Ion scattering: a spectroscopic tool for study of the outermost atomic layer of a solid surface

Voh~mc 14, number

CHEWCAL

3

UN? SCATTERING: THE ~~~TE~~~~S~

t June 1972

PHYSICS LETTERS

A SPECTROSCOPIC TOOL FOR STUDY OF ATOMIC LAYER OF A SOLID SURFACE

H.H. BRONCERShfA and P.M. MUL Philips Research Laimrarories, Eirldhoven, 77~ Netherhds

Noble m.s ions which are back-scattered from s crystal of 3 specific miss. The chemical reaction of B silicon (111) It is shown that this rechnique an and Ne+ ion scatter&. tiwly. Liethods using ion scattering to study vibrations of

1. Introduction

The collision of an ion with an isolated atom can sonl~t~~!es be d~~~rib~d by classical mechanics. Since in the present

experiments

the impact

energies

are

about

1000 eV, the de Broglie wavelength of the reduced mass is so small that :he wave character of the particles can be neglected. If we then first consider an ion (incident energy E, : mass M,) which is scattered over an angle C by 3 target stem (mass hf2) at rest, the finat energy of the ion wit1 be:

.E; = ([cosf? t (y2- sin26)1/2] I( 1 +Y)]‘E 1

*

where y = M2/MI and is assumed & 1. This expression is derived by solving the equations for conservation of energy and rnoItlentu~~. The potentiai for the interac; tion between the ion and the atom d=es not enter into the equation. This implies that the energy loss of the ion is independent elf the electronic configurations of

surface lose 3. specific amount of energy for surface atoms crystal surface with bromine has been followed using He+

bc used to study the first atomic layer of a surface selecsurface

atoms 3re also indicated.

778 and 9 17 eV respectively. For !ar$er scattering angles, the difference is even much larger. The relatively small extra energy losses due to outer-shell excitation of the particles can thus be neglected. The scattering of noble gas ions such as He+ and Ne* from an isolated atom does not differ in principle from that from a crystal surface containing these atoms: the interaction distance Reeded to give an appreciable deflection is small in comparison to the atomic distances in the fattice, so that the path of an ion through a lattice is determined by a sequence of- binary collisions. Tk set-up of the ion-scattering experiment is sketched in fig. 1. When 1000 eV ions such as He* or Ne+ collide with a surface atom, their residence life in the vicinity

Energy AllCllpW

the particles.

During a given experiment E,, Af, and 8 are kept constant and a spectrum of tile number of scattered ions as a function of their final energy is recorded. Each

type

of atom

thus

gives

rise to a characteristic

final energy of the ion. These energies generally differ sufficiently to permit distinction between atom scatterers of different mass. Solving eq. < 1) for scattering of 1000 eV Ne’ ions over 33” by collisions with Si arid l3; atoms, we find the final energies to be 380; : ,. :

.

.

j

frimory

[moss

ion beam

selected

/scattered ion beam

“_..______._

I

Fig. 1. The set-up of the ion-suttcring

experiment.

Volume 14. number 3

CHEbtlCAL PIIYSICS T..XXTERS

of the target atom (about 1O-13 set) is so long that their neutralization probability is high. Only a small number of scattered particles retain a positive charge

after a single collision. Thus the probability that an ion, which collided with two surface atoms, is still in the ionized state, is very small under these circtimstances. Since our detector observes only charged particles, contributions from ions which have collided twice can generally be neglected, This property makes such noble gas ions ideally suitable for analysing the upper Iayer of a surface. Smith, realizing these features, described a few elegant experiments which demonstrated some of the possibilities of ion scattering. For example, Goff and Smith [I] showed that this type of spectroscopy can be used to de:termine the concentrations of the 63Cu

