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An investigation of the YB66(100) surface
Surface Science 419 (1998) 38–47
An investigation of the YB (100) surface 66 J. Gu¨nster a,*, H. Kawanowa b, T. Tanaka b, R. Souda b a Department of Chemistry, Texas A&M University, P.O. Box 300012, College Station, TX 77842-3012, USA b National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan Received 4 May 1998; accepted for publication 28 September 1998
Abstract Scanning tunneling microscopy (STM ), low energy electron diffraction (LEED) and impact collision ion scattering spectroscopy (ICISS) studies were performed on the YB (100) surface. By varying the scanning conditions in STM we were able to identify two 66 ˚ for 4 V positive sample bias and a more complex different surface structures: a regular square lattice with a lattice constant of 11.5 A structure most likely consisting of interconnected B icosahedra for a negative 2 V sample bias. A closer investigation of vacancies 12 and dislocations in the surface square lattice imaged with 4 V positive sample bias indicates that this regular structure does not reflect the ordering of the B supericosahedra. In this context, a surface structure model is offered in which the channels between 156 the giant B supericosahedra, that is, the yttrium atom sites, form the regular square lattice and decomposed B units compose 156 156 the irregular structure imaged with a −2 V sample bias. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Borides; Boron; Ion bombardment; Ion scattering spectroscopy; Low index single crystal surfaces; Surface structure, morphology, roughness and topography; Scanning tunneling microscopy; Yttrium
1. Introduction YB is a boron rich rare-earth metal boride 66 with an extremely complex crystal structure. A schematic of the YB crystal structure, according 66 to a recent investigation by Higashi et al. [1], is shown in Fig. 1. The YB structure is based on a 66 cubic lattice, space group Fm3c, with a lattice ˚ constant of 23.44 A [2–6 ]. Each unit cell contains ca. 1584 boron atoms with 1248 accommodated in B units and the remaining located in inter156 stitial nonicosahedral B and B clusters or, in 48 36 accordance to a more recent study [1], B clusters. 80 Adjacent B units, which are composed of 12 156 B icosahedra bonded to each of the 12 vertices 12 * Corresponding author. Fax: +1 409 8456822; e-mail: [email protected]
Fig. 1. Schematic of the cubic YB crystal structure. 66
˚ apart, of a central B unit, are spaced 11.72 A 12 but due to their different orientation, rotated 90° on an axis perpendicular to the (100) plane, the ˚ . The nonicosahedral lattice constant is 23.44 A
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boron clusters are located together with the yttrium atom sites in fourfold channels between the giant B units. The 48 yttrium atom sites per unit cell 156 ˚ apart, with each appear in pairs spaced 2.55 A pair accommodating ca. one yttrium atom [1,6 ]. Recently Perkins et al. studied the structure of the YB (100) surface by means of scanning tun66 neling microscopy (STM ) [7,8]. This study established the model that the B supericosahedron 156 exist as a stable discrete entity within the YB 66 crystal. This conclusion is drawn from the ˚ square lattice on following observations: a 11.7 A ˚ between terraces terraces, a step height of 11.7 A and step kinks on the scale of complete B units. 156 Furthermore Perkins et al. [7] suggest, with the restriction that additional data are necessary to confirm this particular point, that the B clusters 156 are imaged by STM and that vacancies on the surface are due to an absence of complete B 156 units. However, the goal of this recent study [7] was to confirm that discrete B supericosahedra 156 exist as a basic unit within the YB crystal, but 66 the suggestion that the YB (100) surface is termi66 nated by intact B supericosahedra is not per156 fectly established; therefore we decided to reexamine this surface. The results obtained in this investigation encouraged us to suggest a new interpretation of the STM data reported by Perkins et al. In accordance with the YB structure model, 66 in a (100) plane both the metal atom sites and the B units form a two-dimensional square 156 ˚ . Thus, the fourfold structure lattice of 11.72 A observed on the YB (100) surface can originate 66 either from a two dimensional lattice formed by boron supericosahedra or yttrium atoms. Since only one yttrium atom is accommodated in the near surface site pair aligned perpendicular to the surface (see Fig. 1), one might expect that the lattice formed by yttrium atoms should appear with a high density of vacancies. However, a random distribution of both sites in such a site pair suggests that each site provides nearly the same site energy for yttrium atoms. This picture is valid in an ideal B crystal but not in the vicinity 156 of other interstitial units, that is, nonicosahedral boron clusters and yttrium atoms, or near the surface. It is rather probable that the termination
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of the crystal leads to different site energies between the near surface or upper site relative to the deeper. Thus we expect that close to the (100) surface one of the two sites of the yttrium site pairs is preferentially occupied. The present study provides STM data of the YB (100) surface which 66 give strong evidence that the regular square lattice imaged on this surface with a positive sample bias is formed by yttrium atoms. This suggests a preferential occupation of the near surface yttrium sites. In this context it is unclear whether the near surface sites are really energetically preferred or the appearance of yttrium near the surface is due to the interaction between tip and substrate. In addition, images taken with a negative sample bias suggest that the outermost supericosahedra do not exist as intact units.
