Vacancy-impurity Complexes Research Articles

Formation of vacancy-impurity complexes by kinetic processes in highly As-doped Si.

Positron annihilation experiments have been applied to verify the formation mechanism of electrically inactive vacancy-impurity clusters in highly n-type Si. We show that the migration of V-As pairs at 450 K leads to the formation of V-As2 complexes, which in turn convert to stable V-As3 defects at 700 K. These processes manifest the formation of V-As3 as the dominant vacancy-impurity cluster in highly n-type Si. They further explain the electrical deactivation and clustering of As in epitaxial or ion-implanted Si during postgrowth heat treatment at 700 K.

Radiation Defects in Silicon due to Hadrons and Leptons, their Annealing and Influence on Detector Performance

A phenomenological model was developed to explain quantitatively, without free parameters, the production of primary defects in silicon after particle irradiation, the kinetics of their evolution toward equilibrium and their influence on detector parameters. The type of the projectile particle and its energy is considered in the evaluation of the concentration of primary defects. Vacancy-interstitial annihilation, interstitial migration to sinks, vacancy-impurity complexes (VP, VO, V2O), and divacancy (V2) formation are taken into account in different irradiation conditions, for different concentrations of impurities in the semiconductor material, for 20 and 0°C. The model can be extended to include other vacancy and interstitial complexes. The density of the reverse current in the detector after irradiation is estimated. Comparison with experimental measurements is performed. A special application considered in the paper is the modelled case of the behaviour of silicon detectors operating in the pion field estimated for the LHC accelerator, under continuum generation and annealing.

Influence of low-energy electron beam irradiation on defects in activated Mg-doped GaN

ABSTRACTCL spectroscopy studies at varying temperatures and excitation power densities as well as depth-resolved CL imaging were conducted to investigate the impact of low energy electron beam irradiation (LEEBI) on native defects and residual impurities in metal-organic vapor phase epitaxy (MOVPE) grown Mg-doped p-type GaN. Due to the dissociation of (Mg-H)0 complexes, LEEBI significantly increases the (e,Mg0) emission (3.26 eV) at 300 K and substantially decreases the H-Mg donor-acceptor-pair (DAP) emission (3.27 eV) at 80 K. In-plane and depth-resolved CL imaging indicates that hydrogen dissociation results from electron-hole recombination at H-defect complexes rather than heating by the electron beam. The dissociated hydrogen atoms associate with nitrogen vacancies, forming a deeper donor, i.e. a (H-VN) complex. The corresponding deeper DAP emission with Mg centered at 3.1 eV is clearly observed between 160 and 220 K. Moreover, a broad yellow luminescence (YL) band centered at 2.2 eV is observed in MOVPE-grown Mg-doped GaN after LEEBI-treatment. It is suggested that a combination of LEEBI-induced Fermi-level downshift due to Mg-acceptor activation and simultaneous dissociation of gallium vacancy-impurity complexes, i.e. (VGa-H), is responsible for the observed YL.

Spatial variation of photoluminescence and related defects in InGaN/GaN quantum wells

Spatially and spectrally resolved photoluminescence of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition is studied with near-field scanning optical microscopy (NSOM) and transmission electron microscopy (TEM). High-spatial-resolution NSOM images show bright blue quantum well emission around V defects and yellow emission inside the defects. TEM data suggest that the spatial distribution of blue luminescence is partly due to dislocation gettering by V defects. The yellow emission is attributed to the Ga vacancy-impurity complexes trapped inside V defects.

Photoluminescence properties of ZnTe homoepitaxial films deposited by synchrotron-radiation-excited growth

ZnTe homoepitaxial films have been deposited at substrate temperatures between 27°C and 100°C by synchrotron-radiation-excited growth using diethylzinc and diethyltelluride. Effects of diethylzinc transport rate and substrate temperature upon the photoluminescence properties of the ZnTe films have been clarified. Strong deep level emissions centered at 1.85 and 2.1 eV related to defects such as vacancy-impurity complex become emerged with increasing diethylzinc transport rate or substrate temperature. A sharply excitonic emission at 2.375 eV associated with shallow acceptors is observed and neither a donor–acceptor pair recombination nor a deep level luminescence signal is detected in the spectrum of the film grown under the nearly stoichiometric condition, which indicates that ZnTe films of good quality can be grown even at room temperature by this growth technique.

