Low energy ion scattering (LEIS) and the compositional and structural analysis of solid surfaces. Part I

Vacuum/volume 31 /number 6/pages 259 to 270/l 981 Printed in Great Britain

.0042-207X/81 1060259-l 2$02.00/O @ 1981 Pergamon Press Ltd

Low energy ion scattering (LEIS) and the compositional and structural analysis of solid surfaces. Part I” J A Van den Bergt M5 4WT UK received

4 March

and D G Armour,

Department of Electrical Engineering, University of Salford, Salford

1981

The physics of Low Energy Ion Scattering (LEIS) and its application as a surface analytical technique are reviewed. It is shown that compositional and short-range structural information can be obtained by choosing experimental conditions which optimize the contributions of single and double (or multiple) collisions, respectively. The LEIS technique allows mass analysis in a straightforward way, possessesa high surface selectivity but is unable to provide quantitative information in isolation due to scattering cross-section uncertainties and not easily quantifiable charge exchange effects. Structural information regarding adsorbate positions on single crystal surfaces and the short-range substrate structure (including damaged and reconstructed surfaces) can be obtained by exploiting shadowing andlor multiple scattering phenomena. The progress made in recent years in this area is charted. It is shown that computer simulations often play an important role in this type of study. Effects, such as charge exchange, inelastic energy loss and ion beam surface perturbations, which complicate the use of low energy ion scattering for surface analysis are discussed in detail. The present status of the technique in the different areas of study is indicated.

1. Introduction

Investigations into the fundamental phenomena of Low Energy Ion Scattering (LEIS) and its application for the analysis of solid surfaces have now been conducted for about 15 yr. Information about the scattering process and/or the surface with which the primary particles collide is contained in the energy spectrum of reflected particles. This information may be extracted either straightforwardly or may require the use of computer simulations of varying degrees of complexity. Although some of the surface analytical capabilities of LEIS were recognized and exploited at a very early stage in the development ofthe technique’, its potential for structural analysis of single crystal surfaces only became apparent more recently. Despite the quite distinctivequalities of LEIS in these increasingly important areas of study, the extent to which it has been adopted as an accepted surface analytical technique has been limited in comparison with other techniques such as, for example, AES, SIMS, and LEED. This may be due in part to the basic inability of LEIS to provide quantitative compositional information when used in isolation, but is probably mainly related to the fact that, contrary to other Part II will be published in a subsequent issue of Vacuum. t Present address: Department of Chemistry, University of Manchester Institute of Science and Technology, PO Box 88. Manchester Mb0 IQD. UK l

techniques, no state-of-the-art LEIS instrumentation has been commercially available for most of this period. Another possible reason for its limited application is that its structural analysis capabilities have only recently been exploited and, as for any new technique, there will inevitably be a period of time during which the fundamental aspects of the processes are more fully explored and the range of applications becomes more widely appreciated and adopted. Over the years a number ofexcellent general papers and reviews on LEIS have been published’-“. These contributions have, with perhaps one or two exceptions, largely focused on either the physical aspects or the application for surface analysis and all of them are now some years old. Since the latest of these reviews was published appreciable progress has been made in our understanding of a number of important areas. In the present review, an attempt has been made to provide an up-to-date (final quarter 1980) and comprehensive overview of LEIS by including a discussion of both the physical processes in relation to their contribution to the surface analytical capabilities and the actual application of the technique for surface compositional and structural analysis. 2. Development

and demarcation

The earliest experimental indication of the possibility of performing mass analysis of surface contaminants using energetic ion 259

J A Van

den

Berg

and

D G Armour:

Low energy ion scattering (LEIS). Part I

beams probably goes back to the work of Brown et N/’ 3, published in 1951. In a Rutherford backscattering (RBS) experiment using 1.237 MeV H+ ions and Li targets these authors found peaks in the energy spectra which they ascribed to carbon and oxygen contamination. In 1959, RubinlJ reported experiments in which 1.5-2 MeV proton scattering (RBS) was applied to the analysis of surface contaminants in the surface region of a metal. There too. peaks in the experimental energy spectra were found to result from single elastic collisions of primary particles with target or contaminant nuclei. In the meantime experiments with projectiles of much lower energies had also demonstrated the occurrence of single binary collision events. Brunnee”, ‘n 1957. in a scattering study of 0.4-4 keV alkali ions from a MO surface. noted the contributions of binary elastic collisions to the energy spectra and peaks due to this type of collision were dominant in the spectra observed by Paninlh in a study of the scattering of 7580 keV ions of various species off metal targets, and Datz and Snoek” who investigated 4&80 keV Ar’ ion scattering off copper. Of particular importance was the observation by Panin that upon lowering the ion energy, the spectra of Ar + scattered off a MO target became increasingly like those expected from elastic single binary collision events with target surface atoms. In the mid-1960s Smith, who recognized the potential of low energy ion scattering for the mass analysis of surface atoms, investigated the scattering behaviour of 0.53 keV rare gas ions from MO and Ni surfaces’.‘“. These experiments confirmed that the observed scattering peaks were caused by ions. which had been involved in single binary elastic collisions with surface atoms and suggested that, at these low energies. rare gas ions scattered from sub-surface layers had a very high probability of emerging from the surface as neutrals and therefore did not contribute to the experimentally measured ion spectra. This highly efficient neutralization effect, which occurs when inert gas primary ions are used. is one important characteristic which distinguishes rare gas ion scattering at low energy from that at high energies. The above work has, over the past decade, stimulated a considerable amount of interest in low energy ion scattering and, during this period, both the fundamental scattering processes and their application to the analysis of solid surfaces have been investigated. A parallel development, which, in fact, partly preceded the work of Smith, took place in the Soviet Union from about 1964 onwards. This work. which involved mainly three research groups, was predominantly concerned with the fundamental aspects of the interaction of 2-30 keV inert gas ions with crystalline solid surfaces. It has been reviewed by Arifov“’ and more recently by Mashkova and Molchanov“‘. These studies introduced the concepts of double and multiple scattering off a chain of surface atoms (so-called chain scattering) and showed that under certain conditions scattering off a surface can be described in terms of these processes. Important contributions to the development of the theoretical concepts of chain scattering, experimental studies and computer simulations of this type of scattering are respectively due to Parilis. Kivilis and Turaev”.“, Mashkova and Molchanov”,23 and Yurasova. Shulga and Karpusov’J,25 These studies were, on the whole, not specifically aimed at the application of ion scattering for surface analysis but the results formed the basis of the methods utilized later by other groups for the structural analysis of single crystal surfaces. From the above brief description it is immediately clear that ion scattering studies have been carried out over a wide range of 260