2. Experimental The preparation of the YB single crystal has 66 been described previously [9–11]. Briefly, highpurity single crystals of YB were prepared by 66 multiple zone refining using the floating zone method. YB crystals show semiconductor prop66 erties and a room temperature resistivity of several hundred Vcm [4,5]. Specimens for the STM measurements were prepared by ex situ cleavage from the grown crystal and by several cycles of in situ heating to 2100 K. After this treatment, the X-ray photon spectroscopy ( XPS) measurements of the clean surface exhibited yttrium and boron features only. The surface temperature was measured using an infrared pyrometer (Model TR-630, Minolta). For the STM observations, we have used an ultrahigh-vacuum ( UHV ) STM instrument (JSTM4500XT, JEOL) consisting of two interconnected chambers, one for housing the STM and a scanning electron microscope (SEM ) and the other for sample treatment. In the latter there are facilities for XPS and low-energy electron diffraction (LEED). Both chambers are equipped with sample stages allowing a heat treatment of the sample by electron bombardment. After repeated annealing cycles and waiting an appropriate time (ca. 40 min) in order to minimize thermal drift, scanning was carried out with a gap voltage between 2 and 4 V
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(negative/positive bias on the sample) and a current between 0.2 and 0.5 nA in a constant current mode. Probe tips were made from a 0.3 mm polycrystalline tungsten wire by electrochemical etching and were cleaned in situ by heating. The STM piezo-tube expansion coefficients were determined from atomically resolved images of a Si(111)(7×7) surface. Ion scattering spectroscopy (ISS ) measurements were performed in a different UHV chamber (base pressure of 3×10−10 Torr) equipped with an ion beam line, a sample transfer interlock system and LEED. He+ ions were generated by a dischargetype ion source and were mass analyzed by using a Wien Filter. The ion beam was incident upon the surface at a fixed glancing angle of 80° with respect to the surface. Ions scattered through a laboratory angle of 160° were detected with a constant energy resolution of 2 eV in a constant pass energy mode by means of an electrostatic energy analyzer. Care was taken in order to minimize surface damage during the acquisition of ISS data. In this context we investigated the possible occurrence of selective sputtering by prolonged He+ bombardment of the surface.
3. Results As already mentioned the YB surface has been 66 prepared by ex situ cleavage from the grown crystal and by several cycles of in situ heating to 2100 K. During the annealing procedure the formation of macroscopic clusters in a length-scale between 20 and 60 mm has been observed in a temperature range below 1800 K (see SEM image, Fig. 2). Heating to higher temperatures (up to 2100 K ) results in a disappearance of those structures and finally a flat surface is observed. As shown in the lower left corner of Fig. 2 the approach of the STM tip on those clusters is usually unsuccessful, which suggests that their electronic structure is quite different than that of the substrate. We think that the interaction of oxygen during storage of the surface in air results in the formation of a Y–B-oxide species which agglomerates during annealing of the sample in the observed clusters.
Fig. 2. SEM image of the YB surface after annealing to 66 1800 K. The surface was stored a few months in air after cleavage from the YB crystal. 66
From this observation we deduce that in the near surface region up to a depth of a few micrometers, the structure of the YB crystal is changed during 66 storage in air. This is an important observation since YB is a promising material for the use as 66 a soft X-ray monochromator [12] and thus, changes of the surface properties in a macroscopic scale are highly undesirable. A closer investigation of this phenomena appears necessary. 3.1. STM and LEED results Fig. 3a is a topograph over a 40×20 nm area including two flat terraces separated by a step. Scanning was carried out with a positive sample bias of 4 V and a tunneling current of 0.5 nA. On the flat terraces there are periodic arrays of objects and some defect structures visible. In the ordered ˚ apart, areas, bright spheres spaced 11.5±0.6 A which is half the unit cell length of the YB 66 crystal, form a square lattice. As shown by a single line scan across the step (see Fig. 3b) the step ˚ . These observations are height is ca. 11.2±0.6 A in accordance with a recent STM investigation of the same surface by Perkins et al. [7,8]. In addition, the positioning of the square lattice in the upper terrace relative to the lower reveals, in accordance with the structure model shown in Fig. 1, an on
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˚ ×200 A ˚ ) of the YB (100) surface showing two flat terraces separated by a step. The image was Fig. 3. (a) A STM image (400 A 66 taken at a positive sample bias of 4 V and a tunneling current of 0.1 nA, ca. 40 min after sample preparation. The straight line indicates the position of the line profile shown in (b). Dislocations in the regular surface structure are marked by a d. A surface ˚ 2) (c). The straight line in (c) corresponds to the part of the line scan marked by a circle. defect is shown in the small-scale image (79 A
top stacking of the imaged entities. Further information of the near surface region can be gained by analyzing defects apparent in the regular surface lattice, that is, vacancies and dislocations. As visible in the line scan in Fig. 3b, which is across two defective areas and a step, the absence of a few bright spheres does not necessarily result in the appearance of deep vacancies in the surface. ˚, The measured depth of such a vacancy is ca. 1 A which is in the range of the corrugation induced by the bright spheres on the flat terraces. In the case that a single bright sphere would represent a complete B unit a vacancy depth of ca 1 nm is 156
expected, but was not observed. In the large number of STM images that we have recorded under various scanning conditions such a vacancy always appears as a shallow surface defect. However, we cannot exclude that the shallow appearance of defects is partially due to the convolution of the surface structure with the tip topography. On the other hand, taking the shape of the step apparent in the line scan in Fig. 3b into ˚ deep vacancies consideration we expect at least 3 A for surface defects related to the missing of more than one B unit. Fig. 3c presents a small-scale 156 image of the surface area denoted by a circle in
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Fig. 3b. The lattice defect in this area is related to the absence of five bright spheres. In addition we have taken height profiles across dislocations in the surface lattice. It is found that displaced units, indicated by a d in Fig. 3, do not result in high protrusions in the STM image, which is hardly possible for a dislocated B unit. From 156 these observations we deduce that the bright ˚ square lattice in Fig. 3 spheres forming the 11.5 A are significantly smaller than B units. 156 In order to give a better overview about the topography of the surface shown in Fig. 3, Fig. 4 presents a three-dimensional view of an 40×40 nm area taken at a 4 V positive sample bias on a flat YB (100) terrace. Clearly visible is a small height 66 ˚ of the imaged entities. This modulation of ca 1.5 A modulation can reflect either a real height difference of the imaged protrusions or a change in the local density of states on the surface.
