Surface segregation of implanted ions: Bi, Eu, and Ti at the MgO(100) surface

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  • .Applied Surface Science 130132 1998 534538

    Surface segregation of implanted ions: Bi, Eu, and Ti at the /MgO 100 surface

    T. Suzuki a, S. Hishita b, K. Oyoshi b, R. Souda a,b,)a Department of Material Science, Tsukuba Uniersity, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305, Japan

    b National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, JapanReceived 17 September 1997; accepted 10 November 1997

    Abstract

    .Surface segregation of Bi, Eu, and Ti implanted at the MgO 100 surface has been investigated by using time-of-flight . .coaxial impact-collision ion scattering spectroscopy CAICISS and reflection high energy electron diffraction RHEED . It

    is observed that implanted-Bi is concentrated at subsurface layers of MgO. On the other hand, it is found that Eu segregatesto the outermost surface layer by annealing at 10008C, in addition to the simultaneous segregation of Ca which is included inthe bulk as impurity of 40 ppm. During the implantation of Ti at the MgO substrate, it is found that C makes inroads uponthe substrate. Surface segregation of Ti itself is not observed by annealing the substrate from 4008C to 10008C. It isconcluded that nature of surface segregation of these systems primarily depends on the size of the implanted ions. q 1998Elsevier Science B.V. All rights reserved.

    PACS: 61.18.Bn; 68.35.Bs; 68.90.qg

    Keywords: Coaxial impact-collision ion scattering spectroscopy; Surface segregation; Ion implantation; Bismuth; Europium; Titanium

    1. Introduction

    The composition of a surface few layers is some-times different from the ideal composition of thebulk if the material includes a small amount ofimpurity. Such differences of composition are oftenobserved when the materials with impurity are heatedin vacuum. Especially with metal materials, it iswell-known that a small amount of impurity some-

    times tends to be concentrated at surfaces surface.segregation . The surface composition influences to a

    large extent the oxidation, catalysis, corrosion etc. so

    ) Corresponding author. Fax: q81-298-52-7449.

    that a better understanding of the mechanism ofsurface segregation is indispensable for discussingsuch surface phenomena or modification of surfaceproperties.

    At binary alloy surfaces, it has been revealed thatthe behavior of surface segregation depends on thecombination of segregants and substrate. For exam-ple, Ag tends to concentrate at surfaces of AgAu

    w xbinary alloy 1 , while Au concentrates at surfaces ofw xCuAu binary alloy 2 . Such varieties of segrega-

    tion behavior have been investigated systematicallyby experiment, and several theories have been devel-oped for elucidating the origin of such differencesw x37 . On the other hand, there are very few studies

    0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .PII S0169-4332 98 00110-X

  • ( )T. Suzuki et al.rApplied Surface Science 130132 1998 534538 535

    about segregation at insulating surfaces and no sys-tematic experiments exist.

    In this study, the surface segregation of Bi, Eu,and Ti implanted at the MgO substrate are studied by

    w xusing CAICISS 810 which has the advantage ofthe investigation not only of the surface structure butalso of the composition along the depth direction.The different behavior of surface segregation, de-pending on implanted elements, was found in thisinvestigation. The result acquired in this study isdiscussed in comparison with the theories.

    2. Experimental

    .The MgO 100 substrate was cleaved in air, andthen introduced immediately into the high vacuum

    y7 .chamber base pressure of 5=10 Torr for ionimplantation of Bi, Eu, and Ti. The condition of ionimplantation for each element is indicated in Table 1in addition to the average projected range and theaverage straggling calculated by using the TRIMcode. After ion implantation, the sample was intro-

    duced into an ultrahigh vacuum chamber base pres-y10 .sure of 5=10 Torr equipped with CAICISS and

    RHEED. The CAICISS measurements were madeusing 2 keV Heq ions. The Heq ions were generatedin a discharge-type ion source and its mass wasanalyzed by means of a Wien filter. The ion beamwas chopped by the electrostatic deflection plates,which produced an ion pulse with a full width at halfmaximum of 30 ns. Energy analysis of the backscat-tered ions or neutrals was made by measuring theflight time between the sample and microchannel

    .plate MCP which was placed 60 cm away from thesample. The ion source and MCP were arrangedcoaxially, so that the experimental scattering anglewas very close to 1808. The specimens were an-

    .nealed step by step from room temperature R.T. to

    8008C or 10008C by electron bombardment frombehind. During the measurement of CAICISS usingthe Heq ions, a W filament placed in the rear side ofthe sample was heated to avoid the occurrence ofcharging effect. In this case, the surface temperaturewas raised up to 4008C.

