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Frenken, J.W.M.

Veen, J.F. van der

Vrijmoeth, J.

Zagwijn, P.M.

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Monolayer resolution in medium-energy ion-scattering experiments on the NiSi2(111) surface

Leiden University Scholary Publications

VOLUME 67, NUMBER 9 PH YSICAL REVIEW LETTERS 26 AUGUST 1991Monolayer Resolution in Medium-Energy Ion-Scattering Experiments on the NiSi2(111) SurfaceJ. Vrijmoeth, P. M. Zagwijn, J. W. M. Frenken, and J. F. van der VeenFoundation for Fundamental Research on Matter (FOM) Ins-titute for Atomic and Molecular Physics,KruisIaan 407, )098 SJ Amsterdam, The netherlands(Received 24 April 1991)The surface structure of the epitaxial NiSiq/Si(111) system has been determined applying a new ion-scattering method. Detecting backscattered ions with ultrahigh-energy resolution we resolve the signalsfrom successive atomic layers. From both their intensity and energy, which depends on the specific iontrajectories, we directly deduce the (sub)surface atom coordinates. Applying this new approach, we findthat the NiSi&(111) surface has a bulklike topology, i.e., it is terminated by a Si-Ni-Si triple layer. Theoutermost Ni-Si and Ni-Ni interlayer distances are relaxed from their bulk values.PACS numbers: 61.16.Fk, 34.50.8w, 61.80.Mk, 68.55.3kAlthough the epitaxy of NiSi2 and CoSiq on Si(111)has been investigated in great detail, relatively little at-tention has been paid to the NiSiq(111) surface structure.It has been reported that the NiSiq(111) surface wouldbe bulklike, with the crystal terminated by a Si-Ni-Si tri-ple layer [1,2]. However, recent reports claim that thesurface has an additional Si bilayer on top of the last Si-Ni-Si triple layer [3,4], like the CoSiq(111) surface in itsmost stable form [5,6].We settle this issue employing medium-energy ionscattering (MEIS) in a novel and model-independentfashion. Conventional MEIS is a well-established tool forthe crystallography of surface and interfaces. Atomic po-sitions are determined by comparing angular distributionsof the total surface backscattering intensity ("blockingpatterns") with calculated yields for a variety of structuremodels [7]. The ion intensity generally contains contri-butions from several atomic layers, which, in conventionalMEIS, are not separated but treated as a whole. Thiscomplicates the analysis of, e.g. , multilayer relaxations ina crystal surface.In this experiment we have succeeded in resolving themonolayer contributions in backscattered ion energyspectra taken on the NiSiq(111) surface using anultrahigh-resolution analyzer (dE/F. =9 & 10 ). Angu-lar distributions of the ion yield backscattered mainlyfrom a single atomic layer in the surface region ("mono-layer blocking patterns") directly yield the surface atom-ic positions. Furthermore, we observe the trajectorydependence of the energy loss in a single Si surface layerand use it to locate the outermost Si atoms. The dataconclusively show that the topology of the NiSi2(111)surface is bulklike; the presence of a Si double layer onthe surface is ruled out.Epitaxial NiSi2 films of either the type-3 or type-8orientation were grown on clean Si(111)substrates [8,9].The silicide was checked to be of single orientation()92% of the surface area) using ion scattering [2,9].The data presented here were obtained on type-B orientedfilms; within statistical error, type-2 oriented silicidesyielded identical results. Annealing the silicide Alms to600'C did not affect the surface structure.In the experiments we used a 100-keV proton beamwhich was stable to within 10 eV. Energy spectra of theions backscattered in an angular range of 20' wererecorded simultaneously using a modified version of ourtoroidal electrostatic analyzer (FWHM energy resolution90 eV at 100 keV). Backscattered yields were normal-ized with respect to the random height of Ni in NiSi2[7,10]. The ion energy, energy stability, and resolutionwere determined by steering the ion beam directly intothe analyzer.Protons backscattered from an atom in the outermostlayer of the silicide reach the detector at an energy whichis mainly determined by the elastic and inelastic energylosses during the single ion-atom collision. This results inan extremely sharp peak from that layer in the energyspectrum. Ions backscattered from deeper layers addi-tionally lose energy by electronic interactions along theingoing and outgoing parts