So far, the interpretation of the STM data gives strong evidence that the bright spheres, forming a two-dimensional square lattice on the YB (100) 66 surface, are not boron supericosahedra. However, a more straightforward argument would be possible by imaging the internal structure of single B units or smaller boron cluster, coexisting on 156 the surface with the entities shown in Figs. 3 and 4. For that reason, we have taken images under various scanning conditions in order to identify complete boron supericosahedra but without any success. On the other hand, we found a complex structure on the surface at a 2 V negative sample bias which changes to the regular surface lattice shown in Fig. 3 when a higher negative or positive sample bias is used. In this context, it is noteworthy that the imaging with −2 V was more difficult than scanning conditions with a higher positive sample bias.
˚ 2) of the flat YB (100) surface. The image was taken at a positive sample bias Fig. 4. A STM image (three-dimensional view, 400 A 66 of 4 V and a tunneling current of 0.1 nA, ca. 40 min after sample preparation.
J. Gu¨nster et al. / Surface Science 419 (1998) 38–47
STM images taken with a negative sample bias of 2 V and a tunneling current of 0.5 nA are shown in Figs. 5 and 6. In these images, no square lattice is apparent, on the contrary, a more complex structure is found. At first glance, this structure exhibits no wide range periodicity, and it is difficult to identify single structural elements. In a first step, we compare the lateral density of the imaged entities in Fig. 5 with the scan shown in Fig. 3. An evaluation leads to an approximately two times higher density in Fig. 5. This observation can be explained in different ways: (1) It is possible that the spheres visible in Figs. 5
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and 6 are metal atoms decorating the surface. However, this explanation implies a significantly higher concentration of metal atoms on the surface than the bulk which contradict recent XPS results [7,8] suggesting that the metal concentration near the surface is slightly lower or at least similar to the bulk. (2) Another possibility is that the spheres measured with 2 V negative sample bias are formed by boron. This implies, together with the irregular surface structure, that the boron supericosahedra on the surface do not exist as intact units. However, even
˚ ×200 A ˚ ) of the flat YB (100) surface. The image was taken at a negative sample bias of 2 V and Fig. 5. (a) A STM image (400 A 66 a tunneling current of 0.5 nA, ca. 40 min after sample preparation. The straight line indicates the position of the line profile shown ˚ 2) reveals typical tetragonal elements on the surface. in (b). The small-scale image (c) (3.7 A
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˚ 2) of the flat YB (100) surface. The image was taken at a negative sample bias Fig. 6. STM image (three-dimensional view, 100 A 66 of 2 V and a tunneling current of 0.5 nA, ca. 40 min after sample preparation.
after fragmentation of B units on the surface, 156 some regular structures should be apparent. The straight line in Fig. 5a marks the position of the line profile shown in Fig. 5b. The line profile is taken in a direction parallel to the line profile shown in Fig. 3. Fig. 5b reveals that the surface height modulation has a periodicity of ca 1.1 nm, that is, 5.5 nm divided by five equidistant distinct minima. Since a surface corrugation with a periodicity of 1.1 nm can be found in many line scans (spaced 1.1 nm apart) parallel and perpendicular to the one shown here, the interpretation that the regular surface structure imaged with a positive sample bias interspersed with the more disordered structure shown in Figs. 5 and 6 appears plausible. In addition to this long rage order, preferred binding geometries suggest a strong interaction between the entities imaged with a positive sample bias, for example, five-fold arrangements (see
center of the line in Fig. 5a) and the more complex but symmetric structure shown in Fig. 5c. The LEED image in Fig. 7 taken from the clean YB (100) surface with an electron energy of 30 eV 66 shows a sharp fourfold pattern (see also Fig. 1, Ref. [8]). A comparison with LEED images of ˚ between Si(111) reveals a distance of 23±2 A adjacent scattering centers on the surface. This value is in accordance to the bulk lattice constant in a (100) plane, but it is unclear whether the LEED image reveals the ordering of the boron supericosahedra or the yttrium atoms near the surface. An additional feature of the LEED pattern is the absence of the (h, 0) and (0, k) spots for h and k odd. Despite the fact that the interpretation of the LEED image can not be considered as fully understood, the pattern shows that the surface is characterized by a two-dimensional square lattice with a repeat distance of 2.3 nm.
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Fig. 7. LEED pattern of the clean YB (100) substrate taken with a primary electron energy of 30 eV. 66
3.2. ISS results Fig. 8 presents He-ICISS (impact collision ion scattering spectroscopy) data measured on the YB (100) surface with a primary energy of the 66 He+ projectiles of 1000 eV, an incident angle of 80° with respect to the surface, and a laboratory scattering angle of 160°. The intensity of the backscattered He ions versus their kinetic energy is shown in Fig. 8a. Since the electrostatic analyzer was operated in a constant pass energy mode, the scattered ions were detected with the same efficiency irrespective of the scattered ion energy. Three distinct features are apparent in Fig. 8a: a peak at 830 eV originating from He ions scattered on yttrium, a peak at 219 eV due to scattering on boron and at low kinetic energies a broad feature due to multiple scattering. According to their energetic positions in the spectrum, the features corresponding to boron and yttrium are surface peaks: the measured energies are in accordance with a diatomic collision model. In order to give an estimation of the sputter rate due to He+ impact, Fig. 8b presents the absolute ICISS peak intensities of boron and yttrium versus measure (sputter) time. Taking the different atomic mass
Fig. 8. (a) ICISS spectrum of the clean YB (100) surface. (b) 66 Absolute ICISS yttrium and boron peak intensities versus measure (sputter) time.
