Growth of Bi thin films on quasicrystal surfaces

Transcription

1 Ames Lortory Pulictions Ames Lortory Growth of Bi thin films on qusicrystl surfces H. R. Shrm Ntionl Institute for Mterils Science V. Fournée Ecole des Mines M. Shimod Ntionl Institute for Mterils Science A. R. Ross Iow Stte University Thoms A. Logrsso Iow Stte University, See next pge for dditionl uthors Follow this nd dditionl works t: Prt of the Condensed Mtter Physics Commons, nd the Metllurgy Commons The complete iliogrphic informtion for this item cn e found t mesl_pus/58. For informtion on how to cite this item, plese visit howtocite.html. This Article is rought to you for free nd open ccess y the Ames Lortory t Digitl Iow Stte University. It hs een ccepted for inclusion in Ames Lortory Pulictions y n uthorized dministrtor of Digitl Iow Stte University. For more informtion, plese contct digirep@istte.edu.

2 Authors H. R. Shrm, V. Fournée, M. Shimod, A. R. Ross, Thoms A. Logrsso, P. Gille, nd A. P. Tsi This rticle is ville t Digitl Iow Stte University:

3 PHYSICAL REVIEW B 78, Growth of Bi thin films on qusicrystl surfces H. R. Shrm, 1,, * V. Fournée, 3 M. Shimod, 1, A. R. Ross, 5 T. A. Logrsso, 5 P. Gille, 6 nd A. P. Tsi 1,,7 1 Ntionl Institute for Mterils Science, 1--1 Sengen, Tsuku, Irki , Jpn Deprtment of Physics nd Surfce Science Reserch Centre, The University of Liverpool, Liverpool L69 3BX, United Kingdom 3 LSGM, CNRS-UMR 758, Ecole des Mines, Prc de Surupt, 50 Nncy, Frnce SORST, Jpn Science nd Technology Agency, Jpn 5 Deprtment of Chemistry, Ames Lortory, Iow Stte University, Ames, Iow 50011, USA 6 Deprtment für Geo- und Umweltwissenschften, Sektion Kristllogrphie, Ludwig-Mximilins-Universität München, Theresienstrsse 1, München, Germny 7 Institute of Interdisciplinry Reserch for Advnced Mterils, Tohoku University, Kthir -1-1, , Jpn Received 1 June 008; pulished 1 Octoer 008 We present comprehensive study of Bi thin-film growth on qusicrystl surfces. The sustrtes used for the growth re the fivefold surfce of icoshedrl i -Al-Cu-Fe nd i-al-pd-mn nd the tenfold surfce of decgonl d -Al-Ni-Co qusicrystls. The growth is investigted t 300 nd 55 K sustrte tempertures nd t different coverge rnging from sumonolyer to ten monolyers. The film is chrcterized y scnning tunneling microscopy, reflection high-energy electron diffrction, nd x-ry photoelectron spectroscopy. At 300 K, the deposited Bi yields qusicrystlline film for 1. For 1 5, it forms nnocrystllites with 100 surfce orienttion. The islnds hve mgic heights, which correspond to the stcking of four tomic lyers predominntly. The selection of mgic heights is interpreted in terms of quntum size effects rising from the electron confinement within the film thickness. The islnds estlish rottionl epitxil reltionship with the sustrte. For higher coverge, the film grows with montomic height, not with mgic heights, nd reflects the symmetry of the ulk Bi. When deposition is performed t 55 K, terrce diffusion is more effective, resulting in the ggregtion of Bi dtoms developing into smooth monolyer with qusiperiodic order. At this temperture, multilyers do not dsor. DOI: /PhysRevB PACS numer s : 61..Br, , Ef I. INTRODUCTION Qusicrystls re perfectly ordered structures like regulr crystls, ut they lck periodicity. 1 Their diffrction pttern shows shrp well-defined peks tht often distriute in reciprocl spce ccording to the icoshedrl i or decgonl d symmetry. These noncrystllogrphic rottionl symmetries re reflected in the growth shpe of most single-grin qusicrystls, presenting pentgonl or decgonl fcets. Their unique tomic structure correltes with unexpected electronic properties different from those of oth morphous nd crystlline lloys. For exmple, low vlues of the electricl conductivity re typicl in Al-sed qusicrystls, nd increses with incresing temperture, contrry to the usul metllic ehvior. Their electronic structure is chrcterized y minimum in the electronic density of sttes locted t the Fermi level, termed s pseudogp, which is scried to oth structurl nd hyridiztion effects. 3 The surfces of qusicrystls hve received incresing interest in the pst few yers, which is motivted in prt y their potentilly useful properties such s reduced friction nd dhesion or corrosion resistnce. Developments in the synthesis of lrge single-grin qusicrystls of high structurl qulity hve opened the possiility to investigte the structure nd properties of clen qusicrystlline surfces in ultrhigh vcuum UHV environment. Most studies performed so fr hve focused on the tenfold surfce of the Al-Ni-Co decgonl phse nd the fivefold surfce of the Al-Pd-Mn nd Al-Cu-Fe icoshedrl phses. It hs een found tht under suitle preprtion conditions, the surfce of qusicrystls re well-ordered two-dimensionl D qusiperiodic ojects tht correspond to perfect trunction of the ulk structure, with no chemicl segregtion or surfce reconstruction, except for some interlyer relxtion refer to Ref. for recent review. These progresses hve enled the study of new epitxil systems: solid-film growth on qusiperiodic sustrtes. Here the originl ide ws to use qusicrystls s templte to grow nother qusiperiodic systems of reduced dimensionlity tht could not form otherwise. Different oservtions hve shown tht some of the symmetry elements of the qusiperiodic sustrte re medited into the film during the growth, either