Surface Science 600 (2006) 4058–4061 www.elsevier.com/locate/susc
Temperature-dependent shapeptransformation of Co clusters on p Ag/Ge (1 1 1) 3 · 3 surfaces Tsu-Yi Fu *, Chun-Liang Lin, Sung-Lin Tsay Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan, ROC Available online 9 June 2006
Abstract p p The shapes and structures of cobalt islands on Ag/Ge (1 1 1) 3 · 3 surfaces were studied using a scanning tunnelling microscope and low-energy electron diﬀraction techniques. Cobalt islands with periodic structures on their ﬂat top layers were found after annealing at 200–300 C. This new structure of the islands shows reﬂection symmetry and the mirror planes are along [2 1 1], [1 1 2], and [1 2 1] axes of the Ge (1 1 1) surface. Annealing treatments at temperatures above 200 C cause the cobalt atoms nucleate to form larger and higher islands. As a consequence, the island densities decrease rapidly as the temperature increases. In addition, an obvious structural and shape transformation of the cobalt islands occurs at 400–500 C. These changes include the hexagonal shapes of islands, compact arrangement of the surface atoms with hexagonal symmetry, and looser layer separations. These phenomena can be explained by various atomic processes at diﬀerent temperatures and the interactions between cobalt and the germanium substrate through the silver layer. 2006 Elsevier B.V. All rights reserved. Keywords: Scanning tunnelling microscopy; Nucleation; Surface thermodynamics; Growth; Cobalt; Germanium; Silver; Metal-semiconductor magnetic thin ﬁlm structures
1. Introduction Investigations of the properties of semiconductors with adsorbed transitional metal atoms are interesting for both technological and fundamental reasons [1–3]. Cobalt is one of the most important materials used in magnetic recording media. The system of Co on semiconductors is particularly interesting for the development of spin-based electronic devices. Germanium is a possible material that can improve the performance of Si-based devices because of its high mobility and epitaxy of metal/Ge growth. For room temperature deposition of Co on a Ge surface, the formation of Co–Ge interfacial compounds has been veriﬁed as revealed by the energy shift of Auger lines . Silver is chosen as a buﬀer layer since Ag and Co are immiscible in the bulk phase diagram up to 600 C . Magnetic properties of ultrathin *
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(T.-Y. Fu). 0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.01.166
Co/Ag/Ge (1 1 1) ﬁlms have been reported . The authors proposed that the interfacial roughness changes the homogeneity of the ﬁlm and inﬂuences the squareness of magnetization loops. However, no structural details are presented. In this article, we have investigated the shapes p and p structures of cobalt atom clusters on Ag/Ge (1 1 1) 3 · 3 surfaces after annealing at diﬀerent temperatures using an ultrahigh vacuum (UHV) scanning tunnelling microscope (STM). The temperature dependent structure results can also provide some useful information for the self-organized growth approach, which apparently has diﬃculty in controlling the structure size and chemical homogeneity. 2. Experiment The experiments were carried out in an UHV chamber where the base pressure was less than 5 · 1011 torr. The chamber was also equipped with a variable temperature STM and two well-collimated e-beam evaporators for depositing high purity Ag and Co atoms. A clean
T.-Y. Fu et al. / Surface Science 600 (2006) 4058–4061
3. Results and discussions
24 22 20
Average island volume (nm3)
Ge(1 1 1)-c(2 · 8) surface was prepared by repeated cycles of ion bombardment ( 0.7 keV, Ar+) and annealing up to 800 C for 10 h, then slowly cooling down at apratepof 2 C /min to room temperature. An Ag/Ge (1 1 1) 3 · 3 surface is formed when one monolayer (ML) of Ag is deposited on the clean Ge (1 1 1) surface followed by annealing to 500 C. The structure and cleanness were checked by low energy electron diﬀraction (LEED) and Auger electron spectroscopy (AES). We deposited a suitable amount of Co after the sample was cooled to room temperature. STM observations and measurements were carried out after the deposition and annealing to diﬀerent temperatures.
