Charged Defect States Research Articles

Phonon‐assisted tunnel emission of holes from the double donor level of the EL2 defect

Strong electric‐field‐induced enhancement of the thermal emission rate of holes from the doubly ionised charge state of the ΕL2 defect was revealed with the double‐correlation deep‐level transient spectroscopy in p‐type GaAs crystals of various crystallographic orientations. The observed enhancement, by over four orders of magnitude of the emission rate, was analysed in a model of phonon‐assisted tunnel ionisation. Similar dependences of the hole‐emission rate on the electric‐field intensity received for the electric‐field directions parallel to three main crystallographic axes evidence for tetrahedral symmetry of the defect, which is consistent with its identification as the isolated arsenic antisite defect.

The Effect of Domain Wall on Defect Energetics in Ferroelectric LiNbO3 from Density Functional Theory Calculations

The energetics of defects in the presence of domain walls in LiNbO₃ are characterized using density-functional theory calculations. Domain walls show stronger interactions with antisite defects than with interstitial defects or vacancies. As a result, antisite defects act as a strong pinning center for the domain wall in LiNbO₃. Analysis of migration behavior of the antisite defects across the domain wall shows that the migration barrier of the antisite defects is significantly high, such that the migration of antisite defects across the domain wall is energetically not preferable. However, further study on excess electrons shows that the migration barrier of antisite defects can be lowered by changing the charge states of the antisite defects. So, excess electrons can enhance the migration of antisite defects and thus facilitate domain wall movement by weakening the pinning effect.

Effect of charge on point defect size misfits from ab initio: Aliovalently doped [formula omitted

We demonstrate a computationally efficient method to calculate size misfits of point defects in aliovalently doped SrTiO3 using density functional theory. The size misfits of neutral and charged single point defects are calculated and linearly combined to estimate the effective size misfits of multiple point defects in aliovalently doped SrTiO3. The charge state has a demonstrable effect on point defect size misfits for both isolated and multiple defects. We attribute this to the different charge localization between the defective systems. The extra electron or hole for the charged isolated point defects compensates the difference of charge localization between the neutral point defects and aliovalently doped system. The sum of size misfits of charged isolated point defects accurately estimates size misfits for aliovalently doped SrTiO3, without charge correction which was possibly required due to the additional charge state of point defect.

Metal oxides for optoelectronic applications.

Metal oxides (MOs) are the most abundant materials in the Earth's crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III-V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p-n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

Defect charge states in Si doped hexagonal boron-nitride monolayer

We perform ab initio density functional theory calculations to investigate the energetics, electronic and magnetic properties of isolated stoichiometric and non-stoichiometric substitutional Si complexes in a hexagonal boron-nitride monolayer. The Si impurity atoms substituting the boron atom sites SiB giving non-stoichiometric complexes are found to be the most energetically favourable, and are half-metallic and order ferromagnetically in the neutral charge state. We find that the magnetic moments and magnetization energies increase monotonically when Si defects form a cluster. Partial density of states and standard Mulliken population analysis indicate that the half-metallic character and magnetic moments mainly arise from the Si 3p impurity states. The stoichiometric Si complexes are energetically unfavorable and non-magnetic. When charging the energetically favourable non-stoichiometric Si complexes, we find that the formation energies strongly depend on the impurity charge states and Fermi level position. We also find that the magnetic moments and orderings are tunable by charge state modulation q = −2, −1, 0, +1, +2. The induced half-metallic character is lost (retained) when charging isolated (clustered) Si defect(s). This underlines the potential of a Si doped hexagonal boron-nitride monolayer for novel spin-based applications.

Electrical control of intervalley scattering in graphene via the charge state of defects

We study the intervalley scattering in defected graphene by low-temperature transport measurements. The scattering rate is strongly suppressed when defects are charged. This finding highlights ``screening'' of the short-range part of a potential by the long-range part. Experiments on calcium-adsorbed graphene confirm the role of a long-range Coulomb potential. This effect is applicable to other multivalley systems, provided that the charge state of a defect can be electrically tuned. Our result provides a means to electrically control valley relaxation and has important implications in valley dynamics in valleytronic materials.

