Growth of silicon nanostructures on graphite

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Surface Science 458 (2000) 113–122 www.elsevier.nl/locate/susc Growth of silicon nanostructures on graphite Paul Scheier 1, Bjo ¨ rn Marsen, Manuel Lonfat, Wolf-Dieter Schneider 2, Klaus Sattler * Department of Physics and Astronomy, University of Hawaii at Manoa, 2505 Correa Road, Honolulu, HI 96822, USA Received 23 November 1999; accepted for publication 14 February 2000 Abstract Silicon nanostructures such as small clusters, superclusters, and elongated chains, with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM ) reveals that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized silicon clusters of 0.6±0.2 nm, 5.1±1.2 nm, and 15.4±3 nm diameter. Surface regions with defects such as pits and craters, descending a few layers into the graphite surface, are sparsely covered with silicon. Most of the deposited material, with an average diameter of 2 nm, is found to be attached to the monatomic step edges forming the crater rims. A simulation of the growth process, i.e. deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, convincingly reproduces most of the Si nanostructures observed in the STM topographs. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Computer simulations; Growth; Scanning tunneling microscopy; Silicon; Sputter deposition 1. Introduction clusters by Honig [14], several experimental inves- tigations on silicon clusters have been performed [15–30], including a few STM studies [23,26–30]. Clusters deposited on well-defined surfaces Kuk et al. [23] deposited Si 10 clusters on Au(001) allow the construction of new materials with novel and observed a wide variety of different cluster properties [1]. The current urge for an ever images, even though size-selected clusters were decreasing size of components in the microelec- deposited. McComb et al. [26 ] observed a site- tronics industry renders this particularly relevant specific variation in the electronic characteristics for silicon clusters [2]. Their electronic and optical of Si clusters, which were deposited without size properties are especially sensitive to their size and selection but observed with atomic resolution. structure [3–13]. Since the earliest study on silicon Dinh et al. [27,28], in the context of an investiga- tion of the optical properties of passivated Si * Corresponding author. Tel.: +1-808-956-8941; nanostructures, synthesized Si nanocrystals by fax: +1-808-956-7107. E-mail addresses: paul.scheier@uibk.ac.at (P. Scheier), laser ablation and by thermal evaporation in an sattler@hawaii.edu ( K. Sattler) Ar buffer gas, and determined the size distribution 1 Permanent address: Institut fu ¨ r Ionenphysik, Universita ¨ t of a monolayer of these nanostructures on HOPG Innsbruck, A-6020 Innsbruck, Austria. with an STM. Size-selected Si 30 and Si 39 clusters 2 Permanent address: Institut de Physique de la Matie ` re were imaged with a low-temperature STM on Condense ´ e, Universite ´ de Lausanne, CH-1015 Lausanne, Switzerland. Ag(111) [29]. Manipulation experiments and the 0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 -60 28 ( 00 ) 0 0426-X 114 P. Scheier et al. / Surface Science 458 (2000) 113–122 appearance of the clusters in the images indicated pressure of p<10−6 Pa. This chamber was con- soft-landing of the clusters. Recently, again in an nected via vacuum locks to an analysis chamber STM study, Marsen and Sattler [30] succeeded in (base pressure p<10−8 Pa) equipped with a creating fullerene-structured nanowires of silicon Nanoscope II scanning tunneling microscope by magnetron sputtering on HOPG substrates. (STM) from Digital Instruments. For the synthesis The present STM study intends to investigate in of the Si nanostructures, a magnetron sputter more detail the sub-monolayer and monolayer source (MightyMak, Thin Film Products) was growth regimes of Si nanostructures on defect- used. In an argon atmosphere of 600 Pa at a poor and defect-rich HOPG surfaces. discharge voltage of 600 V and a typical Ar ion current of 0.2 A, a Si deposition rate of 0.3 nm/s was obtained. A quartz crystal micro-balance 2. Experimental mounted