O-rich Conditions Research Articles

Ab-initio studies on Li doping, Li-pairs, and complexes between Li and intrinsic defects in ZnO

First-principles density functional calculations have been performed on Li-doped ZnO using all-electron projector augmented plane wave method. Li was considered at six different interstitial sites (Lii), including anti-bonding and bond-center sites and also in substitutional sites such as at Zn-site (Lizn) and at oxygen site (Lio) in the ZnO matrix. Stability of LiZn over Lii is shown to depend on synthetic condition, viz., LiZn is found to be more stable than Lii under O-rich conditions. Hybrid density functional calculations performed on LiZn indicate that it is a deep acceptor with (0/-) transition taking place at 0.74 eV above valence band maximum. The local vibrational frequencies for Li-dopants are calculated and compared with reported values. In addition, we considered the formation of Li-pair complexes and their role on electronic properties of ZnO. Present study suggests that at extreme oxygen-rich synthesis condition, a pair of acceptor type LiZn-complex is found to be stable over the compensating Lii + LiZn pair. The stability of complexes formed between Li impurities and various intrinsic defects is also investigated and their role on electronic properties of ZnO has been analyzed. We have shown that a complex between LiZn and oxygen vacancy has less formation energy and donor-type character and could compensate the holes generated by Li-doping in ZnO.

Magnetism of Cr-doped ZnO with intrinsic defects

The electronic structure and formation energies of defect in Cr-doped ZnO are investigated by the first-principles calculations. The dopant Cr atom and native defects (O vacancy, Zn vacancy, and Zn interstitial) at different charge states are considered. The calculated results indicate that the substitutive Cr in the oxidation state of +3 and the neutral Zn vacancy in the ZnO:Cr sample is the most favorable defect complex to maintain a high stability of ferromagnetic order. Therefore, a p-type ZnO:Cr prepared under O-rich condition could have a higher Curie temperature. The p-d hybridization and the itinerant d-characteristic n-type carriers are suggested to mediate the long range ferromagnetic exchange interaction between Cr atoms.

Kinetic-Dynamic Properties of Different Monomers and Two-Dimensional Homoepitaxy Growth on the Zn-Polar (0001) ZnO Surface

Homoepitaxy ZnO monolayer growth was investigated from dynamics to kinetics taking ZnO molecules and Zn–O cluster monomers into account in the atomistic growth by first-principles calculations and Monte Carlo simulations and compared with experimental growth by molecular beam epitaxy. Theoretically, the ZnO molecules were found to scatter equivalently on both the wurtzite sites (WSs) and zincblende sites (ZSs) and stick even at high temperatures. The Zn3O1 monomers resulted in a larger island size and a higher compact degree and the growth approached to the two-dimensional mode at high temperature; the film structure finalized in the single wurtzite phase structure with more vacancies, which agreed with the in situ scanning tunnel microscopy observation for the growth in Zn-rich conditions to form Zn3O1 monomers. For the Zn1O3 monomers, the transformation from ZSs to WSs was more difficult even with temperature increase and they could locate at both WSs and ZSs, consistent with the in situ reflection high energy electron diffraction for the growth in O-rich conditions to form Zn1O3 monomers. Combining the advantages of both cluster monomers, a two step-growth technique was developed by alternatively supplying Zn and O. The resultant ZnO films exhibited flat texture and uniform phase structure as indicated by the atomic steps in the STM images and the streaky RHEED patterns.

Strong asymmetrical doping properties of spinel CoAl2O4

Using first-principles density-functional theory, we have investigated the intrinsic and extrinsic doping properties of CoAl2O4 by calculating the transition energies and formation energies of intrinsic and extrinsic defects. We find that CoAl2O4 exhibits strong asymmetrical doping properties: Although excellent p-type conductivity can be achieved by Li or Na doping at O-rich growth condition, n-type conductivity cannot be achieved by any intrinsic or extrinsic dopants at any growth conditions. These asymmetrical doping properties are attributed to the formation of intrinsic defects, particularly AlCo, which has very low formation energy at all growth conditions. Our results suggest that for better use of CoAl2O4, the electronic devices should require CoAl2O4 to exhibit p-type conductivity.

