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Growth properties of ultrathin Fe overlayers grown on a highly stepped Cu(1 1 1) surface
Applied Surface Science 174 (2001) 316±323
Growth properties of ultrathin Fe overlayers grown on a highly stepped Cu(1 1 1) surface Yu-Kwon Kim, Jae Yeol Maeng, Sung Yong Lee, Sehun Kim* Department of Chemistry and School of Molecular Science (BK 21), Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea Received 15 January 2001; accepted 16 February 2001
Abstract The growth properties of Fe overlayers on a stepped Cu(1 1 1) surface with a 88 miscut angle were investigated by the CO titration method, Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). The growth properties of Fe ®lms on the stepped surface are found to be signi®cantly different depending on the deposition temperature. At low substrate temperatures (T s < 200 K), the Fe ®lms grow in a 2D island mode retaining the step periodicity of the stepped Cu(1 1 1) surface, while at room temperature, even the submonolayer of Fe deposition (y 0:3 ML (monolayer)) signi®cantly alters the step structure of the substrate and the Fe ®lms grow in a 3D island mode. # 2001 Elsevier Science B.V. All rights reserved. PACS: 68.55.-a Keywords: Cu(1 1 1); Fe; CO; Thermal desorption spectroscopy (TDS); Thin ®lm
1. Introduction The morphology and structure of ultrathin Fe ®lms correlated with their magnetic properties have been of great interest [1±3]. It has been known that the thermally deposited ultrathin Fe/Cu(1 1 1) ®lms grow in a multilayer mode with a low magnetic moment (0.5 mB), but the ®lms prepared by the pulsed laser deposition grow in a layer-by-layer mode with a magnetic moment larger than 2 mB [1,4]. The structures of ultrathin Fe ®lms on the ¯at Cu(1 1 1) surfaces have been investigated by various surface-sensitive techniques such as low energy electron diffraction *
Corresponding author. Tel.: 82-42-869-2831; fax: 82-42-869-2810. E-mail address: [email protected] (S. Kim).
(LEED) [5±9], X-ray photoelectron diffraction (XPD) [10,11], and scanning tunneling microscopy (STM) [1,4,12±14]. Auger electron spectroscopy (AES) and LEED studies [5±7] claimed that the Fe ®lms grow in a layer-by-layer growth mode at room temperature up to several monolayers [10]. However, Tian et al. [8] and Darici et al. [9], based on their LEED I/V curves, reported that Fe grows ®rst pseudomorphically to a thickness of about 5 ML (monolayers) and then transforms to bcc Fe{1 1 0} domains with Kurdjumov±Sachs (KS) orientation. Kief and Egelhoff [10] found that the growth mode of Fe ®lm depends on the substrate temperature based on their XPD data such that the Fe ®lms grown at 80 K have the bcc structure, but at room temperature, they grow pseudomorphically in the fcc structure and then transform to bcc between 3 and 6 ML. A recent
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 1 9 4 - 5
Y.-K. Kim et al. / Applied Surface Science 174 (2001) 316±323
XPD study [11] also reported a pseudomorphic growth of Fe/Cu(1 1 1) up to 2 ML. However, STM studies [2,12±14] revealed a three-dimensional island growth with a preferential decoration of step edges in the submonolayer regime. The growth of Fe ®lms has been extensively studied for the ¯at Cu substrates, while relatively less for the stepped substrates. Shen et al. [2] studied the structure and the magnetic properties of Fe ®lms on the stepped Cu(1 1 1) with a miscut angle of 1.28 using STM, LEED, and the surface magneto-optic Kerr effect (SMOKE). They found that the Fe ®lms grow in the forms of quasi-one-dimensional stripes along the step edges and then transform to a 2D form between 1.4 and 1.8 ML. The structural transition from the fcc to bcc (1 1 0) structure with the KS orientation between 2.3 and 2.7 ML was also reported as the previous studies mentioned above. Thus, the stepped substrates of Cu could be used as a template for the growth of quasi-one-dimensional Fe ®lms. In order to fabricate high density of 1D wires on the substrate, the surfaces with high step density will be necessary. Thus, it is important to investigate the growth of Fe on the Cu substrates with high density of step. In this work, we report the temperaturedependent growth and structure of Fe overlayers on the stepped Cu(1 1 1) surface with a large miscut angle utilizing LEED and the CO titration method based on the thermal desorption spectroscopy (TDS). 2. Experimental The experiments were performed in a home-built UHV chamber equipped with LEED and a differentially pumped UTI-100 Mass Spectrometer. A stepped Cu(1 1 1) sample with a miscut angle of 88 along [1 1 2] direction and the average terrace width of Ê was employed in the experiments. The clean 16.2 A and well-ordered stepped Cu surface was prepared by Ar ion bombardment (500 eV) and annealing cycles in the UHV chamber, and its cleanliness and ordering was con®rmed by LEED and AES. The average width of terraces was measured by the splitting of diffraction spot. Fe was evaporated onto the cleaned Cu surface by a Fe evaporation source with a deposition rate of 0.4 ML/s keeping the substrate temperatures at 110,