energies (0.1 keV+MeV’s) and a number of scattering regimes have been defined : Low Energy Ion Scattering (LEIS), Medium Energy Ion Scattering (MEIS). and Rutherford Backscattering (RBS). Since the relative contributions of the various types of scattering (single or multiple) and the detailed energy loss processes (elastic or inelastic) depend not only on the primary energy but also on the identity of the collision partners and the scattering geometries used, meaningful demarcation of these scattering regimes necessarily involves more than a consideration of the primary energy alone. The scheme proposed by VerbeekZh takes some of these fundamental properties of the particle solid interaction into account by using Lindhard’s reduced energy. c*‘, as its basis. Of relevance in the present review, the upper limit of LEIS is specified by c 5 0.3. which in more easily recognizable terms, corresponds to 2 keV He’ or 30 keV Ar’-ion scattering off copper. This energy regime is marked by the comparatively large collision crosssections and predominantly elastic energy loss processes which the primary particles suffer in single collisions with surface atoms. For this reason LEIS is otherwise referred to as ISS. Ion Surface Scattering*. In similar terms, the lower limit for RBS would correspond to approximately 60 keV large angle proton scattering off copper. In common with the low energy situation, the RBS regime is characterized by large angle single scattering events with target nuclei. At these energies elastic cross-sections are small. the particles penetrate to considerable depth and follow virtually straight line trajectories on their way into and out of the solid. The rate at which the projectiles lose energy inelastically is well documented and consequently the depth at which the large angle collision occurs can be determined. This technique thus provides information concerning the depth distribution of scattering centres. In the intermediate region (0.5 -CC< lo), referred to as MEIS, the overall scattering process is characterized by multiple collisions inside the solid accompanied by significant inelastic energy losses and the energy spectra of backscattered particles can be very complicated and difficult to interpret. In addition the generation of collision cascades in the surface region causes considerable sputtering and damage and renders this type of scattering relatively unsuitable for surface analysis. Although this reduced energy demarcation scheme can be used as a general guide and in the specific case of LEIS it illustrates the effect of the large cross-section at low energy on the surface specificity of the technique, it does not take into account the important influence of the charge state of the scattered particles and the scattering geometry on the relative contributions of surface and sub-surface scattering to the detected yield. When these factors are considered the LEIS regime can be defined as that in which top surface layer information is acquired by an effective elimination of the contribution of scattering events occurring below the first one or two atomic layers, without exploiting the channelling effect. Following this definition and taking into account experimental data, inert gas ion scattering from 0.1 up to about IO keV can be considered as LEIS (for the lower boundary value see Section 2). This is the demarcation used in the present work. At this point it should be noted that light ion scattering at energies of the order of hundreds of keV (i.e. below the energies l ISS also slands for Ion Scattering Speclrometry. an alternative acronym ‘o LEIS when used in the composition analysis mode (Section 5.1).

J A Van den Berg

and 0 G Armour:

Low

energy

ion scattering

(LEIS).

conventionally associated with RBS) under either single or double alignment conditions using high resolution energy analysing systems. represents an alternative to LEIS for obtaining information about the first one or two atomic layers of the surface. The potential of this technique, which uses channelling and/or blocking to reduce the yield of scattering events deeper in the crystal in order to emphasize the surface scattering contribution, has been explored in recent years. Unlike LEIS the technique is capable of providing quantitative compositional information and has become a powerful method for surface structure (e.g. relaxation) and composition analysis 2*m3’. In this ‘low energy RBS’ regime, typically using 50-600 keV H’and He’-ion scattering, the energy loss rate functions have their largest values3’.33 and hence maximal energy and depth resolution can be obtained. This further enhances the ability to separate surface and bulk information in the spectra. Whatever primary energy is used, ion scattering as an analytical technique is based on the energy selection or analysis of particles backscattered off a target through a specified angle. The experimental system needed for these measurements is, in principle, simple. and basically similar for the different energy regimes. A typical set-up involves a primary ion beam. well-defined in energy. species, direction and size, the target to be investigated in a suitable environment, and an energy analysing and detection system. In LEIS, electrostatic energy analysers (ESA) of various types have been almost exclusively used, although in recent years, time offlight (TOF) spectrometers have made their entry into this field. It is the inability of electrostatic analysers to pass neutral particles, which is an important but not the only factor in enabling LEIS to supply top surface layer information. In contrast to an ESA. when using a TOF spectrometer the energy of scattered neutral particles as well as ions can be measured and the resulting spectra may not be as characteristic of the surface as the ion spectra (Section 5.5). In a particular scattering set-up, the angular position of the energy analyser and hence scattering angle may be either fixed or variable. In general the degree of sophistication of the apparatus will depend on the type of information required (e.g. compositional or structural, qualitative or quantitative). In the following, it will be shown that by careful selection of the experimental conditions, it is often possible to effectively separate single, double or multiple collisions with surface atoms. This is important since compositional information is derived mainly from the former and structural information from the latter type of collision. It is clear that to meaningfully correlate the energy and angular distribution of reflected particles with the identity and the short-range structure ofsurface atoms, a proper understanding of the physical processes of the ion solid interaction and the parameters involved is required. In the interpretation of spectra, particularly in terms of structural information, computer simulations of the interaction often play an important role. This has also been the case in studies of inelastic loss and charge exchange phenomena and the effect of the interaction potential and the role of thermal vibrations on the spectra. All these factors form complications in the use of LEIS for surface analysis. In the following sections these subjects will be considered after a brief development of the physical basis of ion scattering as an analytical technique and a discussion of its use for composition and structural analysis. These applications can be considered as two distinct areas in LEIS and separately, since they are essentially

will therefore be considered based on different scattering

Part I types (single or multiple collisions, respectively) and in practical terms on the different choice of experimental conditions (e.g. scattering geometry. ion/target atom mass ratio) required to optimize the contribution of the type of scattering of interest. Throughout the review the present status of the technique in the various study areas will be indicated and the implications of the different scattering conditions on system design will finally be discussed.

