- Email: [email protected]
Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash
Surface Science 554 (2004) L87–L93 www.elsevier.com/locate/susc
Surface Science Letters
Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash I. Goldfarb
*
Department of Solid Mechanics, Materials and Systems, and University Research Institute for Nanoscience and Nanotechnology, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel Received 26 November 2003; accepted for publication 10 February 2004
Abstract In this work ultrathin iron silicide epilayers were obtained by the reaction of iron contaminants with the Si(1 1 1) substrate atoms during high-temperature flash. After repeated flashing at about 1125 C, reflection high-energy electron diffraction indicated silicide formation. Scanning tunneling microscopy revealed highly ordered surface superstructure interrupted, however, by a number of extended defects. Atomic-resolution bias-dependent imaging demonstrated a p p complex nature of this superstructure with double-hexagonal symmetry and (2 3 2 3)-R30 periodicity. Among the possible candidate phases, including metastable FeSi2 with a CaF2 structure and FeSi1þx with a CsCl structure, the best · 14.4 A unit cell dimensions pointed to the hexagonal Fe2 Si match of the interatomic distances to the measured 14.4 A (Fe2 Si prototype) high-temperature phase. The fact that this phase was obtained by an unusually high-temperature flash, and that neither its reconstruction nor its semiconducting band-gap of about 1.0 ± 0.2 eV (as deduced form the I– V curves obtained by scanning tunneling spectroscopy) has ever been reported, supports such identification. Due to its semiconducting properties, this phase may attract interest, perhaps as an alternative to b-FeSi2 . 2004 Elsevier B.V. All rights reserved. Keywords: Epitaxy; Silicides; Scanning tunneling microscopy; Scanning tunneling spectroscopies; Surface structure, morphology, roughness, and topography; Surface electronic phenomena (work function, surface potential, surface states, etc.); Semiconducting surfaces
Thin metal silicide layers have been subjected to intense studies not only due to their traditional role in microelectronics [1], but more recently due to their ability to self-assemble in the shape of nanodots [2–4] and nanowires [5–7] constituting building blocks in contemporary [8] and futuristic quantum-confined devices [9,10]. Some silicides
*
Tel.: +972-3-6407079; fax: +972-3-6407617. E-mail address: [email protected] (I. Goldfarb).
exhibit favorable electronic and optical properties, such as the direct band-gap of 0.80–1.00 eV in bFeSi2 , making it useful for optoelectronic devices [11,12]. The advantages of Si-based technology in optoelectronics are obvious. The problem in utilization of Fe-silicide, and indeed other optically suitable silicides [12], is to obtain sufficiently flat and homogeneous epitaxial layers of high crystalline quality on silicon, due to the multitude of thermodynamically stable silicide phases, such as Fe9 Si, Fe3 Si, Fe2 Si, Fe5 Si3 , e-FeSi,
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.02.012
CE RF A LE C E TT S E R C IE S N
SU
L88
I. Goldfarb / Surface Science 554 (2004) L87–L93
and two modifications of FeSi2 (a and b) [13,14], and their respective lattice mismatch with silicon. Only few of them have been ever observed in thin epilayers (most notably a- and b-FeSi2 ), and on the other hand structures metastable in the bulk, such as FeSi1þx with a CsCl structure [15,16] and c-FeSi2 with a CaF2 structure [17,18] are strainstabilized in lattice-mismatched heteroepitaxial layers. In other words, metastable structures can stabilize due to their better match with the substrate structure. The phase-formation kinetics and the resulting Fe:Si stoichiometry and crystallography largely depend on the deposition and growth method and coverage. The most popular methods are solid-phase (SPE) where the desired metal is deposited onto a surface at room-temperature (RT) and then annealed to promote phase-formation, and reactive deposition epitaxy (RDE) where the metal is deposited onto substrate preheated to the reaction temperature [2–4]. Molecular beam epitaxy (MBE) in the silicide growth community implies simultaneous or sequential deposition of both metal and silicon onto the substrate, and in the ‘‘template’’ technique a thin metal layer is deposited first, followed by the proportional amounts of metal and silicon to achieve the desired stoichiometry [15,16,19]. In SPE, annealing very low Fe/Si(1 1 1) coverages at about 580 C resulted in a (2 · 2)-terminated metastable c-FeSi2 , pwhereas p similar anneal of thicker films yielded ( 3 3)-R30 periodicity of e-FeSi, according to scanning tunneling microscopy (STM) and spectroscopy (STS) observations of Raunau et al. [17]. This e-FeSi subsequently underwent transformation into b-FeSi2 , after a higher temperature anneal, confirmed by the appearance of 0.85 eV band-gap in I–V curves obtained by STS. A (2 · 2) reconstruction has also been attributed to another metastable (in the bulk) structure, namely CsCl-based FeSi1þx [15], e.g. in low-energy electron diffraction (LEED) [20], or, based on reflection high-energy electron diffraction (RHEED), to some unknown precursor phase to bFeSi2 [21]. Recently, Starke et al. [22–24] have conducted thorough and exceptionally high-resolution STM investigations, combined with LEED and Auger analysis, of the silicide layers prepared by SPE and Fe-Si co-evaporation, as a function of
