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An STM study of Cu on Si(001) in the c(8×8) structure
Surface Science 453 (2000) 137–142 www.elsevier.nl/locate/susc
An STM study of Cu on Si(001) in the c(8×8) structure B.Z. Liu a, M.V. Katkov b, J. Nogami a,b, * a Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824, USA b Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA Received 1 September 1999; accepted for publication 24 January 2000
Abstract We have studied the growth of Cu on the Si(001) surface over a range of growth temperatures and metal coverages. The only ordered phase seen by low energy electron diffraction (LEED), other than the 2×1 substrate pattern, was a c(8×8) phase which occurs at coverages as low as 0.05 monolayers. Scanning tunneling microscopy (STM ) measurements show that the c(8×8) structure consists of an array of bright features, two per unit cell. We propose one possible atomic structure on the basis of the STM images. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper; Scanning tunneling microscopy; Silicon; Surface structure, morphology, roughness, and topography
1. Introduction When copper is grown on silicon, it is known to be more reactive than the other noble metals [1–3]. There is some degree of intermixing at room temperature, and substantial interdiffusion at elevated temperatures, even to the point of Cu depletion from the surface region at above 800°C [4]. In an intermediate temperature regime, Cu forms islands whose morphology varies depending on the substrate orientation [5]. Between the islands, Cu can affect the structure of the substrate in the same way that many metals induce reconstruction of the silicon surface. Early work for Cu grown on Si(111) and Si(001) reported that the only ordered phase detectable by low energy electron diffraction (LEED) was a ‘5×5’ phase for Cu on Si(111). This incommensurate phase has been the subject of extensive study [6 ]. In contrast, comparatively little work has been done on the Cu/Si(001) system. * Corresponding author. Fax: +517-353-9842. E-mail address: [email protected] (J. Nogami)
A fairly recent study of the growth of Cu on Si(001), (111) and (110) by Ikeda et al. [5] has detected several ordered phases by LEED, in contrast to the earliest work. Our study follows up on their work by growing Cu on Si(001) in the same manner, and then examining the atomic structure of the surface with scanning tunneling microscopy (STM ). A very recent STM study of the Cu/Si(001) system has been published by the same group [7]. The two sets of STM results will also be compared.
2. Experimental All experiments were performed in an ultrahigh vacuum chamber with a base pressure <1×10−10 Torr. This chamber includes LEED optics, an STM (Omicron, Gmbh, Taunusstein, Germany) and facilities for metal deposition. Clean 2×1 Si(001) surfaces were prepared by flashing above 1150°C. After flashing, the surfaces were held at 950°C for 10 min, then cooled slowly till
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Fig. 1. Filled state images with (a) 0.05 ML at 450°C and (b) 0.12 ML at 550°C of Cu on Si(001). A c(8×8) unit cell is marked by a white box in (b).
room temperature (RT ). Copper was evaporated from a tungsten filament. Deposition rates were measured with a crystal monitor. Annealing temperatures were monitored with an infrared pyrometer. The Cu films were prepared by two methods. One was depositing copper at RT, then annealing the sample to higher temperatures (300–600°C ). The other was growing copper at various temperatures (400–600°C ), then quenching to RT. The thickness of Cu films ranged from 0.05 to 6.4 monolayers [1 monolayer (ML)=6.78×1014 atoms cm−2]. All STM and LEED observations were at RT.
cell is identified with a white box in Fig. 1b. This periodicity is c(8×8), and is consistent with the LEED pattern in Fig. 2. Fig. 3a and b are close up views of the c(8×8) unit cell in both filled and empty state STM images. It is shown that each unit cell has one bright feature at each corner, and one in the center. The lengths of both sides of the unit cell are 8a ˚ ), which can be seen directly in Fig. 3a (1a=3.83A
3. Results and discussion 3.1. General observations Fig. 1a and b show filled state images with 0.05 and 0.12 ML Cu, respectively, on a 2×1 Si(001) surface. Ordered rows of bright features can be seen running perpendicular to the Si dimer rows of the substrate. Most of the bright features consist of groups of three maxima. Since the density of these features increases with metal coverage, we associate them with the presence of Cu on the surface. At the higher of the two coverages, there is a distinct two dimensional periodicity. The unit
Fig. 2. LEED pattern for 0.38 ML for Cu on Si(001), grown at 600°C. The beam energy is 55 eV. One reciprocal space unit cell is marked.
