Sputtering of ionic clusters from KCl, NaCl and LiF

Nuclear Instruments and Methods 191 (1981) 259-267 North-Holland Pubhshlng Company

259

SPUTTERING OF IONIC CLUSTERS FROM KCI, NaCI AND LiF Z L. KERESTES Mineral Phystcs, CSIR O, North Ryde, Sydney 2113, N S W, Austraha

J C KELLY, R L DALGLISH Phvstcs, Umverstty o f New South Wales, Kensmgton, N S W, Austraha

R J MACDONALD, A R BAYLY and P.J MARTIN Phystcs, Austrahan National Uml erstty, Canberra, A C T, Austraha

Clusters invariably originate from the topmost layers of a solid and thus provide Important reformation concermng the state of the surface We have studied the mass and energy spectra of the cluster component of the positive 1on sputtering yield during 45 keV argon Ion Irradiation of (100) cleavage faces of KC1, NaCI, and LIF The mass spectra, with 1 ainu resolution reveal the relative posmve ion yield to be predominantly composed of the alkah Ion M+ with the second most abundant species (by an order of magnitude) being the dlmer M~ The spectra also show a variety of lower yield metal and alkali halide ion clusters of the form Mn and MnXn_l, where M is the cation and X the anion of the alkah halide During the course of Irradiation of Insulators a positive surface charge develops on the surface and the resultant surface potential distorts the energy spectra We propose a two component surface potential model based on the heating effect on the surface by the irradiation beam The model is used to deconvolute the observed energy spectra. The corrected results of the monomer M÷ energy distribution show an /~-2 dependence at high energies in agreement with simple colhslon cascade theory The cluster energy profiles on the high energy side give an E m dependence with the index m increasing with increasing cluster size The results suggest a colhslonal mechamsm responsible for their ejection

1. Introduction A wide variety of polyatomlc particles (clusters), both in the neutral and Ionic state can be hberated from solid surfaces by r f sparking, sputtering or generally by any technique that imparts sufficient energy to break surface bonds Of the available techniques sputtering provides the greatest control, sensltwlty and flexibility of experimental conditions for generating some unique cluster species The cluster yield is much smaller than that of single atoms and ions Nevertheless the cluster sputter component has recently attracted the Interest of several workers m the field of surface analysis, molecular beams and physical chemIstry Charged sputtered clusters have been examined by Jurela [1 ], Staudenmaler [2], Denms and MacDonald [3] and Herzog et al [4], while neutral sputtered clusters were Investigated by Schmldt-Bleek et al [5], Baede [6] and Konen et al [7,8] In general the cluster distributions appear to be hyperthermal, falling off more rapidly at high energies than the monomer dxstrlbutlon 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 75 © 1981 North-Holland

Several attempts have been made to provide a theoretlcal basis for the observed energy distributions. Baede et al [6] apphed the thermal spike theory to the energy distribution of K2 dlmers sputtered from polycrystalhne K and found the theory to predict an Improbably high spike temperature of 7820 K Staudenmaler [2] proposed a model in wtuch clusters are formed from individual atoms leaving the colhsaon cascade, with approximately the same m o m e n t u m and at the same time, the cluster ejection probabilities being determined by the differences in the binding energies of the various clusters with respect to the surface With a similar set of assumptions Konen et al [7,8] proposed the "statistical model" of molecule ermsslon to explain the energy spectra for both homonuclear and heteronuclear particle clusters Independently, Gerhard [9] and Gerhard and Oechsner [10] proposed a similar model for clusters sputtered from metals and applied it successfully to relatwe yield measurements This paper reports measurements carried out w~th single crystals of KC1, NaC1 and LIF Irradiated with Vl SURFACE SCIENCE

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Z L Kerestes et a l / Sputtering o f tom6 dusters

43 keV Ar ÷ Ions The sputtered positive Ions are analysed m mass and energy, with the specxfic aim of studying the cluster colnponent We also report here the effect of surface charging on energy spectra and propose a two component surface potential model that is used successfully to deconvolute the observed energy spectra

