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Embedded structure of silicon monoxide in SiO2 films
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Applied Surface Science 254 (2008) 2534–2539 www.elsevier.com/locate/apsusc
Embedded structure of silicon monoxide in SiO2 films Kiyoshi Chiba *, Yoshihito Takenaka Department of Nano Material and Bio Engineering, Tokushima Bunri University, Sanuki, Kagawa 769-2193, Japan Received 2 April 2007; accepted 27 September 2007 Available online 2 October 2007
Abstract The structure of SiOx (x = 1.94) films has been investigated using both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The SiOx films were deposited by vacuum evaporation. XPS spectra show that SiO1.94 films are composed of silicon suboxides and the SiO2 matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si3O+ ion species were found in the TOF-SIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO2, and this structure most likely exists as a predominant form of Si3O4. The existence of Si–Si structures in the SiO2 matrix will give rise to dense parts in loose glass networks. # 2007 Elsevier B.V. All rights reserved. PACS : 61.43.Fs; 68.49.Sf; 68.49.Uv; 68.55.Jk Keywords: SiOx films; Silicon suboxides; X-ray photoelectron spectroscopy; Secondary ion species; Time-of-flight secondary ion mass spectrometry
1. Introduction SiOx films are well known and extensively studied materials [1–6] that are very attractive for applications as dielectrics [7,8] and transparent gas barrier films [9,10]. In particular, additional interest in these materials has arisen out of curiosity in their structure in the nanoscale due to the non-stoichiometric silicon oxides [11–14]. Silica glass is thought to be comprised of a randomly networked-structure of Si–O bonds with four-coordination number of silicon atoms surrounded by oxygen atoms and linked to other units through Si–O–Si bridges [1,6]. Although the ratio of silicon to oxygen atoms is 2.0 for the mostenergetically stable, that is, fully-oxidized and stoichiometric silicon oxide, thin film compositions with ratios less than 2.0 are frequently used for applications in electronics [7] and optics [8]. These have the composition SiOx [1–6], wherein x is in the range 0 < x < 2. Due to excess silicon atoms producing optical absorption in the visible range, compositions with x close to 2.0 are preferable in order to attain transparency. For instance, thin films with x between 1.9 and 2.0 have been used as transparent
* Corresponding author. Tel.: +81 87 894 5111; fax: +81 87 894 4201. E-mail address: [email protected] (K. Chiba). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.089
gas barrier films [9,10]. In the Si–SiO2 phase diagram, the intermediate oxidation states of silicon cannot be identified; it is not clear that silicon monoxide exists in the phase diagram [1,15,16]. There is also significant curiosity and interest in the structure of SiOx films, in particular, transparent SiOx films in the nanoscale. Secondary ion species in time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra could play a powerful role in providing information on solid structures in the nanoscale [17,18]. SiO1.94 films were investigated in this study using TOF-SIMS spectra in association with peak analysis in X-ray photoelectron spectroscopy (XPS) spectra [17,19].
2. Experimental Two types of samples were used in this study: 25 nm-thick SiO2 films and 94 nm-thick SiOx films. 25 nm-thick thermallyoxidized as well as plasma-enhanced CVD-prepared SiO2 films on single-crystal n-type (1 0 0) silicon wafers (resistivity of 4–6 V cm) were received from Tohoku Semiconductor Co. SiOx films with 94-nm thickness were deposited onto poly(ethylene terephthalate) (PET) films at room temperature using electron beam evaporation of a mixture of silicon and silicon dioxide. The vacuum pressure was 5.0 10 3 Pa. The atomic
K. Chiba, Y. Takenaka / Applied Surface Science 254 (2008) 2534–2539
composition x of the films was determined by XPS and was shown to be 1.94 0.03. XPS measurements were performed using an ULVAC-PHI Quantera SXM spectrometer with an Al Ka X-ray source.
