Fabricating nanometer-scale Ag lines and islands on Cu(100) surfaces

Surface Science 440 (1999) L835–L840 www.elsevier.nl/locate/susc

Surface Science Letters

Fabricating nanometer-scale Ag lines and islands on Cu(100) surfaces S.L. Silva, F.M. Leibsle * Dept. of Physics, University of Missouri, 5100 Rockhill Rd, Kansas City, MO 64110, USA Received 10 June 1999; accepted for publication 23 July 1999

Abstract We have fabricated epitaxially grown Ag lines on Cu(100) surfaces which are 5 nm wide by several tens of nanometers long by up to four atomic layers high. This was accomplished by first templating the surface with a selfassembled array of islands of an atomic nitrogen-induced structure. These N-islands are sufficiently inert that Ag growth occurs preferentially in the regions of clean surface between the N-islands. Ag growth onto a completely saturated Cu(100)-c(2×2)N surface results in nanometer-scale single crystals of Ag. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper; Epitaxy; Nitrogen; Scanning tunneling microscopy; Silver; Single crystal surfaces

The controlled fabrication of nanometer-scale structures on surfaces is a tremendous challenge. Methodologies based on using scanning probe microscopy to modify surfaces or manipulate atoms are limited both by the difficulty and time required to create these structures over macroscopic distances [1,2]. Epitaxial growth onto vicinal surfaces enables one to mass produce nanometer-scale structures and has the inherent flexibility of being able to choose the offcut angle of the substrate [3,4]. However, the difficulty in using vicinal surfaces is creating structures which are more than a single atomic layer thick. Utilizing periodic features on substrates as preferential nucleation sites again enables mass production * Corresponding author. Fax: +1-816-235-5221. E-mail address: [email protected] (F.M. Leibsle)

[5–7] but may not result in a particularly high degree of crystallinity within the grown material, and again one has difficulties in producing structures with thicknesses of more than a single atomic layer. Producing structures with thicknesses greater than a single atomic layer is extremely important in that a material’s electronic, magnetic and chemical properties depend on its crystalline order. In building nanostructures on surfaces, one desires layer-by-layer (Frank–Van der Merwe) growth to occur in laterally confined regions. In this study, we utilize an alternative method [8] to modify the growth of Ag onto Cu(100) surfaces resulting in the creation of nanometer-scale epitaxially grown Ag lines several layers thick. Basically, we first create a self-assembled array of nanometer-scale islands consisting of an atomic nitrogen-induced

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c(2×2) structure. These relatively inert islands form a template for the epitaxial growth of Ag. We also demonstrate that the growth mode of Ag onto the Cu(100) surface is altered when Ag is grown onto a completely saturated c(2×2)N surface resulting in the formations of nanometer-scale Ag crystals. These experiments were performed in a conventional ultrahigh vacuum ( UHV ) chamber equipped with a commercial room temperature Omicron Vakuumphysik scanning tunneling microscope (STM ) and low energy electron diffraction (LEED). The Cu(100) surfaces were cleaned using cycles of argon ion bombardment followed by annealing to 720 K. Nitrogen deposition was performed by bombarding the surface with 500 eV nitrogen ions followed by annealing (for details, see Ref. [9]). Ag was evaporated from a tantalum crucible heated by a feedback controlled electron beam. Deposition rates were determined by a quartz crystal thickness monitor. Ag thicknesses are measured in monolayers (ML) where 1 ML=1.5×1015 atoms /cm2, the number of atoms per unit area of a Cu(100) surface. We thought it prudent to first begin our studies by reproducing previous STM results for Ag and N deposition onto clean Cu(100) surfaces. Sprunger et al. studied Ag growth on Cu(100) surfaces [10]. Basically, they observed Frank–Van der Merwe growth with the first two Ag layers showing c(10×2) periodicity. Subsequent layers showed hexagonal periodicities indicative of Ag(111) planes. Fig. 1a shows an image of a typical area of a Cu(100) surface following the deposition of 1.5 ML of Ag deposited at room temperature followed by annealing to 425 K. We see periodic features on both the island in the center of the image and on the substrate. These features have a c(10×2) periodicity and the inset in Fig. 1a shows an atomic resolution image. These Fig. 1. Two STM images showing Ag and N structures on Cu(100) surface. (a) A 20×20 nm2 area of a Cu(100) surface following deposition of 1.5 ML of Ag. One sees terraces and oval monatomic islands both displaying c(10×2) periodicities. The contrast in the image has been enhanced to show the c(10×2) structures on the monatomic islands. The inset shows atomic resolution within the c(10×2) structures virtually identical to the work of Ref. [10]. (b) A 50×50 nm2 STM image of an