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200, and 290 K, respectively. The Fe source was made of a pure Fe wire (99.999%) wrapped around a Ta wire which is resistively heated. The Fe ¯ux of this source was calibrated using AES in a separate Perkin-Elmer ESCA-AES chamber prior to being installed in the TDS chamber. Before each Fe deposition, the Cu surface was cleaned by the above cycles and the surface with well-ordered regular steps was regenerated. The line pro®les of (10) LEED spots for the clean and Fe-covered Cu(1 1 1) surfaces are obtained by a charge coupled device (CCD) camera installed in front of the rear view LEED optics. The CO titration experiments were carried out by taking thermal desorption spectra (TDS) from the Fe/Cu(1 1 1) surfaces exposed with 1 l CO after cooling down the substrate from the deposition temperature to 100 K. The TD spectra were obtained by heating the substrate at a heating rate of 2 K/s. The purity of CO used in these experiments was better than 99.999%. The relative numbers of binding sites in steps and terraces of Cu are measured by the areas of CO desorption peaks on the clean Cu surface and the Fe/Cu(1 1 1). 3. Results and discussion The LEED patterns of clean stepped Cu(1 1 1) surface with a miscut angle of 88 along [1 1 2] direction give a clear splitting of (10) beam at the incident electron energy of 108 eV, as shown in Fig. 1(a). It implies that the surface consists of well-ordered arrays Ê [15]. of steps with the average terrace width of 16.2 A Fig. 1(b)±(d) show the line pro®les of (10) beam as a function of Fe coverage for Fe/Cu(1 1 1) surfaces prepared at the substrate temperatures of 110, 200, and 290 K. At low temperatures (110 and 200 K), the splitting of (10) spot persists up to around 1.5 ML. It indicates that the Fe overlayers grow in the registry of steps on the Cu surface up to 1.5 ML. However, at the substrate temperature of 290 K, the spot splitting vanishes for the low coverage of 0.3 ML. It indicates that the deposition of small amount of Fe signi®cantly alters the step structure of the substrate. The disappearance of the beam splitting may be due to the single or mixed contribution from the step meandering, the irregularity of the step width, and the disordered Fe
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Fig. 1. (a) LEED patterns for the stepped Cu(1 1 1) surface with a miscut angle of 88 along [1 1 2] at the electron energies of 108 eV. (b), (c), and (d) are the line pro®les of (10) beam as a function of Fe coverage for Fe/Cu(1 1 1) surfaces prepared at the substrate temperatures of 110, 200, and 290 K, respectively.
atoms on the terrace. The previous STM study [2] for the Fe ®lms on the stepped Cu(1 1 1) with 1.28 miscut angle at room temperature revealed the formation of quasi 1D stripes in the submonolayer coverage followed by 3D island growth. At 0.3 ML, the amount of Fe atoms is so small that the stripes could be virtually formed by segments that are only weakly linked. Ramsperger et al. [16] also reported that the regularly shaped 2D Co islands are found on the ¯at Cu(1 0 0) surface, but an irregular arrangement of Co islands with their shape changing as a function of thickness is found on the stepped Cu(1 0 0) surface with the Ê at room temperature. average terrace width of 30 A These results imply that the presence of the steps obscures the layer-by-layer growth. The disappearance of beam splitting observed in this study is con-
sistent with the above STM results and the CO titration data presented below. It can be speculated as follows. The stepped Cu(1 1 1) used in this work has the Ê , which is much naraverage terrace width of 16.2 A rower than the ones used in the previous STM studies. The narrower terrace of such highly stepped surface may be covered with patch-like Fe stripes. As a result, the straight steps of the original Cu substrate transform into meandering of the steps of Fe layers grown on top to cause the broadening of the diffraction spots. The thermal desorption spectra from the clean stepped Cu(1 1 1) surface exposed with different CO amounts at 100 K are shown in Fig. 2. The CO desorption spectra show two distinct peaks desorbing from the terraces and steps of the Cu surface at about 170 K (a2) and 220 K (b), respectively. The intensities
Y.-K. Kim et al. / Applied Surface Science 174 (2001) 316±323
Fig. 2. CO thermal desorption spectra from the stepped Cu(1 1 1) surface with the heating rate of 2 K/s and CO exposure of 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1.0, 1.2, 1.5, 1.8, and 2.4 L at 100 K.
