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Step flow observed on top of oxidized CoAl(1 0 0) surface
Applied Surface Science 253 (2006) 1796–1800 www.elsevier.com/locate/apsusc
Step flow observed on top of oxidized CoAl(1 0 0) surface V. Podgursky a,*, V. Rose b, J. Costina b, R. Franchy b,ä a
Department of Materials Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia b Institut fu¨r Schichten und Grenzfla¨chen, ISG 3, Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany Received 13 June 2005; received in revised form 2 March 2006; accepted 7 March 2006 Available online 19 April 2006
Abstract Clean and oxidized surfaces of CoAl(1 0 0) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunnelling microscopy (STM). The regrowth or step flow of terraces was observed at 1150 K. The correlation between the growth of oxide and the step flow on the CoAl(1 0 0) surface is discussed in this paper. # 2006 Elsevier B.V. All rights reserved. Keywords: Oxidation; Alumina; Step flow; STM; LEED
1. Introduction Bulk and surface properties of Al2O3 present great interest due to their role in coatings, heterogeneous catalysts, non-linear optics and microelectronics. A crystalline Al2O3 can be grown on the surfaces of intermetallic alloys [1]. A long-range order, low conductivity, no charging effects on the surface, reproducibility of a crystal structure, and thickness are attractive properties of thin Al2O3 films. In the past, low indexed surfaces of NiAl [2–4] were intensively investigated to determine the bulk-termination of single crystal surfaces. Blum et al. [4] claim that the most stable surface of NiAl(1 0 0) after annealing at 800 K is Al terminated with a superstructure indexed by c(H2 3H2)R458, it is altered to Ni terminated, with a (1 1) structure after annealing at 1400 K. Recently, a limited number of studies concerning clean CoAl(1 0 0) and CoAl(1 1 0) surfaces were published [5,6]. Blum et al. [6] state that a LEED pattern taken from the CoAl(1 0 0) surface after annealing at 1300 K exhibits the (1 1) structure and a weak c(H2 3H2)-like superstructure. On the basis of LEED I(E) spectra analyses and ab initio calculations, it was suggested that an origin of the superstructure is Co antisite atoms within the topmost layer of Al, in
˚. addition, Co atoms are outward in respect to Al atoms at 0.08 A The structure of the top layer of CoAl(1 0 0) was treated as a mixture of Al (70%) and Co (30%) atoms (‘‘antisite defects’’). Based on the EELS and AES data, Rose et al. [7,8] believe that an Al-rich surface forms on CoAl(1 0 0) after annealing of the clean CoAl(1 0 0) at 1400 K. The aforementioned studies are of prime importance to understanding initial stages of oxidation, i.e. formation of Al–O bonding. During oxidation, metallic spices move from a bulk to an interface, oxygen might also diffuse into the bulk and, eventually, oxide forms on the surface. To find the cause of the step flow, we refer to [9]. To our knowledge, it is the only study that has analyzed the step flow on the surface of intermetallic alloys. The authors claim that the terraces regrow on an oxidized NiAl(1 0 0) surface at 1200 K. It is believed that at early stages of oxidation, oxygen is chemisorbed with the metallic surface. A strong ionic connection between Al+3 and O2 ions forms within amorphous oxide. The ionic connection presumes a shorter distance between Al and O ions, thus an Al atom is pulled out of a CoAl matrix giving rise to the step flow. The present work shows in more details the step flow investigated under different experimental conditions.
