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Surface etching and enhanced diffusion during the early stages of the growth of Co on Cu(111)
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Surface
ELSEVIER
Science
307-309
(1YY4) 538-543
Surface etching and enhanced diffusion during the early stages of the growth of Co on Cu( 111) J. de la Figuera
*, J.E. Prieto,
C. Ocal, R. Miranda
Departamento de Fhica de la Materia Condensada, Uniclersidad Autdnoma de Madrid, Cantohlanco, 28049 Madrid, Spain (Received
20 August
1993)
Abstract The deposition of Co on Cu(ll1) at room temperature gives rise to the appearance of monoatomically-high, laterally extended holes, as well as room temperature motion of surface features, which is observed in real time by a scanning tunneling microscope. The holes are produced by a spontaneous surface etching process related to the
formation of a surface alloy which, in turn, produces an enhancement of the surface diffusion. As a result of these phenomena there is a noticeable and unexpected mass transport at the surface during growth. WC illustrate how these processes have an impact on the growth mode of Co on Cu(l1 I).
1. Introduction The growth of epitaxial thin films of metals is an attractive topic. Much interest is concentrated nowadays on the growth of magnetic thin films and superlattices, since their magnetic properties can easily differ from their bulk counterparts. The discovery of new phenomena such as the oscillatory magnetic coupling across non-magnetic spacers [l] and the related giant magnetoresistance [2] in metallic superlattices and trilayers have further increased this interest. An in-situ characterization of the dynamical processes taking place during growth is badly needed if an understanding of the magnetic properties is to be achieved. We have shown recently that scanning tunneling microscopy (STM) studies can indeed reveal structural details of the growth process that are crucial to understand the reported discrepancies in the magnetic properties of Co/Cu superlattices [3]. 0039.6028/94/$07.00 0 1994 Elsevier SSDI 003Y-6028(93)E0877-W
Science
On the other hand, even single films of magnetic metals present properties, such as the switching in easy axes of magnetization with thickness [4], which are probably related to subtle structural transitions. Here we report on an STM study on the early stages of the growth of Co on Cu(ll1) that reveals dynamical processes such as surface etching, alloying and enhanced diffusion that have an impact on the growth mode of this system. We believe that many of the findings reported here could be more general than previously thought. Previous studies on the growth of Co on Cu(l 11) by room temperature deposition from the vapor under ultra-high vacuum (UHV) conditions yield contradicting conclusions on the structure of the growing film. In an early work, Gonzhlez et al. [5] concluded a layer-by-layer growth at RT from breaks in Auger uptake curves. The six-fold symmetry of the low energy electron diffraction (LEED) pattern was interpreted as
B.V. All rights reserved
J. de la Figuera et al. /Surface
indicating the formation of an hcp Co film for thicknesses up to 8 ML [5]. More recently, an incomplete wetting of the 0.1 substrate by the Co film was reported by Mankey et al. [6] in an angle resolved photoemission study. Kief and Egelhoff [7] interpreted X-ray photoelectron diffraction (XPD) data as reflecting occupation of fee sites for the Co atoms in the first monolayer. The second layer, however, showed coexistence of fee and hcp stackings. The coexistence was maintained for Co films 5 ML-thick. On the other hand, another XPD study [8] reported the initial
(4
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growth of a first flut monolayer of Co on fee sites. The fee stacking remained up to 2 ML, then mixed fee-hcp layers appeared up to 10 ML with a gradual transition to hcp for higher coverages. We have already demonstrated that the growth of Co/Cu(lll) does not occur in a layer-by-layer fashion [3]. Deposited Co rather forms triangular islands with bilayer height in contradiction to statements that there was no island formation in this system 183. In fact, the first layer does not cover uniformly the surface [6]. Furthermore, there are two types of islands, rotated 60” with respect to each other [3]. In an attempt to clarify some of the open questions, we have performed an STM study on the growth of Co on Cu(ll1). The goal is to directly visualize the structure of the growing film for a variety of coverages. The experiments presented here correspond to the early stages (up to 2 ML) of Co deposition at room temperature CRT). We prove that the bilayer growth occurs even for the lowest Co coverages deposited. Additionally, we report on the observation of spontaneous surface etching by the Co deposition and a substantial increase in the surface diffusion. These features should be considered if a detailed understanding of the interface crystallography in this system is to be achieved. Changes observed in the growth as a function of parameters such as deposition rate, substrate temperature, temperature of annealing, etc., will be reported elsewhere [91.
