Effects of enhanced diffusion on preferred sputtering of homogeneous alloy surfaces

The effects of enhanced diffusion caused by sputter damage on the kinetics of preferred sputtering of alloy surfaces have been analyzed. The diffusion flux is specifically included in considering the mass balance within the altered layer and on the sputtered surface. An analytical solution has been derived by solving the two mass-balance equations simultaneously under certain conditions. The solution is used as the basis of an interative method to obtain exact numerical solutions as a function of sputtering time. The analysis enables one to define an altered layer with respect to the steady state and to express its thickness in terms of the sputter yields and the diffusivity. Illustrative results showing the evolution of the altered layer during sputtering are given for a typical binary alloy surface. Implications of this analysis and possible future experiments are discussed.     

Preferred sputtering on binary alloy surfaces of the AlPdSi system

Auger spectroscopy has been used to investigate the behavior of preferred sputtering on surfaces of homogeneous AlPd, SiPd and AlSi alloy films. These combinations of alloys were chosen for studying the effects of mass and bonding differences on preferred sputtering. Experiments have been carried out using a 1 keV Ar ion beam over a range of alloy compositions. Our results can be summarized as follows: (a) The preferred sputtering of these binary alloys cannot be predicted according to the sputter yields of individual elements, e.g. both Al and Si have been observed to be removed preferentially relative to Pd although pure Pd has a higher sputter yield, (b) In the alloys studied, mass difference appears to dominate over bonding difference in controlling the preferred sputtering behavior since the extent of preferred sputtering of Al and Si relative to Pd is about the same. This observation is interpreted on the basis of the binary alloy sputtering theory formulated by Andersen and Sigmund. (c) Judging from the composition change of the sputtered surface, there is no evidence for formation of compounds with specific compositions as a result of preferred sputtering in the AlPd and SiPd alloys investigated.     

Compositionally modulated ripples induced by sputtering of alloy surfaces

Sputtering of an amorphous or crystalline material by an ion beam often results in the formation of periodic nanoscale ripple patterns on the surface. In this Letter, we show that, in the case of alloy surfaces, the differences in the sputter yields and surface diffusivities of the alloy components will also lead to spontaneous modulations in composition that can be in or out of phase with the ripple topography. The degree of this kinetic alloy decomposition can be altered by varying the flux of the ion beam. In the high-temperature and low-flux regime, the degree of decomposition scales linearly with the ion flux, but it scales inversely with the ion flux in the low-temperature, high-flux regime.     

First-principles-based surface phase diagram of fully relaxed binary alloy surfaces

The combination of density-functional theory (DFT) calculations of geometrically fully relaxed binary alloy surfaces with concepts from statistical physics is applied to construct a DFT-based phase diagram for a binary alloy surface. As a first example, we studied the appearance of Co antisite atoms at CoAl(100) surfaces. The structural parameters as multilayer relaxations, surface buckling, lateral order, and segregation profile of the predicted stable surface phases are in excellent agreement with experimental structure determinations applying low-energy electron diffraction.     

Preferential sputtering of binary compounds: A model study

A model classical dynamics calculation has been performed to ascertain the effect of atomic mass and surface binding energy on preferential sputtering in binary systems. We find the mass effect not to have a simple dependence on the ratio of the masses of the two components. Rather, for two binary systems, both with a mass ratio of ~ 3, the ejection yield of the lighter component can vary from being the same as that of the heavier component to being 100% larger. Over a limited range of surface binding energies the ejection yield is inversely proportional to the binding energy. In addition, we suggest that the effective scattering sizes should be considered as a possible cause for preferential sputtering.     

Auger electron spectroscopy of silicon surfaces

The measurements of the spectra of Auger electrons of the silicon surfaces performed at the pressure of (2–5)×10^?7 Torr are described. In this pressure range rapid oxidation and carbonization of the uppermost layers take place. The changes of characteristic energies in Auger and loss spectra are related to the change of chemical composition of the surface. The combination of the characteristic loss spectroscopy with Auger electron spectroscopy makes possible the determination of the chemical shifts. The measurements of the chemical shifts of the individual energy levels of the silicon atoms in both the pure and contaminated silicon surfaces, in quartz and Fe∶Si alloy are given. Finally, the possibilities and limitations of the heating for the silicon surface cleaning are examined.     

Effects of ion sputtering on semiconductor surfaces

Ion beam sputtering has been combined with Auger spectroscopy to study the effects of ion beams on semiconductor surfaces. Observations on the mass dependence of ion selective sputtering of two component systems are presented. The effects of ion implantation are explained in terms of atomic dilution. Experimental data are presented that illustrate the superposition of selective sputtering and implantation effects on the surface composition. Sample reduction from electron and ion beam interaction is illustrated. Apparent sample changes which one might observe from the effects of residual gas contamination and electric fields are also discussed.     

Disorder broadening of alloy Auger spectra

Auger spectroscopy promises the means to separate initial and final state contributions to the disorder broadening of core XPS spectra in disordered alloys. Auger disorder broadening, deduced from recent ab initio results, is predicted to be greater than XPS disorder broadening for Cu₅₀Pd₅₀ and Ag₅₀Pd₅₀ alloys. Simulations are used to assess whether this effect is observable experimentally despite the greater lifetime broadening of Auger spectra. A number of cases where narrow core–core–core Auger transitions should allow clear experimental identification of this effect are identified. The prospects for determining environment-resolved Auger spectra using APECS have been investigated.     