2. Experimental * Corresponding author. Tel.: +372 620 3358; fax: +372 620 3196. E-mail addresses: [email protected], [email protected] (V. Podgursky). ä Passed away on 8 May 2004. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.018
The investigation was carried out in a UHV apparatus equipped with a cylindrical mirror analyzer (CMA) for AES, LEED optics, an HREEL spectrometer and a home made
V. Podgursky et al. / Applied Surface Science 253 (2006) 1796–1800
Besoke type STM. The base pressure was better than 5 1011 mbar. The CoAl(1 0 0) single crystal was mechanically polished and cleaned in UHV by cycles of Ar+ sputtering (2 mA, 1 kV) and heated at 1470 K for 10 min. The LEED pattern of the clean CoAl(1 0 0) surface showed a (1 1) structure. The clean CoAl(1 0 0) surface was oxidized by 1 L of oxygen at room temperature, followed by annealing up to 1400 K in the step of 100 K for 2 min and EEL spectrum was recorded at each step. Two types of experiments were carried out. First, the clean CoAl(1 0 0) surface was oxidized by 5 L of oxygen at room temperature, after that the sample was annealed at 900 K for 4 min, followed by additional annealing at 1150 K for 2 min. Second, the regrowth of terraces was also investigated at high temperature, namely, 0.5, 0.7, 3, and 5 L of oxygen was deposited at 1150 K. The partial pressure of O2 was different for various exposures, in the case of 0.5 L, it was 5 109 mbar and for 0.7, 1, 3 and 5 L (108) mbar, respectively. The STM measurements were performed in a constant current topography (CCT) mode at room temperature. The sample was grounded during the STM measurement. 3. Results and discussion The present work is restricted to the discussion of the step flow investigated by STM and LEED. More detailed data concerning the structure and growth of oxide on CoAl(1 0 0) investigated by AES, LEED, STM and HREELS can be found in our studies [8,10,11]. Based on thermodynamics a formation of Al2O3 is more favourable than Co oxides, indeed a heat of formation of Al2O3 is DHf = 1675 kJ mol1, which is higher than for CoO (238 kJ mol1) and Co3O4 (891 kJ mol1) [12]. The EEL spectra taken from alumina grown after deposition of 1 L at room temperature and subsequently annealed are shown in Fig. 1. Observed losses at 420, 630 and 900 cm1 are in good
Fig. 1. EEL spectra taken from the clean CoAl(1 0 0) and alumina/CoAl(1 0 0). The oxide was grown by deposition of 1 L of O2 at room temperature, followed by annealing, see text. All spectra are normalized to the elastic peak. The loss at 275 cm1 corresponds to the Al mode of the CoAl(1 0 0) clean surface.
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Fig. 2. STM image of the clean CoAl(1 0 0) surface (1.7 V; 1 nA).
agreement with a number of work dealing with the growth of alumina on various substrates [1,13–15]. In addition, the EEL spectra taken from the oxide grown on CoAl(1 0 0) by deposition of low oxygen exposure at high temperatures are akin to mentioned above [10]. From the similarity of the frequency of the losses found in our experiments, with those in literature, we conclude that only alumina is formed on CoAl(1 0 0). Fig. 2 shows a STM image of the clean CoAl(1 0 0) surface ˚ 6800 A ˚ taken after the cleaning with a scan width of 6800 A procedure, as described above. Terraces are well-defined, with smooth sharp edges. Fig. 3A and B shows alumina grown on the CoAl(1 0 0) surface after deposition of 5 L of oxygen at room temperature, followed by annealing at 900 K for 4 min and 1150 K for 2 min. The terraces are denoted by Tr.1, Tr.2, Tr.3 and Tr.4 in Fig. 3A, respectively. As compared to the clean CoAl(1 0 0), dramatic alterations of the surface, were observed. For instance, the edge of Tr.3 underwent a substantial modification against the primarily shape. Bias voltages of 0.35 V (Fig. 3A) and 4.22 V (Fig. 3B) were chosen to show the distribution of oxide along the surface. At the positive voltage, electrons tunnel from oxide to a STM tip, and at the negative one, the tunnelling of the electrons from the tip to the empty states of alumina and the CoAl(1 0 0) surface sets. It results in different manifestations of the ordered oxide, see an abbreviation of oxide in Fig. 3A and B. Oxide strikes emerge as trenches in Fig. 3A, in contrast to Fig. 3B, as protrusions. Two characteristic peculiarities can be found within oxide, denoted by a hollow and an island. The hollows and islands are rectangular-like shaped trenches and protrusions, respectively. They should be distinguished from the ordered oxide, because of independence on the bias voltage, albeit the manifestation deteriorating, in the case of negative voltage. The step flow was also observed at high temperature oxidation, see Figs. 4–7. STM images were taken after deposition of 0.5, 0.7, 3.0 and 5.0 L at 1150 K, respectively.
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Fig. 5. STM images of alumina layer grown on the CoAl(1 0 0) after 0.7 L of O2 at 1150 K. The tunnelling parameters are U = 2.5 V and I = 0.2 nA, and in the case of higher resolution image are U = 1.7 V and I = 0.1 nA. The higher resolution image (B) is a part of the image (A), which is marked by a black square in (A). Fig. 3. STM images of alumina layer on the CoAl(1 0 0) oxidized by 5 L at room temperature and annealed at 900 K for 4 min following at 1150 K for 2 min. The tunnelling parameters are in: (A) U = 0.35 V, I = 0.4 nA and (B) U = 4.22 V, I = 0.4 nA, respectively.