Investigation and calculation of positron lifetimes of monovacancies in crystals

The first-principles calculations of positron lifetimes of mono-vacancies in crystals were investigated. We use the two-component density functional theory to respectively compute positron lifetimes of neutral charge state of VAl defect in aluminium, VSi defect in silicon, VC, VSi and VC+CSi defects in 3C silicon carbide, VGa and VAs defects in gallium arsenide, taking into account atomic relaxation due to vacancy and electronic structural relaxation due to the presence of the positron. Three different calculation schemes are used. We find that the electron density inside the vacancy more or less increases due to the presence of the positron if the ionic positions are kept fixed, and the positron becomes more localized after the electronic structural relaxation for the case of VAl defect in aluminium and VSi defect in 3C silicon carbide, but it is opposite for the case of VGa defect in gallium arsenide and VC defect in 3C silicon carbide. The results with no consideration of the relaxation are even much closer to the experimental ones, therefore the atomic relaxation due to the position play an important role in calculating the positron lifetime of mono-vacancies in crystals.

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m2/g in pure to 405.1m2/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol–TiO2 interface and finally interact with phenol molecule and degrade it.

A.C. conduction in glassy alloys of Se90Sb10-xAgx

Temperature and frequency dependence of a.c. conductivity is studied in glassy Se90Sb10-xAgx (x=2, 4, 6 and 8) in the temperature range (288–358K) and frequency range (1–500kHz). An agreement between experimental and theoretical results suggests that the behavior of glassy Se–Sb–Ag system can be successfully explained by correlated barrier hopping (CBH) model. The results show that bipolaron hopping dominates over single polaron hopping in present case, which is explained in terms of lower value of potential barrier. By fitting the experimental data to CBH model, the density of charged defect states is calculated for all the glasses studied. The composition dependence of the density of these states is studied and found to show a maxima at 6at% of Ag which is explained in terms of the mechanically stabilized structure at a particular average co-ordination number.

Electrical level of defects in single-layer two-dimensional TiO2.

The remarkable properties of graphene and transition metal dichalcogenides (TMDCs) have attracted increasing attention on two-dimensional materials, but the gate oxide, one of the key components of two-dimensional electronic devices, has rarely reported. We found the single-layer oxide can be used as the two dimensional gate oxide in 2D electronic structure, such as TiO2. However, the electrical performance is seriously influenced by the defects existing in the single-layer oxide. In this paper, a nondestructive and noncontact solution based on spectroscopic ellipsometry has been used to detect the defect states and energy level of single-layer TiO2 films. By fitting the Lorentz oscillator model, the results indicate the exact position of defect energy levels depends on the estimated band gap and the charge state of the point defects of TiO2.

Charge modulation of magnetization in X-doped MgO nanotube clusters (X=C, N)

First-principles calculations based on density functional theory are performed to study the magnetic and electronic properties of X-doped 8×7 MgO nanotube clusters (X=C, N). The N dopant easily occupies the O-site at the edge of MgO nanotube, embracing neutral or charged defect state, and induces notable magnetization in N-doped MgO tubular cluster. More important, this p-electron magnetization can be significantly modulated as the charged state of the defect changes. Regarding C doping, impurity atom readily substitute the Mg atom located at the edge of MgO nanotube to form neutral defect, and net magnetization is found to be zero. The calculated electron densities of states show that the O-site N doping at the edge greatly narrows or even destroys band-gap, while it enlarges somewhat for the Mg-site C doping at the edge. The results are likely to stimulate a promising class of materials for various applications ranging from spintronics to magneto-optics.

ChemInform Abstract: Noncontact Atomic Force Microscopy: An Emerging Tool for Fundamental Catalysis Research

AbstractReview: 53 refs.

Noncontact Atomic Force Microscopy: An Emerging Tool for Fundamental Catalysis Research.

Although atomic force microscopy (AFM) was rapidly adopted as a routine surface imaging apparatus after its introduction in 1986, it has not been widely used in catalysis research. The reason is that common AFM operating modes do not provide the atomic resolution required to follow catalytic processes; rather the more complex noncontact (NC) mode is needed. Thus, scanning tunneling microscopy has been the principal tool for atomic scale catalysis research. In this Account, recent developments in NC-AFM will be presented that offer significant advantages for gaining a complete atomic level view of catalysis. The main advantage of NC-AFM is that the image contrast is due to the very short-range chemical forces that are of interest in catalysis. This motivated our development of 3D-AFM, a method that yields quantitative atomic resolution images of the potential energy surfaces that govern how molecules approach, stick, diffuse, and rebound from surfaces. A variation of 3D-AFM allows the determination of forces required to push atoms and molecules on surfaces, from which diffusion barriers and variations in adsorption strength may be obtained. Pushing molecules towards each other provides access to intermolecular interaction between reaction partners. Following reaction, NC-AFM with CO-terminated tips yields textbook images of intramolecular structure that can be used to identify reaction intermediates and products. Because NC-AFM and STM contrast mechanisms are distinct, combining the two methods can produce unique insight. It is demonstrated for surface-oxidized Cu(100) that simultaneous 3D-AFM/STM yields resolution of both the Cu and O atoms. Moreover, atomic defects in the Cu sublattice lead to variations in the reactivity of the neighboring O atoms. It is shown that NC-AFM also allows a straightforward imaging of work function variations which has been used to identify defect charge states on catalytic surfaces and to map charge transfer within an individual molecule. These advances highlight the potential for NC-AFM-based methods to become the cornerstone upon which a quantitative atomic scale view of each step of a catalytic process may be gained. Realizing this potential will rely on two breakthroughs: (1) development of robust methods for tip functionalization and (2) simplification of NC-AFM instrumentation and control schemes. Quartz force sensors may offer paths forward in both cases. They allow any material with an atomic asperity to be used as a tip, opening the door to a wide range of surface functionalization chemistry. In addition, they do not suffer from the instabilities that motivated the initial adoption of complex control strategies that are still used today.

