January 31, 2020.
Metal oxides are materials of great technological interest due to their rich properties, including optoelectronics, ferroelectricity, photovoltaic and photocatalysis. Some metal oxides including zinc oxide (ZnO), tin dioxide (SnO2) and indium oxide (In2O3) exhibit high optical transparency with high electrical conductivity. These unique properties allow them to be used as transparent electrodes in flat panel displays, solar cells, light-emitting diodes, as transparent heat-mirror coating and as transparent coating for antifogging and self-cleaning windows. In addition, the discovery of photoinduced water splitting on TiO2 surface in 1972, triggers extensive studies on photocatalytic properties of metal oxides. Complex metal oxide such as SrTiO3 has been recognized as a potential candidate for overall photocatalytic water splitting because of its large band gap and suitable band edge positions.
Atomic arrangements in materials, including metal oxides are naturally imperfect. The most common situation is that there would be some atoms missing, some atoms placing in space between other atoms and even some impurities atoms introduced. These imperfections are known as “point defects” which strongly affect the electrical and optical properties of materials and have a significant impact on their performance in applications. These point defects can be present during the growth processes either unintentionally or purposely. For example, in conventional melting methods, oxygen gas is flowed through metals and not all oxygen atoms and metal atoms are placed in their own ideal lattice sites. Oxygen or metal atoms may disappear from their lattice sites, forming oxygen or metal vacancy. Extra oxygen or metal atoms may locate in a space in the lattice, forming oxygen or metal interstitial. In addition, oxygen atoms may replace metal site while metal atom could replace oxygen site, forming oxygen or metal antisites. These class of point defects are native or intrinsic defects. In addition, some background elements such as hydrogen (H), silicon (Si), fluorene (F), chlorine (Cl), and carbon (C) are often present and easily incorporated in the materials. These unintentional impurities are difficult to avoid and can affect the efficiencies of devices.
Most metal oxides exhibit unintentional n-type conductivity, which is repeatedly attributed to the presence of point defects. Understanding the key factors that control the often-observed n-type conductivity can lead to the improvements of the performance of metal-oxide based transparent electrodes. It is also important for efficient p-type doping as the hole-killer defects can be avoided. In general, oxygen vacancy (VO) and metal interstitial (Mi) act as donor defects and easily incorporated in the oxides. Density-functional theory (DFT) has become the most commonly used first-principles approach for calculations of point defects in real materials. Investigation of the effects of point defects on electrical and optical properties of In2O3 using this approach has been recently reported by our team . In2O3 has been widely used as transparent semiconductors but its source of conductivity was still unclear. The role of native defects in In2O3 including oxygen and indium vacancies, interstitials and antisites were explored. The formation energies of the defects, which related to their concentrations, were calculated as a function of Fermi level (Figure 1). It appears that VO act as double donor and is the most dominant defect. Since the O atoms are surrounded by four In atoms, the removal of an O atom results in four dangling bonds and two electrons. Figure 2 shows the local atomic geometry of VO.
Fig. 1 Formation energy as a function of Fermi level position for native point defects in In2O3. Results are shown in (a) In-rich and (b) O-rich limit. The valence band maximum is set to zero and the dashed line indicates the conduction band minimum.
Fig. 2 Local atomic geometry of oxygen vacancy (VO) in the (a) neutral charge state and (b) 2+ charge state.
These two electrons can be readily excited to the conduction band, giving rise to the n-type conductivity. Figure 1 shows that VO likely occurs in oxygen deficient environment (In-rich, O-poor) where oxygen partial pressure is low. On the other hand, when oxygen partial pressure is high (In-poor, O-rich), oxygen interstitials (Oi) and indium vacancies (VIn) have lower formation energy than that of VO when Fermi level is near the conduction band. These defects act as acceptor and can be compensation centers for donors. Under O-rich conditions, formation energy of VO increase and also electron carriers from oxygen vacancy can be compensated, thereby resulting in lower n-type conductivity.