of these two peaks increase simultaneously up to 1 ML and then reached to saturation. Above the saturation coverage of more than 1 ML of Fe, the third peak begins to appear at about 145 K (a1) as a shoulder of the a2 peak due to the multilayers of CO [17]. Since the former two peaks are distinct and well separated by about 50 K, one can measure the relative binding sites of CO in the terraces and steps on the Cu surface by the areas of the two desorption peaks. The measured TD spectra were found to be reproducible such that the integrated peak areas can be used for the quantitative analysis. Fig. 3(a) shows the CO TD spectra desorbed from the exposed Cu substrate on the Fe/Cu(1 1 1) ®lms prepared at the substrate temperatures of 110, 200, and 290 K (Fig. 4(a) shows the rest part of TD spectra of CO desorbed Fe islands). The peak intensities decrease as the coverage of Fe increases and the peak maximum shifts slightly to higher temperature. The TD spectra also indicate that, even well above 2 ML of Fe deposition, the Cu substrate is not fully covered with the Fe layers and the bare Cu substrate can be
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seen. The fraction of the exposed Cu substrate 9, 33.6, and 38.8% for 2, 2.4, and 3.5 ML Fe ®lms were grown at 110, 200, and 290 K, respectively. The fraction of the exposed Cu substrate increases with the growth temperature. The large fraction of the exposed Cu substrate for the ®lms grown at 290 K means that the growth of Fe ®lms on the stepped surface is a multilayer growth mode rather than a layer-by-layer. This growth pattern is in contrast with the LEED studies [5±7] which reported a layer-by-layer growth of Fe ®lms on a ¯at Cu(1 1 1) surface, but agrees well with the STM results [2,12±14] which reveal a 3D island growth. This CO titration results are qualitatively consistent with the CO titration work of Kief and Egellhoff [10] for the growth properties of Fe ®lms on a ¯at Cu(1 1 1) surface. Their data show that the fraction of the exposed Cu surface is 15 and 25% for 2 ML Fe ®lms grown at 80 and 300 K, respectively. They conclude that both Fe agglomeration and Cu segregation occur and are important in the Fe ®lm growth. The fraction of the exposed Cu for the Fe ®lms grown on the stepped surface with a large miscut angle of 88 at room temperature is quite large compared with that determined by the previous studies [10] performed on a ¯at surface. The difference between the ¯at and the stepped surfaces may be ascribed to the different growth morphologies of Fe ®lms on these surfaces. The STM study for Co ®lms grown on the ¯at and stepped Cu(1 0 0) substrates at room temperature shows a completely different surface morphology [16]. For 0.5 ML Co ®lms, the regular shaped 2D Ê are observed on islands with the diameter of 10±250 A the ¯at substrate, but the round and irregular-shaped Ê appear on the islands with the diameter of 20±50 A stepped substrate. For 2 ML of Co, the STM images show well-layered ®lms on the ¯at surface, but display irregular patch-shaped ®lms along the steps on the stepped substrate. The STM work of Shen et al. [1±3] for Fe ®lms on a stepped Cu(1 1 1) with a small miscut angle of 1.28 shows that at the low coverage, the Fe stripes are divided into segments with 1 ML height, but at high coverage they become more continuous stripes with one or double layer height having rougher edges due to the tendency of the Fe edge atoms aligning along all three [0 1 1] directions. Above 2 ML, the ®lm growth shows a multilayer growth mode with increasing the exposed layers (two to three layers at 2.3 ML; ®ve to six layers at 2.7 ML). They
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Fig. 3. (a) CO thermal desorption from exposed Cu steps and terraces of the Fe/stepped Cu(1 1 1) surfaces as a function of Fe coverages (ML) with the heating rate of 2 K/s and CO exposure of 1 L at 100 K. (b) The fraction of exposed step and terrace Cu as a function of Fe coverage.
also observed that the diffusion of Cu leaves some monolayer deep holes on the surface as the previous STM study found [14]. Thus, the observed large fraction of the exposed Cu substrate for the Fe overlayers on the Cu(1 1 1) surface in this study can be explained by the multilayer growth, the Cu segregation, and the hole formation on the substrate.
The desorption peaks originated from the steps and terraces of the exposed Cu surfaces are ®tted with Gaussian functions, and the integrated peak areas for the step and terrace features are plotted as a function of Fe coverage in Fig. 3(b). In Fig. 3(b), the amounts of CO desorption from the steps and terraces of Cu (that is, the bare Cu step and terrace areas on the Fe-covered
Y.-K. Kim et al. / Applied Surface Science 174 (2001) 316±323
surface) decrease as the Fe coverage increases. The decreasing rate is faster at 110 K than at higher growth temperatures (200 and 290 K). For the Fe ®lms grown at 110 K, the peak intensities including the step and terrace decrease rapidly with increasing the Fe coverage. At around 2 ML, the peak resulting from the step sites almost disappears, but the smaller peak from the terrace sites still persists. It indicates that at low temperature, Fe atoms prefer the step sites to the terrace in the submonolayer regime. It is consistent with the STM result [18] for the growth of the Ag ®lms grown on Pt(1 1 1) at low temperatures in which the
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Ag atoms grow along the steps as well as on the terrace-forming dendritic fractals at 110 K. However, for the Fe ®lms grown at 200 and 290 K, the step sites decrease very slightly and persist with increasing Fe exposure even above 2 ML, as shown in Fig. 3(b). The STM results [2] for Fe ®lms grown on a stepped Cu(1 1 1) surface with a miscut angle of 1.28 at room temperature already indicated that Fe atoms grow as stripes along the upper step edges in the submonolayer coverage due to the long diffusion length of Fe atoms. Then we can expect a slower decrease of the Cu step sites than of the terrace sites for room temperature
Fig. 4. (a) CO thermal desorption spectra from Fe overlayers of Fe/Cu(1 1 1) at the growth temperatures of 110, 200, and 290 K. (b) The integrated peak areas of these spectra and (c) maximum desorption temperatures of CO from Fe/stepped Cu(1 1 1) surfaces are plotted as a function of Fe coverage (ML).