3. Physical

basis

for ion scattering

as an analytical

technique

In a detailed analysis of atomic collisions, Bohr3’ showed that, provided the scattering angle is not so small that diffraction effects begin to play a role, the interaction between the incident particle and the target atom can be adequately described in terms of classical mechanics in the energy regime from IO eV to a few MeV. In this classical description, collisions are considered to be elastic. i.e. all kinetic energy lost by the primary particle is acquired by the struck particles(s). However the validity of the classical description does not mean that basically quantum mechanical effects, such as electronic excitation and ionization. can be ignored. These effects are energy dependent and can lead to inelastic energy losses, i.e. a change in internal energy of the collision partners. However it isgenerally assumed that. where they occur, they are to a first approximation uncorrelated and can be assessed separately. It appears that in LEIS using comparatively large scattering angles inelastic energy losses are only a minor effect. A second important aspect of ion solid interactions in the energy range of present interest is that they can be described by single binary collisions (as encountered in ion gas atom collisions) or alternatively as sequences of binary collisions. These points were observed in the experiments quoted in Section I. In this description, the ion, at any particular time, interacts with one individual target atom which is considered to be free, i.e. not interacting with the neighbouring target atoms. The physical basis for this model is hrst of all that for the energy range under consideration the (repulsive) interaction potential decreases so rapidly with distance that during a particular ion solid-atom encounter the interaction with neighbouring atoms is sufficiently small to be negligible. Secondly, the interaction time, for the same reason becomes of the order of 10-‘5~10-‘h s and thus considerably shorter than the vibrational period of the lattice atom which is approximately IO- I3 s. This means that during the interaction with one target atom, in which the energy transfer is far in excess of the binding energy, the target atom can effectively be considered as decoupled from the lattice. Consequently straightforward binary collision theory is applicable and this forms the basis of ion surface scattering as an analytical technique. Energy and momentum conservation laws allow the following expressions to be derived for the energy, E,. retained by an ion with mass M, after scattering through an angle (I with a target atom of mass MZ3$

Et -= EO

cos 9+JA2-sin2 l+A

8 ’

1

(1)

in which E, is the incident ion energy and A =MJM,. The positive sign in equation (I) is found for the case A > I, i.e. when the target atom is heavier than the incident ion and both signs when A < I. In the latter case the scattering angle is limited by the requirement O,,,

< sin - ’

A.

(2) 261

J A Van

den

Berg

and

D G Armour:

Low energy ion scattering

(LEIS).

The energy transferred to the struck target atom (recoil atom), Ez, is given by the expression E, 4A -=mcos ECI

2a

E 1 /‘h-M, EO- M,+M,’ This configuration is frequently used in LEIS for surface compositionalanalysis. The secondimportant parameterin the interpretation of an energyspectrumobtainedat a particular scatteringangleis the particleyield.This yield isrelatedto the arealdensityof scattering centresthrough the differential cross-sectionwhich, unlike the energyafter collision,equation(I), isdependenton the form of the interaction potential. It is definedas do(B) = - Znpdp

(5)

which isthe annularareabetweenimpact parametersp and p + dp for which incidentparticlesscatterinto a conebetweenthe planar angles0 and 0- d0, i.e. a solid angledR=Zn sin0 d0, sincethe situationisrotationally symmetricaroundthe incident beamaxis. The dilferential cross-sectionfor scatteringinto unit solid angle thus becomes (6) To evaluate the cross-section,it is necessaryto specify a relationshipbetweenthe impact parameter and the scattering angle.This is most conveniently derived in the centre of mass system(CM) in whichthe scatteringangle(which differsfrom 0) is denoted by 4 and is subsequentlytransformed back into the laboratory system(Lab).The requiredrelationshipbetweenp and 4 is againderivedfrom conservationconsiderationsand isgiven by the ‘so-called’scatteringintegralI f#J=n-2p

dr

= s‘0

is the interaction potential E, is the relative energy M2

E,=--

ER=Ml+M2

A l+A

EC

and r,,, the distanceof closestapproach,satisfiesthe condition

rOL 262

Finally. the relationshipbetweenthe scatteringanglein the CM systemt$, and the correspondingangle 0, in the Lab systemis given by either

tge =

in which z, the angle between the path of the recoil particle and the incident projectile, is limited to values x< 90’. Equation (I) shows that when the binary collision model applies, for a fixed value of the energy E, and mass M, of the primary particle and scattering angle 0. the energy Et after collision is solely a function of the mass of the atom involved in the collision. There is for example. no dependence of this energy on the interaction potential and this fact greatly simplifies the use of ion scattering for surface compositional analysis, since the energy spectrum can immediately, through the use of equation (I) be converted into a mass spectrum. For a fixed scattering angle 0 = 90L equation (I) reduces to the particularly simple relationship

where V(r) given by

Part I

ER

A sin 4 l+Acosc$

or

$=O+arc

sin 8 sin ~

A ’

The above relations,(&lo), clearly indicate that the differential cross-section,da(@)is not only a function of the incident energy, the massratio andthe scatteringangle,but significantlyasalready mentioned,of the form of the interaction potential. When the latter is known, da(o)can,in principle,beevaluated.However,an exactanalytical expressioncan only beobtainedfor certainforms of the interaction potential, e.g.the pure Coulombpotential used in Rutherford backscattering.For the formsofscreenedCoulomb potentialsuitablefor describingtheinteraction occurringin LEIS, no analytical expressioncan be derived and approximations basedon matchingpotentialsor numericalsolutionsaregenerally invoked. Apart from their relevanceto the evaluationof the crosssection, equations(7-10) are also of considerableimportance whenmultiplescatteringeventsare usedto investigatethe surface structure, aswill be shownin Section 6 (Part II). The validity of the binary collisionapproximation (BCA) and thereforethe reliability of equations(7-10) becomesquestionable when the energyof the incident ionsis reducedto the extent that their trajectoriesareinfluencedby their simultaneousinteraction with a numberof target atoms.Under thesecircumstances crystal bindingeffectscan alsobecomenoticeableand this hasled to the introduction of the conceptof an ‘effectivetarget atom mass’36, whichissomewhathigherthan that ofthe individual target atoms. This resultsin a higherenergy after scatteringthan predictedby the BCA. It is difficult to make theoretical predictions for this lower energy limit where this effect is observed but various authors have tested the applicability of the binary scattering modelexperimentally.However somecaution is necessarywhen comparingexperimentaldata with the predictionsof the BCA theory. An experimentally observeddeviation towards higher scatteringenergiesindicating an apparentbreakdownof the BCA neednot necessarilybe causedby either of theseeffects,sincethe occurrenceof multiplecollisionsconsistingof a sequence of binary collisionsalsoresultsin an increasedfinal energy (seeSection6, Part II). Impact and scatteringanglegeometries,therefore,also needto be taken into consideration.On the other handapparent agreementbetweenan experimentand the BCA theory doesnot necessarilyprove the correctnessof the theory, since in some instancesprocesses which haveopposinginfluencesin the particle energycan compensateeach other, e.g.inelasticenergy lossesvs any of the aboveeffects.Despitethis, experimentalwork hasgiven someindication of the range of validity of the BCA. Smith and Goff3’ in a study of He+ and Ar+ ion scatteringoff Cu through 90”, found equation 4 to be valid down to 100eV, and more recently.Tongsonand Cooper3ereported that, for He+ and Ne+ ionsscatteringthrough 90” off Cu at energiesbelow200 eV, the deviationsfrom equation4 weresmallerthan 0.4”/;,for He++Cu down to 40 eV and lessthan 0.7“<,for Ne++Cu down to 20 eV. While in one experiment Heiland and Taglauer39 found no deviationsfrom binary scatteringbehaviour for Ne+-+Ni down to 50 eV, their moregeneralobservationisthat for He+ and Ne+ ion scatteringthrough 90” off Ni and Ag, deviationsare found for energiesbelow 200eV4’. It is interesting to note that the deviationsobservedare towardslower energieswhich is opposite to that expected on the basisof the increasingeffective mass mode136. The observedadditionalenergylosses may be attributed

