L. M. Howe

Collision cascades in silicon

Abstract Individual damaged regions formed in silicon during low fluence (10 11 –10 12 ions cm −2 ) ion bombardments (∼30–200 amu ions of 15–100 keV energy) were observed using transmission electron microscopy. Both monatomic and diatomic ions were used in order to investigate the role of average deposited energy density θ υ in determining the characteristics of the damaged regions. The efficiency of creating a visible damaged region was for θ υ eV/atom , but increased to 0.7–1.0 for θ υ > 0.4 eV/atom . The fraction of the theoretical collision cascade volume occupied by the damaged regions increased as θ υ increased. During annealing, the number density and the size of the damaged regions decreased but there was no indication of any change in the basic nature of the damaged regions. The damage produced by a diatomic ion was more resistive to annealing than that produced by the corresponding monatomic ion of the same velocity.

Annealing of heavy ion cascade damage in silicon

Abstract Channeling measurements have been performed to investigate the annealing of the irradiation damage produced in monocrystalline silicon by low dose (1012-1013 ions cm−2) implants at 40 K with monatomic and diatomic ions of As, Sb, Te and Bi having incident energies of 15–30 keV. The mass of the bombarding ion has a pronounced effect upon the subsequent annealing behaviour, with the heavier ion implants annealing at the higher temperature. Within the monatomic series of implants, the annealing behaviour correlates well with the average deposited energy density within the cascade. However, this correlation fails to account for the observed increase in anneal temperature in going from monatomic Bi to diatomic Sb2 or Te2 implants.

Irradiation hardening and annealing in copper at high neutron fluences

Abstract Irradiation hardening has been investigated in copper crystals irradiated at ∼325 K with 4 × 1020 and 8 × 1020 n cm−2. In the as-irradiated crystals, cleared channels were produced due to the sweeping up of the irradiation damage by the glide dislocations. The slip band width, spacing and height all increased with increasing testing temperature. Identical post-irradiation annealing treatments produced a larger fractional recovery of the critical resolved shear stress at 4 K than at 295 K. This indicates that the damage removed in the early stages of annealing acts as a more effective obstacle to dislocation glide at 4 K than at 295 K. For certain combinations of annealing treatment and low temperature testing, the initial stage of deformation was characterized by the bowing of the glide dislocations around the defects rather than sweeping them up. In polycrystalline copper, migrating grain and twin boundaries were quite effective in sweeping out the irradiation-produced defects.

The combined effects of lattice vibrations and irradiation-produced defects on dechanneling

Abstract The combined effects of lattice vibrations and irradiation-produced defects on dechanneling have been studied in various crystals by measuring backscattering yields of He + and H + ions for 〈111〉 or 〈111〉 channels in the temperature range 40–300 K. For an Al-0.08 at% Ag crystal examined along 〈110〉 and a Si crystal examined along 〈111〉, the dechanneling contributions from thermal vibrations and from irradiation-produced defects were additive (i.e. the deviation Δ from additivity was essentially zero). However, for the remainder on the crystal systems(Cu-006 at% Au 〈110〉, Ge 〈111〉 and 〈110〉 and Si〈110〉) the contributions of the defects and thermal vibrations to dechanneling were not independent i.e. Δ was found to be appreciable. Also, in general, Δ varied linearly with the mean vibrational amplitude of the crystal atoms.

Irradiation-induced displacement of Ag atoms from lattice sites in an Al-0.2% Mg-0.1% Ag crystal

In irradiated alloys of Al containing approximately 0.1 at.% Ag, Al-Ag (100) dumbbells are created by the trapping of Al interstitial atoms at Ag solute atoms. The results demonstrate that the addition of 0.2% at.% Mg retards not only the formation of Al-Ag dumbbells during annealing from 30 to 100K but also their annihilation during annealing from 180 to 240K. Al interstitials are released from Mg traps at 100-160K, causing further trapping at Ag atoms. Approximately 70% of the Ag atoms return to lattice sites at approximately 200K (stage III) and the remainder at approximately 260K. These results favour migration of Al-Ag dumbbells rather than vacancies during stage III annealing.

