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AFM tip-induced ripple pattern on AIII-BV semiconductor surfaces
Applied Surface Science 254 (2008) 5431–5434
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
AFM tip-induced ripple pattern on AIII-BV semiconductor surfaces B. Such *, F. Krok, M. Szymonski Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 February 2008 Received in revised form 21 February 2008 Accepted 21 February 2008 Available online 26 February 2008
Modification of c(8x2) InSb(0 0 1) surface induced by prolonged scanning with an atomic force microscope tip has been investigated. The experiment performed with loads of few tens of nanoNewtons resulted in creation of ripples perpendicular to the fast scan direction. It was found that terrace edges are acting as initial instabilities leading to development of the ripple pattern. As a result, information about initial surface topography is preserved in the ripple amplitude, even so the final height of the ripples and their periodicity are determined by the tip curvature. ß 2008 Elsevier B.V. All rights reserved.
Keywords: AFM Wear InSb PACS: 62.20.Qp 68.37.Ps 81.40.Pq
1. Introduction Atomic force microscope (AFM) [1] is not only known as a powerful device for surface imaging with nanometer resolution but also as a promising nanostructuring tool. Tip-induced surface patterning has been applied to various materials including semiconductor surfaces, however, exerting high load forces (hundreds of nanoNewtons or larger) resulted in displacement of a large number of atoms at a time [2–5]. Atomic-size features in AFM friction images, such as ‘stick-slip’ phenomenon [6], have been recognized for long time, but deeper insight into the process of tip-induced wear and modification of crystal surfaces on that scale could be obtained only recently [7–10]. Nevertheless, complete understanding of the process is still far from being established. An AFM tip moving over the surface induces displacement of the sample material. It has been reported that prolonged scratching of the surface of different materials, such as polymers [11–14], ionic crystals and metals [8], and adsorbed gold particles on the polymer surface [15] resulted in the formation of periodic ripples. Moreover, the process not only seems to be universal for
* Corresponding author. E-mail address: [email protected] (B. Such). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.067
various materials, but also similar structures can be seen in nature in very different length-scales, since similar ripple pattern covers sand dunes, used rails [16], can result from water jet cutting [17,18], or be created on surfaces due to ion irradiation [19]. In this paper, we would like to clarify whether there are similarities in tip-induced modifications between above-mentioned materials and a covalent semiconductor surface, such as InSb(0 0 1). In contrast to metals and alkali halides that compound has a directional covalent bond between constituents and additionally its (0 0 1) surface is anisotropic due to c(8x2) reconstruction [20–22]. AFM gives a unique opportunity of registering topography changes in real time, and therefore, it gives insight into the very early stages of modification as well as into the kinetics of the process. 2. Experimental The experiment was performed in a vacuum system consisting of three interconnected chambers (sample preparation, surface analysis, scanning probe microscope) with base pressure below 2 10 10 Torr. The samples can be prepared in situ and transferred between the chambers by magnetically coupled linear transfers in UHV. The system was equipped with a standard set of tools for surface preparation and diagnostics, including ion and electron guns, a hemispherical energy analyzer for Auger electron
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B. Such et al. / Applied Surface Science 254 (2008) 5431–5434
spectroscopy, and a 4-grid low energy electron diffractometer for surface structure analysis (LEED). In one of the chambers a scanning probe microscope (Park Scientific Instruments VP2) was installed, for this particular experiment operating in contact AFM mode. Commercially available silicon piezoresistive cantilevers were used as probes. The spring constant of the cantilevers in normal direction was 1 N/m as specified by their suppliers. The signal from piezoresistive cantilevers was obtained by measuring the change in resistivity of a circuit prefabricated on the cantilever arm. The resistivity changed, if the cantilever was deflected but unlike in the optical detection scheme it was impossible to distinguish normal cantilever deflection from torsional deflection. Since cantilever torsional spring constant was not known, both measurements and modification scans were performed in the direction along the cantilever. Therefore, force images collected during modification processes contain both vertical and lateral forces. However, as it will be discussed later, we argue that it was vertical force which was dominant. In order to avoid possible effects of excitation due to too large gain, all surface modifications were performed with a very slow feedback loop maintaining the average tip-surface force over a scanline. Therefore, it was possible to record subsequent images of tip-surface force (force images), depicting development of the ripples. Topographic images prior and after the modification were performed with the same tip in topographic mode (i.e., with a fast feedback loop) and with normal force almost an order of magnitude smaller. All modifications were