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Sputter etching was accomplished using either Ar+ ions at 500 Vor C60+ ions at 10 kV rastered over a 2 mm 2 mm area. This corresponds to an etching rate of about 0.02 nm/s for the SiO2 films. For the C60+ ion source, a differentially pumped C60
Fig. 1. XPS spectra from SiO2 films on Si: (a) as-grown surface of a thermally-oxidized film; (b) 2-nm depth from the surface for the thermally-oxidized film sputteretched using 500 V Ar+ ions; (c) 2-nm depth from the surface for the thermally-oxidized film sputter-etched using 10 kV C60+ ions; (d) as-grown surface of a CVD oxide film; (e) 2-nm depth from the surface for the CVD oxide film sputter-etched using 500 VAr+ ions; and (f) 2-nm depth from the surface for the CVD oxide film sputter-etched using 10 kV C60+ ions.
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ion gun with an integral Wien filter was used. Both thermallyoxidized and CVD-prepared SiO2 films with 25-nm thickness were investigated to examine the spectra of SiO2 films which depend on the preparation conditions as a reference for the SiOx spectra. In addition, two types of sputter etching using 500 V Ar+ and 10 kV C60+ ions were performed to investigate the influence of beam-induced damage; the damage can cause a reduction in the oxidation states of the films which can alter the XPS spectra. All spectra were analyzed after subtracting an integrated background [17,20]. TOF-SIMS measurements were performed using an ULVAC-PHI Trift III TOF-SIMS spectrometer. Sputter etching was accomplished by Ar+ ions at 1 kVand 200 nA rastered over
a 1000 mm 1000 mm area. This corresponds to an etching rate of approximately 0.04 nm/s for the SiO2 films. TOF-SIMS analyses were performed with positive polarity using a pulsed Au3+ primary ion beam operating at 22 kV and 2.0 nA. A bias voltage of +3 kV was applied to the sample holder in this instrument for measurements of positive secondary ions. The beam was rastered over a 100 mm 100 mm surface area. Secondary ions in the mass range from 0 to 1860 atomic mass units (amu) were collected for 300 s. Charge compensation using an electron flood gun was required. The working pressure was in the 10 7 Pa range. Identification of the peaks of the secondary ion species was carried out using the masses of the peaks with an accuracy of within 0.01 amu.
Fig. 2. XPS spectra from SiO1.94 films on a PET film: (a) as-deposited surface; (b) 2-nm and 10-nm depth surfaces sputter-etched using 500 VAr+ ions; (c) 2-nm and 10-nm depth surfaces sputter-etched using 10 kV C60+ ions; and (d) enlarged spectra of the 10-nm depth surface etched using 10 kV C60+ ions. The spectrum is decomposed into two spectra. Peak positions are marked by arrows. Binding energies (EB) of Si 2p1/2 and Si 2p3/2 in silicon are also shown by arrows.
K. Chiba, Y. Takenaka / Applied Surface Science 254 (2008) 2534–2539
MM2 calculations of the SiOx structure were carried out using the program package from Molecular Design Limited. 3. Results and discussion XPS analyses were carried out on the SiO2 and SiO1.94 films. Fig. 1 shows the Si 2p spectra in the SiO2 films for as-grown (before sputtering, Fig. 1a, d) and approximately 2 nm depth from the surfaces (after sputtering, Fig. 1b, c, e, f) of the films. All spectra appear with symmetric peak profiles, corresponding to Si 2p core spectra of silicon atoms surrounded by four oxygen atoms with a binding energy of 103.4 eV [17]. Small differences can be observed in the spectra between the thermally-oxidized and CVD-prepared SiO2 films. A slight increase in the full-width half-maximum (FWHM) of the spectra by about 10–15% can be seen after both Ar+ and C60+ sputtering, which is attributed to lattice damage induced by ion impact on the surfaces. It should be noted that the symmetry of the peak profiles is maintained. These peaks can be fitted with a Lorentzian band modified by the Gaussian shape [17]; they are best fitted by 88% Gaussian–12% Lorentzian bands. XPS analysis was carried out for the SiO1.94 films. Fig. 2 shows the
