Supplemental Material
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. Mori, 2
1
Powder & Ceramics Division, Korea Institute of Materials Science, Changwon, Gyeongnam, 642-831, 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
a)
Electronic mail: [email protected].
1. Evaluation of oxygen content in silicon oxide films
In the present work, the oxygen content in silicon oxide films (i.e., the value of x in SiO x) prepared
by vapor deposition in vacuum was evaluated by the XPS technique.1 The measurements were carried
out using an XPS system (model RDK XPS-7000 by the Rigaku-Denki Co. Japan). The X-ray source
was operated with Al Kα (1486 eV) radiation. Figure S1 depicts a Si 2p XPS spectrum (blue crosses) of
the silicon oxide film. The XPS spectrum was analyzed to obtain quantitative information on the relative
amount of Si oxidation states and then estimate the value of x in SiOx.
After removing the background (dashed red line in Fig. S1), the spectrum was decomposed into five
components, as, in the analysis of oxidized Si, the spectrum analysis is typically based upon five
constituents: Si0, Si1+, Si2+, Si3+, and Si4+,1,2 where Si0 corresponds to Si bonded with four nearest
neighbor (nn) Si atoms, Si1+ with three nn Si atoms (and one nn O atom), Si2+ with two nn Si atoms (and
two nn O atoms), Si3+ with one nn Si atom (and three nn O atoms), and Si4+ with four nn O atoms. In the
practical procedure, the energy axis of the spectrum was first determined by calibration with the carbon
1
1s peak. The peak position of the component Si0 was then fixed at 99.15 eV (see Fig. S1) and,
subsequently, the peak positions of the remaining four components were set so that the positions of the
Si1+, Si2+, Si3+, and Si4+ components were shifted, relatively to the standard position of the component
Si0, by 1.00, 1.80, 2.70, and 3.75 eV, respectively, toward the high binding energy side according to the
literature.3 Using the curve-fitting analysis, the spectrum was then decomposed into five components in
a quantitative manner; the analysis was carried out by means of the least-squares method and each
spectrum component was described by a convolution of Lorentzian shape with a Gaussian due to the
instrumental and phonon broadening. Each of the obtained spectrum component is depicted in Fig. S1 as
a non-solid red line. From the fractional areas of individual spectral components, which are tabulated in
Table S1, it is possible to get quantitative information on the relative amount of the oxidation states of
silicon in the film. By assigning two oxygen atoms per silicon atom for the Si 4+ component, 1.5 for the
Si3+ component, 1.0 for the Si2+ component, and 0.5 for the Si1+ component, the value of x in SiOx can
be evaluated. The value of x thus obtained was equal to 1.48. Therefore, the oxygen content in silicon
oxide films prepared by vapor deposition in vacuum, in the present work, is estimated to be
approximately SiO1.5.
2. Epitaxial growth of a platinum silicide, Pt2Si, on Pt
It was confirmed that a platinum silicide, Pt2Si, being formed at the platinum/silicon oxide interface,
had a crystallographic orientation relationship of (001)Pt//(001)Pt2Si, [110]Pt//[110]Pt2Si with platinum,
as shown in Figs. 2 and 4 in the main text. The epitaxial growth of Pt2Si on platinum can be simply
explained from their crystal structures. Figures S2(a) and S2(b) show a schematic illustration of a unit
cell of fcc Pt and of tetragonal Pt2Si,4 respectively. The arrangement of the platinum atoms on the (001)
plane of fcc Pt at z = 0 is shown in Fig. S2(c), whereas that on the (004) plane of tetragonal Pt2Si at z =
1/4 (or 3/4) is shown Fig. S2(d). A comparison of Fig. S2(c) with S2(d) shows that, on these two planes,
the arrangement of the platinum atoms is the same and the difference in the distance between the
2
platinum atoms is only 0.6%. Therefore, it seems reasonable to consider that Pt2Si epitaxially grew on
platinum with the orientation relationship mentioned above so as to minimize the misfit strain energy at
the Pt/Pt2Si interface. To the authors’ knowledge, this is the first report on the epitaxial growth of Pt2Si
on platinum at, or below, room temperature.