performed on either 1 mm 1 mm or 2 mm 2 mm frame. Density of scanning lines was always 256 lines/1 mm, which gave a line at every 3.9 nm. That was well below projected tip size, and consequently, an assumption that the tip modifies homogeneously all the surface of the frame instead of drawing parallel lines was well justified. In all the images presented in the paper fast scan direction is vertical and parallel to a cantilever, as mentioned above. Collecting a single image means that every line was scanned twice—‘up’ and ‘down’. Therefore, when different stages of modification development are denoted by the number of collected image, the real number of tip scans over every line of the image was two times larger. InSb(0 0 1) epi-ready wafer, purchased from Kelpin Crystals, was used as a sample. The crystal was inserted into vacuum system without any prior chemical treatment. After degassing, the sample was subjected to Ar+ ion bombardment (ion energy - 700 eV, average current - 0.5 mA/cm2, angle 608 off normal, sample temperature 720 K) and annealing for several hours at 720 K. Typically, a few such cycles were sufficient to obtain a wellordered, reconstructed c(8x2) InSb(0 0 1) surface [22]. 3. Results and discussion After the preparation procedure, described in the previous section, the InSb(0 0 1) surface exhibited large, atomically flat terraces separated by mainly monoatomic steps running preferentially along [1 1 0] and [1 1 0] directions. Atomic structure of c(8x2) InSb(0 0 1) surface is strongly anisotropic and comprises atomic rows running along [1 1 0] direction separated by four surface lattice vectors and forming 4x1 symmetry. Indium dimers located in the second bi-layer of the crystal lead to formation of the c(8x2) symmetry [20–22]. A typical topographic image of ripple structure resulting from prolonged scanning of the surface is presented in Fig. 1. The modified area of size 1 mm 1 mm is covered by periodic ripple structure, with ripples perpendicular to the fast scan direction. The square area modified by the tip is surrounded by a frame of accumulated material, which is often aligned in ripple structure
Fig. 1. A topographical AFM image of a modified frame. Fast scan direction in both modification and imaging is vertical. Greyscale corresponds to 7 nm, set point was 3 nN.
along the fast scan direction. However, within the precision of our height measurement, average height of the interior of the modified area does not change during scanning. Creation of the surrounding frame, therefore, is due to the uncontrolled tip behavior at turning points rather, than due to continuous removal of material from the interior area. A series of force images collected during the modification process is presented in Fig. 2. In Fig. 2a there is a topographic image of the surface prior to the modification process with the area of interest marked with a square. Following the recording of the image an AFM microscope tip was zoomed into the marked area, the loading force increased to 37 nN, and the feedback loop gain decreased. Then, the area was scanned continuously and the force images were recorded. In the first scan (Fig. 2b) the flat surface is depicted with clear view of terrace edges (compare topography shown in Fig. 2a). The scanning had no effect on the topography for quite a long time, apart from the creation of troughs at the tip turning points, as seen in Fig. 2c showing 44th scan of the surface. It is not until scan 65 (Fig. 2d) when the first surface modifications within the frame appear. The terrace edges are no longer flat, but there is clear increase of the force sensed by the cantilever on their upper side, and simultaneously depressions in the terraces along the edges can be noticed. The apparent depth of the depressions is similar to the apparent height of a terrace in a force image, i.e., 0.27 nN. In scan 79 (Fig. 2e), clear force modulation around the former step edge is visible, similarly to the area around tip turning points, where modulation is developed, too. On the flat terrace there are now faint traces of created ripples. They become clear in Fig. 2f, depicting the scan 86 and are growing quickly, in the following scans (Fig. 2g - scan no. 93, Fig. 2h - scan no. 100). The diagram shown in Fig. 3 is composed of data obtained by analysis of force images recorded during scanning and it sums up the evolution of the ripple amplitude. In a force image total force acting on a cantilever is recorded. Due to geometry of the experiment that signal includes both normal force (responsible for imaging a topographic relief) and lateral forces arising from friction. However, we claim that normal (topographic) forces are main source of contrast in force images. There are two arguments
B. Such et al. / Applied Surface Science 254 (2008) 5431–5434
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Fig. 2. A series of images showing modification process: (a) a topographic image (set point 7 nN) prior to the modification; an area which will be modified is marked by a square; (b) a force image showing 1st modification scan with loading force of 37 nN; (c) an image showing 44th modification frame; (d) an image showing 65th frame; troughs at turning point are developed and the first ripples at step edge start to be visible; (e) an image showing 79th frame; (f) 86th frame; (g) 93rd frame and (h) 100th frame. Greyscales were adjusted in order to enhance surface features and they correspond to 1.15 nm for (a), 0.82 nN for (b), 1.7 nN for (c), 2.7 nN for (d), 2.7 nN for (e), 3.2 nN for (f), 4.3 nN for (g) and 4.4 nN for (h).
