growth of thin ti films on al single-crystal surfaces at room temperature

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 185È193 (1999)

Growth of Thin Films on Single-crystalTi AlSurfaces at Room Temperature

R. J. Smith,* Y. W. Kim, N. R. Shivaparan, G. A. White and M. A. TeterDepartment of Physics, Montana State University, Bozeman, MT 59717, USA

The growth of thin Ti Ðlms on Al(001), Al(110) and Al(111) surfaces at room temperature has been studied usinghigh-energy Rutherford backscattering spectroscopy (RBS) and channeling, x-ray photoelectron spectroscopy(XPS) and low-energy electron di†raction (LEED). Our results show that Ti atoms form a thin, metastable, fccoverlayer on Al(110) and Al(001) surfaces. The primary evidence for this conclusion is the reduced backscatteringthat occurs as the Ti atoms shadow the Al atoms in the fcc structure of the Al substrate. For the Al(111) surfacethe Al surface peak area in ion channeling, measured as a function of Ti coverage, shows a small decrease for theÐrst monolayer (ML) of Ti coverage but then increases gradually with coverage, a characteristic of alloy forma-tion. However, XPS and low-energy ion scattering (LEIS) results are generally consistent with overlayer growth ofTi on the Al(111) surface, and the LEED pattern indicates an ordered overlayer for Ti coverages from 2 to 12 ML,at which point the Al surface was completely covered by Ti. The results suggest the growth of incommensurate,ordered islands of hcp Ti on Al(111) in a Stranski–Krastanov growth mode, in remarkable contrast to the pseudo-morphic fcc Ti overlayer growth observed for Ti Ðlms on Al(001) and Al(110). Copyright 1999 John Wiley &(

Sons, Ltd.

KEYWORDS: titanium; aluminum; metal Ðlms ; epitaxial growth ; ion scattering ; photoemission

INTRODUCTION

In two earlier papers we presented observations of epi-taxial growth of metastable, fcc Ti Ðlms on Al(110) andAl(001) surfaces at room temperature.1,2 These resultsare particularly interesting because fcc Ti does notoccur in nature at any temperature. This structure isapparently stabilized by the fcc Al substrate, which isable to provide the necessary strain energy to overcomethe small energy di†erence between the fcc and natu-rally occurring hcp Ti structures. In this paper we Ðrstreview the ion scattering and photoemission results forthe (001) and (110) surfaces, and then present newresults for the (111) surface. Our intention is to contrastthe behavior seen for Ti growth on these three low-index Al surfaces. A more detailed description of theresults for Ti growth on Al(111) will be presented in afuture paper.3

The observation of Ti overlayer formation on Al sur-faces is especially interesting because considerations ofthe surface energies for Ti and Al (larger for Ti), and ofthe formation energies for TiÈAl compounds (relativelylarge and exothermic), suggest that Ti atoms depositedon the Al surface should not wet the surface but shouldinstead di†use into the surface to form an alloy. Becausealloy formation is observed when Ti Ðlms on Al(110) areannealed at 350 ¡C,4 we are led to the conclusion thatthere must be a kinetic barrier to Ti di†usion into thesubstrate at room temperature for Ti deposited onAl(001) and Al(110) surfaces. Furthermore, the Al(111)surface provides an excellent template for growth of hcp

* Correspondence to : R. J. Smith, Department of Physics, MontanaState University, Bozeman, MT 59717, USA

