Synthesis of metal silicide at metal/silicon oxide ...

Synthesis of metal silicide at metal/silicon oxide interface by electronic excitation J.-G. Lee, T. Nagase, H. Yasuda, and H. Mori Citation: Journal of Applied Physics 117, 194307 (2015); doi: 10.1063/1.4921429 View online: http://dx.doi.org/10.1063/1.4921429 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An electrical evaluation method for the silicidation of silicon nanowires Appl. Phys. Lett. 95, 023106 (2009); 10.1063/1.3171929 Study on the interface thermal stability of metal-oxide-semiconductor structures by inelastic electron tunneling spectroscopy Appl. Phys. Lett. 88, 262909 (2006); 10.1063/1.2219140 Preparation of bead metal single crystals by electron beam heating J. Vac. Sci. Technol. A 23, 1535 (2005); 10.1116/1.2101793 Synthesis of metallic nanocrystals with size and depth control: A case study J. Vac. Sci. Technol. B 23, 1470 (2005); 10.1116/1.1941248 Low-temperature ordering of FePt by formation of silicides in underlayers J. Appl. Phys. 97, 10H310 (2005); 10.1063/1.1861415

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JOURNAL OF APPLIED PHYSICS 117, 194307 (2015)

Synthesis of metal silicide at metal/silicon oxide interface by electronic excitation J.-G. Lee,1,a) T. Nagase,2,3 H. Yasuda,2,3 and H. Mori2 1

Powder & Ceramics Division, Korea Institute of Materials Science, Changwon, Gyeongnam 642-831, South Korea 2 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567-0047, Japan 3 Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan

(Received 19 January 2015; accepted 11 May 2015; published online 21 May 2015) The synthesis of metal silicide at the metal/silicon oxide interface by electronic excitation was investigated using transmission electron microscopy. A platinum silicide, a-Pt2Si, was successfully formed at the platinum/silicon oxide interface under 25–200 keV electron irradiation. This is of interest since any platinum silicide was not formed at the platinum/silicon oxide interface by simple thermal annealing under no-electron-irradiation conditions. From the electron energy dependence of the cross section for the initiation of the silicide formation, it is clarified that the silicide formation under electron irradiation was not due to a knock-on atom-displacement process, but a process induced by electronic excitation. It is suggested that a mechanism related to the Knotek and Feibelman mechanism may play an important role in silicide formation within the solid. Similar silicide formation was also observed at the palladium/silicon oxide and nickel/silicon oxide interfaces, indicating a wide generality of the silicide formation by electronic excitation. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4921429] V

I. INTRODUCTION

Materials modification by electronic excitation in inorganic materials is now becoming an exciting branch of modern materials science.1 If we focus on the modification of the atomic structure within a solid, the modification may then be divided into two categories: decomposition and synthesis of the materials. Among the electronic-excitation-induced decompositions in inorganic materials occurring within a solid, the most classical example may be the photolysis of silver halide crystals.2 Here, an electron excited by a photon and subsequently trapped at a site successfully combines with a mobile interstitial silver ion and is then de-excited into the metal valence level, resulting in the formation of a silver metal atom.3 The photo-induced formation of small sized clusters of silver atoms plays an essential role in the traditional photographic process.3 Another example is the decomposition of GaSb compound nanoparticles induced by an electronic-excitation process.4 Here, GaSb particles with a size of 10–20 nm can be decomposed into a two-phase mixture of a metallic antimony core and a gallium shell when irradiated with 25 keV electrons in an electron microscope at temperatures around 400 K.4 As the maximum energies transferred from an incident electron to a gallium and antimony atom by the elastic electron-nucleus collision are both well below the threshold energies for atom displacement in typical III-V compounds,5,6 it is evident that the decomposition of GaSb is a result of an inelastic electron-electron collision(s), namely, it is caused by an electronic-excitation-induced process(es).4 a)

