Thermal Stability Studies of DySi2 Nanowires and Nanoislands by in

Article pubs.acs.org/JPCC

Thermal Stability Studies of DySi2 Nanowires and Nanoislands by in Situ GISAXS Anja Seiler,†,‡ Shyjumon Ibrahimkutty,†,‡,§ Peter Wochner,∥ Ramu Pradip,†,‡ Olga Waller,†,‡ Bar̈ bel Krause,† Anton Plech,† Tilo Baumbach,†,‡,⊥ Michael Fiederle,# and Svetoslav Stankov*,†,‡ †

Institute for Photon Science and Synchrotron Radiation, ‡Laboratory for Applications of Synchrotron Radiation, and ⊥ANKA, Angströmquelle Karlsruhe, Karlsruhe Institute of Technology, Karlsruhe, Germany ∥ Max Planck Institute for Solid State Research, Stuttgart, Germany # Freiburg Materials Research Center, Freiburg, Germany ABSTRACT: The thermal stability of parallel, high aspect ratio DySi2 nanowires and nanoislands self-organized on vicinal Si(001) is investigated as a function of the annealing temperature from room temperature up to 760 °C by in situ grazing incidence small-angle X-ray scattering (GISAXS). A transformation of the nanoobjects has been observed above a temperature of 500 °C. The nanowires collapse forming small islands, while the nanoislands grow in size due to Ostwald ripening. The formation of facets is observed during annealing. A comprehensive understanding of the surface morphology changes is obtained by complementing the in situ GISAXS experiment with atomic force microscopy measurements.

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INTRODUCTION Intensive research has been performed on rare earth silicide (RESi) nanostructures since 1998 when Preinesberger et al. demonstrated the self-organization of DySi2 in nanowires, nanoislands, and nanoclusters upon deposition of submonolayer amounts of Dy on the clean Si(001) surface.1 Due to their metallic nature, very low Schottky barrier heights, enhanced chemical stability, and single crystallinity along with the direct integration in the Si technology, the RESi nanostructures became very promising candidates for applications in optoelectronics, light-emitting technology, micro- and nanoelectronics as ohmic contacts, and interconnects.2−6 Hexagonal DySi2 nanowires with the AlB2-type crystal structure were reported to self-assemble parallel to the steps of the vicinal Si(001) surface. The nanowire formation is driven by an anisotropic lattice mismatch between the silicide and the substrate. The epitaxial relationship is such that DySi2[0001] and DySi2[11−20] are oriented along two orthogonal Si⟨110⟩ directions, resulting in a lattice mismatch of 7.3% and −0.3%, respectively.7 However, it has already been demonstrated that the hexagonal DySi2 nanowires are stable in a temperature range of 500−700 °C. Upon annealing at elevated temperatures during growth they tend to transform into more stable nanoislands with tetragonal ThSi2-type crystal structure.1,7−11 In this structure DySi2 grows with a 45° rotation relative to the Si unit cell, i.e., DySi2[100]∥Si[110] on the Si(001) surface, that reduces the lattice mismatch from +25.79% to −4.95%.12 The continuous downscaling of electronic circuits, following Moore’s law, resulted in a drastic augmentation of the heat dissipation, and heat removal became a demanding issue.13 © XXXX American Chemical Society

Therefore, for applications of RESi nanostructures in the nearfuture nanoelectronics it is of crucial importance to extensively investigate their thermal stability. Ding et al. investigated the thermal stability of ErSi2−x nanowires in a room temperature scanning tunneling microscopy (STM) study performed as a function of the annealing time at 630 °C.14 Similarly to the results on DySi2, they reported a transformation of the nanowires exhibiting the metastable AlB2 phase into nanoislands with the more stable ThSi2 structure. Recently, Zeman et al. studied the influence of high-temperature annealing of DySi2 nanostructures during their formation in situ and in real time using photoemission electron microscopy (PEEM).15 They found that evaporation of Dy at 675 °C leads to growth of DySi2 nanowires which decay rapidly upon further annealing above 750 °C. For the DySi2 nanoislands they observed a faceted growth due to Ostwald ripening in the range 900−1200 °C. The thermal stability of grown and quenched RESi2 nanowires and nanoislands upon postgrowth annealing has not yet been investigated, and it is a subject of the present study. Due to its nondestructive nature, the statistically representative sampling over a large surface area and a large number of nanoobjects and due to the ability of performing measurements in situ (under ultrahigh vacuum UHV) grazing incidence smallangle X-ray scattering (GISAXS) has been established as a valuable method for investigation of the morphological changes Received: December 23, 2015 Revised: March 13, 2016