Ti because this close-packed triangular Al lattice closelyresembles the basal plane of hcp Ti, and the interatomicdistance in the Ti basal plane is 2.95 which is onlyÓ,3% larger than that of 2.86 for Al.5 The di†erentia-Ótion between an hcp and an fcc Ti overlayer does notoccur until the third layer of Ti begins to grow and astacking sequence, either A-B-A for hcp or A-B-C-A forfcc Ti, is established in the overlayer. It seems reason-able that the e†ect of the substrate might be consider-ably reduced at this distance from the interface, so hcpTi is expected to result. Yet, based on our earlier studiesthe possibility of fcc Ti growing on Al(111) cannot bedismissed. Also, multilayer TiÈAl Ðlms have been shownto grow with both the hcp and fcc Ti structures,depending on the bilayer thickness.6 X-ray di†ractionmeasurements for TiÈAl multilayers with a bilayerthickness of 108 nm grown by sputter deposition showthe formation of hcp Ti(0001)pAl(111). However, to ourknowledge studies of single bilayer interfaces with thinTi overlayers have not yet been reported for single-crystal Al(111). Understanding the growth of these epi-taxial Ti/Al interfaces will advance our knowledge ofmetalÈmetal epitaxy, and is expected to have applica-tions in the development of di†usion barriers andmetallization schemes on electronic materials.7

EXPERIMENTAL TECHNIQUES

High-energy ion scattering (HEIS) and x-ray photoelec-tron spectroscopy (XPS) were the primary techniquesused in this study. When used in the channeling mode,HEIS provides a powerful tool to probe the substratesurface structure as well as the overlayer structures andgrowth modes.8,9 The MeV He` ions also provide a

CCC 0142È2421/99/040185È09 $17.50 Received 17 June 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 30 September 1998

186 R. J. SMITH ET AL .

direct means for accurately measuring the overlayercoverage when the ion beam is incident on the substratein a random direction. The technique is element speciÐcbecause the ion recoil energy is mass dependent. In thechanneling geometry the He` ion beam is incidentalong a low-index crystallographic direction and mostof the ions channel into the crystal along the relativelyopen areas between the rows of atoms, as shown sche-matically in Fig. 1(b). Ions that backscatter from theÐrst atom in each row give rise to the surface peak (SP)observed in the backscattered ion energy distributioncurve [Fig. 1(c)], while the small-angle forward-scattered ions form a shadow cone that extends alongthe row into the solid [Fig. 1(a)]. For a static model ofan ideally terminated bulk lattice, subsequent atomsalong the row and within the shadow cone do not leadto backscattered ions. However, displacements of theatoms due to thermal vibrations, surface reconstructionor alloy formation can lead to reduced shadowing andincreased ion yield in the surface peak. Similarly, if a Tiadatom is located directly above the Ðrst atom in a row,shadowing by the adatom will result in decreased ionyield in the substrate surface peak [see Figs 1(b) and1(c)]. The measured surface peak areas are converted toareal densities of visible target atoms (atoms cm~2)using the Rutherford scattering cross-section, the solidangle subtended by the detector and the time-integrated

Figure 1. Schematic drawing showing: (a) the shadow coneconcept for ion scattering; (b) the incident ion beam aligned witha single-crystal surface having a single layer of adatoms (solidcircles) ; and (c) the surface peak of backscattered ions for theclean substrate (dotted curve) and the reduction of the surfacepeak associated with shadowing of the substrate atoms by a layerof adatoms.

incident ion current. Quantitative information aboutthe surface structure may be extracted from the data bycomparing measured channeling yields with scatteringyields obtained by computer simulations of the channel-ing experiment for various overlayerÈsubstrate struc-tures.10

In the XPS experiments the attenuation of the Alphotoemission peak intensity as a function of Ti cover-age, as well as the increase of the Ti photoemission peakintensity, are used to characterize the morphology ofthe Ti Ðlms. Photoemission is also used to determinethe amount of contamination on the sample surfaceduring the cleaning process. In some experiments wehave used di†raction of the outgoing photoelectrons toextract complementary structural information about theTi overlayer.2

