Stable Charge States Research Articles

Positive and Negative Oxygen Vacancies in Amorphous Silica

We modeled all stable positive and negative charge states of oxygen vacancies originating from the neutral O3≡Si=Si≡O3 defect in amorphous SiO2 (a-SiO2) using an embedded cluster method on a distribution of structural sites. For the first time, we predict the geometry, electronic structure and spectroscopic properties of doubly ionized and negatively charged oxygen vacancies in a-SiO2 and demonstrate that negatively charged vacancies serve as deep electron traps. The results demonstrate that oxygen vacancies can be responsible for both the electron and hole trapping in silica. We compare our findings with the previous calculations and with the recent experimental data.

First principles investigation of defect energy levels at semiconductor-oxide interfaces: Oxygen vacancies and hydrogen interstitials in the Si–SiO2–HfO2 stack

We introduce a scheme for the calculation of band offsets and defect energy levels at semiconductor-oxide interfaces. Our scheme is based on the use of realistic atomistic models of the interface structure and of hybrid functionals for the evaluation of the electronic structure. This scheme is herein applied to the technologically relevant Si–SiO2–HfO2 stack. Calculated band offsets show a very good agreement with experimental values. In particular, we focus on the energy levels of the oxygen vacancy defect and the interstitial hydrogen impurity. The defect levels are aligned with respect to the interface band structure and determined for varying location in the dielectric stack. The most stable charge states are identified as the Fermi level sweeps through the silicon band gap.

Hf defects inc−Siand their importance for theHfO2∕Siinterface: Density-functional calculations

We have studied, using ab initio density functional theory calculations, substitutional and interstitial Hf impurities in $c\text{\ensuremath{-}}\mathrm{Si}$, for various charge states. Our results indicate that (1) the tetrahedral interstitial defect is energetically more favorable than the substitutional and (2) there are various stable charge states in the Si gap. The possible presence of these charged impurities in the Si channel of $\mathrm{Hf}{\mathrm{O}}_{2}∕\mathrm{Si}$-based metal-oxide-semiconductor devices could lead to a mobility reduction, due to Coulombic scattering.

Oxygen vacancy in monoclinic HfO2: A consistent interpretation of trap assisted conduction, direct electron injection, and optical absorption experiments

The authors calculate energy levels associated with the oxygen vacancy in monoclinic HfO2 using a hybrid density functional which accurately reproduces the experimental band gap. The most stable charge states are obtained for varying Fermi level in the HfO2 band gap. To compare with measured defect levels, they determine total energy differences specific to the considered experiment. Their results show that the oxygen vacancy can consistently account for the defect levels observed in (Poole-Frenkel-type) trap assisted conduction, direct electron injection, and optical absorption experiments.

Charged point defects in semiconductors

Abstract Native point defects control many aspects of semiconductor behavior. Such defects can be electrically charged, both in the bulk and on the surface. This charging can affect numerous defect properties such as structure, thermal diffusion rates, trapping and recombination rates for electrons and holes, and luminescence quenching rates. Charging also introduces new phenomena such as nonthermally photostimulated diffusion, thereby offering distinctive mechanisms for defect engineering. The present work incorporates the first comprehensive account of semiconductor defect charging, identifying correspondences and contrasts between surfaces and the bulk as well as among semiconductor classes (group IV, groups III–V, and metal oxides). For example, small lattice parameters, close-packed unit cells, and basis atoms with large atomic radii all inhibit the formation of ionized interstitials and antisites. The charged defects that exist in III–V and oxide semiconductors can be predicted with surprising accuracy from the chemical potential and oxygen partial pressure of the ambient. The symmetry-lowering relaxations, formation energies, and diffusion mechanisms of bulk and surface defect structures often depend strongly on charge state with similar qualitative behavior, although for a given material surface defects do not typically take on the same configurations or range of stable charge states as their counterparts in the bulk.