coverage and annealing temperature. Under these conditions they observed a variety of surface reconstructions, such as (1 · 1), (2 · 2), c(4 · 2), and c(8 · 4), that have been attributed to a Si-terminated 1:1 stoichiometric c-FeSi (CsCl prototype), tetragonal a-FeSi2 , c-FeSi2 (CaF2 prototype) or a defect CsCl structure with 1:1 and 1:2 stoichiometries, 1:2 orthorhombic b-FeSi2 and, finally, in the ultrathin layers annealed at T P 600 C again defect FeSi1þx (CsCl) structure. The latter has been recently supported by the STM and LEED experiments of Garreau et al. [25]. In this research none of the above-mentioned growth methods, i.e. SPE, RDE or MBE, was used. Instead, the iron atoms were made to diffuse from the holding clamps of an Fe-contaminated sample holder into the Si(1 1 1) substrate by a high-temperature p flash p at about 1125 C. As a result a new, (2 3 2 3)-R30-reconstructed, surface structure appeared, that can be assigned to the high-temperature hexagonal Fe2 Si phase. The experiments were performed in a ultrahigh vacuum (UHV) variable-temperature (VT) scanning probe [scanning tunneling (STM) and atomic force (AFM)] microscope (OmicronÕs AFM 25), equipped with reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED)/Auger spectrometer, and capable of operation up to 1250 C by direct-current heating. Si(1 1 1) wafers were chemically treated ex vacuo, by repeated etch-and-regrowth procedure, to produce clean and homogeneous oxide at the top. The sample holders used were brand new, UHV-clean holders from Omicron, where the sample is only in contact with Mo clamps, however Fe-covered from previous evaporation experiments. Prior to that Fe (99.999% purity) coverage, high-temperature Si(1 1 1) flash at 1125 C always resulted in a high-quality, clean (7 · 7) surface. After thorough degassing in UHV (base pressure 1 · 108 Pa), the samples were repeatedly flashed at 1125 C (5–7 times for 20–30 s each time, with the pressure not exceeding 1 · 107 Pa) until a characteristic silicide pattern appeared in RHEED [see inset in Fig. 1(a)]. Once it appeared, this structure was immensely stable, and was not affected by further anneals. All the images in this work were obtained with etched W tips in a con-
SU N IE SC RS CE E A TT R F LE CE
I. Goldfarb / Surface Science 554 (2004) L87–L93
L89
Fig. 1. Constant-current STM image of a (a) 50 · 50 nm iron-silicide area under )1.5 V bias, and (b) 30 · 30 nm single domain ironsilicide under +1.0 V bias. Black rhombus designates the repeating unit cell in real and Fourier space. (c) RT-STS I–V (continuous) and dI=dV (dotted) curves, averaged over several points at different locations across the image in (b). The spectra were sampled with )2.0 V bias over 15 points, between )3.0 and +3.0 V. Insets: in (a) 20 kV RHEED pattern along h1 1 0i Si azimuth, in (b) FFT from the area shown in (b).
stant-current STM mode with 0.15 nA tunneling current under various biases, and RHEED patterns (20 kV accelerating voltage) along at least two different azimuths where computer-acquired using CCD camera and a ‘‘kSA 400’’ imagegrabber. ‘‘Image SXM’’ software by Steve Barrett was used for image analysis. Fig. 1(a) shows a relatively low-magnification constant-current STM image of the surface after the repeated high-temperatures flashes, as described above. It consists of several rotational domains, each of them highly ordered, as follows from a single domain image in Fig. 1(b), exhibiting
double-hexagonal symmetry consistent with twodimensional fast Fourier transform (2D FFT) in the inset. Two more line-defects were observed on that surface: steps and wavy, dislocation-like or antiphase boundary (APB) defects. The former do not cause any shift of the rows across them [e.g. following thin black line in Fig. 1(b) it can be seen that the hexagon row that crosses the step continues without shifting on the other side of the step], unlike the latter that do [the two thin white lines in the center of Fig. 1(b), that lie on the central axis of the hexagon rows in the bottom part, i.e. before crossing the wavy APB, are
CE RF A LE C E TT S E R C IE S N
SU
L90
I. Goldfarb / Surface Science 554 (2004) L87–L93
located between the hexagon rows on the other side of the APB]. The periodicity of the rows that do not cross the APBs is rendered undisturbed, as exemplified by the right-most white line in Fig. 1(b). I–V spectra acquired at RT in the STS mode from several terraces and domains were of semiconducting character. These spectra were not acquired in a high-resolution mode from various regions of the unit cell, but rather with lower magnification to characterize the band structure of the layer as a whole. Fig. 1(c) displays an average of these I–V and dI=dV curves, with a characteristic band-gap of about 1 ± 0.2 eV. The periodicity inside each domain p waspmeasured to be 14.4 A, comprising a (2 3 2 3)-R30 superstructure (black rhombus in Figs. 1 and 2) relative to the Si(1 1 1)-(1 · 1) cell. The exact nature of this structure is more complex than what, at least at a positive bias p of +2.0 V [Fig. 2(a)], appears as a simple (2 3 p 2 3)-R30, as under negative ()2.0 V) bias there shows an additional trigonal symmetry, in the shape of the rows of black club-like features [Fig. 2(b)]. This trigonal symmetry explains the existence of rotational domains clearly seen in Fig. 1(a), impossible in a normal hexagonal p otherwise p (2 3 2 3)-R30 structure. Under this bias, the bright hexagons with dark in the p depressions p center, comprising the (2 3 2 3)-R30 structure at +2.0 V in Fig. 1(a), break-up into three elongated lobes, and are now surrounded by three additional bright protrusions, clearly observed in the bottom-left inset of Fig. 2(b), showing the location of the filled surface states. In a different rotational domain these protrusions are rotated by 60, as clearly shown in Fig. 3 (the white triangles connecting them are drawn to guide the eye to see the rotation). Three candidate structures could match the measured periodicity of the grown superstructure: (a) defect CsCl-based FeSi1þx structure, (b) CaF2 based c-FeSi2 , and (c) hexagonal Fe2 Si (Fe2 Si prototype), shown in Fig. 4, together with Si(1 1 1) surface. The first two are cubic metastables with FeSi1þx and c-FeSi2 {1 1 1} k Si{1 1 1} and FeSi1þx and c-FeSi2 Æ2 1 1ækSiÆ2 1 1æ orientation relations, and the last one is matched to Si with the Fe2 Si(0 0 1) k Si(1 1 1) and Fe2 Si½ 1 1 0 kSi½ 2 1 1 ori-
Fig. 2. Bias-dependent STM. (a) Atomic-resolution (10 · 10 nm) empty-states (positive surface bias of +2.0 V) STM image p p of the (2 3 2 3)-R30 silicide surface, (b) same area as in (a) but with V ¼ 2:0 V. Black rhombus designates the repeating unit cell on the same spot in both images. Contrast variation of p p (2 3 2 3)-R30 upon changing bias from +2.0 to )2.0 V, such as the appearance of black trigonal clubs and hexagon breaking into three elongated lobes surrounded by three additional white protrusions, are best seen in the enlarged portion of the hexagon image in the respective insets in the left-bottom part of (a) and (b) (see text for details).