The Al single crystals were cut and polished to within1¡ of the respective crystallographic planes, as measuredby x-ray di†raction. The crystals were then chemicallyetched for 15 s in an aqueous solution containing HCl(1.5%), HF (1.5%) and (2.5%), and mounted inHNO3the ultrahigh vacuum (UHV) chamber. Three strands ofhigh-purity Ti wires (99.99%), 0.25 mm in diameter and10 cm in length, were twisted together, wound intosmall coils and then etched in a 20% HF solution. Todeposit Ti on the Al surfaces in a vacuum, these Ðla-ments were resistively heated using a constant currentsupply to maintain a constant Ti sublimation rate. TheTi Ðlaments were mounted 5A away from the Al sampleso that a uniform Ti Ñux was obtained across thesample surface. Deposition rates ranging from 0.15 to0.5 monolayer per minute, as measured by ion scat-tering, were obtained by maintaining a current of 4È4.9A through the Ti wires. The deÐnition of one monolayer(ML) is based on the number density of each surfaceplane : 0.862] 1015 atoms cm~2 for Al(110) ;1.22] 1015 atoms cm~2 for Al(001) ; 1.41] 1015 atomscm~2 for Al(111). All Ti depositions were performedwith the Al sample at room temperature.

The UHV chamber used for the HEIS measurementsis connected to a 2 MV van de Graa† acceleratorthrough a di†erentially pumped beam line as describedelsewhere.11 The Al crystal was mounted in thechamber on a thick Mo block attached to a three-axisgoniometer for channeling measurements. The tem-perature of the Mo block was monitored using a cali-brated Pt resistor mounted inside of the block. Afterbaking the UHV system, a pressure of 1.5 ] 10~10 Torrwas obtained. Energy analysis of the backscattered par-ticles for HEIS was performed using a bakeable, passi-vated, implanted planar silicon (PIPS) detector installedon a rotable arm and located 3A away from the sample.The detector position was set at a scattering angle of105¡ for these experiments.

In a vacuum the Al crystals were cleaned with 1È1.5keV Ar` ion bombardment for several hours with thesample at room temperature, followed by annealing thesample at 450 ¡C for 15 min. The cleaning procedurewas repeated until the photopeak associated with alu-minum oxide was completely removed from the XPSspectrum. The O 1s photopeak could not be used tomonitor reliably the Al surface oxide because the XPSanalysis area included a small portion of the Mo sampleholder surrounding the Al crystal. However, low-energyion scattering (LEIS) experiments performed for the

Surf. Interface Anal. 27, 185È193 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

GROWTH OF THIN Ti FILMS ON Al SINGLE-CRYSTAL SURFACES 187

Al(111) surface conÐrmed that after cleaning the samplenegligible oxygen remained on the surface when the Aloxide peak was absent from the XPS spectrum. Aftercleaning the sample, a collimated beam of MeV He`ions, passing through an aperture of 1.2 mm2 area, wasused to carry out the ion channeling measurements. Thesample was aligned with the ion beam incident alongthe low-index surface normal direction by minimizingthe backscattered ion yield in a small region behind thesurface peak.

Ion scattering and XPS measurements were madeafter each Ti deposition. A total dose of 1.56] 1015ions cm~2 was used to collect each HEIS spectrum. In apreliminary experiment to measure the damage inducedby the incident He` beam, no signiÐcant increase in ionyield was observed after an ion dose of 3.1] 1016 ionscm~2. In addition, an ion scattering spectrum was mea-sured with the sample rotated out of the channelingalignment to determine the total Ti coverage at the dif-ferent stages of the experiment. These measurements ina random-alignment geometry eliminate possible errors,associated with the shadowing of Ti atoms, in determin-ing the Ti coverage. The uncertainty in the ion scat-tering yields reported here is estimated to be ^ 5.6%.Contributions to the uncertainty come from the deter-mination of the detector solid angle, the integratedcharge, the scattering angle and the determination ofthe surface peak area.

Titanium and Al 2p core-level photoemission inten-sities were also monitored during the Ðlm growth usingthe Ka line from an Mg anode for the Al(110) studiesand an Al anode for Al(001) and Al(111) experiments. AÐxed pass energy of 50 eV and a scanning rate of 0.1 eV

s~1 were used for the hemispherical analyzer (VSWHA100). The angle between the sample normal and theelectrostatic analyzer was Ðxed at h \ 30¡ for the inten-sity vs. coverage measurements on Al(001) and Al(111).For Al(110) the XPS emission angle was Ðxed at 0¡ rela-tive to the surface normal. The acceptance angle of thehemispherical analyzer is speciÐed by the manufacturerto be ^ 6¡. Film deposition, channeling measurementsand XPS photopeak intensity measurements were allperformed without moving the sample, althoughoccasionally the sample was rotated slightly to measurethe random alignment backscattering yield from Tiatoms.

