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Alloy formation of Ni ultrathin films on Pt(1 1 1) with Ag buffer layers
Applied Surface Science 237 (2004) 306–310 www.elsevier.com/locate/apsusc
Alloy formation of Ni ultrathin films on Pt(1 1 1) with Ag buffer layers H.Y. Ho, C.W. Su, Y.W. Chu, C.S. Shern* Department of Physics, National Taiwan Normal University, 88 Sec. 4, Ting-chou Road, Taipei 116, Taiwan Available online 30 July 2004
Abstract Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and depth profiling were used to study growth mode and structure in the interfaces of Ni/Ag/Pt(1 1 1). An atomic exchange occurs between Ni and Ag when the annealing temperature is high enough and the starting exchange temperature does not depend on the thickness of Ni. Nevertheless, the complete exchange temperature is higher when the coverage of Ni increases. Experimental evidence shows that the Ni–Pt alloy develops after the atomic exchange between Ag and Ni is complete. The atomic exchange between Ag and Ni, and the formation of Ni–Pt alloy were confirmed by the depth profile. The mechanisms of the atomic exchange are discussed. # 2004 Elsevier B.V. All rights reserved. PACS: 82.80.Pv; 81.70.Jb Keywords: Atomic exchange; Auger electron spectroscopy; Depth profile; Surface alloy; Nickel; Silver; Platinum surface
1. Introduction Interest in growth mechanisms, morphology and formation of alloy of metal-on-metal heteroepitaxial thin films or multilayers is motivated by their unique electronic and magnetic properties, and nanoengineering application. A system containing Ni, Ag and Pt is particularly intriguing. Ni–Pt thin films and alloy have segregation phenomenon and interesting magnetic properties [1]. Ag–Ni alloy has giant magnetoresis* Corresponding author. Tel.: +886 2 29346620x166; fax: +886 2 29326408. E-mail address: [email protected] (C.S. Shern).
tance [2] and can be used as a strain-sensitive material for piezoresistive sensors [3]. The lattice mismatch is 13.9% for Ni/Ag interface and the surface free energy of Ni is much larger than that of Ag. It is interesting to study the initial growth and the evolution of structural changes in the interface of Ni/Ag/Pt(1 1 1) after annealing at high temperature. In this report, the growth mode of Ag on Pt(1 1 1) and Ni on 1 ML Ag/Pt(1 1 1) were studied by lowenergy electron diffraction (LEED) and Auger electron spectroscopy (AES). The evolutions of chemical compositions in the interfaces during high temperature annealing were studied by AES. Ni–Ag atomic exchange and formation of Ni–Pt alloy occured
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.124
H.Y. Ho et al. / Applied Surface Science 237 (2004) 306–310
sequentially during annealing. Atomic exchange and alloy formation were confirmed by the depth profile using Ar ion bombarding.
2. Experimental Experiments were all performed in a stainless UHV chamber. The background pressure was maintained at about 5 1010 Torr and did not exceed 2 109 Torr during the deposition. All samples were fabricated by molecular beam epitaxy. The source of 99.999% pure Ag was evaporated from the alumina crucible wound around a heating 1.0 mm tungsten wire. The other source of 99.995% pure Ni locked on the feedthrough was wound in a coil. The Pt substrate surface was well prepared until a sharp (1 1) LEED pattern was observed at room temperature. Detailed sample cleaning process and equipments in the UHV chamber have been described elsewhere [4,5].