2. Experimental
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Distance
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600
.-L-J. ’
800
(Angstroms)
Fig. 1. (a) STM image of 0.1 ML of Co grown on Cu(ll1) at RT, taken at I = 1.2 nA and V = - 1.34 V. Image size is 2500 x 2500 A*,‘.(b) Profile along the line indicated in (a).
The experimental set up consists of an UHV chamber equipped with AES, LEED and STM facilities. Experimental details concerning the STM equipment can be found elsewhere [lo]. The substrate was a Cu(ll1) single crystal mechanically polished and cleaned by cycles of Arf bombardment at 500°C plus annealing to 850°C until no contamination was present in the AES spectrum. The LEED pattern of this surface presented the expected 1 x 1 three-fold symmetry corresponding to the (111) face of the fee substrate. Due to the absence of contaminants, STM
images of that surface presented straight steps running parallel to high symmetry directions and separating terraces of - 650 A. Co was deposited by electron bombardment onto the Cut1 11) maintained at RT. The coverage. given in monolayers (ML), is obtained directly from the STM images and as a consequence introduces some error due to the details of the growth process itself tsec below).
3. Results and discussion The fowest coverage analyzed corresponds to - 0.1 ML deposited at a rate of 0.5 ML/min. Fig. I shows the surface after such a deposition. Co islands decorate the substrate steps. Additional Co islands have nucleated also at terraces. The total density of islands is 2.24 X 10” cm- ‘. Even at these low coverages all the islandsOvisualized are two layers high, i.e. - 3.9 i 0.2 A high. No island of single-atomic height has ever been observed. This is in contrast to the initial growth of other metal/metal systems [I 1,121. The small-
Fig. 2. Topograph of 0.6 ML of Co evaporated at RT, recorded at a sample bias of V = -- 1.79V and a tunneling current of 1.0 nA. The image size is 2500 X 2500 A*. For the meaning of the marks, see the text.
est islands observed at this stage of the growth (I 40 A) have an irregular shape. On the other hand, islands larger than - 40 A display a clear triangular shape. In agreement with our previous report [3], the triangular islands present two different, 60”-rotated, ~)rientations on the same terrace. Statistics done on the images indicates that at this stage of the growth there arc twice as many islands in one direction than in the opposite one. The rotated triangular shape of Co bilayet islands is consistent with two different nucleation sites (fee and hcp with respect to the Cu suhstratc) for the first layer Co and subsequent fee stacking for the second layer [31. An ~~lternat~vc explanation could be that the Co adsorption site in the first layer is the same in both cases, e.g. fee, and the second Co layer has fee or hcp stacking with respect to the Cu substrate [7]. In both cases, the bilayer Co crystallites are, in fact, fee twins. Compact steps of a given orientation (1111) or {loo}) in twinned fee islands are specularly oriented with respect to their non-finned counterparts, bringing about the observed inverted triangles. The triangular shape can either be the equilibrium shape or be due to kinetic limitations. An example of the latter has been recently presented for homoepitaxial growth of Pt on Ptt 111) [ 131. A kinetically-limited triangular growth shape can bc rationalized in terms of higher surface diffusion along one of the two different { l]O> steps (which present (111) or { 100) microfacets) respectively [13]. Once the critical nuclei are formed, if diffusion along one type of step is faster, the growing islands may adopt the observed triangular shape [131. Already at this point of the growth a very striking fact is observed, namely, the appearance of laterally extended, 1 ML deep holes on the first surface layer after ~l.u~(~r~~t~~ilof Co. They were never observed on the starting. wcll-annealed Cu surface. Holes in the first layer have been also reported for Fe/Cut1 11) [14] and Fe/Cu(lOO) [lS]. In Cut1 1 1), the holes have a hexagonal shape that reflects the symmetry of the substrate. They are monoatomic in height (2 A) and correspond to islands of Cu vacancies X0 A in
J. de la Figuera et al. /Surface
diameter. Generally, these evaporation-produced holes are found close to the steps of the original surface. The hexagonal shape of the Cu vacancy islands is probably an equilibrium shape, already reached at RT. Since the free energy ratio for (11 l} and IlOO] oriented facets in Cu is N 0.95 [16] both compact step edges will have nearly equal length at equilibrium. The appearance of holes implies that Cu vacancies are generated during Co deposition by a sort of surface etching. The Cu vacancies diffuse until coalescence occurs giving rise to the large vacancy holes. If the holes are actually produced by the vacancies created during a surface intermixing of Co and Cu similar to that reported for Au/Ni(llO) 1181,their density and/or size should increase with increasing Co