of their trajectories throughthe crystal ("stopping"). These ions show up at lower en-ergies in the spectrum. The statistical spread on the ener-gy losses in the first few atomic layers proves to be rathersmall, allowing the backscattering contributions fromsuccessive layers to be separated.In conventional lower-resolution MEIS, a channelingdirection must generally be chosen for the incident beamin order to reduce the deeper-layer contributions [7].That restriction is lifted owing to the improved depthresolution; we have deliberately chosen a nonchannelingincident direction in the (110) scattering plane at an an-gle of 22.0' with respect to the (111) surface plane [Fig.1(a)]. Thus the first few atomic layers have about equalhitting probability, giving rise to about equal sensitivityto surface and subsurface layers. These are then separat-ed in the energy spectra, allowing a layerwise structuredetermination.Ion intensities were recorded for energies ranging from95 to 99 keV, and for exit angles with respect to the sur-face plane between 23 and 63 . In the following weconsider difIerent cuts through the resulting two-di-mensional data set.1134 1991 The American Physical SocietyPHYSICAL REVIEW LETTERS 26 AUGUST 1991VOLUME 67, NUMBER 9100 kev H = NiSi2 (111)Nio Si- 1.0- 0,8O30.6 &NQCi2E&F 0.4—& 0.5-N0E~00c 960 96.5 97.0 97.5 98,0 98.5backscattered ion energy (keV)99,0FIG. 1. Monolayer separation by medium-energy ion scatter-ing. (a) Scattering geometry in the (110) plane for Ni and Siatoms, and (b) energy spectrum (circles) taken under blockingconditions ([001] direction, exit angle 35.03' with respect tothe surface (111) plane). The second-layer Ni is visible as adistinct shoulder on the first-layer Ni trailing edge. The resultof a Monte Carlo simulation is given (solid curve), togetherwith the contributions from individual layers (broken curves).The FTHM detector resolution is indicated.The energy spectrum taken in the [0011 direction(35.03' exit angle) is shown in Fig. 1(b). Along thatcrystal axis the signals from deeper layers are attenuateddue to surface blocking. The two different peaks are dueto backscattering from Si (—97 keV) and Ni (—98.6keV) surface atoms. The shape of the Ni peak containsthe clear signature of the first- and second-layer contri-butions (Fig. 1). The second-layer signal is shifted by—300 eV to lower energies and is visible as a shoulder onthe trailing edge of the first-layer signal. The observedwidths of the layer signals are larger than the 90-eV ener-gy resolution [indicated in Fig. 1(b)] and reflect the in-trinsic spreads on the inelastic energy losses.The angular distributions of the individual monolayeryields were used to perform a layerwise structure deter-mination. On their way to the vacuum, ions backscat-tered from different layers are blocked in directions whichdirectly reAect the relative atomic positions in the surfaceregion (Fig. 2, top panel). We present a few selected cutsthrough the Ni part of the two-dimensional data set atconstant backscattering depth (A-E) and at constant an-gle (I-IV) in the bottom panels in Fig. 2. The formerwere obtained approximating the energy loss to be pro-portional to the ion path length through the crystal, andwill be referred to as "constant depth profiles" (CDP's).The profiles 2-F. correspond to backscattering depths of0.50, 1.26, 1.92, 2.56, and 3.43 Ni layers, respectively,using tabulated values for the random stopping power[10]. The absence of blocking features in profile A showsthat the signal at the leading edges of the energy spectrais solely due to the first Ni layer of the silicide crystal.An extra Si double layer [3,4] is ruled out since it wouldunavoidably have resulted in blocking minima in thisprofile. Profile 8 has two blocking minima, characteristicof the presence of a second-layer signal at "depth" 8 inthe energy spectra; see Fig. 2, top panel. Profiles C to Eprobe greater depths in the crystal and the blocking mini-ma increase both in number and in strength.The blocking minima allow a determination of the in-terlayer distances. The angular position of the mainminimum in profile 8 is shifted to lower exit angles by0.4' with respect to the [001] bulk axis, indicating an in-ward relaxation of the atoms in the topmost Ni layer by0.05+ 0.02 A. The minimum in profile F. is observed atthe [001] blocking axis direction (35.0' exit angle).Our layerwise treatment of the data is fully corroborat-ed by