t mesoscopic scle through the formtion of fivefold or tenfold twinning of nnocrystls or t n tomic scle when pseudomorphic growth cn e chieved.,5 The structure of the film strongly depends on the dsorte species nd the growth mode, which in turn depends on oth energetic nd kinetic fctors. The vrious oservtions reported so fr in the field of thin-film growth on qusicrystlline surfces clerly show tht it is not possile to predict the film morphology, either smooth D or rough threedimensionl 3D growth, sed on thermodynmic reltionship etween the sustrte nd dsorte surfce energies nd the interfcil energy, ecuse growth occurs fr from equilirium nd/or unknown interfcil energy. A more tomistic description of the growth phenomen tht would tke into ccounts oth energetic nd kinetic fctors ppers necessry to explin the vrious oservtions reported so fr. Frnke et l. 6 reported the epitxil growth of either Bi or S monolyers on either the tenfold surfce of d-al-ni-co or the fivefold surfce of i-al-pd-mn qusicrystls. The depo /008/78 15 / The Americn Physicl Society

4 SHARMA et l. sition ws performed t K tempertures, which re ove the multiplyer desorption temperture. The qusiperiodic structure of the deposited Bi or S monolyer ws demonstrted y low-energy electron diffrction nd heliumtom scttering HAS. Recent initio studies confirmed the stility of such supported qusiperiodic monolyers nd structure model hs een proposed. 7,8 We recently reported on investigtions y scnning tunneling microscopy STM nd reflection high-energy electron diffrction RHEED of similr system: Bi deposited on the fivefold surfce of i-al-cu-fe. 9 The Al-Pd-Mn nd Al-Cu-Fe icoshedrl phses re lmost isostructurl nd their surfces perpendiculr to fivefold xis re lmost identicl down to the tomic level, 10 lthough their chemicl compositions in the ner surfce region re different. The thin-film growth ws investigted t room temperture. We oserved pseudomorphic growth t sumonolyer coverge. At coverge of out five monolyers MLs, we found the formtion of crystlline Bi islnds with flt tops nd steep edges on top of Bi wetting lyer. The Bi islnds hve uniform heights, corresponding to four tomic lyers or multiple of this height, which re the so-clled mgic heights. Similr phenomen were lso oserved for Ag thin films grown on the fivefold surfce of i-al-pd-mn t 365 K. 9 We rgued tht the driving force for the islnd height selection is relted to quntum size effects QSEs induced y the confinement of the electrons within the film thickness in the direction norml to the surfce. In such systems, quntum well sttes develop nd their position with respect to the Fermi level depends on the film thickness. Then minimiztion of the electronic energy fvors preferred thickness of the Bi islnds. 9 Quntum size effects were lter confirmed y Mors et l. 11 who directly oserved sp-derived quntum well sttes y photoemission spectroscopy in Ag thin films deposited on qusicrystlline templtes. It ws rgued tht the confining rrier is minly driven y the incomptile symmetries of the wve functions t the film sustrte interfce rther thn y the pseudogp in the electronic structure of the sustrte. In this pper, we present extensive studies of Bi thin-film growth oserved y vrious techniques including STM, RHEED, nd x-ry photoelectron spectroscopy XPS. The sustrtes on which the films re grown include the fivefold surfce of oth i-al-pd-mn nd i-al-cu-fe s well s the tenfold surfce of d -Al-Ni-Co. Thin films were grown either t room temperture or t 53 K, temperture which is ove the multilyer desorption temperture. Film thicknesses rnge from sumonolyer coverge up to 10 ML. A wide vriety of structures re oserved depending on experimentl conditions. This includes qusiperiodic Bi wetting lyer, formtion of nnocrystllites with mgic heights, rottionl epitxil reltionship etween the film nd the sustrte, nd terrce-dependent film morphologies s well. II. EXPERIMENT Single-grin icoshedrl Al 63 Cu Fe 13 nd Al 70 Pd 1 Mn 9 qusicrystls were cut perpendiculr to fivefold xis. Similrly, single-grin decgonl Al 71.7 Ni 18.7 Co 9.6 qusicrystl PHYSICAL REVIEW B 78, ws cut perpendiculr to the tenfold xis. The surfces were then mechniclly polished using dimond pste down to 0.5 m. A clen surfce ws prepred in UHV se pressure of mr y repeted cycles of ion sputtering Ar +, 1 3 kev, 30 min nd nneling until no trces of contminnts could e detected y XPS. The nneling tempertures for Al-Cu-Fe, Al-Pd-Mn, nd Al-Ni-Co were, respectively, up to 103, 93, nd 113 K, leding to terrce nd steps surfce termintion s checked y STM prior to deposition experiments. Bi ws evported from fully outgssed Knudsen cell. The pressure ws kept low 10 9 mr during deposition. Bi hs rhomohedrl structure, which cn e considered s slightly distorted simple-cuic structure. It is only poorly metllic with low crrier density, nd its surfce energy is rther low Bi =0.5 J/m. 1 The deposition flux for specific setting of the Bi source ws estimted from the intensity of the XPS core-level spectr. Specificlly, Bi film of thickness d ws deposited on Pd 111 surfce held t room temperture nd the intensity of the Pd 3d core level ws monitored s function of the tkeoff ngle with respect to the surfce norml of the emitted photoelectrons. If I 0 is the intensity of the Pd 3d signl t the sustrte film interfce, the mesured signl I is to