coverage. This is proven to be the case in Fig. 2. For a Co coverage of N 0.6 ML grown at a deposition rate of 2.4 ML/min (Fig. 2), the density of islands has increased only slightly to 3.2 x 10” cm-*. After this deposition, a larger number of (larger) holes are observed indeed. The cleanliness of the steps as well as their catenary shape shows that the edges of the holes are now pinned (not decorated!) by the Co triangular islands. This implies that the holes have grown during formation of the Co islands. The islands are larger than at 0.1 ML, they all preser$ a triangular shape and they are two layers (4 A) high. The triangular shape of the islands, in contrast to the hexagonal shape of the Cu holes, clearly suggests that the islands are Co crystallites. In addition to Co-island formation, if the condensation of Co on Cu(ll1) also results in a surface alloying reaction by which surface Cu atoms are replaced by Co and, since Cu adatoms are highly mobile on (111) surfaces, they would diffuse until trapping at the edges of Co islands takes place. Close inspection of islands near the steps or regions connecting closely-spaced islands reveals indeed an intermediate (2 A high) level (indicated as 1) and 2)) in Fig. 2). This suggests that the diffusing Cu atoms generated by the deposition process are trapped betwe:n Co islands that maintain their double (4 A) height. This is further supported by high resolution STM images of the decorated steps (not shown) that
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image a continuous but non-homogeneous morphology due to the coexistence of bilayer Co islands and a monoatomic Cu layer between them. The phenomenon of surface alloying during epitaxial growth of metals may be more general than expected. Brodde et al. [14,15] also inferred that Fe adsorption on Cu(ll1) and Cu(100) may give rise to ejection of Cu atoms. As mentioned above, Cu vacancies (in addition to Cu adatoms) are also generated during Co deposition by surface etching. They aggregate forming hexagonal vacancy islands. The lateral growth of these monoatomically deep holes continues until the edges are pinned by the immobile Co islands. In fact, the edges of the vacancy holes have been directly viewed while retiring with time and/or temperature [9]. When coalescence of several large vacancy islands occurs, a bilayer Co island may appear isolated in this sea of Cu vacancies. The consequence of the coalescence is that the Co island has now an apparent height of 3 atomic layers (see the 6 A high island in the middle of the hole in left central part of Fig. 2). Fig. 3 shows an effect of these processes in the growth of the Co film. For < 2 ML of Co, the Cu
Fig. 3. Topographic image of 1.9 ML of Co on Cu(ll1). The image size is 1500X 1500 A*. The tunneling current is 0.8 nA and the voltage is - 1.16 V.
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substrate is still visible. The bilayer Co crystallites have grown laterally and still exhibit a triangular shape. Since they are believed to be fee twins [3], most of these crystallites do not coalesce. Notice, however, that those bilayer islands connected by a Cu monolayer are susceptible to coalescence by introducing a dislocation in the third layer. At a coverage of 2 nominal MLs, a new level (one atomic layer thick) appears on top of the bilayer crystallites. It corresponds to the third Co layer being nucleated before complete covering of the Cu substrate occurs. This level preferentially appears on the islands that initially decorate the steps. This makes the step position still recognizable (see lower right part of the image in Fig. 3). On the other hand, the third layer nucleated on the triangular bilayer crystallites does not exhibit a triangular shape anymore. It rather grows with a tendency to form hexagons covering the (small) fraction of bilayer Co islands that have probably collapsed. It is tempting to speculate that this phenomenon is the precursor to the fee-hcp transition in Co(ll1) films and the thickness-dependent change in magnetization direction. In order to illustrate the enhanced atomic mobility on the surface due to the presence of Co, we describe now briefly an experiment on the creation and time evolution of vacancy islands on the surface. More details will appear in a forthcoming publication [193. The starting point for the experiment is the Co-covered Cu(ll1) surface of Fig. 2. In Fig. 4a, a zoom on the same area is presented. A large hole produced spontaneously during the evaporation of Co and placed close to the step is selected as location for the artificially created hole. The hole is fabricated by removing Cu atoms from the surface with the STM tip. This is done reproducibly by increasing in a con-
Fig. 4. Series of topographs of 1000 X 1000 AZ. (a) Zoom on the lower part of Fig. 2, centered on the “pool” (large monoatomic height hole). Mark “1)” indicates the same place as in Fig. 2. (b) A hole is created at the left of this “pool”. Cc) After a time of 1000 s the hole has moved more than 100 A.