J A Van den Berg and D G Armour:

Low

energy

ion scattering

(LEIS).

to the occurrence of inelastic energy loss processes, which will be discussed in Section 8 (Part II). Matsevitch et a14’ found experimental agreement with binary scattering of He+ off PbS, Al and LiF targets for energies down to 100 eV, and Hulpke4’ has shown that for Li+ ion scattering off W and S the binary model still applies for energies below 20 eV if the attractive part of the interaction potential is also considered. Although Heiland et aj4’ observed deviations for Ar+ ion scattering off Ni through 60” below about 600 eV, Hart and Cooper43, using a 90” scattering angle, recently found the binary model to be valid to well below 100 eV for Ar+-Cu. The validity of the description of multiple scattering in terms of a sequence of binary collisions was investigated by Karpusov et al” in a computer simulation study of the scattering of SO-500 eV Ar+ ions off a Cu (100) face. It was found that for primary energies as low as 100 eV no significant changes in the scattering phenomena occurred, if non-binary collisions (i.e. simultaneous multi target-atom interactions with or without consideration of the binding energy) were taken into account. On the basis of the above data, it appears that for a wide range of ion-atom systems the binary collision model provides a valid description of the scattering kinematics in terms of one or more binary collisions for energies as low as 100 eV. This value, then, marks the lower boundary for the LEIS energy regime, quoted in Section 2.

4. The

interaction

interpenetrationof electronclouds, well-known Coulomb potential:

equation

1 I transforms

to the

Z,Z,e’

V(r) = ~

(13)

r

Variousanalytical expressions for the screeningfunctionsand the interaction potentialshave beenreported in the literature44-47. Someof theseare semiempirical,others are analytical approximationsderived from a detailedconsiderationof the changesin electronconfiguration basedon the Thomas-Fermi4*.49 (TF) and Thomas-FermiDira?’ (TFD) statisticaldescriptionsof the atom asa nucleusand a surroundingelectrongas”. Of these,the TF formalismfor the atom has been most frequently usedsinceit allowsa universalscreeningfunction to be derived.In low energy ion scattering,only a limited number of interatomic potentials have beenconsidered.Theseare listed below and are shownin Figure 1 for the caseof argon on copper. (a) The Born-MayerS3 potential-valid for relatively large (say> 1 A) interatomic distancesand of the generalform V(r) = A exp( - r/u).

(14)

potential

The importance of the interaction potential V(r) in ion scattering problems has already been intimated in the previous section not only for the calculation of the cross-section for single scattering but also for the determination of the trajectories and hence the final energies in multiple collision events. In terms of classical mechanics the potential of interaction between two atoms or an atom and ion arises from the mutual forces (repulsive and attractive) between the atomic nuclei and the electrons. It is repulsive for small separations and changes to attractive at large distances. For the energies used in ion scattering (> 100 eV) the particles approach so closely that the attractive part can be neglected, and the interaction may be described in terms of a totally repulsive potential. During a collision the electron clouds overlap to an extent which is determined by the relative kinetic energy. For energies in the LEIS regime, the interpenetration is only partial, the inner electrons continue to screen the nuclei and the interaction potential can thus be described by a Coulomb potential with a screening function to account for electron-nucleus and electronelectron interactions. The effect of these interactions is to reduce the Coulomb repulsion such that the general form of the potential becomes V(r) = ~. Z1T2e2

Part I

qb(r/a)

(11)

whereZ,e andZ,e arethe nuclearchargesof projectileandtarget atom, $+/a) an appropriate screening function and a the screeninglength. For example according to Lindhard” the screeninglength is uL = 0.468(2, 2’3+ Z22’3) - 1’2.

(12)

For high energy collisions (e.g. the RBS regime) where the influenceof the screeningis almostnegligibledue to the complete

r,

a

Figure I. Various representations of the interaction potential function V(r) for Ar-Cu: The Firsov (F), the Born-Mayer using Abrahamson’s constants(A); or those given by Andersen and Sigmund (AS) and the Moliere potential with four different screening lengths: C= 1,0.X75.0.75 and 0.5 in equation 16c (MI, M2, M3, M4 respectively)s’.

Universalconstants,A and a, for this potential havebeenderived by Abrahamsonthrough individual pair-wisecalculationson the basisof the TFD theory, and thesehave, becauseof their ready availability beenusedwidely,despitethe fact that Gunther” with referenceto Abrahamson’scalculationss6 has pointed out errors which resultin too high valuesfor the interaction energy.Figure 1 also indicatesthat comparedto other common potentials the Born-Mayer-Abrahamson V(r) is relatively high. A Born-Mayer potential with empirical constantsobtained by Andersen and Sigmund” has also beenincluded in the figure and is characterized by considerablylower interaction energies. valid (b) The Firsov potential5s*59 based on the TF formalism, only for a range r< 1 A, asit predictably falls off too slowly at 263

J A Van den Berg and D G Armour:

Low energy ion scattering (LEIS). Part I

larger distances due to the neglect of electron effectsho. It has the form:

range energy

V(r)= ~z1;2e’ f#l(I’/NF)

(154

where &Y/Q) is the Thomas-Fermi screening function5’ the Firsov screening length given by

and aF

a,=0.468 (Z,1’z+Z21!2)-2’3. (c) The Moliere TF atom model V(r)

= ~Z1t2e2

(15b)

potential-like

the previous one based on the

4(r/a).