fits to each of the angular profiles with linear com-binations of simulated monolayer contributions from thefirst five Ni layers. The simulations were performed us-ing a well-established Monte Carlo technique [11]. Inthe simulations, we have assumed the outermost Ni-Siand Ni-Ni interplanar distances to be 0.12 A (see below)and 0.05 A smaller than in bulk NiSiq, respectively.Values for vibration amplitudes were taken to be identicalto those used in Refs. [2,9]. The fits are shown as solidcurves in the blocking patterns and energy spectra of Fig.2. The relative contributions from individual monolayersin the fits are shown for energy spectrum III (brokencurves). The fits match the data well, strongly con-firming the bulklike surface termination. Residualdifferences between data and fits are largely a conse-quence of the approximation of a constant energy loss perunit path length, on which the construction of the CDP'sis based.VVe have also attempted to describe the data takinginto account an inelastic energy loss which does dependon the specific ion trajectory in Monte Carlo calculations.In these simulations the inelastic loss due to a single atomis assumed to decay exponentially with increasing ion-atom impact parameter [9,12,13]. The results were sub-jected to energy straggling and folded with our detectorresolution. In Fig. 1(b) we show the calculated spectrum(solid curve) for the geometry of Fig. 1(a), with itsdecomposition into the different layer contributions (bro-1135VOLUME 67, NUMBER 9 PHYSICAL REVIEW LETTERS 26 AUGUST 1991j001j C 10022. 00 Si1.0-zQ)& 0.0-II 111 IVOag ~ ~ ~aK%A~ . ' ~ ~A— 1.01.0—'I.O-~y4E DCB A— 1.003—0.0~ C— 1.0~ 0.0-Nv 0.0iE0c- 0.0-1.0— —0.0D 1.01.0- —0.0E-' 1.00.0~+ ~~0 ')I 00 I 0040 60 1.0 0.5 0,0 —0.5exit angle (deg) inelastic energy loss (keV)FIG. 2. Layerwise structure determination. Top panel: Scattering geometry; the ion beam is incident at an angle of 22.0 withrespect to the surface plane. Bottom panels: Cuts through the two-dimensional data set (Ni signal) at constant angle (energy spec-tra I-IV) and constant backscattering depth (A-F). Data are the points; the curves are the result of a fitting procedure (see text).Profiles 8-F. correspond to backscattering depths of 0.50, 1.26, 1.92, 2.56, and 3.43 Si-Ni-Si triple layers, respectively. The blockingminima in the second- and third-layer signals are indicated by small arrows.ken curves). The agreement with the measured spectrumis rather good, considering that the comparison is madeon an absolute scale and does not involve any free param-eters.We have directly observed the dependence of the back-scattering energy on the specific trajectory along a singleSi surface atom and use it to accurately determine thefirst Ni-Si interplanar distance. A determination of thisdistance using conventional channeling and blocking iscomplicated by the interference of the weak and broadfirst-layer Ni-Si blocking minimum with narrower mini-ma in the deeper-layer yields. By contrast, the energy-loss measurement is sensitive to the first-layer energiesonly, allowing an accurate determination. In this experi-ment, the incident ion beam was aligned with the [001]silicide axis [(110) scattering planel at an angle of35.03' with the surface plane (Fig. 3, inset). The back-scattered ions were energy analyzed in a 20 angularrange around the [1 1 I l direction (dashed line). Ionsbackscattered from the outermost Si layer reach thedetector with a relatively small inelastic energy loss dueto electron excitations in the layer itself; on the otherhand, ions backscattered from Ni additionally lose energyin the Si surface layer. This inelastic loss is large for ionsthat pass a Si atom closely, because there the electrondensity is largest. Additionally, ion deAections awayfrom the Ni-Si internuclear axis to a larger or a smaller0.1 0)0.05~ Ni100 keV H35.03a 0.00Q)v —0.05aN(D—0.10o Si~ ~ ~ ~ ~~ ~~ ~ ~I~ ~~ ~I10I I15 20exit angle (deg)I25 30FIG. 3. Energy loss in a single Si monolayer. The residualenergy differences (see text) of the Ni and Si first-layer signalshave been determined from the leading edges in the energyspectra. Inset: The scattering geometry. The shift of theasymmetric minimum observed at 16.3' (dotted line) withrespect to the direction of the [1 1 1 l bulk axis (dashed line) cor-responds to a 0.12-A contraction of the first Ni-Si interplanardistance. The solid curve is the result of a Monte Carlo calcula-tion (see