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In all of these examples, inorganic compounds that are thermodynamically stable in the ground state can be rather easily decomposed into constituent elements when subjected to an electronic excitation and subsequent de-excitation. The fact that such decomposition can be achieved within the solid by electronic excitation strongly suggests that the synthesis (such as the compound formation) can also be successfully achieved within the solid by electronic excitation, as only the direction of a solid state reaction is reversed between decomposition and synthesis. To the authors’ knowledge, however, examples of electronic-excitation-induced synthesis of an inorganic solid phase within the solid are rare. This work confirms that a new branch of materials modifications, i.e., synthesis of a compound phase, within a solid can be achieved by electronic excitation; namely, experimental evidence for the formation of a metal silicide at the metal/silicon oxide interface within the solid by electronic excitation has been obtained. II. EXPERIMENTAL PROCEDURES A. Sample preparation

A platinum/silicon oxide composite was selected as the vehicle of the study. Details of the sample preparation were as follows. First, Pt particles were grown almost epitaxially on (001) cleaved NaCl substrates at 573 K by magnetron sputtering of a Pt target in argon plasma of high purity (4N purity). The argon pressure in the operation mode was 6 Pa. The particles were then backed with a thin supporting amorphous silicon oxide film. The silicon oxide (SiOx) film was prepared adopting two different techniques. In one case, the pulsed laser deposition (PLD) technique7 was employed.

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Here, pure oxygen was introduced in advance into the PLD chamber at a pressure of 1 102 Pa, and a pulsed Nd:YAG (yttrium aluminum garnet) laser with a wavelength of 355 nm and a pulse duration of 5 ns was focused onto a single-crystal Si wafer at a repetition rate of 10 Hz to deposit a thin supporting silicon oxide film on the Pt particles grown on NaCl. The thickness of the film was approximately 20 nm. In the other case, the vapor deposition technique was employed instead of the PLD. In this occasion, a couple of mm-sized lumps of silicon monoxide, produced by Nisshin EM Corporation, were set in a tungsten wire basket, and then heated in a vacuum chamber under the base pressure of about 5 104 Pa, to achieve the vapor deposition of a silicon oxide film on the Pt particles on NaCl. The thickness of the film was approximately 10 nm. The oxygen content in the latter silicon oxide (SiOx) film was evaluated by X-ray photoelectron spectroscopy (XPS) to be approximately equal to 1.5 (SiO1.5, see Sec. I in the supplementary material).8 The platinum/silicon oxide (Pt/SiOx) composite prepared on NaCl was then floated on distilled water and mounted on a grid for the transmission electron microscopy (TEM) experiments. In the composite, nm-sized Pt particles were embedded on one side of an amorphous silicon oxide film (see Fig. S4(a) in the supplementary material).8 (In figure captions, the silicon oxide film prepared by PLD is designated as SiOx(P), whereas the film prepared by vapor deposition is denoted by SiOx(V)). A typical example of as-produced platinum/silicon oxide composites where Pt particles were supported on an amorphous silicon oxide film is shown in Fig. 1. Figures 1(a) and 1(b) show a bright-field image (BFI) of the composite and the corresponding selected area electron diffraction (SAED) pattern, respectively. Figure 1(a) shows that approximately 10-nm-sized Pt particles, in most cases being connected to each other, were placed on the silicon oxide film. The SAED pattern in Fig. 1(b) indicates that fcc Pt particles exhibited a rather strong preferred orientation along the [001] direction over the random orientation. This is due to the growth mode of Pt during the sample preparation. The SAED pattern can be consistently indexed as the [001] diffraction pattern of [001] oriented fcc Pt particles, which belong to the space-group Fm3m with a lattice constant of a ¼ 0.3923 nm, superposed on the Debye-Scherrer rings of randomly oriented particles. This indicates that, on the (001) cleaved NaCl substrate kept at 573 K, a high fraction of as-

FIG. 1. A typical microstructure of as-produced platinum/silicon oxide composite. (a) BFI of the composite, (b) the corresponding SAED pattern, where a broad halo from amorphous silicon oxide is arrowed by a letter D. The supporting film used was SiOx(P).