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Fig. 3. Close up view of c(8×8) structure in (a) filled states and (b) empty states. In (a) the 2a spacing of the surrounding Si dimer rows is marked in both orthogonal directions.
by comparison with the 2a spacing of surrounding Si dimer rows. Fig. 4a and b are high resolution images of what we define as a single bright feature in both filled and empty states. There are significant differences between the two images. Each maximum in the filled states is split along the Si dimer row direction in the empty states. This splitting is very pronounced, giving the empty
states the appearance of two rows of three features for a total of six maxima. One similar feature for both images is that there are dark regions at both ends of the bright features. From the width of the dark features, it is suggested that each corresponds to two missing Si dimers. In experiments, a LEED pattern of c(8×8) structure can be observed for annealing or depos-
Fig. 4. Close up view of an isolated bright feature in (a) filled states, V =−2.5 V and (b) empty states, V =+1.7 V. T T
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ition temperatures between 450 and 600°C, and for coverages from 0.05 to 6.4 ML. The higher the temperature, the clearer the LEED pattern obtained. The best LEED patterns were observed after annealing or growing at 550°C. 3.2. Maxima position and atomic model Figs. 3 and 4 show Cu related features surrounded by areas of bare Si. Images such as these can be used to measure the registration of the Cu features with respect to the underlying Si. For the clean Si(001) surface, the filled state STM image (Fig. 3a) shows maxima that emphasize the bonding state between the Si atoms in each dimer. Thus the dark lines defining the Si rows in the image correspond to the trenches between the Si dimer rows. In the empty states, the dangling bonds at the ends of the Si dimers are bright, and the dark lines in the image correspond to the center of rows of Si dimers. Keeping the bias polarity dependent appearance of the Si in mind, it can be seen in Figs. 3 and 4 that the Cu maxima within each bright feature are centered over the Si dimer rows, rather than over the trenches. It is also important to know the relative position between maxima and Si dimers along the Si dimer row direction. Since the resolution of the corrugation along the Si dimer row direction is not completely clear in both biases, it is convenient to measure the registration along the row at a step edge, where the features on the adjacent terrace are rotated by 90°. Fig. 5a shows empty state bright features near a step edge. It can be observed that the dark line dividing the bright feature is aligned with the dark lines of the Si dimer rows on the upper terrace, which mark the centers of Si dimer rows. This implies that this dark line lies between the underlying Si dimers, in the direction along the dimer rows on the lower terrace. Fig. 5b shows a schematic of the positions of the empty state maxima with respect to the underlying and surrounding Si dimers. Two missing dimers on either side of the bright features are also shown in the figure. If we assume each empty state maximum in the bright feature corresponds to one Cu atom, or each filled state maximum two Cu atoms, then
(a)
(b)
Fig. 5. (a) Empty state image of bright features near a step edge. Perpendicular arrows mark the centers of Si dimer rows on both terraces. Positions of Si dimers are also shown. (b) Diagram of maxima superimposed on the positions of underlying Si dimers. The black dimers are on the upper terrace, and the gray dimers are on the lower terrace.
each bright feature has six Cu atoms, indicating each c(8×8) unit cell has twelve Cu atoms. Under this assumption, the Cu coverage can be calculated from the STM images. The measured coverages are in good agreement with the nominal coverages deposited, as shown in Fig. 6. It implies that the assumption is reasonable in this coverage range. According to the discussion above, one possible atomic model for the c(8×8) structure can be proposed, as shown in Fig. 7. Each unit cell includes two subunits which corresponds to the bright features in the images, each of which consists of three Cu dimers. The Cu dimers sit on the top of the underlying Si dimer rows, perpendicular to the Si dimer direction. The Si atoms under the
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positions, or that the maxima do not represent exact atomic positions at all. Any structure that is proposed must match the general appearance of the STM data, including symmetry and registration with respect to the surrounding silicon, as well as the constraint that each c(8×8) subunit has six Cu atoms. 3.3. Discussion
Fig. 6. Measured coverage, assuming six Cu atoms per bright feature versus deposited Cu coverage as determined from the crystal thickness monitor.