entation relations (Fig. 4). However the closest periodicity cormatch with the measured 14.4 A rather than responds to the Fe2 Si surface, 14.04 A, 13.20 and 13.57 A in the case of c-FeSi2 and CsCl– FeSi1þx , respectively, even though these differences
SU N IE SC RS CE E A TT R F LE CE
I. Goldfarb / Surface Science 554 (2004) L87–L93
L91
p p Fig. 3. Rotational domains of the (2 3 2 3)-R30 superstructure under )1.0 V bias (30 · 30 nm). Note the 60 rotation between the domains containing the triangles drawn in white. The triangles connect the three additional protrusions that appear in the filled-states image (see text for details).
are beyond the normal spatial 1 A-resolution limit of STM. These three bright protrusions, along with those comprising the hexagon, correspond to Si-atom sites in the (0 0 1) layer of Fe2 Si, while the other Sisites in the layer, designated by and in Fig. 4 appear as depressions, i.e. seem to be missing or represent p empty p surface states. The sites also form (2 3 2 3)-R30 structure (large p white p rhombus) shifted from the filled sites (2 3 2 3)-R30 by ¼ 8.10 A. This can explain 2 · [0 1 0] ¼ 2 · 4.05 A the APBÕs shown in Fig. 1(b), as statistically there p is an equal probability for the filled positions (2 3 p 2 3)-R30 (black rhombus) to form above the empty ones (large white rhombus). p The p sites form a unit cell half as large, i.e. ( 3 3)-R30, marked by a small white rhombus. STM and RHEED are apparently not enough to determine why such a termination takes place, and what is the exact mechanism of its formation. For example, while at )2.0 V only three out of six next-nearest SiSi neighbors are shown [bright round-shape protrusions in the left-bottom inset of Fig. 2(b)], occasionally all six show-up, as shown in the upperright inset of Fig. 2(b). This suggests a possibility of the second layer vacancy ordering and affecting the
Fig. 4. Schematic drawings of the possible candidate structure p p for the (2 3 2 3)-R30 silicide reconstruction (see text for details). Bright and dark circles designate Si and Fe atoms, respectively, and and stand for empty-states and/or missing atoms.
brightness of the observed contrast, similar to the proposed by Krause et al. [22] and Garreau et al. [25] for the defect-structure of FeSi1þx . Detailed theoretical modeling and comparison of simulated bias-dependent STM images with the experimental ones can be useful. The superstructure observed here looks nothing similar to those observed by others on CsCl-based FeSi1þx and CaF2 -based c-FeSi2 [17,20,23,25]. For example, Raunau et al. [17] have found perfectly (2 · 2)-reconstructed c-FeSi2 , a complex,
CE RF A LE C E TT S E R C IE S N
SU
L92
I. Goldfarb / Surface Science 554 (2004) L87–L93
disordered (2 p p · 2)-reconstructed b-FeSi2 , and a simple ( 3 3)-R30-reconstructed e-FeSi. None of these phases even remotely resembles the comp plex, p double-hexagonal, bias-dependent (2 3 2 3)-R30-phase observed here. Furthermore, even though the electronic structure of CsCl-based FeSi1þx is not yet clear, c-FeSi2 is certainly metallic and shows no band-gap in STS-obtainedpI–V spectra [17,18], while the spectra from this (2 3 p 2 3)-R30 structure show distinct band-gap of about 1 ± 0.2 eV [see Fig. 1(c)]. Thus such semiconducting behavior could only be compared to bFeSi2 , however the entirely different surface structure, as appears in STM images, excludes this possibility. The very high temperature, at which this phase was synthesized, corresponds to the stability region of Fe2 Si, also supporting its identification of as Fe2 Si. Lastly, the fact that the pe was closer to the riodicity measured, 14.4 A, than to the 13.30 A unstrained Fe2 Si, i.e. 14.04 A, of Si, indicated that the 5.5% coherency strain was lost, which is entirely reasonable considering the very fast temperature ramp to 1125 C used here. In other words, such a far from equilibrium treatment is not likely to create pseudomorphic layers, and thus metastable structures, such as FeSi1þx and c-FeSi2 , which are only stabilized in epilayers due to better matching and corresponding reduction of pseudomorphic strain. In summary, high-temperature flash was used to synthesize iron-silicide by diffusion of Fe atoms from Fe-covered sample-holder clamps onto the Si(1 The resulting silicide exhibited p 1 1) substrate. p (2 3 2 3)-R30 superstructure. Unlike with the other methods, the entire surface was covered with it, with no islands and bare Si(1 1 1)-(7 · 7) surface patches observed. No differently reconstructed regions were observed either. Thus it could be stated that such a treatment has lead to a very homogeneous and flat surface, even though on the atomic scale extended defects such as rotational domain and antiphase boundaries were present, as well as, of course, surface steps. While three different structures, including the metastable in the bulk form CsCl-based FeSi1þx and CaF2 -based c-FeSi2 could qualify as possible candidates, the majority of evidences pointed towards a high-temperature hexagonal Fe2 Si phase. Based mostly on bias-
dependent STM imaging, a model was proposed p p to account for the above-mentioned (2 3 2 3)R30 termination, that could explain the appearance of both antiphase and rotational domains. One of the most intriguing features of this structure was its semiconducting behavior, with a 1 ± 0.2 eV band-gap, measured by RT-STS, which may turn this phase into an attractive alternative to b-FeSi2 . More quantitative data regarding the nature of this intriguing structure and its formation kinetics, could, perhaps, be obtained by deposition of controlled amounts of iron onto Si surfaces at RT, followed by high-temperature flashes.