The deposition of 0.5 L resulted in the surface shown in Fig. 4A and B. The traces of the ordered oxide are clearly seen, while the steps of the CoAl(1 0 0) surface are hardly recognizable. A layer experiencing the regrowth can be imaged as a net of parts shrinking ‘‘old’’ CoAl(1 0 0) surface and that mentioned above
hollows looks like being formed within a layer located under the regrowing layer. In contrast to 0.5 L, after 0.7 L of O2, the terraces are clearly recognizable, however, they posses saw-like edges oriented along [0 0 1] and [0 1 0] directions, see Fig. 5A and B. The hollows and at least one island are visible, and on analogy with 0.5 L, the hollows are formed under the layer experiencing the step flow. The high resolution image shows a preferable nucleation of amorphous oxide clusters close to the
Fig. 4. STM image of alumina layer grown on the CoAl(1 0 0) after 0.5 L of O2 at 1150 K. The tunnelling parameters are U = 2.2 V and I = 0.3 nA.
Fig. 6. STM images of alumina layer grown on the CoAl(1 0 0) after 3 L of O2 at 1150 K. The tunnelling parameters are U = 2.5 V and I = 0.2 nA.
V. Podgursky et al. / Applied Surface Science 253 (2006) 1796–1800
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Fig. 7. STM images of alumina layer grown on the CoAl(1 0 0) after 5 L of O2 at 1150 K. The tunnelling parameters are U = 1.81 V and I = 0.2 nA. The higher resolution image (B) is a part of the image (A), which is marked by a black square in (A).
step of the terraces, and the ordered oxide stripes grow outwards from these areas. In agreement with the deposition of oxide at room temperature with subsequent annealing, the appearance of the hollows does not depend on the bias voltage, which proves their distinction from the ordered oxide. The next two images taken after the deposition of 3 and 5 L of oxygen exposures, respectively, show reasonably similar structures, see Figs. 6 and 7. Indeed, in the case of 3 L, the terraces start to regrow in single either [0 0 1] or [0 1 0] directions and not in both, see Fig. 6A. Finally, after 5 L of
Fig. 8. LEED pattern taken after annealing of alumina layer on CoAl(1 0 0) grown by deposition of 3 L of O2 at 1150 K.
Fig. 9. Calculated LEED pattern represents the LEED pattern taken from oxide grown after 3 L of O2 at 1150 K. The small filled circles correspond to the Bragg ˚. reflections of the clean CoAl(1 0 0) surface with the lattice constant of 2.86 A The large hollow circles correspond to the (2 1) superstructure reproduced by ˚, a single rectangular domain with the lattice constants of 2.86 and 5.8 A respectively. The small hollow circles correspond to the (8 1) superstructure reproduced by a single rectangular domain with the lattice constants of 2.86 and ˚ , respectively. 23.36 A
oxygen exposure, again a parity between the regrowth along the [0 0 1] and [0 1 0] directions is nearly recovered, see Fig. 7. This phenomenon was observed also by means of LEED. Fig. 8 shows a LEED pattern taken at 70 eV from the oxide layer grown after the deposition of 3 L at 1150 K on the CoAl(1 0 0) surface. It should be noted that the LEED pattern taken after the deposition of 5 L possesses similar intensities [11]. The intensity of the pattern depends on the crystallographic directions, in other words, scattering centres are preferably distributed within one domain. A simulation shown in Fig. 9 might prove such a suggestion. The LEED pattern can be represented by a composition of three different structures, namely, a (1 1) structure of CoAl(1 0 0) with the lattice ˚ , and (2 1) and (8 1) superstructures of constant of 2.86 A alumina. The (2 1) superstructure is reproduced by a single ˚ , which domain with lattice constants of 2.92 and 5.64 A corresponds to monoclinic u oxide [13]. For clarity, the second domain rotated to 908 is not shown, although weak spots corresponding to this domain can be found in the LEED pattern. The (8 1) superstructure could be reproduced by the single ˚, domain with two lattice vectors of 2.92 and 23.36 A ˚ between the two closest respectively. The distance of 24 A white rods is depicted in Figs. 6B and 7B. Unfortunately, due to an insufficient resolution, the distance in the perpendicular direction cannot be measured. The superstructures marked in ˚ between the Figs. 6B and 7B with the distances of 12 and 48 A white rods would result in diffraction spots located between the
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spots corresponding to the (8 1) superstructure, forming solid lines (or strikes) in the LEED pattern. Blum et al. [6] refer to a local deviation from stoichiometrical composition within the CoAl(1 0 0) surface. Therefore, a number of oxidation regimes could be expected and the step flow could advance with different scenarios, depending on the variation of chemical composition along the CoAl(1 0 0) surface. Thus, the observed defect-like islands and hollows could probably form at such specific places. Interestingly, independently of the O2 partial pressure, the islands appear often close to the edges of the regrowing terraces, see Figs. 3–5. Thus, properties of CoAl(1 0 0) might play a significant role during the regrowth. 4. Conclusion
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
The oxidation of the CoAl(1 0 0) surface at 1150 K leads to the regrowth of the terraces. The step flow shows a dependence on the oxygen exposure. Seemingly, to explain the step flow in detail interplay between the temperature, initial termination of the surface, and the partial pressure and exposure of oxygen should be taken into account.