Visible-light photocatalytic activity of Ni-doped TiO2 from ab initio calculations

The geometrical structures and electronic properties of Ni-doped anatase and rutile TiO2 were successfully calculated and simulated by a plane wave pseudopotential method based on density functional theory (DFT). Our results show that most Ni 3d states are located in the forbidden band of substitutional Ni to O-doped anatase and rutile TiO2, and mix with O 2p states, resulting in a decrease of the photon excitation energy and red-shift of absorption edge compared to pure anatase and rutile. For substitutional Ni to Ti-doped anatase TiO2 under O-rich growth condition, the band gap has a slight decline of about 0.05eV compared with pure anatase TiO2 and contains a series of impurity energy levels, which may be responsible for the experimental photocatalytic activity and red-shift of absorption edge.

Enhanced Oxidation Reactivity of WO3(001) Surface through the Formation of Oxygen Radical Centers

The γ-WO3(001) surfaces doped by a series of group VB elements have been investigated by means of first-principles density functional theory (DFT) calculations combined with a slab model. Our results show that the doping of VB element is preferential under O-rich growth conditions and that the replacement of tungsten by Ta atom is energetically favorable among three group VB elements. The introduction of a group VB atom into the surface results in the downward shift of the Fermi level, and in most cases, the 2p states derived from the in-plane oxygen atom are still the dominate components at the Fermi level as before doping. However, the substitution of Ta dopant for 6-fold-coordinated tungsten atom (W6f) at the top layer is a special case in which the 2p states of the top terminal oxygen atom just above Ta become the primary compositions at the Fermi level. Only in this model, the spin densities are mainly located on the terminal oxygen atoms near the Ta site, and the oxygen radical center observed in the gas-phase W3O9+ cluster is reproduced. Therefore, the formation of radical oxygen center in the condensed phase depends on not only the substituent site but also the type of the dopant. Moreover, additional calculations are performed to study the oxidation reaction of CO molecule on the above Ta doped surface, and results indicate that the energy barrier for CO oxidization is obviously reduced compared to the undoped one, which implies that the introduction of Ta at W6f site can efficiently improve the oxidation reactivity of the WO3(001) surface.

First-principles calculation of rutile and anatase TiO2 intrinsic defect

In the paper, we study how the geometry structure and the growing ambience and the Fermi level of several intrinsic point defects effect the defect formation energy of the rutile and anatase TiO2, which type of point defect will be formed, and how to predict the experimental condition of point defect from theory. The key problem is how to calculat the defect formation energy with charge, and correcte the calculation results. The results show the defect type and the defect concentration are related to the nonequilibrium growth condition. In general, under the O-rich condition, VTi will form spontaneously, and under the Ti-rich condition, Ti4+ and VO easily appear in Schottky defects.

Ab initio study of defect properties in YPO 4

Ab initio methods based on density functional theory have been used to calculate the formation energies of intrinsic defects, including vacancies, interstitials, antisites and Frenkel pairs in YPO 4 under the O-rich and Y 2O 3-rich, and the O-rich and Y-rich conditions. The larger size of the yttrium atom may give rise to higher formation energy of the phosphorus antisite defect. In general, the formation energies of anion interstitials are much smaller than those of cation interstitials for both conditions considered. It is of greatly interest to find that the relative stabilities among the same types of interstitials are independent of the reference states. The most stable configuration for oxygen interstitials is an O–O split interstitial near the T a site, while the most stable configuration for cation interstitials is a tetrahedral interstitial near the T a site. The cation split interstitials are unfavorable in YPO 4, with much higher formation energies. Furthermore, the properties of Frenkel pairs are compared with those calculated using empirical potentials. The results reveal that both ab initio and empirical potential calculations show a similar trend in the formation energies of Frenkel pairs, but the formation energies obtained by empirical potentials are much larger than those calculated by ab initio method.