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Si 2p spectra in the SiO1.94 films for the as-deposited surfaces (before sputtering, Fig. 2a) and at approximately 2 nm and 10 nm depths from the surfaces (after sputtering with Ar+ and C60+ shown in Fig. 2b and c, respectively). Asymmetric features, with a tail on the low binding energy side, appeared in the spectra. A slight increase in the FWHM can also be noticed after Ar+ and C60+ sputtering, however, the increase was smaller with C60+ than with Ar+. The enlarged spectrum for approximately 10 nm depth from the surface after sputtering with C60+ is shown in Fig. 2d. This asymmetric Si 2p peak can be decomposed into two spectra using 88% Gaussian–12% Lorentzian bands; it consists of a major peak with a binding energy (EB) of 103.4 eVand a minor peak with a binding energy of 102.2 eV. The ratio of peak areas of the minor peak to the major peak was approximately 0.1. The peak with the binding energy of 103.4 eVand FWHM of 1.9 eV corresponds to silicon atoms surrounded by four oxygen atoms [17], due to the SiO2 matrix. The minor peak with the binding energy of 102.2 eV and FWHM of 2.86 eV can be attributed to the silicon suboxide. Nevertheless, the peak of the suboxide can appear as a single spectrum peak, and the broad FWHM suggests the influence of multiple components
Fig. 3. Positive ion TOF-SIMS spectra from SiO1.94 films on a PET film and SiO2 films on Si. (a) Spectra from SiO1.94 films in the mass range of 0–200 amu. Sputter etching is performed with 1 kVAr+ ions to approximately 10-nm depth from the surface. The inset shows the spectra using an intensity scale enlarged by a factor of 20 in the mass range of 90–200 amu. (b) Spectra from SiO1.94 films with an intensity scale enlarged by a factor of 120. Possible fragment structures are shown. Note that the ratio of the intensities of the ion species marked by an asterisk is used for the analysis of the film structure. (c) Spectra from thermally-oxidized SiO2 films as a reference. Possible fragment structures are shown.
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K. Chiba, Y. Takenaka / Applied Surface Science 254 (2008) 2534–2539
attributable to silicon atoms surrounded by different numbers of oxygen atoms. It has been identified that silicon atoms can have up to five different valence states: Si0+, Si1+, Si2+, Si3+, and Si4+, where these valence states correspond to silicon atoms in the matrix surrounded by 0, 1, 2, 3, and 4 oxygen atoms, respectively [21]. A number of authors [21–25] have reported the chemical shift of Si 2p with these valence states by analyses of Si-very thin (<3-nm thick) SiO2 film interfaces and CVDprepared SiOx (0 < x < 0.8) using both synchrotron radiation and XPS. The values of the chemical shift are mostly obtained as 0 eV, 0.9–1.0 eV, 1.7–1.85 eV, 2.5–2.6 eV, and 3.5–3.7 eV for Si0+, Si1+, Si2+, Si3+, and Si4+, respectively. The variations in these values are small. Taking the values of the chemical shift reported by Shallenberger [24] into consideration and their similarity to the present work, for thick SiOx films using XPS, the binding energies correspond to 102.4 eV and 101.7 eV for Si3+and Si2+ relative to 103.4 eV for Si4+ in this study, respectively. This indicates that the suboxide peak with the binding energy of 102.2 eV is located at a position between Si3+and Si2+, and close to Si3+. It is also to be noted that atomic photo-ionization cross sections for XPS and the escape depths multiplied by the density of Si atoms in silicon oxides [22] are nearly the same for Si3+and Si2+. These data suggest that silicon atoms in the suboxide species in the SiO1.94 films are predominantly surrounded by two and three oxygen atoms, and exist with nearly twice the number of Si3+as Si2+; this can be deduced from the decomposed curves from the peak with the binding energy of 102.2 eV shown with dotted lines in Fig. 2d. Further addition of the Si1+ element does not significantly improve the curve fitting of the spectra. This corresponds to good agreement with a model for SiO1.5 described by Philipp [13], suggesting a mixture of Si–Si and Si–O bonds on an atomic scale in the structures. Si0+, that is, elementary silicon attributable to silicon clusters was found to a negligible extent in the SiO1.94 films. Fig. 3 shows TOF-SIMS spectra for the SiO1.94 films on PET films and thermally-oxidized SiO2 films on Si at a depth of approximately 10 nm after sputtering with 1 kV Ar+ ions. For the SiO1.94 films, the spectrum in the mass range from 0 to 200 is shown in Fig. 3a. Strong Si+ and Si2+ peaks can be observed, whereas the intensities of ions rapidly decreased with increasing mass number. The expansion of the intensity axis of the spectra in the mass range from 90 to 200 is shown in the inset. Notably, a strong peak