[12] [13] [14] [15]
R. Franchy, Surf. Sci. Rep. 38 (2000). H.L. Davis, J.R. Noonan, Phys. Rev. Lett. 54 (1985) 566. J.R. Noonan, H.L. Davis, Phys. Rev. Lett. 59 (1987) 1714. R.-P. Blum, D. Ahlbehrendt, H. Niehus, Surf. Sci. 366 (1996) 107. V. Blum, C. Rath, G.R. Castro, M. Kottcke, L. Hammer, K. Heinz, Surf. Rev. Lett. 3 (1996) 1409. V. Blum, L. Hammer, Ch. Schmidt, W. Wieckhorst, S. Mu¨ller, K. Heinz, Phys. Rev. Lett. 89 (2002) 266102. V. Rose, V. Podgursky, I. Costina, R. Franchy, Surf. Sci. 541 (2003) 128. V. Rose, V. Podgursky, I. Costina, R. Franchy, H. Ibach, Surf. Sci. 577 (2005) 139. R.-P. Blum, D. Ahlbehrendt, H. Niehus, Surf. Sci. 396 (1998) 176. V. Podgursky, V. Rose, I. Costina, R. Franchy, Oxidation of CoAl(1 0 0), in preparation. V. Podgursky, V. Rose, I. Costina, R. Franchy, The coexistence of g(g0 ) and u alumina observed by STM and LEED on top of oxide layer grown on CoAl(1 0 0), Appl. Surf. Sci., doi:10.1016/j.apsusc.2005.10.066. D.R. Lide (Ed.), Handbook of Chemistry and Physics, 75th ed., CRC, London, 1996. P. Gassmann, R. Franchy, H. Ibach, Surf. Sci. 319 (1994) 95. R. Franchy, J. Masuch, P. Gassmann, Appl. Surf. Sci. 93 (1996) 317. V. Podgursky, I. Costina, R. Franchy, Appl. Surf. Sci. 206 (2003) 29.