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grown ®lms. In agreement with this expectation, the decreasing rate of the step sites at room temperature is slower than that at 110 K. This fact can be explained by different growth behaviors on the highly stepped Ê ) of the stepped Cu substrate. The terrace width (16.2 A used in this work is narrow enough to interfere the diffusion of the Fe atoms parallel to the step direction resulting in formation of discontinuous stripes along the steps. The discontinuous Fe stripes along the steps exposed the Cu step sites. Also the Cu steps created by the hole formation on the Cu substrate observed in the STM studies [14] can compensate the decrease of the step sites by Fe deposition. Fig. 4(a) shows CO desorption spectra from Fe overlayers of Fe/Cu(1 1 1) grown at the growth temperatures of 110, 200, and 290 K, respectively. The integrated peak areas of these spectra are plotted as a function of Fe coverage in Fig. 4(b). For the Fe ®lms grown at 110 K, the integrated peak areas increase linearly up to 0.3 ML and then become saturated at about 1 ML. The solid line in the curves is drawn assuming the layer-by-layer growth mode. This observation suggests that at 110 K, Fe atoms start to grow in 2D islands with mono-atomic height up to 0.3 ML and then form 3D islands with multilayers. The peak area curves for the ®lms grown at 200 and 290 K deviates from the solid line right at the beginning of the Fe deposition. The initial slope of the curve decreases with the growth temperature. At 290 K, the curve increases slowly with increasing Fe coverage, which indicates a 3D island growth from the low Fe coverage. In the desorption spectra of 290 K shown in Fig. 4(a), a shoulder peak at about 440 K appears above 1.5 ML Fe coverage. The previous studies reported that a structural transition from fcc to bcc occurs at around 2±5 ML [10]. The 400 K peak and 440 K shoulder peak can be ascribed to the CO desorption from fcc and bcc domains, respectively. In Fig. 4(c), one can notice that the peak position shifts towards higher temperature as Fe coverage increases at low growth temperatures (110 and 200 K). It indicates that the number of defective adsorption sites such as kinks, steps, ad-atoms, and vacancies, which usually have higher adsorption energy, increases as Fe coverage increases. Since the extent of the shift of the peak maximum decreases with increasing the growth temperature, it is concluded that the Fe islands grown at 110 K have more
defective sites than Fe ®lms grown at 200 and 290 K. This is consistent with the photoelectron diffraction result [10]. 4. Conclusion The split spots in LEED patterns of the stepped Cu surface cut 88 from the (1 1 1) plane in the direction [1 1 2] indicated that the surface has a regular array of mono-atomic steps in direction [1 1 0] and (1 1 1) terrace planes which have the average terrace width Ê . The LEED spot analysis shows that at low of 16.2 A substrate temperatures (110 and 200 K), the Fe overlayers on the stepped surface grow in registry of its step structure up to 1.5 ML, but at room temperature, 0.3 ML of Fe deposition signi®cantly affects the step structure of the substrate. The CO desorption spectra obtained from the Fe/Cu(1 1 1) surface show that Fe atoms grown on the surface prefer step sites to terrace sites from the very submonolayer coverage and that Fe atoms grow in 2D islands at low temperatures (110 and 200 K), while in 3D islands at room temperature. The CO titration method in our experiments has shown to be useful to discriminate different facets as well as different chemical identities. Also the CO titration method can be used as a complementary tool to probe the growth properties as well as other surface science techniques. Acknowledgements This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) (no. 98-0501-01-01-3) and Brain Korea 21 project. References [1] J. Shen, P. Ohresser, Ch.V. Mohan, M. Klaua, J. Barthel, J. Kirschner, Phys. Rev. Lett. 80 (1998) 1980. [2] J. Shen, M. Klaua, P. Ohresser, H. Jenniches, J. Barthel, Ch.V. Mohan, J. Kirschner, Phys. Rev. B 56 (1997) 11134. [3] J. Shen, R. Skomski, M. Klaua, H. Jenniches, S. Sundar Manoharan, J. Kirschner, Phys. Rev. B 56 (1997) 2340. [4] H. Jenniches, M. Klaua, H. HoÈche, J. Kirschner, Appl. Phys. Lett. 69 (1996) 3339. [5] U. Gradmann, W. KuÈmmerle, P. Tillmanns, Thin Solid Films 34 (1976) 249.
Y.-K. Kim et al. / Applied Surface Science 174 (2001) 316±323 [6] U. Gradmann, P. Tillmanns, Phys. Stat. Solidi A 44 (1977) 539. [7] W. KuÈmmerle, U. Gradmann, Phys. Stat. Solidi A 45 (1978) 171. [8] D. Tian, F. Jona, P.M. Marcus, Phys. Rev. B 45 (1992) 11216. [9] Y. Darici, J. Marcano, H. Min, O.A. Montano, Surf. Sci. 195 (1988) 566. [10] M.T. Kief, W.F. Egelhoff Jr., Phys. Rev. B 47 (1993) 10785. [11] A. Theobald, O. Schaff, C.J. Hirschmugl, V. Fernandez, K.M. Schindler, M. Polcik, A.M. Bradshaw, Phys. Rev. B 59 (1999) 2313. [12] A. Brodde, H. Neddermeyer, Ultramicroscopy 42±44 (1992) 556.
