Tomographic study of the three-dimensional distribution of a high-fluence implant in a polymer

6 Li⁺ ions were implanted into PMMA at high flux up to fluences of 1×10¹⁵ cm^-2 under angles of 0° to 70° towards the surface normal. The Li depth distributions were determined by means of neutron depth profiling, and compared with theoretical simulations. The three-dimensional Li distribution was reconstructed from the one-dimensional depth profiles by means of a tomographic technique. It turned out that the measured Li depth distributions can be described by a superposition of Gaussian and exponential functions. This points at considerable Li mobility during or after the ion implantation, with trapping in unsaturable traps in the ion-irradiated region which roughly follow the electronic energy transfer distribution. The Li redistribution is more pronounced along the track direction than transversely to it. The normalized Li distributions in various implantation directions were fed into our tomographic program to reconstruct the three-dimensional distribution of the deposited lithium. As expected, the lithium preferentially distributes along the ion tracks. This work is another hint that mobility of implanted ions in solids does not proceed isotropically, but is strongly influenced by the radiation-damage distributions. Received: 11 May 1998 / Accepted: 9 September 1998 / Published online: 24 February 1999     

Incubation of laser ablation in fused silica with 5-fs pulses

The threshold fluences for laser-induced damage of fused silica with single 5-fs pulses from a Ti:sapphire laser system were determined by extrapolating the ablated volume to zero. These thresholds are about 4 times as high as the values previously obtained from multi-shot experiments. This result is interpreted in terms of an irreversible modification of the original material below the single-shot threshold (incubation). Received: 14 June 1999 / Accepted: 24 June 1999 / Published online: 8 September 1999     

Diffusion of hydrogen in heterogeneous systems

The effective long-range long-time tracer diffusivity D_eff for interstitial diffusion of hydrogen through heterogeneous systems was studied theoretically for model systems consisting of isolated grains of material G embedded in a matrix of material M. Different solubilities of hydrogen in these two materials as well as different diffusivities are allowed for. Additionally, modified diffusion barriers at the phase boundaries were included in the diffusion model. The effect of different sizes, arrangements, and forms of the grains was also considered. D_eff was determined by Monte Carlo (MC) simulations on simple lattice models of the systems described above. An equilibrium distribution of hydrogen atoms among the two constituent materials was assumed. Our main interest was focused on whether and how D_eff may be related to mesoscopic or macroscopic quantities characterizing the heterogeneous system and its constituent materials, such as the volume fractions of the two materials, the fraction of lattice sites in the immediate vicinity of the phase boundary, the hydrogen concentrations c_G and c_M in the grains and in the matrix and the respective hydrogen diffusivities D_G(c_G) and D_M(c_M). In order to obtain good estimates for these relations in terms of analytic formulas, we attempted to model a heterogeneous system by a network of diffusion elements connected in series and in parallel, in analogy to an electric network. The properties of the basic connections, in parallel and in series, were studied on layered structures, for which analytic expressions for D_eff could be derived. The network formulas for different grain–matrix systems were tested by comparing with results of MC simulations. In general, the network formulas describe the corresponding MC results for D_eff fairly well. It was found that differences in the hydrogen solubilities in the two phases as well as modified energy barriers at the phase boundaries may have dramatic effects on D_eff. Received: 19 September 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Ab initio study of formation, migration and binding properties of helium-vacancy clusters in aluminum

Ab initio calculations based on density functional theory have been performed to study the dissolution and migration of helium, and the stability of small helium-vacancy clusters He_nV_m (n, m=0-4) in aluminum. The results indicate that the octahedral configuration is more stable than the tetrahedral. Interstitial helium atoms are predicted to have attractive interactions and jump between two octahedral sites via an intermediate tetrahedral site with low migration energy. The binding energies of an interstitial He atom and an isolated vacancy to a He_nV_m cluster are also obtained from the calculated formation energies of the clusters. We find that the di- and tri-vacancy clusters are not stable, but He atoms can increase the stability of vacancy clusters.     

Carbon precipitation and diffusion in SiGeC alloys under silicon self-interstitial injection

In this work we investigate the diffusion and precipitation of supersaturated substitutional carbon in 200-nm-thick SiGeC layers buried under a silicon cap layer of 40 nm. The samples were annealed in either inert (N₂) or oxidizing (O₂) ambient at 850 °C for times ranging from 2 to 10 h. The silicon self-interstitial (I) flux coming from the surface under oxidation enhances the C diffusion with respect to the N₂-annealed samples. In the early stages of the oxidation process, the loss of C from the SiGeC layer by diffusion across the layer/cap interface dominates. This phenomenon saturates after an initial period (2–4 h), which depends on the C concentration. This saturation is due to the formation and growth of C-containing precipitates that are promoted by the I injection and act as a sink for mobile C atoms. The influence of carbon concentration on the competition between precipitation and diffusion is discussed. Received: 19 October 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002     