Step flow observed on top of oxidized CoAl(1 0 0) surface V. Podgursky a,*, V. Rose b, J. Costina b, R. Franchy b,ä a
Department of Materials Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia b Institut fu¨r Schichten und Grenzfla¨chen, ISG 3, Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany Received 13 June 2005; received in revised form 2 March 2006; accepted 7 March 2006 Available online 19 April 2006
Abstract Clean and oxidized surfaces of CoAl(1 0 0) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunnelling microscopy (STM). The regrowth or step flow of terraces was observed at 1150 K. The correlation between the growth of oxide and the step flow on the CoAl(1 0 0) surface is discussed in this paper. # 2006 Elsevier B.V. All rights reserved. Keywords: Oxidation; Alumina; Step flow; STM; LEED
1. Introduction Bulk and surface properties of Al2O3 present great interest due to their role in coatings, heterogeneous catalysts, non-linear optics and microelectronics. A crystalline Al2O3 can be grown on the surfaces of intermetallic alloys [1]. A long-range order, low conductivity, no charging effects on the surface, reproducibility of a crystal structure, and thickness are attractive properties of thin Al2O3 films. In the past, low indexed surfaces of NiAl [2–4] were intensively investigated to determine the bulk-termination of single crystal surfaces. Blum et al. [4] claim that the most stable surface of NiAl(1 0 0) after annealing at 800 K is Al terminated with a superstructure indexed by c(H2 3H2)R458, it is altered to Ni terminated, with a (1 1) structure after annealing at 1400 K. Recently, a limited number of studies concerning clean CoAl(1 0 0) and CoAl(1 1 0) surfaces were published [5,6]. Blum et al. [6] state that a LEED pattern taken from the CoAl(1 0 0) surface after annealing at 1300 K exhibits the (1 1) structure and a weak c(H2 3H2)-like superstructure. On the basis of LEED I(E) spectra analyses and ab initio calculations, it was suggested that an origin of the superstructure is Co antisite atoms within the topmost layer of Al, in
˚. addition, Co atoms are outward in respect to Al atoms at 0.08 A The structure of the top layer of CoAl(1 0 0) was treated as a mixture of Al (70%) and Co (30%) atoms (‘‘antisite defects’’). Based on the EELS and AES data, Rose et al. [7,8] believe that an Al-rich surface forms on CoAl(1 0 0) after annealing of the clean CoAl(1 0 0) at 1400 K. The aforementioned studies are of prime importance to understanding initial stages of oxidation, i.e. formation of Al–O bonding. During oxidation, metallic spices move from a bulk to an interface, oxygen might also diffuse into the bulk and, eventually, oxide forms on the surface. To find the cause of the step flow, we refer to [9]. To our knowledge, it is the only study that has analyzed the step flow on the surface of intermetallic alloys. The authors claim that the terraces regrow on an oxidized NiAl(1 0 0) surface at 1200 K. It is believed that at early stages of oxidation, oxygen is chemisorbed with the metallic surface. A strong ionic connection between Al+3 and O2 ions forms within amorphous oxide. The ionic connection presumes a shorter distance between Al and O ions, thus an Al atom is pulled out of a CoAl matrix giving rise to the step flow. The present work shows in more details the step flow investigated under different experimental conditions.
2. Experimental * Corresponding author. Tel.: +372 620 3358; fax: +372 620 3196. E-mail addresses: [email protected], [email protected] (V. Podgursky). ä Passed away on 8 May 2004. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.018
The investigation was carried out in a UHV apparatus equipped with a cylindrical mirror analyzer (CMA) for AES, LEED optics, an HREEL spectrometer and a home made
V. Podgursky et al. / Applied Surface Science 253 (2006) 1796–1800
Besoke type STM. The base pressure was better than 5 1011 mbar. The CoAl(1 0 0) single crystal was mechanically polished and cleaned in UHV by cycles of Ar+ sputtering (2 mA, 1 kV) and heated at 1470 K for 10 min. The LEED pattern of the clean CoAl(1 0 0) surface showed a (1 1) structure. The clean CoAl(1 0 0) surface was oxidized by 1 L of oxygen at room temperature, followed by annealing up to 1400 K in the step of 100 K for 2 min and EEL spectrum was recorded at each step. Two types of experiments were carried out. First, the clean CoAl(1 0 0) surface was oxidized by 5 L of oxygen at room temperature, after that the sample was annealed at 900 K for 4 min, followed by additional annealing at 1150 K for 2 min. Second, the regrowth of terraces was also investigated at high temperature, namely, 0.5, 0.7, 3, and 5 L of oxygen was deposited at 1150 K. The partial pressure of O2 was different for various exposures, in the case of 0.5 L, it was 5 109 mbar and for 0.7, 1, 3 and 5 L (108) mbar, respectively. The STM measurements were performed in a constant current topography (CCT) mode at room temperature. The sample was grounded during the STM measurement. 3. Results and discussion The present work is restricted to the discussion of the step flow investigated by STM and LEED. More detailed data concerning the structure and growth of oxide on CoAl(1 0 0) investigated by AES, LEED, STM and HREELS can be found in our studies [8,10,11]. Based on thermodynamics a formation of Al2O3 is more favourable than Co oxides, indeed a heat of formation of Al2O3 is DHf = 1675 kJ mol1, which is higher than for CoO (238 kJ mol1) and Co3O4 (891 kJ mol1) [12]. The EEL spectra taken from alumina grown after deposition of 1 L at room temperature and subsequently annealed are shown in Fig. 1. Observed losses at 420, 630 and 900 cm1 are in good
Fig. 1. EEL spectra taken from the clean CoAl(1 0 0) and alumina/CoAl(1 0 0). The oxide was grown by deposition of 1 L of O2 at room temperature, followed by annealing, see text. All spectra are normalized to the elastic peak. The loss at 275 cm1 corresponds to the Al mode of the CoAl(1 0 0) clean surface.