first pproximtion I=I 0 e d/ cos, where is the inelstic men-free pth of the Pd 3d photoelectrons in the Bi film. The deposition flux estimted from XPS experiments ws crosschecked y nlysis of STM imges. Bi ws deposited t sumonolyer coverge on Pd 111 surfce, which results in D islnds. The coverge ws then estimted y clculting the re covered y the Bi islnds. The flux clculted in this wy ws close to the vlue found in XPS mesurements. The Bi thin films were chrcterized using n Omicron room-temperture STM nd y RHEED. III. RESULTS AND DISCUSSION A. Structure of sustrte surfce 1. Fivefold surfce of i-al-cu-fe nd i-al-pd-mn The structure of i-al-cu-fe nd i-al-pd-mn sustrtes hs lredy een discussed elsewhere Here, we only provide rief summry of the min results useful for the understnding of the dt presented in this pper. It is oserved tht the fivefold surfce of oth smples prepred y sputtering nd nneling cycles leds to terrce nd step morphology with tomiclly smooth terrces extending over severl hundreds of nnometers Figs. 1 nd 1 c. STM revels steps of different heights: H=mS+nL with m,n eing consecutive integers of the Fioncci series, L 0.1 nm, nd S=L/, where is the golden men. 15 The nlysis of the step-height distriution, within the specific surfce preprtion explined ove, reveled tht the most frequent step heights re L, S+L, S+L, nd S+3L. The occurrence of these steps of, for exmple, i-al-cu-fe is 1%, 3%, 9%, nd 15%. The step-height distriution cn e explined y ulk trunction t the positions where locks of tomic lyers re seprted y lrger gps ove regions of reltively dense nd high Al content

5 GROWTH OF Bi THIN FILMS ON QUASICRYSTAL SURFACES PHYSICAL REVIEW B 78, c L d e f L FIG.. STM imges of the fivefold i-al-cu-fe surfce fter dosing Bi of pproximtely.5 ML t 53 K : nm,: nm. The mount of Bi dsored on the surfce is in sumonolyer regime. Inset: Fourier trnsform of imge inset. FIG. 1. STM imges of the clen fivefold surfce of i-al-cu-fe nm nd nm, i-al-pd-mn c nm, nd the tenfold surfce of d-al-ni-co d nm, e nm, nd f nm.. Tenfold surfce of d-al-ni-co Compred to the surfces of icoshedrl qusicrystls, the tenfold surfce of decgonl qusicrystl is reltively simpler ecuse of existence of periodic order long the surfce norml. Terrces re seprted y steps of single height of 0. nm consistent with the interlyer spcing of the ulk structure. However, detils of terrce morphology re smple specific or dependent on surfce preprtion or the exct composition of the smple see Ref.. The sustrte used for this study is d-al 71.7 Ni 18.7 Co 9.6. The smple shows the S1-type modifiction nd thus differs from the so-frinvestigted Co-rich phses or type-i superstructure phses, for which the Co nd Ni contents re lmost identicl. STM imges shown in Figs. 1 d nd 1 e were tken from the tenfold d-al 71.7 Ni 18.7 Co 9.6 surfce prepred t 103 K. The surfce of this phse exhiits firly lrge terrces compred to other phses. The typicl size of terrces is in the order of 100 nm. However, terrces up to 1 m wide re lso oserved occsionlly Fig. 1 e. Frequently, we detected smll mount of mteril pining up the steps Fig. 1 f. The reson for this is not cler ut we speculte tht nneling time during the surfce preprtion my not e sufficient to llow the diffusion of the mteril into the terrces. Note tht the surfce ws prepred y flsh nneling to void the increse in pressure in the UHV chmer during surfce preprtion. The nneling time used in other studies ws generlly 0.5 h. High-resolution imges on terrces exhiit protrusions of tomic height like the surfce of other Al-Ni-Co phses not shown. B. High-temperture deposition The growth of Bi t elevted temperture ws investigted y using the Al-Cu-Fe sustrte. The sustrte ws held t 53 K during the deposition nd oservtions were performed t room temperture. Figure shows STM imges of the surfce exposed to pproximtely.5 ML of Bi. The mount of dsored Bi estimted from these imges is clerly much lower in the sumonolyer regime. This is evidenced in Fig. where the smll Bi islnds with right contrst cover only prtilly the sustrte. The fine structure cn still e resolved in uncovered re of the qusiperiodic surfce. As result, the fst-fourier trnsform FFT of the imge Fig. still shows chrcteristics similr to tht of the clen surfce with severl decgonl rings of spots. The dimeters of the rings scle s 1: :, nd the decgonl pttern re consistent with the fivefold symmetry of the qusiperiodic sustrte. The smll mount of dsored Bi indictes tht the sticking coefficient of Bi on the surfce is rther low nd/or re-evportion of Bi dtoms must e esy t 53 K, which is ove the multilyer desorption temperture. The Bi islnds re pproximtely 0. nm in height. Their lterl size is very limited few nnometers nd they re highly irregulr in shpe. Nevertheless, the ggregtion of Bi dtoms into islnds indictes tht terrce diffusion is ctivted within the conditions of the deposit. It is ovious from Fig. tht the Bi coverge is not homogeneous over the entire surfce ut depends on specific terrces. It is possile to estimte for specific terrce from selected re of the STM imges. The result of this nlysis shows tht 5% 30% on lmost every terrce except on those leled L in Fig. where 60%. These terrces re ordered y specific set of step heights. The lel L refers to terrces ordered y 0.66 nm L S step in the uphill direction nd y 0.1 L step on top of 