- Email: [email protected]
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon
Journal Pre-proof
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon ¨ J. Heggemann, S. Appelfeller, T. Niermann, M. Lehmann, M. Dahne PII: DOI: Reference:
S0039-6028(19)30473-X https://doi.org/10.1016/j.susc.2020.121563 SUSC 121563
To appear in:
Surface Science
Received date: Revised date: Accepted date:
25 June 2019 9 October 2019 3 January 2020
¨ Please cite this article as: J. Heggemann, S. Appelfeller, T. Niermann, M. Lehmann, M. Dahne, Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon, Surface Science (2020), doi: https://doi.org/10.1016/j.susc.2020.121563
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
1 Highlights • Atomic structure of terbium silicide nanowires grown on Si(001) capped by silicon
• Scanning tunneling microscopy plus high-resolution transmission electron microscopy • Room temperature capping by amorphous silicon preserves the nanowire structure • Capping at elevated temperatures results in strong shape transitions • Nanowires can consist of hexagonal or tetragonal TbSi2
STM
HRTEM
TbSi2
[001]
Si cap Si substrate
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon J Heggemann1,2 , S Appelfeller3 , T Niermann2 , M Lehmann2 and M D¨ ahne3 1
Universi¨ at Osnabr¨ uck, Fachbereich Physik, Barbarastr. 7, 49076 Osnabr¨ uck, Germany 2 Institut f¨ ur Optik und Atomare Physik, Technische Universit¨ at Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany 3 Institut f¨ ur Festk¨ orperphysik, Technische Universit¨ at Berlin, Hardenbergstraße 36, 10623 Berlin, Germany E-mail: [email protected] Abstract. In a combined scanning tunneling microscopy and high-resolution transmission electron microscopy study, the internal atomic structure of terbium silicide nanowires on Si(001) capped with silicon is determined. Room temperature capping by amorphous silicon preserves the original nanowire structure, and the nanowires with a usual height of two silicide layers are found to consist of hexagonal TbSi2 with the c-axis in nanowire direction, in contrast to previous assumptions. At larger heights, the nanowires are formed from tetragonal TbSi2 . Capping at elevated temperatures results in a shape transition towards higher and more compact nanowires consisting of hexagonal or tetragonal TbSi2 and a crystalline silicon overlayer that shows stacking faults and twin boundaries starting at the nanowires.
Keywords: rare earth silicide nanowires, embedded nanostructures, atomic structure, scanning tunneling microscopy, high resolution transmission electron microscopy
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 4 1. Introduction For two decades, self-organized rare earth silicide nanowires with high structural quality and high aspect ratio are known to form on the Si(001) surface [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. As an example, Fig. 1 shows scanning-tunneling microscopy (STM) images of terbium silicide nanowires, which are also representative for nanowires formed with other trivalent rare earths because of their chemical similarity. Individual nanowires are typically 2 nm wide and can reach lengths of several 100 nm, while the nanowires tend to form wider bundles, in which they are separated by grooves. It was found that they have a quasi-one-dimensional metallic band structure [11, 12, 13, 14, 15], yielding potential for applications in future nanoelectronic devices and for studying physical phenomena that are exclusive for one-dimensional metals, e.g. the Peierls transition [16, 17]. Because of the surplus of substrate silicon during silicide formation, it is generally assumed that the nanowires consist of the silicon richest silicides, which are the hexagonal and the tetragonal rare earth disilicides [3, 6, 8, 9, 18]. Both silicides are observed in large rare earth silicide islands on Si(001) [19, 20] and are thus also candidates for nanowire formation. They are both metallic and show antiferromagnetic ordering at low temperatures, but there are distinct differences in their detailed properties such as the N´eel temperatures [21, 22, 23]. Their atomic structure models are shown in Fig. 2(a–b) together with the lattice parameters of the bulk terbium disilicides
Figure 1. Overview STM image (sample voltage V = –1.5 V and tunneling
current I = 100 pA) of terbium silicide nanowires on the planar Si(001) surface with a terbium deposition of 0.15 nm followed by annealing at 600 ◦ C. The inset (V = −1.5 V and I = 100 pA) shows a detailed STM image highlighting the bundle formation and the surface structure.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 5 [18], but without showing the silicon vacancies usually present in the volume of the bulk silicides. These vacancies actually lead to an orthorhombic instead of a tetragonal phase, but the structure is usually denoted tetragonal in the rare earth silicide nanowire literature [8]. Based on the arrangement of the silicon atoms in hexagonal and tetragonal TbSi2 , both structures fit well on the Si(001) surface, but there are differences in the lattice mismatch. For the hexagonal structure, there is only a negligible lattice mismatch parallel to the ahex -axis (∆ahex = 0.1%), but a large lattice mismatch along the chex -axis (∆chex = 7.4%). In contrast, the tetragonal structure is characterized by a rather large lattice mismatch along both its atet -axis and its btet -axis, which is also less anisotropic (∆atet = 5.4% and ∆btet = 3.3%). Thus, a rather isotropic growth is expected for tetragonal TbSi2 on Si(001), while hexagonal TbSi2 should form highly anisotropic islands on Si(001). This situation, which is similar for the other trivalent rare earths, has lead to the assumption that the strongly anisotropic nanowires consist of the hexagonal disilicides with the ahex -axis oriented along the nanowires and the chex -axis perpendicular to the nanowires. Independent of the rare earth metal, the resulting structure model is widely accepted [1, 3, 5, 6, 7, 9, 10] and is shown in Fig. 2(c–d). In this already previously proposed structure model, the nanowire is depicted as two silicide layers high with the nanowire surface terminated by silicon dimers. Those dimers may form various reconstructions depending on the rare earth metal [2, 3, 6, 9], but dimer rows along the nanowires are typically observed for TbSi2 (see inset in Fig. 1) [5]. The height of two silicide layers is in accordance with the apparent heights observed in STM and
Figure 2. (a–b) Structure models of (a) the hexagonal TbSi2 bulk structure
and (b) the tetragonal TbSi2 bulk structure [8, 18], but without the usually present vacancies. (c–d) Previously proposed structure of rare earth silicide nanowires, (c) in cross-sectional view and (d) in side view [1, 3, 5].
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 6 the amount of deposited rare earth metal [2, 4, 5, 9]. Due to the low height, a first inconsistency in the argumentation for this previously proposed nanowire structure arises. Taking a closer look at the nanowire building blocks [see areas marked by dotted lines in Fig. 2(c–d)], it cannot be exclusively assigned to the hexagonal or tetragonal TbSi2 structure since both structures show the same atomic arrangement for such few silicide layers [see areas limited by the dotted lines in Fig. 2(a– b)]. Thus, there is no basis for the argumentation in favor of or against a specific TbSi2 structure. The structure of the nanowire surface cannot be used to clarify this issue, since it deviates from both TbSi2 bulk structures due to its reconstruction. In the following, a two silicide layers high structure will always be termed hexagonal since its ahex - and chex -axis are easily distinguished in cross-sectional views, in contrast to the atet - and btet -axis of the tetragonal structure. For the previously proposed structure model a further inconsistency in the argumentation arises, when taking into account that no indications for silicon vacancies in the nanowires were reported, and all illustrations of nanowire structures lack silicon vacancies [1, 4, 5, 9, 10]. However, the lattice parameters of the silicide structures without silicon vacancies certainly differ from the ones of the bulk phases with silicon vacancies, so that the lattice parameters measured for the bulk silicides are not applicable for the nanowires and cannot be used for an argumentation regarding the nanowire structure. Thus, there is a great need for further information on the internal structure of rare earth silicide nanowires on Si(001). In most studies, the nanowires were investigated by STM, a very surface sensitive method, but there was almost no analysis of their internal atomic structure. There are only a few earlier studies using high-resolution transmission electron microscopy (HRTEM) with indications of the tetragonal as well as the hexagonal rare earth disilicide structures, however, only much broader nanostructures with heights larger than two silicide layers were investigated [19, 20, 24, 25, 26]. Later we demonstrated that capping by amorphous silicon is possible, and that considerable shape changes occur when annealing the silicon cap for crystallization, but no detailed data were achieved on the atomic structure [27]. In a recent letter we shortly presented the internal structure of the two silicide layers high nanowires capped with amorphous silicon [28]. In the present work, we report in detail on an STM and HRTEM analysis of TbSi2 nanowires on planar and vicinal Si(001) surfaces, capped with amorphous or crystalline silicon. It is found that upon capping with amorphous silicon at room temperature, the nanowires keep their structure characterized by bundle formation and a typical height of two silicide layers. In particular, the HRTEM data clearly demonstrate that nanowires consist of hexagonal TbSi2 with the chex -axis along the nanowires, in disagreement with the previous assumption of a chex -axis perpendicular to the nanowires. However, three silicide layers high nanowires show a tetragonal TbSi2 structure. Nanowires capped at elevated temperatures, in contrast, undergo strong shape changes resulting in more compact cross sections with either a hexagonal or a tetragonal structure, while the silicon overlayer is crystalline, but often showing stacking faults and twin formations.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 7 2. Experimental details All Si(001) samples were cut from n-type wafers. In addition to planar substrates, vicinal substrates with an offcut of 4◦ towards [110] were used. They were cleaned in ultrahigh vacuum (UHV) by degassing at 600 ◦ C for 12 h and subsequent repeated flash annealing up to 1150 ◦ C. To achieve well ordered surface morphologies, the samples were then slowly cooled down from about 800 ◦ C to room temperature with a rate of about 1 ◦ C/s. The samples were heated by direct current and their temperature was controlled via an infrared pyrometer (constant emissivity setting of 0.67 and accurary ±20 ◦ C). Exemplary STM images of the clean substrates are shown in figure 3. Every pristine clean sample was controlled by STM prior to any nanowire study to exclude effects of surface defects or contaminations, which would be visible at steps or as extended missing rows perpendicular to the Si dimer rows. The nanowires were prepared in UHV by depositing 0.15–0.40 nm terbium at room temperature on the surface and subsequent annealing for 2 min at 500–600 ◦ C. First STM images were recorded after this step. For protection against atmospheric influences and to investigate the structural changes upon capping, the nanowires were then capped in UHV with silicon either at room temperature resulting in an amorphous overlayer or at 300–350 ◦ C in order to achieve a crystalline silicon overgrowth. As the STM data often showed holes in the capping layer, possibly penetrating the full capping layer down to the nanowires, further 3 nm silicon were deposited on these samples at room temperature in order to protect the silicide structures for the HRTEM sample preparation. For the STM experiments, a home-built microscope with a SPECS Nanonis control system was used. The preparation for the cross-sectional HRTEM experiments was a standard face-to-face preparation with mechanical thinning and ion milling. For the HRTEM images, we used the image-aberration corrected FEI Titan 80-300 Berlin Holography Special TEM operating at 200–300 kV.