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Fig. 2. STM image of the clean CoAl(1 0 0) surface (1.7 V; 1 nA).
agreement with a number of work dealing with the growth of alumina on various substrates [1,13–15]. In addition, the EEL spectra taken from the oxide grown on CoAl(1 0 0) by deposition of low oxygen exposure at high temperatures are akin to mentioned above [10]. From the similarity of the frequency of the losses found in our experiments, with those in literature, we conclude that only alumina is formed on CoAl(1 0 0). Fig. 2 shows a STM image of the clean CoAl(1 0 0) surface ˚ 6800 A ˚ taken after the cleaning with a scan width of 6800 A procedure, as described above. Terraces are well-defined, with smooth sharp edges. Fig. 3A and B shows alumina grown on the CoAl(1 0 0) surface after deposition of 5 L of oxygen at room temperature, followed by annealing at 900 K for 4 min and 1150 K for 2 min. The terraces are denoted by Tr.1, Tr.2, Tr.3 and Tr.4 in Fig. 3A, respectively. As compared to the clean CoAl(1 0 0), dramatic alterations of the surface, were observed. For instance, the edge of Tr.3 underwent a substantial modification against the primarily shape. Bias voltages of 0.35 V (Fig. 3A) and 4.22 V (Fig. 3B) were chosen to show the distribution of oxide along the surface. At the positive voltage, electrons tunnel from oxide to a STM tip, and at the negative one, the tunnelling of the electrons from the tip to the empty states of alumina and the CoAl(1 0 0) surface sets. It results in different manifestations of the ordered oxide, see an abbreviation of oxide in Fig. 3A and B. Oxide strikes emerge as trenches in Fig. 3A, in contrast to Fig. 3B, as protrusions. Two characteristic peculiarities can be found within oxide, denoted by a hollow and an island. The hollows and islands are rectangular-like shaped trenches and protrusions, respectively. They should be distinguished from the ordered oxide, because of independence on the bias voltage, albeit the manifestation deteriorating, in the case of negative voltage. The step flow was also observed at high temperature oxidation, see Figs. 4–7. STM images were taken after deposition of 0.5, 0.7, 3.0 and 5.0 L at 1150 K, respectively.
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Fig. 5. STM images of alumina layer grown on the CoAl(1 0 0) after 0.7 L of O2 at 1150 K. The tunnelling parameters are U = 2.5 V and I = 0.2 nA, and in the case of higher resolution image are U = 1.7 V and I = 0.1 nA. The higher resolution image (B) is a part of the image (A), which is marked by a black square in (A). Fig. 3. STM images of alumina layer on the CoAl(1 0 0) oxidized by 5 L at room temperature and annealed at 900 K for 4 min following at 1150 K for 2 min. The tunnelling parameters are in: (A) U = 0.35 V, I = 0.4 nA and (B) U = 4.22 V, I = 0.4 nA, respectively.
The deposition of 0.5 L resulted in the surface shown in Fig. 4A and B. The traces of the ordered oxide are clearly seen, while the steps of the CoAl(1 0 0) surface are hardly recognizable. A layer experiencing the regrowth can be imaged as a net of parts shrinking ‘‘old’’ CoAl(1 0 0) surface and that mentioned above
hollows looks like being formed within a layer located under the regrowing layer. In contrast to 0.5 L, after 0.7 L of O2, the terraces are clearly recognizable, however, they posses saw-like edges oriented along [0 0 1] and [0 1 0] directions, see Fig. 5A and B. The hollows and at least one island are visible, and on analogy with 0.5 L, the hollows are formed under the layer experiencing the step flow. The high resolution image shows a preferable nucleation of amorphous oxide clusters close to the
Fig. 4. STM image of alumina layer grown on the CoAl(1 0 0) after 0.5 L of O2 at 1150 K. The tunnelling parameters are U = 2.2 V and I = 0.3 nA.
