A. Oliva

Comments on collisional sputtering theory

The basic relation for collisional sputtering theory can be written Y= 1/8ElCn(0)L0/U, Γ where Γ is a constant with a value near unity, energy E1 is incident, a fraction Cn(x)dx of this energy is deposited at (x,dx), L0 is the calculated characteristic depth of sputtering, and U is the surface binding energy. If L0 is identified with L*, the observed depth of particle origin, then it follows from both experiments and simulations that L0/λ = L*/λ = 0.80 ± 0.10, where λ is the mean atomic spacing. Depending on the speed of atom emission, U should be identified either with (2Z2/Z3) Δ Ha (for rapid ejection) or with the sum Efs + Δ Ha (for slow ejection), where Z2 is the surface coordination number, Z3 is the bulk coordination number, ΔHa is the heat of atomization, and Efs is the surface-vacancy formation energy. U is therefore somewhat larger than Δ Ha, namely − 1.42 Δ Ha or − 1.33 Δ Ha. With these qualifications it follows that collisional sputtering theory, especially if L0 is identified with L*, may seriously underestimate experiment.

Directional ejection of fast excited Si atoms during 10 KeV Ar+ ion bombardment

Abstract Incidence- and detection-angle-resolved Auger spectroscopy has been used to study the spatial and energetic distribution of sputtered energetic Si (∗) target atoms during 10 keV Ar + ion bombardment. The fast-moving Si neutrals and ions with an inner-shell hole de-excite outside the sample emitting characteristic atomic-like Auger electrons which are Doppler-shifted with respect to those emitted from slow-moving sputtered Si species. The results show that for a given incidence angle the Doppler shift assumes a positive maximum at a particular detection angle and decreases on both sides away from it even to reach negative values, and that this direction exhibits a strong dependence on the ion incidence angle. Our experimental findings suggest that the flux of the ejected excited energetic Si particles is higher directional and provide convincing evidence that the primary asymmetric collisions between Ar and Si are an important mechanism for Si 2p hole creation.

Al target 2p electron excitation in asymmetric collisions with very low energy Ne+ projectiles

Abstract We present a study on the 2p electron excitation in Al target atoms in very low energy asymmetric collisions with lighter Ne+ projectile ions. We show that one of the L-shell vacancies originally present in Ne 2p3, created by double 4fσ electron promotion from Ne 2p5 or 2p53s, can be transferred to the Al partner through a two-electron autoexcitation mechanism. Our results provide an interesting interpretation for the previously observed lower threshold energies for target Al LMM Auger emission for Ne+ impact than for Ar+, Kr+, and Xe+.

Ion-induced Auger emission from Si, Al and AlxMg1−x samples

Abstract Angular resolved investigations of Auger atomic lines induced by ion bombarment have been conducted on Si, Al and AlxMg1−x samples. It is confirmed, by direct experimental evidence, that the atomic-like Auger emission occurs outside the sample and new transitions are also observed. All main Auger atomic lines exibit large Doppler shifts which can be accounted by a simple binary collision model in which only excitation of primary recoils by violent asymmetric collisions are considered. Under normal Ar+ ion incidence doubly excited L2MM Auger peaks are observed for AlxMg1−x samples (x = 0.25, 0.50, 0.75, 1.0), such peaks are absent in pure Al bombarded under the same conditions, indicating that such excitations are generated by asymmetric MgAl collisions. The results also show an enrichment of Al within the first layers for an Al0.50Mg0.50 sample as the bombarding energy is increased. This enrichment can be interpreted in terms of preferential sputtering and segregation effects.

Corrections to the collisional sputtering yield

Since the numerical value of the depth of origin of sputtered atoms in transport theory is in conflict with the value from experiment and simulation, we have introduced two corrections to the theoretical sputtering yield. The first relates to a more adequate choice of the value of the cross section for low-energy elastic collisions. The value is inferred by taking propertly into account the correct interpretation of the ejection process and the finite size of an atom. The second correction is the semi-infinite medium correction. Also in this case our correction derives from a modified interpretation of the ejection process.

Atomic versus bulk Al Auger electron emissin induced by noble gas ion bombardment

We present an angle resolved study on the Auger electron emission from an Al surface uneer noble gas ion bombardment along the normal incidence direction for primary energies varying from 5–15 keV. We show that the intensity ratio between the atomic LMM and the bulk LVV features decreases according to an inverse cosine law as the detection goes from normal to grazing directions and that such ratio changes considerably with the primary ions (Ar+, Kr+ and Xe+). These results provide important information on the deexcitation mechanism of the core excited Al particles.

Electron Energy Loss Investigation of Palladium Containing Liquid Crystals

Abstract Electron Energy Loss Spectroscopy was used for the first time to investigate collective properties of metal-containing liquid crystals: the applicability of this experimental technique to investigate electronic properties of liquid crystals is proved. Loss data were collected versus temperature and primary beam energy. Present results show that temperature can change the orientation of molecules with respect to the surface and that the studied material, after few thermal cycles up to 135°C becomes conductor and keeps its electrical conductivity from the solid state up to the isotropic phase.

Composition profiles due to bombardment-induced segregation in Cu-Ni samples as deduced by auger spectroscopy

Abstract Concentration profiles in Cu 0.57 Ni 0.43 samples have been studied as a function of ion fluence (ions/cm 2 ). Samples were bombarded with 7 keV Ar + to a fixed fluence and subsequently profiled by alternately etching with 0.7 keV Ar + and measuring with Auger spectroscopy. The technique used is that of Pons et al., where the surface layer concentration is extracted from the signal difference before and after each etching. We stress that low energy and low current must be used during the etching in order to ensure the minimum perturbation of the profile. In addition, low-energy Auger signals are essential to achieve high surface sensitivity. The measurements indicate a bombardment-induced segregation of copper in agreement with the results obtained by other spectroscopies. Concerning the profiles versus fluence, an increase in surface concentration and in subsurface depletion can be observed as the initial ion fluence increases.

Ar L-shell and metal M-shell Auger electron emission for 14 keV Ar+ ion impact on Ca, Sc, Ti, V, Cr, Fe, Co, Ni, and Cu

Abstract Ar projectile L-shell and metal target M-shell Auger electron emission have been studied by 14 keV Ar+ ion bombardment on Ca, Sc, Ti, V, Cr, Fe, Co, Ni, and Cu targets at an incidence angle of 60° with respect to the surface normal. We observe that the width of the Ar Auger peak progressively reduces and its total intensity decreases exponentially with increase of the target atomic number and that the metal M-shell autoionization peaks are completely absent.

Evidence of alkali adatom island formation induced by ion bombardment on a Na covered Pt surface

Abstract We report on adatom island formation induced by Ne + ion bombardment on a Na covered Pt surface. By using the collisionally excited autoionization electron spectroscopy (CEAES) we observe that, for large initial coverages, ion impact causes the Na adatom aggregation creating large surface areas which are relatively Na-rich and Na-depleted. Decay of excited atoms above these alkali islands and above the more or less clean Pt substrate zone with very different work functions results in the splitting of each atomic transition line in two well separated components.