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trolled way the tunneling current at a relatively low voltage [19]. In the case shown here, the hooe resulting from this process is monoatomic (2 A) in height and with an area of - 4200 f 100 A2, that is a vacancy island where approximately 750 + 20 atoms are missing (Fig. 4b). The island has a nearly perfect hexagonal shape and is indistinguishable from the holes appearing on the surface after Co deposition. Fig. 4c, an image taken 1000 s later, shows the change in position of the vacancy island. The time dependent position of the vacancy island was followed during hours every 100 s. In such a way a real time view of the vacancy island diffusion path was obtained. Analysis of the movement of vacancy islands gives clues to understand surface diffusion in this system [19]. In summary, we have presented evidence of the existence of surface etching and associated mass transport during epitaxial growth of metal on metals. This is the result of an unexpected process of intermixing of species that, while mutually insoluble in the bulk, produce a surface alloy. As a result of this process an enhanced mobility sets in. We show that the phenomena described here have implications in the structure of the grown film. We have illustrated these phenomena with the Co/Cu(lll) system but similar conclusions can probably be applied to Fe/Cu [14,15].
4. Acknowledgments We thank Dr. J.J. de Miguel and Professor J.M. Rojo for fruitful discussions. Financial support by the Fundacion Ramon Areces is gratefully acknowledged.
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5. References [l] See, A. Cebollada, R. Miranda, C.M. Schneider, P. Schuster and J. Kirschner, J. Magn. Magn. Mater. 102 (1991) 25, and references therein. [2] S.S.P. Parkin, R. Bahdra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [3] J. de la Figuera, J.E. Prieto, C. Ocal and R. Miranda, Phys. Rev. B 47 (1993) 13043. [4] G. Luggert and G. Bayreuther, Thin Solid Films 175 (19891 311. [51 L. Gonzalez, R. Miranda, M. Salmeron, J.A. Verges and F. Yndurain, Phys. Rev. B 24 (198113245. I61 G.J. Mankey, R.F. Willis and F.J. Himpsel, Phys. Rev. B 47 (1993) 190. [71 M.T. Kief and W.F. Egelhoff, Jr., Phys. Rev. B 47 (19931 10785. b31B.P. Tonner, Z.-L. Han and J. Zhang, Phys. Rev. B 47 (1993) 9723. [91 J. de la Figuera, J.E. Prieto, C. Ocal and R. Miranda, to be published. HOI D.M. Zeglinski, D.F. Ogletree, T.P. Beebe, Jr., R.Q. Hwang, G.A. Somorjai and M.B. Salmeron, Rev. Sci. Instrum. 61 (1990) 3769. 1111R.Q. Hwang, C. Gunther, J. Schroder, S. Gunther, E. Kopatzki and R.J. Behm, J. Vat. Sci. Technol. A 10 (19921 1970. WI J.J. de Miguel, A. Cebollada, J.M. Gallego, R. Miranda, C.M. Schneider, P. Schuster and J. Kirschner, J. Magn. Magn. Mater. 93 (1991) 1. [131 Th. Michely and G. Comsa, Phys. Rev. Lett. 70 (1993) 3943. [141 A. Brodde, K. Dreps, J. Binder, Ch. Lunau and H. Neddermeyer, Phys. Rev. B 47 (1993) 6609. Ultramicroscopy 42-44 [151 A. Brodde and H. Neddermeyer, (19921 556; Surf. Sci. 287/288 (1993) 988. M G.A. Somorjai, Principles of Surface Chemistry (Prentice-Hall, Englewood Cliffs, NJ, 19721. 1171Th. Michely and G. Comsa, J. Vat. Sci. Technol. B 9 (1991) 862. I. Stensgaard, E. [I81 L. Pleth Nielsen, F. Besenbacher, Laegsgaard, C. Engdahl, P. Soltze, K.W. Jacobsen and J.K. Ngrskov, Phys. Rev. Lett., submitted. [19] J. de la Figuera, J.E. Prieto, C. Ocal and R. Miranda, Solid State Commun., in press.