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[13] A. Brodde, K. Dreps, J. Binder, Ch. Lunau, H. Neddermeyer, Phys. Rev. B 47 (1993) 6609. [14] M. Klaua, H. HoÈche, H. Jenniches, J. Barthel, J. Kirschner, Surf. Sci. 381 (1997) 106. [15] F. Hottier, J.B. Theeten, A. Masson, J.L. Domange, Surf. Sci. 65 (1997) 563. [16] U. Ramsperger, A. Vaterlaus, P. PfaÈf¯i, U. Maier, D. Pescia, Phys. Rev. B 53 (1996) 8001. [17] I. BoÈnicke, W. Kirstein, S. Spinzig, F. Thieme, Surf. Sci. 313 (1994) 231. [18] H. Brune, H. RoÈder, C. Boragno, K. Kern, Phys. Rev. Lett. 73 (1994) 1955.
Growth properties of ultrathin Fe overlayers grown on a highly stepped Cu(1 1 1) surface Yu-Kwon Kim, Jae Yeol Maeng, Sung Yong Lee, Sehun Kim* Department of Chemistry and School of Molecular Science (BK 21), Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea Received 15 January 2001; accepted 16 February 2001
Abstract The growth properties of Fe overlayers on a stepped Cu(1 1 1) surface with a 88 miscut angle were investigated by the CO titration method, Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). The growth properties of Fe ®lms on the stepped surface are found to be signi®cantly different depending on the deposition temperature. At low substrate temperatures (T s < 200 K), the Fe ®lms grow in a 2D island mode retaining the step periodicity of the stepped Cu(1 1 1) surface, while at room temperature, even the submonolayer of Fe deposition (y 0:3 ML (monolayer)) signi®cantly alters the step structure of the substrate and the Fe ®lms grow in a 3D island mode. # 2001 Elsevier Science B.V. All rights reserved. PACS: 68.55.-a Keywords: Cu(1 1 1); Fe; CO; Thermal desorption spectroscopy (TDS); Thin ®lm
1. Introduction The morphology and structure of ultrathin Fe ®lms correlated with their magnetic properties have been of great interest [1±3]. It has been known that the thermally deposited ultrathin Fe/Cu(1 1 1) ®lms grow in a multilayer mode with a low magnetic moment (0.5 mB), but the ®lms prepared by the pulsed laser deposition grow in a layer-by-layer mode with a magnetic moment larger than 2 mB [1,4]. The structures of ultrathin Fe ®lms on the ¯at Cu(1 1 1) surfaces have been investigated by various surface-sensitive techniques such as low energy electron diffraction *
Corresponding author. Tel.: 82-42-869-2831; fax: 82-42-869-2810. E-mail address: [email protected] (S. Kim).
(LEED) [5±9], X-ray photoelectron diffraction (XPD) [10,11], and scanning tunneling microscopy (STM) [1,4,12±14]. Auger electron spectroscopy (AES) and LEED studies [5±7] claimed that the Fe ®lms grow in a layer-by-layer growth mode at room temperature up to several monolayers [10]. However, Tian et al. [8] and Darici et al. [9], based on their LEED I/V curves, reported that Fe grows ®rst pseudomorphically to a thickness of about 5 ML (monolayers) and then transforms to bcc Fe{1 1 0} domains with Kurdjumov±Sachs (KS) orientation. Kief and Egelhoff [10] found that the growth mode of Fe ®lm depends on the substrate temperature based on their XPD data such that the Fe ®lms grown at 80 K have the bcc structure, but at room temperature, they grow pseudomorphically in the fcc structure and then transform to bcc between 3 and 6 ML. A recent
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 1 9 4 - 5
Y.-K. Kim et al. / Applied Surface Science 174 (2001) 316±323
XPD study [11] also reported a pseudomorphic growth of Fe/Cu(1 1 1) up to 2 ML. However, STM studies [2,12±14] revealed a three-dimensional island growth with a preferential decoration of step edges in the submonolayer regime. The growth of Fe ®lms has been extensively studied for the ¯at Cu substrates, while relatively less for the stepped substrates. Shen et al. [2] studied the structure and the magnetic properties of Fe ®lms on the stepped Cu(1 1 1) with a miscut angle of 1.28 using STM, LEED, and the surface magneto-optic Kerr effect (SMOKE). They found that the Fe ®lms grow in the forms of quasi-one-dimensional stripes along the step edges and then transform to a 2D form between 1.4 and 1.8 ML. The structural transition from the fcc to bcc (1 1 0) structure with the KS orientation between 2.3 and 2.7 ML was also reported as the previous studies mentioned above. Thus, the stepped substrates of Cu could be used as a template for the growth of quasi-one-dimensional Fe ®lms. In order to fabricate high density of 1D wires on the substrate, the surfaces with high step density will be necessary. Thus, it is important to investigate the growth of Fe on the Cu substrates with high density of step. In this work, we report the temperaturedependent growth and structure of Fe overlayers on the stepped Cu(1 1 1) surface with a large miscut angle utilizing LEED and the CO titration method based on the thermal desorption spectroscopy (TDS). 2. Experimental The experiments were performed in a home-built UHV chamber equipped with LEED and a differentially pumped UTI-100 Mass Spectrometer. A stepped Cu(1 1 1) sample with a miscut angle of 88 along [1 1 2] direction and the average terrace width of Ê was employed in the experiments. The clean 16.2 A and well-ordered stepped Cu surface was prepared by Ar ion bombardment (500 eV) and annealing cycles in the UHV chamber, and its cleanliness and ordering was con®rmed by LEED and AES. The average width of terraces was measured by the splitting of diffraction spot. Fe was evaporated onto the cleaned Cu surface by a Fe evaporation source with a deposition rate of 0.4 ML/s keeping the substrate temperatures at 110,
