1997 nucleation of homoepitaxial si chains on si(001) at room temperature

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  • 1. __ @ ELSEVIER Surface Science 377-379 (1997) 1006-1009 Nucleation of homoepitaxial Si chains on Si( 001) at room temperature J. van Wingerden, M.J. Haye, P.M.L.O. Scholte *, F. Tuinstra Delft University of Technology, Applied Physics Department, Lorentzweg I, 2628 CJ De& The Netherlands Received 1 August 1996; accepted for publication 15 October 1996 Abstract Results are presented of a microscopic study of the formation of epitaxial Si dimer chains. We lind that dimers adsorb on a limited number of sites only. Interactions between dimers and diffusing adatoms lead to the formation of three-atom clusters which can be extended to diluted lines of dimers. Only two types of lines are observed: lines along [ 1 lo] and lines along [310]. These lines connect to each other and form a random network. Upon further deposition, the diluted dinner lines transform into epitaxial dimer rows. This transition starts at the end of the lines by reorienting the dimers in the line and adding mobile adatoms. Keywords: Adatoms; Clusters; Epitaxy; Growth; Models of surface kinetics; Molecular beam epitaxy; Nucleation; Scanning tunneling microscopy; Self-assembly; Semiconducting tilms; Silicon; Single crystal epitaxy; Stepped single crystal surfaces; Sticking; Surface diffusion; Surface energy; Surface structure, morphology, roughness, and topography; Vicinal single crystal surfaces 1. Introduction In an alternative approach, structures with atomically sharp boundaries can be made by using With the ongoing miniaturisation in electronic the self-assembly of atoms during deposition. devices, standard lithographic techniques such as Examples are the spontaneous formation of Ge electron-beam lithography and masking are near- hut clusters during the epitaxial growth of Ge on ing their limits. To be able to produce conducting Si(OO1) [3], and SiGe quantum dots grown by the structures, even with atomic dimensions, new tech- codeposition of Si and Ge [4]. The morphology niques have to be developed which do not involve and crystal structure of the dots and clusters are the use of statistical processes such as etching. At determined by preferred adsorption sites, diffusion the moment, considerable effort is going on to properties of the adatoms and lattice mismatch. investigate the potential of scanning probe micro- Also, one-dimensional structures can be formed scopes to manipulate atoms and clusters on a by processes which involve self-assembly. In partic- surface into ordered structures (see, for example, ular, during the first stages of the growth of group the quantum corrals of Eigler et al. [l]), or to III, IV and V materials onto Si and Ge (001) modify the surface locally with atomic resolution surfaces, chains of dimers are formed [5-71. VI* In the present paper, we present a detailed study of the formation of Si chains on a Si(OO1) surface. * Corresponding author. Fax: +31 15 278 3251; We will discuss the formation of the chains starting e-mail: scholte@duttncb.tn.tudelft.nl from the formation of single dimers. In Section 2 0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PZZ SOO39-6028(96)01535-X
  • 2. J. van Wingerden et al. j Surface Science 377-379 (1997) 1006-1009 the experimental details are given. Then we will discuss the preferred adsorption sites of single dimers and their extension to long chains, before proceeding to a brief discussion of the electronic structure of a single dimer chain. 2. Experimental The experiments were performed in a UHV system with a base pressure of 5 x lo-l1 Torr. A commercial beetle-type scanning tunnelling micro- scope (STM) was used for measuring constant- current STM images. The surface dynamics at room temperature was studied by capturing STM images at 10 s intervals and combining them into a movie. Clean Si(OOl)-(2 x 1) surfaces with low defect density and monolayer steps were prepared by flashing the surface to 1250°C. Submonolayers Fig. 1. Filled ((a) and (c)) and empty ((b) and (d)) state images of Si were deposited using a commercial miniature of isolated Si dimers on a Si(OOl)-(2 x 1) substrate. The letters electron-beam evaporator. For studies of the room- refer to the adsorption sites of Fig. 2. Tunnelling conditions: temperature growth processes, we waited at least (a)-(d) Itunnel=0 4 nA; (a) and (c) Vbias=1.3 V, (b) 4 h before deposition after flashing. V,,ias=-1.3V, (d) Vbias=-1.15V. 3. Results The mobility of Si adatoms is too high to be able to observe them with STM at room temper- ature. Only adatoms which are adsorbed at surface steps or other surface defects can be observed in empty-state images. Therefore, the pathway for the formation of dimers from single adatoms is not directly accessible by STM. Indirect evidence for this pathway can be obtained from the relative abundance of the preferred adsorption sites of the Fig. 2. Schematic drawings of the orientations of isolated dimers. However, discussion of this formation adsorbed dimers and clusters of two dimers on neighbouring process is beyond the scope of this paper, and will substrate dimer rows with their notations. be undertaken elsewhere [8]. In Fig. 1 both empty-state and filled-state images 67% are at C-positions, while the rest are at either of single Si dimers are shown. The notation for A or B positions. The ratio between the occupan- indicating the different adsorption sites is indicated cies of A and B sites is approximately 1:lO. in Fig. 2. At higher coverages, the dimers start to form At room temperature, the abundance of D clusters. In Fig. 3 two important types of clusters dimers is negligible. Furthermore, we have only are shown: the twin and the cross [9]. The twin observed D dimers in the direct neighbourhood of can be considered as a rearranged BB couple, other dimers or of surface steps, i.e. they are never where two new substrate dimer bonds are formed isolated. Of all the isolated dimers, approximately (see inset in Fig. 3b). It is characterised by the