Fig. 7. Atomic model for the c(8×8) structure.
Cu dimers are undimerized. Two Si dimers at each end of the subunit are missing. This is the simplest possible model that is consistent with the STM data. However, there is nothing in the STM data that specifically excludes other surface structures. For example, it is possible that the maxima are Si atoms, with Cu in subsurface
From the c(8×8) atomic model, it can be decided that each unit cell, including 12 Cu atoms, occupy 64 Si 1×1 unit cells. If the Si(001) surface is covered completely by the perfect c(8×8) structure, then the saturation coverage will be 12/64= 0.1875 ML. That this should be the case can be seen from Fig. 1b, where most of the surface is covered by copper atoms when the coverage is 0.12 ML. But in our experiments, bare surface can still be observed when the coverage is larger than the saturation value. This is because many Cu atoms agglomerate into large three-dimensional islands, which can be observed by scanning electron microscopy (SEM ) at higher coverage, indicating that the growth mode of Cu on Si(001) surface is Stranski–Krastanov mode. From the STM images, the ordered c(8×8) structure can be observed with the order being better with increased annealing temperature, which matches the trend seen in the LEED results. When the processing temperature is <450°C, the bright features can still be observed, but the long range order of the c(8×8) periodicity is absent. The presence of these features even in the absence of long range order implies that they are a favorable local bonding arrangement for Cu and Si. At temperatures >600°C, the amount of Cu on the surface indicated by the fraction of the surface covered by the c(8×8) phase can be less than that expected from the amount of metal deposited. This depletion of Cu could be due to indiffusion away from the surface, or agglomeration of Cu to large three-dimensional islands away from the areas imaged by STM. This complicates the exact determination of the effective amount of Cu on the flat areas of the surface. However, the coverage results reported in Fig. 6 are for temperatures between 450 and 550°C, where these effects are seen to be very small.
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In Fig. 1a, there are rows of missing dimer defects similar to those caused by Ni contamination. These are due to the fact that this particular sample has had Cu on it in the past, and it is not possible to remove all of the Cu by flashing the sample. Sparse coverages of many other metals induce missing dimers on areas of bare Si as well, including Ag [8], and Ga [9]. We cannot absolutely exclude the possibility that the defects play some role in nucleating the c(8×8) features but we think that this is unlikely since many metals are observed to be repelled by missing dimer defects. In any event, any defects on the clean surface do not appear to influence the ordering of the Cu at slightly higher coverages as shown in Fig. 1b. Besides c(8×8) structures, there are still other possible surface structures for Cu/Si(001) system, as reported by Itoh et al. [7]. They suggested that the Cu/Si(001) structure is a function of both deposition temperature and deposited coverage. A phase diagram was proposed, in which 6×2, 2×2 and c(12×10) were seen over the temperature range 500–700°C, and the coverage range 0.1– 10 ML. They did not mention a c(8×8) structure, although they showed similar filled state bright features in Fig. 2 of their paper [7]. These features were interpreted as absorbed-Si dimers, which is totally different from our discussion. The temperature and coverage parameters space covered by our measurements was similar to theirs, and yet c(8×8) was the only ordered phase seen by LEED or STM. In particular, we made a concerted effort to raise the coverage of Cu on the surface; this resulted in the growth of three-dimensional islands that we could see by SEM. Our SEM pictures of three-dimensional islands are similar to those reported in Ref. [5], with regards to island size, shape and density. Thus we have no explanation for our inability to detect the other ordered phases reported by Itoh et al.