Figure 3. STM images of pristine (a) planar and (b) vicinal Si(001) substrates
(V = –1.5 V and I = 100 pA for both images).
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 8 3. Results and Discussion The Tb silicide nanowire growth observed in this report is in nice agreement with earlier reports [5]. In the following, the STM and HRTEM data of the nanowire structures at different capping processes will be shown. First, the nanowires capped at room temperature will be discussed, followed by the development of a refined structure model for the nanowires. Then the data of the nanowire capping at elevated temperatures will be presented. 3.1. Nanowire structure after capping with silicon at room temperature Figure 4(a) shows an STM image of nanowires formed by deposition of 0.33 nm terbium on a planar Si(001) surface. It is found that the nanowires form broad bundles covering a large fraction of the surface and that the nanowires have different lengths and widths. In agreement with the presence of two domains of the nominally planar Si(001) surface separated by monoatomic steps, we also observe two domains of the nanowire orientation with perpendicular directions. The nanowires have a uniform height, but occasionally bright islands are observed on top of the nanowires, which are assigned to an additional silicide layer. The apparent height difference of the bright islands to the residual nanowires of (0.35±0.05) nm is in good agreement with the height of a single hexagonal TbSi2 layer in [¯1100]-direction (shex = 0.33 nm, see Fig. 2) as well as with the height of a single tetragonal TbSi2 layer in [001]-direction (ctet /4 = 0.33 nm). Consequently, the height measurement indicates a TbSi2 nanowire structure, consistent with previous height measurements of (0.32±0.02) nm [5, 9]. Upon capping these structures with 5 nm silicon at room temperature, a rough surface, which is typical for amorphous overlayers, is observed. However, the areas of the bundles are still clearly visible as protrusions of the surface [see Fig. 4(b)]. This is a first indication that the nanowires are conserved by room temperature capping. Strong evidence for intact nanowires below the capping layer is given from HRTEM measurements. Figure 5 shows several measured HRTEM images from the same sample as imaged in STM, which are oriented in such a way that the well ordered crystalline silicon substrate is at the bottom of the images and the disordered amorphous silicon capping layer is at their top. The silicide nanowires can be identified on top of the surface of the crystalline silicon. As expected for a planar substrate, there are two distinctly different appearances of the nanowires. The first one, seen in Fig. 5(a–b), is characterized by many constrictions along the width of the silicide structure, which are absent in the second appearance form [see Fig. 5(c)]. We assume that the constrictions correspond to the grooves, which are observed in STM, i.e. the cross-sections of bundled nanowires are seen in Fig. 5(a– b). Such an interpretation is in agreement with our previous report, in which only the cross-sections of single domain nanowires were accessible [27]. Since the bundle structure is undisturbed by the amorphous silicon cap, we further assume that the internal structure of the nanowires does not change during room temperature capping
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 9
Figure 4. STM images (V = –2.5 V and I = 100 pA) of the nanowire formation
and capping process. (a) Terbium silicide nanowire bundles on planar Si(001) formed by a terbium deposition of 0.33 nm followed by annealing at 575 ◦ C. (b) The same nanowire sample as shown in (a), but after subsequent capping with 5 nm silicon at room temperature.
with silicon. Consequently, Fig. 5(c) is assigned to the side view of a nanowire bundle. It should be noted that in Fig. 5(c) not only one, but multiple nanowires and grooves are imaged due to the narrowness of an individual nanowire with respect to the thickness of the HRTEM sample. Upon closer inspection, the nanowires are characterized by dark spots in the cross-
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 10 sectional views in Fig. 5(a–b) (highlighted by yellow points in the inset). As will be shown later in more detail, these dark spots can be assigned to the terbium columns. Usually, two rows of dark spots are observed indicating that the nanowires have a height of two silicide layers; the height of 0.64 nm is indicated by the white dotted lines in Fig. 5(a). This finding is in agreement with the terbium deposition on this sample and the area filling of the nanowires observed by STM, resulting in an average material consumption in the nanowires corresponding to two monolayers of terbium, as also observed previously [5]. By inspection of several of these images of nanowire cross sections it is found that the widths of the individual nanowires within a bundle amount to 1.1–3.5 nm, also in agreement with previous STM results [5]. Between the nanowires in a bundle and often at the bundle boundaries, single terbium columns are found, as indicated by the red
Figure 5. HRTEM images of terbium silicide nanowires on the planar Si(001)
surface covered with amorphous silicon. The sample is the same as in Fig. 4. The images show nanowire cross sections at (a) Ekin = 300 keV and (b) Ekin = 200 keV, and (c) a side view at Ekin = 300 keV. The white dotted lines in (a) mark the width and height of a nanowire, the red arrows indicate slightly higher single terbium columns, and the terbium column positions are highlighted yellow in the insets.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 11 arrows in Fig. 5(a). These terbium columns are located slightly higher than the lower row of dark spots in the nanowire. It is assumed that these dislocations reduce the stress resulting from the lattice mismatch. The amorphous silicon overlayer is clearly visible above the nanowire bundle in Fig. 5(a). Interestingly, some crystalline silicon is found besides the nanowires even at room temperature, in contrast to the STM data of the uncovered nanowires [see Fig. 4(a)] that show nanowire surfaces higher than those of the surrounding substrate. This observation indicates a partly crystalline overgrowth of the silicon besides the nanowires. Such a behavior is surprising, since it is not observed in silicon homoepitaxy on Si(001)2×1 at room temperature. However, it may be related to the presence of the terbium-induced submonolayer structures, which form 2×4 or 2×7 reconstructions besides the nanowires [5] and might act as surfactants enabling a limited crystalline silicon growth even at room temperature. Figure 5(b) also shows a cross-sectional view of a nanowire bundle, where the nanowires in the bundle are again mainly characterized by two rows of dark spots separated by vertically displaced single dark spots. In this image, also one nanowire is found, which is characterized by a (local) height of three silicide layers. This observation is in excellent agreement with the STM data shown in Fig. 4(a), where the observed islands on top of the nanowires were related to the formation of nanowire sections with an additional layer. In the nanowire side view, shown in Fig. 5(c), two rows of terbium atoms are again observed. In contrast to the cross-sectional view, these rows are not only uninterrupted, but the arrangement of the spots is also different. Here, they are vertically aligned while they are arranged in a zigzag configuration in the cross-sectional views (see insets in Fig. 5). The observation of such arrangements of spots assigned to terbium columns indicates that the previously proposed structure model, which is shown in Fig. 2(c–d), cannot by correct. Identical rows that are aligned with a side shift between lower and upper row, agreeing with the cross-sectional appearance of the nanowires in HRTEM, are characteristic for a hexagonal TbSi2 viewed along its chex -axis. In the following section, these results will be discussed in more detail, and a corresponding refined structure model will be presented. 3.2. Refined structure model of the nanowires Based on the results of the previous section, a refined structure model is developed. It is obvious that the present HRTEM data in Fig. 5 do not agree with the former model shown in Fig. 2(c–d), where an on-top alignment of the terbium atoms in cross-sectional view and a zigzag alignment in side view are expected for nanowires with a height of two silicide layers, which is definitely not observed. In contrast, the HRTEM data show exactly the opposite behavior. Figure 6 shows the refined model for nanowires with a height of two silicide layers based
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 12 on the present HRTEM and STM results (a) in cross section, (b) in side view, and (c) in top view. As a main difference to the previous model, the structural direction of the hexagonal silicide within the nanowires was turned by 90◦ with respect to the previous model. The top silicon atoms of the nanowires, which cannot be distinguished in the HRTEM data due to their interaction with the amorphous silicon cap, are aligned according to the 2×1 reconstructed dimer structure that is frequently observed in STM images {see inset in Fig. 1 and references [2, 5, 6]}. Figure 6(d) shows an alternative surface structure with the dimers aligned according to the c(2×2) periodicity, which was also observed occasionally for other rare earths [2, 3, 9]. The structure model in Fig. 6 is characterized by a zigzag arrangement of the terbium atoms in the cross-sectional view and an on-top arrangement in side view. In order to correctly interpret the HRTEM data, defocus series were recorded, and the respective images were simulated. For this purpose, the imaging parameters of the microscope were determined by a fit to the image sections of the well-known silicon substrate under consideration of the Debye-Waller factor for the different reflections, resulting in values for the imaging intensity, the defocus and its step width, the sample thickness, the twofold astigmatism, the axial coma and the vibrations of the sample, the primitive vectors of the silicon lattice, and the average sample tilt [29, 30]. In Fig. 7, exemplary results of the simulations are shown. The complete simulation series with varying defocus are presented in the Supplemental Material [31]. Figure 7(a) shows the simulated structures, projection A is the view in chex direction and projection B the one in ahex direction of a two silicide layers high TbSi2 film on Si(001). In Fig. 7(b–
Figure 6. Refined model of the terbium silicide nanowires (a) in cross-section,
(b) in side view, and (c) in top view for a 2×1 arrangement of the silicon dimers at the nanowire surface. (d) Corresponding top view for an alternative c(2×2) arrangement of the surface dimers. The respective surface unit cells are indicated in (c–d).