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200, and 290 K, respectively. The Fe source was made of a pure Fe wire (99.999%) wrapped around a Ta wire which is resistively heated. The Fe ¯ux of this source was calibrated using AES in a separate Perkin-Elmer ESCA-AES chamber prior to being installed in the TDS chamber. Before each Fe deposition, the Cu surface was cleaned by the above cycles and the surface with well-ordered regular steps was regenerated. The line pro®les of (10) LEED spots for the clean and Fe-covered Cu(1 1 1) surfaces are obtained by a charge coupled device (CCD) camera installed in front of the rear view LEED optics. The CO titration experiments were carried out by taking thermal desorption spectra (TDS) from the Fe/Cu(1 1 1) surfaces exposed with 1 l CO after cooling down the substrate from the deposition temperature to 100 K. The TD spectra were obtained by heating the substrate at a heating rate of 2 K/s. The purity of CO used in these experiments was better than 99.999%. The relative numbers of binding sites in steps and terraces of Cu are measured by the areas of CO desorption peaks on the clean Cu surface and the Fe/Cu(1 1 1). 3. Results and discussion The LEED patterns of clean stepped Cu(1 1 1) surface with a miscut angle of 88 along [1 1 2] direction give a clear splitting of (10) beam at the incident electron energy of 108 eV, as shown in Fig. 1(a). It implies that the surface consists of well-ordered arrays Ê [15]. of steps with the average terrace width of 16.2 A Fig. 1(b)±(d) show the line pro®les of (10) beam as a function of Fe coverage for Fe/Cu(1 1 1) surfaces prepared at the substrate temperatures of 110, 200, and 290 K. At low temperatures (110 and 200 K), the splitting of (10) spot persists up to around 1.5 ML. It indicates that the Fe overlayers grow in the registry of steps on the Cu surface up to 1.5 ML. However, at the substrate temperature of 290 K, the spot splitting vanishes for the low coverage of 0.3 ML. It indicates that the deposition of small amount of Fe signi®cantly alters the step structure of the substrate. The disappearance of the beam splitting may be due to the single or mixed contribution from the step meandering, the irregularity of the step width, and the disordered Fe
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Fig. 1. (a) LEED patterns for the stepped Cu(1 1 1) surface with a miscut angle of 88 along [1 1 2] at the electron energies of 108 eV. (b), (c), and (d) are the line pro®les of (10) beam as a function of Fe coverage for Fe/Cu(1 1 1) surfaces prepared at the substrate temperatures of 110, 200, and 290 K, respectively.
atoms on the terrace. The previous STM study [2] for the Fe ®lms on the stepped Cu(1 1 1) with 1.28 miscut angle at room temperature revealed the formation of quasi 1D stripes in the submonolayer coverage followed by 3D island growth. At 0.3 ML, the amount of Fe atoms is so small that the stripes could be virtually formed by segments that are only weakly linked. Ramsperger et al. [16] also reported that the regularly shaped 2D Co islands are found on the ¯at Cu(1 0 0) surface, but an irregular arrangement of Co islands with their shape changing as a function of thickness is found on the stepped Cu(1 0 0) surface with the Ê at room temperature. average terrace width of 30 A These results imply that the presence of the steps obscures the layer-by-layer growth. The disappearance of beam splitting observed in this study is con-
sistent with the above STM results and the CO titration data presented below. It can be speculated as follows. The stepped Cu(1 1 1) used in this work has the Ê , which is much naraverage terrace width of 16.2 A rower than the ones used in the previous STM studies. The narrower terrace of such highly stepped surface may be covered with patch-like Fe stripes. As a result, the straight steps of the original Cu substrate transform into meandering of the steps of Fe layers grown on top to cause the broadening of the diffraction spots. The thermal desorption spectra from the clean stepped Cu(1 1 1) surface exposed with different CO amounts at 100 K are shown in Fig. 2. The CO desorption spectra show two distinct peaks desorbing from the terraces and steps of the Cu surface at about 170 K (a2) and 220 K (b), respectively. The intensities
Y.-K. Kim et al. / Applied Surface Science 174 (2001) 316±323
Fig. 2. CO thermal desorption spectra from the stepped Cu(1 1 1) surface with the heating rate of 2 K/s and CO exposure of 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1.0, 1.2, 1.5, 1.8, and 2.4 L at 100 K.