Acknowledgements The author wishes to acknowledge the assistance of M. Levinshtein, S. Grossman and G. Cohen-Taguri with the experiment, and contribution of the Israel Science Foundation (ISF GR. 9043/00) towards the equipment costs.
References [1] S.P. Murarka, Silicides for VLSI Applications, Academic Press, New York, 1983. [2] I. Goldfarb, G.A.D. Briggs, Phys. Rev. B 60 (1999) 4800. [3] I. Goldfarb, G.A.D. Briggs, J. Mater. Res. 16 (2001) 744. [4] I. Goldfarb, G.A.D. Briggs, J. Vac. Sci. Technol. B 20 (2002) 1419. [5] J. Nogami, B.Z. Liu, M.V. Katkov, C. Ohbuchi, N.O. Birge, Phys. Rev. B 63 (2001) 233305. [6] Y. Chen, D.A.A. Ohlberg, R.S. Williams, J. Appl. Phys. 91 (2002) 3213. [7] Y. Chen, D.A.A. Ohlberg, G. Medeiros-Ribeiro, Y.A. Chang, R.S. Williams, Appl. Phys. A 75 (2002) 353. [8] R.W. Frathauer, J.M. Ianelli, C.W. Nieh, S. Hashimoto, Appl. Phys. Lett. 57 (1990) 1419. [9] J. Oh, V. Meunier, H. Ham, R.J. Nemanich, J. Appl. Phys. 92 (2002) 3332. [10] L. Banyai, S. Koch, Semiconductor Quantum Dots, World Scientific, Singapore, 1983. [11] M. De Crescenzi, G. Gaggiotti, N. Motta, F. Patella, A. Balzarotti, J. Derrien, Phys. Rev. B 42 (1990) 5871. [12] H. Lange, Phys. Stat. Sol. (b) 201 (1997) 3. [13] T.B. Massalski, Binary Alloy Phase Diagrams, vol. 2, second ed., ASM International, Materials Park, OH, 1992, p. 1771.
SU N IE SC RS CE E A TT R F LE CE
I. Goldfarb / Surface Science 554 (2004) L87–L93 [14] P. Villars, L.D. Calvert, PearsonÕs Handbook of Crystallographic Data for Intermetallic Phases, vol. 3, second ed., ASM International, Materials Park, OH, 1996, p. 3377. [15] H. von Kanel, K.A. Mader, E. Muller, N. Onda, H. Sirringhaus, Phys. Rev. B 45 (1992) 13807. [16] H. von Kanel, C. Schwarz, S. Goncalves-Conto, E. Muller, L. Miglio, F. Tavazza, G. Malegori, Phys. Rev. Lett. 74 (1995) 1163. [17] W. Raunau, H. Niehus, T. Schilling, G. Comsa, Surf. Sci. 286 (1993) 203. [18] W. Raunau, H. Niehus, G. Comsa, Surf. Sci. Lett. 284 (1993) L375. [19] D.R. Peale, R. Haight, F.K. LeGoues, Thin Solid Films 264 (1995) 28.
L93
[20] J. Alvarez, A.L. Vasquez de Parga, J.J. Hinajeros, J. de la Figuera, E.G. Michel, C. Ocal, R. Miranda, J. Vac. Sci. Technol. A 11 (1993) 929. [21] N. Minami, D. Makino, T. Matsumura, C. Egawa, T. Sato, K. Ota, S. Ino, Surf. Sci. 514 (2002) 211. [22] M. Krause, F. Blobner, L. Hammer, K. Heinz, U. Starke, Phys. Rev. B 68 (2003) 125306. [23] U. Starke, W. Weiss, M. Kutschera, R. Bandorf, K. Heinz, J. Appl. Phys. 91 (2002) 6154. [24] W. Weiß, M. Kutschera, U. Starke, M. Mozaffari, K. Resh€ oft, U. K€ ohler, K. Heinz, Surf. Sci. 377–379 (1997) 861. [25] G. Garreau, S. Hajjar, S. Pelletier, M. Imhoff, C. Pirri, Appl. Surf. Sci. 212–213 (2003) 711.