supporting that statement. Firstly, careful inspection of the force image showing ripples and the subsequently taken topographic image (i.e., with a fast feedback loop) reveals that both show the same features. The topographic image is much less sensitive to lateral forces due to much smaller load forces applied and action of the feedback loop. The friction force is expected to be shifted with respect to the topographic relief and should be inversed while scanning in opposite directions, as shown by Socoliuc et al. [8]. That is not found for force images presented in our work, therefore, it suggests that vertical forces are dominant. The second argument is based on the analysis of cross-sections (not shown) of force images taken over the same line in ‘top to bottom’ and
Fig. 3. Evolution of height of the ripples during modification process. Squares show the height of a ripple created on former step edge, circles mark the height of ripples created on terrace. A dotted line shows an apparent height of a monolayer step in force image, equal to 0.27 nN.
‘bottom to top’ directions. The presence of friction forces induces hysteresis between those two lines, which is much smaller, however, than the modulation due to ripples. Based on both arguments, we are convinced that force images reflect directly surface topography. Although for different experiments, the onset of wear, i.e., appearance of the first ripples happened at various moments, the general wear behavior was repetitive and similar to the one shown in Fig. 3. At first, for several scans, there was hardly any modification visible (apart from the troughs at the tip turning points) and the original step edges were clearly seen. Then, some instabilities occurred on the surface, usually at step edges, and ripples started to grow rapidly until the ripple amplitude achieved a saturation level. However, the proper measurement of the height evolution for such a long time was virtually impossible due to inevitable tip changes occurring during prolonged scanning and affecting both the apparent ripple height and their evolution. Observations by Socoliuc et al. [8] suggest that the final relation between the ripple wavelength is related to tip curvature. That claim was impossible to verify in our experiment, however, the analysis of profiles of fully developed ripples gave the tip curvature estimation of the order of 200 nm, which is plausible for a blunt tip used for surface modification. The height evolution in Fig. 3 is presented separately for ripples appearing at a terrace edge and those created at a flat surface. Ripples at the terrace edge start earlier than those on the flat surface, however, their further evolution follows the same pattern. Despite that the surface undergoes a considerable modification and created structures are as high as several layers of unperturbed crystal, information about initial surface morphology is preserved during the tip-induced modification. A modified 2 mm 2 mm area is presented in Fig. 4a. The contour scheme of the initial terrace edges is superimposed into the image. It can be seen that the ripple pattern correlates with original edges. In the areas where ripples are parallel to the edges the pattern periodicity is alternated and a ripple ridge coincides with the edge. On the other hand, if a ripple is approximately perpendicular to the edge direction, the ripple
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Fig. 4. (a) A modified frame with a scheme of initial step edge positions marked by white lines. Note the correspondence of ripple pattern to initial topography. (b) A frame showing initial stage of surface modification. Note damaged terrace in the lower part of the image with material accumulation on the line of the edge and pits created in the terrace above. (c) A ripple pattern created on a surface with crystallographic directions rotated with respect to the cantilever. Note that the highest slanting ripple follows the line of initial step edge.
