in SnO2. Because the intercationic distances in the oxides are very close to the interatomic distances in the metals, it would be easily supposed that 5s0 orbitals between In3+ or Sn4+ ions in the oxides can largely overlap as in the metals.
Fig. 58.14

Crystal structures of typical n-type TCOs

The large overlap of the orbitals in In2O3 and SnO2 appears as a large energy dispersion of the conduction band in their energy diagrams as shown in Fig. 58.15a,b [58.30, 58.31], where the valence band maximum is set to zero in energy. The large energy dispersion results in the large curvature and large mobility at the bottom of the conduction band. These characteristic conduction band bottoms can be experimentally observed in the photoemission spectra of Fig. 58.16. Because the Fermi energy EF is taken as zero in energy in Fig. 58.16, EF is located at the bottom of the conduction band indicating these materials are n-type TCOs. The large dispersion of the conduction band causes a gentle increase of the density of states, which appears as a gradual increase in the photoemission intensity above EF. The gradual increase at the bottom of the conduction band is in contrast to the steep increase at the top of the valence band, especially in In2O3.
Fig. 58.15a,b

Energy band diagrams of SnO2 (a) and In2O3 (b)

Fig. 58.16

Normal and inverse photoemission spectra of SnO2 and In2O3

Other n-Type TCOs

The characteristics of the crystal and electronic structures preferable to n-type TCOs are commonly seen in other n-type TCOs such as ZnO, Ga2O3, Cd2SnO4 and SrGeO3 [58.32, 58.33, 58.34, 58.35, 58.36]. The edge-sharing octahedra in crystal structures are not always necessary if the overlap of the orbitals for the conduction band is large enough to form a conduction path. Therefore, even amorphous oxides such as a-In2O3-ZnO and a-In2O3-Ga2O3-ZnO can be n-type TCOs because the conduction paths are formed by the orbitals’ overlap in these materials irrespective of the crystal structures [58.37, 58.38, 58.39]. Since the amorphous TCO thin films can be formed at low temperatures and durable during bending, they can be deposited on not only in inorganic glasses but also in organic plastic plates or films. Their advantages are utilized in growing applications such as flexible electronics and printable electronics.

Some TCOs such as TiO2 and KTaO3 have the conduction bands that are not composed of s0 orbitals but d0 orbitals [58.40, 58.41, 58.42]. These materials are considered to have conduction paths consisting of the d0 orbitals. Because the d0 orbitals are not isotropic in shape, the electrical conductivity would be largely influenced by the symmetry of the crystals.

58.2.3 Materials Design for p-Type TCOs

In p-type TCOs, in contrast to n-type TCOs, it is the filled valence bands rather than the the empty conduction bands that form the conduction paths for carriers, namely positive holes. However, the formation of the hole conduction paths are not as easy as that of the electron conduction paths. This is why p-type TCOs are rare [58.43].

The valence bands of oxides are generally composed of filled oxygen 2p orbitals. Accordingly, the oxygen 2p orbitals must be sufficiently overlapped with each other to form the conduction paths for holes. However, the oxygen 2p orbitals at the valence band maximum consist of nonbonding states, which indicate almost no interaction, namely no orbitals overlap between oxygen ions. This makes it very difficult to form the hole conduction paths at the valence band, resulting in considerably small hole mobility even if the holes are doped into the valence band. Hole doping is also substantially difficult in oxides because oxygen has large electronegativity indicating that O2− ions with closed shells are too stable to accept holes. Actually, the energy of oxygen 2p orbitals is very deep from the vacuum level, giving oxygen the large electron affinity. Therefore, it is considered that the formation of a hole conduction path by using only oxygen 2p orbitals is almost impossible.

To improve the nature of the low hole mobility and difficulty in hole-doping in oxides, chemical modulation of the valence band was proposed by hybridizing cations’ orbitals with oxygen 2p orbitals [58.43, 58.44]. The hybridization of orbitals usually occurs between orbitals with similar energies. Therefore, the energy of the candidate cations’ orbitals must be close to that of oxygen 2p orbitals. Figure 58.17 shows the photoemission spectrum of a CdIn2O4 n-type TCO. As similar to In2O3 and SnO2 in Fig. 58.16, the oxygen 2p6 band consists of the valence band. In addition to the oxygen 2p valence band, Cd2+ 4d10 and In3+ 4d10 bands are located at higher binding energies with almost the same energy intervals. Because the Cd and In are located at adjacent positions on periodic table as in Fig. 58.12, the periodicity predicts that the energy of Ag+ 4d10 or Cu+ 3d10 orbitals are very close to the oxygen 2p energy and they are easily hybridized with the oxygen 2p orbitals. From the viewpoint of chemical stability, Cu 3d10 orbitals are considered to be the most favorable for the chemical modulation of the valence band.
Fig. 58.17