Electrical properties and Raman scattering investigation of Ag doped ZnO thin films

Ag-doped ZnO thin films were deposited on quartz glass substrates by a radio-frequency (RF) magnetron sputtering technique at room temperature (RT). The influence of Ag doping content on the electrical and Raman scattering properties of ZnO films were systematically investigated by Hall measurement system and Raman scattering spectrum. Two additional local vibrational modes (LVMs) at 230.0 and 394.5 cm −1 induced by Ag dopant in ZnO:Ag films were observed by Raman analyses at RT, corresponding to Ag atoms located at O sites (LV M Zn−Ag) and Zn sites (LV M Ag−O) in ZnO lattice. Moreover, we further studied the effect of donor Ag O and acceptor Ag Zn defects on the electrical properties of ZnO:Ag films. The results indicate that O-rich condition is preferred to suppress the formation of Ag O defects and enhance Ag Zn defects. The p-type ZnO:Ag film was achieved by properly optimizing the annealing conditions under O-rich condition.

Theoretical Studies on P-Type Conduction in (S,Cu) Co-Doped ZnO

Based on the first-principles calculations with density functional theory, the formation energy and electronic structure of (S, Cu) co-doped ZnO has been investigated, where the doping cases including related defects for Cu mono-doped, S-Cu co-doped, and S-2Cu co-doped ZnO are studied. The calculated results show that the formation energy of S-2Cu complex is lower than that of S-Cu complex under the O-rich condition. From the electronic structure, S-2Cu complex forms a peak of impurity state at the top of valence band. It was further found that heavy doping of Cu, not only enhances the acceptor concentration, but also leads to shallower acceptor energy level. Therefore, we concluded that S-2Cu complex is suitable for yielding better p-type conductivity in ZnO. The results are in good agreement with the experiment results.

First-Principles Calculations for Electronic Structures of Flourine-Doped ZnO

The formation energy and electronic structures of Fluorine-doped ZnO have been calculated by the first principles. The calculated results show that Fi, which is easier to form under O-rich conditions, is helpful to achieve p-type ZnO. The main contribution of accept level are the hybridization between F(2p)，O (2p) and Zn(3d) states which are near the top of the valence band.

Aluminum and nitrogen impurities in Wurtzite ZnO: first-principles studies

First-principles calculations have been performed to investigate the doping behaviors of Al and N dopant impurities in ZnO. According to the results, in the Al mono-doping case, the impurity states are quite delocalized, the corresponding effective masses are small, and the formation energy is as low as −9.71 eV. In the N mono-doping case, the impurity states are localized, the effective masses are large, and the formation energy is high (4.55 eV in the most favorable extreme O-rich conditions). In the Al–N codoping case, the corresponding effective masses are marked decreased compared to the N mono-doping situations, and the formation energy of the N–Al–N system is as low as −2.54 eV in the O-rich condition. The above results can explain the electrical behaviors of the doped or codoped ZnO systems observed in experiments.

Theoretical Study of the Interstitial Oxygen Atom in Anatase and Rutile TiO2: Electron Trapping and Elongation of the r(O−O) Bond

Interstitial oxygen atoms (Oi) in TiO2 were investigated using the density functional theory-based first-principle band structure method. Two polymorphs of TiO2, anatase and rutile structures, were considered. A symmetric O−O structure was obtained in rutile and is quite similar to the reported structure in anatase. In both polymorphs, Oi traps electrons from the conduction band to the σ* orbital of the O−O structure. The bond length r(O−O) is elongated in this process. The interactions between Oi and Nb or Ta impurities were also surveyed. There was weak attraction for both impurities. A simple statistical analysis was conducted to evaluate the Oi formation in anatase. Two conclusions were drawn from the present results. 1) Oi forms in n-type TiO2 in either the anatase or rutile structure under an O-rich condition. 2) The entropy factor, which originates from the distribution of Oi and from the electrons in the conduction band, makes a small contribution to the free energy at room temperature. The discre...