Study of range distribution parameters for bismuth-ion implantation in silver gallium diselenide

Range distributions for bismuth ions implanted in AgGaSe₂ in the energy range 80–300 keV were investigated by using 2.1-MeV He²⁺ Rutherford backscattering spectrometry (RBS). A convolution calculation method was used to extract the true distributions of bismuth from the measured RBS spectra. The range distribution parameters, R_p and ΔR_p, were obtained and compared with those obtained from Monte Carlo simulation. The experimental R_p values agree with the Monte Carlo simulation values very well, but the experimental ΔR_p values are systematically larger than those from the theoretical simulation. Received: 28 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-531/856-5167, E-mail: xdliu@sdu.edu.cn     

Slow positron annihilation studies of vacancy-type defects in the near-surface region of Cu and Nb before and after wear

Received: 22 June 1998 / Accepted: 9 November 1998     

The stability of homogeneous microstructure development under cascade-damage conditions

The development of microstructure, under cascade-damage conditions, in regions far away from any major sink is considered. Within the mean-field theory, a homogeneous distribution of point defects and their clusters is a pre-imposed artificial constraint on the kinetic system. The resulting excessive recombination of the vacancies and interstitials at a high density of accumulated point-defect clusters dictates a low rate of void growth. Considerations beyond the mean-field theory, by taking into account the concentration fluctuations of both the point defects and their clusters, relax the restriction of the homogeneous distribution. In this paper, we consider a system without pre-existing sinks, except the void nuclei, in which vacancies, interstitials and their clusters are continuously produced. Taking into account the mobility of small clusters and the stochastic fluctuations of the point-defect fluxes, a kinetic theory is formulated from first principles. It is rigorously shown that through the stochastic fluctuations, and the positive-feedback action of the mobility of the small clusters on the interstitial concentration, the homogeneous interstitial distribution is unstable at temperatures above stage V, leading to the formation of a spatially heterogeneous microstructure in pure metals at low irradiation doses. The characteristics of the microstructure evolution and void swelling, predicted from the theory, are found to be in good agreement with the experimental results. Received: 17 March 2000 / Accepted: 17 October 2000 / Published online: 25 July 2001     

Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

2 O₃ and nanocrystalline Al₂O₃ specimens. The short-lifetime (170±20 ps), intermediate-lifetime (410±20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1–5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes τ₁=170 ps and τ₂=410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al₂O₃ prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth. Received: 1 April 1997/Accepted: 13 August 1997     

Useful vacancies: Positron beam interrogation of fluorine-vacancy complexes in semiconductor device structures

The formation, migration and agglomeration in silicon of fluorine-vacancy complexes have been monitored by single-detector Doppler broadening spectroscopy. After electronics engineers found that fluorine ion implantation effectively eliminated the transient-enhanced diffusion of dopants in the creation of ultra-shallow junctions, a vital step in the further miniaturization of device structures, positron beams have played a pivotal role in providing an insight into the mechanisms underlying this phenomenon, being able to detect FV complexes in implanted and annealed samples. Secondary Ion Mass Spectrometry has provided complementary information on fluorine concentrations so that the nature of the F_mV_n complexes can be further assessed. New results on Si and SiGe structures are presented.     

Silicon vacancies in 3C-SiC observed by positron lifetime and electron spin resonance

54 , 2512 (1996)]. The trapping coefficient of single-negative silicon vacancies was also derived. Received: 25 August 1997/Accepted: 27 March 1998     

Defect structure and optical damage resistance of Zn:Fe:LiNbO<Subscript>3</Subscript> crystals

A series of Zn:Fe:LiNbO₃ crystals were prepared by the Czochralski technique with 0.015 wt. % Fe₂O₃ content and various concentrations of ZnO. The ultraviolet-visible absorption spectra and the infrared absorption spectra of the Zn:Fe:LiNbO₃ crystals were detected in order to investigate their defect structure. Their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases remarkably when the concentration of ZnO is over its threshold concentration (more than 6.0 mol. %). The effects of defects on the optical damage resistance of the Zn:Fe:LiNbO₃ crystals are discussed in detail. Received: 25 October 2002 / Revised version: 6 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-451/2300-926, E-mail: zzxxhhdoctor@sina.com     

On the sensitivity limit of positron annihilation: detection of vacancies in as-grown silicon

14 cm^-3 in FZ-Si was obtained. Received: 2 June 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

MeV Al+ and Al2+ ions implantation in Si(100): surface roughness and defects in the bulk