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sputter-deposited Pt particles grew with a preferred orientation relationship of (001)Pt//(001)NaCl, [110]Pt//[110]NaCl. This is in agreement with the orientation relationship reported in the literature.9 B. Observation of silicide formation by TEM

The platinum/silicon oxide composite on the grid was then subjected to irradiation of electrons of energies ranging from 25 to 200 keV directly inside a TEM over a temperature range of 93–298 K, and the Pt silicide formation at the platinum/silicon oxide interface was examined by transmission electron microscopy, namely, changes in the BFIs and SAED patterns associated with the electron irradiation were monitored in situ. Actually, one of the two types of TEM was used for the irradiation experiments: either Hitachi H800 type 200 kV TEM or Hitachi H-7000 type 100 kV TEM. When necessary, high-resolution electron micrographs were taken using a Hitachi H-9000NAR type 300 kV microscope or a JEOL JEM-ARM200F type 200 kV microscope. The present experiments showed that a Pt silicide phase, Pt2Si, was formed at the platinum/silicon oxide interface by irradiation, as will be shown below. III. RESULTS AND DISCUSSION A. Formation of a Pt silicide at the platinum/silicon oxide interface

Figure 2 shows a series of SAED patterns of the composite during the 75 keV electron irradiation at 298 K. Figures 2(a)–2(c) show the SAED patterns of a composite before irradiation, and after irradiation for 1200 s and 4800 s at a flux of 1.1 1022 m2 s1, respectively. The SAED pattern in Fig. 2(a) is essentially the same as that in Fig. 1(b)

FIG. 2. A series of SAED patterns of a composite subjected to 75 keV electron irradiation at 298 K. (a)–(c) show the pattern before irradiation, after irradiation for 1200 s, and for 4800 s at a flux of 1.1 1022 m2 s1, respectively. (d) shows a key diagram of diffraction spots appearing in (c). The supporting film used was SiOx(P).


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and can be consistently indexed as the [001] diffraction pattern of [001] oriented fcc Pt particles superposed on the Debye-Scherrer rings of randomly oriented Pt particles. Clearly, after irradiation, a set of regularly arranged extra spots newly appeared, and their intensities increased with increasing total dose of electrons (e.g., spots arrowed in Figs. 2(b) and 2(c)). From a crystallographic analysis, the SAED pattern including the newly appeared extra spots in Figs. 2(b) and 2(c) can be consistently indexed as the [001] diffraction pattern of [001] oriented a-Pt2Si (which have a tetragonal structure belonging to the space-group I4/mmm with lattice constants of a ¼ 0.3948 nm and c ¼ 0.5963 nm) superposed on the [001] diffraction pattern of [001] oriented fcc Pt particles as well as on the Debye-Scherrer rings of randomly oriented Pt particles. Figure 2(d) shows a key diagram of the diffraction pattern shown in Fig. 2(c). An example of irradiation experiments at 93 K is depicted in Fig. 3. Figures 3(a)–3(c) show the SAED patterns of a composite before irradiation, and after irradiation of 200 keV electrons for 1200 s and 4800 s at a flux of 1.1 1022 m2 s1, respectively. The SAED pattern in Fig. 3(a) is again essentially the same as that in Fig. 1(b). As seen from Figs. 3(b) and 3(c), during irradiation at 93 K, extra spots appeared in the same positions as those in Figs. 2(b) and 2(c) (see such spots as arrowed in Figs. 3(b) and 3(c)), and their intensities increased with increasing period of irradiation, indicating that even at such a low temperature as 93 K, the same silicide can be formed in a similar manner to that at 298 K when the composite sample was subjected to irradiation. Figure 3(d) shows a key diagram of the diffraction pattern shown in Fig. 3(c). This observation reminds us of the work by Abbati et al.,10 in which the reaction between silicon and palladium was studied by photoelectron

FIG. 3. A series of SAED patterns of a composite subjected to 200 keV electron irradiation at 93 K. (a)–(c) show the pattern before irradiation, after irradiation for 1200 s, and for 4800 s at a flux of 1.1 1022 m2 s1, respectively. (d) shows a key diagram of diffraction spots appearing in (c). The supporting film used was SiOx(P).