    3. Results and discussion

    Fig. 1 shows the CAICISS spectra of the MgO .substrate implanted Bi. The incidence angle a of

    the Heq ion beam was 908. In this case, the ionbeam hits the sample perpendicular to the surfaceand makes inroad into the bulk through the channel

    w xwhich is parallel to 100 , so that the scattered parti-cles reflect the composition of the topmost surfacelayer as well as the deeper layers. In Fig. 1, thestructures peaked at 4.8 ms and 5.2 ms are deter-mined to be surface peaks of Mg, and O, respec-tively. In addition to these peaks, there is a broadpeak at around 4.5 ms. The surface peak position ofBi is determined to be at 4.2 ms by calculation offlight time T. Fuse et al. reported the result of the

    w xMonte Carlo simulation of CAICISS spectra 11 .They concluded that the scattered ions from atomslocated in the depth from outermost surface down tomore than a hundred atom layers contribute to theTOF spectrum. The scattered ions from deep layerslose their energy due to the nuclear or electronicstopping powers during the passage through thechannel, so that such scattered ions make the peak atthe later position compared with the surface peakposition in the TOF spectra. By considering this, thebroadened peak at 4.5 ms is thought to be due to theexistence of Bi located in deep layers of MgO. Bystep-by-step annealing of the substrate, the broadlyhumped Bi peak grows relatively higher, but thebroadness remains unchanged and the surface peak

    Table 1Condition of ion implantation and calculated average range and straggling

    2 . . . .Implanted ion Implantation energy keV Amount of implantation ionsrcm Average range A Average straggling A

    q 16Bi 200 1=10 438 83q 15Eu 500 1=10 2427 549

    q 15Ti 500 1=10 2585 558

  • ( )T. Suzuki et al.rApplied Surface Science 130132 1998 534538536

    Fig. 1. CAICISS spectra of 2 keV Heq scattered from the .MgO 100 surface before and after annealing. The measurements

    .were made at incidence angle a of 908 normal incidence .

    of Bi is not observed. The CAICISS spectra acquiredbefore and after annealing suggests that the topmostsurface layer is essentially unchanged in both struc-ture and composition although the depth profile of Biis fairly changed by annealing. Indeed, these surfacesshowed a sharp 1=1 pattern in RHEED. Fig. 2shows the intensity of Bi and Mg peaks measured

    w xalong the 110 azimuth before annealing the sub-strate as a function of the polar angle a . The inten-sity variation of Mg corresponds to the crystallo-graphic structure of MgO as determined by universalshadow cones calculated using ThomasFermi

    . w xMoliere TFM potential 12 . On the other hand, theBi peak exhibits very little a dependence except for

    .large a a)808 and its maximum intensity ap-pears at 908. The increase of the Bi peak intensity forlarge a can be explained by taking into account ofpenetration followed by backscattering of the inci-dence ion beam through the channel parallel to

    w xMgO 100 , indicating that Bi is located only in deeplayers of MgO and no marked Bi enrichment occursat the near surface layer. The Mg peak around theas768 is due to the focusing effect of the incidention beam onto the forth layer Mg ions. If the Bi were

    located at the layers shallower than the fourth layer,some corresponding intensity variation should ap-pear. These results clearly indicate that Bi is locatedat deeper than at least the fifth layer. From theseresults, Bi seems not to segregate to the MgO surfaceat the substrate temperature from 4008C to 8008C.After annealing, however, increase of the Bi inten-sity is obvious in the TOF spectra. In addition tothis, the maximum intensity of the Bi peak appears atthe same position. The experimental results showsuch tendency of segregation that implanted-Bi inthe bulk diffuses by annealing, and comes up tosubsurface layers, but is not enriched at the topmostfew layers.

    Probably, desorption of segregated-Bi at MgOsurface occur, and it makes impossible to observesegregated-Bi located at outermost surface byCAICISS. From this point of view, it looks likelythat Bi, being segregated at the shallow layers of theMgO surface, is readily desorbed at temperatureeven below 8008C. However, the possibility of oc-currence of subsurface segregation followed by con-densation of implanted Bi also remain.

    At the CAICISS spectra acquired without heattreatment as shown in Fig. 1, the broadly humped Bipeak is observed. However, such broadened peak isnot observed at the spectra of Eu and Ti implantation .see Figs. 3 and 5 . This difference is due to thefollowing two reasons; one is much smaller amountof average range of Bi implantation as indicated in

    .Fig. 2. CAICISS intensity of He scattered from Mg open squares . .and Bi solid squares at the MgO 100 surface as a function of

    the incidence angle a of the projectile Heq beam; the measure-w xments were made along the 110 azimuth.

  • ( )T. Suzuki et al.rApplied Surface Science 130132 1998 534538 537

    Fig. 3. The normal incidence CAICISS spectra of 2 keV Heq .scattered from the MgO 100 surface before and after annealing at

    several temperature.