This paper deals with the implantation of high-energy (1.0–3.0 MeV) atomic and molecular Al⁺ ions in Si(100) to a fluence of 5×10¹⁴ Al atoms/cm² at room temperature. The molecular effect, i.e. the increase of the displacement yield compared with the sum of the atomic yields, and the damage formation as well as defect behaviour after annealing have been investigated. A detailed experimental study has been made of the evolution of extended secondary defects which form during thermal anneals of Al⁺ or Al₂ ⁺ irradiated silicon. The samples have been examined using combined Rutherford backscattering and channeling experiments together with transmission electron microscopy observations. The surface structure of the implanted wafers has been measured by atomic force microscopy. The results for the implantation-induced roughness at the Si surface, resulting from Al⁺ or Al₂ ⁺ irradiation at the same energy/atom, total atomic fluence, flux rate, and irradiation temperature, are presented and discussed. Received: 19 August 1999 / Accepted: 20 October 1999 / Published online: 23 February 2000     

Spatial stability of a homogeneous system of voids under cascade damage

We consider the microstructural evolution of a system consisting of voids and sessile interstitial clusters continuously produced by irradiation. The stability of the evolution is rigorously analyzed, assuming spatial homogeneity as represented by a mean-field formulation. It is found that, in fully annealed pure metals with a sufficiently low void growth rate, the development of a spatially homogeneous microstructure is unstable. In fact, heterogeneous development already takes place during the void nucleation stage. The characteristic scale of the developing heterogeneity and its temperature dependence are obtained from the analysis, and are found to be consistent with experimental observations. Received: 21 March 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Synthesis of nano-sized SiC precipitates in Si by simultaneous dual-beam implantation of C+ and Si+ ions

Nanometer-sized SiC precipitates were synthesized in situ in Si by simultaneous implantation of two ion beams of C⁺ and Si⁺ ions. The results of simultaneous dual-beam implantation are compared with those of sequential dual-beam ion implantation and of single-beam C⁺ ion implantation. Remarkable differences are observed regarding the content and the crystal quality of SiC precipitates as well as the defect structure of the Si substrate. The SiC precipitation during dual-beam synthesis is found to depend on the ion energy of the second beam and on the implantation mode, simultaneous or sequential. For suitable implantation conditions, simultaneous dual-beam synthesis can improve the in situ SiC formation in comparison to the single-beam synthesis. A higher density of SiC precipitates with better crystal quality was observed, whereas their size was not changed. The second ion beam enables a shift in the dynamic equilibrium of constructive and destructive processes for SiC formation. A model is proposed assuming that SiC precipitation preferentially proceeds in regions with vacancy defects. The implantation process itself creates vacancy-dominated and also interstitial-dominated regions. The balance of the local point-defect composition is shifted under the second ion beam. In this way, the conditions for SiC precipitation can be modified. Received: 18 February 2002 / Accepted: 17 May 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: koegler@fz-rossendorf.de     

IR pulsed laser light interaction with soiled cellulose and paper

Nd:YAG laser irradiation (1064 nm) of cellulose samples does not lead to immediate nor long-term effects on mechanical properties of paper, which renders the method increasingly interesting for cleaning of historical paper artefacts. However, the technique’s usability is so far limited due to discoloration when the treated object’s surface contains carbonaceous dirt. By using diffuse-reflectance Fourier-transform infrared spectroscopy, size-exclusion chromatography and viscometry it is demonstrated that the distinct yellowing is accompanied by formation of ether cross-links and dehydration of the cellulose, as well as its depolymerisation. Furthermore, the origin of the discoloration is discussed and it is proposed that yellow chromophores are formed due to carbon–cellulose interactions during laser irradiation. Received: 10 September 2001 / Accepted: 21 January 2002 / Published online: 3 June 2002     

Photocontrolled mechanical phenomena in photochromic doped polymeric systems

This work examines volume changes at the sub-micron scale, induced photochemically in polymeric matrices doped with photochromic molecules. To achieve this, spiropyran is employed as a photochromic molecule embedded in polyethylmethacrylate-co-methylacrylate (PEMMA) matrices. Spiropyran can be reversibly interconverted to merocyanine, its coloured isomer, by irradiation at 248 nm and 532 nm, correspondingly. It is demonstrated that the interconversion between the two forms activates volume changes in the polymer matrix. To this end, off-axis reflection holographic interferometry is employed as a sensitive probe of the induced volume changes. This scheme provides a novel method for controlling sub-micron volume changes reversibly, as required in several microactuator designs. Received: 17 April 2002 / Accepted: 18 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +30-810/391-318, E-mail: nassia@iesl.forth.gr     

Precision laser ablation of dielectrics in the 10-fs regime

Received: 4 January 1999 / Accepted: 5 January 1999     

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops. Received: 16 November 2001 / Accepted: 12 June 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: yinlw@sdu.edu.cn