amplitude is significantly higher. That corroborates well with the idea of step edges acting as initial instabilities allowing for ripple development. Similar conclusion has been reached by Filleter and co-workers [23] who examined thin layers of KBr grown on Cu(1 0 0). They found that the most significant wear is observed at the borders between Cu and KBr covered areas. Additionally, edges of KBr islands grown on a thin KBr film were also found to be unstable during AFM scanning, in contrast to a smooth KBr film covering Cu steps in carpet-like fashion. However, the first defects on a flat terrace can be created when ripples on step edges are not formed yet. A topographic image of a sample area taken after the first stages of modification is presented in Fig. 4b. The terrace in the lower part of modified frame is already strongly damaged and there is some material accumulation on the former terrace edge. It is clear that the first and the highest ripple is going to be created there. Moreover, a large neighboring terrace is not flat anymore. Pits of monolayer depth formed by a scanning tip are aligned roughly perpendicular to the fast scan direction. It is clear that future ripple morphology is defined at the very beginning of the process. It is not clear yet whether the damage of the flat surface arises due to intrinsic defects present on the surface, or it is induced by stress distribution due to interaction between the tip and the surface and, possibly, a ripple forming at the step edge. The alignment of the pits supports the latter case. As described above, initial stages of ripple creation involve displacement of very limited number of atoms. The c(8x2) InSb surface is strongly anisotropic, comprising rows of atoms running in [1 1 0] direction. It has been shown, that in case of growth of thin layers on that surface, the anisotropy has a profound effect on the adlayer arrangement, since the rows facilitate diffusion of particles along them, while the movement across the rows is hindered [24,25]. All the experiments described above were performed for fast scan movement parallel to [1 1 0] direction, i.e., the ripples were created along the reconstruction rows. To verify a possible effect of crystallographic orientation, additional experiments were performed on the samples with various orientations with respect to the cantilever scan direction. A typical example of the resulting pattern is presented in Fig. 4c. Despite that at the first glance the surface morphology looks different, the process do not differ significantly from modification by scanning along [1 1 0] direction. The main difference stems from the fact that typically directions of step edges are parallel to main crystallographic axis, so they do not coincide with the direction of ripples. However, over the flat areas the ripples are perpendicular to the tip movement, indicating that the ripple orientation is strictly defined by the direction of the tip movement.
4. Conclusions We have investigated the evolution of ripples created at the surface of c(8x2) InSb(0 0 1) by prolonged scanning of AFM tip with loads of few tens of nanoNewtons. After considerably long time when the surface is not affected, the ripple growth is initiated typically at terrace edges and subsequently the process is spread over the entire scanned area. Our results indicate that the maximum height and wavelength of the ripples are likely determined by the tip curvature. Since the step edges constitute initial points of ripple growth some information about the sample original morphology is preserved until very late stages of modification. However, anisotropic reconstruction of c(8x2) InSb(0 0 1) surface does not affect the final topography of the scanned area.
References [1] G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930. [2] R. Magno, B.R. Bennett, Appl. Phys. Lett. 70 (1997) 1855. [3] J. Cortes Rosa, M. Wendel, H. Lorenz, J.P. Kotthaus, M. Thomas, H. Kroemer, Appl. Phys. Lett. 73 (1998) 2684. [4] H.W. Schumacher, U.F. Keyser, U. Zeitler, R.J. Haug, K. Eberl, Appl. Phys. Lett. 75 (1999) 1107. [5] J. Regul, U.F. Keyser, M. Paesler, F. Holhs, U. Zeitler, R.J. Haug, A. Malave, E. Oesterschulze, D. Reuter, A.D. Wieck, Appl. Phys. Lett. 81 (2002) 2023. [6] C.M. Mate, G.M. McClelland, R. Erlandsson, S. Chiang, Phys. Rev. Lett. 59 (1987) 1942. [7] E. Gnecco, R. Bennewitz, E. Meyer, Phys. Rev. Lett. 88 (2002) 215501. [8] A. Socoliuc, E. Gnecco, R. Bennewitz, E. Meyer, Phys. Rev. B 68 (2003) 115416. [9] T. Filleter, S. Maier, R. Bennewitz, Phys. Rev. B 73 (2006) 