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[8]. Therefore, Dg = gNi + gi gAg > 0, where gi is interface energy. Both lattice mismatch and surface free energy favor the Volmer–Weber growth mode. AES was used to monitor the change of compositions at interfaces during the annealing process. The sample temperature was kept constant for 10 min for thermal equilibrium before measuring AES signals. We made sure that all AES signals did not change for over 1 h after annealing 10 min. Fig. 1 (a) shows the evolutions of Ag 351 eV, Ni 102 eV and Pt 237 eV Auger signals with sample temperature for 1 ML Ni/ 1 ML Ag/Pt(1 1 1). Since Auger intensities are related to compositions of the interface. Fig. 1 also shows changes in chemical compositions of the interface. All signals stay constant between 200 K and 400 K. When the temperature is between 400 K and 570 K, the Ag signal increases and the Ni signal decreases linearly,
3. Results and discussion A well-developed (1 1) LEED pattern was observed at any coverage of Ag below 1.5 ML on Pt(1 1 1) during deposition near room temperature. This indicates that Ag atoms accumulate on the first monolayer of the Pt(1 1 1) in a compact arrangement. This observation is consistent with our previous study [6]. Ni ultrathin films were then deposited on 1 ML Ag/Pt(1 1 1) at room temperature. From the curve of Auger peak-to-peak heights versus deposition time (Auger uptake curve) and no oscillation in the intensity of LEED specular beam, we conclude that the growth of Ni thin film is in a Volmer–Weber mode (3D island growth mode). Our observation is consistent with the study that Ni resides near the surface in the form of clusters of various sizes when Ni is directly deposited on a Ag(0 0 1) surface at room temperature [7]. The growth mode of our system can be understood through the lattice mismatch and surface free energies between Ni and Ag. The nearest neighboring distance between atoms of Ni and Ag are 0.249 nm and 0.289 nm, respectively. The lattice mismatch between Ni and Ag is large (about 13.8%). The surface energies of Ag and Ni are gAg = 1.25 J/m2 and gNi = 1.94 J/m2
Fig. 1. (a) The composition of Ag, Ni and Pt in the interface as a function of sample temperature for 1 ML Ni/1 ML Ag/Pt(1 1 1). (b) The composition of Ag, Ni and Pt in the interface as a function of sample temperature for 3 ML Ni/1 ML Ag/Pt(1 1 1).
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but the Pt signal does not change significantly. After the temperature is greater than 570 K, the Ni signal decreases and the Pt signal increases slightly, while the Ag signal stays constant. These results indicate that the atomic exchange between Ag and Ni starts at TE = 400 K and finishes at 570 K. The formation of Ni–Pt alloy starts to develop at TA = 570 K when the atomic exchange is complete. We carefully checked the variation of Ag Auger signal between temperatures 400 K and 570 K. As the sample reached thermal equilibrium after 10 min annealing, the Auger signal maintained a constant value for over 1 h at each temperature. Once the temperature arises, the Ag Auger signal increases immediately. Auger signals versus sample temperature of 3 ML Ni/1 ML Ag/Pt(1 1 1) are shown in Fig. 1 (b). These curves are similar to that of Fig. 1(a). The atomic exchange between Ag and Ni also starts at 400 K, but the formation of Ni–Pt alloy starts at 725 K. The formation of Ni–Pt alloy starts after the atomic exchange is complete. The Ag Auger signal stays constant during the alloy formation of Ni–Pt. Variations of TE and TA for different Ni coverages on 1 ML Ag/Pt(1 1 1) are shown in curve (a) and curve (b) of Fig. 2, respectively. TE is coverage independent and TA is almost linearly dependent on the coverage of Ni. It is interesting that the temperature for alloying of Ni–Pt is higher when the Ag buffer layer is introduced.
Fig. 2. Curve (a): the starting temperature of atomic exchange between Ag and Ni occurs at TE = 400 K for different coverages of Ni on 1 ML Ag/Pt(1 1 1). Note that TE is Ni coverage independent. Curve (b): the starting temperature TA of the formation of Ni– Pt alloy for different coverage of Ni on 1 ML Ag/Pt(1 1 1). TA is the linear relation with the coverage of Ni.
