T.M. Buck

Surface segregation in Cu-Ni and Cu-Pt alloys; A comparison of low-energy ion-scattering results with theory

Abstract Under equilibrium conditions the surfaces of Cu-Ni and Cu-Pt alloys are enriched in Cu. Since low-energy ion scattering is only sensitive to the outermost atomic layer of the surface, it is the ideal technique to study surface segregation. The results are compared with zeroth-and first-order bond-breaking theories. The influences of parameters such as relaxation, crystal face, atomic size and double-layer formation are discussed.

Energy spectra of 6–32 keV neutral and ionized Ar and He scattered from Au targets; ionized fractions as functions of energy

Abstract The neutralization of ions is an important aspect of low energy ion scattering for surface analysis. Electrostatic energy analyzers (ESA) have been used almost exclusively in such work, and information on charge neutralization efficiencies is needed for quantitative interpretation of ESA data. In the past, the occurrence in low energy ion spectra of surface peaks and low backgrounds due to scattering from inside the solid has been attributed to preferential neutralization of ions which penetrate beyond the surface. In the work to be described, a time-of-flight technique was used to measure energy spectra of both neutral and ionized Ar and He scattered at 90° from a polycrystalline gold target. Incident energies of 6–32 keV were used. The energy spectra of neutral Ar scattered from polycrystalline gold exhibit sharp surface peaks, and double scattering shoulders, over this entire energy range. For He there is a gradual downward slope toward lower energy rather than a sharp surface peak. The behavior in both cases is attributed to large scattering cross-sections which cause a loss of beam particles during penetration. A calculation using a 1 r 2 potential illustrates this effect as a function of energy for helium. In the present experiments we find that the ion fraction of scattered argon does indeed depend on depth of penetration. This is in contrast to the behavior of He and H at higher energies, e.g. 100 keV, in which cases the charge state depends on emergent velocity but not on depth of penetration. The characteristic shapes of ion scattering spectra in this energy range appear to result from both neutralization and beam attenuation inside the target.

Selected topics in low-energy ion scattering: Surface segregation in Cu/Ni alloys and ion neutralization

Abstract The usefulness of ion scattering for studies of surface segregation in alloys has been demonstrated for the Cu/Ni system. A strong surface enrichment in Cu is found. As a step towards understanding the mechanism of neutralization of ions at a surface, the scattering of Ne + from Au has been studied. The importance and origin of oscillations in the energy dependence of the ion fraction is discussed.

Low-energy neon-ion scattering and neutralization on first and second layers of a Ni(001) surface

Abstract The scattering and neutralization of 2.4 and 5 keV Ne+ ions on the Ni(001) surface have been studied by time-of-flight (TOF) and electrostatic analyzer (ESA) techniques. The scattering yield of neutrals plus ions (by TOF) is strongly dependent on crystal orientation, in one direction being reduced by the shadowing of 2nd layer atoms by 1st layer atoms, or in another being increased by focussing of ions onto the 2nd layer by 1st layer atoms. Ion yields (by ESA) show little of this variation since the ions are largely neutralized on scattering from the second layer. The results thus demonstrate and explain the first layer selectivity of low-energy ion scattering by ESA for a case in which there is no shadowing of second layer atoms by the first layer. On the other hand, the ability to measure and distinguish first and second layer scattering of neutrals and ions by TOF suggests the possibility of composition analysis of individual layers of single crystal alloys and compound semiconductors.

Investigation of low-energy ion scattering as a surface analytical technique

Abstract The energy spectra of rare gas ions backscattered from various solids have been monitored for primary energies in the range 40 keV to 500 eV. At the higher primary energies peaks in the backscattered energy spectrum, which correspond to the masses of atoms on the sample surface, tend to broaden considerably towards lower energies. As the primary energy is reduced, these peaks become sharp, and below 5 keV primary energy the technique is suitable for elemental surface analysis of solids, including polycrystals and amorphous materials. The sensitivity of the apparatus to trace impurities has been estimated by examination of precalibrated test samples, and is found to be ∼ 5 × 10 −4 monolayers for a heavy element such as gold, and by extrapolation ∼ 10 −1 to 10 −2 monolayer for oxygen. Sputtering by low energy helium ion beams is found not to be a serious problem during analysis. Argon ion beams, on the other hand, may be used to study depth profiles of composition in alloys. This report is intended to bring to light some of the capabilities and present limitations of low energy ion scattering as a surface analytical technique.