155433. [10] M. Urbakh, A.E. Filippov, Presented at 6th ESF Nanotribology Workshop, Santa Margherita di Pula, 2007. [11] O.M. Leung, M.C. Goh, Science 255 (1992) 64. [12] Z. Elkaakour, J.P. Aime, T. Bouhacina, C. Odin, T. Masuda, Phys. Rev. Lett. 73 (1994) 3231. [13] X.P. Wang, M.M.T. Loy, X. Xiao, Nanotechnology 13 (2002) 478. [14] B.D. Beake, P.H. Shipway, G.J. Leggett, Wear 256 (2004) 118. [15] M. Andersson, A. Iline, F. Steitz, F. Traeger, Appl. Phys. A 68 (1999) 609. [16] E. Wild, L. Wang, B. Hasse, T. Wroblewski, G. Goerigk, A. Pyzalla, Wear 254 (2003) 876. [17] R. Friedrich, G. Radons, T. Ditzinger, A. Henning, Phys. Rev. B 85 (2000) 4884. [18] R.T. Deam, E. Lemma, D.H. Ahmed, Wear 257 (2004) 877. [19] D. Datta, S.R. Bhattacharyya, T.K. Chini, M.K. Sanyal, Nucl. Instr. Meth. B 193 (2002) 596. [20] S.-H. Lee, W. Moritz, M. Scheffler, Phys. Rev. Lett. 85 (2000) 3890. [21] C. Kumpf, L.D. Marks, D. Ellis, D. Smilgies, E. Landemark, M. Nielsen, R. Feidenhans’l, J. Zegenhagen, O. Bunk, J.H. Zeysing, Y. Su, R.L. Johnson, Phys. Rev. Lett. 86 (2001) 3586. [22] J.J. Kolodziej, B. Such, M. Szymon´ski, Phys. Rev. Lett. 90 (2003) 226101. [23] T. Filleter, W. Paul, R. Bennewitz, Phys. Rev. B 77 (2008) 035430. [24] J.J. Kolodziej, B. Such, P. Czuba, F. Krok, P. Piatkowski, M. Szymonski, Surf. Sci. 506 (2002) 12. [25] N. Papageorgiou, Y. Ferro, J.M. Layet, L. Giovanelli, A.J. Mayne, G. Dujardin, H. Oughaddou, G. Le Lay, Appl. Phys. Lett. 82 (2003) 2518.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
AFM tip-induced ripple pattern on AIII-BV semiconductor surfaces B. Such *, F. Krok, M. Szymonski Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 February 2008 Received in revised form 21 February 2008 Accepted 21 February 2008 Available online 26 February 2008
Modification of c(8x2) InSb(0 0 1) surface induced by prolonged scanning with an atomic force microscope tip has been investigated. The experiment performed with loads of few tens of nanoNewtons resulted in creation of ripples perpendicular to the fast scan direction. It was found that terrace edges are acting as initial instabilities leading to development of the ripple pattern. As a result, information about initial surface topography is preserved in the ripple amplitude, even so the final height of the ripples and their periodicity are determined by the tip curvature. ß 2008 Elsevier B.V. All rights reserved.
Keywords: AFM Wear InSb PACS: 62.20.Qp 68.37.Ps 81.40.Pq
1. Introduction Atomic force microscope (AFM) [1] is not only known as a powerful device for surface imaging with nanometer resolution but also as a promising nanostructuring tool. Tip-induced surface patterning has been applied to various materials including semiconductor surfaces, however, exerting high load forces (hundreds of nanoNewtons or larger) resulted in displacement of a large number of atoms at a time [2–5]. Atomic-size features in AFM friction images, such as ‘stick-slip’ phenomenon [6], have been recognized for long time, but deeper insight into the process of tip-induced wear and modification of crystal surfaces on that scale could be obtained only recently [7–10]. Nevertheless, complete understanding of the process is still far from being established. An AFM tip moving over the surface induces displacement of the sample material. It has been reported that prolonged scratching of the surface of different materials, such as polymers [11–14], ionic crystals and metals [8], and adsorbed gold particles on the polymer surface [15] resulted in the formation of periodic ripples. Moreover, the process not only seems to be universal for
* Corresponding author. E-mail address: [email protected] (B. Such). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.067
various materials, but also similar structures can be seen in nature in very different length-scales, since similar ripple pattern covers sand dunes, used rails [16], can result from water jet cutting [17,18], or be created on surfaces due to ion irradiation [19]. In this paper, we would like to clarify whether there are similarities in tip-induced modifications between above-mentioned materials and a covalent semiconductor surface, such as InSb(0 0 1). In contrast to metals and alkali halides that compound has a directional covalent bond between constituents and additionally its (0 0 1) surface is anisotropic due to c(8x2) reconstruction [20–22]. AFM gives a unique opportunity of registering topography changes in real time, and therefore, it gives insight into the very early stages of modification as well as into the kinetics of the process. 2. Experimental The experiment was performed in a vacuum system consisting of three interconnected chambers (sample preparation, surface analysis, scanning probe microscope) with base pressure below 2 10 10 Torr. The samples can be prepared in situ and transferred between the chambers by magnetically coupled linear transfers in UHV. The system was equipped with a standard set of tools for surface preparation and diagnostics, including ion and electron guns, a hemispherical energy analyzer for Auger electron