2 Experimental The experimental details have been described previously by Bayly et al [11] In this paper we briefly m e n u o n the mare features A duoplasmatron ton source produces a beam of Ar + Ions (mass selected) with energies up to i000 keV with typical beam current densities of 100/IA/cm 2 and approximately 1° arc colhmatlon The detecnon system consists of a secondary ton collimator, a grid system to produce a planar electrostatic field, Herzog plates, a hemispherical electrostatic energy analyser, a quadrupole mass spectrometer and a channeltron pamcle detector A planar field is placed normal to the secondary ton beam axis between the exit aperture of the drift tube and the entrance aperture to the hemispherical electrostatic analyser The energy and mass analysis systems are operated at a fixed energy and pass band, and across this constant window the energy spectrum IS scanned by a slowly swept hnear bias voltage applied to the target and colhmator The advantage of operating the analysis system at a fixed energy overcomes the problem of the energy dependence of both the energy pass band and the quadrupole mass resolution In all the experiments reported here the angle of incidence to the surface normal as 45 ° and the angle of analyser to surface normal is 45 ° All the samples are single crystals, cleaved in air along tile (100) direction In order to reduce any surface charging effects the surface of the target as flooded with thermal electrons from a tungsten filament

3 Results 3 1 The mass spectra

The mass spectra consist of sputtered particles ot uniform energy, within an energy spread of 1 eV (cor-

lespondmg to the resolution of tile enmgy analyser) The most intense mass hne found from the sputtering of KC1, NaC1 and LIF is the alkali metal monomer, M+ The constant energy pass band used for a particular mass spectrum run is determined from the peak of the energy distribution of the M+ species The energy peaks of the duster species are found to be within this pass band, thus no correcUon to the cluster yields is necessary Fig 1 shows a typical mass &strlbutlon, plotted on a log linear scale, of the sputtered posmve ions obtained m tile low mass (0 140 anm) range ot the quadrupole As tile mass resolution on the low mass range is 1 amu, peak identification IS facilitated by the p~esence ot adJacent isotopic peaks The background counts ate as low as 1 count/s Typically the M+ peak for KCI and NaCI ts ~ I 0 4 counts/s and tol Lt+ ~ 103 counts/s with the predominant cluste~ peaks between one and one and a half orders of magnitude smaller Table 1 shows the relative abundances of tile well resolved isotopes, as well as comparison with their natural abundances Agreements are within 10% Comparison of the yield of the various cluster sputtered species relauve to the total Integrated counts collected during a single mass scan are shown In table 2 The values m the table are obtained from the sulnmatlon ovm all isotopic cont~lbuUons The e~rors associated with the values of both tables are as high as 10T This estmrale is made from the observed effects ot the beam fluctuation on the ymld and the unceztamty associated with the cluster energy peaks within the pass band The main features that emerge from the KCI, NaC1 and LIF posmve ion mass spectral lesults are the following (a) the most intense peaks correspond to the alkah metal monomer M+, (b) L1F, containing the hghtest alkah metal, produces the greatest proliferation of molecular Ions, (c) for all samples exanuned the most prolific alkali hahde cluster Ions are of the form MnXn_l, where n IS an integer with a maximum value of 5 Secondary Ion yields depend on several beam parameters such as the mass and energy of the incident parHcles current density and flux dlstrlbuUons, the angle of primary 1on incidence and surface cond> uons Even thougtl comparison of absolute yMds is not possible, good quahtatlve agreement Is found for our observations with those and Jurela [12] and Rlchards [13]

Z L Kerestes et al / Sputtermg o f tonlc clusters ,o6

261

140KeV Ar+~KCt

K+

41V39

1@

t,e-

%

K+

+

KCt !13 "E ~J

II.~

0 U 101

++ K Ct 2

95~94 915

8o

KCt +

+ ++

K2Ct

~ t_

MOSS t'lg 1 High resolution mass spectra of positive ions sputtered from L1F by 60 keV Ar +

Table 2 Fractional percentage yields of the total integrated slgnal detected

3 2 Mass s e l e c t e d energy spectra In general the clustered alkali h a h d e ions constttute o n l y a few p e r c e n t o f the m o n o m e r yield and although

previous a t t e m p t s

have

confirmed

Species type

their

presence m the mass s p e c t r a , the signal s e n s m v l t t e s were t o o low for f u r t h e r resolution o f their energy c o m p o n e n t s With the signal to notse ratto o f 5 X 106 o f our a p p a r a t u s , g o o d s t a n s t l c s were o b t a i n e d for the l o w e r o r d e r cluster energy d i s t r i b u t i o n s Typical