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 13 c), representative HRTEM images of (b) a cross-sectional view and (c) a side view of the nanowires are overlaid by the simulations (marked by black rectangles) using the respective parameters derived from the substrate fit. The overlaid simulated images are shifted and oriented in such a way that the simulated silicon substrate matches the measured one. Thus, a verification of the proposed structure model can be performed directly by comparing the simulated appearance of the nanowire to the measured one, with the silicon substrate used as a reference. The appearance of the nanowires in Fig. 7(b) is characterized by several dark circular or oval areas (highlighted green and pink). Taking a closer look at the corresponding areas in the simulated images, an excellent agreement with the projection A (along chex ) is found, while the simulation of projection B (along ahex ) differs significantly. This becomes obvious when considering the direct neighborhood of the uppermost silicon substrate bilayer characterized by the dumbbells marked dark blue. Above each
Figure 7. (a) Model of a two silicide layers high film of hexagonal TbSi2
on Si(001) viewed in chex direction (projection A) and in ahex direction (projection B). (b) HRTEM cross sectional view (defocus = –6 nm–c) side view (defocus = 12 nm) taken at Ekin = 300 keV, overlaid in the insets by the simulated images of projections A and B of the structures shown in (a). The HRTEM images and their simulations of the complete defocus series are presented in the Supplemental Material [31].
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 14 dumbbell, first a smaller and then a larger dark spot appears in the experimental image, marked pink and green, respectively. Moreover, when considering the laterally shifted positions, a similar smaller spot is found slightly higher than the first one (marked pink as well), while a similar larger spot is found lower than the respective first one (marked green). It is found that the simulation of projection A reproduces these features very well concerning both their sizes and positions. In the simulation of projection B, in contrast, the order of the larger and smaller spots directly above the substrate dumbbells as well as in the laterally shifted position is reversed, in disagreement with the experimental observation. Since the measured HRTEM image in Fig. 7(b) shows cross sections of nanowires, the simulations verify our proposed structure model and contradict the one reported previously. Since the positions of the atomic columns with respect to the simulated image are known, we can identify the positions of the larger dark spots, which are marked green and are the prominent structures already discussed in the previous section, with the columns of terbium atoms and the smaller dark spots, marked pink, with the columns of central silicon atoms within the nanowires [see Fig. 7(a–b)]. An example that the terbium columns are not always imaged as dark spots is seen in the chosen comparison image for the nanowire side view shown in Fig. 7(c). First of all, now the simulated image of projection B (along ahex ) agrees very well with the measured image, while the intensity maxima observed in the simulation using the projection A (along chex ) are at completely different positions. In the experimental image, directly above each dumbbells from the uppermost substrate bilayer (marked dark blue), two faint bright spots are observed (marked yellow), while in the laterally shifted position a bright spot (marked red) is found directly below an elongated bright feature (marked light blue). Here, all these features are well reproduced by the simulation of projection B, indicating that the elongated feature is related to two atomic silicon columns above each other. The simulation of projection A, in contrast, shows a very bright spot (marked pink) directly above each silicon dumbbell (marked dark blue) and no elongated features in the nanowire region at all, in disagreement with the experiment. Comparing the marked structures with the positions of the atomic columns, the bright spots marked yellow correspond to terbium columns, while the red and blue marked ones are related to silicon columns. It should be noted that the results of the above discussion are further verified when comparing experiment and simulation in the full defocus series shown in the Supplemental Material [31]. Thus, our newly proposed structure model should be preferred over the previous one. While two silicide layers high hexagonal TbSi2 is identical to two silicide layers high tetragonal TbSi2 , provided that their surfaces reconstruct in the same way, a distinction is possible for higher nanowires. A tetragonal configuration has been found for the occasionally observed three silicide layers high nanowire sections [see Fig. 5(b)]. Figure 8(a–b) shows the respective structure model of a three silicide layers high nanowire in hexagonal configuration, and Fig. 8(c–d) the respective one in tetragonal
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 15 configuration. These models can also be extended to the case of even higher nanowires. These structures become also relevant for the nanowires formed by capping at elevated temperatures, which will be discussed in the following section. It should be noted that the three silicide layers high nanowire shown in Fig. 5(b) is characterized by a topmost terbium layer with a slightly smaller width than the terbium layers below, in contrast to the structure model in Fig. 8(c). This finding was not included in the structure model, since the detailed structure of the silicon environment in the edge region of the nanowire could not be resolved in the present study. 3.3. Nanowire structure after capping with silicon at elevated temperatures In the previous sections it was demonstrated that capping by silicon at room temperature leads to amorphous silicon overgrowth, while the original structure of the nanowires seems to be conserved. In contrast, capping at elevated substrate temperatures in UHV is expected to lead to a crystalline silicon overlayer [27]. This issue and the consequences for the nanowire structure will be addressed in the following. Figure 9(a) shows an STM image of nanowires on planar Si(001) before capping. Here, the lower terbium coverage of 0.19 nm and the slightly lower annealing temperature of 550 ◦ C on a planar surface lead to thinner yet longer nanowire bundles as compared with those in Fig. 4(a). After deposition of 4 nm silicon at a temperature of 300 ◦ C, the STM image shown in Fig. 9(b) is characterized by rather cloudy structures. In contrast to the roomtemperature overgrowth shown in Fig. 4(b), it is not possible any more to trace the boundaries of the wire bundles, but many of the clouds appear to be elongated, indicating that nanowire-like structures may still exist underneath. Since holes in the silicon layer were often found after overgrowth at elevated temperatures, additional 3 nm silicon were deposited at room temperature for protecting the nanowires against
Figure 8. Structure models for nanowires consisting of three silicide layers,
(a–b) hexagonal structure (a) in cross-sectional view and (b) in side view, and (c–d) corresponding views for a tetragonal structure.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 16 oxidation, before the samples were prepared for the HRTEM experiments. The corresponding HRTEM data of this sample are shown in Fig. 10. Now, the silicide structures are most of the time embedded in crystalline silicon, but again two distinctly different appearances are found. Based on the extensions of the structures, it is assumed that the images in Fig. 10(a–c) show cross-sectional views, while Fig. 10(d) corresponds to a side view of the observed silicide structures.
Figure 9. STM images (V = –2.5 V and I = 100 pA) of (a) uncapped terbium
silicide nanowire bundles on planar Si(001) with a terbium deposition of 0.19 nm followed by annealing at 550 ◦ C, and (b) the same sample capped with 4 nm silicon at 300 ◦ C.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 17 As revealed from Fig. 10(a–b), now a transformation of the rather flat nanowire bundles to higher and more compact structures with almost circular cross sections is observed. Starting at the compact silicide structure, the crystalline silicon cap often shows stacking faults along {111} lattice planes and twin formation, in agreement with previous observations [27]. The buried structures now have an average height of (1.8±0.5) nm, corresponding to (6±2) silicide layers, and an average width of (2.3±0.8) nm. The almost circular cross sections are probably formed due to a reduction of strain and interface energies. Due to the reduced width of these compact silicide structures as compared to the nanowire bundles, it is impossible to image solely a silicide structure from the side with HRTEM without the neighboring crystalline silicon overlayer, but such side views are still found in form of very long rather homogenous disruptions of the crystalline silicon [see Fig. 10(d)]. Based on their large extension in side view, these silicide structures are also referred to as nanowires in the following.
Figure 10. HRTEM images of terbium silicide nanowires on the planar Si(001)
surface covered with silicon at 300 ◦ C. The sample is the same as in Fig. 9. (a–b) Cross-sectional views at Ekin = 200 keV and (c) another cross-sectional view at Ekin = 300 keV. (d) Nanowire side view at Ekin = 300 keV. In the insets the positions of the terbium columns are highlighted by yellow dots.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 18 The internal structure of the compact nanowires in cross-sectional view shows dark spots, which are again assigned to terbium columns. In some nanowires, these dark spots are aligned in a zigzag configuration characteristic for the hexagonal silicide with the c-axis in nanowire direction [see Fig. 10(a)]. For other nanowires, a tetragonal silicide structure is found, where the alignment of the dark spots alternates from layer to layer between on-top and side shifted [see Fig. 10(b)]. Especially the hexagonal structure appears distorted and slightly rotated. It should also be noted that not all nanowires are completely embedded within crystalline Si, but some are found located at the side of a trough filled with amorphous Si, as shown in Fig. 10(c), indicating an incomplete crystallization of the silicon cap. For nanowires grown on planar substrates, the assignment of HRTEM images to crosssectional or side views was only based on the extensions of the observed structures, which is somehow arbitrary. In order to unambiguously assign these views, nanowires on vicinal Si(001) substrates were studied, which show a single-domain growth along the step edges of the substrate, as shown in Fig. 11(a). Here the nanowires have a similar appearance in terms of widths, lengths, surface structure, and bundle formation, similar to the growth on planar surfaces, but only one orientation is enforced across the entire sample surface. These nanowires, grown by deposition of 0.19 nm terbium and with subsequent annealing at 575 ◦ C, now have an average width of (3±1) nm and are in general longer than on the planar substrate shown in Fig. 9(a). After capping with 5 nm silicon at 350 ◦ C, the STM image of the sample, shown in Fig. 11(b), is characterized by elongated, cloudy structures, which now predominantly run vertically in the image, indicating successful capping. Similar as for the sample on the planar substrate [see Fig. 9(b)], the exact boundaries of nanowires cannot be determined any more. For the HRTEM studies, again additional 3 nm silicon were deposited at room temperature for filling deep holes. The HRTEM images in Fig. 12 show a similar behavior of the nanowires and the silicon overlayer as for the nanowires on planar Si(001). The cross-sectional views again show either a hexagonal [see Fig. 12(a)] or a tetragonal [see Fig. 12(b)] nanowire structure, and they are characterized by similar average heights of (1.8±0.3) nm and average widths of (2.1±0.4) nm. Also the silicon capping layer often shows stacking faults [see Fig. 12(a–b)], or there is an incomplete crystallization with nanowires located at the sides of amorphously filled troughs, similar as observed in Fig. 10(c). In the experiments on vicinal substrates, even defect-free capping layers could be observed above some nanowires, as shown in Fig. 12(c). Figure 12(d), taken in the perpendicular direction, shows a very long homogenous disruption of the crystalline silicon, similar to Fig. 10(d), which is also assigned to a side view of a nanowire. Thus it may be concluded that the growth on vicinal substrates followed by crystalline capping leads essentially to the same nanowire structures as on planar substrates.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 19
Figure 11. STM images (V = –2.5 V and I = 100 pA) of (a) uncapped
terbium silicide nanowire bundles on vicinal Si(001) with a terbium deposition of 0.19 nm followed by annealing at 575 ◦ C, and (b) the same sample capped with 5 nm silicon at 350 ◦ C.