Surface Science Letters
Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash I. Goldfarb
*
Department of Solid Mechanics, Materials and Systems, and University Research Institute for Nanoscience and Nanotechnology, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel Received 26 November 2003; accepted for publication 10 February 2004
Abstract In this work ultrathin iron silicide epilayers were obtained by the reaction of iron contaminants with the Si(1 1 1) substrate atoms during high-temperature flash. After repeated flashing at about 1125 C, reflection high-energy electron diffraction indicated silicide formation. Scanning tunneling microscopy revealed highly ordered surface superstructure interrupted, however, by a number of extended defects. Atomic-resolution bias-dependent imaging demonstrated a p p complex nature of this superstructure with double-hexagonal symmetry and (2 3 2 3)-R30 periodicity. Among the possible candidate phases, including metastable FeSi2 with a CaF2 structure and FeSi1þx with a CsCl structure, the best · 14.4 A unit cell dimensions pointed to the hexagonal Fe2 Si match of the interatomic distances to the measured 14.4 A (Fe2 Si prototype) high-temperature phase. The fact that this phase was obtained by an unusually high-temperature flash, and that neither its reconstruction nor its semiconducting band-gap of about 1.0 ± 0.2 eV (as deduced form the I– V curves obtained by scanning tunneling spectroscopy) has ever been reported, supports such identification. Due to its semiconducting properties, this phase may attract interest, perhaps as an alternative to b-FeSi2 . 2004 Elsevier B.V. All rights reserved. Keywords: Epitaxy; Silicides; Scanning tunneling microscopy; Scanning tunneling spectroscopies; Surface structure, morphology, roughness, and topography; Surface electronic phenomena (work function, surface potential, surface states, etc.); Semiconducting surfaces
Thin metal silicide layers have been subjected to intense studies not only due to their traditional role in microelectronics [1], but more recently due to their ability to self-assemble in the shape of nanodots [2–4] and nanowires [5–7] constituting building blocks in contemporary [8] and futuristic quantum-confined devices [9,10]. Some silicides
*
Tel.: +972-3-6407079; fax: +972-3-6407617. E-mail address: [email protected] (I. Goldfarb).
exhibit favorable electronic and optical properties, such as the direct band-gap of 0.80–1.00 eV in bFeSi2 , making it useful for optoelectronic devices [11,12]. The advantages of Si-based technology in optoelectronics are obvious. The problem in utilization of Fe-silicide, and indeed other optically suitable silicides [12], is to obtain sufficiently flat and homogeneous epitaxial layers of high crystalline quality on silicon, due to the multitude of thermodynamically stable silicide phases, such as Fe9 Si, Fe3 Si, Fe2 Si, Fe5 Si3 , e-FeSi,
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.02.012
CE RF A LE C E TT S E R C IE S N
SU
L88
I. Goldfarb / Surface Science 554 (2004) L87–L93
and two modifications of FeSi2 (a and b) [13,14], and their respective lattice mismatch with silicon. Only few of them have been ever observed in thin epilayers (most notably a- and b-FeSi2 ), and on the other hand structures metastable in the bulk, such as FeSi1þx with a CsCl structure [15,16] and c-FeSi2 with a CaF2 structure [17,18] are strainstabilized in lattice-mismatched heteroepitaxial layers. In other words, metastable structures can stabilize due to their better match with the substrate structure. The phase-formation kinetics and the resulting Fe:Si stoichiometry and crystallography largely depend on the deposition and growth method and coverage. The most popular methods are solid-phase (SPE) where the desired metal is deposited onto a surface at room-temperature (RT) and then annealed to promote phase-formation, and reactive deposition epitaxy (RDE) where the metal is deposited onto substrate preheated to the reaction temperature [2–4]. Molecular beam epitaxy (MBE) in the silicide growth community implies simultaneous or sequential deposition of both metal and silicon onto the substrate, and in the ‘‘template’’ technique a thin metal layer is deposited first, followed by the proportional amounts of metal and silicon to achieve the desired stoichiometry [15,16,19]. In SPE, annealing very low Fe/Si(1 1 1) coverages at about 580 C resulted in a (2 · 2)-terminated metastable c-FeSi2 , pwhereas p similar anneal of thicker films yielded ( 3 3)-R30 periodicity of e-FeSi, according to scanning tunneling microscopy (STM) and spectroscopy (STS) observations of Raunau et al. [17]. This e-FeSi subsequently underwent transformation into b-FeSi2 , after a higher temperature anneal, confirmed by the appearance of 0.85 eV band-gap in I–V curves obtained by STS. A (2 · 2) reconstruction has also been attributed to another metastable (in the bulk) structure, namely CsCl-based FeSi1þx [15], e.g. in low-energy electron diffraction (LEED) [20], or, based on reflection high-energy electron diffraction (RHEED), to some unknown precursor phase to bFeSi2 [21]. Recently, Starke et al. [22–24] have conducted thorough and exceptionally high-resolution STM investigations, combined with LEED and Auger analysis, of the silicide layers prepared by SPE and Fe-Si co-evaporation, as a function of