at a distance of 10 cm from the Si target monitored the flux during deposition. The cleaved HOPG substrate (7×7mm2), used to collect the The synthesis of Si nanostructures was per- formed in a high-vacuum chamber with a base sputtered Si, was mounted in a copper block Fig. 1. (a–c) Room-temperature constant-current topographs of a HOPG surface area at more than 1 ML coverage with silicon clusters. Image size: (a) 1.1×1.1 mm2,(b)44×44 nm2,(c)10×10 nm2; tunneling parameters: (a) U=1.0 V, I t =0.32 nA; (b, c) U= 2.5 V, I t =0.46 nA. (d ) Line-scan along the white line indicated in (c). (e) Size distribution of the silicon nanoclusters determined from the STM images (a–c). 115P. Scheier et al. / Surface Science 458 (2000) 113–122 (equipped with heating and cooling facilities) 5 cm contrast to Ref. [30], the amount of silicon depos- ited in the presently shown examples was muchin front of the sputter source. A manually operated shutter was placed between the sputter source and smaller since both the opening of the shutter and the argon ion current were reduced (<45 s insteadthe substrate holder during precleaning of the Si target and served to control the Si arrival fluences. of 2 min and 50 mA instead of 200 mA). After deposition, the sample was transferred in situ intoThe average size of the Si clusters synthesized by this technique could be varied by changing the the STM chamber in order to characterize the deposited silicon nanostructures under stringentsputter parameters, increasing (or decreasing) the source-to-substrate distance, or a combination of ultra-high-vacuum ( UHV ) conditions. All STM topographs presented in this work were taken withall these parameters [30]. In the present experi- ments, typical exposure times were varied from a Pt/Ir tips on the same sample and were recorded in constant current mode. The bias voltage betweenfew seconds to about a minute, yielding isolated clusters or cluster films with a thickness of 1– tip and sample is taken with respect to the latter. Tunneling resistances in the range between 100 MV3 ML (monolayers) on HOPG, respectively. In Fig. 2. (a–c) Room-temperature constant-current topographs of a HOPG surface area at about 0.1 ML coverage with silicon clusters. Image size: (a) 400 × 400 nm2,(b)94× 94 nm2,(c)41× 41 nm2; tunneling parameters, (a–c) U=1.96 V, I t =0.32 nA. (d ) Section of the HOPG hexagonal surface lattice showing the angle between armchair and zigzag directions. 116 P. Scheier et al. / Surface Science 458 (2000) 113–122 Fig. 3. (a–g) Room-temperature constant-current topographs of a HOPG surface area with nanopits of various depth at about 0.5 ML coverage with silicon clusters. Image size: (a) 350×350 nm2, (b) 150×150 nm2, (c) 100×100 nm2;(d)20×20 nm2; (e) 86×86 nm2; (f ) 100×100 nm2;(g)40×40 nm2; tunneling parameters: (a) U=−1.5 V, I t =0.38 nA; (b) U=1.1 V, I t =4.2 nA; (c) U=−1.3 V, I t =0.38 nA; (d ) U=−1.4 V, I t =0.38 nA; (c) U=1.6 V, I t =0.26 nA; (c) U=0.73 V, I t =0.51 nA; (c) U=1.2 V, I t =0.51 nA. (h) Constant-current topograph of a small island in the center of (f ) showing the graphite surface lattice with atomic resolution. Image size: 3×3nm2; tunneling parameters: U=1.2 V, I t =0.68 nA. and 6 GV yield identical images. Very similar ters. Two step edges of the HOPG substrate are clearly visible in the image due to the denseimages have been obtained from other samples prepared under the same experimental conditions. decoration with a chain of clusters. Fig. 1b and 1c show a 44×44 nm2 and a 10×10 nm2 area, taken across the left step in the bottom of Fig. 1a. These images reveal round Si-structures in the size range3. Results from 1 to several nanometers. A cross section, indicated by a white line in Fig. 1c and shown inFig. 