Our previous study showed that TA = 460 K, 500 K and 650 K for 1 ML, 2 ML and 3 ML Ni/Pt(1 1 1), respectively [5]. When Ag buffer layer exists, TA becomes 570 K, 630 K, 725 K and 780 K for 1 ML, 2 ML, 3 ML, and 4 ML Ni/1 ML Ag/Pt(1 1 1), respectively. Due to the interaction between Ag and Ni, the Ag buffer layer seems to prevent the formation of Ni–Pt alloy. Atomic exchange occurs when the temperature is higher than 400 K. The interaction between Ag atoms and Ni atoms needs more thermal energy to overcome the diffusion barrier between Ni and Pt. To investigate the change of the compositions of 1 ML Ni/1 ML Ag/Pt(1 1 1) after annealing, depth profile by Ar ion sputtering combined with AES was used. Thermal annealing was carried out up to 800 K for 30 min. Ar ions with 2.0-keV kinetic energy were used for the sputtering at room temperature. Auger results are shown in Fig. 3. Ag
Fig. 3. The depth profile for studying the composition at the interfaces of 1 ML Ni/1 ML Ag/Pt(1 1 1) after annealing 30 min at 800 K.
H.Y. Ho et al. / Applied Surface Science 237 (2004) 306–310
351 eV Auger signal decreases very rapidly when the sputtering time is less than 4 min. Ni 102 eV Auger signal increases initially and then decreases to the background value. Pt 237 eV Auger signal increases slowly to a saturated value. The sputtering curves of Ag and Ni indicate that Ag atoms do float on top of Ni layer. Sputtering curves of Ni and Pt confirm that formation of Ni–Pt alloy has developed after annealing. It is interesting to see why the atomic exchange between Ag and Ni can occur. Aufray et al. [9] have shown that a formation of 2 ML Ag burying the Ni atoms can be observed after annealing at 620 K when deposited of 1 ML Ni on Ag(1 0 0) at room temperature. Roussel et al. [10] proved this surfactantlike effect by both mean-field and Monte Carlo kinetic approaches. In our system, the surface free energy of Ag is only about 64.4% that of Ni. The system of Ni/ Ag/Pt(1 1 1) becomes a metastable interface structure. Ag atoms favor floating on the topmost layer to decrease the surface free energy of the system after high temperature annealing [11]. In addition, the lattice mismatch between Ni and Ag is about 13.8%, which is large. The large lattice mismatch results in large strain energy. The surface strain can influence surface diffusion in heteroepitaxy [12]. These two factors drive the atomic exchange between Ag and Ni atoms. Ag sputtering curve in Fig. 3 can also be used to calculate the sputtering rate of Ag. The AES intensity of a flat thin film after N times sputtering can be expressed by IN ¼ I0 ð1 xÞN
(1)
where I0 is the AES intensity before sputtering, and x is the removed percentage of atoms after each sputtering. After taking logarithm in both sides, Eq. (1) becomes lnðIN =I0 Þ ¼ N lnð1 xÞ
(2)
The sputtering rate of Ag is x ¼ 1 es
(3)
where s is the slope of ln(IN/I0) versus N curve. Each sputtering lasted 1 min in our sputtering experiment. The calculated result is shown in Fig. 4. The curve of ln(IN/I0) as a function of N is a perfect straight line. This indicates that the above simple model can
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Fig. 4. Logarithm of the relative residual Ag Auger signal ln(IN/I0) vs. the sputtering number N for the annealed thin film 1 ML Ni/1 ML Ag/Pt(1 1 1).
describe the removal of Ag atoms. It also shows that Ag overlayer is indeed on the topmost layer. Furthermore, Ag atoms and Ni atoms do not form Ag–Ni alloy after the exchange. After calculation, we obtain the sputtering rate of 34.4%. It is a very effective sputtering, and each sputtering can remove over one third of the residual Ag atoms. This is because of the surface of the Ag overlayer became very flat after it was annealed up to 800 K. Ag atoms can be easily removed by Ar ion sputtering due to the lack of shadow effect.
4. Conclusion Ni thin film exhibits the Volmer–Weber growth mode when deposited on 1 ML Ag/Pt(1 1 1). The atomic exchange between Ag and Ni occurs after high temperature annealing for Ni/1 ML Ag/Pt(1 1 1). The starting temperature of the atomic exchange is 400 K, independent of Ni coverages. Complete exchange temperatures are 570 K, 630 K, 725 K, and 780 K for 1 ML, 2 ML, 3 ML, and 4 ML of Ni coverages, respectively. The formation of Ni–Pt alloy develops after the exchange is complete. Large lattice mismatch between Ni and Ag, and especially small surface free energy of Ag are the possible mechanisms of the atomic exchange in this system.