4. Conclusions We have studied the Cu on Si(001) system for growth and annealing temperatures between RT
and 600°C, and metal coverages between 0.05 and 6.4 ML. The only ordered phase seen by LEED, other than the 2×1 substrate pattern, was a c(8×8) phase. STM measurements on these surfaces show that the c(8×8) structure consists of an array of bright features that have a strongly bias dependent appearance, but appear as groups of six maxima in the empty states. Coverage dependent measurements are consistent with each feature having six Cu atoms. Under this assumption, the surface is entirely covered in the c(8×8) phase at 0.1875 ML. We propose one possible atomic structure on the basis of the STM images. At higher Cu coverages, we observe the growth of large three-dimensional islands consistent with prior observations by SEM. These three-dimensional islands coexist with the c(8×8). However, we did not see other two-dimensional ordered phases that were previously reported.
Acknowledgements Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. Support was also provided by the Center for Fundamental Materials Research at Michigan State University.
References [1] G. LeLay, Surf. Sci. 132 (1983) 169–204. [2] M. Hanbucken, G.L. Lay, Surf. Sci. 168 (1986) 122. [3] H. Kemmann, F. Muller, H. Neddermayer, Surf. Sci. 192 (1987) 11. [4] T. Ichinokawa, T. Inoue, H. Izumi, Y. Sakai, Surf. Sci. 241 (1991) 416–424. [5] T. Ikeda, T. Watanabe, H. Itoh, T. Ichinokawa, Surf. Rev. Lett. 3 (1996) 1377–1385. [6 ] J.E. Demuth, U.K. Koehler, R.J. Hamers, P. Kaplan, Phys. Rev. Lett. 62 (1989) 641. [7] H. Itoh, T. Ann, T. Kawasaki, T. Ichinokawa, Surf. Rev. Lett. 5 (1998) 747–753. [8] C.S. Chang, Y.M. Huang, C.C. Chen, T.T. Tsong, Surf. Sci. 367 (1996) L8–L12. [9] J. Nogami, S. Park, C.F. Quate, Appl. Phys. Lett. 53 (21) (1988) 2086.
An STM study of Cu on Si(001) in the c(8×8) structure B.Z. Liu a, M.V. Katkov b, J. Nogami a,b, * a Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824, USA b Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA Received 1 September 1999; accepted for publication 24 January 2000
Abstract We have studied the growth of Cu on the Si(001) surface over a range of growth temperatures and metal coverages. The only ordered phase seen by low energy electron diffraction (LEED), other than the 2×1 substrate pattern, was a c(8×8) phase which occurs at coverages as low as 0.05 monolayers. Scanning tunneling microscopy (STM ) measurements show that the c(8×8) structure consists of an array of bright features, two per unit cell. We propose one possible atomic structure on the basis of the STM images. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Copper; Scanning tunneling microscopy; Silicon; Surface structure, morphology, roughness, and topography
1. Introduction When copper is grown on silicon, it is known to be more reactive than the other noble metals [1–3]. There is some degree of intermixing at room temperature, and substantial interdiffusion at elevated temperatures, even to the point of Cu depletion from the surface region at above 800°C [4]. In an intermediate temperature regime, Cu forms islands whose morphology varies depending on the substrate orientation [5]. Between the islands, Cu can affect the structure of the substrate in the same way that many metals induce reconstruction of the silicon surface. Early work for Cu grown on Si(111) and Si(001) reported that the only ordered phase detectable by low energy electron diffraction (LEED) was a ‘5×5’ phase for Cu on Si(111). This incommensurate phase has been the subject of extensive study [6 ]. In contrast, comparatively little work has been done on the Cu/Si(001) system. * Corresponding author. Fax: +517-353-9842. E-mail address: [email protected] (J. Nogami)
A fairly recent study of the growth of Cu on Si(001), (111) and (110) by Ikeda et al. [5] has detected several ordered phases by LEED, in contrast to the earliest work. Our study follows up on their work by growing Cu on Si(001) in the same manner, and then examining the atomic structure of the surface with scanning tunneling microscopy (STM ). A very recent STM study of the Cu/Si(001) system has been published by the same group [7]. The two sets of STM results will also be compared.