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DOI: 10.1021/acs.jpcc.5b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

and 1.0 nm were studied. The nanostructures were characterized at room temperature by RHEED to ensure their single crystallinity and crystal structure. Figure 1a shows a RHEED image of the clean Si(001) surface recorded with the electron beam being parallel to the Si[110] azimuthal direction (across the steps of the vicinal Si(001) surface). It demonstrates a clean (2 × 1) reconstructed Si(001) surface. The sharp features of the RHEED image of the nanowires (Figure 1b) reveal the formation of epitaxial single-crystalline structure with good quality. In addition, it shows a very characteristic diffraction pattern. The main streaks of the DySi2 are lying on a Laue ring. Many tiny streaks can be seen between the main streaks. These features appear due to a reconstructed surface in between the nanowires. Cui et al. reported that at elevated temperatures the DySi2 nanowires coexist with a reconstructed metal surface.18 The surface was found to be either (2 × 4) or (2 × 7) reconstructed depending on the postgrowth annealing time. The (2 × 4) reconstruction occurs after a postgrowth annealing time of about 5 min, while the (2 × 7) reconstruction appears after postgrowth quenching. Although the STM study of Cui et al. was performed on wider separated nanowires with lower aspect ratio grown at 600 °C it is very likely that in our sample the same reconstructed surface exists that is visible in the RHEED image as small streaks. The in-plane lattice parameter obtained from the RHEED pattern (Figure 1b) is 3.8(1) Å, which is close to the bulk value of 3.83 Å for DySi2 with hexagonal symmetry.19 The RHEED images of the island samples (Figure 1c and 1d) show streak-like patterns having smaller streaks in between the main streaks indicating a reconstructed surface.20 The in-plane lattice parameter obtained from the RHEED patterns of the nanoislands is 4.0(1) Å, which is in agreement with the bulk value of 4.03 Å for DySi2 with tetragonal symmetry.21 GISAXS Experimental Setup. The in situ GISAXS experiment was performed at the MPI beamline22 for hard Xray scattering at ANKA using a portable UHV chamber dedicated to surface diffraction and spectroscopy experiments that is described in detail elsewhere.23 The temperature during the in situ annealing experiment was controlled with a thermocouple placed next to the heater (tungsten filament) and below the sample. The real sample temperature is determined by a calibration performed before the GISAXS experiment with a reference thermocouple directly attached to the Si(001) substrate. The validity of the temperature calibration was verified with a precision of about ±20 °C after the in situ experiment using an optical pyrometer. An Xray beam with an energy of 10 keV and beam sizes of 100 μm × 300 μm (vertical × horizontal) was achieved by a bent mirror and a double monochromator with a sagittally bent second crystal both focused onto the detector. The GISAXS images were recorded using a Pilatus 100 K (Dectris) pixel detector (487 × 195 pixels, 172 × 172 μm2 per pixel) positioned at two different distances from the sample. The distance between the sample and the detector was set to 1.1 m for the nanoislands and 0.3 m for the nanowires. A flight tube filled with He was mounted between the exit window for the X-rays of the UHV chamber and the detector in order to reduce air scattering. A cylindrical beam stop was placed before the detector. A photodiode, which could be moved in and out of the X-ray beam between the Pilatus detector and the beamstop, was used for the alignment of the sample and the beam stop. GISAXS images were recorded at an incidence angle αi ≈ 0.4°, close to

of nanostructures during their formation and transformation upon annealing.16 To obtain a comprehensive understanding of the surface morphological changes, the results from the in situ GISAXS experiments are complemented by in situ atomic force microscopy (AFM) measurements performed before and after the GISAXS experiments.

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EXPERIMENTAL DETAILS Sample Preparation and Characterization. DySi2 nanowires and nanoislands were grown in a chamber with a base pressure of 3 × 10−11 mbar that is a part of an UHV cluster located in the UHV-Analysis lab at the synchrotron radiation facility ANKA, KIT. The n-type (P-doped, 10 Ω cm) Si(001) substrates with a miscut of 4° toward [110], supplied by MaTecK GmbH, were flashed 3 times at a temperature of 1200 °C in a separate UHV chamber with a base pressure of 2 × 10−10 mbar, which is connected to the UHV cluster. During heating the pressure increased up to 5 × 10−9 mbar and recovered quickly to the starting values. The formation of a clean and well-ordered (2 × 1) reconstructed Si(001) surface was verified by reflection high-energy electron diffraction (RHEED) using electrons with an energy of 30 keV (Figure 1a). The surface morphology was investigated before and after

