Wolfgang O. Hofer

RECOIL MIXING IN SOLIDS BY ENERGETIC ION BEAMS

Abstract Within the framework of the transport theoretical description of slowing down of energetic particles in random media, a general relation for recoil mixing is derived and evaluated. It takes into account accumulation of implanted projectiles and compensation of density variations by lattice relaxation. The main emphasis is placed on cascade mixing, both isotropic and anisotropic recoil velocity spectra being treated. Power cross-sections are used in calculating alterations of tracer atom concentrations in the surface and bulk. In addition, the partial flux of sputtered tracer atoms is evaluated in an attempt to determine depth-differential concentration profiles as measured by SIMS or SNMS. The results essentially reproduce the experimental findings, in particular the dependence on the projectile mass, energy and angle of incidence. Inclusion of anisotropic terms in the recoil velocity spectrum gives only a slight shift of the concentration profiles in the beam direction, while a strong opposite shift is found when the injected projectiles are accommodated in the lattice. The importance of over and under-density compensations by lattice relaxation is emphasized, although it is difficult at present to assess the degree to which it will actually take place.

Profiling Hydrogen in Materials Using Ion Beams

Abstract Over the last few years many ion beam techniques have been reported for the profiling of hydrogen in materials. We have evaluated nine of these using similar samples of hydrogen ion-implanted into silicon. When possible the samples were analysed using two or more techniques to confirm the ion-implanted accuracy. We report the results of this work which has produced a consensus profile of H in silicon which is useful as a calibration standard. The analytical techniques used have capabilities ranging from very high depth resolution ( ≈50 A ) and high sensitivity (

A Theoretical Treatment of Cascade Mixing in Depth Profiling by Sputtering

Abstract An analytic expression for mixing in recoil cascades is discussed with particular emphasis on in-depth concentration profiling. Examples presented show that an asymmetric distortion is not necessarily a consequence of direct recoil implantation or an anisotropic velocity distribution.

The emission of neutral clusters in sputtering

The mass, angle, and energy resolved emission of neutral clusters in sputtering was studied for a variety of metals and semiconductors. The main phenomena and results are the following: (i) Cluster emission from a series of transition metals reveals a prominent contribution of clusters to the total flux of ejected particles but there is no simple scaling of cluster intensities with the average sputtering yields. With increasing number of constituents, relative intensities of neutral clusters decrease much faster than those of secondary-ion clusters. (ii) The relative intensities of clusters emitted from amorphous and crystalline semiconductors are identical, but the energy spectra of Gen-clusters (n = 1–4) sputtered from Ge (111) peak at a slightly higher energy (1 eV) as compared to spectra taken from amorphous Ge. The intensities of all Gen-clusters exhibit the same dependence on emission angle; this holds for both the amorphous and crystalline Ge-sample. (iii) The flux of neutral monomers, dimers, and trimers sputtered from Cu(111), Ni(111), and Ag(111) crystals shows a pronouncedly anisotropic emission along the 〈110〉 lattice directions which is ascribed to a momentum alignment in the anisotropic part of the collision cascade. Energy spectra taken along 〈110〉 peak at higher energies than those obtained from a random emission angle.

The Influence of Surface Structures on Sputtering: Angular Distribution and Yield from Faceted Surfaces

Solid surfaces subject to energetic particle bombardment generally develop characteristic structures, which may significantly change the total and differential sputtering yield. The change is due to two competing effects, a yield-increase by an enhanced effective projectile incidence angle, and a yield-reduction by recapture of obliquely ejected particles. Both effects have been included in calculations of the sputtering yields from faceted surfaces in the regime where the plane surface yield follows a cos−v dependence on the incidence angle. Except at very low energies, the total sputtering yield is always increased by faceting. The angular distribution function is mainly influenced at large polar angles and may significantly deviate from that of the corresponding flat surface. This has important consequences both in comparisons between experimental and theoretical distribution functions as well as in applications such as thin film production, plasma contamination, secondary ion mass spectrometry etc.

Analysis of solids by secondary ion and sputtered neutral mass spectrometry

A mass spectrometer is described, which allows the analysis of sputtered neutral and charged particles as well as of residual gas composition. This combined SIMS, SNMS, and RGA instrument consists of a scanning primary ion beam column, an electron impact ionizer, an electrostatic energy filter and an rf quadrupole mass analyzer.Various examples of surface and bulk analysis are presented which demonstrate the beneficial complementary features of these techniques. These are, in particular: a substantial reduction of the matrix effect and fewer complications with samples of low electrical conductivity in SNMS, and the possibility of measuring the depth distribution of gases included in small cavities in the solid in the SNMS/RGA mode. SIMS, on the other hand, allows in many cases higher detection sensitivities.

z1-oscillations in ion-induced kinetic electron emission

Abstract Kinetic electron emission by 12–30 keV ions ranging from Z 1 = 3 to 92 has been studied on clean gold surfaces. With the aid of an ion-electron converter the average electron yield and the emission statistics were measured. Pronounced oscillations of the yield with the shell structure of the projectile ion were found, suggesting a significant contribution of excitation processes of the projectile to electron emission.

Emission of Atoms and Electrons from High-Density Collision Cascades in Metals

Abstract Particle emission from metals subjected to energetic heavy ion bombardment is briefly reviewed. In the case of atom emission, emphasis is placed on the number (yield) and the molecular state (cluster) of ejected particles. The importance of near-surface sub-cascades is stressed. Electron emission is discussed with respect to the influence of bombardment with composite projectiles. It shows a strictly linear behaviour, as evidenced by an electron yield determined solely by the number and velocity of the atoms in the incident molecule/cluster. Thermionic emission as well as effects of molecular disintegration are concluded to be insignificant under the bombardment conditions considered here.

On the chemical sputtering of oxygen-exposed molybdenum

The sputtering of oxygen-exposed molybdenum was studied by means of mass analysis of emitted neutral and charged particles. The irradiation was performed with 8 keV Ar+ ions at temperatures of 25° and 485°C. It was found that the enhanced sputtering yield at elevated temperature during oxygen exposure is due to beam-induced desorption of MoO2 and cascade sputtering of MoO. At this temperature considerable oxygen incorporation also takes place owing to recoil mixing and diffusion.

Depth distributions of low-energy 4He implanted in solids

Abstract Concentration profiles of 4He implanted with energies of 0.25–80 keV in silicon have been measured. The method employed-ion beam sputter erosion in conjunction with the mass spectrometric detection of sputter released He atoms-offers several advantages over other techniques used for helium analysis: He depth distributions in very near-surface regions (