Phonon-assisted optical transitions in GaN with impurities and defects

We have classified all the possible types of phonon-assisted optical processes involving bound or extended initial, final, and virtual (intermediate) electron states and formulated, for the first time, in terms of site symmetry, the selection rules for corresponding transitions. We apply this theory to phonon-assisted transitions in hexagonal GaN involving substitutional impurities and vacancies with C 3v site-symmetry as well as interstitial impurities and molecular point defects (paired impurities, double vacancies, and vacancy-impurity complexes) occupying sites with C 3v, C s, and C 1 ones. We show that phonon-assisted optical recombination is allowed in any polarization for free and bound carriers and excitons whatever is the number of involved phonons. Just, the nature of virtual state(s) and phonon(s) can depend on the polarization of the emitted light. We discuss, in particular, the case of excitons bound to neutral donors or acceptors. Our predictions are in good agreement with experimental optical spectra published in the literature which exhibit numerous lines assigned to one- and multi-phonon-assisted transitions.

Structural properties of fission-product doped ZrO2 and MgAl2O4 single crystals

The structural properties of Zirconia (ZrO2) and spinel (MgAl2O4) single crystals doped by ion implantation with fission products (Cs and I) were investigated by using the Rutherford backscattering and channeling techniques. The implantation process induces a heavy damage in the various sublattices of the ceramic matrices above a fluence of about 1015 at. cm−2, corresponding to a number of displacements per atom (dpa) of about 1–2. Large values of the Cs and I substitutional fractions, attributed to the formation of impurity-vacancy complexes, are obtained at impurity concentrations lower than 0.5 at.%. The substitutional fraction decreases with increasing implantation fluence. This effect, which is more pronounced for I than for Cs ions, is likely due to precipitation or compound formation.

Calculation of the Doppler broadening of the electron-positron annihilation radiation in defect-free bulk materials

Results of a calculation of the Doppler broadening of the positron-electron annihilation radiation and positron lifetimes in a large number of elemental defect-free materials are presented. A simple scheme based on the method of superimposed atoms is used for these calculations. Calculated values of the Doppler broadening are compared with experimental data for a number of elemental materials, and qualitative agreement is obtained. These results provide a database which can be used for characterizing materials and identifying impurityvacancy complexes.

Non-equilibrium intergranular segregation in ultra low carbon steel

Impurity segregation to grain boundaries in ultra low carbon steel was investigated by Auger electron spectroscopy and SEM during isothermal annealing at 900°C and continuous cooling. The results of isothermal annealing at 900°C show that a concentration peak appears at different times for phosphorus, sulphur, and boron, which is contrary to the equilibrium segregation theory of McLean. The phenomena could be satisfactorily explained by the non-equilibrium segregation theory based on the impurity–vacancy complex mechanism. Under continuous cooling, the segregation concentration at the grain boundary largely depends on the cooling rate. At a low cooling rate the concentration of phosphorus and boron at the grain boundary is higher than that of sulphur, while at the higher cooling rate the concentration of sulphur is higher.

Bound-state symmetries and optical transitions in GaAs/AlAs quantum wells and superlattices with impurities and defects

We consider $(\mathrm{GaAs}{)}_{m}(\mathrm{AlAs}{)}_{n}$ superlattices as single crystals whose structure depends on the growth direction and numbers of monolayers within the slabs of constituent materials. We study point defects such as impurities (substitutional and interstitial) or single vacancies and molecular defects such as paired impurities, double vacancies, or vacancy-impurity complexes in $(\mathrm{GaAs}{)}_{m}(\mathrm{AlAs}{)}_{n}$ superlattices grown along the [001], [110], and [111] directions. The possible site symmetries of the defects as well as the state symmetries for carriers bound to them have been determined. In contrast to bulk GaAs or AlAs, no defect can present ${T}_{d}$ symmetry. The atoms located in the center of the slabs occupy, in most of the [001]- and [110]-grown superlattices, sites with higher symmetries ${(D}_{2d}$ and ${C}_{2v},$ respectively). This results in different selection rules for optical transitions involving the same impurity atom which substitutes the same host atom (Ga, Al, or As) at sites with different symmetries. The effect can be important in optical spectra of superlattices with very thin slabs. The modifications of the selection rules when including the spin-orbit interaction have been derived. Quantum wells can be treated as a particular case of superlattices when barriers become very thick. We present their three-dimensional diperiodic space groups. Quantum wells do not differ from superlattices from the points of view of possible site symmetries and selection rules for optical transitions involving defects. All the above results are also valid for any pseudomorphic superlattice or quantum well made of two binary compounds with zinc-blende structures and identical cations or anions, such as in the GaN/AlN system.