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ratios between He projectile and target atoms into consideration, we expect a selective sputtering of boron and thus after a certain sputter time a yttrium enriched surface. In accordance with this model, the yttrium intensity increases versus sputter time. However, the apparent yttrium intensity, even at very short measure times, indicates that yttrium atoms are located in the near surface region. The amount of yttrium relative to boron seen by ICISS can be estimated by taking the different ion scattering cross-sections of boron and yttrium into consideration: the cross-section of yttrium is 12.6 times larger than that for boron. This, together with a B–Y intensity ratio of 4.8 for the shortest measure time in Fig. 8b, leads to an estimated boron to yttrium ratio on the surface of ca 60. Due to the high surface sensitivity of ISS techniques this result gives an estimation of the lateral boron to yttrium ratio near the surface which is, in the error of the measurement, consistent with the bulk value and the XPS data presented by Perkins et al. [8].
4. Discussion The STM images taken with a positive sample bias of 4 V, shown in Figs. 3 and 4, reveal periodic arrays of objects and some defect structures. In ˚ the ordered areas, bright spheres spaced 11.5 A apart form square arrays. Due to the shallow appearance of vacancies in the surface square lattice and the fact that dislocations do not result in high protrusions, we exclude the possibility that the imaged entities are intact boron supericosahedra, that is, B units. STM images showing 156 two terraces separated by a step reveal a three dimensional cubic arrangement of the imaged entities. On the other hand, STM images taken with a negative sample bias of 2 V (see Figs. 5 and 6) reveal a complex structure which changes to the structure shown in Figs. 3 and 4 by switching back to a positive sample bias of 4 V. The greater than two times higher density of structural entities on the more irregular surface suggests that the complex structure imaged with a low negative sample bias originates from a network formed by boron: A surface segregation of yttrium can be excluded,
since our ICISS data show, in accordance with recent XPS results [7,8], that the boron to yttrium ratio near the surface is about the same as the bulk. Furthermore, the size of the imaged spheres and its uniformity in Figs. 5 and 6 suggests that the more disordered surface structure is formed by only one boron species, most likely B icosahedra, 12 the basic unit of amorphous boron [13]. Judging by our data, we think that the B 156 boron supericosahedra do not exist as complete units on the surface. Probably during the surface preparation, a decomposition of boron supericosahedra in the outermost surface layer takes place. In this context, the surface imaged with a negative sample bias of 2 V reveals no compact boron agglomerations compatible with boron supericosahedra but a network of smaller interconnected boron units. It is most likely that these smaller boron clusters are B icosahedra. A more detailed 12 discussion of the observed short range ordering, that is, multiple appearance of specific binding geometries, could provide additional information, but is beyond the framework of the present study. On the other hand, the periodicity of the (100) surface still reveals the periodicity of the bulk especially when we look with a positive sample bias (see Figs. 3 and 4). Since the yttrium atoms exist as cations within the crystal it is rather probable that they appear as bright spheres (high tunneling probability) while imaged with a positive sample bias. Thus one possible explanation for the STM data is that the surface consists of an ordered square array of Y atoms interspersed with the more disordered array of B units with only the 12 former imaged with a +4 V sample bias and only the later imaged with a −2 V bias. However, this does not imply automatically that the metal atoms are located directly on the surface. According to the crystal structure model shown in Fig. 1, the metal atoms are located in channels aligned perpendicular to the (100) surface and it is possible that the STM images ( Figs. 3 and 4) reveal the positioning of these channels between the huge B units (see also Fig. 4 of Ref. [1]). Furthermore 156 we have to take into consideration that an interaction between the negative tip and the yttrium cations can influence the positioning of the metal atoms in those channels and thus relative to the
J. Gu¨nster et al. / Surface Science 419 (1998) 38–47
surface. In a perfect YB crystal, the positions of 66 the metal atoms are well defined, but near the surface, a displacement can be expected. In this context, the slight height variation visible in Fig. 3 can be explained as a variation of the yttrium atom positions relative to the three-dimensional boron network.
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Acknowledgements One of the authors (J. Gu¨nster) thanks the Alexander von Humboldt Foundation for the generous support of his stay at the NIRIM. Critical reading of the manuscript by J. Stultz is gratefully acknowledged.
5. Summary References The structure of the YB (100) surface has been 66 studied by STM together with LEED and ICISS. At different scanning conditions in STM, we were able to identify two different surface structures, that is, a regular square lattice with a lattice ˚ , for 4 V positive sample bias constant of 11.5 A and a more complex structure for 2 V negative sample bias. An investigation of vacancies and dislocations in the imaged square lattice excludes the possibility that the regular surface structure is formed by intact boron supericosahedra. Moreover, STM images together with ICISS data suggest that the surface square lattice reflects the ordering of the yttrium cations on the YB (100) 66 surface. Since the metal atoms are located in channels perpendicular to the surface this does not necessarily mean that yttrium is located directly on the surface. On the other hand, a careful investigation of the more disordered structures imaged with a negative sample bias of 2 V suggest that the B supericosahedra are damaged on the 156 surface and that a boron surface network is formed by interconnected B icosahedra. 12
[1] I. Higashi, K. Kobayashi, T. Tanaka, Y. Ishizawa, J. Solid State Chem. 133 (1997) 16. [2] S.M. Richards, J.S. Kasper, Acta Cryst. Sect. B 25 (1969) 237. [3] A.U. Seybolt, Trans. Am. Soc. Met. 52 (1960) 971. [4] J.S. Kasper, J. Less-Common Metal. 47 (1976) 17. [5] G.A. Slack, D.W. Oliver, G.D. Brower, J.D. Young, J. Phys. Chem. Solids 38 (1977) 45. [6 ] T. Oku, A. Carlsson, L.R. Wallenberg, J.-O. Malm, J.-O. Bovin, I. Higashi, T. Tanaka, Y. Ishizawa, J. Solid State Chem. 135 (1998) 182. [7] C.L. Perkins, M. Trenary, T. Tanaka, Phys. Rev. Lett. 77 (1996) 4772. [8] C.L. Perkins, M. Trenary, T. Tanaka, J. Solid State Chem. 133 (1997) 31. [9] T. Tanaka, S. Otani, Y. Ishizawa, J. Cryst. Growth 73 (1985) 31. [10] T. Tanaka, S. Otani, Y. Ishizawa, J. Cryst. Growth 99 (1990) 994. [11] Y. Kamimura, T. Tanaka, S. Otani, Y. Ishizawa, Z.U. Rek, J. Wong, J. Cryst. Growth 128 (1993) 429. [12] T. Tanaka, T. Aizawa, U. Rowen, Z.U. Rek, Y. Kitajima, I. Higashi, J. Wong, Y. Ishizawa, J. Appl. Cryst. 30 (1997) 87. [13] R.G. Delaplane, U. Dahlborg, W.S. Howels, T. Lundstrøm, J. Non-Crystalline Solids 106 (1988) 66.