5432
B. Such et al. / Applied Surface Science 254 (2008) 5431–5434
spectroscopy, and a 4-grid low energy electron diffractometer for surface structure analysis (LEED). In one of the chambers a scanning probe microscope (Park Scientific Instruments VP2) was installed, for this particular experiment operating in contact AFM mode. Commercially available silicon piezoresistive cantilevers were used as probes. The spring constant of the cantilevers in normal direction was 1 N/m as specified by their suppliers. The signal from piezoresistive cantilevers was obtained by measuring the change in resistivity of a circuit prefabricated on the cantilever arm. The resistivity changed, if the cantilever was deflected but unlike in the optical detection scheme it was impossible to distinguish normal cantilever deflection from torsional deflection. Since cantilever torsional spring constant was not known, both measurements and modification scans were performed in the direction along the cantilever. Therefore, force images collected during modification processes contain both vertical and lateral forces. However, as it will be discussed later, we argue that it was vertical force which was dominant. In order to avoid possible effects of excitation due to too large gain, all surface modifications were performed with a very slow feedback loop maintaining the average tip-surface force over a scanline. Therefore, it was possible to record subsequent images of tip-surface force (force images), depicting development of the ripples. Topographic images prior and after the modification were performed with the same tip in topographic mode (i.e., with a fast feedback loop) and with normal force almost an order of magnitude smaller. All modifications were performed on either 1 mm 1 mm or 2 mm 2 mm frame. Density of scanning lines was always 256 lines/1 mm, which gave a line at every 3.9 nm. That was well below projected tip size, and consequently, an assumption that the tip modifies homogeneously all the surface of the frame instead of drawing parallel lines was well justified. In all the images presented in the paper fast scan direction is vertical and parallel to a cantilever, as mentioned above. Collecting a single image means that every line was scanned twice—‘up’ and ‘down’. Therefore, when different stages of modification development are denoted by the number of collected image, the real number of tip scans over every line of the image was two times larger. InSb(0 0 1) epi-ready wafer, purchased from Kelpin Crystals, was used as a sample. The crystal was inserted into vacuum system without any prior chemical treatment. After degassing, the sample was subjected to Ar+ ion bombardment (ion energy - 700 eV, average current - 0.5 mA/cm2, angle 608 off normal, sample temperature 720 K) and annealing for several hours at 720 K. Typically, a few such cycles were sufficient to obtain a wellordered, reconstructed c(8x2) InSb(0 0 1) surface [22]. 3. Results and discussion After the preparation procedure, described in the previous section, the InSb(0 0 1) surface exhibited large, atomically flat terraces separated by mainly monoatomic steps running preferentially along [1 1 0] and [1 1 0] directions. Atomic structure of c(8x2) InSb(0 0 1) surface is strongly anisotropic and comprises atomic rows running along [1 1 0] direction separated by four surface lattice vectors and forming 4x1 symmetry. Indium dimers located in the second bi-layer of the crystal lead to formation of the c(8x2) symmetry [20–22]. A typical topographic image of ripple structure resulting from prolonged scanning of the surface is presented in Fig. 1. The modified area of size 1 mm 1 mm is covered by periodic ripple structure, with ripples perpendicular to the fast scan direction. The square area modified by the tip is surrounded by a frame of accumulated material, which is often aligned in ripple structure
Fig. 1. A topographical AFM image of a modified frame. Fast scan direction in both modification and imaging is vertical. Greyscale corresponds to 7 nm, set point was 3 nN.
