Thermal evolution of defects produced by implantation of H, D and He in Silicon

Despite decades of study, voids in silicon produced by implantation of H or He followed by annealing continue to be a topic of interest. There are two key applications: gettering of heavy metal impurities, and “ion cutting” used in silicon-on-insulator fabrication. Positron annihilation is one of the few techniques that can probe the vacancies and vacancy clusters that are the precursors to void formation. Data from recent studies will be discussed, including (I) isotopic substitution, in which comparisons of H vs. D implantation permit examination of the impact of primary point defects vs. chemical effects. Remarkable differences exist between H and D in blistering of silicon - ion doses 2-3 times higher are required for blistering with D than with H, despite a higher rate of primary defect production for D; (II) the effect of annealing temperature ramp-rate, in which we show that ramp-rate has a significant impact on residual defects, despite which it is so disregarded as to often be omitted from published reports; and (III) comparisons with electron microscopy which suggest that positron annihilation can be insensitive to large voids. In these studies, positron annihilation augments data from techniques including ion channelling, Raman scattering and electron microscopy; the suite of techniques allows elucidation of the interplay between implanted impurities and the vacancies and interstitials created by implantation.     

Positron annihilation in vacancies at grain boundaries in metals

The behavior of vacancies in selected coherent grain boundaries (GBs) in Fe and Ni is studied by means of molecular dynamics simulations. Corresponding positron lifetimes are calculated using the atomic superposition method. There is a difference between the vacancy behavior in Fe and Ni in dependence on temperature. In Ni, vacancies at GBs appear to diminish substantially their free volume (and lifetime) with the increasing temperature, which can be attributed to ‘vacancy delocalization’. Contrary, GB vacancies remain stable up to apparently higher temperatures in Fe.     

Atomic structure of pre‐Guinier–Preston zones in Al alloys

We propose X‐ray absorption (XAS) measurements as a novel approach to determine the atomic structure of pre‐Guinier–Preston zones. These nano‐clusters are formed during very early stages of aging AlCu alloys, immediately after solution heat treatment and quenching. X‐ray absorption near‐edge structure (XANES) spectra were taken from technical aluminum alloys at the copper K edge, revealing the local atomic environment of copper. The spectra of – after solution heat treatment – freshly quenched and of artificially aged alloys differ significantly from each other. We compare the measured XANES spectra with those calculated by the FEFF‐8 code. We show the importance of employing realistic, i.e. relaxed, atomic coordinates around the scattering atom type from ab‐initio calculations with SIESTA. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Microstructural characterisation of a commercial Al–Cu–Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy

Transmission electron microscopy and positron annihilation spectroscopy techniques were used to characterise the microstructure of the 2024 Al–Cu–Mg commercial alloy artificially aged with and without pre-deformation. In non-deformed samples, a very dense dispersion of small, needle-shaped particles, with mean size in the order of the nanometres, was observed homogeneously distributed in the matrix, together with a coarse distribution of S-phase precipitates. In pre-deformed samples, the needle-shape particles were not seen, only a high density of S-phase precipitates nucleated on dislocations. The needle-shaped particles were identified as solute-rich Guinier–Preston–Bagaryatski (GPB) zones by combining coincidence Doppler broadening positron annihilation measurements with TEM imaging. The relationship between the microstructure and measurements of hardness and positron lifetime evolution during artificial ageing is also discussed.     

Application of positron annihilation spectroscopy on the ion implantation damaged Fe-Cr alloys

Four different Fe-Cr binary alloys with Cr content 2.5-11 wt% were studied in details using various methods. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were applied to obtain basic information, required for standard positron annihilation lifetime spectroscopy (PALS) spectra analysis. Additionally, PALS measurements were performed on as-received state as well as on helium implanted specimens. The He implantation was proposed for simulation of radiation damage and obtain high doses even in near surface areas (up to 1 μm). The implantation was based on the SRIM code simulation and next DPA calculations. Final concentration of vacancy type defects were calculated for 250 keV He²⁺ beam and the maximum was determined in 600 μm depth. Such specimens are very suitable for positron beam study of vacancy type defect mobility as a result of thermal treatment, which will be performed simultaneously in the future.     