Investigation of the interaction between irradiation-induced defects and solute atoms in dilute copper alloys by ion channeling

Abstract The backscattering-channeling method was used to investigate the interactions between irradiation-induced defects and solute atoms of Ag, Sb and Au in dilute copper alloys. Since the solute atoms were not displaced appreciably from lattice sites by irradiation with ∼1016 1.5 MeV He+ ions/cm2 at 40 or 70 K, it is concluded that the Cu self- interstitials created by the irradiation retained their identity when trapped by the solute atoms. During subsequent annealing between 180 and 270 K, the solute atoms were displaced from lattice sites. The irradiation-induced increase in dechanneling recovered only ∼30% in this temperature range. Further low temperature irradiations reduced the solute atom displacements. These results are consistent with multiple trapping of vacancies at the solute atoms.

A channeling investigation of proton and deuteron damage in germanium

Abstract The amount of damage present in germanium (and silicon) following : mplants of 200 keV H+ or 300 keV D+ at temperatures between 40–300°K has been measured using the channeling technique. The damage level is observed to increase strongly with decreasing implantation temperature. Damage profiles were obtained from the data by considering various scattering mechanisms, including single and plural scattering and a steady increase approximation treatment. For equal dose implants, deuterons create considerably more damage in germanium than protons. The ratio of deuteron damage to proton damage is significantly greater than the ratio of energy deposited in nuclear events for the two ions. The results of isochronal anneals at temperatures from 40 to 325°K on Ge crystals implanted at 40°K are also reported. A strong annealing stage is observed around 175°K; this is in good agreement with previous optical absorption studies of proton damage in germanium.

The additivity of dechanneling from lattice vibrations and point defects

Abstract A high energy ion which is directed near close-packed rows or planes of atoms in a single crystal is steered (channeled) into the spaces between these rows or planes.1–3 A channeled ion is deflected out of the channel (is dechanneled) when its transverse energy E ⊥ exceeds a critical value E c ⊥. The ion then becomes part of the random beam, if rechanneling is neglected. An ion can be dechanneled by electronic collisions or by nuclear collisions with displaced atoms. Displacements in the form of thermal vibrations 4–6 or radiation damage7, 8 can cause large increases in dechanneling. We have investigated whether the dechanneling caused by thermal vibrations and that caused by permanent displacements (in the form of radiation-induced point defects) are additive for an Al-0.08% Ag alloy.

High resolution electron microscopy investigation of radiation produced defects in copper and copper-aluminum crystals

Abstract A high resolution electron microscopy investigation has been undertaken in order to obtain additional information regarding the nature of the strain field of small defects (∼10–75 A diameter) produced in copper and copper-16 atomic per cent aluminum crystals by neutron irradiation and by bombardment with 40 keV Cu+ ions. In addition to bright and dark field observations at electron optical magnifications of 250,000–425,000 ×, lattice fringe images were obtained in the vicinity of the radiation produced defects by combining the transmitted beam with one of the diffracted beams. It was found that some of the defects had a principal strain component along only one 〈111〉 whereas other defects had a more complex strain field with strain components along more than one direction. For those defects with a principal strain component along only one 〈111〉, the localized defect concentration in a collision cascade may be sufficiently high for the spontaneous formation of a Frank dislocation loop to occur. In...

4.2. Channeling Studies of Lattice Defects

Publisher Summary Several nuclear techniques have been developed for the study of lattice defects. Most of these methods, such as positron annihilation, hyperfine interactions (Mossbauer effect and perturbed angular correlation), nuclear magnetic resonance, and muon-spin precession as well as other methods, such as electrical resistivity and electron paramagnetic resonance, are used to probe the electronic environment of the nuclei, rather than the actual positions of the atoms in the lattice. However, an important characteristic of lattice defects is that they produce a localized spatial rearrangement of lattice atoms. The technique of ion channeling is especially suited to studying these displacements of lattice atoms in the vicinity of lattice defects. Ion channeling is the steering of a beam of energetic ions into the open spaces (channels) between close-packed rows or planes of atoms in a crystal. The use of ion channeling to study lattice defects in solids is based on the ability of channeled ions to see displacements of atoms from lattice sites. The various lattice defects are characterized by the type of displacement produced by them. Lattice defects can be classified according to their dimension in point defects, line defects, planar defects, and volume defects. Channeling also provides a useful analytical technique for characterizing crystal surfaces; when used in conjunction with low-energy electron diffraction (LEED), Auger spectroscopy, and nuclear microanalysis, it permits an accurate picture of various surface structures to be determined.