M+ Mn MnXm MpO~t x+

Alkah halide samples KC1

NaC1

LIF

94 4 32 2 1 0 1 02

95 8 25 15 02 <0 1

49 1 82 33 8 77 12

c o u n t rates at peak energies for M +, M~ and M2 X+ are Table 1 Isotope abundance ranos ol sputtered alkah halldes Cluster type

Observed isotope ratios

Integrated isotope mass peak ranos

Natural abundance ratios

NaC1÷ Na2CI÷ K+ K~ KCI+ K2CI+ LF L]~ L12F+ LI3| 2

[23,35] [23,37] [23,23,35] [23,23,37] [39] [411 [39,39] [39,41] [39,35] [39,37][41,35] [39,39,35] [ 3 9 , 3 9 , 3 7 ] [ 3 9 , 4 1 , 3 5 ] [7] [6] [7,7] ]7,6] [7,7,19] [7,6,19] [ 6 , 6 , 1 9 ] [7,7,7,19,19] [7,7,6,19,19] [7,6,6,19,19]

77 81 91 88 71 76 81 76 76 68

76 76 93 87 71 66 93 86 86 80

[6,6,6,19,19]

23 19 9 12 29 24 19 24 11 20

13 8 4

24 24 7 13 29 31 7 13 13 <1 18 1 <0 1

VI SURFACE SCIENCE

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Z L Kerestes et al / Sputtering o f tome clusters

104, 103 and 10 2 counts/s, respectively for a 3 5 # A primary beam current In fig 2 we show typical mass selected energy distributions from the sputtering of KC1 In the figure the zero energy position is found by extrapolating the linear p o m o n of the low energy side of the peak to zero yield The curves have also been normahsed to give unit yield at the peak position In table 3 we give the original peak values as well as the full width at half maximum height The only available data for comparison with the energy distribution of sputtered alkah hahdes IS for the alkali cations, M÷, from the works of Rlchards [13] and MIyagawa [14] T a b l e 3 Includes their results for comparison of the energy peak parameter values Comparmon between the peak energy and full width half maximum height values from the table clearly indicate that our values are consmtently higher For methods of analysis using particle beams to probe the surface region of solids by the process o f sputtering, the bombarding beam may induce effects that d~stort the observed mass, energy and angular distributions of the sputtered species These include (a) the field of a charged incident beam (Bryce et al [15,16]), (b) development of a surface charge on Insulators (Rlchards [13,17]), (c) space chmge effects reduced by secondary electron emIssmn (Rlchards [ 13 ])

FWHH (eV) ,+

,+

'2

g5

L2Ct+ ,

++

,3CZ2

O5

The energy and angular distributions for low energy ejected particles (< 10 eV) may be affected by all the above causes, whereas it is possible that the mass spectra will pot correspond to the pre-lrradlated surface stoIchlometly as a result o f field enhanced diffusion processes The 45 ° incidence arid 45 ° detection geometry o f our experiments mInlmmes the effect of (a) and hence this effect is neglected The presence of a space charge is most hkely to affect the angular distributions by providing a virtual electrostatic lens effect This may also have some Influence on the very low energy spectrum However, as our resolution and statistics are poor m this region, effect (c) IS also neglected We thus assume that the major factor contributmg to the dlstortmns to our energy spectra arises from the effects of surface charging Pertinent to our bombarding conditions is the surface charging effect that arises from the high efficiency ot secondary electron production from alkah hahdes The secondary electron coefficient for insulators IS much greater than unity, whereas the secondary ion coefficients are usually of the order o f one or less, and hence charge Imbalance occurs at the surface There are several methods available to reduce the surface charge to varying degrees These methods include the use of high crystal temperatures, low primary current densities, neutral boinbardmg beams, pulsed beams, thermal electron flooding and thin

0 ENERGY SHIFT(eV)

ll

16

6

i4

5

16

5

14

< 113 I,I >-

0

5

10

20

~C

L.C

E N E R G Y (eV)

Fig 2 Mass selected energy distributions ot some sputtered catums trom 40 keY Ar+ Irradiated KCt (See text for detads ot energy shift )

Z L Kerestes et al / Sputtering oftonw clusters

263

Table 3 Results and some comparison of peak energy dlstrlbunon parameters for sputtered alkah catmns Sample

Sputter species

Energy peak (eV)

fwhm (eV)