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Acknowledgement This research was supported by the National Science Council of the Republic of China under Grant No. NSC 92-2112-M-003-019, and by the National Taiwan Normal University under Grant No. ORD93-1. Reference [1] R. Krishnan, H. Lassri, S. Prasad, M. Porte, M. Tessier, J. Appl. Phys. 73 (1993) 6433. [2] D.J. Kubinski, H. Holloway, J. Appl. Phys. 77 (1995) 782. [3] H. Chiriac, M. Urse, F. Rusu, C. Hison, M. Neagu, Sens. Actuators 76 (1999) 376.
[4] F.C. Chen, Y.E. Wu, C.W. Su, C.S. Shern, Phys. Rev. B 66 (2002) 184417. [5] C.W. Su, H.Y. Ho, C.S. Shern, R.H. Chen, Surf. Sci. 499 (2002) 103. [6] J.S. Tsay, Y.D. Yao, C.S. Shern, Phys. Rev. B 58 (1998) 3609. [7] K.S. Lee, S.H. Kim, H.G. Min, J. Seo, J.-S. Kim, Surf. Sci. 377–379 (1997) 918. [8] V.K. Kimikov, K.B. Khokonov, J. Appl. Phys. 54 (1983) 1346. [9] B. Aufray, H. Giordano, B. Legrand, G. Treglia, Surf. Sci. 307– 309 (1994) 531. [10] T.-M. Roussel, A. Saul, G. Treglia, Phys. Rev. B 55 (1997) 10931. [11] C.S. Shern, Y.F. Wu, Y.E. Wu, Surf. Sci. 439 (1999) 779. [12] R. Schuster, H. Roder, K. Bromann, H. Brune, K. Kern, Phys. Rev. B 54 (1996) 13476.
Alloy formation of Ni ultrathin films on Pt(1 1 1) with Ag buffer layers H.Y. Ho, C.W. Su, Y.W. Chu, C.S. Shern* Department of Physics, National Taiwan Normal University, 88 Sec. 4, Ting-chou Road, Taipei 116, Taiwan Available online 30 July 2004
Abstract Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and depth profiling were used to study growth mode and structure in the interfaces of Ni/Ag/Pt(1 1 1). An atomic exchange occurs between Ni and Ag when the annealing temperature is high enough and the starting exchange temperature does not depend on the thickness of Ni. Nevertheless, the complete exchange temperature is higher when the coverage of Ni increases. Experimental evidence shows that the Ni–Pt alloy develops after the atomic exchange between Ag and Ni is complete. The atomic exchange between Ag and Ni, and the formation of Ni–Pt alloy were confirmed by the depth profile. The mechanisms of the atomic exchange are discussed. # 2004 Elsevier B.V. All rights reserved. PACS: 82.80.Pv; 81.70.Jb Keywords: Atomic exchange; Auger electron spectroscopy; Depth profile; Surface alloy; Nickel; Silver; Platinum surface
1. Introduction Interest in growth mechanisms, morphology and formation of alloy of metal-on-metal heteroepitaxial thin films or multilayers is motivated by their unique electronic and magnetic properties, and nanoengineering application. A system containing Ni, Ag and Pt is particularly intriguing. Ni–Pt thin films and alloy have segregation phenomenon and interesting magnetic properties [1]. Ag–Ni alloy has giant magnetoresis* Corresponding author. Tel.: +886 2 29346620x166; fax: +886 2 29326408. E-mail address: [email protected] (C.S. Shern).