and 65C3.1 isotopes at the surface of ~olyc~5tal~ine copper. The surface sensitivity was demonstrated [ f ,2] by studying CdS and ZnTe crystals with He* ions scattered over 90”. The properties of these crystals indicate that only one type of atom should be present on certain surf3ces; and indeed this relation wzs partiaftf confirmed in practice. In the case of CdS, for example, the Cd tb S intensity ratio was I:&3 for the cadmium face and I :0.8 for the sulphur face. It was not clear whether this relatively small difference between the faces was due to surface damage or to an appreciable sensitivity of the method for deeper&ing layers. In the present experiments the chemical reaction between silicon and bromine isused to produce a monolayer of Br atoms on the surface. Experiments are performed under dynamic conditions, so that Br atoms, which are sputtered by the ion beam, are replaced by Br atoms from the gas phase. Under such conditions only scattering from Br atoms is obsemed which shows that the first atomic layer of a surface can be studied selectively. So far, the influence of the lattice and the vibtationaI energy of the target atom has been neglected, This seems reasonable, since the collision time of tO-“S set is short compared with the characteristic vibration times which are of the order of lCPiJ set; there is thus little possibility that the lattice will influence the energy transfer. It would also seem reasonable, at first sight, to neglect the ~b~t~ona~ energies [* I@ ev) of the target atoms. However, as can easify be shown by considering conservation of ener,q and momentums a small kinetic energy E3 of the tar.-

f June 1972

get atom may produce a iarge shift in the final energy of the ion. This shift is a~prox~atefy (Et ., ? *md so for an incident enemy of IO00 eV and a k etic enT ergy of 0.1 eV is about 10 eV. Considering a vibrational mode of the atom along a surface normaf, we see rtrat this energy shift is positive fur atoms which move away from the surface lrnd nesattive for atoms which approach it. Such motion of the surface atoms thus gives rise to broadening of ttie spectral peaks. If it is FeasibIe to reduce other sources of peak broadening, it should be YossibIe to use the peak width to determine the average vibrational energy for each type of surface atom. Further, in contrast to infrared spectroscopy, this methad would provide vibrational energies rather than vibrational excitation energies. The present example of a bromine atom on 3 silicon sub. stntc is, however, not suitable for this a~p~jcat~on due to the folfowing rertson. The zero-point vibrationa3 energy of an isolated Si-Br bond will be small (about 30 meV). Moreover, since the Br atom is much heavier than the Si atom, its maximum kinetic energy will only be ;I fraction of the vibrational energy. In practice, the motion of the Br atom wit! also be influenced by the presence of the lattice and by surface phonons. For a maximum kinetic energy of 30 meV, this would fead to a 6 eV brbadening of the Br peak. For silicon stich a kinetic energy would give a 9 CV hro~den~ng of the silicon peak.

2. Experimental We have designed a spectrometer which enables us to apply various analytical achniques consecutively to the same crystal. Facilities are avaifabfe for arrgular-dependent electron and ion scattering, Auger spectroscopy and optical measurements. The equipment wili be fully described 131 in a forthcoming paper. In the present study, ion scattering afane is used. fans produced by a gas discharge in the source are mass-anafysed by a Wien filter before they impinge on the target. Differential pumping is necessary to maintain a pressure difference between the ion source (IO-L torr) and the scattering chamber fIO-‘0 rorr). A combination of oit diffusion, mercury diffusion, cryogenic, ion getter and Su~I~rt~at~onpumps is used to obtain this pressure difference. The principles used in the design of the electrostatic analyser (for energy 381

Volume

14, number 3

I June 1972

CHEhlICAL PHYSICS LETTERS

selection of the scattered ions) are primarily based on the work of Kuyatt and Simpson [4] although severa: features of the analyacr developed by Lassettre et al. [S] have been incorporated. The ions are decelerated before they are energy-analysed by the electrostatic field of two concentric spheres. The lens properties of the decelerator were determined by numerical solution [6] of Poisson’s equation for the electrostatic field (neglecting space charge) with the lens surfaces giving the boundary conditions. The analyser is suitable for energies in the 5-7000 eV region and both it and the target are rotatable. This allows us to make measurements as a function of scattering angle and crystal orientation.