    Table 1. The other is efficient diffusion of Bi even attemperature below 4008C. The intensity of Bi ac-quired without heat treatment is too high to beascribed to the small values of average range andstraggling. Therefore, it is reasonable to think thatimplanted-Bi diffuses at temperature even below4008C.

    Fig. 3 shows the change of CAICISS spectra dueto step by step annealing of the MgO substrate withimplanted Eu. The measurement was made at as908. A sharp surface peak of Eu appears in thespectra after annealing the substrate at 10008C, inaddition to a surface peak of Ca which is included inthe bulk MgO crystal with amount of 40 ppm. Thissurface showed a sharp 1=1 pattern in RHEED.Fig. 4 shows the intensity of Eu and Mg measured

    w xalong the 110 azimuth after annealing the substrateat 10008C for 30 min. as function of polar angle a .In this figure, the intensity variation of Mg corre-sponds to the crystallographic structure of MgO. Onthe other hand, the Eu peak exhibits very little a

    .dependence except for small a a-208 . This re-sult indicates that segregated-Eu being substitutedfor Mg is located at the outermost surface layer of

    .Fig. 4. CAICISS intensity of He scattered from Mg open squares . .and Eu solid squares at the MgO 100 surface as a function of

    the incidence angle a of the projectile Heq beam; the measure-w xments were made along the 110 azimuth.

    MgO and no marked Eu enrichment occurs at thedeeper layer. In addition to this, the difference of thecritical angle indicates that Eu is protruded from theoriginal MgO surface plane. This assumption wasconfirmed by the experiment of azimuthal angle scanof the incidence ion beam. The protrusion of Eu mayoccur to reduce the strain energy at surfaces due tothe difference in the ionic radii between the Eu2q

    2q

    . .1.12 A and Mg 0.65 A .Fig. 5 shows the CAICISS spectra of the MgO

    substrate to which Ti is implanted. The incidenceangle of the Heq ion was 908. In contrast to theother implants, the surface segregation of Ti was notobserved by annealing the substrate. The spectra

    Fig. 5. The normal incidence CAICISS spectra of 2 keV Heq .scattered from the MgO 100 surface before and after annealing.

  • ( )T. Suzuki et al.rApplied Surface Science 130132 1998 534538538

    acquired without heat treatment indicates the exis-tence of C at the MgO surface. The C remains evenafter annealing at 10008C. In the experiment of Biand Eu implantation, the amount of C before heatingthe sample is much smaller and the small amount ofC disappears after annealing at 8008C. Moreover, thecharging effect during the measurement of CAICISSdid not occur at the substrate with implanted Ti afterheat treatment. The inroads of C during implantation

    w xhave been already reported at the Fe substrate 13,14 .In that study, the source of C is thought to beresidual hydrocarbons. The inroads of C in this studyis thought to be the same kind of phenomenon asinroads at the Fe substrate, but the mechanism ofinroads and the character of implanted C at surfacesis not clear. The segregation of Ti itself was notobserved by annealing from R.T. to 10008C.

    Colbourn and Mackrodt investigated theoreticallythe behavior of dopants in bulk with classification

    w xinto the valence number 15 . They took account ofcombination of electrostatic, elastic and polarizationforces, and predicted that four-valent cations arestable in the bulk of MgO. Ti ions can be usuallydivalent, trivalent, and four-valent cations. More-

    .over, the ionic radii of four-valent Ti 0.68 A is .very close to the ionic radii of divalent Mg 0.65 A .

    Accordingly implanted Ti ions most likely stay inMgO bulk with four-valent state and may not pro-duce the strain in the MgO lattice. In general, one ofthe driving force of surface segregation is thought tobe strain energy. For example, M. Cotter et al.reported that the segregation behavior depends on

    w xthe size of the dopant ions 1618 . Our result agreeswith their investigation. The ionic radii of Eu2q and

    2q Ca is 1.12 A and 0.99 A, respectively, so that the

    total energy can be reduced by making implantedthese ions segregate to the surface. In addition tothis, the protrusion of segregated Eu can be ex-plained on this line. On the other hand, the ionic

    3q 5q radii of Bi and Bi is 1.20 A and 0.74 A,

    respectively, so that Bi ions seem to have advantageto stay at substitutional position in MgO bulk astrivalent cations. Our experimental results indicatethe occurrence of segregation of Bi. This fact sug-gests that not only the difference of the ionic radiibut also the effect of other factors may affect thebehavior of surface segregation. In the case of Bisegregation, the driving force is thought to be a

    disadvantage of electrostatic energy, in addition tothe strain energy in the bulk.

    4. Conclusion

    The surface segregation of Bi, Eu, and Ti im- .planted at the MgO 100 surface has been investi-

    gated by using coaxial impact-collision ion scatteringspectroscopy. Subsurface segregation of Bi is ob-served. On the other hand,...

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