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Zinc oxynitride (ZnON) has the potential to overcome the performance and stability limitations of current amorphous oxide semiconductors because ZnON-based thin-film transistors (TFTs) have a high field-effect mobility of 50 cm

We report a comparative study of the electronic structure and magnetic properties of two cobalt compounds Co3O4 and Co3S4, through first-principles Hubbard-U calculations. Our results indicate that Co3O4 and Co3S4 have similarities in crystal structure (normal spinel), magnetic order (antiferromagnetism), Co spin configuration (high spin Co2+ and low spin Co3+), and comparable band-gap energy. However, the U-dependence on electronic structure in two materials are different from each other. With a change in the applied U values, the band dispersion and the type of band gap are significantly changed in Co3O4, while the band-gap energy only is affected in Co3S4.

The trade-off between performance and stability in amorphous oxide semiconductor-based thin-film transistors (TFTs) has been a critical challenge, meaning that it is difficult to simultaneously ach...

A self-organized n+/n homojunction is proposed to achieve ultrahigh performance of thin film transistors (TFTs) based on an amorphous (Zn,Ba)SnO3 (ZBTO) semiconductor with sufficiently limited scattering centers. A deposited Al layer can induce a highly O-deficient (n+) interface layer in the back channel of a-ZBTO without damaging the front channel layer via the formation of a metal-oxide interlayer between the metal and back channel. The n+ layer can significantly improve the field-effect mobility by providing a relatively high concentration of free electrons in the front n-channel ZBTO, where the scattering of carriers is already controlled. In comparison with a Ti layer, the Al metal layer is superior, as confirmed by first-principles density functional theory (DFT) calculations, due to the stronger metal-O bonds, which make it easier to form a metal oxide AlOx interlayer through the removal of oxygen from ZBTO. The field-effect mobility of a-ZBTO with an Al capping layer can reach 153.4 cm2/Vs, which is higher than that of the pristine device, i.e., 20.8 cm2/Vs. This result paves the way for the realization of a cost-effective method for implementing indium-free ZBTO devices in various applications, such as flat panel displays and large-area electronic circuits. A layer of aluminum improves the performance of electronic devices useful for high-resolution flat displays, according to researchers in South Korea. Thin-film transistors are ultrafast electrical switches comprising stacked layers of semiconductors, metals and oxide materials. Their tiny size and planar architecture make them useful for large-area electronics and displays. A key performance metric for these devices is mobility: a measure of how quickly a charged particle can move through the semiconductor. Ji-Min Park from Chungnam National University in Daejeon and colleagues have shown how adding a capping layer of a highly oxidizing metal such as aluminum can greatly enhance the mobility of zinc barium tin oxide thin-film transistors. This cost-effective and useful improvement was achieved without introducing significant defects into the semiconductor. The formation of the conductive region (n+ layer) in (Zn,Ba)SnO3 (ZBTO) semiconductor is proposed to achieve high performance of thin-film transistors (TFTs). The aluminum metal capping layer is adopted to enhance the field-effect mobility of ZBTO TFTs. The capped Al layer takes out oxygen in the back-channel region, where the AlOx interlayer is formed. As a result, the field-effect mobility of Al-capped ZBTO TFTs is remarkably increased from 20.8 to 153.4 cm2/Vs. Furthermore, the Al-capped ZBTO TFTs are stable even when exposed to air for 3 months.

High-speed, high-performance electrochromic devices can be designed using unique solid-solid phase changes induced by oxygen flow., Mobile oxygen vacancies offer a substantial potential to broaden the range of optical functionalities of complex transition metal oxides due to their high mobility and the interplay with correlated electrons. Here, we report a large electro-absorptive optical variation induced by a topotactic transition via oxygen vacancy fluidic motion in calcium ferrite with large-scale uniformity. The coloration efficiency reaches ~80 cm2 C−1, which means that a 300-nm-thick layer blocks 99% of transmitted visible light by the electrical switching. By tracking the color propagation, oxygen vacancy mobility can be estimated to be 10−8 cm2 s−1 V−1 near 300°C, which is a giant value attained due to the mosaic pseudomonoclinic film stabilized on LaAlO3 substrate. First-principles calculations reveal that the defect density modulation associated with hole charge injection causes a prominent change in electron correlation, resulting in the light absorption modulation. Our findings will pave the pathway for practical topotactic electrochromic applications.