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Surface
ELSEVIER
Science
307-309
(1YY4) 538-543
Surface etching and enhanced diffusion during the early stages of the growth of Co on Cu( 111) J. de la Figuera
*, J.E. Prieto,
C. Ocal, R. Miranda
Departamento de Fhica de la Materia Condensada, Uniclersidad Autdnoma de Madrid, Cantohlanco, 28049 Madrid, Spain (Received
20 August
1993)
Abstract The deposition of Co on Cu(ll1) at room temperature gives rise to the appearance of monoatomically-high, laterally extended holes, as well as room temperature motion of surface features, which is observed in real time by a scanning tunneling microscope. The holes are produced by a spontaneous surface etching process related to the
formation of a surface alloy which, in turn, produces an enhancement of the surface diffusion. As a result of these phenomena there is a noticeable and unexpected mass transport at the surface during growth. WC illustrate how these processes have an impact on the growth mode of Co on Cu(l1 I).
1. Introduction The growth of epitaxial thin films of metals is an attractive topic. Much interest is concentrated nowadays on the growth of magnetic thin films and superlattices, since their magnetic properties can easily differ from their bulk counterparts. The discovery of new phenomena such as the oscillatory magnetic coupling across non-magnetic spacers [l] and the related giant magnetoresistance [2] in metallic superlattices and trilayers have further increased this interest. An in-situ characterization of the dynamical processes taking place during growth is badly needed if an understanding of the magnetic properties is to be achieved. We have shown recently that scanning tunneling microscopy (STM) studies can indeed reveal structural details of the growth process that are crucial to understand the reported discrepancies in the magnetic properties of Co/Cu superlattices [3]. 0039.6028/94/$07.00 0 1994 Elsevier SSDI 003Y-6028(93)E0877-W
Science
On the other hand, even single films of magnetic metals present properties, such as the switching in easy axes of magnetization with thickness [4], which are probably related to subtle structural transitions. Here we report on an STM study on the early stages of the growth of Co on Cu(ll1) that reveals dynamical processes such as surface etching, alloying and enhanced diffusion that have an impact on the growth mode of this system. We believe that many of the findings reported here could be more general than previously thought. Previous studies on the growth of Co on Cu(l 11) by room temperature deposition from the vapor under ultra-high vacuum (UHV) conditions yield contradicting conclusions on the structure of the growing film. In an early work, Gonzhlez et al. [5] concluded a layer-by-layer growth at RT from breaks in Auger uptake curves. The six-fold symmetry of the low energy electron diffraction (LEED) pattern was interpreted as
B.V. All rights reserved
J. de la Figuera et al. /Surface
indicating the formation of an hcp Co film for thicknesses up to 8 ML [5]. More recently, an incomplete wetting of the 0.1 substrate by the Co film was reported by Mankey et al. [6] in an angle resolved photoemission study. Kief and Egelhoff [7] interpreted X-ray photoelectron diffraction (XPD) data as reflecting occupation of fee sites for the Co atoms in the first monolayer. The second layer, however, showed coexistence of fee and hcp stackings. The coexistence was maintained for Co films 5 ML-thick. On the other hand, another XPD study [8] reported the initial
(4
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growth of a first flut monolayer of Co on fee sites. The fee stacking remained up to 2 ML, then mixed fee-hcp layers appeared up to 10 ML with a gradual transition to hcp for higher coverages. We have already demonstrated that the growth of Co/Cu(lll) does not occur in a layer-by-layer fashion [3]. Deposited Co rather forms triangular islands with bilayer height in contradiction to statements that there was no island formation in this system 183. In fact, the first layer does not cover uniformly the surface [6]. Furthermore, there are two types of islands, rotated 60” with respect to each other [3]. In an attempt to clarify some of the open questions, we have performed an STM study on the growth of Co on Cu(ll1). The goal is to directly visualize the structure of the growing film for a variety of coverages. The experiments presented here correspond to the early stages (up to 2 ML) of Co deposition at room temperature CRT). We prove that the bilayer growth occurs even for the lowest Co coverages deposited. Additionally, we report on the observation of spontaneous surface etching by the Co deposition and a substantial increase in the surface diffusion. These features should be considered if a detailed understanding of the interface crystallography in this system is to be achieved. Changes observed in the growth as a function of parameters such as deposition rate, substrate temperature, temperature of annealing, etc., will be reported elsewhere [91.