Ultraviolet photoemission spectrum of CdIn2O4

Hybridization between Cu 3d10 and oxygen 2p nonbonding orbitals newly forms Cu–O bonding and antibonding states, and the Cu–O antibonding states become the top of the valence band as shown in Fig. 58.18 [58.45]. Therefore, the valence band is mainly composed of a Cu–O bonding band (A), an M–O bonding band (B), a Cu nonbonding band (C) and a Cu–O antibonding band (D). The chemical modulation by Cu 3d10 orbitals varies the nonbonding oxygen 2p orbitals to the Cu–O antibonding band at the top of the valence band and elevates the energy of the valence band maximum simultaneously. Accordingly, the shallow Cu–O antibonding band is expected to increase the hole mobility and make hole doping easier.
Fig. 58.18

Chemical modulation of the valence band by Cu 3d10 orbitals in oxides shown in a schematic molecular orbital diagram

The chemical modulation works more effectively in oxysulfides, in which Cu 3d orbitals hybridize not oxygen 2p orbitals but sulfur 3p orbitals forming Cu–S bonding (A) and antibonding (D) band [58.46]. Because sulfur 3p orbitals are shallower in energy and more covalent than oxygen 2p orbitals, larger hybridization is expected at the top of the valence band. The larger modulation leads to higher hole mobility by forming a steady hole conduction path at the valence band maximum.

The chemical modulation usually narrows the bandgap by the formation of the shallow antibonding band at the top of the valence band. Because the degree of the modulation is usually dependent on the dimension of crystal structures, lower dimensional crystal structure gives a moderate hybridization, namely a moderately shallow antibonding band. Therefore, layered structures and chain structures are preferable to maintain wide bandgap and transparency.

Typical p-Type TCOs and Their Electronic Structure

From the materials design for p-type TCOs, the Cu+ containing oxides with layered structures or chain structures can be leading candidates. CuAlO2 with delafossite structure was proposed as a prototype of p-type TCOs [58.43, 58.47]. CuAlO2 is a layered structure composed of Cu+ and AlO6 octahedra layers as shown in Fig. 58.19. Although p-type Cu2O, in which an O2− ion bonds to four Cu+ ions, shows deep dark red color, CuAlO2 is almost transparent in the visible region because an O2− ion in CuAlO2 bonds to only one Cu+ and three Al3+ ions resulting in a moderate Cu–O hybridization. Another example of a transparent p-type material is LaCuOS, which is also a layered structure composed of CuS and LaO layers [58.48]. The hybridization between Cu 3d and S 3p orbitals occurs in the CuS layer separated by the LaO layer. Because the LaO layers are inserted between CuS layers reducing dimensionality, the layered structure brings about a moderate Cu–S hybridization to maintain transparency.
Fig. 58.19

Crystal structures of typical p-type TCOs

The chemical modulation at the top of the valence band in CuAlO2 and LaCuOS is seen in the energy diagram of Fig. 58.20a,b [58.47, 58.49, 58.50]. In contrast to the energy diagram of the valence band for n-type TCOs, especially In2O3, dispersed bands are observed in the energy range from 0 to −2 eV above several flat bands densely gathered at around −3 eV. The dispersed bands and flat bands are derived from the Cu–O/S antibonding bands and Cu nonbonding bands respectively. The dispersed bands are considered to form a hole conduction path and increase hole mobility at the top of the valence band. These features in the valence band can be experimentally observed in the photoemission spectra of Cu2O, CuGaO2 delafossite and LaCuOS as shown in Fig. 58.21. The Fermi energy EF is located near the top of the valence band indicating these materials show p-type conductions. The narrow bandgap of Cu2O is also observed in contrast to wide bandgaps of CuGaO2 and LaCuOS. In the valence band, the main four bands A–D, explained in molecular orbital diagram of Fig. 58.18, can be found in all materials. The Cu–O hybridized band D is located just below EF and the presence of the band D demonstrates that the chemical modulation by Cu 3d10 orbitals is effective in the materials design of p-type TCOs. The typical electrical properties of some p-type materials are listed in Table 58.2.
Table 58.2

Electrical properties of Cu+-based p-type TCOs

Materials

σ a

(S cm−1)

μ b

(\(\mathrm{cm^{2}{\,}V^{-1}{\,}s^{-1}}\))

E g

(eV)

Reference

CuAlO2

0.34

0.13

3.5d, 1.8c

[58.47]

CuGaO2

0.063

0.23

3.6 d

[58.51]

SrCu2O2

0.048

0.46

3.3 c

[58.52]

LaCuOS

5.9

0.5

3.1 c

[58.53, 58.54]

LaCuOSe

910

8.0

2.8 c

[58.54, 58.55]

