Surface Science 454­456 (2000) 802­806 www.elsevier.nl/locate/susc A kinetic scanning tunneling microscopy study of iron silicide growth on Si(113) M. Kneppe, V. Dorna, P. Kohstall, E. Kot, U. Ko¨hler * Experimentalphysik IV/Oberfla¨chenphysik, Ruhr-Universita¨t Bochum, D-44780 Bochum, Germany Abstract High-temperature kinetic scanning tunneling microscopy (STM) studies are used to investigate the surface morphology and growth mode of iron silicide on Si(113) formed by gas-source reactive iron deposition with Fe(CO)5 as precursor. The first monolayer of silicide on Si(113) forms a (4×n) reconstruction that covers the surface completely before growth proceeds via the formation of strongly anisotropic, three-dimensional silicide islands. After the first monolayer is closed, growth is slowed down by a blocked interdiffusion with the silicon substrate and a reduced sticking probability for the precursor. Lateral spreading of the islands is achieved by a stoichiometric codeposition of iron and silicon using Fe(CO)5 and Si2H6. In this way, nearly closed layers of silicide can be grown. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Low energy electron diffraction (LEED); Metal­semiconductor nonmagnetic thin film structures; Scanning tunneling microscopy; Silicides; Single crystal epitaxy 1. Introduction (111) to (001) a number of stable faces have been found [3]. In particular, Si(113), being the most Despite the interesting physical properties of stable of these faces, has attracted considerable iron silicide, there is still a lack of technological interest in past years. Whereas the atomic arrange- applications. Problems regarding wetting of the ment of the Si(113)-(3×2) reconstruction seems silicon substrates, rough interfaces and the pres- to be resolved by a model containing a subsurface ence of different competing silicide modifications interstitial [4], little is known about the growth diminish the quality of the layers. Especially on behavior on this surface. In homoepitaxial growth Si(111), three-dimensional (3D) silicide islands on Si(113), a strong anisotropy in the island shape form already in the submonolayer range [1,2]. was found [5] that led to nearly one-dimensional Higher-index or vicinal silicon surfaces may offer structures along the [11:0] direction. Another pos- one way to overcome part of these problems. sible application of Si(113) is given by the faceting When the orientation is varied continuously from behavior when the surface is annealed [6]. The resulting periodic groove arrangement may serve as a template during heteroepitaxial growth to produce nanostructured layers. This paper presents * Corresponding author. E-mail address: ulrich.koehler@rz.ruhr-uni-bochum.de an overview on the nucleation and initial growth (U. Ko¨hler) behavior of iron silicide on the Si(113) surface. 0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0039-6028 ( 00 ) 00275-2 M. Kneppe et al. / Surface Science 454­456 (2000) 802­806 803 2. Experimental 3. Results All experiments were performed in an ultrahigh Fig. 1 shows the morphology of the Si(113) vacuum (UHV ) chamber equipped with optics for surface after an Fe(CO)5 exposure of 5 min at low-energy electron diffraction (LEED) and a p=1×10-7 mbar at 400°C. Whereas on Si(111) high-temperature scanning tunneling microscope iron deposition at elevated temperatures leads (STM) that was specially designed for in situ immediately to the formation of 3D islands of iron measurements directly during growth. The temper- silicide, which cover only a fraction of the surface ature of the substrate could be adjusted between [1,2], on Si(113) a two-dimensional (2D) iron- room temperature (RT) and 850°C, while induced reconstruction is formed. Fig. 1a displays sequences of scanning tunneling microscopy details of this structure. On the left the clean (STM) images of a selected location on the surface Si(113) surface is visible and, on the right, an were taken to study the kinetics of epitaxial iron-induced chain structure along the [11:0] direc- growth. The gas-phase precursors Fe(CO)5 and tion. The protrusions in the chains have a separa- Si tion of 15 A along [11:0] which, by comparison 2H6 were supplied through a standard UHV leak valve controlled by an ion gauge and a quadru- with the (3×2) reconstruction of the silicon sub- pople mass analyzer (QMA). The Si(113) sub- strate, can be identified as a four-fold periodicity. strate samples were prepared by outgassing the In the [332:] direction a six-fold periodicity is samples at 600°C for 8 h, followed by slow oxide visible. If the same sticking coefficient of #0.03 removal at 800°C and several short flashes up to for the Fe(CO)5 precursor is assumed on Si(113) 1250°C. To exclude any influence of the STM as on the Si(111) surface, each (4×6) unit cell scanning process on the deposition from the gas contains 10­15 iron atoms. If these iron atoms are phase, bias voltages below 2.5 V were used. A located in the outermost surface layer, the iron- comparison of ex situ (not in the STM) and in induced chain reconstruction can be considered as situ grown silicides shows exactly the same struc- a silicide structure. Because of the complex struc- ture of the layers. All STM images shown in this ture of the reconstruction that contains chain paper were taken in the constant-current mode structures also in lower levels in between the main with a tunneling current of 0.5 nA and a sample chains (see Fig. 1a), it is impossible to make an bias of -2 V. atomic model on the basis of the present data set. Fig. 1. Structure of the submonolayer iron silicide on Si(113) grown at 400°C and scanned at RT. (a) STM image (165 A ×150 A ): chains of protrusions along [11:0] show a four-fold periodicity. (b) Larger area (370 A ×440 A ): in the [332:] direction, two-fold, four- fold and [as seen in (a)] six-fold periodicities are found. (c) LEED pattern of a surface completely covered by the chain structure. The mixture of periodicities along [332:] results in the formation of streaks. 