3. Results

and discussion

aw)

700 Energy

The chemical reaction between Br2 and an Si (111) has been chosen to investigate whether ion scattering can be used to study the top atomic layer of the surface region. This choice is based on several considerations. On the Si (1 II) surface, each atom is bonded to three deeper-lying Si atoms. A fourth valence electron can be considered, to a first approximation, as an unpaired electron in an orbital (“dangling bond”) along an outward normal to the surface. Tflis property ensures a high reactivity of the (11 1 j surface, with each Si atom capable of reacting with one Br atom. The spacing between adjacent dangling bonds is 3.85 a while the vxl der Wnals diameter of a Br atom is 3.90 A. .4 full monoatomic coverage of the surface should thus be starically possible. This view is supported by LEED measurements by Lander and Morrison [7], and Florio and Robertson [S] , showing that chlorine and iodine give a full monoatomic coverage on the Si (1 11) surface. The bromine atom is too large to diffuse into the bulk of the crystal and it is also highly unlikely under the present experimental conditions that Br2 molecules will be physisorped on top of the cllemically-bonded Br atoms. This will be clear if we realize that the adsorption energy for Br molecules on such a surface is comparable to that for adsorption on the surface of liquid bromine. The equilibrium pressure of bromine above its liquid at room temperature is 160 torr, while the highest pressure used in the present experiments was 3 X 10m6 torr, so that physisorption of Br2 is unlikely. surface

900

1000

of scattered Ne* ions ieV1 __b

Fig. 2. Partial sccondnry ion energy distribution sholring the silicon. and bromine peaks for two bromine pressures. At 3x 106 torr the occurrence of silicon unnot be detected, indicztiny that the method is specific for the top atomic layer.

The reaction of Br, with Si (11 1) can thus be expected to yield a pure Si crystal with a monoatomic bromine surface covering. The present experiments were performed under dynamic conditions so that any Br atoms sputtered from the surface would be replaced from the gas phase. Fig. 2 shows two spectra from a series where the bromine pressure was increased stepwise. The surface was analyscd with a primary beam of 1000 eV Ne+ ions and a scattering angle of 33”. The ions were incident along the (375) directions which is not a special channelling direction. At a bromine pressure of 3 X 10d6 torr the signal due to silicon atoms is less than

2% of that

experiments

were

from

a clean

performed

silicon

surface.

under

specular

Similar reflection

conditions with 1000 eV Nef ions scattered over 45’ and with 1000 eV and 2000 eV He+ ions scattered over 77” and 90”. In all cases, ion scattering was found to depend only on the top atomic layer. In fig. 3 the heights of the Si and Br peaks are plotted as a function of the bromine pressure. The experiments are carried out under dynamic equilibrium conditions. The sputter efficiency is about one Br atom/incident ion. At low bromine pressures, the

volume 14, number 3

CHEhlICriL

Fi& 3. The intensity of the Br and Si peaks pfottcd as a function of the bromine pressure. intensity of the Br peak is directly proportional to this pressure. At higher pressures Only a few adsorption sites remain and pressure increases lead to only small increases in the Br peak height as saturation is approached. The shape of the curve is similar to a J-angmuir isotherm for reversible adsorption, For small coverages, the relative decrease in the silicon peak intensity is somewhat faster than the relative increase for the Br peak. For instance, at a pressure of 3 X IO-+ torr, the Br peak is only 15% of its final value, while the Si peak has already decreased to 80% of its original value. This behaviour is not observed for the specular reflection of He+ over 77’ and 90’ and is observed only to a small extent for the specular reflection Of Ne+ ions over 4F. h similar phenomenon is observed when experiments are carried out with He* ions entering making a small a.n$e with the normal to an Si (1 I 1) surface covered with a smail fracticn of a bromine monolayer. The energy analyser is ii1 a position to detect ions Ieaving the crystal at low angies. A small change in the incident angle should not influence the sputtering yield in this case. Rotating the crystal sIightly in a direction, such that the trajectory of the reflected ion makes a still lower angle with the crystal surface, increases the intensity of the Fr with respect to the Si spectral peak hy a factor of four. These observations contrast with experiments where other types of anaIytica1 tools such as W absorption or NMR are used to follow chemical rcactions. We believe that this effect is genuine and can be