3. Evidence for the silicide formation at the platinum/silicon oxide interface within the solid
As described in the main text, under electron irradiation conditions, a silicide Pt2Si was formed at the
Pt/SiOx interface (i.e., at such an interface as marked by A in Fig. S3(a)). One way to get additional
evidence for the fact that the silicide was certainly formed at the platinum/silicon oxide interface within
the solid, was to study whether or not the silicide was formed also in such a specially-designed
composite sample where the top surface of the original platinum/silicon oxide composite (e.g., that
marked by B in Fig. S3(a)) was completely covered with a layer of additionally deposited silicon oxide
(SiOx) and consequently each platinum particle was fully embedded in (and enclosed with) a silicon
oxide (SiOx) matrix, with no free platinum surface present in the composite, as illustrated in Fig. S3(b).
Thus, supplementary electron irradiation experiments were carried out on such SiOx/Pt/SiOx
composite samples (i.e., sandwich-type composite samples) as shown in Fig. S3(b), in order to confirm
the silicide formation within the solid, using the electron microscope operated at 200 kV. The irradiation
experiments were done at room temperature. Figures S3(c) and S3(d) show a BFI of an as-prepared
sandwich-type composite and the corresponding SAED pattern, respectively. The SAED pattern in Fig.
S3(d) is consistently identified as the [001] diffraction pattern of fcc Pt, superposed with a broad halo
ring (albeit weak in intensity) from the matrix of amorphous silicon oxide. This is essentially identical to
the SAED patterns shown in Figs. 1(b) and 2(a) in the main text, except that in the pattern in Fig. S3(d)
the intensities of the Debye-Scherrer rings of randomly oriented platinum particles are too weak to be
recognized in the photograph because of the significantly reduced fraction of such particles compared to
3
the well-aligned, cubic oriented [001] particles. Figures S3(e) and S3(f) show a BFI of the composite
after irradiation for 1200 s at a flux of 4.4 x 1023 m-2s-1, and the corresponding SAED pattern,
respectively. The SAED pattern in Fig. S3(f), which includes extra spots newly appeared during the
irradiation (such spots as marked by Pt2Si 110 and Pt2Si 1̅ 10), is consistently identified as the [001]
diffraction pattern of α-Pt2Si superposed with the [001] diffraction pattern of fcc Pt as well as the broad
halo from amorphous silicon oxide. This feature is essentially the same as that seen in Figs. 2(b) and
2(c) in the main text. This result indicates that the same silicide was formed also in the sandwich-type
SiOx/Pt/SiOx composite (i.e., in such a sample as shown in Fig. S3(b)) with no free surface of platinum
particles. Therefore, under electron irradiation conditions, the Pt2Si silicide was formed at the
platinum/silicon oxide interface within the solid. This provides a strong support for the authors’ view
that an electronic-excitation-induced process, which is responsible for the silicide formation, is certainly
operating within the solid.
4. Comparison with the equilibrium phases in the Pt-Si binary alloy system
It is interesting to compare the silicide formed by electron irradiation (or electronic excitation) in the
present study (i.e., Pt2Si) with the thermodynamically stable equilibrium phases in the Pt-Si binary alloy
system that can be formed by conventional thermal annealing of a diffusion couple of Pt-Si alloys. There
are five equilibrium intermediate phases with different stoichiometries in the Pt-Si system, namely, Pt3Si,
Pt7Si3, Pt2Si, Pt6Si5, and PtSi.5 Experiments on Pt-Si thin-film diffusion couples have confirmed that, of
the five phases, Pt2Si is the first phase formed at the Pt/Si interface in the phase formation sequence
during thermal annealing.6,
7
The formation of Pt2Si as the first phase in the sequence is in good
agreement with the prediction of the effective heat of formation (EHF) model proposed by Pretorius et
al.8 In the model, it is envisaged that, at the metal/silicon interface, upon heating, the greatest mobility of
the constituent atoms, and consequently the most effective mixing and reaction between constituent
atoms, will occur at such sites where the alloy composition is that of the liquidus minimum of the binary
4
system. Consequently, it is then predicted that the first phase nucleated and formed at the metal/silicon
interface is the phase for which the effective heat of formation becomes the lowest (namely, the most
negative) at the alloy composition of the liquidus minimum, among the possible equilibrium phases. 8 In
the particular system of Pt-Si, the first phase predicted from the model is Pt2Si.8 Thus, the present
experimental result is in complete agreement with the prediction.