804 M. Kneppe et al. / Surface Science 454­456 (2000) 802­806 Fig. 2. Sequence of STM images (derivative image) showing details of the completion of the first monolayer of silicide (a and b) before 3D islands are formed (c and d). Fig. 1b shows a larger arrangement of chains. The place. The growth is slowed down dramatically distance between the chains along [332:] varies when the first monolayer is closed. The 3D islands between 2 and 6 silicon unit cells. The chain grow slowly in height but lateral spreading is structure is therefore referred to as a (4×n). largely suppressed. The presence of the completely Whereas in the [11:0] direction the four-fold period- closed 2D silicide layer seems to block silicon out- icity is typically maintained in domains of five to diffusion in contrast to iron silicide formation on 15 protrusions, in the [332:] direction an average Si(111), where the substrate is removed layer-by- domain size of only two or three chains is present. layer to provide silicon for the formation of iron This low degree of order leads to the formation of silicide islands [2]. Besides the blocked silicon streaks in the LEED pattern (Fig. 1c) perpendicu- diffusion, the slowing down of the silicide growth lar to [11:0]. Along [11:0], on the other hand, after completion of the 2D layers has also to be narrow diffraction features are visible with a four- connected to a dramatically decreased sticking fold periodicity. probability for the Fe(CO)5 precursor, since no Upon further iron deposition the surface is piling up of bare iron is found on the surface. completely covered by the (4×n) structure. Fig. 2 Further experiments are in progress to quantify shows the completion of the 2D silicide structure this behavior. at 485°C and the subsequent growth of 3D silicide This blocked silicon diffusion, on the other islands (Fig. 2c and d). Obviously, iron silicide hand, should result in a sharp interface between grows on the Si(113) surface in the Stranski­ the substrate and the silicide layers when both iron Krastanov mode. The 3D islands are strongly and silicon are supplied. Fig. 4 shows the resulting elongated in the [11:0] direction, displaying an growth at 485°C when iron is supplied by aspect ratio of more than 8 (see also Fig. 3. A p=2.1×10-7 mbar of Fe(CO)5 and silicon by similar anisotropy in growth has also been found p=1.9×10-7 mbar of Si2H6. for homoepitaxial growth on Si(113) [5]. In contrast to pure iron deposition, the codepos- Rarely, holes in the 2D silicide layer can be ition of iron and silicon leads to a lateral spreading found in the neighborhood of 3D islands (see of the 3D islands. Additionally, island coalescence arrows in Figs. 2 and 3) where the first monolayer can be found (see Fig. 4b and c). Eventually an of silicide is removed. During further growth these interconnected net of islands results with only holes (see Fig. 2d) deepen and the silicon of the small gaps in between (Fig. 4e). The surface of the substrate is consumed to proceed in silicide forma- islands is flat and presents an arrangement as tion. Fig. 3 shows the temporal evolution of 3D shown in Fig. 4f, with a periodicity of #12.5 A in silicide islands during growth at 470°C. Only a the [11:0] direction. In the perpendicular direction minority of the islands are connected to holes in two different periodicities are found as marked in the substrate where silicon out-diffusion takes Fig. 4f. The step height on the islands is 2 A . The M. Kneppe et al. / Surface Science 454­456 (2000) 802­806 805 Fig. 3. Sequence of STM images (derivative image) giving an overview of the growth of the highly anisotropic 3D islands after the submonolayer chain structure completely covers the surface. In only a few locations (see arrows), out-diffusion of silicon leads to the formation of holes in the first silicide layer. Growth is slowed down dramatically after the first monolayer is closed. Fig. 4. Sequence of STM images (derivative image) showing the codeposition of silicon and iron with (Si2H6) and Fe(CO)5. In contrast to pure iron deposition (see Fig. 3), the 3D islands spread out laterally. (f ) shows a zoom-in on one of the islands. (g) shows a height profile along the line marked in (a). height profile, Fig. 4g, along the line in Fig. 4a, surface is roughly consistent with the present set shows that the surface of the 3D silicide islands is of data. Further studies are needed to determine inclined by 5° in the [332:] direction with respect the crystalline structure of the iron silicide on to the substrate. Assuming the same epitactic Si(113) unambiguously. relationship between silicon and iron silicide as The present study has shown that the use of found on Si(111) and the same temperature-depen- higher-index silicon surfaces may provide a means dent phase behavior [1,2], a CsCl defect structure to improve the interface quality between substrate (composition FeSi and silicide layer by blocking the interdiffusion 1+x, x#0.7) exposing a (225) 806 M. Kneppe et al. / Surface Science 454­456 (2000) 802­806 using an intrinsic 2D silicide layer. In contrast to References low-index silicon surfaces, a complete 2D wetting layer is formed on Si(113) before the formation [1] F. Thibaudau, L. Masson, A. Chemam, J.R. Roche, F. Salvan, J. Vac. Sci. Technol. A 16 (1998) 2967. of 3D islands starts. Further studies on Si(114) [2] U. Ko¨hler, P. Kohstall, V. Dorna, in preparation. and Si(5 5 12) are currently in progress. [3] A.A. Baski, S.C. Erwin, L.J. Whitman, Surf. Sci. 392 (1997) 69. [4] J. Dabrowski, H.J. Mu¨ssig, G. Wolff, Phys. Rev. Lett. 73 (1994) 1660. Acknowledgement [5] V. Dorna, Z. Wang, U. Ko¨hler, Surf. Sci. Lett. 401 (1998) L375. This work was supported by the Volkswagen- [6] S. Song, M. Jong, S.G.J. Mochrie, G.B. Stephenson, S.T. stiftung (Germany). Milner, Surf. Sci. 372 (1996) 37.