along the fast scan direction. However, within the precision of our height measurement, average height of the interior of the modified area does not change during scanning. Creation of the surrounding frame, therefore, is due to the uncontrolled tip behavior at turning points rather, than due to continuous removal of material from the interior area. A series of force images collected during the modification process is presented in Fig. 2. In Fig. 2a there is a topographic image of the surface prior to the modification process with the area of interest marked with a square. Following the recording of the image an AFM microscope tip was zoomed into the marked area, the loading force increased to 37 nN, and the feedback loop gain decreased. Then, the area was scanned continuously and the force images were recorded. In the first scan (Fig. 2b) the flat surface is depicted with clear view of terrace edges (compare topography shown in Fig. 2a). The scanning had no effect on the topography for quite a long time, apart from the creation of troughs at the tip turning points, as seen in Fig. 2c showing 44th scan of the surface. It is not until scan 65 (Fig. 2d) when the first surface modifications within the frame appear. The terrace edges are no longer flat, but there is clear increase of the force sensed by the cantilever on their upper side, and simultaneously depressions in the terraces along the edges can be noticed. The apparent depth of the depressions is similar to the apparent height of a terrace in a force image, i.e., 0.27 nN. In scan 79 (Fig. 2e), clear force modulation around the former step edge is visible, similarly to the area around tip turning points, where modulation is developed, too. On the flat terrace there are now faint traces of created ripples. They become clear in Fig. 2f, depicting the scan 86 and are growing quickly, in the following scans (Fig. 2g - scan no. 93, Fig. 2h - scan no. 100). The diagram shown in Fig. 3 is composed of data obtained by analysis of force images recorded during scanning and it sums up the evolution of the ripple amplitude. In a force image total force acting on a cantilever is recorded. Due to geometry of the experiment that signal includes both normal force (responsible for imaging a topographic relief) and lateral forces arising from friction. However, we claim that normal (topographic) forces are main source of contrast in force images. There are two arguments
B. Such et al. / Applied Surface Science 254 (2008) 5431–5434
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Fig. 2. A series of images showing modification process: (a) a topographic image (set point 7 nN) prior to the modification; an area which will be modified is marked by a square; (b) a force image showing 1st modification scan with loading force of 37 nN; (c) an image showing 44th modification frame; (d) an image showing 65th frame; troughs at turning point are developed and the first ripples at step edge start to be visible; (e) an image showing 79th frame; (f) 86th frame; (g) 93rd frame and (h) 100th frame. Greyscales were adjusted in order to enhance surface features and they correspond to 1.15 nm for (a), 0.82 nN for (b), 1.7 nN for (c), 2.7 nN for (d), 2.7 nN for (e), 3.2 nN for (f), 4.3 nN for (g) and 4.4 nN for (h).
supporting that statement. Firstly, careful inspection of the force image showing ripples and the subsequently taken topographic image (i.e., with a fast feedback loop) reveals that both show the same features. The topographic image is much less sensitive to lateral forces due to much smaller load forces applied and action of the feedback loop. The friction force is expected to be shifted with respect to the topographic relief and should be inversed while scanning in opposite directions, as shown by Socoliuc et al. [8]. That is not found for force images presented in our work, therefore, it suggests that vertical forces are dominant. The second argument is based on the analysis of cross-sections (not shown) of force images taken over the same line in ‘top to bottom’ and
Fig. 3. Evolution of height of the ripples during modification process. Squares show the height of a ripple created on former step edge, circles mark the height of ripples created on terrace. A dotted line shows an apparent height of a monolayer step in force image, equal to 0.27 nN.