5. Confirmation of silicide formation at the Pt/SiO2 (stoichiometric composition) interface
In understanding the effect of off-stoichiometry on the silicide formation, it is of significance to
confirm experimentally whether or not a silicide is formed not only at the Pt/SiOx (x  1.5) interface (as
shown in the main text) but also at the Pt/SiO2(stoichiometric composition) interface under electron
irradiation conditions. From such results, it will become possible to discriminate whether or not the
deviation from the stoichiometry, (i.e., in other words, the excess of silicon atoms from the
stoichiometric composition SiO2,) is a prerequisite for the silicide formation at the platinum/silicon
oxide interface under electron irradiation conditions.
Based upon the premise, electron irradiation experiments on Pt/SiO2 composite samples were
performed using the same electron microscope. TEM foils of single crystalline quartz were first
prepared by a combination of ion milling and chemical polishing techniques. The quartz crystal was
purchased from KYOCERA Crystal Device Corporation, Japan. The ion milling was carried out with
4.5 keV argon ions at an incident angle of 4°, using an instrument of model 691 Gatan. The chemical
polishing was done to furnish the samples with a non-ion-damaged surface using a mixture of HF,
HNO3, and H2O with a volume ratio of 3:2:60. Platinum was then deposited onto the surface of the
quartz foil by magnetron sputtering and platinum particles were grown on the surface. The Pt/SiO2
(quartz) composite samples were subjected to irradiation experiments with 200 keV electrons in the
microscope at room temperature, while the microstructural changes during irradiation were monitored in
5
situ. The electron flux employed was 4.4 × 1023 m-2 s-1. An example of the results is depicted in Fig. S4.
Figures S4(a) and S4(b) show a BFI of an as-prepared composite and the corresponding SAED pattern,
respectively. A number of platinum islands, which appear somewhat grey, and channels between islands,
which appear bright, can be seen in the BFI. The SAED pattern in Fig. S4(b) is consistently identified as
the [0001] diffraction pattern of α-quartz superposed with Debye-Scherrer rings from randomly oriented
particles of fcc Pt. Although irradiation for 10 s did not cause any essential change in the BFI (Fig.
S4(c)), except for the slight coarsening in the microstructure of the platinum particle aggregates, it
produced, however, a quite remarkable change in the SAED pattern: the appearance of a broad halo ring
(arrowed by a letter D in Fig. S4(d)) in exchange for the complete disappearance of the spot pattern of
crystalline α-quartz, as seen from Fig. S4(d). The halo ring with a radius of 2.4 nm-1, is consistently
identified as the first halo ring of amorphous SiO2, as reported in the literature.9 This fact indicates that
α-quartz is rendered amorphous by the irradiation. The occurrence of a crystalline to amorphous
transition in quartz that is induced by a small dose of ~100 keV electrons (i.e., the high susceptibility to
radiation-induced amorphization) is in agreement with the descriptions reported in the literature.10 With
continued irradiation, additional Debye-Scherrer rings appeared in the SAED pattern, as shown in Fig.
S4(f). The most inner ring (r1) and the second inner ring (r2) of the newly appeared rings can be
consistently indexed as the rings of 110 and 112 reflections of α-Pt2Si, respectively, whereas, in the BFI,
only slight microstructural evolution was noticed (Fig. S4(e)). The results then confirmed that the same
silicide, α-Pt2Si, was formed at the Pt/SiO2 (stoichiometric composition) interface under electron
irradiation conditions, in a manner similar to that observed at the Pt/SiOx (x  1.5) interface. Therefore,
it is reasonable to conclude that the excess of silicon atoms over the stoichiometric composition SiO 2 is
not a prerequisite for the silicide formation at the platinum/silicon oxide (Pt/SiOx) interface observed in
the present work.
6
References for supplemental material
1
F. J. Himpsel, F. R. McFeely, A. Taleb-Ibrahimi, J. A. Yarmoff, and G. Hollinger, Phys. Rev. B 38,
6084 (1988).
2
S. Ogawa, A. Yoshigoe, S. Ishidzuka, and Y. Teraoka, Japanese J. Appl. Phys. 46, 3244 (2007).
3
A. Yoshigoe and Y. Teraoka, Japanese J. Appl. Phys. 49,115704 (2010).
4
P. Villars and L. D. Calvert, Pearson's Handbook of Crystallographic Data for Intermetallic Phases,
(ASM International, 2nd Ed., Materials Park, Ohio, U. S., 1991) Vol.4, p.4983 (for Pt) and p.4995 (for
Pt2Si)
5
T. B. Massalski, Editor-in-Chief, Binary Alloy Phase Diagrams (American Society for Metals, Metals
Park, Ohio, U. S., 1986).