  • 3. J. van Wingerden et al. / Surface Science 377-379 (1997) 1006-1009 Fig. 4. A V-shaped chain of C dimers along [310] directions. (a) Filled-state image, (b) empty-state image. (c) Metastable adsorption sites for adatoms near to a C dimer. Tunnelling conditions: Itunnel= 0.4nA, (a) Vbias=1.3V, (b) Vbias= -1.3 v. Fig. 3. Filled- and empty-state images of a twin and a cross structure. (a) Filled-state image of a twin, (b) empty-state image of a twin, (c) filled-state image of a cross structure, (d) empty- state image of a cross structure. The insets in (b) and (d) show this process will lead to a chain of C dimers in the a schematic drawing of a twin and a cross. The horizontal grey [ 3 lo] direction [ 81. An example is shown in Fig. 4. lines indicate substrate dimer bonds. Upon further deposition, the diluted dimer rows are transformed into epitaxial chains of BD dimers. fainter appearance of the B dimers which are We observed that this conversion takes place exclu- involved in the twin, relative to non-bonded B sively at the end of the [ 1lo] dimer rows. Both B dimers (see Fig. 3a). and D dimers were observed as the last dimers in The cross structure is essentially a C dimer with the chain. This corroborates a model in which the two adatoms attached to it, as shown in the inset conversion initiates by forming a B dimer from an of Fig. 3d. The twin is a “closed” structure which atom of the outer C dimer with the adatom at the cannot be easily extended into a chain of dimers. end of the diluted dimer row. Repeating this In contrast, the cross structure can. By adding process once more results in the BB twin. Finally, extra atoms at the adatoms of the cross, a so-called a collapse of the twin leads to the formation of an diluted dimer row of C dimers along [ 1101 is epitaxial BD segment, yielding an end sequence of formed [9]. At the ends of the diluted dimer row, C-adatom-B-D. single adatoms can be attached. It should be noted The conductivity of the chains demands special that this diluted dimer row is essentially non- attention, since it is not trivial that a single row epitaxial, since the orientation of the dimers is of atoms will be conducting or semiconducting, parallel to the substrate dimers. even if the element is. For example, ab-initio Also, the C dimers may form ordered structures. calculations by Brocks et al. [lo] indicate that a There are two preferred adsorption sites for single single chain of Al dimers may be semiconducting. adatoms in the neighbourhood of a C dimer. The On the other hand, in the case of Au on Si( 11l), first was indicated above in the cross structure. It 1D conduction in a cluster of lines has been is in line with the C-dimer bond (see Fig. 3d). The observed [ 111. second preferred adsorption site is indicated in We have calculated the band structure of a single Fig. 4c. If two adatoms are subsequently adsorbed epitaxial dimer chain on a Si(OO1) surface, using at this site, a new C dimer is formed. Repeating a Car-Parrinello ab-initio method. A detailed
  • 4. J. van Wingerden et al. / Surface Science 377-379 (1997) 1006-1009 1009 discussion of the results will be published elsewhere along the row. These states appear to be localized [ 121. From the electron density obtained, the corre- on the chain. However, the chains are semicon- sponding STM image can be computed. The result ducting and not metallic. Our calculations show is shown in the inset of Fig. 5. The surface was that it is possible to fill the empty states by doping prepared by depositing Si at a slightly elevated the Si chains with alkali atoms. We find that the temperature, causing the dimers to line up in dimer band structure of the chain essentially does not rows. The calculation result, placed in line with a alter, while the Fermi level shifts upward, resulting dimer row, shows a good agreement with the in a Si dimer chain which conducts in the direction experiment. We find that the epitaxial dimer chains along the chain [ 131. have empty electron states with a high dispersion References [l] M.F. Crommie, C.P. Lutz and D.M. Eigler, Science 262 (1993) 218. [2] D.H. Huang, H. Uchida and M. Aono, Jpn. J. Appl. Phys. 31 (1992) 4501. [3] Y.-W. MO. D.E. Savage, B.S. Swartzenruber and M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020. [4] J. Tersoff, C. Teichert and M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675. [5] J. Nogami, A.A. Baski and C.F. Quate, Phys. Rev. B 44 (1991) 1415. 61 Y.-W. MO, R. Kariotis, B.S. Swartzenruber, M.B. Webb and M.G. Lagally, J. Vat. Sci. Technol. A 8 (1990) 201. 71 J. Nogami, A.A. Baski and C.F. Quate, Appl. Phys. Lett. 58 (1991) 475. 81 J. van Wingerden, A. van Dam, M.J. Haye, P.M.L.O. Scholte and F. Tuinstra, Phys. Rev. B 55 (1997) 4723. 91 A. van Dam, J. van Wingerden, M.J. Haye, P.M.L.O. Scholte and F. Tuinstra, Phys. Rev. B 54 (1996) 1557. lo] G. Brocks, P.J. Kelly and R. Car, J. Vat. Sci. Technol. B 12 (1994) 2705. 111 I.R. Collins, J.T. Moran, P.T. Andrews, R. Cosso, J.D. Fig. 5. Filled-state image of epitaxial dimer chains on Si(OOl)- O’ Mahony, J.F. McGilp and G. Margaritondo, Surf. Sci. (2 x 1). The inset shows the result of an ab-initio calculation. 325 (1995) 45. Tunnelling conditions: Itunnel 0.4 nA, Vbias= 1.3 V. = [ 121 M. Haye, to be published.