D.A. Thompson

High density cascade effects

Abstract The objective of this article is to review the effects of high density cascades in solids. Such cascades can be separated into those occurring in which “high density” refers to either the density of atomic collision events or to the density of ionization. Both types of cascade are discussed with reference to their effects in the bulk (i.e. damage production and inert gas detrapping) and on the surface properties (i.e. sputtering, secondary ion and electron emission). The appropriate experimental data are reviewed and discussed in relation to the various proposed spike models; i.e. displacement, thermal, plasticity and ionization spikes.

Energy spikes in Si and Ge due to heavy ion bombardment

Abstract The number of displaced atoms/ion, N∗D , produced by heavy ion bombardment (10-250 keV) of Si and Ge at 35 K is reported. A semi-empirical formulation is given, based upon the separation of the damage into a “spike” component and a “collisional” component, which adequately predicts N∗D , for all ions and energies investigated. The “spike” concept is also tested by investigation of damage enhancement effects occuring for diatomic bombardment with equal atom dose and equal energy/atom corresponding to the monatomic bombardments. The semiempirical model is found to adequately predict the diatomic damage enhancement for heavy ions, e.g. Te+2, but shows discrepancies for lower mass ions. The reasons for such discrepancies are discussed. For Si targets, the “spike” component appears to fit a thermal-spike model, whereas for Ge it is suggested that a damage-spike (localized lattice collapse within the individual cascade) better interprets the experimental results. Spike diameters, DS, for various ions a...

Computer simulation of ion bombardment collision cascades

Abstract A Monte-Carlo code is described which simulates features of collision cascades due to ion bombardment. The effects of including an energy loss, U, from the recoiling atom during the displacement process is investigated for 0 ≤ U ≤ Ed . Depending on whether U = 0 or U = Ed is used, the fraction of replacement collisions is determined as 0.28 or 0.19 respectively. Also the fraction of energy expended in elastic collisions and displacement events, v(E) is determined dependent upon the value of U. These results are compared to transport theory solutions. Individual collision cascades are generated and quantitative values determined for the ratio between the individual cascade volume and the cascade volume obtained using the longitudinal and transverse moments of the deposited energy distributions evaluated from transport theory by Winterbon. These volume ratios are compared to the equivalent ratios calculated by Sigmund.

Disorder production and amorphisation in ion implanted silicon

Abstract Measurements of the depth profiles of disorder and the depth integrated disorder produced in Si at 40 K or 300 K by low energy (15 keV-60 keV) ions of a range of masses from the light (N+ and P+), intermediate (Ar+ and As+) to heavy (In+, Sb+, As+ 2, Bi and Sb+ 2) were made using Rutherford scattering-dechannelling techniques. Studies of both the ‘surface peak’ disorder and the dechannelling minimum indicate a gradual change in the process of disorder production from one of point defect generation for light projectiles to direct impact amorphisation for heavy projectiles. In the lower mass ion cases it is concluded that amorphisation is produced by overlap of more lightly disordered regions and it is shown how all the data can be encompassed within a generalized model of amorphousness production which occurs when local defect densities lie in the range 2–18%.

Deuterium permeation and diffusion in high-purity beryllium

Abstract The permeation rate of deuterium through high-purity beryllium membranes was measured using the gas-driven permeation technique. The time-dependent and the steady-state deuterium flux data were analyzed and the effective diffusivities of the samples were determined. Using multilayer permeation theory the effects of surface oxide were eliminated and the diffusion coefficients of the bulk beryllium determined. The diffusion parameters obtained for the extra-grade beryllium samples (99.8%) are D0 = 6.7 × 10−9m2/s and ED = 28.4 kJ/mol. For the high-grade beryllium samples (99%) the parameters are D0 = 8.0 × 10−9m2/s and ED = 35.1 kJ/mol.

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.

Measurement of damage distributions in ion bombarded Si, GaP and GaAs at 50 K

Abstract A combined accelerator system is described in which implantations (10–150 keV) can be carried out at temperatures as low as ∼50 K and in situ channeling-backscattering analysis can be undertaken using 0.5–3.5 MeV He+ ions or protons. Damage profiles have been obtained for Si, GaP and GaAs bombarded at 50 K with He+, N+, and Zn+. The analytical procedure by which the profiles are extracted from the backscatter spectra is detailed.

A first order diffusion approximation to atomic redistribution during ion bombardment of solids, II. finite range approximation

Abstract An improved diffusion approximation, in which diffusion enhancement operates only over a finite depth, to model atomic redistribution resulting from recoil and cascade mixing during ion bombardment of solids is described, it is shown that this model correctly simulates the main results of the more complex linear cascade atomic collision calculations of positive mean depth shifts, negative peak shifts, broadenings and exponential tails in surface concentration of diffusant marker species during sputter erosion. The analytic method of generalisation to any arbitrary and more realistic diffusivity-depth profiles is also outlined.

1.4-μm InGaAsP-InP strained multiple-quantum-well laser for broad-wavelength tunability

InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings.

Low energy ion induced damage in silicon at 50 K

Abstract A systematic investigation has been undertaken to study the damage created in silicon due to bombardments by light and medium mass ions at 50 K. The results are interpreted in terms of the average cascade damage density. For cascade damage densities ≲ 10 −3 , the results are consistent with linear cascade theory. For higher cascade damage densities, non-linear effects suggest the existence of spike phenomena.