Energy and mass spectra of neutral and charged particles scattered and desorbed from gold surfaces

Surface analysis by low energy (~0.1–10 keV) ion scattering has, in the past, been performed primarily by means of electrostatic energy analysis. This technique though very useful, ignores the large neutral fraction of the scattered particles. In this paper a low energy time-of-flight (TOF) spectrometer is described, capable of providing TOF spectra of both charged ions and neutrals. This system has been used to obtain energy spectra of 8 keV Ar+ ions scattered off polycrystalline gold targets. Unexpected features of these spectra were shown to be due to the sputtering of hydrogen, and other species, off the gold surface and into the TOF channel. The mass to charge ratios of these atomic species were determined by measuring the flight times of ions through an electrostatic analyzer. An alternative technique for TOF analysis of beam-desorbed neutrals, as well as ions, is also proposed.

Charge states of 25–150 keV H and 4He backscattered from solid surfaces

Abstract Measurements have been made of the ion-fractions of H and 4 He backscattered with energies of 25–160 keV from Cu, Au, and Si surfaces which were etched and washed but not atomically clean. The ion-fractions for H range from 0.37 at 25keV to 0.92 at 160 keV, and for 4 He from 0.10 at 30keV to 0.58 at 150 keV, depending to a small extent on the target material. Where comparisons can be made the data agree rather closely with results of others for particles traversing thin foils. The data are useful for calibration of an electrostatic analyzer in surface analysis. Plots of ion-fraction against particle velocity show a primary dependence on velocity, as expected, but there is a small difference in slope between the H and He curves. Charge states of particles scattered from surface impurities did not deviate significantly from those of particles scattered from the substrate at the same energy.

Comparison of a time-of-flight system with an electrostatic analyzer in low-energy ion scattering

Abstract Neon ions with primary energy of 5 keV were scattered from gold and silicon and energy spectra were obtained by both time-of-flight (TOF) and electrostatic analyzer (ESA) methods. At this energy the neutral plus ion spectra obtained by the TOF method are not so sharply peaked as the ion spectra of the ESA, owing to neutralization effects on natural linewidth and also to somewhat poorer energy resolution. However, the ion dose required for the TOF technique was substantially lower and a significant reduction in surface disorder on silicon was detected by Leed. Neon ions were neutralized more efficiently on a gold surface than on silicon. Evidence of sputtered Si + ions having relatively high energy, e.g. 800 eV, was found by a combination ESA-TOF measurement. The Si + ions are evidently knocked out of the first atom layer by Ne reflected from deeper layers.

A rutherford scattering study of the diffusion of heavy metal impurities in silicon to ion-damaged surface layers

Abstract Rutherford backscattering of 1.75 and 2 MeV 4 He + ions has been utilized to study the high temperature gettering of Fe, Co, Ni, Cu and Au from silicon by ion-damaged surface layers. In a typical experiment a metal film was evaporated onto one side of a silicon wafer (125 microm thick) which had received ion implantation damage (10 16 /cm 2 Si + ions at 100 keV) on the opposite side; the wafer was then annealed at 900°C, usually for 30 min. The results of such experiments show that the metals studied may be divided into two classes, those which are gettered slowly - Fe, Co, and Au, and those gettered rapidly - Cu and Ni. Fe, Co, and Au were found at levels of 1×10 13 −1×10 14 /cm 2 in the damaged layer, whereas Cu and Ni appeared at levels of 6×10 14 to 5 ×10 16 cm 2 . The gettered level of Au, one of the “slow” group, was increased ten-fold by an equal increase in the anneal time to 300 min. The gettered Cu and Au exhibited double peaks in the scattered ion spectra, corresponding to metal concentrated at the most heavily damaged region (end of range for Si implant) and also at the outer surface, with a separation of ~ 1300 A. A simple model is proposed to explain the slow and fast gettering, based on published interstitial diffusivities and solubilities of the five elements studied. Rutherford scattering has proven to be well suited for the quantitative identification of low levels of impurities on Si surfaces and for impurities gettered at damaged layers close to the surface.

Low-energy ion scattering (LEIS) for composition and structure analysis of the outer surface

Abstract The energy distribution of low-energy ions (1–2 keV) scattered at some specific angle from a solid surface, provides information on the mass, or identify, and the number of surface atoms, through the energy position and magnitude, respectively, of peaks in the spectrum. The sampling depth is restricted to one or two atom layers. In the case of single-crystal targets, surface structure or atom location information can be derived from shadowing and multiple scattering effects. Examples are included of applications to equilibrium surface segregation in alloys and the location of sulphur atoms on the Ni (001) surface. The importance of inelastic effects is discussed.