Figure 1. RHEED images recorded along the Si[110] azimuth (across the steps of the vicinal Si(001) surface) of (a) clean Si(001) and for Dy coverages of (b) 0.1 (nanowires), (c) 0.26, and (d) 1.0 nm (both nanoislands).

the in situ GISAXS experiment with an Omicron SPM using contact mode AFM with tips (Contr-20, NanoAndMore GmbH) having a radius of r < 8 nm. The instrument is located in a separate UHV chamber with a base pressure of 3 × 10−10 mbar connected to the UHV cluster. Vacuum-outgassed Dy metal, supplied by Ames Laboratory,17 was sublimated from an electron beam evaporator using a Ta crucible. The pressure increased up to 3 × 10−10 mbar during sample growth. The nanostructures were formed following well-established growth conditions1−3,7,8 keeping the substrate temperature during deposition at 500 °C. The nanowires were quenched to room temperature (RT) directly after growth to avoid formation of nanoislands. The nanoislands were postgrowth annealed for 10 min at about 600 °C. The deposition rate was ∼0.1 monolayer (ML)/min. One monolayer is defined as 6.78 × 1014 atoms/cm2 (the number of atoms per unit area on the Si(001) surface), which corresponds to a Dy coverage of 0.28 nm. Nanowires with a nominal Dy coverage of 0.1 nm and nanoislands with Dy coverages of 0.26 B


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Figure 2. GISAXS images of the DySi2 nanowires recorded at αi ≈ 0.4° and at the indicated temperatures with ki∥DySi2[0001]∥Si[1−10] (along the steps of the vicinal Si(001) surface). The lines in the first image indicate the cuts made for deducing the correlation length, width, and height of the nanowires (see explanation in the text). RT* shows the GISAXS image obtained at RT after annealing. The white arrow points toward a streak-like scattering feature indicating the formation of a facet.

samples were heated in steps of either 50 or 100 °C up to about 760 °C. For every temperature GISAXS images were recorded after reaching a thermal equilibrium on the sample surface (about 30 min) as well as again at room temperature after the last annealing step at 757 °C. The GISAXS experiment was performed with the wave vector of the X-ray beam being parallel to the steps of the vicinal Si(001) surface, i.e., k in ∥Si[1− 10]∥DySi 2 [0001] for the nanowires and k in ∥Si[1− 10]∥DySi2[010] for the nanoislands. Thus, the width of the nanowires can be explored, while their length is beyond the resolution of the GISAXS setup. Additionally, side maxima of the scattering distribution allow for derivation of the packing factor from the nearest neighbor distance. The data were converted into 1D scattering curves by making cuts in the GISAXS images. The sizes of the nanostructures are determined from these cuts by applying a less model-dependent unified fitting procedure to the 1D scattering intensity I(q), incorporating simultaneous Guinier and Porod fits25−27

the critical angle (αc = 0.179°) of Si for X-rays with an energy of 10 keV. In GISAXS geometry the incident wave vector ki impinges at grazing angle αi on the sample surface to minimize the scattering from the bulk and to enhance the near-surface scattering signal. The X-rays are scattered off the sample surface with a wave vector k f at an angle αf and at an in-plane angle 2Θf with respect to the transmitted beam. The wave vector transfer is defined as q = k f − ki . The angles αi, αf, and 2Θf define the components of the wave vector transfer q. The parallel (qx and qy ) and perpendicular qz components of qare given by24 qx = k 0[cos(2θf )cos(αf ) − cos(αi)] qy = k 0[sin(2θf )cos(αf )] qz⃗ = k 0[sin(αf ) + sin(αi)]

(1)

with k0 = 2π/λ and λ being the wavelength of the X-rays. In the UHV chamber the sample can be rotated around its surface normal by an angle ω, which defines the orientation of the incident X-ray beam with respect to the in-plane crystallographic directions. In order to investigate the changes of the surface morphology upon high-temperature annealing of DySi2 nanostructures the

⎛ − q 2R 2 ⎞ g −d f ⎟ I(q) = G exp⎜⎜ ⎟ + B(q*) 3 ⎝ ⎠

(2)

where G = NΔρ2e V2 is the Guinier prefactor with N the number density, Δρe is the scattering contrast, V is the average particle C


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The Journal of Physical Chemistry C volume, B is a constant scattering power law specific prefactor, df is the mass fractal dimension, and q* = q[erf(qR g / 6 )]−3 is the lower limit of the power law decay regime with erf() being the error function and Rg the radius of gyration (average particle size).