0.66 nm LS or SL lock in the downhill direction. These L terrces ccount for limited surfce re ecuse L steps represent only 1% of ll steps nd the L terrces re generlly rther smll. Relted oservtions were reported recently y Unl et l. 1 for Ag thin films grown on the fivefold surfce of i-al-pd-mn t 365 K. At sumonolyer coverge, it ws found tht some specific terrces were decorted y much lower density of Ag islnds s compred to other terrces. It is striking tht terrces exhiiting this prticulr ehvior for Ag/Al-Pd-Mn re the sme specific L terrces presenting lrger coverge compred to other terrces for Bi/Al-Cu-Fe. Three possile explntions for the different islnd densities were discussed in Ref. 1. The first is terrce width, which cn ffect islnd densities due to competition etween islnd nucletion nd cpture t step edge, s step flow growth cn led to lower islnd densities on smller terrces. However, lthough L terrces re smll s noted ove, we cn lwys find in STM imges terrces of similr size ut with lower Bi coverge. Thus terrce width cnnot ccount for the terrce dependence of the coverge. The second possiility is the Ehrlich-Schwoeel rrier ssocited with dtom diffu

6 SHARMA et l. PHYSICAL REVIEW B 78, FIG. 3. STM imge of the fivefold i-al-cu-fe surfce covered y complete monolyer of Bi. The deposition temperture ws 53 K nm. sion downwrd step. In our cse, the lrger coverge oserved on L terrces would suggest tht prticle diffusing on this terrce will hve to overcome higher energy rrier for downhill diffusion thn on other terrces. However in this cse, one should oserve region of incresed coverge ordering the downwrd step, which is in contrdiction with the experiment. The third possile explntion relies on the intrinsic nture of the L terrces. Terrces of icoshedrl qusicrystls re expected to differ in their exct tomic structures, oth in chemistry nd tomic densities, which cn ffect the verge distriution of diffusion rrier. In ddition, lthough ll terrces re different, they shre some common locl configurtions tht pper t different densities. For exmple the density of the drk strs which hs een identified s nucletion sites in numer of studies vries from terrce to terrce in greement with ulk structure models. Bsed on rte eqution nlysis of the experimentl dt, it ws rgued tht the different islnd densities oserved for Ag/Al-Pd-Mn deposited t 365 K is most likely due to terrce dependence in the diffusion rrier. In our cse, the sitution is different ecuse the deposition occurs t much higher temperture 53 K nd desorption of dtoms must e tken into ccount. If one ssumes tht the density of trp sites is higher on L terrce, esier nucletion should result in less desorption nd higher on L terrces. Another phenomenon tht my shre some similrities with the present cse ws reported y Ci et l. 3 It concerns the morphology of 1-ML-thick Al film deposited t room temperture on the sme fivefold Al-Cu-Fe surfce. It ws found tht the Al thin film hs lumpy spect with no welldefined islnd shpe on the vst mjority of the terrces except on few terrces where lrger islnds with tomiclly flt tops re oserved. As ws noted in Ref. 3, these specil terrces my correspond to tomic plnes contining no nucletion sites identified on other terrces. In this experiment, the coverge ws uniform ecuse no evportion of Al dtoms occurs t room temperture. Shrm et l. lso reported the oservtion of terrce-dependent morphology of Sn film deposited on the i-al-cu-fe surfce. Figure 3 shows n STM imge of the surfce covered y complete Bi monolyer on the fivefold i-al-cu-fe. In this cse, the deposition time ws long enough to produce the expected Bi monolyer, nd the deposition temperture ws high enough to chieve lmost complete desorption of Bi multilyer. Therefore the film is reltively smooth nd the terrce nd step morphology of the sustrte re not modified y the film. However, the roughness on the terrces is still FIG.. RHEED ptterns from the clen fivefold surfce of i-al-cu-fe nd the sme surfce fter the deposition of complete monolyer. lrger for the clen sustrte minly due to the remining smll mount of dtoms on top of the monolyer ppering s white dots in Fig. 3. This incresed roughness is detrimentl to the STM resolution nd we could not otin informtion on the tomic structure of the Bi monolyer. Anneling of the Bi monolyer could e used in lter studies to improve its structurl qulity, s demonstrted in the recent studies of qusiperiodic P monolyers formed on the fivefold i-al-pd-mn surfce. 5 Figure shows RHEED pttern otined either efore or fter the formtion of the Bi wetting lyer. Both diffrction ptterns re chrcteristic of qusicrystlline surfce with streks seprted y long L nd short S spcing with L/S. This result does not prove unmiguously tht the Bi monolyer hs dopted the qusiperiodic sustrte structure ecuse the thickness proed y RHEED is typiclly severl nnometers, i.e., much lrger thn the film thickness. It is consistent with previous oservtion y HAS reported y Frnke et l. 6 demonstrting the qusiperiodic nture of the Bi monolyer formed on the isostructurl fivefold surfce of i-al-pd-mn. Finlly we mention the recent report y Smerdon et l. 6 demonstrting the qusiperiodic structure of similr Bi monolyer grown on the fivefold surfce of i-al-pd-mn y STM. C. Room-temperture deposition 1. i-al-cu-fe sustrte. Growth of qusiperiodic monolyer. By depositing Bi t lower sustrte temperture, one expects reduced proility for the re-evportion of dtoms s well s reduced dtom moility. The STM imge in Fig. 5 encompsses one single terrce of the surfce dosed with pproximtely 0.35 ML of Bi. The coverge estimted from such imges is out third of monolyer, which is very close to the deposited mount. Therefore the sticking coefficient of Bi on the qusicrystlline surfce is close to unity t room temperture, nd re-evportion of dtoms is inhiited s