2. Experimental All experiments were performed in an ultrahigh vacuum chamber with a base pressure <1×10−10 Torr. This chamber includes LEED optics, an STM (Omicron, Gmbh, Taunusstein, Germany) and facilities for metal deposition. Clean 2×1 Si(001) surfaces were prepared by flashing above 1150°C. After flashing, the surfaces were held at 950°C for 10 min, then cooled slowly till
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Fig. 1. Filled state images with (a) 0.05 ML at 450°C and (b) 0.12 ML at 550°C of Cu on Si(001). A c(8×8) unit cell is marked by a white box in (b).
room temperature (RT ). Copper was evaporated from a tungsten filament. Deposition rates were measured with a crystal monitor. Annealing temperatures were monitored with an infrared pyrometer. The Cu films were prepared by two methods. One was depositing copper at RT, then annealing the sample to higher temperatures (300–600°C ). The other was growing copper at various temperatures (400–600°C ), then quenching to RT. The thickness of Cu films ranged from 0.05 to 6.4 monolayers [1 monolayer (ML)=6.78×1014 atoms cm−2]. All STM and LEED observations were at RT.
cell is identified with a white box in Fig. 1b. This periodicity is c(8×8), and is consistent with the LEED pattern in Fig. 2. Fig. 3a and b are close up views of the c(8×8) unit cell in both filled and empty state STM images. It is shown that each unit cell has one bright feature at each corner, and one in the center. The lengths of both sides of the unit cell are 8a ˚ ), which can be seen directly in Fig. 3a (1a=3.83A
3. Results and discussion 3.1. General observations Fig. 1a and b show filled state images with 0.05 and 0.12 ML Cu, respectively, on a 2×1 Si(001) surface. Ordered rows of bright features can be seen running perpendicular to the Si dimer rows of the substrate. Most of the bright features consist of groups of three maxima. Since the density of these features increases with metal coverage, we associate them with the presence of Cu on the surface. At the higher of the two coverages, there is a distinct two dimensional periodicity. The unit
Fig. 2. LEED pattern for 0.38 ML for Cu on Si(001), grown at 600°C. The beam energy is 55 eV. One reciprocal space unit cell is marked.
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Fig. 3. Close up view of c(8×8) structure in (a) filled states and (b) empty states. In (a) the 2a spacing of the surrounding Si dimer rows is marked in both orthogonal directions.
by comparison with the 2a spacing of surrounding Si dimer rows. Fig. 4a and b are high resolution images of what we define as a single bright feature in both filled and empty states. There are significant differences between the two images. Each maximum in the filled states is split along the Si dimer row direction in the empty states. This splitting is very pronounced, giving the empty
states the appearance of two rows of three features for a total of six maxima. One similar feature for both images is that there are dark regions at both ends of the bright features. From the width of the dark features, it is suggested that each corresponds to two missing Si dimers. In experiments, a LEED pattern of c(8×8) structure can be observed for annealing or depos-
Fig. 4. Close up view of an isolated bright feature in (a) filled states, V =−2.5 V and (b) empty states, V =+1.7 V. T T
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ition temperatures between 450 and 600°C, and for coverages from 0.05 to 6.4 ML. The higher the temperature, the clearer the LEED pattern obtained. The best LEED patterns were observed after annealing or growing at 550°C. 3.2. Maxima position and atomic model Figs. 3 and 4 show Cu related features surrounded by areas of bare Si. Images such as these can be used to measure the registration of the Cu features with respect to the underlying Si. For the clean Si(001) surface, the filled state STM image (Fig. 3a) shows maxima that emphasize the bonding state between the Si atoms in each dimer. Thus the dark lines defining the Si rows in the image correspond to the trenches between the Si dimer rows. In the empty states, the dangling bonds at the ends of the Si dimers are bright, and the dark lines in the image correspond to the center of rows of Si dimers. Keeping the bias polarity dependent appearance of the Si in mind, it can be seen in Figs. 3 and 4 that the Cu maxima within each bright feature are centered over the Si dimer rows, rather than over the trenches. It is also important to know the relative position between maxima and Si dimers along the Si dimer row direction. Since the resolution of the corrugation along the Si dimer row direction is not