Negative Charge State Research Articles

Scanning probe microscopy of single Au ion implants in Si

We have studied 5 MeV Au 2+ ion implantation with fluences between 7 × 10 7 and 2 × 10 8 cm − 2 in Si by deep level transient spectroscopy (DLTS) and scanning capacitance microscopy (SCM). The DLTS measurements show formation of electrically active defects such as the two negative charge states of the divacancy ( V 2( /–) and V 2(–/0)) and the vacancy–oxygen (VO) center. It is observed that the intensity of the V 2( /–) peak is lower compared to that of V 2(–/0) by a factor of 5. This has been attributed to a highly localized distribution of the defects along the ion tracks, which results in trapping of the carriers at V 2(–/0) and incomplete occupancy of V 2( /–). The SCM measurements obtained in a plan view show a random pattern of regions with a reduced SCM signal for the samples implanted with fluence above 2 × 10 8 cm − 2 . The reduced SCM signal is attributed to extra charges associated with acceptor states, such as V 2(–/0), formed along the ion tracks in the bulk Si. Indeed, the electron emission rate from the V 2(–/0) state is in the range of 10 kHz at room temperature, which is well below the probing frequency of the SCM measurements, resulting in “freezing” of electrons at V 2(–/0).

First-principles study of the structure and stability of oxygen defects in zinc oxide

A comparative study on the structure and stability of oxygen defects in ZnO is presented. By means of first-principles calculations based on local density functional theory we investigate the oxygen vacancy and different interstitial configurations of oxygen in various charge states. Our results reveal that dumbbell-like structures are thermodynamically the most stable interstitial configurations for neutral and positive charge states due to the formation of a strongly covalent oxygen--oxygen bond. For negative charge states the system prefers a split-interstitial configuration with two oxygen atoms in almost symmetric positions with respect to the associated perfect lattice site. The calculated defect formation energies imply that interstitial oxygen atoms may provide both donor- and acceptor-like defects.

Direct Correlation of the Crystal Structure of Proteins with the Maximum Positive and Negative Charge States of Gaseous Protein Ions Produced by Electrospray Ionization

Electrospray mass spectrometric studies in native folded forms of several proteins in aqueous solution have been performed in the positive and negative ion modes. The mass spectra of the proteins show peaks corresponding to multiple charge states of the gaseous protein ions. The results have been analyzed using the known crystal structures of these proteins. Crystal structure analysis shows that among the surface exposed residues some are involved in hydrogen-bonding or salt-bridge interactions while some are free. The maximum positive charge state of the gaseous protein ions was directly related to the number of free surface exposed basic groups whereas the maximum negative charge state was related to the number of free surface exposed acidic groups of the proteins. The surface exposed basic groups, which are involved in hydrogen bonding, have lower propensity to contribute to the positive charge of the protein. Similarly, the surface exposed acidic groups involved in salt bridges have lower propensity to contribute to the negative charge of the protein. Analysis of the crystal structure to determine the maximum charge state of protein in the electrospray mass spectrum was also used to interpret the reported mass spectra of several proteins. The results show that both the positive and the negative ion mass spectra of the proteins could be interpreted by simple consideration of the crystal structure of the folded proteins. Moreover, unfolding of the protein was shown to increase the positive charge-state because of the availability of larger number of free basic groups at the surface of the unfolded protein.