4. Summary and Conclusion In summary, this study presents detailed insights into the structure of terbium silicide nanowires on Si(001) overgrown with silicon. Based on the STM and HRTEM results of nanowires capped with amorphous silicon at room temperature, the previous structure model of the nanowires had to be refined. It was found that they mostly consist of
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 20
Figure 12. HRTEM images of terbium silicide nanowires on the vicinal Si(001)
surface covered with silicon at 350 ◦ C. The sample is the same as in Fig. 11. (a– c) Cross-sectional views (Ekin = 200 keV), and (d) side view (Ekin = 300 keV). In the insets the positions of the terbium columns within a nanowire are highlighted by yellow dots.
two layers high hexagonal TbSi2 , but with the chex -axis along the nanowire direction. HRTEM image simulations indicate that the refined model correctly reproduces the experimental results. Capping with silicon at 300–350 ◦ C results in a shape transition to nanowires with more compact cross sections consisting of either hexagonal or tetragonal TbSi2 . These nanowires are usually overgrown by crystalline silicon, which is characterized by stacking faults along the {111} planes and twin formation. It should be noted that the present results indicate an analogous atomic structure of nanowires formed on Si(001) from other trivalent rare earths because of their chemical similarity.
Acknowledgments This work was supported by the Deutsche Forschungsgemeinschaft, FOR1700 project E2 and FOR 1282 project D. The structure models in this article were made with the
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 21 molecule editor Avogadro [32]. We gratefully acknowledge Philipp Rahe for his comments on our manuscript. [1] M. D¨ahne and M. Wanke, Metallic rare-earth silicide nanowires on silicon surfaces, J. Phys.: Condensed Matter 25 (2013) 014012. https://doi.org/10.1088%2F0953-8984%2F25%2F1%2F014012 [2] V. Iancu, P. R. C. Kent, S. Hus, H. Hu, C. G. Zeng, and H. H. Weitering, Structure and Growth of Quasi One-Dimensional YSi2 Nanophases on Si(100), J. Phys.: Condensed Matter 25 (2013) 014011. https://dx.doi.org/10.1088%2F0953-8984%2F25%2F1%2F014011 [3] Y. Chen, D. A. A. Ohlberg, G. Medeiros-Ribeiro, Y. A. Chang, and R. S. Williams, Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001), Appl. Phys. Lett. 76 (2000) 4004. https://doi.org/10.1063/1.126848 [4] C. Preinesberger, S. Vandr´e, T. Kalka, and M. D¨ ahne-Prietsch, Formation of dysprosium silicide wires on Si(001), J. Phys. D: Appl. Phys. 31 (1998) L43. https://doi.org/10.1088%2F00223727%2F31%2F12%2F001 [5] S. Appelfeller, S. Kuls, and M. D¨ ahne, Tb silicide nanowire growth on planar and vicinal Si(001) surfaces, Surf. Sci. 641 (2015) 180. https://doi.org/10.1016/j.susc.2015.07.001 [6] C. Preinesberger, S. K. Becker, S. Vandr´e, T. Kalka, and M. D¨ ahne, Structure of DySi2 nanowires on Si(001), J. Appl. Phys. 91 (2002) 1695. https://doi.org/10.1063/1.1430540 [7] J. Nogami, B. Z. Liu, M. V. Katkov, C. Ohbuchi, and N. O. Birge, Selfassembled rare-earth silicide nanowires on Si(001), Phys. Rev. B 63 (2001) 233305. https://link.aps.org/doi/10.1103/PhysRevB.63.233305 [8] C. Eames, M. I. J. Probert, and S. P. Tear, The structure and growth direction of rare earth silicide nanowires on Si(100), Appl. Phys. Lett. 96 (2010) 241903. https://doi.org/10.1063/1.3453865 [9] B. Z. Liu and J. Nogami, A scanning tunneling microscopy study of dysprosium silicide nanowire growth on Si(001), J. Appl. Phys. 93 (2003) 593. https://doi.org/10.1063/1.1516621 [10] Y. Chen, D. A. A. Ohlberg, and R. S. Williams, Nanowires of four epitaxial hexagonal silicides grown on Si(001), J. Appl. Phys. 91 (2002) 3213. https://doi.org/10.1063/1.1428807 [11] M. Wanke, K. L¨ oser, G. Pruskil, D. V. Vyalikh, S. L. Molodtsov, S. Danzenb¨ acher, C. Laubschat, and M. D¨ahne, Electronic properties of self-assembled rare-earth silicide nanowires on Si(001), Phys. Rev. B 83 (2011) 205417. https://doi.org/10.1103/PhysRevB.83.205417 [12] C. Preinesberger, G. Pruskil, S. K. Becker, M. D¨ ahne, D. V. Vyalikh, S. L. Molodtsov, C. Laubschat, and F. Schiller, Structure and electronic properties of dysprosium-silicide nanowires on vicinal Si(001), Appl. Phys. Lett. 87 (2005) 083107. https://doi.org/10.1063/1.2032620 [13] H. W. Yeom, Y. K. Kim, E. Y. Lee, K.-D. Ryang, and P. G. Kang, Robust OneDimensional Metallic Band Structure of Silicide Nanowires, Phys. Rev. Lett. 95 (2005) 205504. https://doi.org/10.1103/PhysRevLett.95.205504 [14] S. Appelfeller, M. Franz, H.-F. Jirschik, J. Große, and M. D¨ ahne, The electronic structure of Tb silicide nanowires on Si(001), New. J. Phys. 18 (2016) 113005. https://doi.org/10.1088%2F13672630%2F18%2F11%2F113005 [15] K. Holtgrewe, S. Appelfeller, M. Franz, M. Dhne, and S. Sanna, Structure and one-dimensional metallicity of rare earth silicide nanowires on Si(001), Phys. Rev. B. 99 (2019) 214104. https://doi.org/10.1103/PhysRevB.99.214104 [16] C. Zeng, P. R. C. Kent, T.-H. Kim, A.-P. Li, and H. H. Weitering, Charge-order fluctuations in one-dimensional silicides, Nature Mater. 7 (2008) 539. https://doi.org/10.1038/nmat2209 [17] S. Kagoshima, Peierls Phase Transition, Jpn. J. Appl. Phys. 20 (1981) 1617. https://doi.org/10.1143%2Fjjap.20.1617 [18] M. V. Bulanova, A. N. Mikolenko, K. A. Meleshevich, G. Effenberg, and P. A. Saltykov, , TerbiumSilicon System, Z. Metallkd. 90 (1999) 216. [19] G. Ye, J. Nogami, and M. A. Crimp, Dysprosium disilicide nanostructures on silicon(001) studied by scanning tunneling microscopy and transmission electron microscopy, Thin Solid Films 497 (2006) 48. https://doi.org/10.1016/j.tsf.2005.09.180
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 22 [20] G. Ye, M. A. Crimp, and J. Nogami, Crystallographic study of self-assembled dysprosium silicide nanostructures on Si(001), Phys. Rev. B 74 (2006) 033104. https://doi.org/10.1103/PhysRevB.74.033104 [21] F. H. Kaatz, W. R. Graham, J. Van der Spiegel, W. Joss, and J. A. Chroboczek, J. Vac. Sci. Technol. A 9 (1991) 426. https://doi.org/10.1116/1.577426 [22] K. Narasimhan and H. Steinfink, J. Solid State Chem. 10 (1974) 137. https://doi.org/10.1016/0022-4596(74)90019-X [23] J. Pierre, E. Siaud, and D. Frachon, J. Less Comm. Met. 139 (1988) 321. https://doi.org/10.1016/0022-5088(88)90014-8 [24] Z. He, D. J. Smith, and P. A. Bennett, Faulted surface layers in dysprosium silicide nanowires, Phys. Rev. B 70 (2004) 241402(R). https://doi.org/10.1103/PhysRevB.70.241402 [25] J. Zhang, M. A. Crimp, Y. Cui, and J. Nogami, Self-assembled thulium silicide nanostructures on silicon(001) studied by scanning tunneling microscopy and transmission electron microscopy, J. Appl. Phys. 103 (2008) 064308. https://doi.org/10.1063/1.2896414 [26] G. Ye, M. A. Crimp, and J. Nogami, Self-assembled Gd silicide nanostructures grown on Si(001), J. Appl. Phys. 105 (2009) 104304. https://doi.org/10.1063/1.3118573 [27] S. Appelfeller, M. Franz, M. Kubicki, P. Reiß, T. Niermann, M. A. Schubert, M. Lehmann, and M. D¨ahne, Capping of rare earth silicide nanowires on Si(001), Appl. Phys. Lett. 108 (2016) 013109. https://doi.org/10.1063/1.4939693 [28] S. Appelfeller, J. Heggemann, T. Niermann, M. Lehmann, and M. Dhne, Refined structure model of rare earth silicide nanowires on Si(001), Appl. Phys. Lett. 114 (2019) 093104. https://doi.org/10.1063/1.5086369 [29] T. Niermann and M. Lehmann, in: Proc. Microscopy Conf. 2015, M. Laue (Ed.), https://epub.uniregensburg.de/32876/1/MC2015 Proceedings.pdf, p. 482 (2015). [30] T. Niermann, Microsc. Microanal. 24(S1) (2018) 568. [31] See Supplemental Material at http://**** for the complete defocus series and their simulations [32] Jekyll & Minimal Mistakes, Avogadro Chemistry, https://avogadro.cc/ (2017), viewed 20.03.2018
Jonas Heggemann Universität Osnabrück Fachbereich Physik Barbarastr. 7 D-49076 Osnabrück Tel.: +49 (0)541-969-2679 Fax.: +49 (0)541-969-3472 E-Mail: [email protected] 09. October 2019
Conflict of Interest by Jonas Heggemann, Stephan Appelfeller, Tore Niermann, Michael Lehmann and Mario Dähne.