Positron annihilation studies on the migration of deformation induced vacancies in stainless steel AISI 316 L

Positron lifetime measurements were carried out at room temperature before and after isochronous annealing of cylindrical, machined fatigue specimens and of round slabs of austenitic stainless steel AISI 316 L deformed in compression. Annealing experiments are evaluated in terms of vacancy migration and sinking to grain boundaries and dislocations. The model assumes spherical grains with a homogeneous initial distribution of vacancies. A vacancy migration enthalpy of H^M _V=(0.9±0.15) eV was found. It is concluded that positron trapping at dislocation lines does not significantly contribute to positron lifetime measurements at room temperature and that single vacancies are the dominating positron traps. Positron annihilation depth profiling on cross-sectional areas prepared from machined specimens using a positron microprobe with 10 μm spatial resolution shows that machining of cylindrical specimens creates vacancies up to 5 mm below the surface. Received: 11 August 2000 / Accepted: 13 November 2000 / Published online: 28 February 2001     

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al₂CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.     

Precursor effects of the Rhombohedral-to-Cubic Phase Transition in Indium Selenide

We report on the observation of precursor effects of the rhombohedral-to-cubic phase transition in Indium Selenide (InSe) with several experimental techniques. The pressure at which these precursor defects are first observed depends on the sensitivity of the experimental technique. In transport measurements, which are very sensitive to low defect concentrations, precursor effects are observed 5 to 6 GPa below the phase transition pressure whereas in X-ray diffraction measurements precursor effects are only observed 2 GPa below the phase transition pressure. We report optical absorption measurements, in which the precursor effects are shown by the growth and propagation of dark linear defects appearing 3 GPa below the phase transition pressure. On the base of a simple model of the stress field around edge dislocations, we attribute the darkening of the InSe samples to local phase transitions to a high-pressure modification along linear dislocations. These results agree with room-pressure and high-pressure Raman spectra of samples compressed up to 7-8 GPa, which show new phonon lines not corresponding to the low-pressure phase.     

The role of quenched‐in vacancies for the decomposition of aluminium alloys

The very early stages of decomposition during room temperature storage, i.e. just a few minutes after quenching, are investigated by positron annihilation lifetime spectroscopy for both an AlMgSi alloy and an AlCuMg alloy. It turns out that by freezing the decomposition kinetics during measurements we can detect vacancy–solute atom pairs. The formation of larger solute clusters with structural vacancies is seen by an increase of the mean positron lifetime in the course of storage at room temperature (RT). Earlier findings concerning aging at RT were unable to discover this effect. The detected changes are interpreted in terms of cluster formation. Thus we show that positron annihilation spectroscopy (PAS) is one of the very few methods to access early stages of decomposition in metallic alloys. Moreover, the lower limit of the concentration of quenched‐in vacancy‐like defects is calculated to be at least 2 × 10^–5 per atom. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Status of an R&D project of a positron gun at “Horia Hulubei” NIPNE Bucharest

A new positron gun (PG) will enable high sensitivity measurements in applications of positron annihilation spectroscopy in Romania. Some data concerning the design of a modular system for focussing, transport and acceleration of mono-energetic positrons in the range 0.8-50 keV have been obtained and experimenting on moderators and CDBS was performed. We present a short overview of the present status of the project and preliminary results from Coincidence Doppler Broadening Spectroscopy with a ²²NaCl source, on Al samples. The entire positron gun system will be designed as a high-vacuum dedicated extension operating with two options: a 50 mCi ²²NaCl source and in-line with the NIPNE cyclotron or a new intense compact cyclotron.     

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Structural and Magnetic Properties of Fe-Doped Anatase TiO2 Films Annealed in Vacuum

Structural and magnetic properties of Fe-doped anatase TiO2 films fabricated by sol-gel spin coating are investigated. X-ray diffraction measurements reveal that Fe^3＋ ions are incorporated into the TiO2 lattice. No ferromagnetism-related secondary phases and magnetic nanopaxticles are observed in the films. The presence of electron paramagnetic resonance signals at 9- 2.0 supports oxygen vacancies and/or defects generated in the films after annealing in vacuum. Magnetic measurements indicate that Fe-doped anatase TiO2 films are ferromagnetic at room temperature. These observations suggest that oxygen vacancies and/or defects axe energetically favorable for the long range Fe^3＋-Fe^3＋ ferromagnetic coupling in Fe-doped anatase TiO2 films.     

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1?nm.     