First-principles embedded-cluster calculations of the neutral and charged oxygen vacancy at the rutileTiO2(110) surface

We perform full-potential screened-hybrid density-functional theory (DFT) calculations to compare the thermodynamic stability of neutral and charged states of the surface oxygen vacancy at the rutile TiO$_2$(110) surface. Solid-state (QM/MM) embedded-cluster calculations are employed to account for the strong TiO$_2$ polarization response to the charged defect states. Similar to the situation for the bulk O vacancy, the +2 charge state $V_{\rm O}^{2+}$ is found to be energetically by far most stable. Only for Fermi-level positions very close to the conduction band, small polarons may at best be trapped by the charged vacancy. The large decrease of the $V_{\rm O}^{2+}$ formation energy with decreasing Fermi-level position indicates strongly enhanced surface O vacancy concentrations for $p$-doped samples.

Charge states of point defects in plutonium oxide: A first-principles study

The stability of charged defects in PuO2 are systematically investigated by quasi-annealing simulations based on local spin density approximation (LSDA) + U calculations. The formation energies of intrinsic (Frenkel and Schottky pairs) and extrinsic (single vacancy or interstitial) defects at different charged states are calculated and presented. According to our results, the noninteracting oxygen Frenkel pairs made of the association of doubly charged defects are the most favorable form of intrinsic defects, the formation energy of which is in good agreement with experimental values. The negative charged oxygen interstitial (IO2−) and positive charged oxygen vacancy (VO2+) defects have the lowest formation energies at oxygen-rich and oxygen-poor environments, respectively, and thus are the most commonly seen extrinsic defects. Both the existences of oxygen vacancy and oxygen interstitial induce new electronic states in the band gap of stoichiometric PuO2, showing doping effects.

First-principles survey of the structure, formation energies, and transition levels of As-interstitial defects in InGaAs

While point defects in elemental (Si) and compound (GaAs, GaN, AlN) semiconductors have been extensively studied both experimentally and theoretically, only limited theoretical studies of these defects exist for technologically important binary $(\mathrm{S}{\mathrm{i}}_{x}\mathrm{G}{\mathrm{e}}_{1\text{\ensuremath{-}}x})$ and pseudobinary $(\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{As}$, $\mathrm{I}{\mathrm{n}}_{\mathrm{x}}\mathrm{G}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{N}$, $\mathrm{A}{\mathrm{l}}_{x}\mathrm{G}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{N})$ semiconductor alloys. Here, we use density-functional theory and a recently developed bounds-analysis approach to survey the atomic structures, formation energies, and charge-state transition levels of the stable and metastable states of As interstitials in the pseudobinary alloy $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$. Our studies consider seven different candidate defect structures for the As interstitial, with calculations performed for selected defect charge states in the range $q=\ensuremath{-}2$ to $+3$. In each case, the mean and standard deviations of the defect-formation energy are determined using statistical sampling methods that place the defect into a wide variety of differing local-alloy environments. When examined from the point of view of the mean formation energy of the defect, the stable configurations of the As interstitial in $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$ are found to resemble previous findings for GaAs, with a ${\mathrm{C}}_{1\mathrm{h}}\text{\ensuremath{-}}\mathrm{p}{001}_{\mathrm{III}}$ interstitial structure in a $q=+1$ charge state favored near midgap and below, and a ${\mathrm{C}}_{2\mathrm{v}}\text{\ensuremath{-}}11{0}_{\mathrm{a}}$ split-interstitial structure in a $q=\ensuremath{-}1$ charge state favored above midgap (the named point-group symmetries refer to the underlying symmetry that the alloy defect would possess if within GaAs). The statistical sampling reveals a strong dependence of the defect-formation energy on the local-alloy environment, with the standard deviation $\ensuremath{\sigma}$ of the formation energy approaching 0.21 eV for the most stable As-interstitial structures. Because the range of ground-state energies encountered by an As-interstitial defect when moving through the alloy is found to be quite large, approaching $\ensuremath{\sim}1.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}(\ifmmode\pm\else\textpm\fi{}3\ensuremath{\sigma})$, defect-diffusion pathways in $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$ will have spatial and temporal complexities not found in GaAs.