RESULTS

Titanium on Al(001)

Channeling spectra taken for the clean Al surface andafter the deposition of 1.89 ML of Ti are shown in Fig.2. Both spectra were taken with the 0.57 MeV He` ionbeam incident along the direction, i.e. at normal[0016 ]incidence. The crystal was aligned with the ion beam byminimizing the integrated backscattering yield to theleft of the Al surface peak (SP) in Fig. 2. The measuredSP area yields a value of 9.7] 1015 atoms cm~2 for theclean (001) surface, or 4.0 Al atoms per row visible tothe incident ion beam normal to the surface. This valueis in excellent agreement with computer simulations for

Figure 2. Helium ion backscattering spectra at 0.57 MeV incident ion energy along the direction for a clean Al(001) surface (solidÍ0016 Ëcircles) and after a deposition of 1.89 ML of Ti (open circles). The Al and Ti surface peak energies are indicated by the vertical arrows.

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 185È193 (1999)


the clean surface, as discussed below. After deposition of1.89 ML of Ti, the Al yield has decreased to 8.3 ] 1015atoms cm~2, associated with the shadowing of Alsurface atoms by Ti adatoms.

Figure 3 illustrates the basic growth characteristics ofthe Ti Ðlms on the Al(001) surface as measured usingion channeling. The open circles in the Ðgure representthe measured ion scattering yield from Al atoms, i.e. theAl SP area from Fig. 2, plotted as a function of the Ticoverage as determined from the Ti yield in the ionscattering spectra recorded for a non-channeling direc-tion of incidence. For the Ðrst half monolayer of Ticoverage the trend in the SP area is not very clear, andmay be assumed to be constant to within experimentaluncertainty. However, after this coverage and up to 5.5ML of Ti deposited on the substrate, a decrease in theAl SP area was observed. An increase in the Al peakarea is observed at higher Ti coverages.

The results of computer simulations of the ion scat-tering experiment are indicated by the solid circles inFig. 3. The simulations were done using the VEGAScode10 with lattice parameters for bulk Al.12 In thesesimulations the Ti atoms were arranged in a Ñat over-layer and placed on the Al fcc lattice sites above the Alsurface to simulate layer-by-layer growth of fcc Ti. TheAl(001) interplanar distance of 2.025 and a vibrationÓamplitude of 0.096 based on a Debye temperature ofÓ,428 K, were used for the Ti atoms.5

Figure 4 shows the Al 2p photoemission peak area,normalized to the value for the clean surface, plotted as

a function of Ti coverage as determined from the ionscattering yield. The attenuation in the Al photopeak isnot signiÐcant until the Ti coverage exceeds a thicknessof D1.5 ML. After this coverage the emission peakdecreased in area throughout the experiment. Thedecrease in the Al peak area is compared with a modelexponential decay represented by the solid curve.13 Thisparticular curve is for a model of layer-by-layer growthwith an electron attenuation length of 20.3 calculatedÓ,using the results of Seah and Dench.14 The data andmodel calculations agree well at higher Ti coverages,but there is a noticeable lack of attenuation at low Ticoverages. This may be the result of some alloy forma-tion in the surface layer of Ti on Al(001). However, theion scattering results show that there is still consider-able order at the surface as the Ti coverage increases, aresult further supported by low-energy electron di†rac-tion (LEED) studies.2,15

The results presented in Figs 3 and 4 support a modelin which Ti Ðlms grow on the Al(001) surface in an epi-taxial fcc structure.2 The primary evidence for this is thereduction in Al yield seen in Fig. 3, which can onlyoccur if Ti atoms sit directly above Al atoms in apseudomorphic structure. Any other arrangement of Tiatoms will not result in this amount of Al shadowingbecause the radius of the shadow cone for these He ionsincident on Ti, as indicated in Fig. 1(a), is only 0.1 Ó.Displacement of the Ti atoms by much more than theradius of the shadow cone would result in no shadow-ing of Al atoms, and a Ñat curve in Fig. 3. Quantitative