Experimental condltmns

KC1

K+

16 3 14 16 14

11 6 6 5 3

45 keV Ar+ with thermal electron flooding 15 keV Ar°, crystal temperature 180-320°C

KS K2CI K3CI~ NaC1

Na+

Na~ Na2C1+ L1t,

L1+ Ll~ L12F+

-+0 3 b)

17 2 9 + 0 4 a) 2 e) 16 14 19 3 17 17

10 6 4 7 3

20 keV Ar÷, lp = 10.4/aA/cm 2

8 -+1 8 b) 6 5

a) Rlchards [13] b) Rxchards [ 17] c) Mlyagawa [14]

crystal samples White et al [18] have compared the extent of the surface charge produced by neutral beams, ionic beams and ionic beams together with thermal electrons Their results show that the main effect of thermal electron flooding of the target is to reduce the macroscopic field across the insulator, but not lo completely eliminate it In view of the above discussions we can explain why our peak energy values are higher than those of Rachards and Mayagawa, listed in table 3 Rlchards' use of a neutral sputtering beam and Mxyagawa's use of elevated crystal temperatures and low primary current densities are simply more effective methods of neutrahsmg the surface charge than our thermal electron flooding method The shift of the energy peak can be taken to indicate the change In surface potential The energy peak does not correspond to the maximum potential on the surface, as this is expected to occur only at the central region of the irradiated area, but it can be taken to correspond to a r e p r o ducIble fraction of the maximum potential The results also show that our energy peaks are broader by at least 4 eV than those of Rlchards and Mlyagawa This second order effect, we also attribute to the effect of surface charging The excess of charge that accumulates on the surface is hkely to leak away across the surface rather than through the bulk, Cherynyshev [19] This picture is supported by the

lower activation energy (0 38 eV) for charge carrier motion found by Rachards [13] as compared to the value (0 75 eV) found by Karkhoff [20] for c o n d u c tlon through the bulk Surface leakage is also likely to be enhanced in the beam lrra&ated zone where local heating by the beam occurs A lower potential drop is then expected across this central zone with the effect of broadening the energy peaks

3 3 The two c o m p o n e n t surface potenttal model The form of the surface potential distribution for a circular region, irradiated by a constant, uniform positive beam is readily obtained by the use of the following simplifying assumptions First, the surface charge is assumed to be accounted for by the b o r n birding Ions depositing all their charge on the surface Second, this surface charge is then assumed to leak away uniformly across the surface, with no charge leakage to the bulk The associated surface current density ls wall then be that of the beam ]p For an incremental area bounded by the radn r and r + dr, within the irradiated region, one can associate a resistance R a t = dr/o2s nr where o s is the surface conductivity The third assumption is that o s is constant, independent of r The surface current through Rat is then simply nr2ls and to a good approximation the incremental potential frop across this area is A V = rls dr/2Os Thus the potential difference between any VI SURFACESCIENCE

Z L Keresteset a l / Sputtermg of tomc clusters

264

two points r~ and r2 (rl > r2) on the target surface is by simple integration given by

lsr~ V(r,)- V(r2)-~o s [1 _

(rl/r2) 21

(1)

For the alkah hahdes the first assumptions is hkely to underestimate the resultant surface charge density and hence the surface current density This is because the sputtering coefficients for secondary ions are roughly 0 5 whereas the secondary electron coefficient (43 keV Ar +) may be as high as 10 The second assumption should be reasonably valid for thick crystal samples The third assumption, the constancy o f as with respect to r, is however likely to be invalid The ion beam is expected to heat up the surface region For a well defined uniform beam the surface temperature should be highest near the beam centre and decrease towards the edges of the beam The surface conductivity then changes appropriately across the irradiated area To take account of the heating effect of the beam, at least to a first order approximation, the m a d l a t e d area is divided Into two regions each charactensed by a mean surface conductivity As the observed energy peak is some fraction of the maximum potential at the centre, the component of the potential distribution responsible for the peak broadening is locahsed around the centre of the Irradiated zone The gross shift of tire observed energy spectra is then due to a large potential drop near the edges of the central zone Tire net sulface potential distribution is thus described in terms of two components each described by a distribution of the form given by eq (1) The general expression relating the observed energy distribution to the energy distribution characterlstlc of emission from an uncharged surface is then relatively straightforward to obtain We have found that the final computation on the data is considerably simplified if the various distributions are expressed in terms of probability distributions The subsequent discussion thus follows this convention Consider the set of positive, single charged ions o f a particular species to be sputtered from a circular surface region of radius r a Let the number of such particles with energies in the interval (E, E + dfi) and in a unit interval of time be given by N(E) dE The corresponding probability function is defined in the usual manner, EnLax p(E) dE = N(E) dE/ f N(E) dE (2) 0