Identifying open-volume defects in doped and undoped perovskite-typeLaCoO3,PbTiO3,andBaTiO3

Dopants, vacancies, and impurity-vacancy clusters have a substantial impact on the properties of perovskite-type metal oxides (general formula $\mathrm{AB}{\mathrm{O}}_{3}).$ In order to determine synthesis and processing conditions that optimize the desirable properties of these materials a careful study of these defects is required. It is essential to identify the defects and to map the defect densities. Positron annihilation spectroscopy has often been used to identify vacancy-type defects. Calculations of the positron lifetime and Doppler-broadened profiles of the positron-electron annihilation radiation in undoped and doped ${\mathrm{LaCoO}}_{3},$ ${\mathrm{PbTiO}}_{3},$ and ${\mathrm{BaTiO}}_{3}$ are reported, and compared with available experimental data. The results show that these positron techniques are excellent for studying open-volume defects, vacancy-impurity complexes, and for identifying the sublattice occupied by the dopants.

The structure of vacancy–impurity complexes in highly n-type Si

We show that the detailed atomic structure of vacancy–impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V–P and V–As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020cm−3. The defects are identified as monovacancies surrounded by three As atoms. The formation of V–As3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As doped Si.

Electron irradiation of heavily doped silicon: group-III impurity ion pairs

Defect annealing processes in near-degenerate p-Si subjected to electron irradiation at cryogenic temperatures are investigated. A marked difference in the annealing behavior of defects in p-Si doped with boron and gallium is observed. It has been found that this effect is primarily due to the different charge states and stability of interstitial impurity ions. If mobile these ions form interstitial ion-substitutional ion pairs stable up to T=600°C. The charge states and stability of impurity–vacancy complexes are also discussed.

Defects in NTD InP probed by positron annihilation spectroscopy

Two samples of high purity InP extracted from the same wafer were examined by positron annihilation spectrum analysis after having been, processed by means of thermal Neutron Transmutation Doping (NTD). Compared with the as grown sample with an average positron lifetime of 246 ps at 300 K, the high dose doped one has an average lifetime of 251 ps and the lower dose doped one 248 ps measured under the same condition, indicating that some defects have been introduced in the NTD process. Annealing experimental results show a steady decrease in the average lifetime with increasing annealing temperature up to 550°C. And a peak in lifetime curve around 500°C was observed which may be attributed to defects related structure conversion. Temperature experiments conducted on the low dose doped sample from 150K to 290 K suggest the existence of vacancy-impurity complex which have given rise to an abnormal reduction of average lifetime with increasing temperature. Also a n-type InP sample (A61) was irradiated with thermal neutrons in another reactor and the lifetime results display an increase of 15 ps. Furthermore, to study epithermal neutron irradiation effects on InP, measurements were performed on an n-type InP sample (N119) along with one p-type sample (P118) after having been irradiated with high fluence of epithermal neutrons. The former has an average lifetime of 262 ps and the latter 247 ps after irradiation. The results prove that on some occassion epithermal neutrons can produce sizable defects in InP.

Electron-irradiation-induced deep levels in n-type 6H–SiC

The fluence-dependent properties and the annealing behavior of electron-irradiation-induced deep levels in n-type 6H–SiC have been studied using deep-level transient spectroscopy (DLTS). Sample annealing reveals that the dominant DLTS signal at EC−0.36 eV (labeled as E1 by others) consists of two overlapping deep levels (labeled as ED3L and ED3H). The breakup temperature of the defect ED3L is about 700 °C. The ED3H center together with another deep level located at EC−0.44 eV (so-called E2) can withstand high-temperature annealing up to 1600 °C. It is argued that the involvement of the defect ED3L is the reason that various concentration ratios of E1/E2 were observed in the previous work. The revised value of the capture cross section of the deep-level ED3H has been measured after removing ED3L by annealing. A deep level found at EC−0.50 eV is identified as a vacancy–impurity complex since it was found to have a lower saturated concentration and weak thermal stability. Two other deep levels, EC−0.27 eV and EC−0.32 eV, which were not observed by others because of the carrier freeze-out effect, are also reported.

Defect energy levels in electron-irradiated and deuterium-implanted6Hsilicon carbide