around 100 amu appeared in this spectrum. The 96 amu peak arises from Ti2+ due to the boat material being made of Ti in the evaporation. The ion species in Fig. 3b were determined based on data analyzed for thermally grown SiO2 on Si and the interface [26]. This indicates that a large number of Si3O+ species, likely attributable to Si–Si–Si– O+, can be found in the film in addition to the species originating from the SiO2 matrix. For the SiO2 films, the spectrum in the mass range from 90 to 160 is shown in Fig. 3c. The intensity of Si3O+ is seen to be negligibly small. Relative intensities of the Si3O+, Si3O2+, and Si3O3+ peaks normalized to the intensity of the Si3O4+ peak were calculated, and the results are listed in Table 1. The Si3O4+ peak identified is likely attributable to Si–O–Si(O)–O–Si(O)+ due to the SiO2 matrix
Fig. 4. Molecular modelling of the structure of SiO1.94 films. Silicon monoxide is embedded in SiO2 matrix as a predominant form of Si3O4. This structure was calculated and energetically minimized using the MM2 package.
[26]. For the SiO1.94 films, the ratio of the intensity of Si3O+ to that of Si3O4+ is 20.3, which is much larger than that observed for the SiO2 films. The data obtained with XPS and TOF-SIMS suggest that silicon atoms are surrounded by both two and three oxygen atoms and partial structures of Si–Si–Si–O may exist in the suboxides in the SiO1.94 films. We can thus postulate the chemical formula of the suboxide species in the SiO1.94 films to be SiO1.5–(Si–O)n–SiO1.5. The silicon monoxide structure (Si– O)n is linked to the SiO2 matrix via oxygen atom bridges. Considering the intensity factor in the spectra of the silicon suboxides in the XPS data, n seems most likely to be 1.0, but is distributed as greater or smaller numbers in the structure corresponding to peak broadening. A simple calculation suggests that SiO1.94 corresponds to approximately 9 at.% of Si3O4 embedded in the SiO2 matrix. It is important to note that this value is consistent with the value of approximately 9.2% of silicon suboxides in the SiO2 matrix obtained by XPS analysis, as shown in Fig. 2d. Fig. 4 illustrates a molecular model of the structure with the nearest neighbours of the matrix structures. This is calculated Table 1 Comparison between relative intensities of Si3On+ (n = 1, 2, 3, 4) ions emitted from SiO2 film (10-nm depth surface) and SiO1.94 film (10-nm depth surface) Source
SiO2 SiO1.94 a b
Ion species 28
Si3O+ (100)a
28
Si3O2+ (116)
28
Si3O3+ (132)
28
–b 20.3
0.20 2.9
0.28 1.3
1.0 1.0
The values in parentheses represent mass number m/z. Negligibly small.
Si3O4+ (148)
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and energetically minimized using the MM2 force field. This model indicates that the structure of the SiO1.94 films consists of silicon monoxide embedded in SiO2, and most likely exists as a predominant form of Si3O4. The existence of Si–Si structures in the SiO2 matrix will give rise to dense parts in loose glass networks. This model may provide a more preferable explanation for the data obtained than the model wherein various amounts of Si1+ are postulated to exist in the films [27].
References
4. Conclusions
[9] [10]
The structure of SiOx (x = 1.94) films has been investigated using both X-ray photoemission spectroscopy and time-offlight secondary ion mass spectrometry. The SiOx films were deposited by vacuum evaporation. XPS spectra show that SiO1.94 films are composed of silicon suboxides and the SiO2 matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si3O+ ion species were found in the TOFSIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO2, and this structure most likely exists as a predominant form of Si3O4. The embedded structure of silicon monoxide in SiO2 films is presented in this study. This will expand the scope of nanocrystals, for instance, of Si and Ge embedded in a matrix that exhibits unique and attractive behaviours and properties [28,29]. Furthermore, the existence of silicon monoxide in equilibrium can be inferred, not only for a single atomic layer in the Si–SiO2 interface [30], but also for thick materials. Acknowledgements We thank Tohoku Semiconductor Co. and Oike & Co. for supplying silicon dioxide films on silicon wafers and PET/SiOx films, respectively. We are indebted to ULVAC-PHI, Inc. for measurements of XPS spectra, and to Mr. M. Kawamura of Tokushima Bunri University for his help in TOF-SIMS measurements.