1a shows a 1.1×1.1 mm2 area of an HOPG surface covered with about 3 ML of silicon clus- Fig. 1d reveals that the smallest round structures 117P. Scheier et al. / Surface Science 458 (2000) 113–122 Fig. 3. (continued) are semi-spherical with a diameter (FWHM ) of thermal evaporation in an Ar buffer gas and collected on HOPG, where the gathering of the Siabout 1 nm. Due to the convolution of tip and object geometries the clusters appear larger as in nanoclusters at step edges as well as their self- assembly into superclusters has been noted [31].reality. To correct for this effect we evaluated the tip dimensions on the widths of monatomic steps Fig. 2a shows an STM topograph of a 400×400 nm2 area of HOPG taken at a lateralof pure HOPG yielding a tip contribution of 0.3 nm. Fig. 1f shows the corrected size distribu- distance of several micrometers from the region shown in Fig. 1. Three step edges cross the imagetion of about 1000 Si-clusters obtained from an analysis of Fig. 1a -e. It follows that all observed from the bottom to the top. The two uppermost layers of graphite are partially folded back onnanostructures fall into three relatively narrow size ranges. The smallest structures have an average their left-hand side, a phenomenon already well known from earlier STM studies of HOPG [32,33].diameter of 0.6±0.2 nm, containing up to 10 Si atoms [3–13,29]. Larger aggregates exhibit diame- In contrast to the observations made in Fig. 1, the silicon coverage at this new position with a higherters of 5.1±1.2 nm, and the largest superclusters have sizes in the range of 15.4±3 nm. This obser- density of defects is significantly smaller (about 0.1 ML), and the step edges are less densely decor-vation suggests that the small clusters of 1 nm diameter constitute building blocks for the larger ated, although the flux of silicon atoms is expected to be homogeneous over much larger surface areas.aggregates. These findings confirm similar observa- tions made in a recent atomic force microscopy In the lower part of the uppermost terrace, an elongated Si structure is visible. A close-up of a(AFM) study of Si nanocrystals synthesized by 118 P. Scheier et al. / Surface Science 458 (2000) 113–122 94×94 nm2 area of this region reveals a chain of the Si-step decoration of the upper step edge (see Fig. 3c). silicon clusters at an angle of 41.3° with respect to We summarize our main experimental observa- the step edge. A combination of armchair and tions on the growth of Si nanostructures on HOPG zigzag directions in the 2D-graphite hexagonal as follows. The average silicon coverage varies by network yields an angle close to this value, as a factor of more than 10 between the surface illustrated in Fig. 2d. We conclude that the regions of different defect densities, separated by arrangement of the carbon surface atoms in this only 0.1 mm. The diameter of the clusters formed crystallographic direction provides favorable bind- onto defect-poor, flat surface regions is about ing sites for such a chain-like structure. An closer 0.6 nm, while clusters attached to step edges or look at this structure (Fig. 2c) reveals that the defects have diameters of about 2 nm and, occa- segments of this cluster chain have an average sionally, are found to be fused into rod- or tubelike thickness of 3.1±0.3 nm (see line scan) with structures. In the coverage range between 0.5 and lengths varying from 2.3 to 7.5 nm (uncorrected 5 ML, these small clusters often form superclusters. values). In addition, on the two terraces shown in The density and size of the clusters attached to the Fig. 2b and c, uniformly sized (1.1±0.1 nm) Si step edges forming HOPG nanopits are indepen- clusters are distributed randomly. Most of these dent of the width of their ‘feeding’ terraces. The small silicon clusters form distinct, loosely packed rims of nano-sized graphite islands on HOPG are groups (only 10% of the small clusters have no practically free of silicon decoration. neighbors). Fig. 3a–f shows constant current images of sur- face areas containing craters and pits [34] with 4. Simulation depth down to 10 ML. Every step edge of the descending terraces is decorated with a chain of In order to rationalize the above observations, silicon clusters. The average diameter of all cluster we simulate the growth process, i.e. adsorption, chains in this area is 3.1±1.1 nm (see Fig. 2c, line surface diffusion, and clustering of the silicon scan) and thus seem not to depend on the width atoms on HOPG within a simple two-dimensional of the terraces limited by the steps. This value model, as sketched in the flow diagram shown corresponds