coverage and annealing temperature. Under these conditions they observed a variety of surface reconstructions, such as (1 · 1), (2 · 2), c(4 · 2), and c(8 · 4), that have been attributed to a Si-terminated 1:1 stoichiometric c-FeSi (CsCl prototype), tetragonal a-FeSi2 , c-FeSi2 (CaF2 prototype) or a defect CsCl structure with 1:1 and 1:2 stoichiometries, 1:2 orthorhombic b-FeSi2 and, finally, in the ultrathin layers annealed at T P 600 C again defect FeSi1þx (CsCl) structure. The latter has been recently supported by the STM and LEED experiments of Garreau et al. [25]. In this research none of the above-mentioned growth methods, i.e. SPE, RDE or MBE, was used. Instead, the iron atoms were made to diffuse from the holding clamps of an Fe-contaminated sample holder into the Si(1 1 1) substrate by a high-temperature p flash p at about 1125 C. As a result a new, (2 3 2 3)-R30-reconstructed, surface structure appeared, that can be assigned to the high-temperature hexagonal Fe2 Si phase. The experiments were performed in a ultrahigh vacuum (UHV) variable-temperature (VT) scanning probe [scanning tunneling (STM) and atomic force (AFM)] microscope (OmicronÕs AFM 25), equipped with reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED)/Auger spectrometer, and capable of operation up to 1250 C by direct-current heating. Si(1 1 1) wafers were chemically treated ex vacuo, by repeated etch-and-regrowth procedure, to produce clean and homogeneous oxide at the top. The sample holders used were brand new, UHV-clean holders from Omicron, where the sample is only in contact with Mo clamps, however Fe-covered from previous evaporation experiments. Prior to that Fe (99.999% purity) coverage, high-temperature Si(1 1 1) flash at 1125 C always resulted in a high-quality, clean (7 · 7) surface. After thorough degassing in UHV (base pressure 1 · 108 Pa), the samples were repeatedly flashed at 1125 C (5–7 times for 20–30 s each time, with the pressure not exceeding 1 · 107 Pa) until a characteristic silicide pattern appeared in RHEED [see inset in Fig. 1(a)]. Once it appeared, this structure was immensely stable, and was not affected by further anneals. All the images in this work were obtained with etched W tips in a con-
SU N IE SC RS CE E A TT R F LE CE
I. Goldfarb / Surface Science 554 (2004) L87–L93
L89
Fig. 1. Constant-current STM image of a (a) 50 · 50 nm iron-silicide area under )1.5 V bias, and (b) 30 · 30 nm single domain ironsilicide under +1.0 V bias. Black rhombus designates the repeating unit cell in real and Fourier space. (c) RT-STS I–V (continuous) and dI=dV (dotted) curves, averaged over several points at different locations across the image in (b). The spectra were sampled with )2.0 V bias over 15 points, between )3.0 and +3.0 V. Insets: in (a) 20 kV RHEED pattern along h1 1 0i Si azimuth, in (b) FFT from the area shown in (b).
stant-current STM mode with 0.15 nA tunneling current under various biases, and RHEED patterns (20 kV accelerating voltage) along at least two different azimuths where computer-acquired using CCD camera and a ‘‘kSA 400’’ imagegrabber. ‘‘Image SXM’’ software by Steve Barrett was used for image analysis. Fig. 1(a) shows a relatively low-magnification constant-current STM image of the surface after the repeated high-temperatures flashes, as described above. It consists of several rotational domains, each of them highly ordered, as follows from a single domain image in Fig. 1(b), exhibiting
double-hexagonal symmetry consistent with twodimensional fast Fourier transform (2D FFT) in the inset. Two more line-defects were observed on that surface: steps and wavy, dislocation-like or antiphase boundary (APB) defects. The former do not cause any shift of the rows across them [e.g. following thin black line in Fig. 1(b) it can be seen that the hexagon row that crosses the step continues without shifting on the other side of the step], unlike the latter that do [the two thin white lines in the center of Fig. 1(b), that lie on the central axis of the hexagon rows in the bottom part, i.e. before crossing the wavy APB, are
CE RF A LE C E TT S E R C IE S N
SU
L90
I. Goldfarb / Surface Science 554 (2004) L87–L93
located between the hexagon rows on the other side of the APB]. The periodicity of the rows that do not cross the APBs is rendered undisturbed, as exemplified by the right-most white line in Fig. 1(b). I–V spectra acquired at RT in the STS mode from several terraces and domains were of semiconducting character. These spectra were not acquired in a high-resolution mode from various regions of the unit cell, but rather with lower magnification to characterize the band structure of the layer as a whole. Fig. 1(c) displays an average of these I–V and dI=dV curves, with a characteristic band-gap of about 1 ± 0.2 eV. The periodicity inside each domain p waspmeasured to be 14.4 A, comprising a (2 3 2 3)-R30 superstructure (black rhombus in Figs. 1 and 2) relative to the Si(1 1 1)-(1 · 1) cell. The exact nature of this structure is more complex than what, at least at a positive bias p of +2.0 V [Fig. 2(a)], appears as a simple (2 3 p 2 3)-R30, as under negative ()2.0 V) bias there shows an additional trigonal symmetry, in the shape of the rows of black club-like features [Fig. 2(b)]. This trigonal symmetry explains the existence of rotational domains clearly seen in Fig. 1(a), impossible in a normal hexagonal p otherwise p (2 3 2 3)-R30 structure. Under this bias, the bright hexagons with dark in the p depressions p center, comprising the (2 3 2 3)-R30 structure at +2.0 V in Fig. 1(a), break-up into three elongated lobes, and are now surrounded by three additional bright protrusions, clearly observed in the bottom-left inset of Fig. 2(b), showing the location of the filled surface states. In a different rotational domain these protrusions are rotated by 60, as clearly shown in Fig. 3 (the white triangles connecting them are drawn to guide the eye to see the rotation). Three candidate structures could match the measured periodicity of the grown superstructure: (a) defect CsCl-based FeSi1þx structure, (b) CaF2 based c-FeSi2 , and (c) hexagonal Fe2 Si (Fe2 Si prototype), shown in Fig. 4, together with Si(1 1 1) surface. The first two are cubic metastables with FeSi1þx and c-FeSi2 {1 1 1} k Si{1 1 1} and FeSi1þx and c-FeSi2 Æ2 1 1ækSiÆ2 1 1æ orientation relations, and the last one is matched to Si with the Fe2 Si(0 0 1) k Si(1 1 1) and Fe2 Si½ 1 1 0 kSi½ 2 1 1 ori-
Fig. 2. Bias-dependent STM. (a) Atomic-resolution (10 · 10 nm) empty-states (positive surface bias of +2.0 V) STM image p p of the (2 3 2 3)-R30 silicide surface, (b) same area as in (a) but with V ¼ 2:0 V. Black rhombus designates the repeating unit cell on the same spot in both images. Contrast variation of p p (2 3 2 3)-R30 upon changing bias from +2.0 to )2.0 V, such as the appearance of black trigonal clubs and hexagon breaking into three elongated lobes surrounded by three additional white protrusions, are best seen in the enlarged portion of the hexagon image in the respective insets in the left-bottom part of (a) and (b) (see text for details).