PHYSICS

LETTERS

1 June

1972

described as a “shadow effect”. For ions at glancing incidence one bromine atom may conceal the presence of more than one Si atom. One should realize that this “shadowing” action of a brornine atom is an unusual one in that it conceals not onfy Si atoms whicl~ are behind it, but also Si atoms which are in front of it. This is due to the fact that an Nc” ion which impinges upon 3 silicon atom just in front of a bromine atom cannot reach the detector without colliding with the Br atom too. The high neutralization probability for the ion makes it unlikely that such an ion will be observed. Thus an atom on top of a surface throws a “double-sided shadow” onto the crystal, parallel and antiparrllcl to the ion beam. For high Br coverages the Br atoms will partly screen each other. The peaks in fig. 2 are very broad. This can be attributed to several factors: spread in the scattering angie, spread in the energy of the primary beam, motion of the surface atoms, electronic excitation of the ion and/or target atom, contriblltios~s from ions which collided more than once with target atoms [9], occurrence of natural bromine as a mixture of its 7QBr and 81 Br isotopes, etc. One of the most important factors is believed to be the angular spread. The acceptance angle for the analyser is I .4”. The angular spread in the incident beam is not known, but simple geometric considerations show that the spread could be as tnuch as 6’. By insertion of 0 in eq. (1) it may be calculated that under the present experimental conditions a 5” spread leads to a peak broadening of 22 eV for bromine and 77 CV for silicon. This is large in comparison with the inftuencc of the motion of the atoms (possibly G and 9 eV respectively). The peak broadening was considered as being unilnport~t in this study, as the emphasis was laid on speed and easy operation rather than high resolution. Recently [31, under more favourable conditions, we have been able to detect broadening of a spectral peak due to vibrational motion of the surface atom.

4. conclusions Scattering of Hei and Nef ions has been used to determine the identity of atoms on top of a crystal surface. The results are not intluenced by the chemical state of the atoms. It seems possible that the

method can be uad to obtain information about the modes of surface atoms and

grow-id-state vibrational about surface phonons.

1 June 1972

CHEMICAL PHYSICS LETTERS

Volume 14, number 3

Such information

very helpful in studies of heterogeneously

[31 H.H. Brongersma and P.M. hiul, to be published. [41 CL Kuyatt and J. Arol Simpson, Rev. Sci. Instr. 38 (1967) 103.

may be

151 E.N. Lzsettie,

A. Shrbele, WA. Dillon and K.J. Ross, J. Chem. Phys. 48 (1968) 5066. L61 C. Weber, in: Focussing of chnG,ed particles, Vol. 1, ed. A. Septier (Academic Press, New York, 1967) p. 45. 171 J.J. Lander and J. Morrison, J. Chem. Phys. 37 (1962) 729. ISI J.V. Florio and W.D. Robertson, Surface Sci. 18 (1969) 398. 191 S.H.A. Begcmnnn and A.L. Boers, Surface Sci., to lx published.

catalyzed

reactions.

References [I] R.F. Goff and D.P. Smith, J. Vat. Sci. Tech. 7 (1970) 72. [2j D.P. Smith, Surface Sci. 25 (1971) I7i.

ERRATUM

A.B. Callear and J.H. Conucr, Attachment of NH3 clusters to Hg (@PO), Chem. Phys. Letters 13 (1972) 245. In fig. 1 caption ‘+ [NH31 = 200 torr’ should be replaced by ‘+ [N2] = 200 torr’.

On p. 246, the beginning of the first paragraph of the second column should read ‘With [NH, ] Z 20 torr . ..‘. In the next paragraph, the last two sentences should read ‘However the functional dependence of the luminescent intensity on the [NH3], to the long and short wavelength extremes of the observed bands, could not be rationalised with 3.sirnple model involving monomer and dimer alone. Furthermore, a two state system requires the occurrence of an isobiestic

point

of intersection

of the various

is not found as shown in fig. 1’.

384 ._ :: ,.

profiles,which