Doping properties of monoclinic BiVO4studied by first-principles density-functional theory

The intrinsic and extrinsic doping properties of BiVO${}_{4}$, i.e., the formation energies and transition energy levels of defects and impurities, have been studied systematically by first-principles density-functional theory. We find that for doping caused by intrinsic defects, O vacancies are shallow donors and Bi vacancies are shallow acceptors. However, these defects compensate each other and can only lead to moderate $n$-type and $p$-type conductivities at Bi-rich and O-rich growth conditions, respectively. To obtain BiVO${}_{4}$ with high $n$-type and $p$-type conductivities, which are required for forming Ohmic contacts, extrinsic doping using foreign impurities is necessary. Our results reveal that Sr, Ca, Na, and K atoms on Bi sites are very shallow acceptors and have rather low formation energies. The calculated Fermi-level pinning positions predict that doping of these impurities under oxygen-rich growth conditions should result in outstanding $p$-type conductivity. Substitutional Mo and W atoms on V sites are very shallow donors and have very low formation energies. Fermi-level pinning position calculations expect the doping of Mo and W under oxygen-poor growth conditions to produce excellent $n$-type conductivity. Also discussed is the dependence of formation energies and transition energies of defects on the atomic size and atomic chemical potential trends.

Structural, Electronic, and Optical Properties of Ag-Doped ZnO Nanowires: First Principles Study

First-principles calculations have been performed to determine the effects of Ag doping to the structural, electronic, and optical properties of ZnO NWs. The calculated formation energies are very low for Ag dopants at substitutional-Zn sites (both under low and high Ag concentration), but rather high at substitutional-O and interstitial sites under O-rich conditions. The AgZn and 2AgZn defects all prefer the edge of the NW and the formation energy of 2AgZn in the favorable O-rich conditions is only 0.40 eV. The calculated acceptor levels of AgZn and 2AgZn ones are 0.60 and 0.44 eV respectively, indicating Ag may be a good candidate for producing p-type ZnO NW. From the optical properties calculations, strong absorptions have been found in the visible-light region for both the 2AgZn and AgO-doped ZnO NWs. It provides evidence that, except for the usage as short-wavelength optoelectronic devices, Ag-doped ZnO NWs could also display potential application for photocatalysis due to the increase of the visible-photocatalytic activity.

Theoretical Study of the Effect of (F, Li) Codoping on P-Type Tendency in ZnO

Based on the first-principle calculations, we present a study for p-type ZnO doping. The calculated results show that (F, Li) codoping can suppress the formation of interstitial Li because the formation energy of FO-LiZn is lower than that of FO-Lii under O-rich condition. However, it is also found that FO-LiZn codoping could not realize p-type ZnO because FO-LiZn forms a fully passive complex. Interestingly, we further find FO-2LiZn is a stable acceptor that has lower formation energy and shallower transition level under O-rich condition. We expect this study might be helpful for synthesizing good p-type ZnO by controlling (F, Li) codoping.

Suppression of the formation of interstitial Li through (F, Li) codoping ZnO

Based on the first-principles calculations with density functional theory, the formation energy and electronic structure of (F, Li) codoping ZnO has been investigated, where the configurations including related defects for the ratio of 1:1 of (F, Li) codoping ZnO, and the ratio of 1:2 of (F, Li) codoping ZnO are studied. The calculated results show that the formation energy of Fo-LiZn is lower than that of Fo-Lii when the Fermi level approaches the valence band under the O-rich condition, which indicates that the (F, Li) codoping can suppress the Li interstitial. Furthermore, it is found the formation energy of Fo-2LiZn reduces more dramatically than that of Fo-LiZn. Moreover, the ionization energy of Fo-2LiZn is also lower than that of Fo-LiZn. It is found, for Fo-2LiZn complex, an occupied state at the top of valence band is formed, meaning that the presence of F, not only enhances the acceptor concentration, but also leads to shallower acceptor level. Therefore, we draw a conclusion that the presence of F atom could suppress the formation of interstitial Li, and Fo-2LiZn may form good p-type tendency in ZnO. Our results are in good agreement with the experiment results.