7 GROWTH OF Bi THIN FILMS ON QUASICRYSTAL SURFACES PHYSICAL REVIEW B 78, () 5 o 7 o c d () c (c) FIG. 6. c STM imges of the fivefold surfce of i-al-cu-fe fter deposition of out.5 ML of Bi t room temperture: nm, nm, nd c nm. d RHEED pttern from the sme surfce. FIG. 5. STM topogrphy nm of the fivefold Al 63 Cu Fe 13 surfce dosed with 0.35 ML of Bi. Locl pentgonl motif formed y five Bi dtoms.5.5 nm. c FFT of the imge in. expected. The film morphology is significntly different from tht in Fig.. There is no islnd formtion nd dots of right contrst in Fig. 5 must correspond to single Bi dtom. As expected, terrce diffusion is not ctivted t room temperture in the sumonolyer regime. Height profiles tken cross Bi dtoms give vlue of out 0.15 nm, which is significntly smller thn the islnd height mesured for high-temperture growth 0. nm. This effect must result from different Bi-sustrte onding depending on whether the Bi dtoms is isolted or emedded in n islnd. At this sumonolyer coverge, the fine structure of the qusiperiodic sustrte cn still e resolved. As result, the FFT of the STM imges is tenfold symmetric pttern Fig. 5 c. FFT of STM imge, otined y pplying n pproprite height threshold to select the Bi dtoms, lso demonstrtes the tenfold symmetric rrngement of the dtoms. This s well s the oservtion of locl fivefold configurtion of Bi dtoms Fig. 5 is consistent with qusiperiodic growth of the first Bi monolyer.. Nnocrystllites nd rottionl epitxy. For coverge lrger thn one monolyer, once the first Bi wetting lyer hs een completed, we oserve the formtion of islnds. Figures 6 6 c shows n STM imge of the surfce exposed to.5 ML of Bi. Most of the islnds hve shrp edges nd smooth tops. The RHEED pttern of the Bi film is shown in Fig. 6 d. The pttern consists of diffrction spots not streks ligned long stright lines, indicting trnsmissionreflection diffrction through the 3D Bi islnds. The distriution of spots shows tht the film grows with pseudocuic 100 orienttion, which is equivlent to 01 in the rhomohedrl system. 7 The smll-spot size lso shows the good crystllinity of the islnds. Our system llows for in situ zimuthl rottion round the surfce norml nd it is oserved tht the sme diffrction pttern ppers every 7. This demonstrtes tht the epitxil reltionship within the surfce plne is defined y the lignment of crystllogrphic xis of the Bi islnds with one of five equivlent directions within the Bi wetting lyer. The fivefold twinning of the Bi crystllites cn lso e deduced from ngles etween step edges of djcent Bi islnds, which re multiples of /10, consistent with the fivefold rottionl symmetry of the first Bi lyer. c. Mgic heights. Most interesting is the fct tht the Bi crystllites hve specific height of 1.3 nm or multiple of this height. In few cses, islnds with pproximtely hlf of this height could lso e oserved ut never one-lyerthick islnds. These islnds re of irregulr shpe nd re only oserved immeditely fter the deposition, ut they disppered with time. In the rhomohedrl system, the interlyer spcing long the 01 direction is d 01 =0.38 nm nd therefore we conclude tht the vst mjority of islnds hs thickness corresponding to the stcking of four tomic lyers in the 01 orienttion or multiple of this height. The occurrence of mgic islnd heights revels specil stility for certin thickness, which we re going to discuss in Sec. III C.. i-al-pd-mn sustrte Figure 7 shows the i-al-pd-mn surfce fter Bi deposition t 1.0 nd.5 ML, which covers the mount of Bi occupied y islnds nd the underneth Bi wetting lyer. The growth mode of Bi on this sustrte is similr to tht oserved on the i-al-cu-fe sustrte. For oth coverge, the deposited Bi forms islnds corresponding to the stcking of four 01 lyers L islnds or multiple of this height. The ilyer islnds were lso oserved t erly stges of the growth ut gin they disppered fterwrd. Similrly, the islnds grow

8 SHARMA et l. PHYSICAL REVIEW B 78, o 0.37nm nm FIG. 7. STM imges of the fivefold surfce of i -Al-Pd-Mn fter deposition of 1.0 ML nm nd.5 ML nm of Bi t room temperture. The height of the islnds in ML units is indicted in. with the 01 rhomohedrl xis i.e., pseudocuic 100 xis eing prllel to the surfce norml. Within the surfce plne, islnds hve five different possile orienttions with respect to the sustrte rotted y ngles of n /10 from ech others. Furthermore, the stility of islnds ws investigted y nneling the surfce t different tempertures, which will e descried in Sec. III C d-al-ni-co sustrte. Coverge-dependent growth mode. The d-al-ni-co sustrte ws used to investigte the growth of Bi t wider coverge rnges. STM imges from the surfce for different coverge up to 10 ML re shown in Fig. 8. Three different growth modes re oserved depending on. For 1, smooth growth is oserved s shown in Fig. 8. The Fourier trnsform of the imge shows tenfold symmetry nd scling reltionships Fig. 8 inset suggesting