18 16 14 12 10 8 6 4 2 0
p Copatoms seem to sit randomly on the Ag/Ge (1 1 1) 3 · 3 surface at room temperature.pAs the p Co coverage increases to 0.35 ML, the LEED 3 · 3 pattern is dimmed. The STM images also show small clusters of various sizes and heights distributed randomly on the surface as shown in Fig. 1(a). After annealing the surface to 100 C, the behavior is similar to that of room temperature deposition. No diﬀusion and nucleation are found below 100 C. After 200 C annealing, the Co atoms condense and nucleate to form 2D islands as shown in Fig. 1(b). The islands with diameters bigger than 1.5 nm clearly show periodic structure on their ﬂat top layer. After 1 h of annealing at 300 C, the sizes of the islands with periodic structure become bigger as shown in Fig. 1(c). Fig. 1 (d) and (e) show the results of continuing to anneal the sample to higher temperatures, as the temperature increases, the density of islands decreases while the islands become bigger and higher. The average island area increases from 5 nm2 after 200 C annealing to 60 nm2 after 500 C annealing. The heights of the islands with one and two atomic layers
Temperature ( C)
Fig. 2. Temperature dependence of the average Co island volume on the p p Ag/Ge (1 1 1) 3 · 3 surfaces. The average volume increases promptly as the temperature increases between 200 and 500 C.
are 0.05 and 0.1 nm higher than the substrate. For islands with three and four atomic layers, the heights are 0.3 and 0.5 nm, i.e., the layer separation changes to 0.2 nm from 0.05 nm when the third layer grows. Fig. 2 shows that the average Co island volume increases quickly as the annealing temperature increases from less than 1 nm3 at 200 C to 22 nm3 at 500 C. Furthermore, obvious structure and shape transformations of the Co islands occur at 400–500 C. We give the name shape 1 to one type of island which appears at lower temperatures and p p with only one or two atomic layers and looser 13 · 13 R14 periodic structure. We give the name shape 2 to the other type of island with more than 3 atomic layers and 2 · 2 periodic structure which appears
Fig. 1. STM images showing the temperature dependent morphology of Co islands on Ag/Ge (1 1 1) 100 · 100 nm2, (f) 50 · 50 nm2, and sample bias, +2 V, for all images.
3 surfaces. Size of images: (a)–(e)
T.-Y. Fu et al. / Surface Science 600 (2006) 4058–4061
Fig. 3. Temperature dependence of two types of Co islands total area percentage shows the shape transformation starts at 300 C. Insert STM images shows the diﬀerent structures of two types of Co islands. Size of images: 15 · 15 nm2.
at higher temperatures. Fig. 3 shows STM images of the two types of Co islands and their total area percentage at various annealing temperatures. The shape transformation which starts at 300 C is observed clearly at 400 C, and almost ﬁnishes converting at 500 C. In Fig. 4, we observe shape 1 islands more clearly. Two features are noted. First, no evidence shows that the sub-
Fig. 4. STM image of the shape 1 Co islands. The structures of island B to island A show a reﬂection symmetry with mirror axes [2 1 1], [1 1 2], and [1 2 1]. Image size: 13 · 19 nm2.
p p strate, Ag/Ge (1 1 1) 3 · 3, has changed after the islands formed. The islands should be constituted with Co atoms only. Second, there are two diﬀerent directions for the shape 1 islands. The direction diﬀerence is 28. They can be seen as a reﬂection symmetry growth and the mirror planes are along [2 1 1], [1 1 2], and [1 2 1] axes of the Ge (1 1 1) surface. The middle Ag buﬀer layer retards compound formation of the Co overlayer and the underlying Ge surface. However, the electron density state distribution of the Ge (1 1 1) surface dominates the growth structure of the Co overlayer. The honeycomb-chained-trimer (HCT) model p p of Ag/Ge (1 1 1) 3 · 3 surface has the follow structural characteristics. The Ag atoms replace the top Ge layer. They are shifted laterally from bulk sites by 0.062 nm. The Ge atoms in the second layer are displaced by 0.074 nm to form trimers which are centered above fourth-layer atoms. The formation of Ge trimers satisﬁes two of the three dangling bounds, while the remaining dangling bond is used to bond with an Ag atom . So, the surface cannot compound with Co atoms easily. And the centers of the trimers, i.e., the original sites (rest-atom sites or ﬁrst layer Ge atom sites) of Ge (1 1 1) surface, become the preferred sites for Co atoms. We ﬁnd that the Co atoms occupy the trimer center sites regardless of