It has long been controversial whether or not electrochromism with the color change due to applied voltage is caused by small polarons. Recently, the coloration efficiency of Ca-doped $\mathrm{BiFe}{\mathrm{O}}_{3}$ (CBFO) was reported to be more prominent over a wider energy range than that of a conventional oxide. However, only an interpretation based on oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}$), which cannot account for the wide energy dependence of absorption, has been attempted. Here, we show that using first-principles hybrid-functional calculations, hole-trap centers in CBFO can be produced by a variety of small hole polarons and bipolarons around substitutional Ca ($\mathrm{C}{\mathrm{a}}_{\mathrm{Bi}}$) and ultimately play a significant role of color change. The polaron formation is attributed to the fact that up to two excess holes are trapped to enhance the Bi--O $sp\ensuremath{\sigma}$ bond. It is consistent with the experimental results that under the electroforming condition, electrochromism occurs well in the $p$-type region when CBFO is separated into two discrete regions relatively rich in ${\mathrm{V}}_{\mathrm{O}}$ ($n$ type) and $\mathrm{C}{\mathrm{a}}_{\mathrm{Bi}}$ ($p$ type). We, therefore, propose that identifying the diversity of the carrier-trap polarons provides a crucial clue to a deeper understanding of the origin of electrochromism.

The trade-off between performance and stability in amorphous oxide semiconductor-based thin-film transistors (TFTs) has been a critical challenge, meaning that it is difficult to simultaneously achieve high mobility and stability under bias and light stresses. Here, an amorphous mixture of two indium-free BaSnO

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO2 or TiO2-SrO) with oxygen partial pressure (PO2) during PLD. We found that a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure (Ptotal), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures., Comment: 27 pages, 6 figures, Supporting Information

Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO

In order to identify the source of charge trapping sites causing the device instability, we carry out ab initio calculations on the interface between amorphous SiO and InGaZnO. The interface structure is modeled by joining the two amorphous phases with additional annealing steps. The theoretical band offset is obtained by aligning oxygen 2s levels and shows good agreement with experiment. For the stoichiometric interface, we could not identify any defects within the gap that can capture positive holes. However, when oxygen vacancies are introduced at the interface, the Si–metal bonds are formed, resulting in the defect levels within the band gap. When positively charged with holes, the Si–metal bonds undergo huge relaxations, implying that the recovery to the original neutral state should involve a large energy barrier. Such oxygen vacancies at the interface may play as charge-trapping sites, affecting the long-term device instability.

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO

Zinc oxynitride (ZnON) semiconductors are suitable for high performance thin-film transistors (TFTs) with excellent device stability under negative bias illumination stress (NBIS). The present work provides a first approach on the optimization of electrical performance and stability of the TFTs via studying the resonant interaction between anions or vacancies in ZnON. It is found that the incorporation of nitrogen increases the concentration of nitrogen vacancies (VN+s), which generate larger concentrations of free electrons with increased mobility. However, a critical amount of nitrogen exists, above which electrically inactive divacancy (VN-VN)0 forms, thus reducing the number of carriers and their mobility. The presence of nitrogen anions also reduces the relative content of oxygen anions, therefore diminishing the probability of forming O-O dimers (peroxides). The latter is well known to accelerate device degradation under NBIS. Calculations indicate that a balance between device performance and NBIS stability may be achieved by optimizing the nitrogen to oxygen anion ratio. Experimental results confirm that the degradation of the TFTs with respect to NBIS becomes less severe as the nitrogen content in the film increases, while the device performance reaches an intermediate peak, with field effect mobility exceeding 50 cm2/Vs.

Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO3 thin films. Depending on the substrate, the lattice of epitaxial WO3 expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, instead of the defect concentration, plays the dominant role in determining the thermal conductivity. In particular, the thermal conductivity increases significantly with proton intercalation, which is contrary to the expectation that point defects typically lower the lattice thermal conductivity. The thermal conductivity can be dynamically varied by a factor of ≈1.7 via electrolyte gating, and tuned over a larger range, from 7.8 to 1.1 W m-1 K-1 , by adjusting the oxygen pressure during film growth. The electrolyte-gating-induced changes in thermal conductivity and lattice dimensions are reversible through multiple cycles. These findings not only expand the basic understanding of thermal transport in complex oxides, but also provide a path to dynamically control the thermal conductivity.

Throughout the past decades, doped-ZnO has been widely used in various optical, electrical, magnetic, and energy devices. While almost every element in the Periodic Table was doped in ZnO, the systematic computational study is still limited to a small number of dopants, which may hinder a firm understanding of experimental observations. In this report, we systematically calculate the single-element doping property of ZnO using first-principles calculations. We develop an automation code that enables efficient and reliable high-throughput calculations on thousands of possible dopant configurations. As a result, we obtain formation-energy diagrams for total 61 dopants, ranging from Li to Bi. Furthermore, we evaluate each dopant in terms of n-type/p-type behaviors by identifying the major dopant configurations and calculating carrier concentrations at a specific dopant density. The existence of localized magnetic moment is also examined for spintronic applications. The property database obtained here for doped ZnO will serve as a useful reference in engineering the material property of ZnO through doping.

Magnons and phonons are fundamental quasiparticles in a solid and can be coupled together to form a hybrid quasi-particle. However, detailed experimental studies on the underlying Hamiltonian of this particle are rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored, although it is essential for a proper understanding of their diverse thermodynamic behaviour and intrinsic zero-temperature decay. Here we show that in non-collinear antiferromagnets, a strong magnon–phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon–phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it., The properties of magnetic, crystalline solids can be described in terms of quantum particles of spin-wave and lattice-vibration energy, known as magnons and phonons respectively. Here, the authors show that strong magnon-phonon coupling in a noncollinear antiferromagnet can create magnetoelastic excitations.

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. In this study, it is demonstrated that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO

The negative bias illumination stress (NBIS)-induced VT instability of amorphous InGaZnO thin-film transistors (TFTs) is quantitatively investigated and a shallow donor state-creation model is proposed as a physical mechanism. Furthermore, the difference between InGaZnO and HfInZnO TFTs in perspective of NBIS-induced instability is consistently elucidated. We expect that the proposed model and simulation platform are potentially powerful tools for high-level instability-aware design of oxide TFT devices and circuits.

Trademark rights in Korea are based on registration. The Trademark Decree was promulgated in 1908 and the Trademark Act was firstly enacted and enforced on November 28, 1949. Since that time, the Trademark Act has been amended 39 times, with the most recent amendment on June 11, 2014 (this amendment took effect as of June 11, 2014). The Trademark Act is composed of ten chapters divided into General Provisions, Requirements and Application for Trademark Registration, Examination, Trademark Registration Fees and Trademark Registration, Trademark Rights, Protection of Owners of Trademark Rights, Trial, Retrial and Litigation, International Application under the Protocol, and Supplementary Provisions.