if it iS assumed that the sputtering of an Ion is independent of its position within the region considered then the probability of an ion m the energy range (E, /~. + dE.) being sputtered from air area bounded by the interval (r, r + dr) is snnply 2arP(E) dE dr, where P(E) =(l/Trr2a)P(E) If now a certain potential V is associated with the point r then the observed energy w l l l b e E ' = E + Vand P ( E ' ) P(r) dE' dr = P(E) P(r) dE dr = P(E'

V(r)) P(r) d (E'

V(r)) dr (31

For a continuous, well defined function V(r) a suitably small dr may be found such that d(E' - V(r)) ~d E ' Then integrating both sides of eq (2) over the region (0, ra) p ( E ' ) = j a P(E' - V(r)) P(r) dr

(4)

o

From probablhty theory, for functions of a continuous variable, if y =¢(x) is a strictly increasing function and x = g(v) is its inverse function and ~(x) and f ( y ) are the corresponding probablhty density functions then the following relationship holds ~(y) = ~g(y) Ig'(v)[ This equality is useful m defining the probability dlsm b u t l o n for V(r), namely

P(v) =P(r)

dr

d--V

(5)

Thus the distribution P(V), for the range (0, Vm~x) is given by 2

p(v) = 'a;Kt r~l = iK ra l

VI < V < gma x

(6a)

0 < V < VI

(6b)

where kz = l~z/Os, Thus the potential expectation distribution reduces to a simple two step function Furthermore, from eq (4) the observed energy distribution, p(E') can be written finally In the form Vmax

p(E') = .f 0

P(E'

V) P(V) dV

(71

Z L Kerestes et al / Sputtering of tome clusters Thus the observed distrlbutmn is the convolution of the true distribution taken with the surface potential distribution The deconvolutlon method used on our data to obtain the true energy distribution uses no Fourier transforms and In fact the operation is a series of convolutlons As shown by previous workers, Konen et al [7,8], an analytical expression is assumed for the energy distribution

~o

.....

P(E) -

(E + mEpk )

~

~

K+

P(E) ~0

I

NE

265

k

I

I

I

I

I I J

k

10

m + 1,

(8)

where N is the normalization constant satisfying f P(E) dE = 1 and Epk is the peak energy. This distribution is then taken in convolution with the two-step surface potential expectation transfer function, eq (6), and the result is compared with the observed energy dlstributton The step-potential probability function P(V) IS obtained from the position of the onset of the observed energy peak, Vl, the peak position Vmax and the ratio of the step heights Pexp(Vmax)/ Pexp(Vl) This ratio is found to be 10 .3 From eq 8 the parameter Epk and the high energy asymptotic index are used as fitting parameters

3 4 Deconvolutlon results Fig 3 shows a typical example of the various distributions involved m the "deconvolutlon" program The fitting of Pc(/:), the result of the convolution, to the experimental distribution IS via the fitting parameters discussed in the previous sections, with the goodness of fit determined visually Table 4 lists the parameters rn a n d / : p k of the distribution Pc(E) that provide the curves of best fit to the M÷, M; and MX + dlstlibutIons The results of our convolution analysis on the various sputtered species show that the general shapes of the distributions are in close agreement with our experimental profiles As expected the convolution results fit well on the high energy side Generally Vmax is approximately 20 eV and hence the effect of the surface potential may be neglected for ejected particles of high energy Near the peak and low energy side we find the agreement is not so good This is especially pronounced for the dlmers M~ and MX÷ For consistency, in the convolution procedure we have insisted that the potential distribution for the central irradiation zone should be kept fixed for all

i

i

I

I

i

i i [

i

10 2

ENERGY (eV)