Dear Editor, There is no conflict of interest. Sincerely Yours, Jonas Heggemann on behalf of all authors
Journal Pre-proof
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon ¨ J. Heggemann, S. Appelfeller, T. Niermann, M. Lehmann, M. Dahne PII: DOI: Reference:
S0039-6028(19)30473-X https://doi.org/10.1016/j.susc.2020.121563 SUSC 121563
To appear in:
Surface Science
Received date: Revised date: Accepted date:
25 June 2019 9 October 2019 3 January 2020
¨ Please cite this article as: J. Heggemann, S. Appelfeller, T. Niermann, M. Lehmann, M. Dahne, Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon, Surface Science (2020), doi: https://doi.org/10.1016/j.susc.2020.121563
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
1 Highlights • Atomic structure of terbium silicide nanowires grown on Si(001) capped by silicon
• Scanning tunneling microscopy plus high-resolution transmission electron microscopy • Room temperature capping by amorphous silicon preserves the nanowire structure • Capping at elevated temperatures results in strong shape transitions • Nanowires can consist of hexagonal or tetragonal TbSi2
STM
HRTEM
TbSi2
[001]
Si cap Si substrate
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon J Heggemann1,2 , S Appelfeller3 , T Niermann2 , M Lehmann2 and M D¨ ahne3 1
Universi¨ at Osnabr¨ uck, Fachbereich Physik, Barbarastr. 7, 49076 Osnabr¨ uck, Germany 2 Institut f¨ ur Optik und Atomare Physik, Technische Universit¨ at Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany 3 Institut f¨ ur Festk¨ orperphysik, Technische Universit¨ at Berlin, Hardenbergstraße 36, 10623 Berlin, Germany E-mail: [email protected] Abstract. In a combined scanning tunneling microscopy and high-resolution transmission electron microscopy study, the internal atomic structure of terbium silicide nanowires on Si(001) capped with silicon is determined. Room temperature capping by amorphous silicon preserves the original nanowire structure, and the nanowires with a usual height of two silicide layers are found to consist of hexagonal TbSi2 with the c-axis in nanowire direction, in contrast to previous assumptions. At larger heights, the nanowires are formed from tetragonal TbSi2 . Capping at elevated temperatures results in a shape transition towards higher and more compact nanowires consisting of hexagonal or tetragonal TbSi2 and a crystalline silicon overlayer that shows stacking faults and twin boundaries starting at the nanowires.
Keywords: rare earth silicide nanowires, embedded nanostructures, atomic structure, scanning tunneling microscopy, high resolution transmission electron microscopy
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 4 1. Introduction For two decades, self-organized rare earth silicide nanowires with high structural quality and high aspect ratio are known to form on the Si(001) surface [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. As an example, Fig. 1 shows scanning-tunneling microscopy (STM) images of terbium silicide nanowires, which are also representative for nanowires formed with other trivalent rare earths because of their chemical similarity. Individual nanowires are typically 2 nm wide and can reach lengths of several 100 nm, while the nanowires tend to form wider bundles, in which they are separated by grooves. It was found that they have a quasi-one-dimensional metallic band structure [11, 12, 13, 14, 15], yielding potential for applications in future nanoelectronic devices and for studying physical phenomena that are exclusive for one-dimensional metals, e.g. the Peierls transition [16, 17]. Because of the surplus of substrate silicon during silicide formation, it is generally assumed that the nanowires consist of the silicon richest silicides, which are the hexagonal and the tetragonal rare earth disilicides [3, 6, 8, 9, 18]. Both silicides are observed in large rare earth silicide islands on Si(001) [19, 20] and are thus also candidates for nanowire formation. They are both metallic and show antiferromagnetic ordering at low temperatures, but there are distinct differences in their detailed properties such as the N´eel temperatures [21, 22, 23]. Their atomic structure models are shown in Fig. 2(a–b) together with the lattice parameters of the bulk terbium disilicides
Figure 1. Overview STM image (sample voltage V = –1.5 V and tunneling
current I = 100 pA) of terbium silicide nanowires on the planar Si(001) surface with a terbium deposition of 0.15 nm followed by annealing at 600 ◦ C. The inset (V = −1.5 V and I = 100 pA) shows a detailed STM image highlighting the bundle formation and the surface structure.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 5 [18], but without showing the silicon vacancies usually present in the volume of the bulk silicides. These vacancies actually lead to an orthorhombic instead of a tetragonal phase, but the structure is usually denoted tetragonal in the rare earth silicide nanowire literature [8]. Based on the arrangement of the silicon atoms in hexagonal and tetragonal TbSi2 , both structures fit well on the Si(001) surface, but there are differences in the lattice mismatch. For the hexagonal structure, there is only a negligible lattice mismatch parallel to the ahex -axis (∆ahex = 0.1%), but a large lattice mismatch along the chex -axis (∆chex = 7.4%). In contrast, the tetragonal structure is characterized by a rather large lattice mismatch along both its atet -axis and its btet -axis, which is also less anisotropic (∆atet = 5.4% and ∆btet = 3.3%). Thus, a rather isotropic growth is expected for tetragonal TbSi2 on Si(001), while hexagonal TbSi2 should form highly anisotropic islands on Si(001). This situation, which is similar for the other trivalent rare earths, has lead to the assumption that the strongly anisotropic nanowires consist of the hexagonal disilicides with the ahex -axis oriented along the nanowires and the chex -axis perpendicular to the nanowires. Independent of the rare earth metal, the resulting structure model is widely accepted [1, 3, 5, 6, 7, 9, 10] and is shown in Fig. 2(c–d). In this already previously proposed structure model, the nanowire is depicted as two silicide layers high with the nanowire surface terminated by silicon dimers. Those dimers may form various reconstructions depending on the rare earth metal [2, 3, 6, 9], but dimer rows along the nanowires are typically observed for TbSi2 (see inset in Fig. 1) [5]. The height of two silicide layers is in accordance with the apparent heights observed in STM and
Figure 2. (a–b) Structure models of (a) the hexagonal TbSi2 bulk structure
and (b) the tetragonal TbSi2 bulk structure [8, 18], but without the usually present vacancies. (c–d) Previously proposed structure of rare earth silicide nanowires, (c) in cross-sectional view and (d) in side view [1, 3, 5].
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 6 the amount of deposited rare earth metal [2, 4, 5, 9]. Due to the low height, a first inconsistency in the argumentation for this previously proposed nanowire structure arises. Taking a closer look at the nanowire building blocks [see areas marked by dotted lines in Fig. 2(c–d)], it cannot be exclusively assigned to the hexagonal or tetragonal TbSi2 structure since both structures show the same atomic arrangement for such few silicide layers [see areas limited by the dotted lines in Fig. 2(a– b)]. Thus, there is no basis for the argumentation in favor of or against a specific TbSi2 structure. The structure of the nanowire surface cannot be used to clarify this issue, since it deviates from both TbSi2 bulk structures due to its reconstruction. In the following, a two silicide layers high structure will always be termed hexagonal since its ahex - and chex -axis are easily distinguished in cross-sectional views, in contrast to the atet - and btet -axis of the tetragonal structure. For the previously proposed structure model a further inconsistency in the argumentation arises, when taking into account that no indications for silicon vacancies in the nanowires were reported, and all illustrations of nanowire structures lack silicon vacancies [1, 4, 5, 9, 10]. However, the lattice parameters of the silicide structures without silicon vacancies certainly differ from the ones of the bulk phases with silicon vacancies, so that the lattice parameters measured for the bulk silicides are not applicable for the nanowires and cannot be used for an argumentation regarding the nanowire structure. Thus, there is a great need for further information on the internal structure of rare earth silicide nanowires on Si(001). In most studies, the nanowires were investigated by STM, a very surface sensitive method, but there was almost no analysis of their internal atomic structure. There are only a few earlier studies using high-resolution transmission electron microscopy (HRTEM) with indications of the tetragonal as well as the hexagonal rare earth disilicide structures, however, only much broader nanostructures with heights larger than two silicide layers were investigated [19, 20, 24, 25, 26]. Later we demonstrated that capping by amorphous silicon is possible, and that considerable shape changes occur when annealing the silicon cap for crystallization, but no detailed data were achieved on the atomic structure [27]. In a recent letter we shortly presented the internal structure of the two silicide layers high nanowires capped with amorphous silicon [28]. In the present work, we report in detail on an STM and HRTEM analysis of TbSi2 nanowires on planar and vicinal Si(001) surfaces, capped with amorphous or crystalline silicon. It is found that upon capping with amorphous silicon at room temperature, the nanowires keep their structure characterized by bundle formation and a typical height of two silicide layers. In particular, the HRTEM data clearly demonstrate that nanowires consist of hexagonal TbSi2 with the chex -axis along the nanowires, in disagreement with the previous assumption of a chex -axis perpendicular to the nanowires. However, three silicide layers high nanowires show a tetragonal TbSi2 structure. Nanowires capped at elevated temperatures, in contrast, undergo strong shape changes resulting in more compact cross sections with either a hexagonal or a tetragonal structure, while the silicon overlayer is crystalline, but often showing stacking faults and twin formations.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 7 2. Experimental details All Si(001) samples were cut from n-type wafers. In addition to planar substrates, vicinal substrates with an offcut of 4◦ towards [110] were used. They were cleaned in ultrahigh vacuum (UHV) by degassing at 600 ◦ C for 12 h and subsequent repeated flash annealing up to 1150 ◦ C. To achieve well ordered surface morphologies, the samples were then slowly cooled down from about 800 ◦ C to room temperature with a rate of about 1 ◦ C/s. The samples were heated by direct current and their temperature was controlled via an infrared pyrometer (constant emissivity setting of 0.67 and accurary ±20 ◦ C). Exemplary STM images of the clean substrates are shown in figure 3. Every pristine clean sample was controlled by STM prior to any nanowire study to exclude effects of surface defects or contaminations, which would be visible at steps or as extended missing rows perpendicular to the Si dimer rows. The nanowires were prepared in UHV by depositing 0.15–0.40 nm terbium at room temperature on the surface and subsequent annealing for 2 min at 500–600 ◦ C. First STM images were recorded after this step. For protection against atmospheric influences and to investigate the structural changes upon capping, the nanowires were then capped in UHV with silicon either at room temperature resulting in an amorphous overlayer or at 300–350 ◦ C in order to achieve a crystalline silicon overgrowth. As the STM data often showed holes in the capping layer, possibly penetrating the full capping layer down to the nanowires, further 3 nm silicon were deposited on these samples at room temperature in order to protect the silicide structures for the HRTEM sample preparation. For the STM experiments, a home-built microscope with a SPECS Nanonis control system was used. The preparation for the cross-sectional HRTEM experiments was a standard face-to-face preparation with mechanical thinning and ion milling. For the HRTEM images, we used the image-aberration corrected FEI Titan 80-300 Berlin Holography Special TEM operating at 200–300 kV.
