A. P. Knights

Impurity gettering to secondary defects created by MeV ion implantation in silicon

Impurities in MeV-implanted and annealed silicon may be trapped at interstitial defects near the projected ion range, Rp, and also at vacancy-related defects at approximately Rp/2. We have investigated the temperature dependence of impurity trapping at these secondary defects, which were preformed by annealing at 900u2009°C. The binding energies of Fe, Ni, and Cu are greater at the vacancy-related defects than at extrinsic dislocation loops. During subsequent processing at temperatures up to 900u2009°C, the amount of these impurities trapped at Rp/2 increases with decreasing temperature while the amount trapped at Rp decreases, with most of the trapped metals located at Rp/2 in samples processed at temperatures ≲ 700u2009°C. However, intrinsic oxygen is trapped at both types of defects; this appears to have little effect on the trapping of metallic impurities at extrinsic dislocations, but may inhibit or completely suppress the trapping at vacancy-related defects.

Defect structure of carbon rich a-SiC:H films and the influence of gas and heat treatments

A comprehensive study of carbon rich a-SiC:H films using optical absorption measurements, Fourier transform infrared spectroscopy, thermal desorption measurements, atomic force microscopy, and positron lifetime and Doppler-broadening techniques suggests that open volumes are formed in the films, due to incomplete breaking of the source molecule during film deposition. These open volumes are interconnected and can effectively trap gases from the ambient, during the film growth or after deposition. With increasing temperature the gases are desorbed from the internal surfaces of these open volumes and are released from the sample. This increases the areal density of the defects and is observable in positronium formation and annihilations of positrons with surface electrons. The growth of a nanocrystalline structure is observable upon annealing. At sufficiently high temperatures thermal breaking of Si–H and C–H bonds occurs and results in irreversible structural changes and film densification due to new C–C b...

Si ion implantation‐induced damage in fused silica probed by variable‐energy positrons

Samples of synthetic fused silica have been implanted at room temperature with silicon ions of energy 1.5 MeV. Fluences ranged from 1011 to 1013 cm−2. Samples were probed using variable‐energy positron annihilation spectroscopy. The Doppler‐broadening S parameter corresponding to the implanted region decreased with increasing fluence and saturated at a fluence of 1013 cm−2. It is shown that the decrease in the S parameter is due to the suppression of positronium (Ps) which is formed in the preimplanted material, due to the competing process of implantation‐induced trapping of positrons. In order to satisfactorily model the positron data it was necessary to account for positron trapping due to defects created by both electronic and nuclear stopping of the implanted ions. Annealing of the 1013 cm−2 sample resulted in measurable recovery of the preimplanted S parameter spectrum at 350u2009°C and complete recovery to the preimplanted condition at 600u2009°C. Volume compaction was also observed after implantation. Upon annealing, the compaction was seen to decrease by 75%.

Enhanced depth resolution in positron analysis of ion irradiated SiO2 films

We report the observation of defects following 1.7 MeV Si+ through-implantation of SiO2 films thermally grown on Si(100) substrates. Films were irradiated to fluences of 1012,u200a1013, and 1014u2009ions/cm2, and analyzed using a variable energy positron beam. Enhanced depth resolution was achieved via iterative chemical etching and measurement, for the sample irradiated to 1012u2009Si+/cm2, and for an unirradiated control sample. The positron annihilation “S parameter” is reduced in the implanted SiO2, due to a reduction in the formation of positronium (the atom-like positron–electron bound pair), and is increased in the Si substrate, due to the creation of vacancy-type defects. The concentration of implant-induced defects was found to be very nearly constant as a function of depth in the SiO2 film, indicating a significant contribution of both the electronic and nuclear stopping of the implanted Si+ ions to the production of positron-trapping defects. We find that the maximum overlayer thickness which allows simult...

Assessment of the normalization procedure used for interlaboratory comparisons of positron beam measurements

Variable-energy positron annihilation data from ion implanted and unirradiated Si and SiO2 were obtained at five separate laboratories. Line-shape analysis of the 511 keV annihilation γ rays yielded normalized S parameter signatures for radiation defect distributions in both types of samples. Laboratory-to-laboratory variations are found which, although small, lie outside the expected range of reproducibility. Large variations found in the extracted values for positron diffusion lengths L+ in silicon are identified and thought to arise from differences in sample surface conditions. Possible sources of the observed discrepancies are discussed, together with methods for reducing them.

ANNEALING OF DEFECTS INDUCED BY GE IRRADIATION OF SILICA PROBED WITH VARIABLE ENERGY POSITRONS

Abstract Fused silica (Suprasil 2) has been implanted with 2 MeV Ge ions. Defects produced by the implantation have been studied using a variable energy positron beam. The measurements allow depth resolved characterisation of the implantation induced defects, which are found to be produced by both the electronic and nuclear stopping of the ions. It is suggested that the previously unidentified positron-trapping defect associated with the nuclear stopping is an oxygen-decorated vacancy. Heating the sample to a temperature of 600°C demonstrates an annealing stage for this defect between 350–400°C. A region with a low concentration of positron-trapping defects is found to be present close to the sample surface, even in the as-irradiated sample prior to annealing. The depth of this region increases with increasing annealing temperature.

Positron characterization of defects formed during solid phase epitaxy of cobalt silicide

Cobalt films of thickness nm have been deposited on Si(100) substrates and subsequently annealed to temperatures of 400, 500, 600 and for 2 h. These anneals resulted in the formation of overlayers consisting of Co , CoSi and (600 and ). The various phases are easily distinguishable using both Rutherford backscattering (RBS) and positron annihilation spectroscopy (PAS). The values of the Doppler broadening S parameter for CoSi and (formed at ) are equal to the stoichiometrically weighted averages of the S parameters of the constituents. Consequently, it is proposed that positron trapping sites previously detected in using re-emitted positron spectroscopy are most likely grain boundaries and dislocations. Open volume defects (possibly voids) were detected by PAS in the sample formed at . Using in situ PAS measurements during annealing of an initially unreacted 60 nm Co film, transitions between the various phases were observed.

Modification of the properties of silica glasses by ion implantation

High energy MeV ion implantation of fused silica and Ge- doped silica renders these materials photosensitive. The physical processes involved are closely related to the photosensitization of Ge-doped silica by UV irradiation but present certain characteristics that are different. We discuss the results of studied of the induced absorption and refractive index changes under different preparation conditions, annealing sequences and subsequent bleaching by ArF and KrF excimer radiation. We include the results of a study using positron annihilation spectroscopy of the defects introduced by ion implantation and subsequent annealing and bleaching.

Defect Profiling of Oxygen-Related Defects Using a Slow Positron Beam

Variable-energy positron annihilation spectroscopy (PAS) is a relatively new technique for probing subsurface defects, and has provided novel insights into defects associated with silicon-based systems such as SiO2/Si, silicon nitrides, SIMOX and ion-irradiated Si. The technique entails measurement of Doppler broadening of the annihilation radiation from positrons implanted monoenergetically and subsequently thermalised in the sample, which is in turn dependent on the electronic environment. Positrons trapped by defects can thus be distinguished from those in undefected material. By controlling the incident positron energy, depth profiling of defects to several microns can be achieved. The technique is described, together with a study of defects induced by oxygen implantation for fluences of 5xl011, lxl013 and lxl014cm-2. The positrons are trapped at open-volume type defects (probably divacancies) in the as-implanted samples. Changes in defect structure are observed and the sensitivity of the technique to the chemical environment of the defects is demonstrated.