Fig 3 Results of the convolution technique apphed to the K+ energy distribution - Experiment, - - assumed true dlstnbutton, - . . . . . result of convolution, twostep potential

sputtered species The results show however that for the dlmers the resultant convolutions are consistently too wide below the peaks There may be several reasons for this behavlour (1) In developing our two component surface potential model we have assumed that there is a well defined boundary between the two surface zones In reality the transition from the central to the outer zone is likely to be more gradual Thus the distribution P(V) will not be a pure step-function as postulated but rather, near V1 a smoother roll off in P(V) is expected This corrected distribution would then give a narrower peak for the convolution curve (2) The applicability of eq (8) to the low energy profile of the true distribution can at this stage be only speculatwe We cannot expect a high correlation at the low energy end other than the reproduction of the general shape of the profile (3) For the convolution procedure we have kept the width of the step, Vmax - VI, of P(V) constant For the M~ species a much better curve fit is obtained if the peak width is reduced from ItS normal value of 6 eV to 3 eV If our surface potential model is valid then tt indicates that the dlmers do not have ejection probablhties independent of their point of ejection on the surface The elevated temperatures expected in the central irradiation zone may reduce the number of clusters from this region as they are more likely to dissociate Our energy resolution and the ambiguity of the convolution operation prevents us from distinguishing among the above possibilities VI SURFACE SCIENCE

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z L Kerestes et al / Sputtering of ionic clusters

3 5 High energy behavtour

cluster to be

The results for the M÷ distributions show good agreement with the experimental values of Rlchards [13,17] and Mlyagawa [14] as well as with the E -2 prediction of the simple collision cascade theory The variance of the high energy fall off for the case of L1+ may be related to two causes According to Slgmund's analysis [21] the E -2 behaviour should only hold for mass ratios M2/MI < 1 For the case of LIF, M2/M1 = 2 7 The second possibility is that Irradiation of L1F surface is accompanied by surface stolchiometrlc changes (Carroll and Blrnbaum [22]) rendering the surface metal enriched Posltwe ion energy distributions from metals generally show a lower energy dependence than E -2 because the speira are modulated by the probability that a sputtered Ion can recapture an electron trom the conduction band This probability Is proportional to the velocity of the escaping ion, thus an k 1/2 difference between atomic and ionic eJection may be expected Thus the E- I 7 behavlour of L1+ may in fact be due to metal enrichment of the surface Konen et al [7,8] have proposed a statistical emission model for the cluster energy spectrum The model assumes that the constituent particles of a cluster are emitted from a surface independently Partlcles coalesce to form a cluster if the sum of their kinetic energy in the centre of mass frame and their potential energy is below the binding energy of the corresponding molecule The assumption of independent ejection leads to the factorlzatton of the cluster energy distribution Into a product of the individual one particle distributions Assuming a one particle distribution to be of the form o f eq (8) Konen et al have shown that the statistical model predicts the high energy asymptote, for a homonuclear K-particle

__oc

ds

E

Km--O

SOn--1 )

dE and for a heteronuclear cluster XklYk2 to be ds

--

¢c E - k l m l - k 2 m 2

-0

5(k1+k2--l)

dE To compare these predictions with our results for the M~ species we must assume that the metal dlmer is formed from a M+ and M° We do not have values )or the energy distribution of the neutral monomers o f our investigation, however the work of Overeljnder [23,24] on the neutral emission of heavy alkali hahdes shows that the enelgy distribution of the neutrals do not deviate greatly from the ionic distributions Hence if we assume that the two distributions are the same then we would expect an £-4 s fall off for the M~ species From table 4 we see that this prediction falls within our error limits The statistical model is not very successful near peak energy values We have considered a surface correlated M2 dlmer emission model [25] whereby the constituent particles of the dnner are not emitted from the surface Independently, being correlated by secondary colhslons with nearest surface nelghbours The model is simulated In a computer and the results + show the Mz energy distribution near peak energies for the KCI and NaC1 systems to be In good agreement with our experimental values Although energy spectra were taken of species of higher order than two the convolution procedure cannot be applied to them The high energy fall off for these species Is very steep and hence the surface potential is expected to make a significant contribution to this part of the distribution as well Indeed it is

Table 4 The results obtained by the convolution method lor determining the parameters rn and b.pk ot the true energy distributions ol the lorm [k/(E + mkpk) ] rn + 1 m

Sample

M÷ m

M~ /:pk (eV)

KC1

20 ±01

30 -+04

NaC1

20-+01

22 ±05

LW

17 ±01

20 -+04

m

50 ± 0 8 03 O2 02 35 -+ O3

43 ±

MX+ Epk

m

Epk

25 -+03

54±06

28±03

28 ±03 20 +03

267

Z L Kerestes et al / Sputtermg o f tonic clusters

Table 5 High energy index [eq (8)] for some high order clusters, with no correction for surface charging Index