2. Experimental
(W
01 0
’
’
200
’ 400
Distance
’
600
.-L-J. ’
800
(Angstroms)
Fig. 1. (a) STM image of 0.1 ML of Co grown on Cu(ll1) at RT, taken at I = 1.2 nA and V = - 1.34 V. Image size is 2500 x 2500 A*,‘.(b) Profile along the line indicated in (a).
The experimental set up consists of an UHV chamber equipped with AES, LEED and STM facilities. Experimental details concerning the STM equipment can be found elsewhere [lo]. The substrate was a Cu(ll1) single crystal mechanically polished and cleaned by cycles of Arf bombardment at 500°C plus annealing to 850°C until no contamination was present in the AES spectrum. The LEED pattern of this surface presented the expected 1 x 1 three-fold symmetry corresponding to the (111) face of the fee substrate. Due to the absence of contaminants, STM
images of that surface presented straight steps running parallel to high symmetry directions and separating terraces of - 650 A. Co was deposited by electron bombardment onto the Cut1 11) maintained at RT. The coverage. given in monolayers (ML), is obtained directly from the STM images and as a consequence introduces some error due to the details of the growth process itself tsec below).
3. Results and discussion The fowest coverage analyzed corresponds to - 0.1 ML deposited at a rate of 0.5 ML/min. Fig. I shows the surface after such a deposition. Co islands decorate the substrate steps. Additional Co islands have nucleated also at terraces. The total density of islands is 2.24 X 10” cm- ‘. Even at these low coverages all the islandsOvisualized are two layers high, i.e. - 3.9 i 0.2 A high. No island of single-atomic height has ever been observed. This is in contrast to the initial growth of other metal/metal systems [I 1,121. The small-
Fig. 2. Topograph of 0.6 ML of Co evaporated at RT, recorded at a sample bias of V = -- 1.79V and a tunneling current of 1.0 nA. The image size is 2500 X 2500 A*. For the meaning of the marks, see the text.
est islands observed at this stage of the growth (I 40 A) have an irregular shape. On the other hand, islands larger than - 40 A display a clear triangular shape. In agreement with our previous report [3], the triangular islands present two different, 60”-rotated, ~)rientations on the same terrace. Statistics done on the images indicates that at this stage of the growth there arc twice as many islands in one direction than in the opposite one. The rotated triangular shape of Co bilayet islands is consistent with two different nucleation sites (fee and hcp with respect to the Cu suhstratc) for the first layer Co and subsequent fee stacking for the second layer [31. An ~~lternat~vc explanation could be that the Co adsorption site in the first layer is the same in both cases, e.g. fee, and the second Co layer has fee or hcp stacking with respect to the Cu substrate [7]. In both cases, the bilayer Co crystallites are, in fact, fee twins. Compact steps of a given orientation (1111) or {loo}) in twinned fee islands are specularly oriented with respect to their non-finned counterparts, bringing about the observed inverted triangles. The triangular shape can either be the equilibrium shape or be due to kinetic limitations. An example of the latter has been recently presented for homoepitaxial growth of Pt on Ptt 111) [ 131. A kinetically-limited triangular growth shape can bc rationalized in terms of higher surface diffusion along one of the two different { l]O> steps (which present (111) or { 100) microfacets) respectively [13]. Once the critical nuclei are formed, if diffusion along one type of step is faster, the growing islands may adopt the observed triangular shape [131. Already at this point of the growth a very striking fact is observed, namely, the appearance of laterally extended, 1 ML deep holes on the first surface layer after ~l.u~(~r~~t~~ilof Co. They were never observed on the starting. wcll-annealed Cu surface. Holes in the first layer have been also reported for Fe/Cut1 11) [14] and Fe/Cu(lOO) [lS]. In Cut1 1 1), the holes have a hexagonal shape that reflects the symmetry of the substrate. They are monoatomic in height (2 A) and correspond to islands of Cu vacancies X0 A in
J. de la Figuera et al. /Surface
diameter. Generally, these evaporation-produced holes are found close to the steps of the original surface. The hexagonal shape of the Cu vacancy islands is probably an equilibrium shape, already reached at RT. Since the free energy ratio for (11 l} and IlOO] oriented facets in Cu is N 0.95 [16] both compact step edges will have nearly equal length at equilibrium. The appearance of holes implies that Cu vacancies are generated during Co deposition by a sort of surface etching. The Cu vacancies diffuse until coalescence occurs giving rise to the large vacancy holes. If the holes are actually produced by the vacancies created during a surface intermixing of Co and Cu similar to that reported for Au/Ni(llO) 1181,their density and/or size should increase with increasing Co coverage. This is proven to be the case