Theory study of Ag-S codoping in ZnO

All calculations, regarding S-monodoped and Ag-S codoped ZnO, have been performed by first-principles density-functional calculations. In S-monodoped ZnO, the formation energies show that SZn and SO is easier to form in O-rich conditions while S dopant prefers to occupy the substitutional O site in Zn-rich conditions. In Ag-S codoped ZnO, the electrical structures show that 2AgZn+SO complex is better for p-type doping while 2AgZn+SZn and 2AgZn+Si complexes are not suitable for p-doping. Moreover, 2AgZn+SO complex is easier to form than the other two complexes according to the formation energy. Therefore, in 2Ag-S codoped ZnO, the Zn-rich condition is helpful to gain p-type ZnO.

Energetics of intrinsic defects and their complexes in ZnO investigated by density functional calculations

Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated using a density-functional-theory-based pseudopotential all-electron method. The various defects considered are oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}$), zinc vacancy (${\mathrm{V}}_{\mathrm{Zn}}$), oxygen at an interstitial site (${\mathrm{O}}_{i}$), Zn at an interstitial site (${\mathrm{Zn}}_{i}$), Zn at ${\mathrm{V}}_{\mathrm{O}}$ (${\mathrm{Zn}}_{\mathrm{O}}$), O at ${\mathrm{V}}_{\mathrm{Zn}}$(${\mathrm{O}}_{\mathrm{Zn}}$), and an antisite pair (combination of the preceding two defects). In addition, defect complexes like (${\mathrm{V}}_{\mathrm{O}}+{\mathrm{Zn}}_{i}$) and Zn-vacancy clusters are studied. The Schokkty pair (${\mathrm{V}}_{\mathrm{O}}+{\mathrm{V}}_{\mathrm{Zn}}$) and Frenkel pairs [(${\mathrm{V}}_{\mathrm{O}}+{\mathrm{O}}_{i}$) and (${\mathrm{V}}_{\mathrm{Zn}}+{\mathrm{Zn}}_{i}$)] are considered theoretically for the first time. Upon comparing the formation energies of these defects, we find that ${\mathrm{V}}_{\mathrm{O}}$ would be the dominant intrinsic defect under both Zn-rich and O-rich conditions and it is a deep double donor. Both ${\mathrm{Zn}}_{\mathrm{O}}$ and ${\mathrm{Zn}}_{i}$ are found to be shallow donors. The low formation energy of donor-type intrinsic defects could lead to difficulty in achieving $p$-type conductivity in ZnO. Defect complexes have charge transitions deep inside the band gap. The red, yellow, and green photoluminescence peaks of undoped samples can be assigned to some of the defect complexes considered. It is believed that the red luminescence originates from an electronic transition in ${\mathrm{V}}_{\mathrm{O}}$, but we find that it can originate from the antisite ${\mathrm{Zn}}_{\mathrm{O}}$ defect. Charge density and electron-localization function analyses have been used to understand the effect of these defects on the ZnO lattice. The electronic structure of ZnO with intrinsic defects has been studied using density-of-states and electronic band structure plots. The acceptor levels introduced by ${\mathrm{V}}_{\mathrm{Zn}}$ are relatively localized, making it difficult to achieve $p$-type conductivity with sufficient hole mobility.

Ab initio Study of the Hydrogen Molecule on ZnO Surfaces

ABSTRACTWe conduct first-principles total-energy density functional calculations to study the interaction of H2 on ZnO surfaces. Four surface models of Zn-terminated (0001)-, O-terminated (0001)-, $(10{\bar 1}0)-$, and $(2{\bar 1}{\bar 1}0)-$oriented ZnO planes in the presence of H2 are evaluated. The relative stability of four different surface models is examined as a function of the chemical potentials of oxygen and hydrogen. We find that only surfaces of O-terminated (0001)-oriented ZnO models exhibit active sites for the dissociation of H2, which in turn enables the formation of water from dissociative chemisorption of 2H on the O-terminated ZnO(0001) surface. The surface energy of O-terminated ZnO(0001) surface in the presence of water was found to be negative under the O-rich and H-rich condition. The findings agree with the experimental observations that ZnO epitaxial layers are easily etched by hydrogen at typical growth temperatures.