Using deep-level transient spectroscopy, we studied defect energy levels and their annealing behavior in nitrogen-doped $6H\ensuremath{-}\mathrm{SiC}$ epitaxial layers irradiated with 2-MeV electrons and implanted with 300-KeV deuterium or hydrogen at room temperature. Five levels located at ${E}_{c}\ensuremath{-}0.34,$ ${E}_{c}\ensuremath{-}0.41,$ ${E}_{c}\ensuremath{-}0.51,$ ${E}_{c}\ensuremath{-}0.62,$ and ${E}_{c}\ensuremath{-}0.64\mathrm{eV}$ consistently appear in various samples grown by chemical vapor deposition, showing they are characteristic defects in n-type $6H\ensuremath{-}\mathrm{SiC}$ epitaxial layers. It is suggested that the ${E}_{c}\ensuremath{-}0.51\mathrm{eV}$ level originates from a carbon vacancy, and that the two levels at ${E}_{c}\ensuremath{-}0.34$ and ${E}_{c}\ensuremath{-}0.41\mathrm{eV},$ which likely arise from the occupation of inequivalent lattice sites, and the level at ${E}_{c}\ensuremath{-}0.51\mathrm{eV}$ are different charge states of the carbon vacancy. The annealing kinetics of the ${E}_{c}\ensuremath{-}0.51\mathrm{eV}$ level are first order with an activation energy of 1.45 eV, and a level at ${E}_{c}\ensuremath{-}0.87\mathrm{eV}$ growing upon its decay arises most likely from a vacancy-impurity complex. The results for the ${E}_{c}\ensuremath{-}0.62\mathrm{eV}$ and ${E}_{c}\ensuremath{-}0.64\mathrm{eV}$ levels are consistent with a defect model involving a silicon vacancy on inequivalent sites in the $6H$ lattice. Furthermore, the present results show that at hydrogen doses of ${10}^{11}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ no interaction between hydrogen and the irradiation-induced silicon vacancy takes place even after annealing at temperatures up to 800 \ifmmode^\circ\else\textdegree\fi{}C, in contrast to the results reported for n-type silicon.

Numerical calculations of the radiation-induced strain rate of interstitial solid solutions. II Allowance for the main channels for the influence of impurities on radiationinduced creep

A previously developed numerical method for calculating the radiation-induced creep rate [Yu. S. Pyatiletov and A. D. Lopuga, Tech. Phys. 45 (1999)] is used to study the influence of impurity atmospheres around dislocations and pores, impurity traps, and mobile impurity-vacancy and impurity-interstitial complexes on the radiation-induced strain rate of interstitial alloys. Quantitative data are obtained on the creep rate as a function of impurity concentration, and a physical interpretation allowing for the recombination of interstitial atoms and vacancies directly with one another, on impurity traps, and on mobile complexes is put forward.

Identification of Vacancy-Impurity Complexes in Highlyn-Type Si

We show that the detailed atomic structure of vacancy-impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V -P and V -As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020 cm23. The defects are identified as monovacancies surrounded by three As atoms. The formation of a V -As3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As-doped Si. [S0031-9007(99)08546-4]

First-Principles Calculations of Positron Annihilation in Solids

ABSTRACTWe present first-principles approaches based on density functional theory for calculating positron states and annihilation characteristics in condensed matter. The treatment of the electron-positron correlation effects (the enhancement of the electron density at the positron with respect to mean-field density) is shown to play a crucial role when calculating the annihilation rates. A generalized gradient approximation (GGA) takes the strong inhomogeneities of the electron density in the ion core region into account and reproduces well the experimental total annihilation rates (inverses of the positron lifetimes) by suppressing the rates given by a local density approximation (LDA). The GGA combined with an electron-state-dependent enhancement scheme gives a good description for the momentum distributions of the annihilating positron-electron pairs reproducing accurately the trends observed in the angular correlation (ACAR) or Doppler broadening measurements of the annihilation radiation. The combination of the present positron lifetime and momentum density calculations with the corresponding measurements yields a unique tool for defect identification. Especially, the investigation of various vacancy-type defects in semiconductors able to trap positrons will be an important field for these methods. We will show that the identification of vacancy-impurity complexes in highly n-Type Si and the study of the SiO2/Si interface are particularly interesting applications.

Low-temperature photoluminescence of n-InSe layer semiconductor crystals

Low-temperature photoluminescence spectra of n-InSe layered single crystals were studied in the temperature range 10–210 K. Photoluminescence of n-InSe showed peaks at 1.334, 1.306, 1.288, and 1.232 eV at 10 K. These four peaks were attributed to radiative recombination of the direct free excitons, an impurity-band transition, a donor–acceptor recombination channel, and the transition within an impurity-vacancy complex, respectively. To determine the temperature dependence of direct band gap of n-InSe, we estimated the exciton binding energy to be 15 meV by assuming an isotropic approximation for the anisotropy parameter γ = 1. From these peak positions and the estimated band gap, the donor and acceptor levels associated with these centers were estimated to be approximately 43 and 18 meV, respectively. The temperature variations of the peak energy and linewidth of the excitons in n-InSe were explained by taking into account both the exciton–acoustic-phonon and the exciton–optical-phonon interactions. Below ≈60 K, these variations are due mainly to exciton scattering by acoustic phonons via the deformation potential in InSe.