K2CI+

Na2C1+

L12 F+ L12OH+

Ll3k ~

LI2 O+

L13O+

K2C1+

m

6-+1

5-+1

65-+15

7-+2

54-+16

75-+7

32-+7

very likely that most o f these higher order clusters are a result o f thermal ejection processes In table 5 we list the index m directly from the experimental data with no surface potential correction The results show qualitatively that the high energy slope increases with increase in cluster size

4 Conclusions In this w o r k we have e x a m i n e d the sputtering o f posltwely charged clusters from alkali halide surfaces Mass analysis o f the sputtered Ions has shown the presence o f various clustered atoms A l t h o u g h from tile KC1 and NaC1 targets the cluster yield relative to the M + c o m p o n e n t is only a few percent, the sputterlng o f LIF has shown cluster species to be as n m c h as 50% o f the M ÷ yield This Illustrates that previous w o r k on sputtering that has considered only the M ÷ m o n o m e r needs to be reconsidered Mass selected energy spectra show the lower order clusters to have a high energy tail The analysis of this c o m p o n e n t correlates well with the statistical emission m o d e l This supports colhslonal rather than thermal emtssmn models o f ejectmn The low resolution o f our energy analysis below the peak has prevented firm conclusions being drawn regarding the mechanism responsible for the high order clusters It seems likely however that these larger species have a thermal origin Distortions o f the energy spectra have suggested that a surface potentml exists across the surface o f our samples We have analysed and corrected for this effect F u r t h e r m o r e an Instrument has been developed In our laboratories (Dalghsh et al [26]) with which we intend to explore the surface potential phenonrenon further

References [ll Z Jurelaand B Perovlt~,Can J Phys 46 (1968) 773 [21 G Staudenmamr, Rad Effects 13 (1972) 87 [3] E Denms and R J MacDonald, Rad Effects 13 (1972) 243 [41 R K F Herzog, W P Po%henrmder and F G Satklewacz, Rad Effects 18 (1973) 199 [5] F Schmldt-Bleek, G Ostrom and S Dats, Rev Scl Instr 40 (1969) 1351 [ 6 1 A P M Beade, WF Jungman and J Los, Physlca 54 (1971) 459 [ 7 ] G P Konen, J Grosser, A Harmg, A E d e V n e s a n d J Kastemaker, Rad Effects 21 (1974) 171 [8] G P Konen, A Tip and A E de Vrms, Rad Effects 21 (1974) 269 [9]W Gerhard, Z PhyslkB22(1975) 31 [10] W Gerhard and H Oechsner, Z Physlk B22 (1975) 41 [llIAR Bayly, P J Martin and R J MacDonald, Nucl Instr and Meth 132 (1976) 459 [12] J Jurela, Summer School and Symposmm on Physics of lomzed gases, Herzognovl (1970) p 67 [13] J Rlchards, PhD Thesis, University of New South Wales (1972) [14] S Mlyagawa, J Appl Phys 44 (1973) 5617 [15] P Bryce, PhD Thesis, Umverslty of New South Wales (1971) [161 P Bryce and J C Kelly, J Phys C5 (1972) 1604 [171 J Rlchards and J C Kelly, Rad Fffects 19 (1973) 185 [18] C W White, R A Kushner, D V McCaughan,N H Tolk and D L Simms, in Characterization of solid surfaces ed P F , Kame and Larrabee (Plenum, New York, 1974) 636 [19] A Cherynyshev, Bull Acad Sol U S S R Phys Ser 29 no 1 (1966) 71 [20] F Klrkhoff, Z Physlk 130 (1951) 449 [211 P Slgmund, Phys Rev 184 (1969) 383 [221 D.B Carroll and H K Blrnbaum, J Appl Phys 36 (1965) 2658 [23[ H Overmjnder, M Szymonskl, A Harlng and A.E de Vries, Rad Effects 36 (1978) 63 [24] H Overeljnder, A Haring and A de Vrles, Rad Effects 35 (1978) 205 [25] Z Kerestes, R Dalghsh and J C Kelly, to be published [26] R Dalgllsh, R Katsh and Z Kerestes, J Phys E 13 (1980) 1047

VI SURFACE SCIENCE