ordered array of N-islands. The small 5×5 nm2 islands, which appear dark in the image, align in long rows separated by areas of the clean surface. These rows even align across step edges. The inset shows atomic resolution of the c(2×2) structures within the islands. The associated linescan across several islands and a single monatomic step shows that the islands are imaged as depressions relative to the clean surface.

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images are virtually identical to the images obtained in Ref. [10] and demonstrate the ability to grow Ag on Cu(100) surfaces. Fig. 1b demonstrates the ability to create arrays of N-islands on this surface, as has been reported in the literature [8,9]. The surface imaged here was created by bombarding the crystal with 500 eV nitrogen ions for a short period of time so that a total integrated ion flux of 80 mC was reached. Here, we annealed the surface at 600 K followed by a slow cooling (10 K/min) to room temperature. We have found that this slow cooling results in surfaces on which the N-islands show a high degree of alignment. One sees from Fig. 1b long, equally spaced rows of N islands, running in the 001 directions, which even continue across step edges. We note that such long range periodic self-organization has also been observed in the Cu(110)-(2×1)O system [11]. In this, and other images of the N-islands, the square shaped N-islands appear dark in the STM images (the inset in Fig. 1b shows the internal c(2×2) structure of the islands). The c(2×2)N structure is believed to be incommensurate and the small islands formed as a strain relief mechanism [9]. The dark appearance of the islands is attributed to electronic effects — namely the reduction of the local density of states near the Fermi level [9]. Depending on the STM tip, linescans taken across the N-islands show them imaged 0.03– 0.1 nm below neighboring areas of the clean surface. It has been previously observed that Fe and Cu deposited onto surfaces such as that shown in Fig. 1b will preferentially grow on the regions of clean surface between the N-islands [8]. This preliminary study concerned itself with growth on room temperature surfaces and did not examine the effects of annealing nor was growth beyond the first atomic layer satisfactorily investigated, nor were atomic resolution images obtained from the epitaxially grown islands. Fig. 2a shows a typical area of the surface following a 1 ML Ag deposition at RT followed by annealing to 425 K. One sees that growth has occurred in the regions between the N-islands. Due to the alignment of the N-islands across step edges, one also sees the alignment of Ag islands across step edges. Linescans show that the tip now retracts 0.4 nm

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as it moves from the N-islands to the surrounding Ag regions, indicative that the Ag islands are second layer growth. Fig. 2b shows an array of Ag lines created by depositing 2 ML of Ag onto a N-templated surface followed by annealing to 425 K. Linescans indicate that these Ag lines are four Ag layers thick. For thicknesses greater than four layers, substantial overgrowth of the N-islands begins to occur. Fig. 2c shows a collage of atomic resolution images, starting at the lower left going clockwise one sees c(2×2)N structures which are found between the Ag lines, c(10×2) Ag structures which are observed between the N-islands for the first two layers of Ag growth, and hexagonal Ag(111) structures found on top of the Ag lines for the third and fourth Ag layers. The observation of c(10×2) and hexagonal periodicities on the Ag lines is indicative that Ag is growing in a layer-by-layer fashion between the islands which implies that these lines are small single crystals of Ag. In terms commonly used by the semiconductor industry, these islands shown in Fig. 2 are 0.005 mm wide. We wish to emphasize the importance of creating nanometer-scale single crystals in that a material’s electronic, magnetic and chemical properties are strongly dependent on crystalline order. We also examined the growth of Ag onto completely saturated Cu(100)-c(2×2)N surfaces. Fig. 3a shows an area of a surface which was exposed to a 400 mC dose of nitrogen followed by annealing to 600 K. This image shows an essentially flat area of a terrace which contains several atomically straight trenches and several small monolayer high islands. These structures are characteristic of a saturated c(2×2)N surface and the trenches are believed to be a way of dealing with the induced surface strain of the c(2×2) structure at high N coverages [9]. Atomic resolution images show the c(2×2)N structures. Fig. 3b shows the surface after a 2 ML Ag deposition at RT without any annealing after the evaporation. The surface now appears covered by small islands, many of which appear hexagonally or trapezoidally shaped. On the top of the islands, as the inset in Fig. 3b shows, one can observe atomic features arrayed in hexagonal symmetry similar to that observed in Fig. 2c. Linescans across these structures indicate