In this case the screening function” analytical expression of the form”

is approximated

exp( - 6.~)+ 0.55 exp( + 0.35 exp( - 0.3.~)

#(r/a)

= c#J(.u) = 0.1

by an

1.2s)

(16b)

in which the screening length a is given by equation (12) or (15b). For reasons discussed below, the expression for the screening length (12) is frequently modified to

a=0.468 C (Z,2’3+Z22’3)-‘r2.

(16~)

The Moliere potential is at present the most widely used form in ion-surface scattering since it is appropriate over a large part of the relevant interaction range and is found togive good agreement between computer simulations and experiments if a somewhat reduced screening length is used 52.62-65. This is achieved by reducing the value of C in equation (16~) to a value smaller than unity. the result of which is a lowering of the interatomic potential as is shown in Figure I for four different values of C (C= I. 0.875. 0.75 and 0.5). In the studies referred to above, it has been found that the empirically derived values for C lie within the range 0.61C10.8. It is interesting to note that another screened Coulomb potential, the Lenz Jensen potentialh6.67 for Ar-Cu virtually coincides with the Moliere for C=O.75 (M3) over the entire range shown in Figure I. A final comment on the interatomic potentials applied concerns the consequences of the use of screening functions based on the TF (and TFD) models. Since these are statistical descriptions of the atom as a rotationally symmetric, totally degenerate electron gas surrounding a nucleus, the screening functions are smoothly varying and do not contain shell effects. Discrete inelasticelTects in scattering, such as charge exchange and excitation, are thus inherently ignored and must be described separately. 5. Low

energy

ion scattering

for surface

composition

analysis

In Section 3 the fundamental importance of single binary collision events to the use of LEIS as a surface compositional analysis technique was discussed. As mentioned this application is often referred to as Ion Scattering Spectrometry (ISS). The extent to which single scattering can be made to dominate the collision events depends on the choice of scattering conditions, but as a general rule large scattering angles (2 60 ) and light projectiles lead to energy spectra dominated by the contributions of single collisions. For a given projectile-target surface combination the critical angles for scattering, incidence and emergence above which the scattering, from experimental point of view, can still be characterized as single scattering can, for 5.1.

264

Introduction.

a certain choice of the interaction potential, be calculated2’.h8. It appears that these angles become larger for increasing projectile mass and decreasing primary energy. In general a value of 90 for the scattering angle (and about equal angles of incidence and emergence) not only ensures single scattering events but also makes the conversion of the recorded energy scale into a mass scale very straightforward, equation (4). and, for this reason, this geometry is often used. In fact a commercially available ISS apparatus uses this scattering angle. A typical example of composition analysis using the 0 = 90 scattering configurations is given in Figure 2 which shows the energy spectrum of 300 eV Ne’ ions scattered off a halogenated Ni( 100) surface obtained by Brongersma”. Apart from a scattering peak for the target material, peaks for Ne+ scattering off 3sCI, “Cl, Br and I present on the surface can be seen. The directness of the technique is evident but from a practical point of view it must be noted that spectra such as that shown in Figure 2 are only obtained in LEIS under well defined uhv conditions when using low intensity ion beams. Heavily contaminated surfaces tend in general not to give spectra as clear cut as the one shown but exhibit rather heavy low energy tails, particularly for oxidized surfaces, and sometimes no distinct peaks at all. The peak positions in Figure 2 and those predicted by equation (4) are found to be in very good agreement. Previously Smith’ investigated the validity of equations (I) and (3) which give the energy after collision of the primary backscattered El/E,, and recoil particles EJE,, for the case of scattering of I.8 keV He’ and Ar’ ions offan oxygen contaminated Ni surface, over a large range of angles and he also found good agreement between theory and experiment. Br NI Ne’

300

eV

NI (100) 8=!xl* Halogen

E II

Figure

2.

halogenated

Energy spectrum Ni

(100)

of 300 surface”g.

adsorption

ev

eV Ne’

ions

scattered

through

90 08 a

Energy spectra such as that shown in Figure 2 give an immediate indication of the resolution and sensitivity capabilities of the LEIS technique from the point of view of composition analysis. However, to fully assess the potential of the technique questions concerning the extent to which the ion yield or intensity of a particular peak in the spectrum gives an absolute measure of the surface concentration of an observed species, the elemental sensitivity or detection limit, and the surface selectivity as well as the dependence of these factors on the chosen conditions, require discussion.

5.2. Mass resolution. It is immediately clear that, because of the straightforward relationship between the energy after collision El and the mass of the struck atom M,, as expressed in equation (l),

JA

Van den Berg and D G Armour

Low energy ion scattering (LEG). Part I

Figure

150

400

e

3. The dependenceof the function y1.J. 0) in equation (17) on thescattering anglefor various collision partners

the mass resolution is also closely related to the energy resolution of the overall ion scattering apparatus. The relationship can be expressed for il> 1, i.e. M2 > M, as:

80% Au 20% NI ED: 18 keV

(17a) where A+sin’ d-cos @AZ-sin’ 2A A+ 1 ’ AZ-sin2 e+c0s @AZ-sin2

dA, e) = -

0)“’ ep2’

(1%)

The dependence of the function y( A, 0) on the scattering angle 8 is plotted in Figure 3 for various projectile-target mass combinations This figure clearly shows that a high mass separation is obtained for larger scattering angles and small mass ratios, in other words heavy projectile mass. The latter point has been illustrated experimentally by Smith in the spectra of He’ and Ne’ ions scattered off a MO contaminated Au-Ni alloy as shown in Figure 42. The separation between the peaks in the Ne+ spectra has increased considerably. The requirement of large scattering angles in order to achieve single scattering conditions (Section 5.1) thus turns out to be advantageous from the point of view of the mass resolution. However, scattering yields decrease for larger scattering angles due to the falling cross section and this situation in ISS is yet another example of the trend generally found in mass spectrometry that an increased mass resolution is obtained at the cost of a

1 04

05

06

Scattered

07

ion

Figure 4. Energy spectra of I .X keV contaminated Au Ni alloy showing heavier primary ions’.