tance [2] and can be used as a strain-sensitive material for piezoresistive sensors [3]. The lattice mismatch is 13.9% for Ni/Ag interface and the surface free energy of Ni is much larger than that of Ag. It is interesting to study the initial growth and the evolution of structural changes in the interface of Ni/Ag/Pt(1 1 1) after annealing at high temperature. In this report, the growth mode of Ag on Pt(1 1 1) and Ni on 1 ML Ag/Pt(1 1 1) were studied by lowenergy electron diffraction (LEED) and Auger electron spectroscopy (AES). The evolutions of chemical compositions in the interfaces during high temperature annealing were studied by AES. Ni–Ag atomic exchange and formation of Ni–Pt alloy occured
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.124
H.Y. Ho et al. / Applied Surface Science 237 (2004) 306–310
sequentially during annealing. Atomic exchange and alloy formation were confirmed by the depth profile using Ar ion bombarding.
2. Experimental Experiments were all performed in a stainless UHV chamber. The background pressure was maintained at about 5 1010 Torr and did not exceed 2 109 Torr during the deposition. All samples were fabricated by molecular beam epitaxy. The source of 99.999% pure Ag was evaporated from the alumina crucible wound around a heating 1.0 mm tungsten wire. The other source of 99.995% pure Ni locked on the feedthrough was wound in a coil. The Pt substrate surface was well prepared until a sharp (1 1) LEED pattern was observed at room temperature. Detailed sample cleaning process and equipments in the UHV chamber have been described elsewhere [4,5].
307
[8]. Therefore, Dg = gNi + gi gAg > 0, where gi is interface energy. Both lattice mismatch and surface free energy favor the Volmer–Weber growth mode. AES was used to monitor the change of compositions at interfaces during the annealing process. The sample temperature was kept constant for 10 min for thermal equilibrium before measuring AES signals. We made sure that all AES signals did not change for over 1 h after annealing 10 min. Fig. 1 (a) shows the evolutions of Ag 351 eV, Ni 102 eV and Pt 237 eV Auger signals with sample temperature for 1 ML Ni/ 1 ML Ag/Pt(1 1 1). Since Auger intensities are related to compositions of the interface. Fig. 1 also shows changes in chemical compositions of the interface. All signals stay constant between 200 K and 400 K. When the temperature is between 400 K and 570 K, the Ag signal increases and the Ni signal decreases linearly,
3. Results and discussion A well-developed (1 1) LEED pattern was observed at any coverage of Ag below 1.5 ML on Pt(1 1 1) during deposition near room temperature. This indicates that Ag atoms accumulate on the first monolayer of the Pt(1 1 1) in a compact arrangement. This observation is consistent with our previous study [6]. Ni ultrathin films were then deposited on 1 ML Ag/Pt(1 1 1) at room temperature. From the curve of Auger peak-to-peak heights versus deposition time (Auger uptake curve) and no oscillation in the intensity of LEED specular beam, we conclude that the growth of Ni thin film is in a Volmer–Weber mode (3D island growth mode). Our observation is consistent with the study that Ni resides near the surface in the form of clusters of various sizes when Ni is directly deposited on a Ag(0 0 1) surface at room temperature [7]. The growth mode of our system can be understood through the lattice mismatch and surface free energies between Ni and Ag. The nearest neighboring distance between atoms of Ni and Ag are 0.249 nm and 0.289 nm, respectively. The lattice mismatch between Ni and Ag is large (about 13.8%). The surface energies of Ag and Ni are gAg = 1.25 J/m2 and gNi = 1.94 J/m2
Fig. 1. (a) The composition of Ag, Ni and Pt in the interface as a function of sample temperature for 1 ML Ni/1 ML Ag/Pt(1 1 1). (b) The composition of Ag, Ni and Pt in the interface as a function of sample temperature for 3 ML Ni/1 ML Ag/Pt(1 1 1).