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Fig. 2. STM images and associated linescans demonstrating that the N-islands serve to create templates for Ag growth. (a) A 50×50 nm2 region after 1 ML Ag deposition followed by annealing to 425 K. Ag has grown forming 0.005 mm wide lines, primarily two atomic layers thick, between the N-islands. Due to the alignment of the N-islands across step edges, the Ag lines also align across step edges and one can see instances of third layer growth as well. The linescans show that these second and third Ag layers are imaged 0.4 and 0.6 nm above the N-islands. (b) A 50×50 nm2 area after 2 ML Ag deposition resulting in an array of Ag lines four layers thick as evidenced by the 1.0 nm corrugation observed in the corresponding linescan. (c) Three STM images with atomic resolution. From bottom left going clockwise, we see a 7×7 nm2 image of a c(2×2)N island found between the Ag lines, a 4×4 nm2 image of c(10×2) Ag structures found between the N-islands for the first two layers of Ag growth, and a 4×8 nm2 image of the hexagonal Ag structures found on the tops of Ag lines with thicknesses greater than or equal to 3 layers.

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Fig. 3. STM and LEED results for Ag growth onto completely saturated Cu(100)-c(2×2)N surfaces which demonstrate the formation of nanometer-scale Ag single crystals. (a) A 50×50 nm2 image of a completely saturated c(2×2)N surface. The surface shows c(2×2) periodicity (1.5×1.5 nm2 inset). For saturation N-coverages, networks of narrow trenches are formed, apparently as a strain relief mechanism [9]. Material ejected from the trenches forms small single layer islands (bright areas). N-induced c(2×2) structures are present on the terraces, islands and bottoms of the trenches. (b) A 50×50 nm2 image and associated linescan following 2 ML Ag deposition. Ag forms small islands several layers thick. The top faces of the islands are flat and hexagonal atomic features (1.0×1.0 nm2 inset) can be observed, indicative of Ag(111) planes. The N structures appear to be intact between the Ag islands as evidenced by the network of trenches as seen in the image. (c) Schematic of the LEED pattern observed from the surface shown in (b) at 54 eV. Open circles represent integral order spots, dashed circles represent the half-order spots of the c(2×2)N periodicities, and the filled circles represent the additional spots due to two rotationally equivalent domains of the hexagonal Ag structures.

that they are imaged 1.0–1.8 nm above the dark areas between the islands. This is indicative that the islands are several atomic layers thick. To

reveal details within these dark areas, we subjected the image to a high pass filter. One can clearly see the trench-like structures indicative of the

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Cu(100)-c(2×2)N surface suggesting that this surface is intact between, and possibly under, the Ag islands. Subsequent annealing to 425 K failed to produce significant change. LEED patterns from this surface (shown schematically in Fig. 3c) were similar to those obtained from the surface shown in Fig. 2b. At 54 eV, one sees a c(2×2) periodicity coexisting with evidence of two rotationally equivalent hexagonal structures [12]. The atomic resolution images of the tops of the islands and the hexagonal periodicities in the LEED patterns demonstrate that the small islands observed in Fig. 3b are due to small single crystals of Ag. The density of these small single crystals is 2×1011cm−2. We also note that the growth mode of Ag onto saturated Cu(100)-c(2×2)N surfaces is Volmer–Weber instead of Frank–Van der Merwe. In conclusion, we have shown that Ag grown onto N-templated Cu(100) surfaces can result in lateral confinement resulting in single crystals of Ag which are 5 nm wide by several tens of nanometers long by up to four atomic layers thick. Ag growth onto completely saturated c(2×2)N surfaces results in the observation of nanometer-scale Ag single crystals. We believe that methodology employed here may serve as an attractive alternative for creating nanometer-scale structures on surfaces.

Acknowledgement This work was partially supported by the University of Missouri Research Board.

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