Positron trapping in electron-irradiated silicon crystals

The positron lifetime in electron-irradiated undoped and doped silicon crystals is studied as a function of temperature between 90 and 300 K. We show that the temperature dependence of the two lifetime components does not arise from the escape, but from the trapping rate at defects. The temperature dependences of the capture cross sections are deduced. It is concluded that in undoped crystals the positrons interact with negatively charged and neutral defects, probably divacancies and vacancy-oxygen complexes, respectively. In strongly P-doped crystals positron trapping occurs preferably in negatively charged centers.     

Illumination induced annealing in electron-irradiated p-type silicon

Illumination with white light during isochronal annealing in p-type Si irradiated with 2 MeV electrons at 77 K introduces stages of recovery at around 85 and 140 K. These stages recover about 90% of carriers removed by the irradiation.     

Li-defect interactions in electron-irradiated n-type silicon by EPR measurements

Single crystal silicon, both with and without oxygen, has been diffused with lithium to concentrations ～10¹⁷/cm², irradiated with 1 to 1.5 MeV electrons, and the ensuing defects studies by EPR measurements. The presence of oxygen strongly affects the properties of these defects. Measurements have indicated the presence of two new defects which involve Li-one in O-containing material and one in O-free material. The defects are observed in their electron-filled state, and indicate a net electron spinof ½. The defect spectra disappear (with time) at room temperature, and can be explained by the formation of other Li-involved defects which lie deeper in the energy bandgap and are not visible by EPR. Electron irradiation at 40 °K followed by annealing at higher temperatures show that both EPR defects described above begin to form at about 200 °K and begin to decrease at about 275 °K-just as does the 250 °K reverse annealing observed generally for n-type Si. Based on these data, and the work of others, it is suggested that both defects form as a result of the motion of Si interstitials which produce a (Li-O-interstitial) complex in O-containing Si, and a (Li-interstitial) complex in O-free Si.     

The kinetics of formation and the parameters of radiation defect clusters in silicon

An approximate analytic solution of the system of equations describing the kinetics of formation and the parameters of radiation defect clusters in crystals is obtained. A model corresponding to the real parameters of Si is assumed as a basis for calculation and it is shown that two types of secondary radiation defect clusters may be realized corresponding to the congealing and spreading of the initial vacancy clusters. The cluster type depends on the incident energy of the particle which creates the effects and on the physical parameters of the crystal under irradiation. For a congealing cluster most of the initial vacancies react within the original volume of the damage cascade and for a spreading cluster the final concentration of divacancies and A-centres within the original cascade volume is much less than the initial vacancy concentration, i.e. most of the vacancies form divacancies and A-centres dispersed throughout the crystal volume.

The definition of the concept “threshold energy of cluster formation” as the minimum primarily displaced atom energy for the creation of a “congealing” vacancy cluster is proposed.

It is shown that A-centres form a belt surrounding a central divacancy cluster.     

Temperature characteristics of positron trapping at defects in electron-irradiated silicon

Positron-annihilation lifetime and Doppler-broadening measurements are used to investigate defects in silicon irradiated at 373 K with 6 MeV electrons to a dose of 1×l0¹⁹e/cm². In the unirradiated silicon sample (p type) a temperature-independent behaviour of the bulk-lifetime is observed in the temperature interval 110–500 K with a constant value of 220±1 ps. The slight effect observed on the S-parameter evolution is explained taking into account the thermal expansion of the lattice. The lifetime results obtained at 80 K and at 300 K after isochronal annealing as well as the behaviour of the intensity of the second lifetime componentI ₂ during lifetime measurements below the irradiation temperature in the irradiated silicon sample (n type), clearly indicate the temperature dependent characteristics of the positron trapping cross section _t(T) T ⁿ withn= –1.905±0.016. From isochronal annealing results, an annealing stage is observed in which di-vacancies agglomerate into quadri-vacancies. The mean positron lifetime in those quadri-vacancies is 350 ps.A.B.O.S., on leave from University of Kinshasa, Zaïre     

Radiation friction in fractal clusters

The classical problem of radiation damping is formulated for fractal clusters in terms of interference of radiation friction forces within a small number of aggregated particles, with their interactions being described in the dipole-dipole approximation. The general theory is developed for spherical particles with arbitrary sizes. For particular configurations, the interference nature of the radiation widths of the normal dipole oscillations of aggregates is demonstrated.     

Cubic defect clusters in fcc metals

The number of cubic defects in In-implanted copper, silver and platinum has been determined by decorating the cubic defects with He atoms. For this, the samples were post-implanted with He⁺ ions at subthreshold energies. Our results, obtained by DPAC measurements on¹¹¹In, are compared with data from Mössbauer spectroscopy and channeling experiments.     

An EPR study of optical absorption of the oxygen-vacancy pair in electron-irradiated silicon

The negative charge state of the vacancy-oxygen pair (Si-B1) in irradiated silicon was populated by illumination with polarized light, from which the direction of the electric dipole moment was determined to be near 〈110〉 perpendicular to the (Si-O-Si) bond axis. Energy dependence of the alignment suggests the presence of an optical absorption band at λ = 1.30 μm.     

Radiation breakdown in silicon wafers

Radiation breakdown in silicon slabs is observed and studied as revealed in anomalous behavior of the dose characteristics of their radiation defects when the radiative intensity is varied. A theory is constructed for reversible radiation breakdown due to the bistability which develops in a gas of radiation vacancies when the gas can be regarded as quasi-two-dimensional. In order to explain the exponential saturation of the dose characteristics as the irradiation intensity is increased, scenarios are proposed in which different forms of the constituent radiation defects develop. Some parameters of the bistable gas of primary vacancies are estimated, including diffusion coefficients, dimensions of inhomogeneity regions, and the rate of movement of the stratification line. On the whole, satisfactory agreement with experiment is obtained. Discrepancies between the diffusion coefficient for neutral vacancies obtained here and in the literature are attributed to the role of interband recombination accompanying radiation defect formation during electron bombardment. Zh. éksp. Teor. Fiz. 114, 1067–1078 (September 1998)     

An orientation-dependent defect in ion-implanted silicon

A new EPR spectrum is resolved in the N⁺-implanted silicon, and this center can be produced only by the 〈110〉 channeling ions in the region underneath the amorphous layer.     

Annealing of defect clusters in irradiated tungsten

Transmission electron microscopy has been used to study the thermal annealing of radiation induced defect clusters in tungsten irradiated at reactor ambient tempeaature, ～ 70°C. Annealing, consisting of the growth and eventual elimination of the clusters, is observed to occur in two stages, one at temperatures below 435°C and one at temperatures above 740°C, which are interpreted as being due to interstitial and vacancy migration, respectively. The changes in defect cluster size and density are paralleled by similar changes in the measured strength of the material, and can be quantitatively correlated with the shear stress on the basis of a model of dislocation movement through a random array of obstacles.     

Topological defect motifs in two-dimensional Coulomb clusters

We study the distribution of topological defects in two-dimensional Coulomb clusters with parabolic lateral confinement. The minima hopping algorithm based on molecular dynamics is used to efficiently locate the ground-?and low-energy metastable states, and their structure is analysed by means of the Delaunay triangulation. The size, structure and distribution of geometry-induced lattice imperfections strongly depends on the system size and the energetic state. Besides isolated disclinations and dislocations, classification of defect motifs includes defect compounds-grain boundaries, rosette defects, vacancies and interstitial particles. Proliferation of defects in metastable configurations destroys the orientational order of the Wigner lattice.