Theory of boron-vacancy complexes in silicon

The substitutional boron-vacancy ${\mathrm{B}}_{s}\mathrm{V}$ complex in silicon is investigated using the local density functional theory. These theoretical results give an explanation of the experimentally reported, well established metastability of the boron-related defect observed in $p$-type silicon irradiated at low temperature and of the two hole transitions that are observed to be associated with one of the configurations of the metastable defect. ${\mathrm{B}}_{s}\mathrm{V}$ is found to have several stable configurations, depending on charge state. In the positive charge state the second nearest neighbor configuration with ${C}_{1}$ symmetry is almost degenerate with the second nearest neighbor configuration that has ${C}_{1h}$ symmetry since the bond reconstruction is weakened by the removal of electrons from the center. A third nearest neighbor configuration of ${\mathrm{B}}_{s}\mathrm{V}$ has the lowest energy in the negative charge state. An assignment of the three energy levels associated with ${\mathrm{B}}_{s}\mathrm{V}$ is made. The experimentally observed ${E}_{v}+0.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${E}_{v}+0.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ levels are related to the donor levels of second nearest neighbor ${\mathrm{B}}_{s}\mathrm{V}$ with ${C}_{1}$ and ${C}_{1h}$ symmetry respectively. The observed ${E}_{v}+0.11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ level is assigned to the vertical donor level of the third nearest neighbor configuration. The boron-divacancy complex ${\mathrm{B}}_{s}{\mathrm{V}}_{2}$ is also studied and is found to be stable with a binding energy between ${\mathrm{V}}_{2}$ and ${\mathrm{B}}_{s}$ of around $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Its energy levels lie close to those of the ${\mathrm{V}}_{2}$. However, the defect is likely to be an important defect only in heavily doped material.

Radiation-induced defects in MOS structures after irradiation with high-energy Ar, Kr, Bi heavy ions

Radiation damage in the MOS (metal-oxide-semiconductor) structures irradiated with Ar, Kr and Bi heavy ions with energies from 1 to 7.5 MeV/nucl. has been studied using charge deep level transient spectroscopy (Q-DLTS) and feedback charge capacitance voltage ( C – V ) techniques. Four dominant well-known deep levels such as vacancy–oxygen (VO) complex, divacancy (VV 2−) in the double negative charge state and composite VV −+VSb complex were registered versus ion fluence and annealing temperature. It was shown that VO center concentration at the near-surface silicon substrate layer is determined by ion energy loss in elastic collisions and no effect of high-level electronic stopping power was detected. At the same time, the value of the positive radiation-induced defect charge in the oxide layer was decreased under Bi ions irradiation. Thermal treatment in air ambient at 250 °C restores CV characteristics of the MOS samples to that of un-irradiated state.

Path dependence of the charge state distributions of low-energy Fq+ ions backscattered from RbI(1 0 0)

Projectile neutralization during backscattering from RbI(100) of F multicharged ions in the keV energy range was investigated utilizing a time-of-flight technique. The energy and charge state distributions of the scattered ions were measured as a function of the polar incidence angle and the target azimuthal orientation. We found significant variations in the neutralization degree for incident projectiles of different charge states. The charge state distribution of scattered ions, including negative charge states, was found to depend on both the polar incidence and azimuthal orientation angles. These variations are attributed to the particular hard and soft encounters with neighboring lattice sites at the target surface along the path of the ion. Sample data for few-keV F2+ and F7+ incident projectiles are presented to illustrate the underlying concepts.

Competing fragmentation processes in tandem mass spectra of heparin-like glycosaminoglycans

Heparin-like glycosaminoglycans (HLGAGs) are highly sulfated, linear carbohydrates attached to proteoglycan core proteins and expressed on cell surfaces and in basement membranes. These carbohydrates bind several families of growth factors and growth factor receptors and act as coreceptors for these molecules. Tandem mass spectrometry has the potential to increase our understanding of the biological significance of HLGAG expression by providing a facile means for sequencing these molecules without the need for time-consuming total purification. The challenge for tandem mass spectrometric analysis of HLGAGs is to produce abundant ions derived via glycosidic bond cleavages while minimizing the abundances of ions produced from elimination of the fragile sulfate groups. This work describes the competing fragmentation pathways that result from dissociation of high negative charge state ions generated from HLGAGs. Glycosidic bond cleavage ion formation competes with losses of equivalents of H2SO4, resulting in complex ion patterns. For the most highly sulfated structure examined, an octasulfated tetramer, an unusual loss of charge from the precursor ion was observed, accompanied by low abundance ions originating from subsequent backbone cleavages. These results demonstrate that fragmentation processes competing with glycosidic bond cleavages are more favored for highly sulfated HLGAG ions. In conclusion, reduction of charge-charge repulsions, such as is achieved by pairing the HLGAG ions with metal cations, is necessary in order to minimize the abundances of ions derived via fragmentation processes that compete with glycosidic bond cleavages.