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spectroscopy on silicon (111)/palladium interfaces at low temperatures down to 85 K. In the paper, spectra suggesting that some intermixing between silicon and palladium took place when the interface was prepared at 85 K and that, after preparation at 85 K, the intermixing increased at room temperature, have been presented.10 It is suggested by Abbati et al. that the considerable intermixing at 85 K can be explained by an extra energy such as the condensation energy of the metal. It is of interest to note here that although the possible contribution from the condensation energy can be ruled out in the present work where no deposition is involved in the reaction, an interface possessing an intermixing ability similar to that of the as-in-situ-cleaved clean silicon/as-deposited metal interface employed in that paper10 may be realized also in the present work under an electronirradiation condition, albeit short in individual durations. This point will be again touched later in Sec. III D. In order to have more evidence for the fact that the silicide newly formed is certainly a-Pt2Si, TEM experiments to get SAED patterns of an irradiated composite taken not only from the [001]Pt direction but also from another direction were carried out, and an example of the results is depicted in Fig. 4. Figure 4(a) shows the SAED pattern before irradiation, taken from the [001]Pt direction, and is essentially the same as that in Fig. 2(a), except that in the pattern in Fig. 4(a) the intensities of the Debye-Scherrer rings of randomly oriented Pt particles are too weak to be recognized in the photograph because of the significantly reduced fraction of such particles compared to the well-aligned, cubic oriented [001] particles in this particular sample. A key diagram corresponding to the pattern in Fig. 4(a) is illustrated in Fig. 4(d). Figures 4(b) and 4(c) show the SAED patterns after 125 keV electron irradiation for 1200 s at a flux of 4.4 1023 m2 s1, taken from the [001]Pt direction (i.e., from the [001]Pt2Si direction) and from the [201]Pt2Si direction, respectively. The SAED pattern in Fig. 4(b) is essentially the same as that in Fig. 2(b) (or in Fig. 2(c)), as seen from the corresponding key diagram in Fig. 4(e). The SAED pattern in Fig. 4(c) was taken after tilting the sample around the [020] Pt (i.e., the [020] Pt2Si) axis by approximately 50 , relative to the orientation corresponding to Fig. 4(a) (or Fig. 4(b)). The SAED pattern in Fig. 4(c) can be consistently indexed as the [201] diffraction pattern of a-Pt2Si, as illustrated in the corresponding key diagram in Fig. 4(f). From the key diagrams in Figs. 2 and 4, it is then concluded that the silicide newly formed under irradiation is a-Pt2Si and that the silicide has a crystallographic orientation relationship of (001)Pt//(001)Pt2Si, [110]Pt//[110]Pt2Si with Pt. The epitaxial growth of Pt2Si on platinum can be easily explained based on the crystallographic structures (see Sec. II in the supplementary material).8 As the next step, to have information on the site of the silicide formed by irradiation, HREM observations were performed. Figures 5(a) and 5(b) show high-resolution electron microscope images of a platinum/silicon oxide composite before irradiation and after irradiation with 50 keV electrons to a dose of 4.8 1026 m2 at 298 K, respectively. As shown in Fig. 5(a), highly oriented crystalline Pt particles are supported on a thin continuous amorphous silicon oxide film.


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FIG. 4. (a) and (d) show the SAED pattern before irradiation and the corresponding key diagram, respectively. (b) and (c) show the SAED patterns after 125 keV electron irradiation for 1200 s at a flux of 4.4 1023 m2 s1 at room temperature, taken from the [001]Pt direction (i.e., from the [001]Pt2Si direction) and from the [201]Pt2Si direction, respectively. The SAED pattern in (c) was taken after tilting the sample around the [020] Pt (i.e., the [020] Pt2Si) axis by approximately 50 , relative to the orientation corresponding to (a) (or (b)). (e) and (f) are the key diagrams corresponding to (b) and (c), respectively. The tilting axis employed in taking the SAED in (c) is illustrated as broken lines in the key diagrams. The supporting film used was SiOx(V).

The right-angled 0.2-nm-spaced lattice fringes appearing on the particles correspond to the {200} planes of fcc Pt. A Fourier transformed (FT) pattern taken from frame A in Fig. 5(a) is shown in Fig. 5(c); the diffractogram is essentially identical to the [001] diffraction pattern of [001] oriented fcc Pt particles shown in Fig. 2(a). Figure 5(b) shows that islands of a new solid phase, which appear somewhat dark compared to Pt particles, were formed on the Pt particles. A FT pattern taken from a portion of the new solid phase (i.e., from frame B in Fig. 5(b)) is depicted in Fig. 5(d). The diffractogram is essentially in accordance with the [001] diffraction pattern of [001] oriented a-Pt2Si (Fig. 2(c)) and with the key diagram of the diffraction pattern (Fig. 2(d)). This result indicates that islands of a Pt silicide, a-Pt2Si, were formed under irradiation at the platinum/silicon oxide interface in an epitaxial manner with Pt. With regard to the data shown in Fig. 5, another point to be noted is that no indication of the presence of pure silicon aggregates can be observed in Fig. 5(b) and only the presence of the Pt2Si phase is evident in addition to the original