Figure 3. Visible Al atoms, at 0.57 MeV incident ion energy, as a function of Ti coverage deposited at room temperature on the Al(001)surface (open circles). The solid circles indicate the scattering yield from Al atoms for a flat, pseudomorphic fcc Ti film, calculated using theVEGAS simulation code. The solid lines are linear fits to the two regions indicated, and are provided to guide the eye.



Figure 4. Normalized Al 2p photoelectron intensities (open circles) plotted as a function of Ti coverage on the Al(001) surface. The solidline is the expected result for Al 2p intensity based on a model calculation for a layer-by-layer growth mode, using an attenuation length of20.3 A� .

LEED studies have also been reported, supporting theexistence of an epitaxial fcc structure of Ti on Al(001).15There is an apparent small coverage delay before Alshadowing occurs. The delay is also manifested in thelack of attenuation in the Al 2p photopeak at low Ticoverages shown in Fig. 4. This behavior is consistentwith a small amount of TiÈAl interchange in the Ðrstsurface layer of the Al(001) substrate.2,15

After the critical thickness of 5.5 ML is reached, webelieve that the strain energy in the Ti Ðlm exceeds theTi/Al interfacial energy, resulting in the interruption ofpseudomorphic growth. Although the atomic structureafter 5.5 ML cannot be determined completely on thebasis of our results, we believe that misÐt dislocations inthe thicker Ti Ðlms allow Ti atoms to shift graduallyparallel to the surface, relieving strain in the Ðlm,uncovering Al atoms in the substrate and causing the Alyield to slowly increase. It is important to note that atno time in our experiments did the yield from Al atomsexceed the value for the clean surface, which wouldoccur, for example, if Al atoms were moving o† fcclattice sites.16

Titanium on Al(110)

In Fig. 5 we plot the number of Al atoms cm~2 visibleto the incident ion beam as a function of Ti coverage forthe Al(110) surface.1 The incident ion beam energy inthese experiments was 0.96 MeV. Similar to the obser-vations for the Al(001) surface, the ion yield initiallydecreases with Ti deposition, indicating a lower hitting

probability for Al atoms. This decrease is again attrib-uted to shadowing of the surface Al atoms by Tiadatoms, forming a pseudomorphic fcc overlayer. Thisinterpretation is supported by results from computersimulations (VEGAS) of the ion scattering yield for amodel of layer-by-layer growth of fcc Ti shown by thesolid circles in Fig. 5.1 Note that at this higher ion beamenergy, the amount of Al shadowing by Ti is smallerthan that seen in Fig. 3 for Al(001), primarily becausethe Ti shadow cone radius is smaller at the higherenergy. As more Ti is deposited on the substrate,exceeding a critical thickness of D5.5 ML, the initialgrowth regime is followed by one in which the numberof visible substrate atoms begins to increase. Theincrease continues up to a total coverage of 12.5 ML ofTi, at which coverage the experiment was terminated.Again we note that the scattering yield from Al atomsnever exceeds that for the clean surface. A quantitativeLEED study for this system was attempted,15 butLEED patterns useful for quantitative analysis were notobtained for Ðlms of more than a few monolayers thick-ness. That work did conÐrm the pseudomorphism of asingle Ti monolayer on Al(110).

Figure 6 shows the results for the Al 2p photoemiss-ion intensity measured normal to the Al(110) surface asa function of Ti coverage (open circles). The solid line isagain the result of model calculations for layer-by-layergrowth of Ti on this surface using an attenuation lengthof 18.5 (collected with an Mg x-ray source). The rapidÓattenuation of substrate photoemission intensity for lowTi coverages is clearly not consistent with the calcu-lations. We also show in Fig. 6 the results of calcu-lations that Ðt the initial decay by using a much shorter



Figure 5. Visible Al atoms, at 0.96 MeV incident ion energy, as a function of Ti coverage deposited at room temperature on the Al(110)surface (open circles). The solid circles indicate the scattering yield expected from Al atoms for a flat, pseudomorphic fcc Ti film, calculatedusing the VEGAS simulation code. The solid lines are linear fits to the two regions indicated, and are provided to guide the eye.