of these two peaks increase simultaneously up to 1 ML and then reached to saturation. Above the saturation coverage of more than 1 ML of Fe, the third peak begins to appear at about 145 K (a1) as a shoulder of the a2 peak due to the multilayers of CO [17]. Since the former two peaks are distinct and well separated by about 50 K, one can measure the relative binding sites of CO in the terraces and steps on the Cu surface by the areas of the two desorption peaks. The measured TD spectra were found to be reproducible such that the integrated peak areas can be used for the quantitative analysis. Fig. 3(a) shows the CO TD spectra desorbed from the exposed Cu substrate on the Fe/Cu(1 1 1) ®lms prepared at the substrate temperatures of 110, 200, and 290 K (Fig. 4(a) shows the rest part of TD spectra of CO desorbed Fe islands). The peak intensities decrease as the coverage of Fe increases and the peak maximum shifts slightly to higher temperature. The TD spectra also indicate that, even well above 2 ML of Fe deposition, the Cu substrate is not fully covered with the Fe layers and the bare Cu substrate can be
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seen. The fraction of the exposed Cu substrate 9, 33.6, and 38.8% for 2, 2.4, and 3.5 ML Fe ®lms were grown at 110, 200, and 290 K, respectively. The fraction of the exposed Cu substrate increases with the growth temperature. The large fraction of the exposed Cu substrate for the ®lms grown at 290 K means that the growth of Fe ®lms on the stepped surface is a multilayer growth mode rather than a layer-by-layer. This growth pattern is in contrast with the LEED studies [5±7] which reported a layer-by-layer growth of Fe ®lms on a ¯at Cu(1 1 1) surface, but agrees well with the STM results [2,12±14] which reveal a 3D island growth. This CO titration results are qualitatively consistent with the CO titration work of Kief and Egellhoff [10] for the growth properties of Fe ®lms on a ¯at Cu(1 1 1) surface. Their data show that the fraction of the exposed Cu surface is 15 and 25% for 2 ML Fe ®lms grown at 80 and 300 K, respectively. They conclude that both Fe agglomeration and Cu segregation occur and are important in the Fe ®lm growth. The fraction of the exposed Cu for the Fe ®lms grown on the stepped surface with a large miscut angle of 88 at room temperature is quite large compared with that determined by the previous studies [10] performed on a ¯at surface. The difference between the ¯at and the stepped surfaces may be ascribed to the different growth morphologies of Fe ®lms on these surfaces. The STM study for Co ®lms grown on the ¯at and stepped Cu(1 0 0) substrates at room temperature shows a completely different surface morphology [16]. For 0.5 ML Co ®lms, the regular shaped 2D Ê are observed on islands with the diameter of 10±250 A the ¯at substrate, but the round and irregular-shaped Ê appear on the islands with the diameter of 20±50 A stepped substrate. For 2 ML of Co, the STM images show well-layered ®lms on the ¯at surface, but display irregular patch-shaped ®lms along the steps on the stepped substrate. The STM work of Shen et al. [1±3] for Fe ®lms on a stepped Cu(1 1 1) with a small miscut angle of 1.28 shows that at the low coverage, the Fe stripes are divided into segments with 1 ML height, but at high coverage they become more continuous stripes with one or double layer height having rougher edges due to the tendency of the Fe edge atoms aligning along all three [0 1 1] directions. Above 2 ML, the ®lm growth shows a multilayer growth mode with increasing the exposed layers (two to three layers at 2.3 ML; ®ve to six layers at 2.7 ML). They
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Fig. 3. (a) CO thermal desorption from exposed Cu steps and terraces of the Fe/stepped Cu(1 1 1) surfaces as a function of Fe coverages (ML) with the heating rate of 2 K/s and CO exposure of 1 L at 100 K. (b) The fraction of exposed step and terrace Cu as a function of Fe coverage.
also observed that the diffusion of Cu leaves some monolayer deep holes on the surface as the previous STM study found [14]. Thus, the observed large fraction of the exposed Cu substrate for the Fe overlayers on the Cu(1 1 1) surface in this study can be explained by the multilayer growth, the Cu segregation, and the hole formation on the substrate.
The desorption peaks originated from the steps and terraces of the exposed Cu surfaces are ®tted with Gaussian functions, and the integrated peak areas for the step and terrace features are plotted as a function of Fe coverage in Fig. 3(b). In Fig. 3(b), the amounts of CO desorption from the steps and terraces of Cu (that is, the bare Cu step and terrace areas on the Fe-covered
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surface) decrease as the Fe coverage increases. The decreasing rate is faster at 110 K than at higher growth temperatures (200 and 290 K). For the Fe ®lms grown at 110 K, the peak intensities including the step and terrace decrease rapidly with increasing the Fe coverage. At around 2 ML, the peak resulting from the step sites almost disappears, but the smaller peak from the terrace sites still persists. It indicates that at low temperature, Fe atoms prefer the step sites to the terrace in the submonolayer regime. It is consistent with the STM result [18] for the growth of the Ag ®lms grown on Pt(1 1 1) at low temperatures in which the
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Ag atoms grow along the steps as well as on the terrace-forming dendritic fractals at 110 K. However, for the Fe ®lms grown at 200 and 290 K, the step sites decrease very slightly and persist with increasing Fe exposure even above 2 ML, as shown in Fig. 3(b). The STM results [2] for Fe ®lms grown on a stepped Cu(1 1 1) surface with a miscut angle of 1.28 at room temperature already indicated that Fe atoms grow as stripes along the upper step edges in the submonolayer coverage due to the long diffusion length of Fe atoms. Then we can expect a slower decrease of the Cu step sites than of the terrace sites for room temperature
Fig. 4. (a) CO thermal desorption spectra from Fe overlayers of Fe/Cu(1 1 1) at the growth temperatures of 110, 200, and 290 K. (b) The integrated peak areas of these spectra and (c) maximum desorption temperatures of CO from Fe/stepped Cu(1 1 1) surfaces are plotted as a function of Fe coverage (ML).