Effect of high energy ion irradiation on silicon substrate in a pulsed plasma device

We have performed an experimental analysis on the investigation of high energy ion beam irradiation on Si(1 0 0) substrates at room temperature using a low energy plasma focus (PF) device operating in methane gas. The surface modifications induced by the ion beams are characterized using standard surface science diagnostic tools, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), photothermal beam deflection, energy-dispersive X-ray (EDX) analysis and atomic force microscope (AFM) and the results are reported. In particular, it has been found that with silicon targets, the application of PF carbon ion beams results in the formation of a surface layer of hexagonal (6H) silicon carbide, with embedded self-organized step/terrace structures.     

First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

Pressure effects on EXAFS Debye-Waller factor and melting curve of solid krypton

The pressure effects on atomic mean-square displacement, extended X-ray absorption fine structure (EXAFS) Debye-Waller factor and melting temperature of solid krypton have been investigated in within the statistical moment method scheme in quantum statistical mechanics. By assuming the interaction between atoms can be described by Buckingham potential, we performed the numerical calculations for krypton up to pressure 120?GPa. Our calculations show that the atomic mean-square displacement and EXAFS Debye-Waller factor of krypton crystal depend strongly on pressure. They make the robust reduction of the EXAFS peak height. Our results are in good and reasonable agreements with available experimental data. This approach gives us a relatively simple method for qualitatively calculating high-pressure thermo-physical properties of materials. Moreover, it can be used to verify future high-pressure experimental and theoretical works.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

The first-principle calculation of structures and defect energies in tetragonal PbTiO3

The first-principle calculation had been adopted to investigate various neutral vacancies and vacancy pairs under seven thermodynamic conditions in bulk PbTiO₃. The electronic structures, atomic relaxations, and formation energies of vacancies were obtained. Depending on the thermodynamic condition, the main and stable defects are different. It was found that V_O is the main defect under the reducing condition, whereas V_Pb becomes dominating under the oxidizing condition. The Pb-O vacancy pair forms more easily than the isolated vacancies under certain thermodynamic condition. Due to the introducing of vacancies, the acceptorlike levels and donorlike levels appear in the cases of the cation and anion vacancies, respectively.     

Surface chemistry study of Mn-doped germanium nanowires

Single crystal germanium nanowires have been grown by vapour-liquid-solid deposition onto silicon oxide substrates with Au catalyst nanoparticles. They have been doped by two different techniques: Ge and Mn co-evaporation during growth and post-growth Mn implantation. Scanning electron microscopy images show that Mn-implanted nanowires have a lower surface density and a smaller average diameter (18.8 nm) than the un-doped ones and those Mn doped by co-deposition. The effectiveness of Mn doping has been verified by X-ray photoemission spectroscopy and by energy-dispersive X-ray measurements, indicating in the two cases significant Mn atomic concentration in the nanowire. X-ray diffraction indicates that the nanowires are single crystals and that they do not contain precipitates of Mn extrinsic phases. Both SEM and XPS experimental evidences are in line to indicate that the Mn doping by ion implantation is preferable with respect to that one performed by co-evaporation as it reduces the thickness of the outer oxide sheath of the nanowires and their diameter.     

Atomic-registry-dependent electronic structures of sulfur vacancies in ReS2 studied by scanning tunneling microscopy/spectroscopy

Rhenium disulfide (ReS₂) is regarded as a promising candidate for optoelectronic applications (e.g., infrared photodetector), as it maintains a direct bandgap regardless of the number of layers unlike other typical transition metal dichalcogenides. Therefore, it is very important to understand and control the defects of ReS₂ for enhancing the performance of photodevices. In this work, we studied the electronic structures of ReS₂ affected by sulfur vacancies of different atomic registries at the atomic scale. The atomic and electronic structures of the mechanically exfoliated ReS₂ flakes were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), and were confirmed using density functional theory (DFT) calculations. The atomic structural models indicate four distinguishable atomic registries of sulfur vacancies on one face of ReS₂. Energetically, these atomic vacancies prefer to locate on the bottom side of the top monolayer of ReS₂ flakes. Only two among four possible kinds of vacancies could be observed using STM and STS, and they were identified using additional DFT calculations. We believe that our results regarding the identification of the defects and understanding the corresponding effects for electronic structures will provide important insights to enhance the performances of ReS₂-based optoelectronic devices in the future.