As the scale of transistors and capacitors in electronics is reduced to less than a few nanometers, leakage currents pose a serious problem to the device's reliability. To overcome this dilemma, high-kappa materials that exhibit a larger permittivity and band gap are introduced as gate dielectrics to enhance both the capacitance and block leakage simultaneously. Currently, HfO2 is widely used as a high-kappa dielectric; however, a higher-kappa material remains desired for further enhancement. To find new high-kappa materials, we conduct a high-throughput ab initio calculation for band gap and permittivity. The accurate and efficient calculation is enabled by newly developed automation codes that fully automate a series of delicate methods in a highly optimized manner. We can, thus, calculate 41800 structures of binary and ternary oxides from the Inorganic Crystal Structure Database and obtain a total property map. We confirm that the inverse correlation relationship between the band gap and permittivity is roughly valid for most oxides. However, new candidate materials exhibit interesting properties, such as large permittivity, despite their large band gaps. Analyzing these materials, we discuss the origin of large. values and suggest design rules to find new high-kappa materials that have not yet been discovered.

The microscopic origin of the negative bias and illumination stress (NBIS) instability of amorphous oxide semiconductors is revealed by first-principles calculations. We find that the excited holes during the stresses generate O2 peroxide defects in the oxide channel with increasing the Fermi level. The peroxide defects exhibit meta-stability.

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. In this study, it is demonstrated that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) ferroelectric (FE) capacitors. The SRO/BTO/SRO heterostructures are grown by a pulsed laser deposition method. The top SRO/BTO interface, grown at high PO2 (around 150 mTorr), usually exhibits a mixture of RuO2–BaO and SrO–TiO2 terminations. By reducing PO2, the authors obtain atomically sharp SRO/BTO top interfaces with uniform SrO–TiO2 termination. Using capacitor devices with symmetric and uniform interfacial termination, it is demonstrated for the first time that the FE critical thickness can reach the theoretical limit of 3.5 unit cells.

Substitutional hydrogen at oxygen site (H-O) is well-known to be a robust source of n-type conductivity in ZnO, but a puzzling aspect is that the doping limit by hydrogen is only about 10(18) cm(-3), even if solubility limit is much higher. Another puzzling aspect of ZnO is persistent photoconductivity, which prevents the wide applications of the ZnO-based thin film transistor. Up to now, there is no satisfactory theory about two puzzles. We report the bistability of H-O in ZnO through first-principles electronic structure calculations. We find that as Fermi level is close to conduction bands, the H-O can undergo a large lattice relaxation, through which a deep level can be induced, capturing electrons and the deep state can be transformed into shallow donor state by a photon absorption. We suggest that the bistability can give explanations to two puzzling aspects.

We provide a first-principles description of the crystalline and oxygen-deficient Ta2O5 using refined computational methods and models. By performing calculations on a number of candidate structures, we determined the low-temperature phase and several stable oxygen vacancy configurations, which are notably different from the previous results. The most stable charge-neutral vacancy site induces a shallow level near the bottom of conduction band. Stability of different charge states is studied. Based on the results, we discuss the implications of the level structures on experiments, including the leakage current in Ta2O5-based electronic devices and catalysts., Comment: 23 pages, 4 figures and 2 tables in the body texts plus 4 figures in the supplementary material

The excited holes occupying the valence band tail states in amorphous oxide semiconductors are found to induce formation of meta-stable O$_2^{2-}$ peroxide defects. The valence band tail states are at least partly characterized by the O-O pp{\sigma}* molecular orbital, and the localized-hole-mediated lattice instability results in the formation of the peroxide defects. Along with the O-O bond formation, the pp{\sigma}* state is heightened up into the conduction bands, and two electrons are accordingly doped in the electronic ground state. The energy barrier from the O$_2^{2-}$ peroxide state to the normal disorder state is found to be 0.97 eV in hybrid density functional theory. The hole-mediated formation of the meta-stable peroxide defects and their meta-stability is suggested as an origin of the negative bias and/or illumination stress instability in amorphous oxide semiconductors., Comment: 19 pages, 7 figures

We investigate the microscopic structures of interstitial and substitutional hydrogen impurities in LiNbO3 through the first-principles pseudopotential total-energy calculations. The interstitial hydrogen is located between two O atoms and bonds to one of the oxygen atoms. The hydrogen impurity substituting Li is significantly displaced from the Li-site and strongly bonds to an oxygen atom and is located on a biaxial axis of an O triangle. We discuss the effect of hydrogen to the ferroelectric polarization and the energetics for the trap of the interstitial hydrogen at a Li vacancy.