Irradiation-induced defects in ZnO studied by positron annihilation spectroscopy

We have used positron annihilation spectroscopy to study the point defects induced by 2 MeV electron irradiation (fluence 6 × 1017 cm–2) in single crystal n-type ZnO samples. The positron lifetime measurements have shown that the zinc vacancies in their doubly negative charge state, which act as dominant compensating centers in the as-grown material, are produced in the irradiation and their contribution to the electrical compensation is important. The lifetime measurements reveal also the presence of competing positron traps with low binding energy and lifetime close to that of the bulk lattice. The analysis of the Doppler broadening of the 511 keV annihilation line indicates that these defects can be identified as neutral oxygen vacancies. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogen Incorporation in Diamond: The Vacancy-Hydrogen Complex

We report the identification of the vacancy-hydrogen complex in single crystal diamond synthesized by chemical vapor deposition. The S=1 defect is observed by electron paramagnetic resonance in the negative charge state. The hydrogen atom is bonded to one of the carbon atoms neighboring the vacancy. Unlike the analogous defect in silicon, no symmetry lowering reconstruction occurs between the three remaining carbon dangling orbitals. The very small measured hydrogen hyperfine interaction is explained by dipolar coupling between the hydrogen and the unpaired electron probability density delocalized on the three equivalent carbon neighbors.

Polar Mesospheric Summer Echoes (PMSE) overshoot effect due to cycling of artificial electron heating

We study the time‐dependent behavior of dust and plasma parameters for artificial electron heating experiments during Polar Mesospheric Summer Echoes (PMSE) conditions, by the use of the model for dusty plasmas introduced by Havnes and Morfill [1984] and Havnes et al. [1990, 1992]. We find that if the heater is run in a long series of cycles with equal and comparatively short (e.g., 10–20 s) intervals in the on and off mode, the difference between the dusty plasma parameters in subsequent heater off phases will be small. This will give the impression that the PMSE, which can be severely weakened during the heater on phase, returns more or less exactly to its value in the former heater off phase. However, we also find that if the heater off phase is kept long (e.g., 160 s), during which the dusty plasma in the radar beam will relax to its undisturbed value or be replaced by unaffected dusty plasma due to horizontal transport by wind, we will see a substantial difference in the plasma parameters from just before the heater is switched on and just after it is switched off. Owing to an additional charging of the dust during the heater on phase and which takes a long time to decharge, the negative charge state of the dust is higher when the heater is switched off than before it was on. This will increase electron gradients in the dusty plasma and also the value of dust charge density to electron density, and we propose that this should lead to a stronger PMSE, or an PMSE overshoot, when the heater is switched off, compared with the value just before it was switched on.

Effect of uniaxial stress on the electronic state of a platinum-dihydrogen complex in silicon and charge-state-dependent motion of hydrogen during stress-induced reorientation

We have combined isothermal deep-level transient spectroscopy (IT-DLTS) technique with the application of uniaxial compressive stress along $〈111〉$ direction to study the effect of stress on the electronic state of a platinum-dihydrogen complex in Si and the kinetics of charge-state-dependent motion of hydrogen around the Pt atom during stress-induced reorientation. We have found that the application of stress splits the IT-DLTS peak into two components and the electronic energy of the short-time component increases linearly with $〈111〉$ compressive stress as $35\ifmmode\pm\else\textpm\fi{}4\mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{G}\mathrm{P}\mathrm{a},$ indicating antibonding character. We have also studied the reorientation kinetics of the complex under the applied stress, and have found that the defect aligned at 78--88 K in the configuration with the smallest activation energy of the level only when the level was not occupied by an electron. This indicates a clear charge-state effect on the local motion of hydrogen around the Pt atom, that is, hydrogen is mobile only in the singly negative charge state of the complex. We have estimated an activation energy 0.27 eV for the hydrogen motion around the Pt atom under a stress of 0.6 GPa. We have examined three structural models, among which a model where the two hydrogen atoms are directly bonded to the platinum atom may be the most plausible candidate. In this structure, defect reorientation needs no bond switching but only the rotation of the whole $\mathrm{Pt}\ensuremath{-}{\mathrm{H}}_{2}$ entity. A possible mechanism of the charge-state-dependent reorientation observed may be that if the electronic state with antibonding character is occupied by an electron, the two hydrogen atoms may be displaced outward, probably retarding their motion for the reorientation.