well with the average diameter of the in Fig. 4. cluster chain observed in Fig. 2. Almost no silicon In a first step, the topography of the surface is clusters are found on the flat terraces. Only in the defined. Within a two-dimensional array of upper left-hand corner of Fig. 3a is the density of 400×400 pixels, the location of a step or defect is the silicon clusters large enough to cover more assigned a value of 1 (black pixel ), and all other than just the steps. Images taken of adjacent positions are set to zero (white pixel ). Fig. 5a surface regions in this direction exhibit structures displays a typical example of such a model surface that are identical to those found in Fig. 1. corresponding closely with the experimental STM In Fig. 3b, c, f, and g, flat islands with diameters image shown in Fig. 3a. The number, n, of indivi- between 5 and 20 nm are visible on the larger dual Si atoms adsorbed on the surface is chosen. terraces. A high-resolution image of one of these Furthermore, the number of diffusion jumps of a islands is shown in Fig. 3h. A periodic lattice single atom before desorption is considered (max- identical to that of graphite is observed, which steps), representing the residence time at the allows us to identify these small islands as genuine surface. graphite nanoflakes. We note that we were able to All adsorbed atoms move randomly along the obtain this pattern only on very few HOPG flakes, surface. Whenever an atom desorbs or attaches to indicating a shift and/or rotation of these flakes a nucleation site, such as a cluster or defect, it is with respect to the underlying graphite layers. replaced by a new one. In case of attachment, the Finally, we note that in the STM images shown number of Si atoms at the nucleation site is increased by one. Collision with another adsorbedin Figs. 1–3, the respective line scans clearly reveal 119P. Scheier et al. / Surface Science 458 (2000) 113–122 Fig. 4. Flow diagram of a two-dimensional growth simulation. n is the number of single atoms that are attached to the surface at all times. If an atom desorbs, diffuses out of the defined surface area or fuses to a cluster or defect, it is replaced by a new incoming atom. n is proportional to the flux of impinging atoms. maxsteps is the number of diffusion steps that a single atom can perform before it desorbs from the surface. atom leads to the formation of the smallest cluster, f displays simulated growth patterns for a surface exhibiting the characteristic topography of Fig. 3a.a dimer. The probability for the desorption and the diffusion of clusters is set to zero. Therefore, The total number of adsorbed atoms that form clusters and decorations of defects is one milliononce a cluster is formed on a terrace, its migration to a defect is excluded, and a new nucleation site for all four simulations. An atom density of n=2 atoms was used in the case of images Fig. 5c andis created. Fig. 5b shows the number of atoms attached to each pixel of a surface area exhibiting d and n=1000 atoms for images Fig. 5e and f. In Fig. 5c and e, the residence time was short (max-five step edges in terms of a bar diagram. The height of each column represents the number of steps=1) whereas in Fig. 5d and f, an infinite residence time was assumed (maxsteps=2). Foratoms at this location at the end of a simulation. For a direct comparison of the simulated results the low atom density, clusters are formed almost exclusively along the steps and defects (Fig. 5c andwith the STM images, semi-spherical clusters were plotted where the cube of the radius is proportional d). In contrast, at high atom density, clusters are also formed on the terraces (Fig. 5e and f ). Atto the number of atoms within the cluster. Fig. 5c– 120 P. Scheier et al. / Surface Science 458 (2000) 113–122 Fig. 5. Simulation of the Si-cluster growth (see text). (a) Model surface with defects (black lines). (b) Bar diagram of the number of accumulated atoms at a few step edges. (c, d) Growth pattern after low-Si-atom density deposition at short (c) and long (d) residence times of the Si-atoms at the surface. (e, f ) Growth pattern after high-Si-atom density deposition at short (e) and long (f ) residence times. Note the excellent agreement of the results of these simulations with the observed growth patterns shown in Fig. 3. 