Figure 3. STM images of pristine (a) planar and (b) vicinal Si(001) substrates
(V = –1.5 V and I = 100 pA for both images).
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 8 3. Results and Discussion The Tb silicide nanowire growth observed in this report is in nice agreement with earlier reports [5]. In the following, the STM and HRTEM data of the nanowire structures at different capping processes will be shown. First, the nanowires capped at room temperature will be discussed, followed by the development of a refined structure model for the nanowires. Then the data of the nanowire capping at elevated temperatures will be presented. 3.1. Nanowire structure after capping with silicon at room temperature Figure 4(a) shows an STM image of nanowires formed by deposition of 0.33 nm terbium on a planar Si(001) surface. It is found that the nanowires form broad bundles covering a large fraction of the surface and that the nanowires have different lengths and widths. In agreement with the presence of two domains of the nominally planar Si(001) surface separated by monoatomic steps, we also observe two domains of the nanowire orientation with perpendicular directions. The nanowires have a uniform height, but occasionally bright islands are observed on top of the nanowires, which are assigned to an additional silicide layer. The apparent height difference of the bright islands to the residual nanowires of (0.35±0.05) nm is in good agreement with the height of a single hexagonal TbSi2 layer in [¯1100]-direction (shex = 0.33 nm, see Fig. 2) as well as with the height of a single tetragonal TbSi2 layer in [001]-direction (ctet /4 = 0.33 nm). Consequently, the height measurement indicates a TbSi2 nanowire structure, consistent with previous height measurements of (0.32±0.02) nm [5, 9]. Upon capping these structures with 5 nm silicon at room temperature, a rough surface, which is typical for amorphous overlayers, is observed. However, the areas of the bundles are still clearly visible as protrusions of the surface [see Fig. 4(b)]. This is a first indication that the nanowires are conserved by room temperature capping. Strong evidence for intact nanowires below the capping layer is given from HRTEM measurements. Figure 5 shows several measured HRTEM images from the same sample as imaged in STM, which are oriented in such a way that the well ordered crystalline silicon substrate is at the bottom of the images and the disordered amorphous silicon capping layer is at their top. The silicide nanowires can be identified on top of the surface of the crystalline silicon. As expected for a planar substrate, there are two distinctly different appearances of the nanowires. The first one, seen in Fig. 5(a–b), is characterized by many constrictions along the width of the silicide structure, which are absent in the second appearance form [see Fig. 5(c)]. We assume that the constrictions correspond to the grooves, which are observed in STM, i.e. the cross-sections of bundled nanowires are seen in Fig. 5(a– b). Such an interpretation is in agreement with our previous report, in which only the cross-sections of single domain nanowires were accessible [27]. Since the bundle structure is undisturbed by the amorphous silicon cap, we further assume that the internal structure of the nanowires does not change during room temperature capping
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 9
Figure 4. STM images (V = –2.5 V and I = 100 pA) of the nanowire formation
and capping process. (a) Terbium silicide nanowire bundles on planar Si(001) formed by a terbium deposition of 0.33 nm followed by annealing at 575 ◦ C. (b) The same nanowire sample as shown in (a), but after subsequent capping with 5 nm silicon at room temperature.
with silicon. Consequently, Fig. 5(c) is assigned to the side view of a nanowire bundle. It should be noted that in Fig. 5(c) not only one, but multiple nanowires and grooves are imaged due to the narrowness of an individual nanowire with respect to the thickness of the HRTEM sample. Upon closer inspection, the nanowires are characterized by dark spots in the cross-
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 10 sectional views in Fig. 5(a–b) (highlighted by yellow points in the inset). As will be shown later in more detail, these dark spots can be assigned to the terbium columns. Usually, two rows of dark spots are observed indicating that the nanowires have a height of two silicide layers; the height of 0.64 nm is indicated by the white dotted lines in Fig. 5(a). This finding is in agreement with the terbium deposition on this sample and the area filling of the nanowires observed by STM, resulting in an average material consumption in the nanowires corresponding to two monolayers of terbium, as also observed previously [5]. By inspection of several of these images of nanowire cross sections it is found that the widths of the individual nanowires within a bundle amount to 1.1–3.5 nm, also in agreement with previous STM results [5]. Between the nanowires in a bundle and often at the bundle boundaries, single terbium columns are found, as indicated by the red
Figure 5. HRTEM images of terbium silicide nanowires on the planar Si(001)
surface covered with amorphous silicon. The sample is the same as in Fig. 4. The images show nanowire cross sections at (a) Ekin = 300 keV and (b) Ekin = 200 keV, and (c) a side view at Ekin = 300 keV. The white dotted lines in (a) mark the width and height of a nanowire, the red arrows indicate slightly higher single terbium columns, and the terbium column positions are highlighted yellow in the insets.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 11 arrows in Fig. 5(a). These terbium columns are located slightly higher than the lower row of dark spots in the nanowire. It is assumed that these dislocations reduce the stress resulting from the lattice mismatch. The amorphous silicon overlayer is clearly visible above the nanowire bundle in Fig. 5(a). Interestingly, some crystalline silicon is found besides the nanowires even at room temperature, in contrast to the STM data of the uncovered nanowires [see Fig. 4(a)] that show nanowire surfaces higher than those of the surrounding substrate. This observation indicates a partly crystalline overgrowth of the silicon besides the nanowires. Such a behavior is surprising, since it is not observed in silicon homoepitaxy on Si(001)2×1 at room temperature. However, it may be related to the presence of the terbium-induced submonolayer structures, which form 2×4 or 2×7 reconstructions besides the nanowires [5] and might act as surfactants enabling a limited crystalline silicon growth even at room temperature. Figure 5(b) also shows a cross-sectional view of a nanowire bundle, where the nanowires in the bundle are again mainly characterized by two rows of dark spots separated by vertically displaced single dark spots. In this image, also one nanowire is found, which is characterized by a (local) height of three silicide layers. This observation is in excellent agreement with the STM data shown in Fig. 4(a), where the observed islands on top of the nanowires were related to the formation of nanowire sections with an additional layer. In the nanowire side view, shown in Fig. 5(c), two rows of terbium atoms are again observed. In contrast to the cross-sectional view, these rows are not only uninterrupted, but the arrangement of the spots is also different. Here, they are vertically aligned while they are arranged in a zigzag configuration in the cross-sectional views (see insets in Fig. 5). The observation of such arrangements of spots assigned to terbium columns indicates that the previously proposed structure model, which is shown in Fig. 2(c–d), cannot by correct. Identical rows that are aligned with a side shift between lower and upper row, agreeing with the cross-sectional appearance of the nanowires in HRTEM, are characteristic for a hexagonal TbSi2 viewed along its chex -axis. In the following section, these results will be discussed in more detail, and a corresponding refined structure model will be presented. 3.2. Refined structure model of the nanowires Based on the results of the previous section, a refined structure model is developed. It is obvious that the present HRTEM data in Fig. 5 do not agree with the former model shown in Fig. 2(c–d), where an on-top alignment of the terbium atoms in cross-sectional view and a zigzag alignment in side view are expected for nanowires with a height of two silicide layers, which is definitely not observed. In contrast, the HRTEM data show exactly the opposite behavior. Figure 6 shows the refined model for nanowires with a height of two silicide layers based
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 12 on the present HRTEM and STM results (a) in cross section, (b) in side view, and (c) in top view. As a main difference to the previous model, the structural direction of the hexagonal silicide within the nanowires was turned by 90◦ with respect to the previous model. The top silicon atoms of the nanowires, which cannot be distinguished in the HRTEM data due to their interaction with the amorphous silicon cap, are aligned according to the 2×1 reconstructed dimer structure that is frequently observed in STM images {see inset in Fig. 1 and references [2, 5, 6]}. Figure 6(d) shows an alternative surface structure with the dimers aligned according to the c(2×2) periodicity, which was also observed occasionally for other rare earths [2, 3, 9]. The structure model in Fig. 6 is characterized by a zigzag arrangement of the terbium atoms in the cross-sectional view and an on-top arrangement in side view. In order to correctly interpret the HRTEM data, defocus series were recorded, and the respective images were simulated. For this purpose, the imaging parameters of the microscope were determined by a fit to the image sections of the well-known silicon substrate under consideration of the Debye-Waller factor for the different reflections, resulting in values for the imaging intensity, the defocus and its step width, the sample thickness, the twofold astigmatism, the axial coma and the vibrations of the sample, the primitive vectors of the silicon lattice, and the average sample tilt [29, 30]. In Fig. 7, exemplary results of the simulations are shown. The complete simulation series with varying defocus are presented in the Supplemental Material [31]. Figure 7(a) shows the simulated structures, projection A is the view in chex direction and projection B the one in ahex direction of a two silicide layers high TbSi2 film on Si(001). In Fig. 7(b–
Figure 6. Refined model of the terbium silicide nanowires (a) in cross-section,
(b) in side view, and (c) in top view for a 2×1 arrangement of the silicon dimers at the nanowire surface. (d) Corresponding top view for an alternative c(2×2) arrangement of the surface dimers. The respective surface unit cells are indicated in (c–d).