FIG. 5. High-resolution electron microscope images of a platinum/silicon oxide composite (a) before irradiation and (b) after irradiation to a dose of 4.8 1026 m 2. FT patterns taken from frame A (in (a)) and B (in (b)) are shown in (c) and (d), respectively. The electron energy, flux, and irradiation temperature employed were 50 keV, 4.4 1022 m2 s1, and 298 K, respectively. The supporting film used was SiOx(V).

two phases: the amorphous silicon oxide and the pure Pt phases. This observation is at variance with a previous work in the literature where the presence of clusters or aggregates of pure silicon, formed from amorphous silicon dioxide by 100 keV electron irradiation, is reported.11 A possible cause for the difference (i.e., absence and presence of pure silicon in this work and in the previous work, respectively) may be ascribed to the large difference in the electron flux, employed during the irradiations, between this work (i.e., on the order of 1022–1023 m2 s1) and the previous work (i.e., on the order of 1027–1028 m2 s1). This point will be again touched later in Sec. III D.


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In an attempt to get information on the dynamic aspect of the silicide formation under irradiation, in situ HREM observations were carried out. An example of the results is depicted in Fig. 6. Figures 6(a)–6(d) show a series of sequential HREM images of a fixed area including one (same) isolated particle, taken at times of 0 s, 10 s, 430 s, and 510 s after the start of the observation, respectively. In Fig. 6(a), a [001] oriented Pt particle is illustrated, along with the corresponding FT pattern (inset). These are essentially the same with those shown in Figs. 5(a) and 5(c). Figure 6(b), which was taken at a time of 10 s, shows that already at this very beginning stage of irradiation, the regularly spaced lattice fringes corresponding to the {200} planes of fcc Pt became distorted in places (e.g., indicated by a white arrow), suggesting the initiation of a reaction between platinum and silicon. This stage may be associated with the initial dissolution of silicon into platinum. With continued irradiation, an approximately 3-nm-sized region with the right-angled 0.28nm-spaced lattice fringes appeared at the down-right corner of the particle (i.e., encircled), as seen from Fig. 6(c) taken at a time of 430 s. Since the right-angled 0.28-nm-spaced lattice fringes can be identified as those corresponding to the {110} lattice planes of Pt2Si from the data depicted in Fig. 5, the image in Fig. 6(c) suggests that a 3-nm-sized Pt2Si island was formed at the down-right corner of the interface between the platinum particle and amorphous silicon oxide. At present, it is not clear whether the nucleation of Pt2Si occurred in a much smaller scale within the 3-nm-sized region or it occurred over the whole 3-nm-sized interfacial region. With further irradiation, the region with the right-angled 0.28-nmspaced lattice fringes extended over almost the whole image of the particle, as seen from Fig. 6(d) taken at a time of 510 s. This fact indicates that, by that time, the island of

FIG. 6. Successive stages of silicide formation on one isolated particle examined by a 200 kV HREM. (a)–(d) show a series of sequential HREM images, taken at times of 0 s, 10 s, 430 s, and 510 s after the start of the observation, respectively. The electron flux used was 1.66 1024 m2 s1 and the observation was carried out at room temperature. See text for details. The supporting film used was SiOx(V).

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Pt2Si made growth and extended over almost the whole interface between the platinum particle and the supporting amorphous silicon oxide in this case. Additional evidence for the silicide formation at the platinum/silicon oxide interface within the solid is given in Sec. 3 in the supplementary material.8 Furthermore, it is of interest to note the following two points here: (1) the Pt silicide, a-Pt2Si, formed at the platinum/silicon oxide interface was in agreement with the first phase produced at the Pt/Si interface in the phase formation sequence in the Pt-Si simple binary alloy system during thermal annealing (details are given in Sec. 4 in the supplementary material),8 and (2) it was confirmed by experiments that the same silicide, aPt2Si, was formed at the platinum/silicon dioxide (i.e., SiO2 in the stoichiometric composition) interface under electron irradiation conditions, in a manner similar to that observed at the platinum/silicon oxide (i.e., SiOx with x 1.5) interface, indicating that the excess of silicon atoms over the stoichiometric composition SiO2 is not a prerequisite for the silicide formation at the platinum/silicon oxide (Pt/SiOx) interface observed in the present work (details are given in Sec. 5 in the supplementary material).8 B. Absence of silicide formation by simple thermal annealing