An investigation of the YB (100) surface 66 J. Gu¨nster a,*, H. Kawanowa b, T. Tanaka b, R. Souda b a Department of Chemistry, Texas A&M University, P.O. Box 300012, College Station, TX 77842-3012, USA b National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan Received 4 May 1998; accepted for publication 28 September 1998
Abstract Scanning tunneling microscopy (STM ), low energy electron diffraction (LEED) and impact collision ion scattering spectroscopy (ICISS) studies were performed on the YB (100) surface. By varying the scanning conditions in STM we were able to identify two 66 ˚ for 4 V positive sample bias and a more complex different surface structures: a regular square lattice with a lattice constant of 11.5 A structure most likely consisting of interconnected B icosahedra for a negative 2 V sample bias. A closer investigation of vacancies 12 and dislocations in the surface square lattice imaged with 4 V positive sample bias indicates that this regular structure does not reflect the ordering of the B supericosahedra. In this context, a surface structure model is offered in which the channels between 156 the giant B supericosahedra, that is, the yttrium atom sites, form the regular square lattice and decomposed B units compose 156 156 the irregular structure imaged with a −2 V sample bias. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Borides; Boron; Ion bombardment; Ion scattering spectroscopy; Low index single crystal surfaces; Surface structure, morphology, roughness and topography; Scanning tunneling microscopy; Yttrium
1. Introduction YB is a boron rich rare-earth metal boride 66 with an extremely complex crystal structure. A schematic of the YB crystal structure, according 66 to a recent investigation by Higashi et al. [1], is shown in Fig. 1. The YB structure is based on a 66 cubic lattice, space group Fm3c, with a lattice ˚ constant of 23.44 A [2–6 ]. Each unit cell contains ca. 1584 boron atoms with 1248 accommodated in B units and the remaining located in inter156 stitial nonicosahedral B and B clusters or, in 48 36 accordance to a more recent study [1], B clusters. 80 Adjacent B units, which are composed of 12 156 B icosahedra bonded to each of the 12 vertices 12 * Corresponding author. Fax: +1 409 8456822; e-mail: [email protected]
Fig. 1. Schematic of the cubic YB crystal structure. 66
˚ apart, of a central B unit, are spaced 11.72 A 12 but due to their different orientation, rotated 90° on an axis perpendicular to the (100) plane, the ˚ . The nonicosahedral lattice constant is 23.44 A
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boron clusters are located together with the yttrium atom sites in fourfold channels between the giant B units. The 48 yttrium atom sites per unit cell 156 ˚ apart, with each appear in pairs spaced 2.55 A pair accommodating ca. one yttrium atom [1,6 ]. Recently Perkins et al. studied the structure of the YB (100) surface by means of scanning tun66 neling microscopy (STM ) [7,8]. This study established the model that the B supericosahedron 156 exist as a stable discrete entity within the YB 66 crystal. This conclusion is drawn from the ˚ square lattice on following observations: a 11.7 A ˚ between terraces terraces, a step height of 11.7 A and step kinks on the scale of complete B units. 156 Furthermore Perkins et al. [7] suggest, with the restriction that additional data are necessary to confirm this particular point, that the B clusters 156 are imaged by STM and that vacancies on the surface are due to an absence of complete B 156 units. However, the goal of this recent study [7] was to confirm that discrete B supericosahedra 156 exist as a basic unit within the YB crystal, but 66 the suggestion that the YB (100) surface is termi66 nated by intact B supericosahedra is not per156 fectly established; therefore we decided to reexamine this surface. The results obtained in this investigation encouraged us to suggest a new interpretation of the STM data reported by Perkins et al. In accordance with the YB structure model, 66 in a (100) plane both the metal atom sites and the B units form a two-dimensional square 156 ˚ . Thus, the fourfold structure lattice of 11.72 A observed on the YB (100) surface can originate 66 either from a two dimensional lattice formed by boron supericosahedra or yttrium atoms. Since only one yttrium atom is accommodated in the near surface site pair aligned perpendicular to the surface (see Fig. 1), one might expect that the lattice formed by yttrium atoms should appear with a high density of vacancies. However, a random distribution of both sites in such a site pair suggests that each site provides nearly the same site energy for yttrium atoms. This picture is valid in an ideal B crystal but not in the vicinity 156 of other interstitial units, that is, nonicosahedral boron clusters and yttrium atoms, or near the surface. It is rather probable that the termination
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of the crystal leads to different site energies between the near surface or upper site relative to the deeper. Thus we expect that close to the (100) surface one of the two sites of the yttrium site pairs is preferentially occupied. The present study provides STM data of the YB (100) surface which 66 give strong evidence that the regular square lattice imaged on this surface with a positive sample bias is formed by yttrium atoms. This suggests a preferential occupation of the near surface yttrium sites. In this context it is unclear whether the near surface sites are really energetically preferred or the appearance of yttrium near the surface is due to the interaction between tip and substrate. In addition, images taken with a negative sample bias suggest that the outermost supericosahedra do not exist as intact units.