shape 1 or shape 2 Co islands. From an atomic kinetics point of view, the diﬀusion activationpenergy p is rather high for Co atoms on the Ag/Ge (1 1 1) 3 · 3 surface. The Co atoms have to anneal above 100 C to overcome the diﬀusion barrier to nucleate and form bigger islands. There is very strong coupling between Co atoms and the Ge (1 1 1) surface even though the Ag buffer layer exists. We ﬁnd that the periodic property of the Co island structures is inﬂuenced by the property of the Ge (1 1 1) surface. This ispwhy p the activation energy of Co atoms on the Ag/Ge (1 1 1) 3 · 3 surface is far greater than that of metal atoms on metal surfaces and near semiconductor atoms on semiconductor surfaces [8–10]. A higher temperature annealing above 300 C provides enough energy to overcome the edge trapping energy and start ascending motion [11,12]. In other words, it makes the Co atoms move up to the upper terrace to form higher Co islands. While the Co islands grow higher, the surface free energy p pdecreases due to the larger area of the Ag/Ge (1 1 1) 3 · 3 surface. This upward movement triggers the island shape transformation. The Co–Ge coupling decreases as the Co islands grow upward. So, above the third layer of the Co islands, the surface structures of the shape 2 islands change to something like metal cobalt which has more compact arrangements and wider layer separations than shape 1 islands. In summary, when the temperature is well below 100 C, no diﬀusion can occur. The growth is random condensation of deposited Co atoms, or the morphology of the ﬁlm has a columnar fractal structure. As the temperature is raised above 100 C, Co atoms can diﬀuse to ﬁnd the most stable sites and sit to form ﬂat shape 1 islands. When the temperature exceeds 300 C, ascending motion can occur and play the dominant role. The creation of higher shape 2 islands decreases surface free energy. If the temperature is further
T.-Y. Fu et al. / Surface Science 600 (2006) 4058–4061
raised above 500 C, the Ag layer will exhibit thermal desorption and Co atoms will diﬀuse into Ge bulk and form an alloy. No cobalt islands will be found on the surface. Fig. 1(f) shows the result after 600 C annealing. The Ge(1 1 1) substrate without the Ag layer is observed. This result is consistant with previous study , which found Ag atoms on the Ge (1 1 1) surface desorb at around 550–600 C. 4. Conclusion p
The deposited cobalt clusters on Ag/Ge (1 1 1) 3 · 3 surfaces form Co islands with periodic structures after annealing above 200 C. Thep surfacepstructure of Co islands thinner than two layers is 13 · 13 R14 with mirror symmetry. The shape transformation of Co islands starts about 300 C when the trapping strength is driven oﬀ. The surface structure of Co islands thicker than three layers is 2 · 2. The ﬁrst two and the upper layers are 0.05 nm and 0.2 nm higher than the lower layers, respectively. In general, annealing treatments at temperatures above 200 C cause the cobalt atoms nucleate to form larger and higher islands. The island densities decrease rapidly as the temperature increases. The Co atoms have to overcome the diﬀusion barrier to nucleate and form larger islands. The upward movement of cobalt atoms causes an increase of the island height and triggers island shape transformation.
Acknowledgements The research is supported by National Science Council of ROC under Grant No. NSC 93-2112-M-003-002. Our gratitude also goes to the Academic Paper Editing Clinic, National Taiwan Normal University. References  R.K.K. Chong, M. Yeadon, W.K. Choi, E.A. Stach, C.S. Boothroyd, Appl. Phys. Lett. 82 (2003) 1833.  R.A. McKee, F.J. Walker, M.B. Nardelli, W.A. Shelton, G.M. Stocks, Science 300 (2003) 1726.  J.S. Tsay, Y.D. Yao, C.S. Yang, W.C. Cheng, T.K. Tseng, K.C. Wang, Surf. Sci. 513 (2002) 93.  J.S. Tsay, H.Y. Nieh, Y.D. Yao, Y.T. Chen, W.C. Cheng, Surf. Sci. 566–568 (2004) 226.  W.G. MoﬀattThe Handbook of Binary phase Diagram, Vol. 1, Genium Press, New York, 1990.  J.S. Tsay, H.Y. Nieh, Y.D. Yao, T.S. Chin, J. Magn. Magn. Mater. 282 (2004) 78.  H. Hung, H. Over, S.Y. Tong, Phys. Rev. B 49 (1994) 13483.  T.Y. Fu, Y.J. Hwang, T.T. Tsong, Appl. Surf. Sci. 219 (2003) 143.  C.E. Allen, R. Ditchﬁeld, E.G. Seebauer, Phys. Rev. B 55 (1997) 13304.  V. Cherepanov, B. Voiglander, Phys. Rev. B 69 (2004) 125331.  T.Y. Fu, L.C. Cheng, Y.J. Hwang, T.T. Tsong, Surf. Sci. 507–510 (2002) 103.  T.Y. Fu, H.T. Wu, T.T. Tsong, Phys. Rev. B 58 (1998) 2340.  D. Grozea, E. Bengu, L.D. Marks, Surf. Sci. 461 (2000) 23.