08

energy,

09

08

09

10

El/E,

Ne + and He + ions scattered the increased mass resolulion

ON a MO for the

yield and sensitivity. This feature therefore emphasizes the advantage of the availability of a variable scattering angle which allows either the mass resolution or the sensitivity to be optimized and indicates the severe restrictions of a fixed geometry system. A fundamental factor which limits the maximum obtainable

poorer

265

J A Van den Berg and D G Armour:

Low

energy ion scattering

(LEIS).

mass resolution, apart from the already mentioned instrumental energy resolution, is the natural linewidth of a peak which is determined by the thermal motion of an atom in the solid”. This quantity would appear to be about 0.5-0.8”;“. Scattered ion yield. The sensitivity of ion scattering in a particular experimental situation is related to the measured ion yield, in that it expresses what yield is to be expected for a target species of a certain concentration. An attempt to quantify the scattered ion yield or intensity of a peak in the spectrum has to take several factors into consideration, among them obviously the cross-section, equation (6). In a typical low energy ion scattering set-up in which inert gas ions are used, charge exchange phenomena also have to be taken into account, and the ion yield Ii for scattering olfaparticular species with surface concentration Ni can be expressed as3.“-” 5.3.

Part I potentialsmentionedin Section4. Howeversuitableapproximate expressionsfor the cross-sectionhave beenderived and reported in the literature e.g. for the Firsov potential” and the Born-Mayer potential” and have been tabulated for various potentialsby Robinson”. Usingdata from that work, Ecksteinrr ale2plotted cross-sections for 4.8and 15.1keV Ne+ ion scattering off Ni, asa function of the scatteringangle.Theseplots areshown in Figure 5 which servesto illustrate the generalbehaviourof the cross-section.It showstwo trends valid for all potentialsusedin LEIS (and in fact all repulsivepotentials); (I) a rapid drop in the cross-section for increasingscatteringangleup to about 90” after

IO” 50 20 IO’ 50

where 1, A

dai dR AR T pi

is the primary ion current on the area A (A cme2); the overlap betweenthe bombardedarea and the area sampledby the analyser(cm’);

20 100

rot PVI-

50

the differential crosssectionfor scatteringinto unit solid 2c

angle(cm’/sr) ; the acceptanceangleof the analyser(sr); the overall transmissionfactor of the energy analyserand detector; the probability that a particle escapes in the ionized state, sometimes referredto as(1 - P,) whereP, is the neutralization probability.

Equation 18 is obviously only valid if speciesNi is not shadowedby other species.The situation where this occurs is discussed in Section 5.6. All parametersin equation(I 8) except for da/da and Pi can be accuratelyspecifiedfor a given experimentalsystemand if it were not for these,ion yields could be directly related to the surface concentration, hence allowing the surface composition to be determinedquantitatively. It is interesting to note that in RBS measurements the scatteringcross-sections are accuratelyknown andboth ionsand neutralsare analysedand detectedthrough the use of solid state detectors and this technique is, therefore, uniquely capableof yielding quantitative compositiondata. The secondunknown, the ion escapeprobability Pi or rather the neutralizationprobability P,, standsfor net overall effectof a numberof different chargeexchangeprocesses75-77 which may take placeduring the interaction of the ion with the surface,and which eventually lead to the projectile leaving the surfacein a neutralstate.PN maybeashighas99.97~7*.7g but cannot easilybe quantified theoretically. Scattering and neutralization may, to a good approximation, be regarded as independent and can thereforebe treatedseparately.Chargeexchangephenomenawill be discussed in moredetail in Section 6 (Part II). The problemsassociatedwith a quantitative determinationof the differential cross-sectionda have already been indicated in Section 4. The preciseform of the screenedCoulomb potential appropriatefor interaction in this energyrangeis not known and an analytica! solution of this integral is impossiblefor the 266

&

10. 5C 20 10.'L

0

20

40

60

60 ICO I20140

160 100

Scatteringangle, LS Figure 5. DitTerential cross-sections for thescatteringof 4.8 and Ne+ OR Ni as a function of the (lab) scattering angle”.

15.1 keV

which decreaseis muchslowerand (2) a decrease in cross-section for increasingprojectile energy, at a fixed scattering angle. A similar behaviour to that shown in Figure 5 is also observed experimentallyfor the ion yield in thoseinstanceswherethere is no overriding influenceof neutralization2s3.84.Consequentlyfor high sensitivity composition analysis, small scattering angles (30”-60”) aremostprofitably used,keepingthe condition ofsingle scattering in mind. However if, on the other hand, high mass resolution is required it is advantageousto utilize very large scatteringanglessincethe drop in sensitivityfor scatteringangles larger than 90” is only relatively small.An interestingtechnical development which experimentally compensatesfor the low sensitivity at large scattering anglesis the NODUS scattering apparatus, describedby Brongersmaet aI”*a5. It employs a modifiedcylindrical mirror analyser(CMA) equippedwith a ring detector which allowsthe incident ion beamto passthrough its centreand the completeconeof scatteringanglesto be accepted. This high acceptancefacility offerssignificantsensitivityimprovementsover the more commonly usedsector field arrangements in which, through the use of aperturesto define the scattering anglein the plane,the analysersolid angleis only of the order of 1O-3-1O-5 sr. In fact for a scatteringangleof 142”.improvement in sensitivity, of a factor of 100-1000over other typesisobtained

J A Van den Berg

and D G Armour:

Low energy ion scattering (LEIS). Part I

with, at the same time, high mass resolution due to the large scattering angle employed. A somewhat similar type of instrument using a 138” scattering angle geometry has been commercially available for a number of years. In order to obtain an estimate of the size of the ion yields to be expected in a conventional scattering apparatus with a small aperture, high resolution energy analyser, the ion yield equation (18) might be evaluated ignoring charge exchange effects and transmission losses. Assuming typical values for the cross-section da/dR= 0.1 - 1 A2 (see Figure S), a target area A = 0.1 cm2, a surface density N = 10” atoms cm2 and a solid angle AR= lo-’ sr then it follows that the particle yield is ]=I,.

is illustrated in Figure 6 for the case of He+ scattering off 0 on Ni (111). A second observation also shown in this figure was the linear decrease of the Ni substrate scattering signal due to shadowing (Section 5.6) by adsorbed 0 and S atoms. A close to linear relationship for the He’ scattering yield off Be on W up to about f monolayer (ML) coverage was also observed by Niehus et 01”. For higher coverage however, deviations were found which were ascribed to a change in neutralization caused by surface rearrangement and a change in the work function. The nonlinear relationship observed by these authors for the ion scattering yield

(10-6-10-7).