gin qusiperiodic structure of the first wetting lyer. For 1 5, islnds re formed. The height of lmost ll islnds is 1.6 nm =0.03, corresponding to L islnds with 01 orienttion. In rre cses, 8L islnds were lso oserved. These results re consistent with those oserved on the i-al-cu-fe nd i-al-pd-mn surfces. For incresing coverge, the islnds predominntly grow lterlly. FIG. 9. A section of STM imge in Fig. 8 g in 3D view nm. Aove 5 ML, Bi grows with montomic step height. Indictions of n erly stge of this third growth mode re pprent lredy for =.3 ML s montomic steps lredy ppers mrked y rrows in Fig. 8 f. Figure 9 shows 3D view of n STM imge for 10 ML coverge showing pyrmidlike islnd with tringulr shpe on top of smooth film. The step height mesured on tringulr islnds is 0.37 nm =0.01, which is close the interlyer spcing d 001 = 0.39 nm in the rhomohedrl nottion. RHEED ptterns from the surfce confirm the 001 surfce orienttion of the film t this stge Fig. 10. The chnge from the 01 to the 001 orienttion ws lso oserved for Bi growth on the Si 111 sustrte. 7 The tringulr symmetry is expected from the ulk Bi. 7 These tringulr islnds hve different in-plne orienttions rotted y n /10 with ech other. This results from the fivefold twinning of the Bi domins with 01 orienttion on which the growth strted. As consequence, grin oundries cn e oserved on STM imges c d e 0.90 ML 1.0 ML 1.3 ML 1.80 ML f g h 3.00 ML.3 ML 10 ML FIG. 8. g STM imges of the tenfold surfce of d-al-ni-co fter deposition of Bi t different coverge t room temperture: nm ; inset: Fourier trnsform of the imge, nm, c nm, d nm, e nm, f nm, nd g nm. h Fourier trnsform of imge. FIG. 10. RHEED ptterns from the tenfold surfce of d-al-ni-co. Clen surfce nd fter the deposition of 10 ML Bi

9 GROWTH OF Bi THIN FILMS ON QUASICRYSTAL SURFACES PHYSICAL REVIEW B 78, c 0 min 8 min hrs hrs d FIG. 11. STM imges recorded t different time intervls given from certin plce of the tenfold d-al-ni-co surfce fter the deposition of 1.0 ML Bi nm. The height of the islnds is given. White rrows in d indicte step edges of Bi islnds. seprting djcent domins with different orienttions.. Evolution of islnds with mgic heights. In our experimentl systems, the deposition of Bi nd the STM mesurements were crried out in two different chmers. Therefore, it ws not possile to monitor the growth of the Bi film simultneously during deposition. In order to oserve the erly stge of the growth, we minimized the time etween deposition nd the first cquisition of STM imges down to 10 min compred to n hour or more for previous experiments on i-al-cu-fe nd i-al-pd-mn. Just fter deposition, high density of two tomic height islnds could e oserved. The precise height estimted from lrge numer of L islnds is found to e 0.68 nm. The expected height of the L islnds should thus e equl to 1.36 nm, which is significntly lrger thn their mesured height 1.6 nm using the sme tunneling prmeters. The evolution of the film morphology into L islnds could e followed y recording STM imges of the sme re over severl hours. We found tht two different processes occur. The L islnds either reshped themselves into L islnds or they colesce with neighoring L islnds s descried elow. Figures d show STM imges fter the deposition of 1.0 ML nd recorded t the sme plce with sme tunneling prmeters ut t different times given in the figure. We collected 50 imges in h. The imges recorded t selected time intervls re given in the figure. As previously oserved for Bi on i-al-cu-fe or i-al-pd-mn surfces, L islnds re irregulr in shpe, wheres L islnds re highly fceted. As cn e seen, the shpe of the L islnds remin unchnged in the given mesurement time, ut the L islnds mrked y circles in the figure grdully rerrnge themselves. There is mss trnsporttion within the islnds in order to reorder themselves into L islnds with highly oriented edges. Islnds re often emedded t step edges of the sustrte, suggesting esy diffusion of Bi dtoms on the wetting lyer nd preferentil nucletion t step edges. Sometimes, islnds extend over step edges indicted y rrows in Fig. 11 d nd the step height on the islnds reproduces the step height of the sustrte. The colescence of islnds is illustrted in Fig. 1 showing sequence of STM imges recorded on the sme re of single terrce fter the deposition of 1.3 ML on freshly prepred surfce. Agin, we cn identify two types of islnds: elongted L islnds nd irregulrly shped L islnds. It is ovious tht Bi toms diffuse from L islnds to L islnds resulting in lrger L islnds nd lower density of L islnds with time. Interestingly, Bi toms tend to ttch preferentilly t the end of the islnds rther thn t the sides, nd therefore the islnds grow nisotropiclly. As noted ove, the diffusion of Bi dtoms on the wetting lyer is esy, contrsting with the reduced moility on the clen surfce s explined in Sec. III C 1. This is consistent with erlier oservtion y He-tom scttering reveling the inertness of the wetting lyer. 6 We found tht the L islnds hve completely disppered from ll prt of the surfce severl hours fter the deposition. An exmple of such STM imges is shown in Fig. 8 c. The few islnds with lumpy spects oserved t the erly stte of the growth middle left right prt of Fig. 1 lso disppered fter this time. c. Growth direction. As explined in Sec. III C 3, the growth of L islnds is nisotropic. The growth direction cn e determined with respect to the sustrte high-symmetry c d 0 min 31 min 35 min 75 min FIG. 1. STM imges recorded t different time intervl given from single terrce of the tenfold d-al-ni-co surfce fter the deposition of 1.3 ML Bi nm