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 13 c), representative HRTEM images of (b) a cross-sectional view and (c) a side view of the nanowires are overlaid by the simulations (marked by black rectangles) using the respective parameters derived from the substrate fit. The overlaid simulated images are shifted and oriented in such a way that the simulated silicon substrate matches the measured one. Thus, a verification of the proposed structure model can be performed directly by comparing the simulated appearance of the nanowire to the measured one, with the silicon substrate used as a reference. The appearance of the nanowires in Fig. 7(b) is characterized by several dark circular or oval areas (highlighted green and pink). Taking a closer look at the corresponding areas in the simulated images, an excellent agreement with the projection A (along chex ) is found, while the simulation of projection B (along ahex ) differs significantly. This becomes obvious when considering the direct neighborhood of the uppermost silicon substrate bilayer characterized by the dumbbells marked dark blue. Above each
Figure 7. (a) Model of a two silicide layers high film of hexagonal TbSi2
on Si(001) viewed in chex direction (projection A) and in ahex direction (projection B). (b) HRTEM cross sectional view (defocus = –6 nm–c) side view (defocus = 12 nm) taken at Ekin = 300 keV, overlaid in the insets by the simulated images of projections A and B of the structures shown in (a). The HRTEM images and their simulations of the complete defocus series are presented in the Supplemental Material [31].
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 14 dumbbell, first a smaller and then a larger dark spot appears in the experimental image, marked pink and green, respectively. Moreover, when considering the laterally shifted positions, a similar smaller spot is found slightly higher than the first one (marked pink as well), while a similar larger spot is found lower than the respective first one (marked green). It is found that the simulation of projection A reproduces these features very well concerning both their sizes and positions. In the simulation of projection B, in contrast, the order of the larger and smaller spots directly above the substrate dumbbells as well as in the laterally shifted position is reversed, in disagreement with the experimental observation. Since the measured HRTEM image in Fig. 7(b) shows cross sections of nanowires, the simulations verify our proposed structure model and contradict the one reported previously. Since the positions of the atomic columns with respect to the simulated image are known, we can identify the positions of the larger dark spots, which are marked green and are the prominent structures already discussed in the previous section, with the columns of terbium atoms and the smaller dark spots, marked pink, with the columns of central silicon atoms within the nanowires [see Fig. 7(a–b)]. An example that the terbium columns are not always imaged as dark spots is seen in the chosen comparison image for the nanowire side view shown in Fig. 7(c). First of all, now the simulated image of projection B (along ahex ) agrees very well with the measured image, while the intensity maxima observed in the simulation using the projection A (along chex ) are at completely different positions. In the experimental image, directly above each dumbbells from the uppermost substrate bilayer (marked dark blue), two faint bright spots are observed (marked yellow), while in the laterally shifted position a bright spot (marked red) is found directly below an elongated bright feature (marked light blue). Here, all these features are well reproduced by the simulation of projection B, indicating that the elongated feature is related to two atomic silicon columns above each other. The simulation of projection A, in contrast, shows a very bright spot (marked pink) directly above each silicon dumbbell (marked dark blue) and no elongated features in the nanowire region at all, in disagreement with the experiment. Comparing the marked structures with the positions of the atomic columns, the bright spots marked yellow correspond to terbium columns, while the red and blue marked ones are related to silicon columns. It should be noted that the results of the above discussion are further verified when comparing experiment and simulation in the full defocus series shown in the Supplemental Material [31]. Thus, our newly proposed structure model should be preferred over the previous one. While two silicide layers high hexagonal TbSi2 is identical to two silicide layers high tetragonal TbSi2 , provided that their surfaces reconstruct in the same way, a distinction is possible for higher nanowires. A tetragonal configuration has been found for the occasionally observed three silicide layers high nanowire sections [see Fig. 5(b)]. Figure 8(a–b) shows the respective structure model of a three silicide layers high nanowire in hexagonal configuration, and Fig. 8(c–d) the respective one in tetragonal
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 15 configuration. These models can also be extended to the case of even higher nanowires. These structures become also relevant for the nanowires formed by capping at elevated temperatures, which will be discussed in the following section. It should be noted that the three silicide layers high nanowire shown in Fig. 5(b) is characterized by a topmost terbium layer with a slightly smaller width than the terbium layers below, in contrast to the structure model in Fig. 8(c). This finding was not included in the structure model, since the detailed structure of the silicon environment in the edge region of the nanowire could not be resolved in the present study. 3.3. Nanowire structure after capping with silicon at elevated temperatures In the previous sections it was demonstrated that capping by silicon at room temperature leads to amorphous silicon overgrowth, while the original structure of the nanowires seems to be conserved. In contrast, capping at elevated substrate temperatures in UHV is expected to lead to a crystalline silicon overlayer [27]. This issue and the consequences for the nanowire structure will be addressed in the following. Figure 9(a) shows an STM image of nanowires on planar Si(001) before capping. Here, the lower terbium coverage of 0.19 nm and the slightly lower annealing temperature of 550 ◦ C on a planar surface lead to thinner yet longer nanowire bundles as compared with those in Fig. 4(a). After deposition of 4 nm silicon at a temperature of 300 ◦ C, the STM image shown in Fig. 9(b) is characterized by rather cloudy structures. In contrast to the roomtemperature overgrowth shown in Fig. 4(b), it is not possible any more to trace the boundaries of the wire bundles, but many of the clouds appear to be elongated, indicating that nanowire-like structures may still exist underneath. Since holes in the silicon layer were often found after overgrowth at elevated temperatures, additional 3 nm silicon were deposited at room temperature for protecting the nanowires against
Figure 8. Structure models for nanowires consisting of three silicide layers,
(a–b) hexagonal structure (a) in cross-sectional view and (b) in side view, and (c–d) corresponding views for a tetragonal structure.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 16 oxidation, before the samples were prepared for the HRTEM experiments. The corresponding HRTEM data of this sample are shown in Fig. 10. Now, the silicide structures are most of the time embedded in crystalline silicon, but again two distinctly different appearances are found. Based on the extensions of the structures, it is assumed that the images in Fig. 10(a–c) show cross-sectional views, while Fig. 10(d) corresponds to a side view of the observed silicide structures.
Figure 9. STM images (V = –2.5 V and I = 100 pA) of (a) uncapped terbium
silicide nanowire bundles on planar Si(001) with a terbium deposition of 0.19 nm followed by annealing at 550 ◦ C, and (b) the same sample capped with 4 nm silicon at 300 ◦ C.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 17 As revealed from Fig. 10(a–b), now a transformation of the rather flat nanowire bundles to higher and more compact structures with almost circular cross sections is observed. Starting at the compact silicide structure, the crystalline silicon cap often shows stacking faults along {111} lattice planes and twin formation, in agreement with previous observations [27]. The buried structures now have an average height of (1.8±0.5) nm, corresponding to (6±2) silicide layers, and an average width of (2.3±0.8) nm. The almost circular cross sections are probably formed due to a reduction of strain and interface energies. Due to the reduced width of these compact silicide structures as compared to the nanowire bundles, it is impossible to image solely a silicide structure from the side with HRTEM without the neighboring crystalline silicon overlayer, but such side views are still found in form of very long rather homogenous disruptions of the crystalline silicon [see Fig. 10(d)]. Based on their large extension in side view, these silicide structures are also referred to as nanowires in the following.
Figure 10. HRTEM images of terbium silicide nanowires on the planar Si(001)
surface covered with silicon at 300 ◦ C. The sample is the same as in Fig. 9. (a–b) Cross-sectional views at Ekin = 200 keV and (c) another cross-sectional view at Ekin = 300 keV. (d) Nanowire side view at Ekin = 300 keV. In the insets the positions of the terbium columns are highlighted by yellow dots.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 18 The internal structure of the compact nanowires in cross-sectional view shows dark spots, which are again assigned to terbium columns. In some nanowires, these dark spots are aligned in a zigzag configuration characteristic for the hexagonal silicide with the c-axis in nanowire direction [see Fig. 10(a)]. For other nanowires, a tetragonal silicide structure is found, where the alignment of the dark spots alternates from layer to layer between on-top and side shifted [see Fig. 10(b)]. Especially the hexagonal structure appears distorted and slightly rotated. It should also be noted that not all nanowires are completely embedded within crystalline Si, but some are found located at the side of a trough filled with amorphous Si, as shown in Fig. 10(c), indicating an incomplete crystallization of the silicon cap. For nanowires grown on planar substrates, the assignment of HRTEM images to crosssectional or side views was only based on the extensions of the observed structures, which is somehow arbitrary. In order to unambiguously assign these views, nanowires on vicinal Si(001) substrates were studied, which show a single-domain growth along the step edges of the substrate, as shown in Fig. 11(a). Here the nanowires have a similar appearance in terms of widths, lengths, surface structure, and bundle formation, similar to the growth on planar surfaces, but only one orientation is enforced across the entire sample surface. These nanowires, grown by deposition of 0.19 nm terbium and with subsequent annealing at 575 ◦ C, now have an average width of (3±1) nm and are in general longer than on the planar substrate shown in Fig. 9(a). After capping with 5 nm silicon at 350 ◦ C, the STM image of the sample, shown in Fig. 11(b), is characterized by elongated, cloudy structures, which now predominantly run vertically in the image, indicating successful capping. Similar as for the sample on the planar substrate [see Fig. 9(b)], the exact boundaries of nanowires cannot be determined any more. For the HRTEM studies, again additional 3 nm silicon were deposited at room temperature for filling deep holes. The HRTEM images in Fig. 12 show a similar behavior of the nanowires and the silicon overlayer as for the nanowires on planar Si(001). The cross-sectional views again show either a hexagonal [see Fig. 12(a)] or a tetragonal [see Fig. 12(b)] nanowire structure, and they are characterized by similar average heights of (1.8±0.3) nm and average widths of (2.1±0.4) nm. Also the silicon capping layer often shows stacking faults [see Fig. 12(a–b)], or there is an incomplete crystallization with nanowires located at the sides of amorphously filled troughs, similar as observed in Fig. 10(c). In the experiments on vicinal substrates, even defect-free capping layers could be observed above some nanowires, as shown in Fig. 12(c). Figure 12(d), taken in the perpendicular direction, shows a very long homogenous disruption of the crystalline silicon, similar to Fig. 10(d), which is also assigned to a side view of a nanowire. Thus it may be concluded that the growth on vicinal substrates followed by crystalline capping leads essentially to the same nanowire structures as on planar substrates.