Apart from unintentional impurities, one can deliberately introduce external elements into the metal oxides in order to improve their physical properties. An example is doping with anion impurities in metal-oxide photocatalysts including TiO2 and SrTiO3. This method has been widely adopted to shift the absorption edge of these metal oxides to the visible-light region. In general, the optical absorption process requires energy which can excite an electron from the valence band to the conduction band. Since these metal oxides have wide band gap, they only absorb and operate in the ultraviolet (UV) region which is only a small portion of the solar spectrum. Experiments have been performed on nitrogen (N) doping in SrTiO3 and it was reported that N impurities introduced visible-light absorption. However, the actual mechanism of the optical absorption was ambiguous. Recently, our team performed hybrid-density functional calculations to study the energetics, electrical and optical properties of the possible N configurations in SrTiO3, including NO, (NO)O, (NO2)2O, (N2)O split-interstitial and (N2)2O complex defects as shown in Figure 3 . The optical and thermodynamic transitions are determined from calculations of formation energies and configuration coordinate diagrams. We find that N prefers to occupy the O site (NO) in n-type SrTiO3, where the Fermi level position is near the conduction band, under O-poor, intermediate, and O-rich conditions. The localized states induced by NO, above the valence band, give rise to the optical transitions , explaining the visible-light absorption observed in the experiments (see Figure 4). In addition, we consider the effects of hydrogen impurity, which is ubiquitous and could be unintentionally incorporated during growth or processing, by examining the interactions between H and N impurities. The calculations show that H binds to NO, forming NO-Hi complex. Unlike in TiO2, where this type of complex eliminates the effects of NO on the visible-light absorption, in SrTiO3 it only results in a relatively small blueshift of the optical absorption. (NO2)2O, (N2)O split-interstitial and (N2)2O defect complexes are also predicted to absorb light in the visible range. Finally, we calculated vibrational frequencies of N-related defects in order to aid in experimental identification.
Fig. 3 Local atomic structures of N impurities in SrTiO3: (a) NO, (b) (NO)O, (c) (NO2)2O, (d) (N2)O, and (e) (N2)2O.
Fig. 4 (a) Configuration coordinate diagram for NO. (b) Charge density of NO 2p states associated with the optical transition.
More recently, we also investigate hole carrier self-trapping in BaTiO3. Our calculated results reveal that a hole prefers self-trapping to delocalizing in BaTiO3  (as seen in Figure 5). The self-trapped hole is even more stable in BaTiO3 than in SrTiO3, and likely to be observed at relatively higher temperatures. The emission peak energy from the recombination of an electron in the conduction band and a self-trapped hole was determined by constructing a configuration coordinate diagram, and found to give rise to the luminescence at 2.17 eV. Finally, we determined the migration of the self-trapped hole and drew comparisons with available experimental data on transport properties.
Figure 5 Configuration coordinate diagram for the transition from the delocalized hole to the localized hole in BaTiO3.
These defects and impurities, including small polarons can modify both of electrical properties and optical properties by introducing states in the band gap, adding electron and hole carriers in the conduction and valence band. Even though unintentional defect is difficult to avoid, we must learn how to control them in the growth processes. Since doping with external elements has purpose to improve desirable properties of oxides, scientists are exploring possibility for doping techniques to extend the range of applications and enhance the efficiency of metal oxides. For example, nowadays metal oxides that can be doped both p-type and n-type are scarce due to the characteristic of their band structures and low-lying valence band which do not facilitate efficient p-type doping. The difficulty to make p-type metal oxides limits their potential applications. This challenge is a current topic which has been discussed for several years and many researchers around the globe still find a way to break this limit.
1. Chatratin, I., Sabino, F.P., Reunchan, P., Limpijumnong, S., Varley, J.B., Van de Walle, C.G. and Janotti, A., 2019. “Role of point defects in the electrical and optical properties of In2O3”. Physical Review Materials, 3(7), p. 074604.
2. Reunchan, P., Umezawa, N., Janotti, A., Jiraroj, T. and Limpijumnong, S., 2017. “Energetics and optical properties of nitrogen impurities in SrTiO3 from hybrid density-functional calculations”. Physical Review B, 95(20), p. 205204.
3. Traiwattanapong, W., Janotti, A., Umezawa, N., Limpijumnong, S., T-Thienprasert, J. and Reunchan, P., 2018. “Self-trapped holes in BaTiO3”. Journal of Applied Physics, 124(8), p. 085703.
Assoc. Prof. Dr. Sirichok Jungthawan
School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima – 30000, Thailand