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H.Y. Ho et al. / Applied Surface Science 237 (2004) 306–310
but the Pt signal does not change significantly. After the temperature is greater than 570 K, the Ni signal decreases and the Pt signal increases slightly, while the Ag signal stays constant. These results indicate that the atomic exchange between Ag and Ni starts at TE = 400 K and finishes at 570 K. The formation of Ni–Pt alloy starts to develop at TA = 570 K when the atomic exchange is complete. We carefully checked the variation of Ag Auger signal between temperatures 400 K and 570 K. As the sample reached thermal equilibrium after 10 min annealing, the Auger signal maintained a constant value for over 1 h at each temperature. Once the temperature arises, the Ag Auger signal increases immediately. Auger signals versus sample temperature of 3 ML Ni/1 ML Ag/Pt(1 1 1) are shown in Fig. 1 (b). These curves are similar to that of Fig. 1(a). The atomic exchange between Ag and Ni also starts at 400 K, but the formation of Ni–Pt alloy starts at 725 K. The formation of Ni–Pt alloy starts after the atomic exchange is complete. The Ag Auger signal stays constant during the alloy formation of Ni–Pt. Variations of TE and TA for different Ni coverages on 1 ML Ag/Pt(1 1 1) are shown in curve (a) and curve (b) of Fig. 2, respectively. TE is coverage independent and TA is almost linearly dependent on the coverage of Ni. It is interesting that the temperature for alloying of Ni–Pt is higher when the Ag buffer layer is introduced.
Fig. 2. Curve (a): the starting temperature of atomic exchange between Ag and Ni occurs at TE = 400 K for different coverages of Ni on 1 ML Ag/Pt(1 1 1). Note that TE is Ni coverage independent. Curve (b): the starting temperature TA of the formation of Ni– Pt alloy for different coverage of Ni on 1 ML Ag/Pt(1 1 1). TA is the linear relation with the coverage of Ni.
Our previous study showed that TA = 460 K, 500 K and 650 K for 1 ML, 2 ML and 3 ML Ni/Pt(1 1 1), respectively [5]. When Ag buffer layer exists, TA becomes 570 K, 630 K, 725 K and 780 K for 1 ML, 2 ML, 3 ML, and 4 ML Ni/1 ML Ag/Pt(1 1 1), respectively. Due to the interaction between Ag and Ni, the Ag buffer layer seems to prevent the formation of Ni–Pt alloy. Atomic exchange occurs when the temperature is higher than 400 K. The interaction between Ag atoms and Ni atoms needs more thermal energy to overcome the diffusion barrier between Ni and Pt. To investigate the change of the compositions of 1 ML Ni/1 ML Ag/Pt(1 1 1) after annealing, depth profile by Ar ion sputtering combined with AES was used. Thermal annealing was carried out up to 800 K for 30 min. Ar ions with 2.0-keV kinetic energy were used for the sputtering at room temperature. Auger results are shown in Fig. 3. Ag
Fig. 3. The depth profile for studying the composition at the interfaces of 1 ML Ni/1 ML Ag/Pt(1 1 1) after annealing 30 min at 800 K.
H.Y. Ho et al. / Applied Surface Science 237 (2004) 306–310
351 eV Auger signal decreases very rapidly when the sputtering time is less than 4 min. Ni 102 eV Auger signal increases initially and then decreases to the background value. Pt 237 eV Auger signal increases slowly to a saturated value. The sputtering curves of Ag and Ni indicate that Ag atoms do float on top of Ni layer. Sputtering curves of Ni and Pt confirm that formation of Ni–Pt alloy has developed after annealing. It is interesting to see why the atomic exchange between Ag and Ni can occur. Aufray et al. [9] have shown that a formation of 2 ML Ag burying the Ni atoms can be observed after annealing at 620 K when deposited of 1 ML Ni on Ag(1 0 0) at room temperature. Roussel et al. [10] proved this surfactantlike effect by both mean-field and Monte Carlo kinetic approaches. In our system, the surface free energy of Ag is only about 64.4% that of Ni. The system of Ni/ Ag/Pt(1 1 1) becomes a metastable interface structure. Ag atoms favor floating on the topmost layer to decrease the surface free energy of the system after high temperature annealing [11]. In addition, the lattice mismatch between Ni and Ag is about 13.8%, which is large. The large lattice mismatch results in large strain energy. The surface strain can influence surface diffusion in heteroepitaxy [12]. These two factors drive the atomic exchange between Ag and Ni atoms. Ag sputtering curve in Fig. 3 can also be used to calculate the sputtering rate of Ag. The AES intensity of a flat thin film after N times sputtering can be expressed by IN ¼ I0 ð1 xÞN
(1)
where I0 is the AES intensity before sputtering, and x is the removed percentage of atoms after each sputtering. After taking logarithm in both sides, Eq. (1) becomes lnðIN =I0 Þ ¼ N lnð1 xÞ
(2)
The sputtering rate of Ag is x ¼ 1 es
(3)
where s is the slope of ln(IN/I0) versus N curve. Each sputtering lasted 1 min in our sputtering experiment. The calculated result is shown in Fig. 4. The curve of ln(IN/I0) as a function of N is a perfect straight line. This indicates that the above simple model can
309
Fig. 4. Logarithm of the relative residual Ag Auger signal ln(IN/I0) vs. the sputtering number N for the annealed thin film 1 ML Ni/1 ML Ag/Pt(1 1 1).