BaCuSeF

43

3.0 c

[58.56]

a Conductivity, b Mobility, c Direct transition, d Indirect transition

Fig. 58.20a,b

Energy band diagrams of CuAlO2 delafossite (a) and LaCuOS (b)

Fig. 58.21

Normal and inverse photoemission spectra of Cu2O, CuGaO2 and LaCuOS

Other p-Type TCOs

The concept of the chemical modulation by Cu 3d10 orbitals is also applicable to other Cu-containing materials such as SrCu2O2, LaCuOSe and BaCuFSe as listed in Table 58.2 [58.47, 58.51, 58.52, 58.53, 58.54, 58.55, 58.56]. The crystal structure of SrCu2O2 has low dimensional zigzag chains of Cu–O bonds, which enables a moderate Cu–O hybridization and transparency. LaCuOSe and BaCuFSe have the similar crystal structures to LaCuOS, in which CuSe and BaF layers replace CuS and LaO layers respectively.

From other concepts of materials design for p-type TCOs, SnO [58.57, 58.58] and ZnRh2O4 [58.59] have been developed. Sn2+ in SnO has a lone pair of 5s2 electrons, which mainly occupy the bottom of the valence band. However, partial densities of Sn 5s orbitals also compose the top of the valence band along with Sn 5p orbitals and O 2p orbitals. This feature is similar to the modulation by Cu 3d orbitals and can be interpreted that the top of the valence band is composed of their hybridized states. The bandgap of SnO is as small as 0.2–0.7 eV for indirect transition, but as large as 2.7–3.1 eV for direct transition. Although the conductivity and transparency is not as high as Cu+-based materials, SnO shows transparent p-type conductivity of approximately \({\mathrm{10^{-1}}}\,{\mathrm{S{\,}cm^{-1}}}\).

ZnRh2O4 with spinel structure includes Rh3+ ions with open shells of 4d6, which is not preferable for transparency. Rh3+ ions occupy the octahedral sites in the spinel structure and are influenced by the crystal field. Because the crystal field in ZnRh2O4 is sufficiently strong, the Rh3+ 4d orbitals are no longer usual open shells but two largely separated bands of occupied 4d6 and empty 4d0 orbitals, namely t 2g 6 band and e g 0 band, with the bandgap as large as 2.1 eV. The p-type conductivity by holes, which probably originate from cationic vacancies, is reported to be approximately 1 S cm−1.

There are several controversial reports on novel p-type TCOs such as p-type SnO2. The experimental determination of p- or n-type conduction is sometimes difficult when carrier concentration is extremely high/low or both electrons and holes are present in conductors. Theoretical verification such as evaluation of acceptor formation energy becomes important, as well as experimental demonstration, to identify novel TCOs [58.60].

58.3 New Approach to Explore Candidate Materials: Materials Genome Approach

The materials design of TCOs explained above is empirically deduced from simple theories and some experimental data on electronic structures. However, recent remarkable progress in computational ability has enabled the calculation of essential parameters for TCOs such as bandgaps and carrier effective masses in almost all oxides (except strongly electron correlated systems) accumulated in databases. Because the candidates for TCOs are extracted from materials databases, this process is a kind of materials genome approach, which is similar to the biomedical approach in the human genome project.

For instance, by targeting new p-type TCOs through high-throughput computations and screening, groups at Université Catholique de Louvain (IMCN) and the Massachusetts Institute of Technology predicted several candidates of novel p-type TCOs such as K2Sn2O3 as shown in Fig. 58.22 [58.61]. The electronic structures of more than 3000 binary and ternary oxides in the inorganic crystal structure database (ICSD ) were computed and oxides with exceptionally low effective mass less than 1.5 were selected. The bandgaps of the selected oxides were estimated more accurately by a calculation based on many-body perturbation theory. In addition to the calculation of bandgaps and hole effective masses, the p-type dopability was also evaluated by calculating the defect formation energies. From the viewpoint of the electronic structure and p-type dopability, several candidates were selected and two design principles for p-type TCOs were drawn; one is to hybridize cations’ ( n − 1 ) d10ns2 (n = 5,6) orbitals such as Sn2+ 5s2 with oxygen 2p6 orbitals; and the other is to include anions’ p orbitals such as S2− and P3− 3p6 orbitals, which are more delocalized than oxygen 2p6 orbitals. This approach looks promising and having applicability as methodology. However, one has to keep in mind that practical TCO materials have to meet various fundamental properties such as chemical stability and etching ability.
Fig. 58.22

Candidates for p-type TCOs selected from materials database through high-throughput computations

References

  1. 58.1
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Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.Materials Research Center of Element StrategyTokyo Institute of TechnologyYokohamaJapan
  2. 2.Dept. of Materials ScienceKyushu Institute of TechnologyKitakyushuJapan

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