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RESULTS AND DISCUSSION DySi2 Nanowires. Figure 2 shows selected GISAXS images of the DySi2 nanowires obtained at the indicated temperatures.

Figure 3. Width, height, and correlation length of the DySi2 nanowires as a function of the annealing temperature obtained from analysis (see Figure 4) of the GISAXS data. (a), (b), and (c) correspond to the width, height and correlation length, respectively. Figure 4. Data analysis (solid/red lines) using eq 2 of the 1D scattering curves (symbols) obtained from the GISAXS data shown in Figure 2 at the indicated temperatures.

The images from RT up to about 500 °C exhibit very similar features without any pronounced changes. Above 582 °C the scattering signal around the critical angle (αf = αc) is getting narrower, indicating the formation of bigger nanostructures. At 655 °C a streak-like scattering feature, marked with a white arrow, appears and remains until the end of the annealing experiment at 757 °C. Such streaks indicate the formation of facets in the scattering objects. The streaks are asymmetric, forming angles of 21° ± 1° (left-hand side) and 18° ± 1° (right-hand side) with respect to the vertical axis. The formation of facets upon annealing has already been investigated for vertically aligned nanowires,28−31 but only a few investigations on in-plane nanowires have been performed until now. For instance, Kim et al. investigated ZnO nanowires, concluding that no facets could be observed. 32 Our investigations also do not reveal the presence of facets in the quenched DySi2 nanowires. The faceting only appears after the nanowires destruction and their transformation into nanoislands take place at temperatures above 500 °C (see Figure 3). The lateral dimensions of the nanowires are obtained by analyzing the horizontal cuts through the GISAXS images close to the critical angle qz ≈ 0.448 nm−1, indicated by a solid/white line in the first image in Figure 2. The height of the nanowires is obtained by analyzing the cuts at qy ≈ 0.35 nm−1, indicated by vertical solid/white lines in the same image, and the correlation length is obtained by analyzing the cuts at around αf = 2° (qz ≈ 2.14 nm−1), indicated by a horizontal dashed/white line (Figure 2). Figure 3 summarizes the evolution of the nanowires’ width, height, and correlation length as a function of

Figure 5. 3D projections of contact mode AFM images (1 × 1 μm2) of DySi2 nanowires with nominal coverage of 0.1 nm Dy obtained at RT (a) before and (b) after the in situ GISAXS experiment.

the annealing temperature obtained from the performed analysis (see Figure 4). This figure reveals that the surface morphology does not change upon annealing at temperatures up to the growth temperature. However, drastic changes are triggered by annealing the nanowires at and above 500 °C. The GISAXS data analysis (Figure 4) shows that upon annealing in the temperature interval from 500 to 582 °C the nanowire width increased by a factor of 2 from 3.9 ± 0.3 to 8.3 ± 0.3 nm, while the correlation length has doubled upon annealing from 3.4 ± 0.1 to 6.0 ± 0.1 nm, suggesting their collapsing. The height of the nanowires increases only slightly from 3.4 ± 0.1 to 3.9 ± 0.2 nm. The observed changes in morphology by in situ GISAXS are confirmed by the AFM study. Figure 5 shows AFM D


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Figure 6. GISAXS images of the DySi2 nanoislands with a nominal Dy coverage 0.26 nm recorded at αi ≈ 0.4° at the indicated temperatures with ki∥DySi2[010]∥Si[1−10] (along the steps of the vicinal Si(001) surface). RT* shows the GISAXS image obtained at RT after heat treatment. The white arrow points toward a streak-like scattering feature indicating formation of a facet.