Deconvolution method for composition profiling by Auger sputtering technique

Some of the sputter broadening effects on profile measurements using the Auger sputtering technique have been quantitatively investigated for $\text{Ag}\text{Au}$ and $\text{Cu}\text{Ni}$ systems. Two methods have been used to measure the resolution function as a function of the sputter distance for these two systems. The broadening can be separated into contributions from original surface roughness and sputtering effects, the latter amounting to about 7% of the sputter depth for $\text{Cu}\text{Ni}$ and 14% for $\text{Ag}\text{Au}$ interfaces. Based on the measured resolution function, a deconvolution method has been developed to facilitate the retrieval of the actual profile from the observed profile by reducing the sputter broadening. Using this method, we found that the measured interfacial profile can be substantially sharpened. The implications of using the deconvolution method for interdiffisuion studies in thin films are discussed.     

Auger electron spectroscopic study of CO2 adsorption on Zircaloy-4 surfaces

We investigate the adsorption of CO₂ onto Zircaloy-4 (Zry-4) surfaces at 150, 300 and 600 K using Auger electron spectroscopy (AES). Following CO₂ adsorption at 150 K the graphitic form of carbon is detected, whereas upon chemisorption at 300 and 600 K we detect the carbidic phase. As the adsorption temperature is increased, the carbon Auger signal increases, whereas the oxygen signal decreases. Adsorption at all three temperatures results in a shift of the Zr Auger features, indicating surface oxidation. The effect of adsorbed CO₂ on the Zr(MVV) and Zr(MNV) transitions depends on adsorption temperature and is less pronounced at higher temperatures. On the other hand, changes in the Zr(MNN) feature are similar for all three adsorption temperatures. The changes in the Zr Auger peak shapes and positions are attributed to oxygen from dissociated CO₂, with the differences observed at various temperatures indicative of the diffusion of oxygen into the subsurface region.     

Investigation of partial sputtering of lithium from a binary Al/Li alloy with laser induced fluorescence

Sputtering investigations of an Al/Li alloy containing 9.1 at-% of lithium have been performed for 6 keV helium ion bombardment. Absolute particle densities and velocity distributions of the sputtered neutral lithium atoms were measured with laserinduced fluorescence. The amount of sputtered lithium was found to be constant for target temperatures ranging from room temperature up to 500° C. The mean transport velocity and the sputtering yield of the Li component have been calculated from the measurements. Thermal evaporation of neutral Li atoms could be measured independently of the presence of the helium beam for target temperatures above 300° C. The experimental results indicate that the surface is covered by lithium with at least several atomic layers even under highcurrent ion irradiation.Preliminary results have been presented at the SYMPOSIUM ON SPUTTERING, Spitz/Austria (1986)     

Surface miller index dependence of Auger neutralization of ions on surfaces

Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories is performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.     

Simulation of sputtering from ion-beam-roughened carbon surfaces

Effects of ion-induced surface roughness on sputtering of amorphous carbon under ion bombardment are studied by means of binary-collision computer simulation in a wide range of incidence angles. Most simulations refer to 1–10?keV Ar ion bombardment, and sinusoidal ripple morphology is assumed. It is shown that surface roughness is a key factor to achieve quantitative agreement with experiment. The simulation results are compared with the analytical estimates of the yield from sine-shaped and ridged surfaces based on continuum models of ion sputtering. Some deviations between the results are discussed.     

Auger and electron energy loss spectroscopic study of surfaces of iron-sulfur alloy,Fe7S8 and FeS2 cleaved in ultra high vacuum

The surfaces of an iron-210 at. ppm sulfur alloy, Fe₇S₈ and FeS₂ cleaved in an ultra high vacuum were studied by Auger (AES) and electron energy loss Spectroscopy (EELS). The S LVV Auger transition for the intergranular fracture plane of the alloy indicates that the sulfur is bonded to the surface as though it were adsorbed. The loss energies of the transition from valence to conduction bands for the surface are identical to those for the transgranular fracture planes. The trans- and intergranular fracture planes have very similar fine structure in the Fe MVV Auger transition profile. This indicates that the interaction between iron and sulfur is too weak to perturb the electronic structure at the fracture surface. The spectral features of the electron transitions having kinetic energies between approximately 40 and 50 eV are explained by a normal Fe MVV Auger transition and an autoionization process after excitation of Fe 3p electrons. The low spin ferrous ion in FeS₂ results in triplet peaks for the Fe MVV transition and doublet peaks for the autoionization event, but similar transitions for Fe₇S₈ exhibit singlet peak for each process.     

Auger analysis of etched (100) GaAs surfaces

The Auger depth profiling technique has been used to study the surface oxygen coverage and stoichiometry of (100) GaAs surfaces etched in various etching solutions. The quality of the surface varies with the etching solution, the etching time and the relative concentrations of the agents in each solution. The electrical behaviour of Schottky contacts deposited on the etched surfaces, was clearly affected by the characteristics of the surface.     

Diffusion of adsorbates on random alloy surfaces

In this work the diffusion of non-interacting adsorbates on a random AB alloy surface is considered. For this purpose a simple cubic (sc), body-centered cubic (bcc) or face-centered cubic (fcc) auxiliary metal lattice is introduced. The auxiliary lattice is truncated parallel to its (100) plane in such a way that the fourfold hollow positions of the metal surface form a regular net of adsorption sites with square symmetry. The adsorption energy of each adsorption site is determined by its own environment, i.e. by the numbers of direct A or B neighbors. The Monte-Carlo method has been utilized to simulate surface diffusion of adsorbates on such energetically heterogeneous alloy surfaces and to calculate the tracer, jump and chemical diffusion coefficients. The chemical diffusion coefficient was calculated via two different approaches: the fluctuation and the Kubo-Green method. The influence of energetical heterogeneities on the surface diffusion is largely pronounced at low temperatures and low surface coverages, where most of the adatoms are trapped by deep adsorption sites. It was found that at low temperatures the sequential occupation of the different types of adsorption sites can be observed. Received: 24 October 1997 / Accepted: 17 December 1997