entation relations (Fig. 4). However the closest periodicity cormatch with the measured 14.4 A rather than responds to the Fe2 Si surface, 14.04 A, 13.20 and 13.57 A in the case of c-FeSi2 and CsCl– FeSi1þx , respectively, even though these differences
SU N IE SC RS CE E A TT R F LE CE
I. Goldfarb / Surface Science 554 (2004) L87–L93
L91
p p Fig. 3. Rotational domains of the (2 3 2 3)-R30 superstructure under )1.0 V bias (30 · 30 nm). Note the 60 rotation between the domains containing the triangles drawn in white. The triangles connect the three additional protrusions that appear in the filled-states image (see text for details).
are beyond the normal spatial 1 A-resolution limit of STM. These three bright protrusions, along with those comprising the hexagon, correspond to Si-atom sites in the (0 0 1) layer of Fe2 Si, while the other Sisites in the layer, designated by and in Fig. 4 appear as depressions, i.e. seem to be missing or represent p empty p surface states. The sites also form (2 3 2 3)-R30 structure (large p white p rhombus) shifted from the filled sites (2 3 2 3)-R30 by ¼ 8.10 A. This can explain 2 · [0 1 0] ¼ 2 · 4.05 A the APBÕs shown in Fig. 1(b), as statistically there p is an equal probability for the filled positions (2 3 p 2 3)-R30 (black rhombus) to form above the empty ones (large white rhombus). p The p sites form a unit cell half as large, i.e. ( 3 3)-R30, marked by a small white rhombus. STM and RHEED are apparently not enough to determine why such a termination takes place, and what is the exact mechanism of its formation. For example, while at )2.0 V only three out of six next-nearest SiSi neighbors are shown [bright round-shape protrusions in the left-bottom inset of Fig. 2(b)], occasionally all six show-up, as shown in the upperright inset of Fig. 2(b). This suggests a possibility of the second layer vacancy ordering and affecting the
Fig. 4. Schematic drawings of the possible candidate structure p p for the (2 3 2 3)-R30 silicide reconstruction (see text for details). Bright and dark circles designate Si and Fe atoms, respectively, and and stand for empty-states and/or missing atoms.
brightness of the observed contrast, similar to the proposed by Krause et al. [22] and Garreau et al. [25] for the defect-structure of FeSi1þx . Detailed theoretical modeling and comparison of simulated bias-dependent STM images with the experimental ones can be useful. The superstructure observed here looks nothing similar to those observed by others on CsCl-based FeSi1þx and CaF2 -based c-FeSi2 [17,20,23,25]. For example, Raunau et al. [17] have found perfectly (2 · 2)-reconstructed c-FeSi2 , a complex,
CE RF A LE C E TT S E R C IE S N
SU
L92
I. Goldfarb / Surface Science 554 (2004) L87–L93
disordered (2 p p · 2)-reconstructed b-FeSi2 , and a simple ( 3 3)-R30-reconstructed e-FeSi. None of these phases even remotely resembles the comp plex, p double-hexagonal, bias-dependent (2 3 2 3)-R30-phase observed here. Furthermore, even though the electronic structure of CsCl-based FeSi1þx is not yet clear, c-FeSi2 is certainly metallic and shows no band-gap in STS-obtainedpI–V spectra [17,18], while the spectra from this (2 3 p 2 3)-R30 structure show distinct band-gap of about 1 ± 0.2 eV [see Fig. 1(c)]. Thus such semiconducting behavior could only be compared to bFeSi2 , however the entirely different surface structure, as appears in STM images, excludes this possibility. The very high temperature, at which this phase was synthesized, corresponds to the stability region of Fe2 Si, also supporting its identification of as Fe2 Si. Lastly, the fact that the pe was closer to the riodicity measured, 14.4 A, than to the 13.30 A unstrained Fe2 Si, i.e. 14.04 A, of Si, indicated that the 5.5% coherency strain was lost, which is entirely reasonable considering the very fast temperature ramp to 1125 C used here. In other words, such a far from equilibrium treatment is not likely to create pseudomorphic layers, and thus metastable structures, such as FeSi1þx and c-FeSi2 , which are only stabilized in epilayers due to better matching and corresponding reduction of pseudomorphic strain. In summary, high-temperature flash was used to synthesize iron-silicide by diffusion of Fe atoms from Fe-covered sample-holder clamps onto the Si(1 The resulting silicide exhibited p 1 1) substrate. p (2 3 2 3)-R30 superstructure. Unlike with the other methods, the entire surface was covered with it, with no islands and bare Si(1 1 1)-(7 · 7) surface patches observed. No differently reconstructed regions were observed either. Thus it could be stated that such a treatment has lead to a very homogeneous and flat surface, even though on the atomic scale extended defects such as rotational domain and antiphase boundaries were present, as well as, of course, surface steps. While three different structures, including the metastable in the bulk form CsCl-based FeSi1þx and CaF2 -based c-FeSi2 could qualify as possible candidates, the majority of evidences pointed towards a high-temperature hexagonal Fe2 Si phase. Based mostly on bias-
dependent STM imaging, a model was proposed p p to account for the above-mentioned (2 3 2 3)R30 termination, that could explain the appearance of both antiphase and rotational domains. One of the most intriguing features of this structure was its semiconducting behavior, with a 1 ± 0.2 eV band-gap, measured by RT-STS, which may turn this phase into an attractive alternative to b-FeSi2 . More quantitative data regarding the nature of this intriguing structure and its formation kinetics, could, perhaps, be obtained by deposition of controlled amounts of iron onto Si surfaces at RT, followed by high-temperature flashes.