text).1136VOLUME 67, NUMBER 9 PHYSICAL REVIEW LETTERS 26 AUGUST 1991detection angle cause an apparent energy gain or loss.The small variations in the first-layer Ni backscatteringenergy are detected using the Si energy as a reference, sothat possible angular variations in the detector energyscale cancel out.We have derived the first-layer backscattering energiesfrom the leading edges of the Ni and Si surface peaks, atwhich the yield comes from the respective first layersonly. Figure 3 shows the angular dependence of thedifference of the so-called energy residuals of Ni and Si(circles). The residuals are the differences between theobserved leading edges and the (calculated) purely elasticbackscattering energies. The negative sign of the energydifference demonstrates the Si termination of the surface.The decrease for smaller exit angles is a natural conse-quence of the larger energy loss at larger ion pathlengths. The inelastic energy losses and ion deflectionsgive rise to a strongly asymmetric minimum at 16.3 exitangle (dotted line). From this angle, we conclude thatthe outermost Ni-Si interlayer distance is relaxed inward-ly from its bulk value by 0.12+ 0.02 A. In a LEEDanalysis Yang, Jona, and Marcus [1] found a relaxationof 0.19 A; however, they did not specify an error margin.The solid curve represents the result from a Monte Carlocomputer calculation using the impact-parameter-de-pendent inelastic energy loss described above [12]. Thesimulation accurately reproduces the asymmetric min-imum in the data, both in depth and in angular position.The constant-energy difference (-40 eV) between dataand calculation should probably be attributed to thedifference in energy straggling between calculated andobserved first-layer Ni signals, which effectively gives riseto an apparent energy shift of the leading edge of the ex-perimental Ni peaks. A Si double layer on the surfacewould have resulted in a residual energy difference largerthan 0.2 keV [in an earlier experiment on annealedCoSi2(111) we have measured a large shift indicative of aSi double layer on top [6]]. In this case, such a shift isnot observed.Both blocking spectra and backscattering energy mea-surements show that the NiSiq(111) surface has a bulk-like topology, without an additional bilayer on top [3,4].The outermost Ni-Ni and Ni-Si interlayer distances arerelaxed inwardly by 0.05+ 0.02 and 0.12~0.02 A, re-spectively.We have demonstrated that MEIS measurements withsufficiently high-energy resolution can be analyzed in alayerwise fashion. This significantly improves the analyt-ical strength of the technique. In addition, energy lossesin a single layer of atoms may be used to determine sur-face structures under circumstances where blocking ef-fects are not strong enough.We thank P. F. A. Alkemade for the development ofthe code for simulation of the trajectory-dependent ener-gy loss. E. Vlieg is acknowledged for critically readingthe manuscript. This work is part of the research pro-gram of the Foundation for Fundamental Research onMatter (FOM) and was made possible by financial sup-port from the Netherlands Organisation for the Advance-ment of Research (NWO).[1] W. S. Yang, F. Jona, and P. M. Marcus, Phys. Rev. B 2$,7377 (1983).[2] E. J. van Loenen, A. E. M. J. Fischer, J. F. van der Veen,and F. Legoues, Surf. Sci. 154, 52 (1985).[3] T. L. Porter, D. M. Cornelison, C. S. Chang, and I. S. T.Tsong, J. Vac. Sci. Technol. A 8, 2497 (1990).[4] J. E. Rowe, R. S. Becker, G. K. Wertheim, and R. T.Tung (to be published).[5] S. A. Chambers, S. B. Anderson, H. W. Chen, and J. H.Weaver, Phys. Rev. B 34, 913 (1986).[6] J. Vrijmoeth, A. G. Schins, and J. F. van der Veen, Phys.Rev. B 40, 3121 (1989).[7] J. F. van der Veen, Surf. Sci. Rep. 5, 199 (1985).[81 R. T. Tung, J. Vac. Sci. Technol. A 5, 1840 (1987).[9] J. Vrijmoeth, J. F. van der Veen, D. R. Heslinga, and T.M. Klapwijk, Phys. Rev. B 42, 9598 (1990).[10] H. H. Andersen and J. F. Ziegler, The Stopping andRanges of lons in Matter (Pergamon, New York, 1977).[11]J. W. M. Frenken, R. M. Tromp, and J. F. van der Veen,Nucl. Instrum. Methods Phys. Res. , Sect. B 17, 334(1986).[12] O. S. Oen and M. T. Robinson, Nucl. Instrum. Methods132, 647 (1976).[13]P. F. A. Alkemade (to be published).1137

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Monolayer resolution in medium-energy ion-scattering experiments on the NiSi2(111) surface

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