121P. Scheier et al. / Surface Science 458 (2000) 113–122 short residence times, clusters are very uniform in flakes has a cluster attached to its edge. We attri- bute this observation to two effects:size ( Fig. 5c and d ), whereas for an infinite resi- dence time, their size depends strongly on the size 1. A vanishing Schwoebel–Ehrlich barrier on the step edge of these very small HOPG islandsof the feeding terrace. allowing for interlayer diffusion. At a critical island size, such an effect has been invoked to be responsible for ‘landsliding’ on small Cu 5. Discussion islands [36,37]. 2. Bond weakening of the HOPG nanoflakes A comparison of the experimental STM image towards the adsorbed Si atoms due to weak of Fig. 3a with the results of the simulation dis- coupling of these flakes to the underlying graph- played in Fig. 5 clearly suggests that our simulation ite surface. In the case of Pt on HOPG, the captures the essential physics of the growth pro- perfect stacking of the graphite layers has been cess. After adsorption of single silicon atoms on shown to be important for optimal bonding the HOPG surface, these adatoms move randomly [38,39]. by thermal diffusion along the surface. Collisions among them lead to the growth of Si clusters. Step edges with unsaturated or dangling bonds consti- 6. Summary and conclusions tute preferred nucleation sites and exhibit an effec- tive Schwoebel–Ehrlich [35] barrier for interlayer Silicon nanoparticles were synthesized using diffusion of Si adatoms. On surface regions with magnetron sputtering deposition onto cleaved an increased defect density, we observe a much HOPG. The resulting Si nanostructures were inves- smaller coverage of silicon (see Figs. 2 and 3: tigated with STM. On defect-poor, flat regions of 0.1 ML) than on surface regions with a low defect the HOPG surface, Si clusters with a mean diame- density (see Fig. 1: 3 ML). For the simulation ter of about 0.6 nm and a narrow size distribution shown in Fig. 5c (which gives the best agreement were found. On defect-rich surface regions, step with the STM topograph), the residence time of edge decoration was observed almost exclusively, the adatoms was short. In other words, most of while the terraces were free of attached particles. the diffusing Si atoms desorb from the surface A simple two-dimensional simulation of the Si before they encounter a defect or collide with cluster growth successfully describes most of the another Si atom and form a cluster on a terrace. experimental observations, e.g. the gathering of For Fig. 5c, the second parameter used in the clusters on step edges and the formation of clusters simulation (the density of silicon atoms) was low. and superclusters on the terraces. The simulation This parameter can be interpreted as a combina- leads to the conclusion that the sticking coefficient tion of the flux from the sputter source and the of the HOPG surface depends on the density of sticking coefficient. In order to end up with the the defects. The STM topographs reveal that the same number of 1 million silicon atoms attached silicon coverage on a defect-rich surface region is to the surface (either in the form of clusters on smaller by a factor of ~30 than on a defect-poor terraces or decorations of edges and defects), the region of the same sample. In view of the present actual number of initial silicon atoms that hit our results, magnetron sputtering might provide an model surface was, in the case of Fig. 5c, about interesting alternative route towards the pro- 100 times larger than for Fig 5f. This agrees very duction of Si nanostructures with potential appli- well with the experimental observation that a cations in future silicon nanotechnology. defect-poor region has about 30 times more silicon attached than the defect-rich region of the same sample. Acknowledgements The areas that exhibit pits and craters also contain small HOPG islands or flakes that are practically free of adsorbed silicon particles. 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