Structural and electronic properties ofFe3+andFe2+centers in GaN from optical and EPR experiments

This work provides a consistent picture of the structural, optical, and electronic properties of Fe-doped GaN. A set of high-quality GaN crystals doped with Fe at concentrations ranging from $5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is systematically investigated by means of electron paramagnetic resonance and various optical techniques. ${\mathrm{Fe}}^{3+}$ is shown to be a stable charge state at concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The fine structure of its midgap states is successfully established, including an effective-mass-like state consisting of a hole bound to ${\mathrm{Fe}}^{2+}$ with a binding energy of $50\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. A major excitation mechanism of the ${\mathrm{Fe}}^{3+}(^{4}T_{1}\ensuremath{\rightarrow}^{6}A_{1})$ luminescence is identified to be the capture of free holes by ${\mathrm{Fe}}^{2+}$ centers. The holes are generated in a two-step process via the intrinsic defects involved in the yellow luminescence. The ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level is found $2.863\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band, suggesting that the internal reference rule does not hold for the prediction of band offsets of heterojunctions between GaN and other III-V materials. The ${\mathrm{Fe}}^{2+}(^{5}E\ensuremath{\rightarrow}^{5}T_{2})$ transition is observed around $390\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at any studied Fe concentration by means of Fourier transform infrared spectroscopy. Charge-transfer processes and the effective-mass-like state involving both ${\mathrm{Fe}}^{2+}$ states are observed. At Fe concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, additional lines occur in electron paramagnetic resonance and photoluminescence spectra which are attributed to defect complexes involving ${\mathrm{Fe}}^{3+}$. With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level, where it stays pinned for concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction.

Formation of Vacancies and Divacancies in Plane-Stressed Silicon

The effect of compressive and tensile plane-stress loading on formation energies and electronic properties of vacancies and divacancies in silicon are studied by first-principles approach for in-plane strains up to 0.7%. It is demonstrated that contributions to defect formation energies from the elastic lattice relaxation and from the band structure modification respond to stress in a different manner, leading to noticeable different behaviour of formation energies for different charges states. The most stable vacancy charge states at different Fermi level are shown to be sensitive to strain magnitude and sign. This results in the strain-induced shifts and even disappearance of some of thermal ionization levels of vacancies and divacancies in the band gap.

Relative concentration and structure of native defects in GaP

The native defects in the compound semiconductor GaP have been studied using a pseudopotential density functional theory method in order to determine their relative concentrations and the most stable charge states. The electronic and atomic structures are presented and the defect concentrations are estimated using calculated formation energies. Relaxation effects are taken into account fully and produce negative-U charge transfer levels for ${\mathrm{V}}_{\mathrm{P}}$ and ${\mathrm{P}}_{\mathrm{Ga}}$. The concentration of ${\mathrm{V}}_{\mathrm{Ga}}$ is in good agreement with the results of positron annihilation experiments. The charge transfer levels presented compare qualitatively well with experiments where available. The effect of stoichiometry on the defect concentrations is also described and is shown to be considerable. The lowest formation energies are found for ${\mathrm{P}}_{\mathrm{Ga}}^{+2}$ in $p$-type and ${\mathrm{V}}_{\mathrm{Ga}}^{\ensuremath{-}3}$ in $n$-type GaP under P-rich conditions, and for ${\mathrm{Ga}}_{\mathrm{P}}^{\ensuremath{-}2}$ in $n$-type GaP under Ga-rich conditions. Finally, the finite size errors arising from the use of supercells with periodic boundary conditions are examined.

Diffusivity of native defects in GaN

The diffusion of relevant native point defects in wurtzite GaN crystals is investigated using first-principles density-functional pseudopotential calculations. Our reexamination of the ground state of the defects, using a higher level of convergence than was previously used, yields results in good agreement with earlier published results [J. Neugebauer and C. G. Van de Walle, Phys. Rev. B $50,$ 8067 (1994)]. Gallium interstitials are stable at the octahedral interstitial site and can occur in 1+, 2+ (metastable), or 3+ charge states. They migrate via an interstitialcy mechanism with an unexpectedly low barrier of 0.9 eV, consistent with the annealing of the $L5$ signal in electron-paramagnetic-resonance experiments [K. H. Chow et al., Phys. Rev. Lett. $85,$ 2761 (2000)]. For the nitrogen interstitial the ground-state configuration is a split interstitial, occurring in charge states ranging from $1\ensuremath{-}$ to 3+. Migration also proceeds via an interstitialcy mechanism, with barriers of 2.4 eV or lower, depending on the charge state. The nitrogen vacancy has two stable charge states 1+ and 3+. The migration barrier for ${V}_{\mathrm{N}}^{+}$ is high (4.3 eV), while the migration barrier for ${V}_{\mathrm{N}}^{3+}$ is significantly lower, at 2.6 eV, consistent with recent positron-annihilation experiments [S. Hautakangas et al., Phys. Rev. Lett. $90,$ 137402 (2003)]. The gallium vacancy, finally, can occur in charge states 0, $1\ensuremath{-},$ $2\ensuremath{-},$ and $3\ensuremath{-},$ and migrates with a barrier of 1.9 eV. For all these defects the lowest-energy migration path results in motion both parallel and perpendicular to the c axis; no anisotropy in the diffusion will therefore be observed. Applications to point-defect-assisted impurity diffusion will also be discussed.