completely clear in both biases, it is convenient to measure the registration along the row at a step edge, where the features on the adjacent terrace are rotated by 90°. Fig. 5a shows empty state bright features near a step edge. It can be observed that the dark line dividing the bright feature is aligned with the dark lines of the Si dimer rows on the upper terrace, which mark the centers of Si dimer rows. This implies that this dark line lies between the underlying Si dimers, in the direction along the dimer rows on the lower terrace. Fig. 5b shows a schematic of the positions of the empty state maxima with respect to the underlying and surrounding Si dimers. Two missing dimers on either side of the bright features are also shown in the figure. If we assume each empty state maximum in the bright feature corresponds to one Cu atom, or each filled state maximum two Cu atoms, then
(a)
(b)
Fig. 5. (a) Empty state image of bright features near a step edge. Perpendicular arrows mark the centers of Si dimer rows on both terraces. Positions of Si dimers are also shown. (b) Diagram of maxima superimposed on the positions of underlying Si dimers. The black dimers are on the upper terrace, and the gray dimers are on the lower terrace.
each bright feature has six Cu atoms, indicating each c(8×8) unit cell has twelve Cu atoms. Under this assumption, the Cu coverage can be calculated from the STM images. The measured coverages are in good agreement with the nominal coverages deposited, as shown in Fig. 6. It implies that the assumption is reasonable in this coverage range. According to the discussion above, one possible atomic model for the c(8×8) structure can be proposed, as shown in Fig. 7. Each unit cell includes two subunits which corresponds to the bright features in the images, each of which consists of three Cu dimers. The Cu dimers sit on the top of the underlying Si dimer rows, perpendicular to the Si dimer direction. The Si atoms under the
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positions, or that the maxima do not represent exact atomic positions at all. Any structure that is proposed must match the general appearance of the STM data, including symmetry and registration with respect to the surrounding silicon, as well as the constraint that each c(8×8) subunit has six Cu atoms. 3.3. Discussion
Fig. 6. Measured coverage, assuming six Cu atoms per bright feature versus deposited Cu coverage as determined from the crystal thickness monitor.
Fig. 7. Atomic model for the c(8×8) structure.
Cu dimers are undimerized. Two Si dimers at each end of the subunit are missing. This is the simplest possible model that is consistent with the STM data. However, there is nothing in the STM data that specifically excludes other surface structures. For example, it is possible that the maxima are Si atoms, with Cu in subsurface
From the c(8×8) atomic model, it can be decided that each unit cell, including 12 Cu atoms, occupy 64 Si 1×1 unit cells. If the Si(001) surface is covered completely by the perfect c(8×8) structure, then the saturation coverage will be 12/64= 0.1875 ML. That this should be the case can be seen from Fig. 1b, where most of the surface is covered by copper atoms when the coverage is 0.12 ML. But in our experiments, bare surface can still be observed when the coverage is larger than the saturation value. This is because many Cu atoms agglomerate into large three-dimensional islands, which can be observed by scanning electron microscopy (SEM ) at higher coverage, indicating that the growth mode of Cu on Si(001) surface is Stranski–Krastanov mode. From the STM images, the ordered c(8×8) structure can be observed with the order being better with increased annealing temperature, which matches the trend seen in the LEED results. When the processing temperature is <450°C, the bright features can still be observed, but the long range order of the c(8×8) periodicity is absent. The presence of these features even in the absence of long range order implies that they are a favorable local bonding arrangement for Cu and Si. At temperatures >600°C, the amount of Cu on the surface indicated by the fraction of the surface covered by the c(8×8) phase can be less than that expected from the amount of metal deposited. This depletion of Cu could be due to indiffusion away from the surface, or agglomeration of Cu to large three-dimensional islands away from the areas imaged by STM. This complicates the exact determination of the effective amount of Cu on the flat areas of the surface. However, the coverage results reported in Fig. 6 are for temperatures between 450 and 550°C, where these effects are seen to be very small.