2. Experimental The preparation of the YB single crystal has 66 been described previously [9–11]. Briefly, highpurity single crystals of YB were prepared by 66 multiple zone refining using the floating zone method. YB crystals show semiconductor prop66 erties and a room temperature resistivity of several hundred Vcm [4,5]. Specimens for the STM measurements were prepared by ex situ cleavage from the grown crystal and by several cycles of in situ heating to 2100 K. After this treatment, the X-ray photon spectroscopy ( XPS) measurements of the clean surface exhibited yttrium and boron features only. The surface temperature was measured using an infrared pyrometer (Model TR-630, Minolta). For the STM observations, we have used an ultrahigh-vacuum ( UHV ) STM instrument (JSTM4500XT, JEOL) consisting of two interconnected chambers, one for housing the STM and a scanning electron microscope (SEM ) and the other for sample treatment. In the latter there are facilities for XPS and low-energy electron diffraction (LEED). Both chambers are equipped with sample stages allowing a heat treatment of the sample by electron bombardment. After repeated annealing cycles and waiting an appropriate time (ca. 40 min) in order to minimize thermal drift, scanning was carried out with a gap voltage between 2 and 4 V
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(negative/positive bias on the sample) and a current between 0.2 and 0.5 nA in a constant current mode. Probe tips were made from a 0.3 mm polycrystalline tungsten wire by electrochemical etching and were cleaned in situ by heating. The STM piezo-tube expansion coefficients were determined from atomically resolved images of a Si(111)(7×7) surface. Ion scattering spectroscopy (ISS ) measurements were performed in a different UHV chamber (base pressure of 3×10−10 Torr) equipped with an ion beam line, a sample transfer interlock system and LEED. He+ ions were generated by a dischargetype ion source and were mass analyzed by using a Wien Filter. The ion beam was incident upon the surface at a fixed glancing angle of 80° with respect to the surface. Ions scattered through a laboratory angle of 160° were detected with a constant energy resolution of 2 eV in a constant pass energy mode by means of an electrostatic energy analyzer. Care was taken in order to minimize surface damage during the acquisition of ISS data. In this context we investigated the possible occurrence of selective sputtering by prolonged He+ bombardment of the surface.
3. Results As already mentioned the YB surface has been 66 prepared by ex situ cleavage from the grown crystal and by several cycles of in situ heating to 2100 K. During the annealing procedure the formation of macroscopic clusters in a length-scale between 20 and 60 mm has been observed in a temperature range below 1800 K (see SEM image, Fig. 2). Heating to higher temperatures (up to 2100 K ) results in a disappearance of those structures and finally a flat surface is observed. As shown in the lower left corner of Fig. 2 the approach of the STM tip on those clusters is usually unsuccessful, which suggests that their electronic structure is quite different than that of the substrate. We think that the interaction of oxygen during storage of the surface in air results in the formation of a Y–B-oxide species which agglomerates during annealing of the sample in the observed clusters.
Fig. 2. SEM image of the YB surface after annealing to 66 1800 K. The surface was stored a few months in air after cleavage from the YB crystal. 66
From this observation we deduce that in the near surface region up to a depth of a few micrometers, the structure of the YB crystal is changed during 66 storage in air. This is an important observation since YB is a promising material for the use as 66 a soft X-ray monochromator [12] and thus, changes of the surface properties in a macroscopic scale are highly undesirable. A closer investigation of this phenomena appears necessary. 3.1. STM and LEED results Fig. 3a is a topograph over a 40×20 nm area including two flat terraces separated by a step. Scanning was carried out with a positive sample bias of 4 V and a tunneling current of 0.5 nA. On the flat terraces there are periodic arrays of objects and some defect structures visible. In the ordered ˚ apart, areas, bright spheres spaced 11.5±0.6 A which is half the unit cell length of the YB 66 crystal, form a square lattice. As shown by a single line scan across the step (see Fig. 3b) the step ˚ . These observations are height is ca. 11.2±0.6 A in accordance with a recent STM investigation of the same surface by Perkins et al. [7,8]. In addition, the positioning of the square lattice in the upper terrace relative to the lower reveals, in accordance with the structure model shown in Fig. 1, an on
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˚ ×200 A ˚ ) of the YB (100) surface showing two flat terraces separated by a step. The image was Fig. 3. (a) A STM image (400 A 66 taken at a positive sample bias of 4 V and a tunneling current of 0.1 nA, ca. 40 min after sample preparation. The straight line indicates the position of the line profile shown in (b). Dislocations in the regular surface structure are marked by a d. A surface ˚ 2) (c). The straight line in (c) corresponds to the part of the line scan marked by a circle. defect is shown in the small-scale image (79 A
top stacking of the imaged entities. Further information of the near surface region can be gained by analyzing defects apparent in the regular surface lattice, that is, vacancies and dislocations. As visible in the line scan in Fig. 3b, which is across two defective areas and a step, the absence of a few bright spheres does not necessarily result in the appearance of deep vacancies in the surface. ˚, The measured depth of such a vacancy is ca. 1 A which is in the range of the corrugation induced by the bright spheres on the flat terraces. In the case that a single bright sphere would represent a complete B unit a vacancy depth of ca 1 nm is 156
expected, but was not observed. In the large number of STM images that we have recorded under various scanning conditions such a vacancy always appears as a shallow surface defect. However, we cannot exclude that the shallow appearance of defects is partially due to the convolution of the surface structure with the tip topography. On the other hand, taking the shape of the step apparent in the line scan in Fig. 3b into ˚ deep vacancies consideration we expect at least 3 A for surface defects related to the missing of more than one B unit. Fig. 3c presents a small-scale 156 image of the surface area denoted by a circle in
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Fig. 3b. The lattice defect in this area is related to the absence of five bright spheres. In addition we have taken height profiles across dislocations in the surface lattice. It is found that displaced units, indicated by a d in Fig. 3, do not result in high protrusions in the STM image, which is hardly possible for a dislocated B unit. From 156 these observations we deduce that the bright ˚ square lattice in Fig. 3 spheres forming the 11.5 A are significantly smaller than B units. 156 In order to give a better overview about the topography of the surface shown in Fig. 3, Fig. 4 presents a three-dimensional view of an 40×40 nm area taken at a 4 V positive sample bias on a flat YB (100) terrace. Clearly visible is a small height 66 ˚ of the imaged entities. This modulation of ca 1.5 A modulation can reflect either a real height difference of the imaged protrusions or a change in the local density of states on the surface.