As described above in Sec. III A, a platinum silicide, aPt2Si, was successfully formed at the platinum/silicon oxide interface under electron irradiation conditions. To clarify the effect of electron irradiation (or of electronic excitation) on the silicide formation reaction, it is valuable to get information on the natural direction of reaction under no electron irradiation. Based upon this premise, a series of simple annealing experiments were carried out with the same platinum/silicon oxide composite samples, using a hot sample holder in the electron microscope with the electron beam off. The results are depicted in Fig. 7. Figures 7(a) and 7(b) show a BFI of an as-prepared platinum/silicon oxide composite and the corresponding SAED pattern, respectively. The SAED pattern is consistently analyzed as the [001] diffraction pattern of fcc Pt, superposed with the Debye-Scherrer rings from randomly oriented Pt particles and with a broad halo from amorphous silicon oxide. This feature is similar to that of the SAED pattern depicted in Fig. 1(b). Figures 7(c) and 7(d) show a BFI of the same sample after being gradually heated up to 873 K and subsequently cooled down to room temperature, and the corresponding SAED pattern, respectively. The black arrows in Figs. 7(a) and 7(c) point at the same position on the sample. A comparison of Fig. 7(a) with Fig. 7(c) and of Fig. 7(b) with Fig. 7(d) reveals that essentially no changes occurred either in the BFI or in the SAED pattern after annealing, except for the slight coarsening of the platinum particles. It should be emphasized here that any additional diffraction spots coming from a platinum silicide did not appear after annealing, as seen from Fig. 7(d). All these experimental results provide evidence for the fact that platinum silicide was not formed in the platinum/silicon oxide composite by simple thermal annealing with no electron irradiation (or no electronic excitation)


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FIG. 7. Absence of silicide formation after simple thermal annealing. (a) and (b) show a BFI of an as-produced platinum/silicon oxide composite sample and the corresponding SAED pattern, respectively. (c) and (d) show a BFI of the same sample after being gradually heated in the microscope with no electron illumination up to 873 K and subsequently cooled down to room temperature and the corresponding SAED, respectively. The black arrows in (a) and (c) point at the same position of the sample. Note that no additional diffraction spots attributed to a platinum silicide appeared after annealing (see (d)), indicating the absence of silicide formation by simple thermal annealing without electron irradiation. Observations were done as quickly as possible at an accelerating voltage of 200 kV and at 298 K, so as to avoid electron-irradiation-induced silicide formation during the TEM observation. The supporting film used was SiOx(V).

involved. Namely, when the system was under a conventional no-electron-irradiation condition, the platinum silicide formation at the platinum/silicon oxide interface was not allowed to occur. Here, it is of significance to point out that the silicide formation, however, can occur under electron irradiation (or electronic excitation) conditions, as described above in Sec. III A. C. Electron energy dependence of the cross section for the initiation of the silicide formation

Logically, there are two possible causes for the silicide formation under electron irradiation: one is the atomic displacement through elastic scattering of incident electrons, whereas the other is the electronic excitation through inelastic scattering of incident electrons. To determine which is the case, the critical dose of electrons required to form the silicide was measured as a function of the electron energy; the results are shown in Fig. 8(a). In the measurements, the critical dose was defined as a dose at which the intensity of a 110 spot of a-Pt2Si reached a low fixed level in the SAED pattern. Figure 8(a) shows that the critical dose of electrons required to synthesize the silicide at the interface decreased with decreasing electron energy. That is, the silicide was formed more easily when the composite was irradiated with electrons of lower energies than with electrons of higher energies, at least over a range of 25–200 keV. Next, the inverse of the critical dose, which may correspond to the

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FIG. 8. (a) The critical dose of electrons required to form a-Pt2Si at the platinum/silicon oxide interface within the solid, plotted as a function of the electron energy. Flux and irradiation temperature employed were 4.4 1023 m2s1 and 298 K, respectively. The supporting film used was SiOx(V). (b) The inverse of the critical dose shown in (a), plotted as a function of the electron energy. (c) Calculated total cross sections for the inelastic scattering by silicon and oxygen atoms and those for the atomic displacement of silicon and oxygen in silicon dioxide, as functions of the electron energy.