Zinc-based metal oxide semiconductors have attracted attention as an alternative to current silicon-based semiconductors for applications in transparent and fl exible electronics. Despite this, metal oxide transistors require signifi cant improvements in performance and electrical reliability before they can be applied widely in optoelectronics. Amorphous indium‐zinc‐tin oxide (a-IZTO) has been considered an alternative channel layer to a prototypical indium‐gallium‐zinc oxide (IGZO) with the aim of achieving a high mobility (>40 cm 2 Vs −1 ) transistors. The effects of the gate bias and light stress on the resulting a-IZTO fi eldeffect transistors are examined in detail. Hydrogen impurities in the a-IZTO semiconductor are found to play a direct role in determining the photo-bias stability of the resulting transistors. The Al 2 O 3 -inserted IZTO thin-fi lm transistors (TFTs) are hydrogen-poor, and consequently show better resistance to the external-bias-thermal stress and photo-bias-thermal stress than the hydrogenrich control IZTO TFTs. First-principles calculations show that even in the amorphous phase, hydrogen impurities including interstitial H and substitutional H can be bistable centers with an electronic deep-to-shallow transition through large lattice relaxation. The negative threshold voltage shift of the a-IZTO transistors under a negative-bias-thermal stress and negative-bias-illumination stress condition is attributed to the transition from the acceptor-like deep interstitial H i − (or substitutional H-DX − ) to the shallow H i + (or H O + ) with a high (low) activation energy barrier. Conclusively, the delicate controllability of hydrogen is a key factor to achieve the high performance and stability of the metal oxide transistors.

Undercoordinated indium (In*) is found to be an intrinsic defect that acts as a strong electron trap in amorphous InGaZnO4. Conduction electrons couple with the under-coordinated In* via Coulomb attraction, which is the driving force for the formation of an In*–M (M=In, Ga, or Zn) bond. The new structure is stable in the electron-trapped (2–) charge state, and we designate it as an intrinsic (In*–M)2− center in amorphous InGaZnO4. The (In*–M)2− centers are preferentially formed in heavily n-doped samples, resulting in a doping limit. They are also formed by electrical/optical stresses, which generate excited electrons, resulting in a metastable change in their electrical properties. Korean scientists have identified a key clue that can enhance the long-term stability of transparent semiconductors in ‘wearable’ computers. Recently, a glass-like oxide containing the metals indium, gallium and zinc (InGaZnO4) has found wide use in flexible electronic devices because its speedy transistor characteristics can drive high-resolution optical displays. However, the capabilities of this see-through semiconductor often degrade over time due to a phenomenon called charge trapping. Yong-Sung Kim from the Korea Research Institute of Standards and Science and co-workers simulated the amorphous structure of InGaZnO4 by performing first-principles quantum computations and discovered a previously unnoticed trapping site — ‘under-coordinated’ indium atoms that snare extra electrons through strong electron–ion interactions. Processing conditions that specifically supress populations of under-coordinated indium should be an essential part of future manufacturing efforts, suggest the authors. Undercoordinated indium (In*) is found to be an intrinsic defect that acts as a strong electron trap in amorphous InGaZnO4. Conduction electrons couple with the under-coordinated In* via Coulomb attraction, which is the driving force for the formation of an In*–M (M=In, Ga, or Zn) bond. The new structure is stable in the electron-trapped (2–) charge state, and we designate it as an intrinsic (In*–M)2− center in amorphous InGaZnO4. The (In*–M)2− centers are preferentially formed in heavily n-doped samples, resulting in a doping limit. They are also formed by electrical/optical stresses, which generate excited electrons, resulting in a metastable change in their electrical properties.