10 SHARMA et l. xes y nlyzing Fourier trnsform of STM imges. The Fourier trnsform of the STM imge given in Fig. 8, which ws otined t 1.0 ML coverge, is shown in Fig. 8 h. This shows two distinct fetures: rings of right spots nd slit, which is perpendiculr to the Bi rod. The spots result from the structure of the wetting lyer while the slit is relted to the Bi rod. The two rings of spots hve scling reltionship nd exhiit tenfold symmetry. This further confirms tht the wetting lyer underneth the islnds is qusiperiodic. The sustrte comprises two nonequivlent set of twofold xes. In the Fourier trnsform, the directions long the rightest spots from the center correspond to one set of the twofold xes, while nother set is rotted y 18 with respect to this set. Representtive of these xes is indicted y rrows in the Fourier trnsform imge. The slit is rotted y 9 from these high-symmetry xes of the sustrte. This revels tht growth direction of the Bi islnds is not long the twofold xes of the sustrte ut in etween them. The highsymmetry xes of the ulk Bi in the islnds could not e determined ecuse our ttempts to resolve tomic structure of the islnds y STM were unsuccessful. PHYSICAL REVIEW B 78, FIG. 13. STM imges of the fivefold i-al-pd-mn surfce fter the deposition of.5 ML Bi nd nneling t 63 K nm nd nm.. Interprettion of mgic heights The interprettion of the specil stility ssocited with specific film thicknesses or islnd heights relies on QSE. As the thickness of film is reduced to the nnometer scle, the confinement of the electron t the film/sustrte nd film/ vcuum interfces leds to the formtion of discrete electronic sttes quntum well sttes. Becuse the energy of the quntum well sttes depends on the film thickness t, the physicl properties of the film lso vry with t. This is true lso for the film stility ecuse the electronic contriution to the totl energy of the film depends on t. As result, films or islnds of specific thicknesses cn hve n enhnced stility over other thicknesses due to QSEs. The existence of quntum well sttes close to the Fermi level will enhnce the DOS t the Fermi level, incresing the electronic contriution to the totl energy of the film. Preferred films or islnds thicknesses with enhnced stility re those for which the energies of the quntum well sttes re well elow the Fermi level. The occurrence of specific islnd height selection hs een reported not only in severl metl thin films grown on semiconductor sustrtes such s Ag, P, nd Bi on Si 111 Refs. 7 9 or Ag on GAs 110 Ref. 30 ut lso in metl heterostructures such s P on Cu 111 Ref. 31 or Ag on Fe For recent review on QSE in metllic thin films with more exmples, see Refs. 33 nd 3. In our erlier report, we interpreted the occurrence of mgic islnd heights oserved on Ag/Al-Pd-Mn nd Bi/Al- Cu-Fe s mnifesttion of QSE. We hypothesized tht the origin of the confinement t the film/sustrte interfce could e due to the reduced density of sttes t the Fermi chrcteristic of these lloys. Lter, Mors et l. 11 directly oserved y photoemission spectroscopy the quntum well sttes in Ag thin films deposited on either the fivefold surfce of i-al-pd-mn or the tenfold surfce of d-al-ni-co, thus confirming the suggested electronic growth mechnism. However, it ws found tht these quntum well sttes did not disply ny dependence on the density of sttes of the qusicrystlline sustrtes, which re significntly different for oth sustrtes. It ws therefore concluded tht the confinement my e driven y the incomptile symmetries of the electronic sttes of the crystlline film nd the qusicrystlline sustrte. Our oservtions of height selection mechnism in the cse of Bi deposited on three different systems shows tht QSE ppers to e rther generl on qusiperiodic sustrtes independent of their detiled electronic structure. We mentioned erlier tht the height of the unstle L islnds on Al-Ni-Co sustrte mesured y STM is significntly lrger thn hlf of the vlue for the L islnds. This is consistent with lrger locl density of sttes in the vicinity of the Fermi level for L islnds compred to L islnds. Therefore the driven force for the islnds to reshpe into L islnds would e to lower N E F nd thus the electronic energy of the system. 5. Therml stility of the islnds with mgic heights The stility of the Bi islnds ws exmined y nneling the.5-ml-thick Bi film Fig. 7 t different tempertures up to 63 K. The XPS spectr tken from the surfce during nneling suggest tht Bi strts desoring ove room temperture. However, significnt mount of Bi ws still detected fter nneling the surfce t 63 K. The smple ws then cooled down to room temperture nd the surfce ws imged y STM. As seen in Fig. 13, the Bi islnds completely desored nd the first lyer remins. The desorption of the islnds ws expected ecuse the employed temperture ws ove the multilyer desorption temperture of 53 K found y He-tom scttering dt. 6 A prt of motivtion of our nneling experiments ws to otin monolyer of Bi y desoring top islnds nd to resolve tomic structure y STM. However, the film otined fter nneling ws found rougher thn the monolyer otined y direct deposition on the i-al-pd-mn surfce t elevted temperture Sec. III B or on the d-al-ni-co surfce t room temperture Sec. III C 3, mking it difficult to get tomiclly resolved STM imges. The film otined fter desoring the islnds cn e seen in mgnified view in Fig. 13. The RMS roughness is 0. nm. 6. Comprison with Bi growth on Si(111) Previous studies y STM nd initio hve shown tht Bi growth on Si 111 lso yields islnds of mgic heights. 7,