Electronic structure calculations of intrinsic and extrinsic hydrogen point defects inKH2PO4

We report first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of H intrinsic (Frenkel pair) and extrinsic (H vacancy or interstitial) point defects in ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}.$ The relaxed atomic structures, the formation energy, the ionization energy, and electron and hole affinities for the various defects have been calculated. For the Frenkel pair, the additional hole leads to a decrease of the $\mathrm{O}---\mathrm{O}$ bond length between the two O atoms next to the H vacancy, while the effect of the additional electron is small. For the H vacancy, the added hole is trapped and shared by the two O atoms adjacent to the vacancy, reducing dramatically the $\mathrm{O}---\mathrm{O}$ bond length, thus forming a molecular-type polaron. We find that the positively charged H vacancy introduces states in the gap, in contrast with its neutral state, confirming the experimental suggestion that it is a relevant absorbing center. The negatively charged H vacancy leads to an increase of the two O atoms close to the H vacancy, and does not induce states in the gap. The H interstitial does not interact with the host atoms in the neutral state. However, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of a ${\mathrm{H}}_{2}$ interstitial molecule and a H vacancy, in agreement with experimental suggestions. In the positively charged state the H interstitial binds to its nearest-neighbor O atom forming a hydroxyl bond. The H interstitial in both positive and negative charge states induces no defect states in the band gap, in contrast with its neutral state. The calculations provide insights into the role of the charged and neutral defects on the transient optical absorption under irradiation by high-intensity laser beam.

Electrical and multifrequency EPR study of nonstoichiometric defects in 4H-SiC

Two paramagnetic intrinsic defects P and ND1 have been studied in both C-rich n-type 4H-SiC and undoped semi-insulating (s.i.) 4H-SiC in the dark and under illumination of the s.i. sample with light at 140 and 37GHz in the temperature interval from 4.2 to 77K. Photo EPR and Hall effect measurements indicate that P is a deep donor defect localized at EC−1.15eV, while ND1 is shallow donor defect localized at EC−0.07eV.Based on the observed 13C hyperfine structure, and the C-rich growth conditions, we identify the P center with the silicon vacancy, while ND1, which exhibited a strong central hyperfine interaction with one 13C atom, is attributed to the carbon antisite CSi. Considering that the defects have spin S=12, C3V symmetry of the EPR spectrum and appear to be donor-like defects, P is attributed to the silicon vacancy in the −3 charge state (VSi3−), whereas ND1 is suggested to be the carbon antisite in the single negative charge state (CSi−).

Piezospectroscopic studies of the re-orientation process of defect complexes

The stress-induced electronic level splitting pattern can provide information regarding the symmetry of a defect complex. If the components of the piezospectroscopic tensor are known it is also possible to deduce information about the lattice relaxation in the vicinity of the defect. Based on a complete piezospectroscopic analysis of the vacancy–oxygen complex in silicon we confirm that this defect has an orthorhombic symmetry in the stable configuration while in the unstable configuration of the saddle point on the reconfiguration trajectory we demonstrate that it has trigonal symmetry. The piezospectroscopic description of static defects is, in some cases, inadequate to deal with some defect complexes when they are observed at relatively high temperatures. This is often the case for the high-resolution Laplace deep level transient spectroscopy (DLTS) measurements. At very low temperatures the vacancy–oxygen–hydrogen complex has monoclinic symmetry while at the temperatures used by us for Laplace DLTS measurements the observed symmetry is orthorhombic due to fast hydrogen jumps. A similar re-orientation (position averaging) process occurs for the divacancy in silicon. However, this is true only for the divacancy in the single negative charge state while the symmetry of the complex in the double negative state is stable trigonal. We discuss how these re-orientation processes can be identified from the observed piezospectroscopic parameters for both cases.