in Fig. 2. For a Co coverage of N 0.6 ML grown at a deposition rate of 2.4 ML/min (Fig. 2), the density of islands has increased only slightly to 3.2 x 10” cm-*. After this deposition, a larger number of (larger) holes are observed indeed. The cleanliness of the steps as well as their catenary shape shows that the edges of the holes are now pinned (not decorated!) by the Co triangular islands. This implies that the holes have grown during formation of the Co islands. The islands are larger than at 0.1 ML, they all preser$ a triangular shape and they are two layers (4 A) high. The triangular shape of the islands, in contrast to the hexagonal shape of the Cu holes, clearly suggests that the islands are Co crystallites. In addition to Co-island formation, if the condensation of Co on Cu(ll1) also results in a surface alloying reaction by which surface Cu atoms are replaced by Co and, since Cu adatoms are highly mobile on (111) surfaces, they would diffuse until trapping at the edges of Co islands takes place. Close inspection of islands near the steps or regions connecting closely-spaced islands reveals indeed an intermediate (2 A high) level (indicated as 1) and 2)) in Fig. 2). This suggests that the diffusing Cu atoms generated by the deposition process are trapped betwe:n Co islands that maintain their double (4 A) height. This is further supported by high resolution STM images of the decorated steps (not shown) that
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image a continuous but non-homogeneous morphology due to the coexistence of bilayer Co islands and a monoatomic Cu layer between them. The phenomenon of surface alloying during epitaxial growth of metals may be more general than expected. Brodde et al. [14,15] also inferred that Fe adsorption on Cu(ll1) and Cu(100) may give rise to ejection of Cu atoms. As mentioned above, Cu vacancies (in addition to Cu adatoms) are also generated during Co deposition by surface etching. They aggregate forming hexagonal vacancy islands. The lateral growth of these monoatomically deep holes continues until the edges are pinned by the immobile Co islands. In fact, the edges of the vacancy holes have been directly viewed while retiring with time and/or temperature [9]. When coalescence of several large vacancy islands occurs, a bilayer Co island may appear isolated in this sea of Cu vacancies. The consequence of the coalescence is that the Co island has now an apparent height of 3 atomic layers (see the 6 A high island in the middle of the hole in left central part of Fig. 2). Fig. 3 shows an effect of these processes in the growth of the Co film. For < 2 ML of Co, the Cu
Fig. 3. Topographic image of 1.9 ML of Co on Cu(ll1). The image size is 1500X 1500 A*. The tunneling current is 0.8 nA and the voltage is - 1.16 V.
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substrate is still visible. The bilayer Co crystallites have grown laterally and still exhibit a triangular shape. Since they are believed to be fee twins [3], most of these crystallites do not coalesce. Notice, however, that those bilayer islands connected by a Cu monolayer are susceptible to coalescence by introducing a dislocation in the third layer. At a coverage of 2 nominal MLs, a new level (one atomic layer thick) appears on top of the bilayer crystallites. It corresponds to the third Co layer being nucleated before complete covering of the Cu substrate occurs. This level preferentially appears on the islands that initially decorate the steps. This makes the step position still recognizable (see lower right part of the image in Fig. 3). On the other hand, the third layer nucleated on the triangular bilayer crystallites does not exhibit a triangular shape anymore. It rather grows with a tendency to form hexagons covering the (small) fraction of bilayer Co islands that have probably collapsed. It is tempting to speculate that this phenomenon is the precursor to the fee-hcp transition in Co(ll1) films and the thickness-dependent change in magnetization direction. In order to illustrate the enhanced atomic mobility on the surface due to the presence of Co, we describe now briefly an experiment on the creation and time evolution of vacancy islands on the surface. More details will appear in a forthcoming publication [193. The starting point for the experiment is the Co-covered Cu(ll1) surface of Fig. 2. In Fig. 4a, a zoom on the same area is presented. A large hole produced spontaneously during the evaporation of Co and placed close to the step is selected as location for the artificially created hole. The hole is fabricated by removing Cu atoms from the surface with the STM tip. This is done reproducibly by increasing in a con-
Fig. 4. Series of topographs of 1000 X 1000 AZ. (a) Zoom on the lower part of Fig. 2, centered on the “pool” (large monoatomic height hole). Mark “1)” indicates the same place as in Fig. 2. (b) A hole is created at the left of this “pool”. Cc) After a time of 1000 s the hole has moved more than 100 A.