“effective” cross section for the initiation of silicide formation, was calculated from the data given in Fig. 8(a), and the results are depicted in Fig. 8(b). The “effective” cross section continuously increases with decreasing electron energy. Figure 8(c) shows the calculated total cross sections for the inelastic scattering of incident electrons by silicon and oxygen atoms along with those for the atomic displacement of silicon and oxygen in silicon dioxide, as functions of the electron energy.12 The former cross sections were calculated according to an equation by Egerton,13 whereas the latter atomic displacement cross sections were calculated according to the McKinley-Feshbach formula5 under the assumption that the threshold displacement energies of silicon and oxygen in silicon dioxide are approximately equal to 19 and 9 eV, respectively.14 Comparing Fig. 8(b) with Fig. 8(c), the observed electron energy dependence of the “effective” cross section for the initiation of the silicide formation clearly does not match that of the atomic displacement cross sections, but rather follows the energy dependence of the total cross sections for inelastic scattering of incident electrons. Thus, Fig. 8 clearly indicates that the observed silicide formation under electron irradiation was not due to a knock-on atom-displacement process (i.e., the elastic electron-nucleus


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collision) but due to an atomic process induced by electronic excitation (i.e., the inelastic electron-electron collision). D. Discussion on the mechanism

When the SAED data in Fig. 2 through Fig. 4 are combined with the HREM data in Figs. 5 and 6, they clearly indicate that a silicide, a-Pt2Si, was formed at the platinum/silicon oxide interface under electron irradiation. Furthermore, the data on the cross section for the silicide formation in Fig. 8 indicate that the silicide was formed due to a process induced by electronic excitation. Discussions on the mechanism behind the silicide formation will be made below, focusing on the role of the electronic excitation. In an attempt to analyze the electronic excitation that is responsible for the silicide formation, a series of photon irradiation experiments are in progress in our laboratory using a beam line of a synchrotron radiation facility. Preliminary results show that the excitation of electrons in the valence band (VB) only, which lies in a binding energy range of approximately 5–17 eV in the case of silicon dioxide,15 is insufficient to promote the silicide formation and that, at least, the excitation of electrons in the highest-lying silicon core level below the VB (i.e., Si 2p band at around 100 eV) is required for silicide formation. This reminds us of the core hole Auger decay mechanism of Knotek and Feibelman (KF) for ion desorption from surface:16 there is a possibility that a mechanism related to the K-F mechanism may be operating also at the platinum/silicon oxide interface within the solid to realize a “reduced” state of silicon necessary for the initiation of silicide formation. In fact, in the paper by Chen et al. on the electron-beam-induced damage in amorphous silicon dioxide, it is suggested that oxygen is internally displaced with the assistance of the K-F related mechanism and the Coulomb explosion mechanism.11 From all these results and suggestions, it seems reasonable to consider a sequence as follows; that is, in the present work, owing to a mechanism related to the K-F one, oxygen is forcibly removed from silicon at the platinum/silicon oxide interface (i.e., breakage of Si-O bonds); this then allows silicon to bind with platinum nearby (i.e., formation of Si-Pt bonds), resulting in the formation of Pt silicide at the interface. Therefore, the multiple ionization of oxygen may play an essential role in the silicide formation. The sequence envisaged above can explain the present observation that clusters or aggregates of pure silicon were absent while islands of Pt2Si were present at the interface in a sample subjected to the electronic excitation (see Fig. 5(b)), in the following manner. It is considered that for an aggregate of silicon atoms composed of, say, two silicon atoms to be formed, the two silicon atoms in the neighboring positions need to be reduced at the same time under irradiation at a flux, whereas for a bond between platinum and silicon to be formed at the platinum/silicon oxide interface, only one silicon atom needs to be reduced under irradiation at a somewhat lower flux because a high fraction of platinum atoms at the interface are already in a reduced state (i.e., in a metallic state) and to be ready to bond with a nearby silicon atom as soon as it is put in a reduced state. Such a situation will establish a