describe the removal of Ag atoms. It also shows that Ag overlayer is indeed on the topmost layer. Furthermore, Ag atoms and Ni atoms do not form Ag–Ni alloy after the exchange. After calculation, we obtain the sputtering rate of 34.4%. It is a very effective sputtering, and each sputtering can remove over one third of the residual Ag atoms. This is because of the surface of the Ag overlayer became very flat after it was annealed up to 800 K. Ag atoms can be easily removed by Ar ion sputtering due to the lack of shadow effect.
4. Conclusion Ni thin film exhibits the Volmer–Weber growth mode when deposited on 1 ML Ag/Pt(1 1 1). The atomic exchange between Ag and Ni occurs after high temperature annealing for Ni/1 ML Ag/Pt(1 1 1). The starting temperature of the atomic exchange is 400 K, independent of Ni coverages. Complete exchange temperatures are 570 K, 630 K, 725 K, and 780 K for 1 ML, 2 ML, 3 ML, and 4 ML of Ni coverages, respectively. The formation of Ni–Pt alloy develops after the exchange is complete. Large lattice mismatch between Ni and Ag, and especially small surface free energy of Ag are the possible mechanisms of the atomic exchange in this system.
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H.Y. Ho et al. / Applied Surface Science 237 (2004) 306–310
Acknowledgement This research was supported by the National Science Council of the Republic of China under Grant No. NSC 92-2112-M-003-019, and by the National Taiwan Normal University under Grant No. ORD93-1. Reference [1] R. Krishnan, H. Lassri, S. Prasad, M. Porte, M. Tessier, J. Appl. Phys. 73 (1993) 6433. [2] D.J. Kubinski, H. Holloway, J. Appl. Phys. 77 (1995) 782. [3] H. Chiriac, M. Urse, F. Rusu, C. Hison, M. Neagu, Sens. Actuators 76 (1999) 376.
[4] F.C. Chen, Y.E. Wu, C.W. Su, C.S. Shern, Phys. Rev. B 66 (2002) 184417. [5] C.W. Su, H.Y. Ho, C.S. Shern, R.H. Chen, Surf. Sci. 499 (2002) 103. [6] J.S. Tsay, Y.D. Yao, C.S. Shern, Phys. Rev. B 58 (1998) 3609. [7] K.S. Lee, S.H. Kim, H.G. Min, J. Seo, J.-S. Kim, Surf. Sci. 377–379 (1997) 918. [8] V.K. Kimikov, K.B. Khokonov, J. Appl. Phys. 54 (1983) 1346. [9] B. Aufray, H. Giordano, B. Legrand, G. Treglia, Surf. Sci. 307– 309 (1994) 531. [10] T.-M. Roussel, A. Saul, G. Treglia, Phys. Rev. B 55 (1997) 10931. [11] C.S. Shern, Y.F. Wu, Y.E. Wu, Surf. Sci. 439 (1999) 779. [12] R. Schuster, H. Roder, K. Bromann, H. Brune, K. Kern, Phys. Rev. B 54 (1996) 13476.