This is of course further reduced by about a factor in the range of lO-IO’, taking neutralization effects into account. Primary ion beam intensities have, for reasons of minimizing target perturbation (Section 9, Part II) to be kept below 10e6 and preferably to lo-’ A cm2; this, combined with the above estimate of 10-7-10-‘o, clearly indicates the need for a high transmission energy analyser and a sensitive particle detection system, having low background counts, i.e. well below I cps, to obtain acceptable signal to noise ratios. The application of TOF techniques to the energy analysis of reflected particles, which allow adequate energy resolutions to be obtained for the energies used in LEIS, effectively overcomes this sensitivity problem, since both neutrals and ions are analyseda~“. In addition lower ion fluences are required for a spectrum and this ensures less target perturbation”*. The extra information from deeper layers inherent with the analysis of neutral particles can be reduced by a suitable choice of the scattering conditions*‘s’. The use of alkali primary ions, a technique extensively used in early Russian work”, and recently revived89.90, represents an alternative means of performing low intensity, low dose surface analysis because of the low neutralization probability of these particles. However regular sputter cleaning with rare gas ions is required to restore surface cleanliness. The problems mentioned in connection with quantifying the yield equation, mean that the ion yield and thus the sensitivity for scattering of a particular species cannot be predicted and this renders LEIS essentially a non-quantitative composition analysis technique. Nonetheless, there are as discussed below, some quantitative aspects to the sensitivity of ion scattering at low energies. aspects. Despite the fact that the sensitivity of LEIS cannot be specified quantitatively i.e. there is not a universally applicable relationship between ion yield and surface atomic density for each element, there are situations involving either comparison of ion scattering data with those obtained in situ, using a reference technique or the use of a reference material in which the ion yield may be calibrated to give quantitative compositional information. The quantitative capabilities of ion scattering spectrometry have been quite extensively investigated and exploited using both the above modes of operation. Low energy ion scattering was combined with Auger electron spectrosscopy (AES) by Taglauer et a/ in a study of the contamination of a Ni (111) surface with various amounts of oxygen and sulphur. For submonolayer coverages the He+ and Ne+ ion yields for scattering OF the adsorbed species were shown to depend linearly on the coverage determined by (calibrated) AES74*9’. This result 5.4. Quantitative

. IS: He -Ni (Ill) ? :b$ J, z LSQ

.

I,

i 51.10‘~A

AES: E, = 214 I.

=SO/JA

2VptP .

\

1 2 3 L Oxygen-exposure (au) Figure 6. The dependenceof the ion scattering and AES signals on the oxygen coverage during exposure”.

off 0 on W may be connected with their method of depositing tungsten oxide, which possibly resulted in the partial shadowing (Section 5.6) of 0 by the W atoms. Smith2 compared the ion scattering signals obtained from Au-Ni alloys of varying ratios with electron microprobe data giving bulk compositions and found a nearly linear relationship between the two methods. However, some Au enrichment on the surface was observed. Brongersma et a/a4 in a more recent investigation of the surface enrichment of Cu in Ni-Cu and Cu-Pt alloys of different composition ratios, reported that for the Cu-Pt case the surface composition could be determined with high accuracy by the ion yields for the alloys against the ion yield obtained for the pure materials and that the concentrations found added up to 1004; within experimental error. This is an important result since it shows, as do the above mentioned linear relationships between the ion yield and coverage, that at least in these systems there is no matrix effect i.e. the ion yield is not, or not strongly, affected by the chemical environment of the atom offwhich the ions are scattered. These instances demonstrate that ion scattering yields can give a quantitative measure of the concentration of a species on the surface and also that the sensitivity for a particular species can be quantified provided provision is made for external calibration. A further example of the use of LEIS for quantitative analysis in the 267

J A Van den Berg

and D

G Armour:

Low

energy ion scattering (LEIS). Part I

work by Tongson et 01yL in which the Nb/Ge ratio in superconducting Nb,Ge films was found to agree with data obtained using other techniques. The use of reference techniques or reference materials is also essential when trying to obtain values for theelemental sensitivity or detection limit of LEIS for a particular species. Although this parameter is of prime interest it is impossible to give a figure of general applicability since, apart from the basic problems associated with quantifying the ion yield. there is an obvious dependence on the performance details of the particular experimental system used. Various studies have been carried out, however, from which empirical data on detection limits have been derived. Using neutrons activation measurements for the calibration of the scattered ion yield. Ball er a/93 using He and Ar primary ions, found a detection limit of 5 x 10e4 ML for Au deposited onto Si. For Br on Si, Brongersma” using He ion scattering, estimated a value of 10m3 ML. In the surface contamination work by Taglauer Eda19’ a detection limit of better than lo-’ ML was reported for the scattering of Ne’ of!% For 0 on Ni a value of3 x 10m3 MLwasderived”. For C.a sensitivity of IO-’ ML may be estimated after extrapolation of the above figures. It can be seen from these data that for lighter masses the sensitivity decreases, as is expected from the decrease in the scattering cross-section. Although hydrogen cannot normally be detected its presence on the surface can cause a decrease in substrate signal due to shadowing and this effect has been observed for He’ scattering below 1 keV off Niss and W’. The latter results re-emphasize the need for well defined clean target conditions in this type of measurement. The use of TOF energy analysis results in two important improvements in this connection in addition to the increased sensitivity already mentioned. It allows the detection of sputtered recoil H-atoms, as recently shown by Luitjens er a/s* and also opens up the possibility of performing quantitative composition analysis after a single scattering cross-section determination for each element since neutralization effects no longer form a complication. 5.5. Surface selectivity. The specificity of the scattered ion yield to the outermost surface layer when using low energy inert gas ions has already been mentioned and this feature of the LEIS techniques is one of its major assets for surface composition analysis. This unique surface specificity is primarily the result of the neutralization of particles which penetrate beyond the surface before being backscattered. The use in conventional scattering systems, of electrostatic analysers which do not transmit neutral particles effectively filters out this ‘sub-surface’ yield. The neutralization process is particularly efficient for projectile particles which have a sufficiently large ionization potential that their outer electron levels lie well below the Fermi level of the bombarded solid, since under these circumstances tunnelling of electrons from the valence or conduction bands into an unoccupied state of the projectile occurs with high probability. Hence Auger and resonance neutralization of rare gas ions are highly efficient processes and these and other charge exchange effects will be further discussed in Section 7, Part II). The use of alkali primary ions, which has been briefly mentioned, results in high scattered ion yields and thus high sensitivity since neutralization is now very improbable due to the small ionization potential, in general smaller than the work function of a solid. The difference in energy spectra between active and inert gas ion scattering was 268