aspect ratio of about 250 (1000 nm length/4 nm width). After the in situ annealing experiment the surface morphology has changed significantly, as evidenced by Figure 5b. The nanowires have collapsed upon annealing, forming small islands that, however, still tend to be elongated and parallel to the steps of the vicinal Si(001) surface (note the lower part of Figure 5b). Some of these islands coalesce, forming bigger islands. The in situ GISAXS experiment demonstrates that postgrowth annealing of the quenched nanowires at their growth temperature and above (range 500−580 °C) leads to their collapse. At about 580 °C the process is already completed, and no further changes take place up to the final annealing step at 757 °C. In their in situ PEEM study Zeman et al. reported a similar behavior for DySi2 nanowires formed on flat Si(001) surface,15 namely, the nanowires deposited at 700 °C start to decay already at 750 °C. They investigated nanowires exhibiting widths of about 10 nm and lengths of about 500 nm and observed that narrower wires decay more rapidly compared to the wider ones. The nanowires investigated in our experiment exhibit widths that are by more than a factor of 2 smaller and lengths by a factor of 2 bigger (about 5 times larger aspect ratio), which is most likely the reason for their rapid decay at relatively low annealing temperatures. DySi 2 Nanoislands. The thermal stability of DySi2 nanoislands formed upon deposition of 0.26 and 1.0 nm of Dy has been investigated by in situ GISAXS. Figure 6 shows selected GISAXS images from the sample with a nominal Dy coverage of 0.26 nm. The experimental data do not reveal obvious changes up to 655 °C. At this temperatures a broad streak-like scattering feature appears, indicated by the white arrow that is getting more pronounced with increasing temperature. This feature indicates the formation of a facet of the nanoislands upon annealing. The streak forms an angle of 28° ± 1° with respect to the vertical axis. The faceting of the nanoislands upon annealing has been intensively studied, for instance, for Ge/Si(001)33,34 Pd/MgO(100),35 Pt/W(111),36 or Ag/MgO(001)37 and appears to be a common phenomenon for various materials. The formation of facets is observed only

Figure 7. 3D projections of contact mode AFM images (1 ×1 μm2) of DySi2 nanoislands with a nominal coverage of 0.26 nm Dy obtained at RT (a) before and (b) after the in situ GISAXS experiment.

Figure 8. Width of the nanoislands with nominal Dy coverages of 0.26 and 1.0 nm as a function of the annealing temperature obtained from analysis of the GISAXS images from Figures 6 and 9, respectively (see text for details).

images of the nanowires obtained at RT before (a) and after (b) the GISAXS annealing experiment. The AFM image in Figure 5a reveals the presence of parallel nanowires with an E


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Figure 9. GISAXS images of the DySi2 nanoislands with a nominal Dy coverage of 1.0 nm recorded at αi ≈ 0.4° at the indicated temperatures with kin∥DySi2[010]. RT* shows the GISAXS image obtained at RT after heat treatment. The red arrow points toward a broad streak-like scattering feature indicating the presence of a facet, while the white arrow points toward a very narrow streak that indicated the presence of a second, larger facet.

at elevated temperatures since it is a thermodynamically driven process that is attributed to the surface free energy anisotropy.38 The AFM image (Figure 7a) obtained before the annealing experiment shows the formation of small nanoislands with high surface density. The AFM image obtained at room temperature after the in situ annealing experiment (Figure 7b) is dominated by nanoislands that are characterized by rectangular shape and much larger lateral dimensions. At the same time the number of smaller islands is reduced. The lateral sizes of the nanoislands, obtained from analysis of the horizontal scattering curve at qz ≈ 0.45 nm−1, indicated with a solid/white line in Figure 6, increased by about a factor of 2 from 19.0 ± 0.4 to 45.4 ± 0.6 nm, see Figure 8. The islands growth most likely occurs due to Ostwald ripening as also observed by Zeman et al., who

Figure 10. (a and b) 3D projections of contact mode AFM images (1 × 1 μm2) of DySi2 nanoislands with a nominal coverage of 1.0 nm Dy obtained at RT, respectively, before and after the in situ GISAXS experiment.

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growth rate due to Oswald ripening crucially depends on the initial size and surface density of the nanostructures.15 Our experiment confirms this fact and also shows that at very low coverages the coarsening dynamics starts at lower temperatures.