‘bottom to top’ directions. The presence of friction forces induces hysteresis between those two lines, which is much smaller, however, than the modulation due to ripples. Based on both arguments, we are convinced that force images reflect directly surface topography. Although for different experiments, the onset of wear, i.e., appearance of the first ripples happened at various moments, the general wear behavior was repetitive and similar to the one shown in Fig. 3. At first, for several scans, there was hardly any modification visible (apart from the troughs at the tip turning points) and the original step edges were clearly seen. Then, some instabilities occurred on the surface, usually at step edges, and ripples started to grow rapidly until the ripple amplitude achieved a saturation level. However, the proper measurement of the height evolution for such a long time was virtually impossible due to inevitable tip changes occurring during prolonged scanning and affecting both the apparent ripple height and their evolution. Observations by Socoliuc et al. [8] suggest that the final relation between the ripple wavelength is related to tip curvature. That claim was impossible to verify in our experiment, however, the analysis of profiles of fully developed ripples gave the tip curvature estimation of the order of 200 nm, which is plausible for a blunt tip used for surface modification. The height evolution in Fig. 3 is presented separately for ripples appearing at a terrace edge and those created at a flat surface. Ripples at the terrace edge start earlier than those on the flat surface, however, their further evolution follows the same pattern. Despite that the surface undergoes a considerable modification and created structures are as high as several layers of unperturbed crystal, information about initial surface morphology is preserved during the tip-induced modification. A modified 2 mm 2 mm area is presented in Fig. 4a. The contour scheme of the initial terrace edges is superimposed into the image. It can be seen that the ripple pattern correlates with original edges. In the areas where ripples are parallel to the edges the pattern periodicity is alternated and a ripple ridge coincides with the edge. On the other hand, if a ripple is approximately perpendicular to the edge direction, the ripple
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Fig. 4. (a) A modified frame with a scheme of initial step edge positions marked by white lines. Note the correspondence of ripple pattern to initial topography. (b) A frame showing initial stage of surface modification. Note damaged terrace in the lower part of the image with material accumulation on the line of the edge and pits created in the terrace above. (c) A ripple pattern created on a surface with crystallographic directions rotated with respect to the cantilever. Note that the highest slanting ripple follows the line of initial step edge.
amplitude is significantly higher. That corroborates well with the idea of step edges acting as initial instabilities allowing for ripple development. Similar conclusion has been reached by Filleter and co-workers [23] who examined thin layers of KBr grown on Cu(1 0 0). They found that the most significant wear is observed at the borders between Cu and KBr covered areas. Additionally, edges of KBr islands grown on a thin KBr film were also found to be unstable during AFM scanning, in contrast to a smooth KBr film covering Cu steps in carpet-like fashion. However, the first defects on a flat terrace can be created when ripples on step edges are not formed yet. A topographic image of a sample area taken after the first stages of modification is presented in Fig. 4b. The terrace in the lower part of modified frame is already strongly damaged and there is some material accumulation on the former terrace edge. It is clear that the first and the highest ripple is going to be created there. Moreover, a large neighboring terrace is not flat anymore. Pits of monolayer depth formed by a scanning tip are aligned roughly perpendicular to the fast scan direction. It is clear that future ripple morphology is defined at the very beginning of the process. It is not clear yet whether the damage of the flat surface arises due to intrinsic defects present on the surface, or it is induced by stress distribution due to interaction between the tip and the surface and, possibly, a ripple forming at the step edge. The alignment of the pits supports the latter case. As described above, initial stages of ripple creation involve displacement of very limited number of atoms. The c(8x2) InSb surface is strongly anisotropic, comprising rows of atoms running in [1 1 0] direction. It has been shown, that in case of growth of thin layers on that surface, the anisotropy has a profound effect on the adlayer arrangement, since the rows facilitate diffusion of particles along them, while the movement across the rows is hindered [24,25]. All the experiments described above were performed for fast scan movement parallel to [1 1 0] direction, i.e., the ripples were created along the reconstruction rows. To verify a possible effect of crystallographic orientation, additional experiments were performed on the samples with various orientations with respect to the cantilever scan direction. A typical example of the resulting pattern is presented in Fig. 4c. Despite that at the first glance the surface morphology looks different, the process do not differ significantly from modification by scanning along [1 1 0] direction. The main difference stems from the fact that typically directions of step edges are parallel to main crystallographic axis, so they do not coincide with the direction of ripples. However, over the flat areas the ripples are perpendicular to the tip movement, indicating that the ripple orientation is strictly defined by the direction of the tip movement.
4. Conclusions We have investigated the evolution of ripples created at the surface of c(8x2) InSb(0 0 1) by prolonged scanning of AFM tip with loads of few tens of nanoNewtons. After considerably long time when the surface is not affected, the ripple growth is initiated typically at terrace edges and subsequently the process is spread over the entire scanned area. Our results indicate that the maximum height and wavelength of the ripples are likely determined by the tip curvature. Since the step edges constitute initial points of ripple growth some information about the sample original morphology is preserved until very late stages of modification. However, anisotropic reconstruction of c(8x2) InSb(0 0 1) surface does not affect the final topography of the scanned area.
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