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characteristic range in the intensity of electron flux, over which only silicide can be formed at the interface with no formation of aggregates of pure silicon. Consequently, if we assume that the electron flux employed here was within this range (namely, the flux was high enough to form the Pt-Si bonds but too low to form the Si-Si bonds), then, the present observation mentioned above can be explained reasonably. The electronic excitation in the sequence mentioned above, must be so efficient as to overcome a barrier that prohibited the silicide formation under simple thermal annealing, according to the data presented in Sec. III B. As mentioned beforehand in Sec. III A, the silicide formation at such a low temperature as 93 K observed here suggests a possibility that an interface exhibiting an intermixing ability similar to that of the as-in-situ-cleaved clean silicon/ as-deposited metal interface employed in the previous paper10 was realized also in the present work as a result of electronic excitation, albeit short in individual durations. Details of the atomic process behind such enhanced intermixing under electronic excitation conditions, along with the kinetics of silicide formation, will be studied in a future work. Lastly, it is of interest to note that similar silicide formation induced by electronic excitation has been observed also at interfaces such as those between palladium and silicon oxide (i.e., formation of amorphous Pd-Si alloy) and between nickel and silicon oxide (i.e., formation of Ni3Si). These observations indicate that the silicide formation by electronic excitation is of wide generality in a series of transition metal/silicon oxide interfaces. It is worth noting that the present observation regarding the silicide formation at the internal interface provides a piece of firm evidence for the realization of a “reduced” state of silicon within the solid by electronic excitation. IV. CONCLUSION

A platinum silicide, a-Pt2Si, was successfully formed at the platinum/silicon oxide interface within the solid under 25–200 keV electron irradiation. This observation is of interest since any platinum silicide was not formed at the platinum/silicon oxide interface by simple thermal annealing under no-electron-irradiation conditions. It is clarified that the silicide formation under electron irradiation was not due to a knock-on atom-displacement process but a process induced by electronic excitation. Similar silicide formation was also observed at the palladium/silicon oxide and nickel/ silicon oxide interfaces, indicating a wide generality of the silicide formation by electronic excitation. ACKNOWLEDGMENTS

A part of this work was supported by “Advanced Characterization Nanotechnology Platform, Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan” at the Research Center for Ultra-High Voltage Electron Microscopy (Nanotechnology Open Facilities) in Osaka University. 1

N. Itoh and A. M. Stoneham, Materials Modification by Electronic Excitation (Cambridge University Press, Cambridge, 2001).


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N. Itoh and A. M. Stoneham, Radiat. Eff. Defects Solids 155, 277 (2001). J. F. Hamilton, Adv. Phys. 37, 359 (1988). 4 H. Yasuda, A. Tanaka, K. Matsumoto, N. Nitta, and H. Mori, Phys. Rev. Lett. 100, 105506 (2008). 5 J. W. Corbett, Electron Radiation Damage in Semiconductors and Metals, Series of Solid State Physics, Suppl. 7, edited by F. Seitz and D. Turnbull (Academic Press, New York, 1966). 6 R. B€auerlein, “Radiation damage in solids,” in Proceedings of the International School of Physics {Enrico Fermi} XVIII Course (Academic Press, New York, 1962), p. 358. 7 T. Ikuno, S. Honda, T. Yasuda, K. Oura, M. Katayama, J.-G. Lee, and H. Mori, Appl. Phys. Lett. 87, 213104 (2005). 8 See supplementary material at http://dx.doi.org/10.1063/1.4921429 for sample preparation and others. 3

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G. Rupprechter, K. Hayek, L. Rendon, and M. Jose-Yacaman, Thin Solid Films 260, 148 (1995). 10 I. Abbati, L. Braicovich, B. De Michelis, and U. del Pennino, J. Vac. Sci. Technol. 17, 1303 (1980). 11 G. S. Chen, C. B. Boothroyd, and C. J. Humphreys, Philos. Mag. A 78, 491 (1998). 12 L. Reimer, Transmission Electron Microscopy (Springer-Verlag, Berlin, 1989). 13 R. F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope (Plenum Press, New York, 1986). 14 R. L. Pfeffer, J. Appl. Phys. 57, 5176 (1985). 15 F. G. Bell and L. Ley, Phys. Rev. 37, 8383 (1988). 16 M. L. Knotek and P. J. Feibelman, Phys. Rev. Lett. 40, 964 (1978).