Acknowledgements The author wishes to acknowledge the assistance of M. Levinshtein, S. Grossman and G. Cohen-Taguri with the experiment, and contribution of the Israel Science Foundation (ISF GR. 9043/00) towards the equipment costs.
References [1] S.P. Murarka, Silicides for VLSI Applications, Academic Press, New York, 1983. [2] I. Goldfarb, G.A.D. Briggs, Phys. Rev. B 60 (1999) 4800. [3] I. Goldfarb, G.A.D. Briggs, J. Mater. Res. 16 (2001) 744. [4] I. Goldfarb, G.A.D. Briggs, J. Vac. Sci. Technol. B 20 (2002) 1419. [5] J. Nogami, B.Z. Liu, M.V. Katkov, C. Ohbuchi, N.O. Birge, Phys. Rev. B 63 (2001) 233305. [6] Y. Chen, D.A.A. Ohlberg, R.S. Williams, J. Appl. Phys. 91 (2002) 3213. [7] Y. Chen, D.A.A. Ohlberg, G. Medeiros-Ribeiro, Y.A. Chang, R.S. Williams, Appl. Phys. A 75 (2002) 353. [8] R.W. Frathauer, J.M. Ianelli, C.W. Nieh, S. Hashimoto, Appl. Phys. Lett. 57 (1990) 1419. [9] J. Oh, V. Meunier, H. Ham, R.J. Nemanich, J. Appl. Phys. 92 (2002) 3332. [10] L. Banyai, S. Koch, Semiconductor Quantum Dots, World Scientific, Singapore, 1983. [11] M. De Crescenzi, G. Gaggiotti, N. Motta, F. Patella, A. Balzarotti, J. Derrien, Phys. Rev. B 42 (1990) 5871. [12] H. Lange, Phys. Stat. Sol. (b) 201 (1997) 3. [13] T.B. Massalski, Binary Alloy Phase Diagrams, vol. 2, second ed., ASM International, Materials Park, OH, 1992, p. 1771.
SU N IE SC RS CE E A TT R F LE CE
I. Goldfarb / Surface Science 554 (2004) L87–L93 [14] P. Villars, L.D. Calvert, PearsonÕs Handbook of Crystallographic Data for Intermetallic Phases, vol. 3, second ed., ASM International, Materials Park, OH, 1996, p. 3377. [15] H. von Kanel, K.A. Mader, E. Muller, N. Onda, H. Sirringhaus, Phys. Rev. B 45 (1992) 13807. [16] H. von Kanel, C. Schwarz, S. Goncalves-Conto, E. Muller, L. Miglio, F. Tavazza, G. Malegori, Phys. Rev. Lett. 74 (1995) 1163. [17] W. Raunau, H. Niehus, T. Schilling, G. Comsa, Surf. Sci. 286 (1993) 203. [18] W. Raunau, H. Niehus, G. Comsa, Surf. Sci. Lett. 284 (1993) L375. [19] D.R. Peale, R. Haight, F.K. LeGoues, Thin Solid Films 264 (1995) 28.
L93
[20] J. Alvarez, A.L. Vasquez de Parga, J.J. Hinajeros, J. de la Figuera, E.G. Michel, C. Ocal, R. Miranda, J. Vac. Sci. Technol. A 11 (1993) 929. [21] N. Minami, D. Makino, T. Matsumura, C. Egawa, T. Sato, K. Ota, S. Ino, Surf. Sci. 514 (2002) 211. [22] M. Krause, F. Blobner, L. Hammer, K. Heinz, U. Starke, Phys. Rev. B 68 (2003) 125306. [23] U. Starke, W. Weiss, M. Kutschera, R. Bandorf, K. Heinz, J. Appl. Phys. 91 (2002) 6154. [24] W. Weiß, M. Kutschera, U. Starke, M. Mozaffari, K. Resh€ oft, U. K€ ohler, K. Heinz, Surf. Sci. 377–379 (1997) 861. [25] G. Garreau, S. Hajjar, S. Pelletier, M. Imhoff, C. Pirri, Appl. Surf. Sci. 212–213 (2003) 711.