Figure 6. Normalized Al 2p photoelectron intensities (open circles) plotted as a function of Ti coverage on the Al(110) surface. The solidline is the expected result for Al 2p intensity based on a model calculation for the layer-by-layer growth mode, using an attenuation lengthof 18.5 The dashed line shows the calculated intensities for the same model with a shorter attenuation length, as discussed in the text.A� .



attenuation length (dashed line).4 At larger Ti cover-ages, however, the rate of decrease appears to be morelike that expected for island growth, or characteristic ofthe larger attenuation length. Although we do not com-pletely understand this intensity behavior at low Ticoverage, it may be associated with the Ti-inducedremoval of enhanced forward scattering of photoelec-trons along the [110] direction normal to the surface.We note that the results for Fig. 6 were collected atnormal emission, while those for Al(001) and Al(111)were collected at an emission angle of 30¡ from normalemission. Such anomolous attenuation of normal emis-sion photoelectrons was also reported for fcc Co Ðlmsgrown on Cu(001).17 On the other hand, Chambers hasshown that the growth of Ge Ðlms on GaAs(001) doesnot result in such an anomolously short attenuationlength for the Ga 3d emission normal to the surface.18

Titanium on Al(111)

The ion scattering results for Ti Ðlm growth on theAl(111) surface are shown in Fig. 7. Here we again plotthe number of Al atoms visible to the incident ionbeam, determined from the Al surface peak area, as afunction of Ti coverage, determined by the Ti surfacepeak in a non-channelling geometry. The incident He`ion energy is 1.09 MeV. A small amount of Al shadow-ing occurs for the Ðrst 1.5 ML of Ti coverage, where thenumber of visible Al atoms decreases slightly. However,

at larger Ti coverages the Al yield begins to increaseand continues to do so in a linear fashion up to D10ML of Ti coverage, where the experiment ended. Thisbehavior is remarkably di†erent from that seen for theAl(001) and Al(110) surfaces discussed above. The evi-dence of a critical thickness of 5 ML is not observed,and the ion yield from Al atoms rapidly increases abovethat for the clean surface. Increases in the Al ion yieldwere also observed for Pd, Fe and Ni deposition on theother Al surfaces. Those results were attributed to alloyformation at the transition metal/Al interface.16,19,20Such an increase might also occur if the growth of theTi overlayer caused Al atoms to move o† the substratelattice sites below the interface because of overlayer-induced strain in the Al lattice near the interface.However, we do not believe that such displacementswould continue to occur in a linear fashion over such alarge range of Ti coverages. Furthermore, the photoe-mission results shown below do not support a model ofalloy formation at the interface.

In Fig. 8 we show the measured photoemission inten-sity of the Al 2p core level plotted as a function of Ticoverage. The attenuation of the substrate emissionbegins immediately with the smallest Ti coverages. Thesolid line in the Ðgure shows the results of a model cal-culation for layer-by-layer growth, calculated onceusing an attenuation length of 20.3 At intermediateÓ.14coverages we see that the data are somewhat above thesolid line. We also show with a dashed line the resultsfor a model of StronskiÈKrastinov growth, whereislands of Ti grow on top of a Ñat initial ML of Ti.13

Figure 7. Visible Al atoms, at 1.09 MeV incident ion energy, as a function of Ti coverage deposited at room temperature on the Al(111)surface (open circles). The solid lines are linear fits to the two regions indicated, and are provided to guide the eye.



Figure 8. Normalized Al 2p photoelectron intensities (open circles) plotted as a function of Ti coverage on the Al(111) surface. The solidline is the expected result for Al 2p intensity based on a model calculation for the layer-by-layer growth mode, using an attenuation lengthof 20.3 The dashed line shows the expected result for Al 2p intensity based on a model calculation of Ti island growth following theA� .growth of a 2 ML thick, flat Ti film, with the islands coalescing at 9 ML of Ti coverage.