Effect of oxygen vacancy and zinc interstitial on the spontaneous polarization of wurtzite ZnO: maximally localized Wannier functions analysis

First-principles calculations are employed to obtain the maximally localized Wannier functions of wurtzite ZnO with O vacancy and Zn interstitial. Through analyzing the shape of Wannier functions and the distribution of Wannier function centers, the influence of O vacancy and Zn interstitial in different charge states on the spontaneous polarization are investigated. We found that polarization appears in 010 and [20] orientations when the defects are charged, and the absolute polarization value will get larger as the defect charge state increases. Neutral O vacancy decreases the spontaneous polarization along the [0 0 0 1] direction. For O vacancy in +1 and +2 charge states, an additional electronic field is introduced, and it results in a dramatic change in the electronic structure, which reverses the polarization. However, in the case of Zn interstitial in +2 charge state, we find that it is the ionic relaxation that plays a dominant role in the polarization reversal.

Colloidal synthesis and electrical behaviour of n-ZnGdO/p-Si heterojunction diodes

Studies on manoeuvring the optoelectronic characteristics of a semiconducting nanostructure are of recent specific interest for a wide range of photonic applications. In this regard, the optical and electrical characteristics of ZnO nanostructures have been tuned and studied systematically using Gd ions. The structural and morphological characteristics of the solution processed ZnGdO nanostructures were studied in detail using the results of X-ray diffraction and microscopic measurements. The absorption band edge in ZnO was noted to shift towards the lower wavelength values on Gd substitution, suggesting an increase in its energy band gap. The blue emissivity from ZnO complexes was also noted to improve as a function of Gd composition in ZnO. The potential of ZnGdO nanostructures for optoelectronic functions was evaluated by fabricating heterojunction diodes based on n-ZnGdO/p-Si. The diode characteristics revealed an improved electrical conductivity and rectifying behaviour from the fabricated architectures upon Gd substitution and photon illumination. The findings are correlated with the increased charge carrier concentration and defect states existing within ZnGdO species, through appropriate mechanisms.

Synergistic ultraviolet photoresponse of a nanostructured ZnO film with gate bias and ultraviolet illumination

We report large enhancement of photoresponse in a nanostructured ZnO film using an electrolyte as a gate dielectric with gate bias and ultraviolet (UV) illumination. The gate bias used for observing the large response is low and ≤5 V. The resultant current enhancement by the combined effect of two effects (field effect and illumination) is much larger than the simple addition of two effects when they act individually. The polymeric electrolyte used as gate dielectric forms an electric double layer at the interface between the nanostructured film and gate dielectric, which enables accumulation of large charge carrier density (≈1013–1014) at the interface. The field effect mobility, μFE, of the carriers is enhanced under modest gate bias in the presence of UV illumination due to the reduction of barrier EB in between the nano crystallites because of large surface charge induced by field effect. It is proposed that the gate induced carriers also passivate the charged defect states in the ZnO, thereby reducing carrier scattering and enhancing the photocurrent. The mechanism proposed has been validated by observation of gate controlled Photoluminescence, where the passivation of charged defect states by gate induced carriers led to suppression of visible photoluminescence, which arises from charged oxygen vacancy defect states.

Application of the bounds-analysis approach to arsenic and gallium antisite defects in gallium arsenide

A recently developed bounds-analysis approach has been used to interpret density-functional-theory (DFT) results for the As and Ga antisites in GaAs. The bounds analysis and subsequent processing of DFT results for the As antisite yielded levels---defined as the Fermi levels at which the defect charge state changes---in very good agreement with measurements, including the \ensuremath{-}1/0 level which is within 0.1 eV of the conduction-band edge. Good agreement was also obtained for the activation energies to transform the $\mathrm{A}{\mathrm{s}}_{\mathrm{Ga}}$ from its metastable state to its stable state. For the Ga antisite, the bounds analysis revealed that the \ensuremath{-}1 and 0 charge states are hole states weakly bound to a localized \ensuremath{-}2 charge state. The calculated levels are in good agreement with measurements.