6
C. Canali, G. Majni, G. Ottaviani, and G. Celotti, J. Appl. Phys. 50, 255 (1979).
7
M. A. E. Wandt, C. M. Comrie, J. E. McLeod, and R. Pretorius, J. Appl. Phys. 67, 230 (1990).
8
R. Pretorius, T. K. Marais, and C. C. Theron, Mater. Sci. Eng. Rep. 10, 1 (1993).
9
B. E. Warren, J. Appl. Phys. 8, 645 (1937).
10
H. Inui, H. Mori, T. Sakata, and H. Fujita, J. of Non-Cryst. Solids 116, 1 (1990).
7
Figures and a table
FIG. S1. Core-level spectrum of a SiOx film prepared by vacuum deposition of silicon monoxide. The
top curve (blue crosses) shows the raw photoemission data for the Si 2p core level. After removing the
background (dashed red line), the spectrum was decomposed into five components: Si 0, Si1+, Si2+, Si3+,
and Si4+ (non-solid red lines). The relative intensities of the five components were calculated from the
fractions of the areas and were tabulated in Table S1. From the relative intensities, the value of x in SiO x
was evaluated to be approximately equal to 1.5.
8
FIG. S2. Crystal structures of fcc Pt and tetragonal α-Pt2Si; (a) and (b) schematically illustrate unit cells
of Pt and α-Pt2Si, respectively. Pt atoms marked A, B, and C in (a) correspond to those in (c),
respectively. Similarly, Pt atoms marked A’, B’, and C’ in (b) correspond to those in (d), respectively.
Here, spheres in blue and red correspond to Pt and Si atoms, respectively.
9
FIG. S3. Preparation of a SiOx/Pt/SiOx sandwich-type composite sample and confirmation of electronirradiation-induced silicide formation at the Pt/SiOx interface within the solid. (a) Schematic illustration
of a Pt/SiOx composite sample used in the experiments described in the main text. (b) Schematic
illustration of a SiOx/Pt/SiOx sandwich-type composite sample used in the experiments for the present
supplementary online material. (c) and (d) show a bright-field image (BFI) of an as-prepared sample and
the corresponding selected area electron diffraction (SAED) pattern, respectively. (e) and (f) show a BFI
of the same sample after electron irradiation for 1200 s and the corresponding SAED pattern,
respectively. The appearance of diffraction spots such as 110 Pt2Si and 1̅10 Pt2Si in the SAED (f) after
irradiation indicates the formation of Pt2Si at the Pt/SiOx interface within the solid. The incident energy
of electrons, flux, and irradiation temperature were 200 keV, 4.4 × 1023 m-2 s-1, and room temperature,
respectively.
10
FIG. S4. Electron-irradiation-induced silicide formation at the Pt/SiO2 interface. (a) and (b) show a
bright-field image (BFI) of an as-prepared Pt/SiO2(quartz) composite and the corresponding selected
area electron diffraction (SAED) pattern, respectively. (c) and (d) show a BFI of the same sample after
electron irradiation for 10 s and the corresponding SAED pattern, respectively. A comparison of (b) with
(d) indicates that SiO2 (quartz) underwent a crystalline to amorphous transition to become amorphous
SiO2 by irradiation. (e) and (f) show a BFI of the same sample after irradiation for 1800 s and the
corresponding SAED pattern, respectively. The appearance of the Debye-Scherrer rings attributed to
Pt2Si in the SAED pattern after irradiation (i.e., (f)) indicates the formation of Pt2Si at the Pt/SiO2
(amorphous) interface. The incident energy of electrons, flux, and irradiation temperature were 200 keV,
4.4 × 1023 m-2 s-1, and room temperature, respectively.
11
TABLE S1. Peak energy positions, heights, full widths at half maximum (FWHMs), and areas for the
individual components, Si0 through Si4+, obtained from the component spectra (non-solid red lines)
shown in Fig. S1.
Component
Energy of peak (eV)
Height (count)
FWHM (eV)
0
99.15
81.52
1.20
113.98
[ 4.95]
1+
100.15
121.49
1.60
226.50
[ 9.84]
2+
100.95
162.37
1.61
304.61
[13.23]
3+
101.85
379.33
1.51
667.42
[29.00]
4+
102.90
451.53
1.88
989.12
[42.97]
Si
Si
Si
Si
Si
12
Area (count・eV)
[fraction%]