Fig. 6. STM images of alumina layer grown on the CoAl(1 0 0) after 3 L of O2 at 1150 K. The tunnelling parameters are U = 2.5 V and I = 0.2 nA.
V. Podgursky et al. / Applied Surface Science 253 (2006) 1796–1800
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Fig. 7. STM images of alumina layer grown on the CoAl(1 0 0) after 5 L of O2 at 1150 K. The tunnelling parameters are U = 1.81 V and I = 0.2 nA. The higher resolution image (B) is a part of the image (A), which is marked by a black square in (A).
step of the terraces, and the ordered oxide stripes grow outwards from these areas. In agreement with the deposition of oxide at room temperature with subsequent annealing, the appearance of the hollows does not depend on the bias voltage, which proves their distinction from the ordered oxide. The next two images taken after the deposition of 3 and 5 L of oxygen exposures, respectively, show reasonably similar structures, see Figs. 6 and 7. Indeed, in the case of 3 L, the terraces start to regrow in single either [0 0 1] or [0 1 0] directions and not in both, see Fig. 6A. Finally, after 5 L of
Fig. 8. LEED pattern taken after annealing of alumina layer on CoAl(1 0 0) grown by deposition of 3 L of O2 at 1150 K.
Fig. 9. Calculated LEED pattern represents the LEED pattern taken from oxide grown after 3 L of O2 at 1150 K. The small filled circles correspond to the Bragg ˚. reflections of the clean CoAl(1 0 0) surface with the lattice constant of 2.86 A The large hollow circles correspond to the (2 1) superstructure reproduced by ˚, a single rectangular domain with the lattice constants of 2.86 and 5.8 A respectively. The small hollow circles correspond to the (8 1) superstructure reproduced by a single rectangular domain with the lattice constants of 2.86 and ˚ , respectively. 23.36 A
oxygen exposure, again a parity between the regrowth along the [0 0 1] and [0 1 0] directions is nearly recovered, see Fig. 7. This phenomenon was observed also by means of LEED. Fig. 8 shows a LEED pattern taken at 70 eV from the oxide layer grown after the deposition of 3 L at 1150 K on the CoAl(1 0 0) surface. It should be noted that the LEED pattern taken after the deposition of 5 L possesses similar intensities [11]. The intensity of the pattern depends on the crystallographic directions, in other words, scattering centres are preferably distributed within one domain. A simulation shown in Fig. 9 might prove such a suggestion. The LEED pattern can be represented by a composition of three different structures, namely, a (1 1) structure of CoAl(1 0 0) with the lattice ˚ , and (2 1) and (8 1) superstructures of constant of 2.86 A alumina. The (2 1) superstructure is reproduced by a single ˚ , which domain with lattice constants of 2.92 and 5.64 A corresponds to monoclinic u oxide [13]. For clarity, the second domain rotated to 908 is not shown, although weak spots corresponding to this domain can be found in the LEED pattern. The (8 1) superstructure could be reproduced by the single ˚, domain with two lattice vectors of 2.92 and 23.36 A ˚ between the two closest respectively. The distance of 24 A white rods is depicted in Figs. 6B and 7B. Unfortunately, due to an insufficient resolution, the distance in the perpendicular direction cannot be measured. The superstructures marked in ˚ between the Figs. 6B and 7B with the distances of 12 and 48 A white rods would result in diffraction spots located between the
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spots corresponding to the (8 1) superstructure, forming solid lines (or strikes) in the LEED pattern. Blum et al. [6] refer to a local deviation from stoichiometrical composition within the CoAl(1 0 0) surface. Therefore, a number of oxidation regimes could be expected and the step flow could advance with different scenarios, depending on the variation of chemical composition along the CoAl(1 0 0) surface. Thus, the observed defect-like islands and hollows could probably form at such specific places. Interestingly, independently of the O2 partial pressure, the islands appear often close to the edges of the regrowing terraces, see Figs. 3–5. Thus, properties of CoAl(1 0 0) might play a significant role during the regrowth. 4. Conclusion
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
The oxidation of the CoAl(1 0 0) surface at 1150 K leads to the regrowth of the terraces. The step flow shows a dependence on the oxygen exposure. Seemingly, to explain the step flow in detail interplay between the temperature, initial termination of the surface, and the partial pressure and exposure of oxygen should be taken into account.
[12] [13] [14] [15]
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