illustrated by Smith’ for I.8 keV He’ and H+ ion scattering of Au. With He+ ions there is a clear surface peak and no low energy background but with H+ ions this background is so high that it immerses the surface peak and little or no surface information can be extracted. Similar differences are observed when the spectra of large angle (fl= 135”) backscattered He’ ions and He atoms off Ni (using a stripping cell) are comparedgJ. A second important reason for the occurrence of surface peaks and hence surface specificity of low energy ion scattering was pointed out by Buck rr alg6. At low primary energies, the crosssections for backscattering are so high, particularly when heavier primary ions are used, that a rapid beam attenuation inside the solid occurs resulting in a general yield reduction from deeper layers. The surface selectivity of LEIS has also been demonstrated in studies of the polar faces of non-centro-symmetric crystals as CdS. CdSe, ZnS and Gap. Strehlow er a/95 unambiguously identified the (I 1 I) as the Cd and the (iii) as the S face of CdS by the difference in Cd/S peak height ratios for the two faces. However, both faces showed large Cd scattering peaks. Honig rr al’ observed similar results for CdSe. The results of Efremenkova et n/96 on CdS were inconclusive, probably because of the high beam current used. Brongersma and MuI” found that the LEIS spectra of CdS when --using low ion current densities only showed an S peak for the (I 1 I) face and no Cd while spectra for the (1 I I) face were entirely dominated by the Cd peak. This experiment not only demonstrated the extreme top layer sensitivity attainable but also emphasized the necessity of using low ion currents and fluences to exploit this feature of the technique. Similar results have also been reported for ZnS and Gaps’. Perhaps the most elegant demonstration of the surface selectivity of ion scattering was the study, again by Brongersma era/ ‘1.98 of the adsorption of bromine onto Si (I 1 I) where, from bonding arguments, it is concluded that the surface is saturated after the formation of a single monolayer. There is, as shown in Figure 7, no He+ backscattering off Si after adsorption, in other words, the substrate is completely masked by the presence of this single layer of adsorbate. It is this sensitivity to the topmost layer or surface specificity which makes LEIS a very valuable tool in surface analysis despite its general inability to provide quantitative compositional information. It is therefore a particularly suitable technique for the study of phenomena which takes place essentially in the top surface layer, such as adsorption’~6~99-10’catalysis’0~’06,adhesive bonding’07~‘08,segregation’ 1.85 and activation effects in tungsten-impregnated dispenser cathodes’09*1i0. Si \

I

\

Br

\ I5

\

1

\ \

i

II II

-

II

--

Sill11

\

5

\

Sil 111 I +Br 2

\ t 73 01 = : In

\

-I 0

L--. 250

500

750

E,leVI

1000

-

Figure 7. Energy spectra of 1 keV He+ ions scattered through 142” off a clean Si (11 I) surface after adsorption of I ML Brg8.

J A Van

den

Berg

and

Low

D G Armour:

energy

ion scattering

(LEIS).

5.6. Shadowing. The presence of adsorbates on the surface will in most cases cause a reduction of the ion scattering yield from the substrate and this effect is commonly referred to as shadowing or masking. To formally describe this situation, the ion yield equation (18) for scattering of the substrate has to be modified to’OJ

2 ARTP,(N,-aNj)

li=l,A

(19)

where Ni is the surface density of the clean surface, Nj that of the adsorbate and ix the shadowing coefficient. The decrease in substrate ion yield can, in principle, be caused by two effects; the actual physical masking due to scattering by the adsorbate atoms or a change in neutralization behaviour caused by the presence of the adsorbate. This shadowing effect has found application in surface structure analysis (Section 6.3, Part II) and has been used, instance, to determine the relative position of the C and 0 atom following CO adsorption on metals’.“*. In this type of study it is important to assume that the ion escape probability P, for scattering off the substrate does not change with coverage. as otherwise the analysis of shadowing effects becomes difficult. This assumption can be tested experimentally in situations in which the surface density of a species j, Nj is varied. An example is the sputtering by 600 eV He’ ions of CO adsorbed on Ni (I I O)‘04. By considering the yield for scattering off clean Ni, I,vo, equation (18) CO adsorbed Ni. lNi, equation (19) and the 0 scattering yield, I,,, equation (18) it can be shown that

I

ox 0INi

*-

1 a

1-N’ (

I I,y

(20) )

when it is assumed that the ion escape probability P,JP,vi is constant. From Figure 8. where the ion yield ratios for scattering off 0 (from CO) and Ni as a function of the coverage are shown, it is seen that this proportionality does hold. Compliance with this condition, although necessary for the P,JP,, ratio to be coverage independent, is not sufficient to prove such an independence since compensating changes in the two factors may occur. The figure also shows that if P,JP,, is constant, x is also constant. However the shadowing coefficient has been found to change under large coverage conditions’ ’ ’ and can also be expected to change if for instance surface rearrangement occurs during adsorption. Interesting applications of the shadowing effect are the preferen-

He+NI (IlO)+ CO E,=600eV @:300

0

I

I

I

I

1

1

2

L

6

8

IO

I2

Dose

w

( pA.sec)

Figure 8. Dose dependence of the He+ ion signal for scattering of 0 (in adsorbed CO) and OR the Ni (I IO) substrate. The Ni signal is plotted as 1 -I,,//,+,’ equation (20)‘OJ.

Part I tial adsorption of CO at low coverages onto Ni in a Ni-Cu alloy’ and the possibility of detecting adsorbed Hbs5. However the major application of the shadowing effect has been in the area of surface structure analysis which will be further considered in Section 6.3. Part II.

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” A B Brown,

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J A Van den Berg

and D G Armour:

Low

energy

ion

scattering

(LEIS).

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270

Part

I

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