investigated the coarsening dynamics of DySi2 nanoislands on flat Si(001) using PEEM.15 They showed that DySi2 nanoislands are coarsening via Ostwald ripening upon annealing at 1077 °C. Our results demonstrate that this process is activated at much lower temperatures and similarly to the nanowires proceeds in the temperature interval 500−582 °C. Further notable changes in morphology upon annealing above 582 °C up to 757 °C have not been revealed. A similar behavior is observed for the sample with a nominal Dy coverage of 1.0 nm. However, in this case the analysis of the GISAXS data (Figure 9) alone is not enough to get a comprehensive understanding about the surface morphology evolution. The AFM image taken before the annealing experiment (Figure 10a) reveals the coexistence of islands having two different characteristic sizes and being equally distributed over the surface. The characteristic widths derived from the GISAXS data (see Figure 8), however, revealed only the sizes of the bigger nanoislands having a width of 73.5 ± 1.6 nm. During annealing, both kinds of islands have grown via Ostwald ripening that is visible in Figure 10b. The amount of the smaller islands on the surface, however, is dominating that of the bigger ones. Furthermore, due to the fixed 1.1 m sample to detector distance, the q range of the larger islands is no longer accessible, implying that the GISAXS signal is dominated by the islands with smaller dimensions. From Figure 8 it is visible that while the initially formed large islands grow in size, a trend indicated by the arrow at 580 °C, islands with a new size of 30.1 ± 1.0 nm dominate the scattering at temperatures above 582 °C. An indication for having islands with two different sizes is given by the scattering streaks marked with arrows in Figure 9. The dashed/red arrow points toward a rather broad streak that occurs upon annealing above 655 °C, indicating faceted small islands. This streak forms an angle of 20° ± 1° with respect to the vertical axis and is similar to the one observed for the sample with lower Dy coverage. The white arrow points toward a very sharp scattering streak occurring above 690 °C that is characteristic for big facets and thus bigger islands. The streak forms an angle of 39° ± 1° with respect to the vertical axis. Figure 8 summarizes the results from the performed analysis of the GISAXS data for both samples consisting of DySi2 nanoislands. The graph shows the evolution of their average widths as a function of the annealing temperature. The width of the nanoislands of the sample with Dy coverage of 0.26 nm is increasing by a factor of 2 in the temperature range between 500 and 582 °C. In the sample with Dy coverage 1.0 nm nanoislands with two different average sizes coexists, as evidenced by the AFM study (Figure 10a and 10b), with the bigger nanoislands dominating the scattering signal before annealing. Although the sizes of both kinds of islands have increased due to Ostwald ripening, the smaller nanoislands dominate the scattering signal after annealing at 582 °C and higher temperatures. Due to the q resolution, defined by the fixed sample to detector distance, the GISAXS experiment reveals only the average dimensions of the smaller nanoislands formed upon annealing. The data plotted in Figure 8 also indicate that the coarsening dynamics in the sample with lower Dy coverage is triggered at their growth temperature, despite the fact that the sample has been postgrowth annealed at about 600 °C for 10 min. We attribute this behavior to the high surface density of the rather small islands, as evidenced by the AFM image in Figure 7a, formed upon deposition of a subnanometer Dy coverage. Zeman et al. demonstrated that the

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CONCLUSIONS The thermal stability of parallel DySi2 nanowires with very high aspect ratio and of DySi2 nanoislands upon postgrowth annealing has been investigated by means of in situ GISAXS. Data analysis reveals a rather rapid decay of the nanowires during annealing at and slightly above the growth temperature (500 °C). Annealing at up to about 580 °C leads to their collapse and transformation into nanoislands doubling their width and correlation length. A faceting of the newly formed islands is observed at 655 °C and higher temperatures. A comparison with other studies suggests that the thermal stability is inversely proportional to the width of the nanowires. Additionally, the faceted growth of DySi2 nanoislands due to Ostwald ripening investigated by Zeman et al.15 has also been observed in our GISAXS experiment starting, however, at lower temperatures. We found that nanoislands coarsening already takes place in the temperature interval 500−582 °C for smaller and 582−655 °C for bigger nanoislands. We attribute this observation to the very high surface density of the initially formed small DySi2 nanoislands at subnanometer Dy coverage. In both cases broad facets are formed above about 660 °C, while the nanoislands with higher Dy coverage additionally reveal a very narrow facet above 690 °C. Furthermore, we demonstrate that the combination of in situ GISAXS with in situ AFM is a powerful approach for obtaining a comprehensive understanding of the surface morphology modification upon thermal treatment of inhomogeneous and complex nanolength-scaled objects.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

S.I.: Max Planck Institute for Solid State Research, Stuttgart, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to A. Weißhardt and H.-H. Gräfe for support in the UHV-Analysis lab at ANKA. S.S. acknowledges the financial support by the Initiative and Networking funds of the President of the Helmholtz Association and the Karlsruhe Institute of Technology via the Helmholtz-University Young Investigators Group “Interplay between structure and dynamics in epitaxial rare-earth nanostructures” contract VH-NG-625. The financial support for the UHV-Analysis lab via the Excellence Initiative within the project KIT-Nanolab@ANKA is acknowledged.

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Thermal Stability Studies of DySi2 Nanowires and Nanoislands by in

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