11 GROWTH OF Bi THIN FILMS ON QUASICRYSTAL SURFACES However, our results show the following fetures distinct from these reports. First, these studies found tht even numer lyer films re prominently stle, where L islnds re expected to e the most stle. The mechnism tht induces even numer lyer films ws scried to lrge tomic relxtion, which pired ech two neighoring lyers nd voids dngling onds. 7 However, we clerly oserve tht the L islnds re unstle in our cse. They either reshpe into L islnds or colesce to the neighoring L islnds Sec. IIIC3. Second, the model predicted tht the piring of the lyers occurs independently from the nture of the sustrte. However, we deposited Bi on the Pd 111 surfce nd found no evidence of mgic thicknesses or lyer piring effect. Therefore, we elieve tht the origin of the mgic height thickness on qusicrystl surfces is not the piring effect ut rther the quntum size effect s descried in Sec. III C. IV. SUMMARY AND CONCLUSION PHYSICAL REVIEW B 78, We investigted the growth of Bi on three different qusicrystl surfces t two different growth tempertures 300 nd 55 K nd t different coverge from sumonolyer to 10 ML. The sustrtes were the fivefold i-al-cu-fe nd i-al-pd-mn surfces nd the tenfold d-al-ni-co surfce. We comined vrious techniques including STM, RHEED, nd XPS to chrcterize the growth. A wide vriety of structures were oserved depending on experimentl conditions. This includes qusiperiodic Bi lyer, nnocrystllites of mgic heights, rottionl epitxil reltion of the film with the sustrte, nd terrce-dependent film morphologies. At 300 K, three different growth modes were oserved depending on. For 1, it grows smoothly forming qusiperiodic lyer. For 1 5, nnoislnds with 100 surfce orienttion re formed. The islnds hve mgic height of four tomic lyer predominntly. The islnds re ligned long certin directions of the sustrtes resulting in twinning structures with the symmetry of the sustrte. The islnds predominntly grow lterlly with incresing coverge nd form lmost complete lyer of four tomic height. For higher coverge, it grows with montomic height, not with mgic height, nd reflects tringulr symmetry expected from the ulk Bi. We were le to monitor the growth of islnds with respect to time y depositing Bi t converge slightly ove 1 ML nd mesuring STM imges of the sme re in severlminute intervls over severl hours. We found tht the fourlyer islnds re most stle. The two-lyer islnds oserved immeditely fter deposition either reshped themselves into four-tom-high islnds or colesced with neighoring fourlyer islnds. The selection of four tomic height is scried to quntum size effects, rising from the confinement of the electron within the film, which is mnifested y the electronic structure of the qusicrystlline sustrtes. At 55 K, only the first lyer cn e dsored, not the multilyers. This temperture is enough to ctivte terrce diffusion resulting in the ggregtion of Bi dtoms nd yielding smooth lyer of qusiperiodic order t monolyer coverge. At sumonolyer coverge, the mount of Bi oserved on different terrces is found to e different, which is explined in terms of re-evportion of Bi from selective terrces. ACKNOWLEDGMENT H.R.S. is grteful to EPSRC for funding Grnt No. EP/ D07188/1. *Corresponding uthor. Present ddress: Deprtment of Physics, The University of Liverpool, Liverpool L69 3BX, UK; h.r.shrm@liv.c.uk 1 D. Shechtmn, I. Blech, D. Grtis, nd J. W. Chn, Phys. Rev. Lett. 53, Z. M. Stdnik, Physicl Properties of Qusicrystls Springer, New York, G. Trmly de Lissrdiere, D. Nguyen-Mnh, nd D. Myou, Prog. Mter. Sci. 50, H. R. Shrm, M. Shimod, nd A. P. Tsi, Adv. Phys. 56, V. Fournee nd P. A. Thiel, J. Phys. D 38, R K. J. Frnke, H. R. Shrm, W. Theis, P. Gille, Ph. Eert, nd K. H. Rieder, Phys. Rev. Lett. 89, M. Krjci nd J. Hfner, Phys. Rev. B 71, M. Krjci nd J. Hfner, Phys. Rev. B 73, V. Fournée, H. R. Shrm, M. Shimod, A. P. Tsi, B. Unl, A. R. Ross, T. A. Logrsso, nd P. A. Thiel, Phys. Rev. Lett. 95, H. R. Shrm, M. Shimod, nd A. P. Tsi, Jpn. J. Appl. Phys., Prt 1 5, P. Mors, Y. Weisskopf, J.-N. Longchmp, M. Erudk, P. H. Zhou, L. Ferrri, nd C. Crone, Phys. Rev. B 7, 1105 R L. Vitos, A. V. Run, H. L. Skriver, nd J. Kollr, Surf. Sci. 11, M. Gierer, M. A. Vn Hove, A. I. Goldmn, Z. Shen, S.-L. Chng, P. J. Pinhero, C. J. Jenks, J. W. Anderegg, C.-M. Zhng, nd P. A. Thiel, Phys. Rev. B 57, Z. Ppdopolos, G. Ksner, J. Ledieu, E. J. Cox, N. V. Richrdson, Q. Chen, R. D. Diehl, T. A. Logrsso, A. R. Ross, nd R. McGrth, Phys. Rev. B 66, H. R. Shrm, V. Fournée, M. Shimod, A. R. Ross, T. A. Logrsso, A. P. Tsi, nd A. Ymmoto, Phys. Rev. Lett. 93, T. Ci, V. Fournée, T. Logrsso, A. Ross, nd P. A. Thiel, Phys. Rev. B 65, A. Ymmoto, H. Tkkur, nd A. P. Tsi, Phys. Rev. B 58, A. Ymmoto, H. Tkkur, nd A. P. Tsi, J. Alloys Compd. 3, A. Ymmoto, H. Tkkur, nd A. P. Tsi, Ferroelectrics 305, H. Tkkur, A. Ymmoto, nd A. P. Tsi, Act Crystllogr.,

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