So far, the interpretation of the STM data gives strong evidence that the bright spheres, forming a two-dimensional square lattice on the YB (100) 66 surface, are not boron supericosahedra. However, a more straightforward argument would be possible by imaging the internal structure of single B units or smaller boron cluster, coexisting on 156 the surface with the entities shown in Figs. 3 and 4. For that reason, we have taken images under various scanning conditions in order to identify complete boron supericosahedra but without any success. On the other hand, we found a complex structure on the surface at a 2 V negative sample bias which changes to the regular surface lattice shown in Fig. 3 when a higher negative or positive sample bias is used. In this context, it is noteworthy that the imaging with −2 V was more difficult than scanning conditions with a higher positive sample bias.
˚ 2) of the flat YB (100) surface. The image was taken at a positive sample bias Fig. 4. A STM image (three-dimensional view, 400 A 66 of 4 V and a tunneling current of 0.1 nA, ca. 40 min after sample preparation.
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STM images taken with a negative sample bias of 2 V and a tunneling current of 0.5 nA are shown in Figs. 5 and 6. In these images, no square lattice is apparent, on the contrary, a more complex structure is found. At first glance, this structure exhibits no wide range periodicity, and it is difficult to identify single structural elements. In a first step, we compare the lateral density of the imaged entities in Fig. 5 with the scan shown in Fig. 3. An evaluation leads to an approximately two times higher density in Fig. 5. This observation can be explained in different ways: (1) It is possible that the spheres visible in Figs. 5
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and 6 are metal atoms decorating the surface. However, this explanation implies a significantly higher concentration of metal atoms on the surface than the bulk which contradict recent XPS results [7,8] suggesting that the metal concentration near the surface is slightly lower or at least similar to the bulk. (2) Another possibility is that the spheres measured with 2 V negative sample bias are formed by boron. This implies, together with the irregular surface structure, that the boron supericosahedra on the surface do not exist as intact units. However, even
˚ ×200 A ˚ ) of the flat YB (100) surface. The image was taken at a negative sample bias of 2 V and Fig. 5. (a) A STM image (400 A 66 a tunneling current of 0.5 nA, ca. 40 min after sample preparation. The straight line indicates the position of the line profile shown ˚ 2) reveals typical tetragonal elements on the surface. in (b). The small-scale image (c) (3.7 A
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˚ 2) of the flat YB (100) surface. The image was taken at a negative sample bias Fig. 6. STM image (three-dimensional view, 100 A 66 of 2 V and a tunneling current of 0.5 nA, ca. 40 min after sample preparation.
after fragmentation of B units on the surface, 156 some regular structures should be apparent. The straight line in Fig. 5a marks the position of the line profile shown in Fig. 5b. The line profile is taken in a direction parallel to the line profile shown in Fig. 3. Fig. 5b reveals that the surface height modulation has a periodicity of ca 1.1 nm, that is, 5.5 nm divided by five equidistant distinct minima. Since a surface corrugation with a periodicity of 1.1 nm can be found in many line scans (spaced 1.1 nm apart) parallel and perpendicular to the one shown here, the interpretation that the regular surface structure imaged with a positive sample bias interspersed with the more disordered structure shown in Figs. 5 and 6 appears plausible. In addition to this long rage order, preferred binding geometries suggest a strong interaction between the entities imaged with a positive sample bias, for example, five-fold arrangements (see
center of the line in Fig. 5a) and the more complex but symmetric structure shown in Fig. 5c. The LEED image in Fig. 7 taken from the clean YB (100) surface with an electron energy of 30 eV 66 shows a sharp fourfold pattern (see also Fig. 1, Ref. [8]). A comparison with LEED images of ˚ between Si(111) reveals a distance of 23±2 A adjacent scattering centers on the surface. This value is in accordance to the bulk lattice constant in a (100) plane, but it is unclear whether the LEED image reveals the ordering of the boron supericosahedra or the yttrium atoms near the surface. An additional feature of the LEED pattern is the absence of the (h, 0) and (0, k) spots for h and k odd. Despite the fact that the interpretation of the LEED image can not be considered as fully understood, the pattern shows that the surface is characterized by a two-dimensional square lattice with a repeat distance of 2.3 nm.