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 19
Figure 11. STM images (V = –2.5 V and I = 100 pA) of (a) uncapped
terbium silicide nanowire bundles on vicinal Si(001) with a terbium deposition of 0.19 nm followed by annealing at 575 ◦ C, and (b) the same sample capped with 5 nm silicon at 350 ◦ C.
4. Summary and Conclusion In summary, this study presents detailed insights into the structure of terbium silicide nanowires on Si(001) overgrown with silicon. Based on the STM and HRTEM results of nanowires capped with amorphous silicon at room temperature, the previous structure model of the nanowires had to be refined. It was found that they mostly consist of
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 20
Figure 12. HRTEM images of terbium silicide nanowires on the vicinal Si(001)
surface covered with silicon at 350 ◦ C. The sample is the same as in Fig. 11. (a– c) Cross-sectional views (Ekin = 200 keV), and (d) side view (Ekin = 300 keV). In the insets the positions of the terbium columns within a nanowire are highlighted by yellow dots.
two layers high hexagonal TbSi2 , but with the chex -axis along the nanowire direction. HRTEM image simulations indicate that the refined model correctly reproduces the experimental results. Capping with silicon at 300–350 ◦ C results in a shape transition to nanowires with more compact cross sections consisting of either hexagonal or tetragonal TbSi2 . These nanowires are usually overgrown by crystalline silicon, which is characterized by stacking faults along the {111} planes and twin formation. It should be noted that the present results indicate an analogous atomic structure of nanowires formed on Si(001) from other trivalent rare earths because of their chemical similarity.
Acknowledgments This work was supported by the Deutsche Forschungsgemeinschaft, FOR1700 project E2 and FOR 1282 project D. The structure models in this article were made with the
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 21 molecule editor Avogadro [32]. We gratefully acknowledge Philipp Rahe for his comments on our manuscript. [1] M. D¨ahne and M. Wanke, Metallic rare-earth silicide nanowires on silicon surfaces, J. Phys.: Condensed Matter 25 (2013) 014012. https://doi.org/10.1088%2F0953-8984%2F25%2F1%2F014012 [2] V. Iancu, P. R. C. Kent, S. Hus, H. Hu, C. G. Zeng, and H. H. Weitering, Structure and Growth of Quasi One-Dimensional YSi2 Nanophases on Si(100), J. Phys.: Condensed Matter 25 (2013) 014011. https://dx.doi.org/10.1088%2F0953-8984%2F25%2F1%2F014011 [3] Y. Chen, D. A. A. Ohlberg, G. Medeiros-Ribeiro, Y. A. Chang, and R. S. Williams, Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001), Appl. Phys. Lett. 76 (2000) 4004. https://doi.org/10.1063/1.126848 [4] C. Preinesberger, S. Vandr´e, T. Kalka, and M. D¨ ahne-Prietsch, Formation of dysprosium silicide wires on Si(001), J. Phys. D: Appl. Phys. 31 (1998) L43. https://doi.org/10.1088%2F00223727%2F31%2F12%2F001 [5] S. Appelfeller, S. Kuls, and M. D¨ ahne, Tb silicide nanowire growth on planar and vicinal Si(001) surfaces, Surf. Sci. 641 (2015) 180. https://doi.org/10.1016/j.susc.2015.07.001 [6] C. Preinesberger, S. K. Becker, S. Vandr´e, T. Kalka, and M. D¨ ahne, Structure of DySi2 nanowires on Si(001), J. Appl. Phys. 91 (2002) 1695. https://doi.org/10.1063/1.1430540 [7] J. Nogami, B. Z. Liu, M. V. Katkov, C. Ohbuchi, and N. O. Birge, Selfassembled rare-earth silicide nanowires on Si(001), Phys. Rev. B 63 (2001) 233305. https://link.aps.org/doi/10.1103/PhysRevB.63.233305 [8] C. Eames, M. I. J. Probert, and S. P. Tear, The structure and growth direction of rare earth silicide nanowires on Si(100), Appl. Phys. Lett. 96 (2010) 241903. https://doi.org/10.1063/1.3453865 [9] B. Z. Liu and J. Nogami, A scanning tunneling microscopy study of dysprosium silicide nanowire growth on Si(001), J. Appl. Phys. 93 (2003) 593. https://doi.org/10.1063/1.1516621 [10] Y. Chen, D. A. A. Ohlberg, and R. S. Williams, Nanowires of four epitaxial hexagonal silicides grown on Si(001), J. Appl. Phys. 91 (2002) 3213. https://doi.org/10.1063/1.1428807 [11] M. Wanke, K. L¨ oser, G. Pruskil, D. V. Vyalikh, S. L. Molodtsov, S. Danzenb¨ acher, C. Laubschat, and M. D¨ahne, Electronic properties of self-assembled rare-earth silicide nanowires on Si(001), Phys. Rev. B 83 (2011) 205417. https://doi.org/10.1103/PhysRevB.83.205417 [12] C. Preinesberger, G. Pruskil, S. K. Becker, M. D¨ ahne, D. V. Vyalikh, S. L. Molodtsov, C. Laubschat, and F. Schiller, Structure and electronic properties of dysprosium-silicide nanowires on vicinal Si(001), Appl. Phys. Lett. 87 (2005) 083107. https://doi.org/10.1063/1.2032620 [13] H. W. Yeom, Y. K. Kim, E. Y. Lee, K.-D. Ryang, and P. G. Kang, Robust OneDimensional Metallic Band Structure of Silicide Nanowires, Phys. Rev. Lett. 95 (2005) 205504. https://doi.org/10.1103/PhysRevLett.95.205504 [14] S. Appelfeller, M. Franz, H.-F. Jirschik, J. Große, and M. D¨ ahne, The electronic structure of Tb silicide nanowires on Si(001), New. J. Phys. 18 (2016) 113005. https://doi.org/10.1088%2F13672630%2F18%2F11%2F113005 [15] K. Holtgrewe, S. Appelfeller, M. Franz, M. Dhne, and S. Sanna, Structure and one-dimensional metallicity of rare earth silicide nanowires on Si(001), Phys. Rev. B. 99 (2019) 214104. https://doi.org/10.1103/PhysRevB.99.214104 [16] C. Zeng, P. R. C. Kent, T.-H. Kim, A.-P. Li, and H. H. Weitering, Charge-order fluctuations in one-dimensional silicides, Nature Mater. 7 (2008) 539. https://doi.org/10.1038/nmat2209 [17] S. Kagoshima, Peierls Phase Transition, Jpn. J. Appl. Phys. 20 (1981) 1617. https://doi.org/10.1143%2Fjjap.20.1617 [18] M. V. Bulanova, A. N. Mikolenko, K. A. Meleshevich, G. Effenberg, and P. A. Saltykov, , TerbiumSilicon System, Z. Metallkd. 90 (1999) 216. [19] G. Ye, J. Nogami, and M. A. Crimp, Dysprosium disilicide nanostructures on silicon(001) studied by scanning tunneling microscopy and transmission electron microscopy, Thin Solid Films 497 (2006) 48. https://doi.org/10.1016/j.tsf.2005.09.180
Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon 22 [20] G. Ye, M. A. Crimp, and J. Nogami, Crystallographic study of self-assembled dysprosium silicide nanostructures on Si(001), Phys. Rev. B 74 (2006) 033104. https://doi.org/10.1103/PhysRevB.74.033104 [21] F. H. Kaatz, W. R. Graham, J. Van der Spiegel, W. Joss, and J. A. Chroboczek, J. Vac. Sci. Technol. A 9 (1991) 426. https://doi.org/10.1116/1.577426 [22] K. Narasimhan and H. Steinfink, J. Solid State Chem. 10 (1974) 137. https://doi.org/10.1016/0022-4596(74)90019-X [23] J. Pierre, E. Siaud, and D. Frachon, J. Less Comm. Met. 139 (1988) 321. https://doi.org/10.1016/0022-5088(88)90014-8 [24] Z. He, D. J. Smith, and P. A. Bennett, Faulted surface layers in dysprosium silicide nanowires, Phys. Rev. B 70 (2004) 241402(R). https://doi.org/10.1103/PhysRevB.70.241402 [25] J. Zhang, M. A. Crimp, Y. Cui, and J. Nogami, Self-assembled thulium silicide nanostructures on silicon(001) studied by scanning tunneling microscopy and transmission electron microscopy, J. Appl. Phys. 103 (2008) 064308. https://doi.org/10.1063/1.2896414 [26] G. Ye, M. A. Crimp, and J. Nogami, Self-assembled Gd silicide nanostructures grown on Si(001), J. Appl. Phys. 105 (2009) 104304. https://doi.org/10.1063/1.3118573 [27] S. Appelfeller, M. Franz, M. Kubicki, P. Reiß, T. Niermann, M. A. Schubert, M. Lehmann, and M. D¨ahne, Capping of rare earth silicide nanowires on Si(001), Appl. Phys. Lett. 108 (2016) 013109. https://doi.org/10.1063/1.4939693 [28] S. Appelfeller, J. Heggemann, T. Niermann, M. Lehmann, and M. Dhne, Refined structure model of rare earth silicide nanowires on Si(001), Appl. Phys. Lett. 114 (2019) 093104. https://doi.org/10.1063/1.5086369 [29] T. Niermann and M. Lehmann, in: Proc. Microscopy Conf. 2015, M. Laue (Ed.), https://epub.uniregensburg.de/32876/1/MC2015 Proceedings.pdf, p. 482 (2015). [30] T. Niermann, Microsc. Microanal. 24(S1) (2018) 568. [31] See Supplemental Material at http://**** for the complete defocus series and their simulations [32] Jekyll & Minimal Mistakes, Avogadro Chemistry, https://avogadro.cc/ (2017), viewed 20.03.2018
Jonas Heggemann Universität Osnabrück Fachbereich Physik Barbarastr. 7 D-49076 Osnabrück Tel.: +49 (0)541-969-2679 Fax.: +49 (0)541-969-3472 E-Mail: [email protected] 09. October 2019
Conflict of Interest by Jonas Heggemann, Stephan Appelfeller, Tore Niermann, Michael Lehmann and Mario Dähne.
Dear Editor, There is no conflict of interest. Sincerely Yours, Jonas Heggemann on behalf of all authors