The agreement between measurement and the dashedline is quite good. We present elsewhere a more detaileddiscussion of these results along with LEED and LEISmeasurements.3 The LEED pattern is observed toweaken considerably for the Ðrst 1.5 ML of Ti coverage,but then gradually returns by D4 ML and continues toshow a hexagonal symmetry throughout the experi-ment. The LEIS results show that the Al signal isreduced at a rate that seems too slow to be consistentwith layer-by-layer growth. Some Al signal persists evenat a coverage of 7 ML. Finally, a small amount of shad-owing of Ti atoms is also observed for HEIS channel-ling along the [111] direction normal to the surface.For the 10 ML Ti Ðlm, the yield from Ti atoms isreduced by 30% when the ion beam is directed normalto the Al(111) surface. From all of these results we areled to the conclusion that a monolayer of Ti initiallycovers the Al(111) surface, followed by the growth ofordered Ti islands (StranskiÈKrastinov growth) up to acoverage of D9 ML, at which point the islands coalesceto a relatively Ñat overlayer.

Finally, we return to the increase in the Al ion yieldshown in Fig. 7. Calculated ion scattering results showthat the growth of commensurate, ordered fcc islands ofTi on the Al(111) surface would result in shadowing ofthe Al atoms similar to that seen for the other Al sur-faces. At the present time we are left with only oneexplanation, which assumes that the Ti islands areincommensurate with the Al(111) substrate, associatedwith the small (3%) lattice mismatch. We have per-formed ion scattering simulations that show that the

shadow cones formed by the Ti overlayer may result inadditional ion Ñux at the sites of the Al atoms at theinterface. If the islands are not commensurate with theAl substrate, the rows of Ti atoms normal to the surfacewill be displaced slightly from the lattice sites of the Alsubstrate with a small range of displacement values. Asthe Ti layer thickness increases, the width of the shadowcone at the interface increases, placing more and moreof the associated Ñux peak at the site of the Al interfaceatoms, resulting in increasing ion scattering yield as afunction of Ti coverage. The increasing ion yield associ-ated with such a focusing e†ect will eventually saturateand should be energy dependent. Future experiments inour laboratory will investigate this possibility. Similarfocusing and shadowing arguments may also explainthe increase in ion scattering yields at Ti coveragesexceeding the critical thickness for Ti Ðlms on Al(001)and Al(110) (see Figs 3 and 5). Additional photoemiss-ion di†raction experiments are in progress to determinebetter whether the Ti overlayer on Al(111) has the fcc orhcp structure.

CONCLUSION

In conclusion, we have shown remarkably di†erentbehavior for ion channelling studies of Ti Ðlms grownon three low-index orientations of Al surfaces. TitaniumÐlms grow in a pseudomorphic fcc structure on Al(001)and Al(110) surfaces. The Ðlms grow to a critical thick-



ness of D5.5 ML and then begin to relax and graduallyuncover the fcc Al lattice sites as the Ti Ðlm thicknessincreases. On the Al(111) surface the Ti appears to growin a StranskiÈKrastanov mode, with a single layer of Ticovering the surface, followed by growth of Ti islands.The Ti islands are well ordered and appear to coalesceat a Ti coverage of D9 ML. An unusual increase in theion scattering yield is attributed to focusing e†ectsassociated with growth of incommensurate islands of Tion the Al(111) surface. Additional experiments are

under way to deÐne further the structure of the Ti Ðlmson Al(111).

Acknowledgements

The authors are pleased to acknowledge the technical support of ErikAndersen and Norm Williams, and the early contributions to thiswork by Dr Adli A. Saleh and Dr V. Shutthanandan. This work wassupported by the National Science Foundation under Grant DMR-9710092, and by NASA EPSCoR Grant NCCW-0058.

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growth of thin ti films on al single-crystal surfaces at room temperature

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