Based on density-functional theory calculations, the effects of cation compositions on electronic structures of In-Sn-Zn-O amorphous semiconductors were investigated. We considered various composition ratios of In, Sn, and Zn in O stoichiometric condition, and found that the conduction band minimum (CBM) energy level decreases and the valence band tail (VBT) energy level extent increases as the sum of In and Sn ratios (RIn+RSn) increases. The CBM lowering is attributed to the increased overlap of the In-5s and Sn-5s orbitals as the RIn+RSn increases, and correspondingly the electron effective masses (me*) are found to be reduced. The VBT increase is found to be due to the increased density of the In and Sn atoms, near which the O-2p inter-site ppσ* coupling is larger than that near the Zn atoms. The acute O-(In,Sn)-O angles are suggested to be structurally important, giving the stronger O-O ppσ* coupling.

Transparent amorphous oxide semiconductors (TAOS's) are of practical importance for applications including oxide electronics and displays. Here we show the lone-pair s-electrons incorporated by for example Sb-doping can suppress the hole-induced lattice instability, which has been a major obstacle to commercial application of the TAOS-based thin film transistors. The Sb(III)-O spσ* hybridization in the top-most valence states makes the lone-pair s-electrons to capture the excited holes, the Sb(V)O6 octahedral bonding configuration by which formed is easily dissociated into the stable lone-pair Sb(III) state by recapturing conduction electrons.

An inversion domain boundary (IDB) related to an interstitial stacking layer (ISL) was observed on the {0002} planes of the wurtzite (WZ) structure of tin (Sn)-doped ZnO nanobelts. Quantitative STEM analysis confirmed that the ISL was composed of Sn element. Oxygen related to the ISL was in a triangular coordination as determined by analyzing the electron energy-loss spectra. Expansion of the interplanar spacing along the c-axis of a WZ structure was observed near the IDB while that along the a-axis was constrained. Density functional theory calculations were carried out to elucidate the origin of microstructural evolution.

For highly H injected ZnCoO achieved by simultaneous high pressure (1000 bar) and annealing using a hot isostatic pressure (HIP), we report electrical and magnetic properties with first-principles calculation results. The HIP process increased the carrier concentration by ∼103 times and restored the conductivity up to that of H injected ZnO. Interestingly, with maintaining high conductivity, the extended HIP processing time significantly enhanced the short-ranged spin orderings of Co-H-Co complexes. Based on the experimental and theoretical results, we proposed the explanation for the relation magnetic characteristics and the behavior of hydrogen triggering spin ordering for spintronic applications.

We report a reversible manipulation of short-range spin ordering in Co-doped ZnO through hydrogenation and dehydrogenation processes. In both magnetic-circular dichroism and superconducting quantum interference device measurements, the ferromagnetism was clearly induced and removed by the injection and ejection of hydrogen, respectively. The x-ray photoelectron spectroscopy results and the first-principles electronic structure calculations consistently support the dependence of the ferromagnetism on the hydrogen position and the contribution of transition metal ions. The results suggest the ferromagnetic interaction between Co ions can be reversibly controlled by the hydrogen-mediated intrinsic spin ordering in Co doped ZnO.

Through first-principles pseudopotential total-energy calculations, we investigate the formation energetics and the electronic structures and structural properties of the native defects available in LiNbO3. We found that the formation enthalpies of Li-vacancy and Nb-antisite defects where lower compared to other defects. We examined the detailed electronic structures and structural properties of Nb-antisite. We identified a large Jahn-Teller distortion of Nb-antisite that can be induced by light illumination, by which the electronic structure is also remarkably changed. We discuss the role of the bistability of Nb-antisite in the photorefractivity.

We investigate the structural and electronic properties of hydrogen impurity in HgO through first-principles pseudo-potential total-energy calculations. The most stable position of the interstitial hydrogen is identified to be at the location between two O atoms, each of which is in two adjacent chains, forming a H-O bonding. Our electronic-structure calculations indicate that the interstitial hydrogen is a shallow-donor-type impurity. We also examined the formation enthalpy of the interstitial hydrogen. This indicates that HgO can be easily contaminated by H.