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grown ®lms. In agreement with this expectation, the decreasing rate of the step sites at room temperature is slower than that at 110 K. This fact can be explained by different growth behaviors on the highly stepped Ê ) of the stepped Cu substrate. The terrace width (16.2 A used in this work is narrow enough to interfere the diffusion of the Fe atoms parallel to the step direction resulting in formation of discontinuous stripes along the steps. The discontinuous Fe stripes along the steps exposed the Cu step sites. Also the Cu steps created by the hole formation on the Cu substrate observed in the STM studies [14] can compensate the decrease of the step sites by Fe deposition. Fig. 4(a) shows CO desorption spectra from Fe overlayers of Fe/Cu(1 1 1) grown at the growth temperatures of 110, 200, and 290 K, respectively. The integrated peak areas of these spectra are plotted as a function of Fe coverage in Fig. 4(b). For the Fe ®lms grown at 110 K, the integrated peak areas increase linearly up to 0.3 ML and then become saturated at about 1 ML. The solid line in the curves is drawn assuming the layer-by-layer growth mode. This observation suggests that at 110 K, Fe atoms start to grow in 2D islands with mono-atomic height up to 0.3 ML and then form 3D islands with multilayers. The peak area curves for the ®lms grown at 200 and 290 K deviates from the solid line right at the beginning of the Fe deposition. The initial slope of the curve decreases with the growth temperature. At 290 K, the curve increases slowly with increasing Fe coverage, which indicates a 3D island growth from the low Fe coverage. In the desorption spectra of 290 K shown in Fig. 4(a), a shoulder peak at about 440 K appears above 1.5 ML Fe coverage. The previous studies reported that a structural transition from fcc to bcc occurs at around 2±5 ML [10]. The 400 K peak and 440 K shoulder peak can be ascribed to the CO desorption from fcc and bcc domains, respectively. In Fig. 4(c), one can notice that the peak position shifts towards higher temperature as Fe coverage increases at low growth temperatures (110 and 200 K). It indicates that the number of defective adsorption sites such as kinks, steps, ad-atoms, and vacancies, which usually have higher adsorption energy, increases as Fe coverage increases. Since the extent of the shift of the peak maximum decreases with increasing the growth temperature, it is concluded that the Fe islands grown at 110 K have more
defective sites than Fe ®lms grown at 200 and 290 K. This is consistent with the photoelectron diffraction result [10]. 4. Conclusion The split spots in LEED patterns of the stepped Cu surface cut 88 from the (1 1 1) plane in the direction [1 1 2] indicated that the surface has a regular array of mono-atomic steps in direction [1 1 0] and (1 1 1) terrace planes which have the average terrace width Ê . The LEED spot analysis shows that at low of 16.2 A substrate temperatures (110 and 200 K), the Fe overlayers on the stepped surface grow in registry of its step structure up to 1.5 ML, but at room temperature, 0.3 ML of Fe deposition signi®cantly affects the step structure of the substrate. The CO desorption spectra obtained from the Fe/Cu(1 1 1) surface show that Fe atoms grown on the surface prefer step sites to terrace sites from the very submonolayer coverage and that Fe atoms grow in 2D islands at low temperatures (110 and 200 K), while in 3D islands at room temperature. The CO titration method in our experiments has shown to be useful to discriminate different facets as well as different chemical identities. Also the CO titration method can be used as a complementary tool to probe the growth properties as well as other surface science techniques. Acknowledgements This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) (no. 98-0501-01-01-3) and Brain Korea 21 project. References [1] J. Shen, P. Ohresser, Ch.V. Mohan, M. Klaua, J. Barthel, J. Kirschner, Phys. Rev. Lett. 80 (1998) 1980. [2] J. Shen, M. Klaua, P. Ohresser, H. Jenniches, J. Barthel, Ch.V. Mohan, J. Kirschner, Phys. Rev. B 56 (1997) 11134. [3] J. Shen, R. Skomski, M. Klaua, H. Jenniches, S. Sundar Manoharan, J. Kirschner, Phys. Rev. B 56 (1997) 2340. [4] H. Jenniches, M. Klaua, H. HoÈche, J. Kirschner, Appl. Phys. Lett. 69 (1996) 3339. [5] U. Gradmann, W. KuÈmmerle, P. Tillmanns, Thin Solid Films 34 (1976) 249.
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