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R.B. Sosman, Trans. Br. Ceram. Soc. 54 (1955) 655. G. Hass, J. Am. Ceram. Soc. 33 (1950) 353. V.H. Scha¨fer, R. Ho¨rnle, Z. Anorg. Allg. Chem. 263 (1950) 261. M. Hoch, H.L. Johnston, J. Am. Chem. Soc. 75 (1953) 5224. von H.-H. Emons, P. Hellmold, Z. Anorg. Allg. Chem. 341 (1965) 78. R.K. Iler, The Chemistry of Silica, A Wiley-Interscience Publication, New York, 1979. J.H. Thomas III, A.M. Goodman, J. Electrochem. Soc. 126 (1979) 1766. H.A. Macleod, Thin-Film Optical Filters, third ed., Institute of Physics Publishing, Bristol, 2001. H. Chatham, Surf. Coat. Technol. 78 (1996) 1. A.P. Roberts, B.M. Henry, A.P. Sutton, C.R.M. Grovenor, G.A.D. Briggs, T. Miyamoto, M. Kano, Y. Tsukahara, M. Yanaka, J. Memb. Sci. 208 (2002) 75. G.W. Brady, J. Phys. Chem. 63 (1959) 1119. J. Benyon, Vacuum 20 (1970) 293. H.R. Philipp, J. Phys. Chem. Solids 32 (1971) 1935. S.I. Rauder, S.I. Flitsch, J. Electrochem. Soc. 123 (1976) 1754. R.E. Johnson, A. Muan, J. Am. Ceram. Soc. 51 (1968) 430. E.M. Levin, H.F. McMurdie, Phase Diagrams for Ceramists 1975 Supplement, The American Ceramics Society, Columbus, 1975. D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, vols. 1 and 2, Wiley, Chichester, 1990. K. Chiba, T. Akamatsu, M. Kawamura, Chem. Phys. Lett. 419 (2006) 506. S. Hu¨fner, Photoelectron Spectroscopy, third ed., Springer, Heidelberg, 2003. D.A. Shirley, Phys. Rev. B5 (1972) 4709. P.J. Grunthaner, M.H. Hecht, F.J. Grunthaner, N.M. Johnson, J. Appl. Phys. 61 (1987) 629. F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Phys. Rev. B38 (1988) 6084. M.T. Sieger, D.A. Luh, T. Miller, T.-C. Chiang, Phys. Rev. Lett. 77 (1996) 2758. J.R. Shallenberger, J. Vac. Sci. Technol. A14 (1996) 693. S. Dreiner, M. Schu¨rmann, C. Westphal, H. Zacharias, Phys. Rev. Lett. 86 (2001) 4068. K. Chiba, S. Nakamura, Appl. Surf. Sci. 253 (2006) 412. A. Barranco, J.A. Mejı´as, J.P. Espino´s, A. Caballero, A.R. Gonza´lez-Elipe, F.J. Yubero, J. Vac. Sci. Technol. A19 (2001) 136. M.D. Ventra, S. Evoy, J.R. Heflin (Eds.), Introduction of Nanoscale Science and Technology, Kluwer Academic Publishers, Boston, 2004. Q. Xu, I.D. Sharp, C.W. Yuan, D.O. Yi, C.Y. Liao, A.M. Glaeser, A.M. Minor, J.W. Beeman, M.C. Ridgway, P.W. Kluth, J.W. Ager III, D.C. Chrzan, E.E. Haller, Phys. Rev. Lett. 97 (2006) 155701. Y. Tu, J. Tersoff, Phys. Rev. Lett. 89 (2002) 086102.