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trolled way the tunneling current at a relatively low voltage [19]. In the case shown here, the hooe resulting from this process is monoatomic (2 A) in height and with an area of - 4200 f 100 A2, that is a vacancy island where approximately 750 + 20 atoms are missing (Fig. 4b). The island has a nearly perfect hexagonal shape and is indistinguishable from the holes appearing on the surface after Co deposition. Fig. 4c, an image taken 1000 s later, shows the change in position of the vacancy island. The time dependent position of the vacancy island was followed during hours every 100 s. In such a way a real time view of the vacancy island diffusion path was obtained. Analysis of the movement of vacancy islands gives clues to understand surface diffusion in this system [19]. In summary, we have presented evidence of the existence of surface etching and associated mass transport during epitaxial growth of metal on metals. This is the result of an unexpected process of intermixing of species that, while mutually insoluble in the bulk, produce a surface alloy. As a result of this process an enhanced mobility sets in. We show that the phenomena described here have implications in the structure of the grown film. We have illustrated these phenomena with the Co/Cu(lll) system but similar conclusions can probably be applied to Fe/Cu [14,15].
4. Acknowledgments We thank Dr. J.J. de Miguel and Professor J.M. Rojo for fruitful discussions. Financial support by the Fundacion Ramon Areces is gratefully acknowledged.
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5. References [l] See, A. Cebollada, R. Miranda, C.M. Schneider, P. Schuster and J. Kirschner, J. Magn. Magn. Mater. 102 (1991) 25, and references therein. [2] S.S.P. Parkin, R. Bahdra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [3] J. de la Figuera, J.E. Prieto, C. Ocal and R. Miranda, Phys. Rev. B 47 (1993) 13043. [4] G. Luggert and G. Bayreuther, Thin Solid Films 175 (19891 311. [51 L. Gonzalez, R. Miranda, M. Salmeron, J.A. Verges and F. Yndurain, Phys. Rev. B 24 (198113245. I61 G.J. Mankey, R.F. Willis and F.J. Himpsel, Phys. Rev. B 47 (1993) 190. [71 M.T. Kief and W.F. Egelhoff, Jr., Phys. Rev. B 47 (19931 10785. b31B.P. Tonner, Z.-L. Han and J. Zhang, Phys. Rev. B 47 (1993) 9723. [91 J. de la Figuera, J.E. Prieto, C. Ocal and R. Miranda, to be published. HOI D.M. Zeglinski, D.F. Ogletree, T.P. Beebe, Jr., R.Q. Hwang, G.A. Somorjai and M.B. Salmeron, Rev. Sci. Instrum. 61 (1990) 3769. 1111R.Q. Hwang, C. Gunther, J. Schroder, S. Gunther, E. Kopatzki and R.J. Behm, J. Vat. Sci. Technol. A 10 (19921 1970. WI J.J. de Miguel, A. Cebollada, J.M. Gallego, R. Miranda, C.M. Schneider, P. Schuster and J. Kirschner, J. Magn. Magn. Mater. 93 (1991) 1. [131 Th. Michely and G. Comsa, Phys. Rev. Lett. 70 (1993) 3943. [141 A. Brodde, K. Dreps, J. Binder, Ch. Lunau and H. Neddermeyer, Phys. Rev. B 47 (1993) 6609. Ultramicroscopy 42-44 [151 A. Brodde and H. Neddermeyer, (19921 556; Surf. Sci. 287/288 (1993) 988. M G.A. Somorjai, Principles of Surface Chemistry (Prentice-Hall, Englewood Cliffs, NJ, 19721. 1171Th. Michely and G. Comsa, J. Vat. Sci. Technol. B 9 (1991) 862. I. Stensgaard, E. [I81 L. Pleth Nielsen, F. Besenbacher, Laegsgaard, C. Engdahl, P. Soltze, K.W. Jacobsen and J.K. Ngrskov, Phys. Rev. Lett., submitted. [19] J. de la Figuera, J.E. Prieto, C. Ocal and R. Miranda, Solid State Commun., in press.