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Fig. 7. LEED pattern of the clean YB (100) substrate taken with a primary electron energy of 30 eV. 66
3.2. ISS results Fig. 8 presents He-ICISS (impact collision ion scattering spectroscopy) data measured on the YB (100) surface with a primary energy of the 66 He+ projectiles of 1000 eV, an incident angle of 80° with respect to the surface, and a laboratory scattering angle of 160°. The intensity of the backscattered He ions versus their kinetic energy is shown in Fig. 8a. Since the electrostatic analyzer was operated in a constant pass energy mode, the scattered ions were detected with the same efficiency irrespective of the scattered ion energy. Three distinct features are apparent in Fig. 8a: a peak at 830 eV originating from He ions scattered on yttrium, a peak at 219 eV due to scattering on boron and at low kinetic energies a broad feature due to multiple scattering. According to their energetic positions in the spectrum, the features corresponding to boron and yttrium are surface peaks: the measured energies are in accordance with a diatomic collision model. In order to give an estimation of the sputter rate due to He+ impact, Fig. 8b presents the absolute ICISS peak intensities of boron and yttrium versus measure (sputter) time. Taking the different atomic mass
Fig. 8. (a) ICISS spectrum of the clean YB (100) surface. (b) 66 Absolute ICISS yttrium and boron peak intensities versus measure (sputter) time.
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ratios between He projectile and target atoms into consideration, we expect a selective sputtering of boron and thus after a certain sputter time a yttrium enriched surface. In accordance with this model, the yttrium intensity increases versus sputter time. However, the apparent yttrium intensity, even at very short measure times, indicates that yttrium atoms are located in the near surface region. The amount of yttrium relative to boron seen by ICISS can be estimated by taking the different ion scattering cross-sections of boron and yttrium into consideration: the cross-section of yttrium is 12.6 times larger than that for boron. This, together with a B–Y intensity ratio of 4.8 for the shortest measure time in Fig. 8b, leads to an estimated boron to yttrium ratio on the surface of ca 60. Due to the high surface sensitivity of ISS techniques this result gives an estimation of the lateral boron to yttrium ratio near the surface which is, in the error of the measurement, consistent with the bulk value and the XPS data presented by Perkins et al. [8].
4. Discussion The STM images taken with a positive sample bias of 4 V, shown in Figs. 3 and 4, reveal periodic arrays of objects and some defect structures. In ˚ the ordered areas, bright spheres spaced 11.5 A apart form square arrays. Due to the shallow appearance of vacancies in the surface square lattice and the fact that dislocations do not result in high protrusions, we exclude the possibility that the imaged entities are intact boron supericosahedra, that is, B units. STM images showing 156 two terraces separated by a step reveal a three dimensional cubic arrangement of the imaged entities. On the other hand, STM images taken with a negative sample bias of 2 V (see Figs. 5 and 6) reveal a complex structure which changes to the structure shown in Figs. 3 and 4 by switching back to a positive sample bias of 4 V. The greater than two times higher density of structural entities on the more irregular surface suggests that the complex structure imaged with a low negative sample bias originates from a network formed by boron: A surface segregation of yttrium can be excluded,
since our ICISS data show, in accordance with recent XPS results [7,8], that the boron to yttrium ratio near the surface is about the same as the bulk. Furthermore, the size of the imaged spheres and its uniformity in Figs. 5 and 6 suggests that the more disordered surface structure is formed by only one boron species, most likely B icosahedra, 12 the basic unit of amorphous boron [13]. Judging by our data, we think that the B 156 boron supericosahedra do not exist as complete units on the surface. Probably during the surface preparation, a decomposition of boron supericosahedra in the outermost surface layer takes place. In this context, the surface imaged with a negative sample bias of 2 V reveals no compact boron agglomerations compatible with boron supericosahedra but a network of smaller interconnected boron units. It is most likely that these smaller boron clusters are B icosahedra. A more detailed 12 discussion of the observed short range ordering, that is, multiple appearance of specific binding geometries, could provide additional information, but is beyond the framework of the present study. On the other hand, the periodicity of the (100) surface still reveals the periodicity of the bulk especially when we look with a positive sample bias (see Figs. 3 and 4). Since the yttrium atoms exist as cations within the crystal it is rather probable that they appear as bright spheres (high tunneling probability) while imaged with a positive sample bias. Thus one possible explanation for the STM data is that the surface consists of an ordered square array of Y atoms interspersed with the more disordered array of B units with only the 12 former imaged with a +4 V sample bias and only the later imaged with a −2 V bias. However, this does not imply automatically that the metal atoms are located directly on the surface. According to the crystal structure model shown in Fig. 1, the metal atoms are located in channels aligned perpendicular to the (100) surface and it is possible that the STM images ( Figs. 3 and 4) reveal the positioning of these channels between the huge B units (see also Fig. 4 of Ref. [1]). Furthermore 156 we have to take into consideration that an interaction between the negative tip and the yttrium cations can influence the positioning of the metal atoms in those channels and thus relative to the
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surface. In a perfect YB crystal, the positions of 66 the metal atoms are well defined, but near the surface, a displacement can be expected. In this context, the slight height variation visible in Fig. 3 can be explained as a variation of the yttrium atom positions relative to the three-dimensional boron network.
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Acknowledgements One of the authors (J. Gu¨nster) thanks the Alexander von Humboldt Foundation for the generous support of his stay at the NIRIM. Critical reading of the manuscript by J. Stultz is gratefully acknowledged.
5. Summary References The structure of the YB (100) surface has been 66 studied by STM together with LEED and ICISS. At different scanning conditions in STM, we were able to identify two different surface structures, that is, a regular square lattice with a lattice ˚ , for 4 V positive sample bias constant of 11.5 A and a more complex structure for 2 V negative sample bias. An investigation of vacancies and dislocations in the imaged square lattice excludes the possibility that the regular surface structure is formed by intact boron supericosahedra. Moreover, STM images together with ICISS data suggest that the surface square lattice reflects the ordering of the yttrium cations on the YB (100) 66 surface. Since the metal atoms are located in channels perpendicular to the surface this does not necessarily mean that yttrium is located directly on the surface. On the other hand, a careful investigation of the more disordered structures imaged with a negative sample bias of 2 V suggest that the B supericosahedra are damaged on the 156 surface and that a boron surface network is formed by interconnected B icosahedra. 12
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