Laplace transform transient spectroscopy study of a divacancy-related double acceptorcentre in Si

Radiation-induced divacancy-related levels in high-purity oxygen-enriched n-typesilicon have been studied with the use of deep level transient spectroscopy(DLTS) and Laplace-DLTS. It has been shown that heat treatment at250 °C results in a shift of thedivacancy (V2)-relatedpeaks observed by ‘standard’ DLTS. Using Laplace-DLTS it is demonstrated that the shift is due to annealingof V2 and formation of a new acceptor centre. The new centre has presumablytwo negative charge states: singly and doubly negative. The formation ofthe new centre holds a close one-to-one correlation with the annealing ofV2,indicating that the new centre is a result of divacancy interaction with an impurity or adefect. The close position of the electronic levels of the new centre to that ofV2 suggests a similar electronic and microscopic structure of the new centre toV2,and a tentative identification is a divacancy–oxygen centre.

Interstitial H and H2 in SiC

The properties of hydrogen in 3C and 4H type silicon carbide (SiC) are studied theoretically at the density functional level. We find that only singly positive or negative charge states of hydrogen are thermodynamically stable in SiC. The transition from the positive to the negative charge state (+/−) is at 0.9 and 1.3 eV above the valence band maximum in 3C and 4H structures, respectively. The diffusion barrier for the proton is 0.5 eV (being, however, anisotropic in 4H). For the negative H− the diffusion barrier is found to be considerably higher, of the order of 3 eV.

Spectral signatures of positive and negative charged states in doped and photoexcited low band-gap polydithienothiophenes

Spectroelectrochemical studies of polydithienothiophenes (pDTTs), low band-gap conjugated polymers with polythiophene-like chain, using in situ FTIR-ATR and ESR spectroscopy, are shown. Spectral results obtained during p- and n-doping, as well as photoinduced spectral effects, are compared. The formation of paramagnetic positive and negative charge carriers with unusually high g-values is described.

Hydrogen incorporation in diamond: the nitrogen-vacancy-hydrogen complex.

We report the identification of the nitrogen-vacancy-hydrogen complex in a freestanding nitrogen-doped isotopically engineered single crystal diamond synthesized by chemical vapor deposition. The hydrogen atom is located in the vacancy of a nearest-neighbor nitrogen-vacancy defect and appears to be bonded to the nitrogen atom maintaining the trigonal symmetry of the center. The defect is observed by electron paramagnetic resonance in the negative charge state in samples containing a suitable electron donor (e.g., substitutional nitrogen N(0)(S)).

First-principle theoretical study on the electronic properties of SiO 2 models with hydrogenated impurities and charges

We adopt first-principle approach to calculate potential energy surfaces for α-quartz and -cristobalite silicon dioxide (SiO 2) with hydrogenated impurities in the neutral, positive, and negative charged states. It is expected that H atoms in SiO 2 have an important role to generate leakage current paths. In this paper, we calculate stable and unstable positions of a migrating H atom and discuss electrons or holes trapping mechanism. These results clarify that the dynamics of the H atom in the neutral, negative, and positive charged states should be different from one another, and therefore, the mechanism of the dielectric break down of SiO 2 also depends on the charged environment. Based on the regional density functional theory, we have calculated the effective charge tensor density of the migrating H atom in the neutral state.

Spatial correlation effects in the charged impurity distribution on the electronic properties of δ‐doped structures

AbstractIn this paper we will show that the δ‐doped GaAs/AlGaAs/GaAs quantum barrier is an ideal system to study deep centers in narrow doping layers with high doping density. By varying the Al content in the barrier, the distance between the Fermi‐level and the deep level can be tuned and therefore the number of populated states. By applying hydrostatic pressure this number can be further increased. Our measurements show that in these δ‐doped barrier structures electrons are trapped under hydrostatic pressure and that the quantum mobility is enhanced. This mobility enhancement can be explained by MC calculations which include the effect of the spatial correlation of the charge distribution when the deep state is the DX center and has a negative charge state. The fact that we do not observe a PPC effect or a significant change in the mobility after illumination at high pressure suggest that the Fermi‐level is pinned by a non‐metastable deep state like the A1 state or that the DX center is modified.