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In Fig. 1a, there are rows of missing dimer defects similar to those caused by Ni contamination. These are due to the fact that this particular sample has had Cu on it in the past, and it is not possible to remove all of the Cu by flashing the sample. Sparse coverages of many other metals induce missing dimers on areas of bare Si as well, including Ag [8], and Ga [9]. We cannot absolutely exclude the possibility that the defects play some role in nucleating the c(8×8) features but we think that this is unlikely since many metals are observed to be repelled by missing dimer defects. In any event, any defects on the clean surface do not appear to influence the ordering of the Cu at slightly higher coverages as shown in Fig. 1b. Besides c(8×8) structures, there are still other possible surface structures for Cu/Si(001) system, as reported by Itoh et al. [7]. They suggested that the Cu/Si(001) structure is a function of both deposition temperature and deposited coverage. A phase diagram was proposed, in which 6×2, 2×2 and c(12×10) were seen over the temperature range 500–700°C, and the coverage range 0.1– 10 ML. They did not mention a c(8×8) structure, although they showed similar filled state bright features in Fig. 2 of their paper [7]. These features were interpreted as absorbed-Si dimers, which is totally different from our discussion. The temperature and coverage parameters space covered by our measurements was similar to theirs, and yet c(8×8) was the only ordered phase seen by LEED or STM. In particular, we made a concerted effort to raise the coverage of Cu on the surface; this resulted in the growth of three-dimensional islands that we could see by SEM. Our SEM pictures of three-dimensional islands are similar to those reported in Ref. [5], with regards to island size, shape and density. Thus we have no explanation for our inability to detect the other ordered phases reported by Itoh et al.
4. Conclusions We have studied the Cu on Si(001) system for growth and annealing temperatures between RT
and 600°C, and metal coverages between 0.05 and 6.4 ML. The only ordered phase seen by LEED, other than the 2×1 substrate pattern, was a c(8×8) phase. STM measurements on these surfaces show that the c(8×8) structure consists of an array of bright features that have a strongly bias dependent appearance, but appear as groups of six maxima in the empty states. Coverage dependent measurements are consistent with each feature having six Cu atoms. Under this assumption, the surface is entirely covered in the c(8×8) phase at 0.1875 ML. We propose one possible atomic structure on the basis of the STM images. At higher Cu coverages, we observe the growth of large three-dimensional islands consistent with prior observations by SEM. These three-dimensional islands coexist with the c(8×8). However, we did not see other two-dimensional ordered phases that were previously reported.
Acknowledgements Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. Support was also provided by the Center for Fundamental Materials Research at Michigan State University.
References [1] G. LeLay, Surf. Sci. 132 (1983) 169–204. [2] M. Hanbucken, G.L. Lay, Surf. Sci. 168 (1986) 122. [3] H. Kemmann, F. Muller, H. Neddermayer, Surf. Sci. 192 (1987) 11. [4] T. Ichinokawa, T. Inoue, H. Izumi, Y. Sakai, Surf. Sci. 241 (1991) 416–424. [5] T. Ikeda, T. Watanabe, H. Itoh, T. Ichinokawa, Surf. Rev. Lett. 3 (1996) 1377–1385. [6 ] J.E. Demuth, U.K. Koehler, R.J. Hamers, P. Kaplan, Phys. Rev. Lett. 62 (1989) 641. [7] H. Itoh, T. Ann, T. Kawasaki, T. Ichinokawa, Surf. Rev. Lett. 5 (1998) 747–753. [8] C.S. Chang, Y.M. Huang, C.C. Chen, T.T. Tsong, Surf. Sci. 367 (1996) L8–L12. [9] J. Nogami, S. Park, C.F. Quate, Appl. Phys. Lett. 53 (21) (1988) 2086.