Vacancy charging on Si(100)-(2×1): Consequences for surface diffusion and STM imaging

The present work investigates the structure and energetics of charged vacancies on Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) by calculations using density functional theory. The calculations predict multiple stable charge states for all vacancy structures investigated, although the neutral state is most stable for typical Si(100) surfaces. The multiplicity of possible states lends significant support to a hypothesized mechanism for nonthermal illumination influences on surface diffusion. The calculations also show that the +1 state of the upper dimer monovacancy is destabilized by structural relaxations, leading to negative-U properties. Implications of this work for possible artifacts in imaging by scanning tunneling microscopy are discussed.

Atomic charge states in a weakly coupled plasma environment.

The relative stability of various charge states of a number of selected elements has been studied as a function of increasing screening due to a weakly bound plasma represented by the statically screened Coulomb potential of the Yukawa-type. Elements with positive electron affinity in a vacuum are predicted to undergo a sequential electron detachment process from anionic state to the cationic one as the inverse screening length of plasma increases. Conversely, elements with negative electron affinity in a vacuum present only two charge states, neutral and positive. At small screening the neutral is the most stable charge state, and as the screening increases the cation develops as the most stable charge state. Values of the inverse screening length for each of the transitions have been calculated and discussed.

Proton-induced defect generation at the Si-SiO/sub 2/ interface

We report first-principles calculations of interface trap formation in MOS structures. Hydrogen is known to passivate Si dangling bonds at the Si-SiO/sub 2/ interface, but the subsequent arrival of H+ at the interface causes depassivation of Si-H bonds. We show that, contrary to conventional assumptions, depassivation is not a two-step process, namely, neutralization of H+ by a Si electron, reaction with a Si-H bond, and subsequent formation of an H/sub 2/ molecule. Instead, we establish that H+ is the only stable charge state of hydrogen at the interface, and that H+ reacts directly with Si-H. The products of this reaction are an H/sub 2/ molecule and a positively charged dangling bond center (D/sup +/), formed via the reaction SiH + H/sup +/ /spl rArr/ D/sup +/ + H/sub 2/. Here the D/sup +/ center is most likely the positive charge state of the P/sub b/ defect. As a result, H-induced interface-trap formation depends on the electric field in the oxide to establish a preferred direction for proton drift, but does not depend on the availability of Si electrons to enable the interface reaction to occur. After the dangling bond center is formed via this process, the subsequent charge state of the interface traps is controlled by the Si surface potential. A hydrogen catalytic cycle can lead to reversible passivation and depassivation reactions at or near the interface, depending on experimental conditions.

Defect generation by hydrogen at the Si- SiO(2) interface.

Hydrogen is known to passivate Si dangling bonds at the Si-SiO(2) interface, but the subsequent arrival of H(+) at the interface causes depassivation of Si-H bonds. Here we report first-principles density functional calculations, showing that, contrary to conventional assumptions, depassivation is not a two-step process, namely, neutralization of H(+) by a Si electron and subsequent formation of an H(2) molecule. Instead, we establish that H(+) is the only stable charge state at the interface and that H(+) reacts directly with Si-H, forming an H(2) molecule and a positively charged dangling bond (P(b) center). As a result, H-induced interface-trap formation does not depend on the availability of Si electrons.

Quantum chemical studies on Cs 3C 60 and Cs 6C 60

The three-dimensional EHMO crystal orbital calculations for crystalline C 60, Cs 3C 60 and Cs 6C 60 are reported. The ground states of both undoped solid C 60 and fully doped Cs 6C 60 are found to be insulating with an indirect energy gap of 1.2–1.7 and 0.9 eV, respectively. In contrast, Cs 3C 60 forms a metallic-conducting phase with a set of three half-filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state (DOS). The character of crystal orbital near the Fermi level for both Cs 3C 60 and Cs 6C 60 is completely carbonlike. In both cases, the Cs atoms are almost fully ionized and C 60 molecules form a stable negative charge state with three to six additional electrons. All the results obtained by us are in excellent agreement with experimental results.

Stable site and stable charge state of a fluorine atom in Si

We investigated a stable state of a fluorine (F) atom in Si bulk by using first-principles calculations. We considered various interstitial sites and three charge states from +1 to −1. We found that over a wide range of Fermi levels, the bond center site in the +1 charge state is the most stable for an F atom. The present calculations suggest that the experimentally observed conductivity-dependent etching properties of F for Si are intimately related to the charge-state-dependent stable site of F in the Si bulk.

First principles study of Pb vacancies in PbTiO3

Electronic and ionic structures of lead vacancies in PbTiO3 were studied using an ab initio approach. Even though the lead vacancy is found to be an acceptor with stable charge states ranging from 2- to 4- it does not form a tightly bound defect pair with a double donor oxygen vacancy. The formation of distant and nearby lead-vacancy–oxygen-vacancy pairs is shown to be exothermic under certain growth conditions.

Band structure studies on polymeric fullerenes

The three-dimensional EHMO crystal orbital program has been used to study both quasi-one-dimensional neutral polymers (C 60) n and orthorhombic doped polymers (KC 60) n and (RbC 60) n . Our calculated results show that metallic conducting phases are formed in (KC 60) n and (RbC 60) n . The characteristics of the crystal orbitals near the Fermi level for all doped polymeric fullerenes are completely carbon like. These dopant K and Rb atoms are thoroughly ionized and the C 60 molecules form stable negative charge states with one additional electron in each C 60 molecule.

Design and Synthesis of Dendrimers with Enhanced Two-Photon Absorption

ABSTRACTModel dendrimers based on bis-(diphenylamino)stilbene repeat units have been synthesized and shown to have very stable bipolaronic charge states. Calculations at the AM-1 level predict two orders magnitude enhancement in the third-order nonlinearity in going from the neutral to bipolaron state. Measurement of two-photon cross-sections for PPV dimers with attached diphenylamino donor groups show enhanced two-photon cross-sections for ns pulses. Preliminary evaluation of one of the model dendrimers indicates even larger two-photon cross-sections are possible.

Electronic and structural properties of the As vacancy on the (110) surface of GaAs

The isolated As vacancy on the GaAs (110) surface is investigated using ab initio pseudopotentials. The relaxed atomic structure reveals an inward relaxation of the neighboring surface Ga atoms regardless of the vacancy charge state. The stable charge state is determined to be (+1) in p-type material from the formation energies. Three vacancy states, two located within the band gap, are identified. The character of the vacancy wavefunctions is described in detail and discussed in connection with the charge-state dependent relaxation. Despite the inward relaxation of surface Ga neighbors, the theoretical scanning tunneling microscopy topography shows enhanced tunneling into these sites, which is in agreement with existing experiments. The origin of this apparent contradiction is attributed to the nature of the wavefunction for one of the unoccupied vacancy states.

Threefold coordinated oxygen atom in silica glass

The threefold coordinated oxygen atom in solid silicon dioxide is studied in a cluster model using the MNDO method. Atomic configuration, electronic structure, optical and vibrational properties of three charge states of the defect are calculated. The positively-charged threefold coordinated oxygen atom is found to be the only stable charge state of the defect. UV optical absorption with energy ⪰ 7.5 eV and IR optical absorption near 410 cm −1, 460 cm −1 and 990 cm −1 and depolarized Raman scattering in two latter bands are typical for this charge state. A model of photobleaching of oxygen-deficient centers in silica glass is suggested.