Applied Surface Science 254 (2008) 2534–2539 www.elsevier.com/locate/apsusc
Embedded structure of silicon monoxide in SiO2 films Kiyoshi Chiba *, Yoshihito Takenaka Department of Nano Material and Bio Engineering, Tokushima Bunri University, Sanuki, Kagawa 769-2193, Japan Received 2 April 2007; accepted 27 September 2007 Available online 2 October 2007
Abstract The structure of SiOx (x = 1.94) films has been investigated using both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The SiOx films were deposited by vacuum evaporation. XPS spectra show that SiO1.94 films are composed of silicon suboxides and the SiO2 matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si3O+ ion species were found in the TOF-SIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO2, and this structure most likely exists as a predominant form of Si3O4. The existence of Si–Si structures in the SiO2 matrix will give rise to dense parts in loose glass networks. # 2007 Elsevier B.V. All rights reserved. PACS : 61.43.Fs; 68.49.Sf; 68.49.Uv; 68.55.Jk Keywords: SiOx films; Silicon suboxides; X-ray photoelectron spectroscopy; Secondary ion species; Time-of-flight secondary ion mass spectrometry
1. Introduction SiOx films are well known and extensively studied materials [1–6] that are very attractive for applications as dielectrics [7,8] and transparent gas barrier films [9,10]. In particular, additional interest in these materials has arisen out of curiosity in their structure in the nanoscale due to the non-stoichiometric silicon oxides [11–14]. Silica glass is thought to be comprised of a randomly networked-structure of Si–O bonds with four-coordination number of silicon atoms surrounded by oxygen atoms and linked to other units through Si–O–Si bridges [1,6]. Although the ratio of silicon to oxygen atoms is 2.0 for the mostenergetically stable, that is, fully-oxidized and stoichiometric silicon oxide, thin film compositions with ratios less than 2.0 are frequently used for applications in electronics [7] and optics [8]. These have the composition SiOx [1–6], wherein x is in the range 0 < x < 2. Due to excess silicon atoms producing optical absorption in the visible range, compositions with x close to 2.0 are preferable in order to attain transparency. For instance, thin films with x between 1.9 and 2.0 have been used as transparent
* Corresponding author. Tel.: +81 87 894 5111; fax: +81 87 894 4201. E-mail address: [email protected] (K. Chiba). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.089
gas barrier films [9,10]. In the Si–SiO2 phase diagram, the intermediate oxidation states of silicon cannot be identified; it is not clear that silicon monoxide exists in the phase diagram [1,15,16]. There is also significant curiosity and interest in the structure of SiOx films, in particular, transparent SiOx films in the nanoscale. Secondary ion species in time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra could play a powerful role in providing information on solid structures in the nanoscale [17,18]. SiO1.94 films were investigated in this study using TOF-SIMS spectra in association with peak analysis in X-ray photoelectron spectroscopy (XPS) spectra [17,19].
2. Experimental Two types of samples were used in this study: 25 nm-thick SiO2 films and 94 nm-thick SiOx films. 25 nm-thick thermallyoxidized as well as plasma-enhanced CVD-prepared SiO2 films on single-crystal n-type (1 0 0) silicon wafers (resistivity of 4–6 V cm) were received from Tohoku Semiconductor Co. SiOx films with 94-nm thickness were deposited onto poly(ethylene terephthalate) (PET) films at room temperature using electron beam evaporation of a mixture of silicon and silicon dioxide. The vacuum pressure was 5.0 10 3 Pa. The atomic
K. Chiba, Y. Takenaka / Applied Surface Science 254 (2008) 2534–2539
composition x of the films was determined by XPS and was shown to be 1.94 0.03. XPS measurements were performed using an ULVAC-PHI Quantera SXM spectrometer with an Al Ka X-ray source.
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Sputter etching was accomplished using either Ar+ ions at 500 Vor C60+ ions at 10 kV rastered over a 2 mm 2 mm area. This corresponds to an etching rate of about 0.02 nm/s for the SiO2 films. For the C60+ ion source, a differentially pumped C60
Fig. 1. XPS spectra from SiO2 films on Si: (a) as-grown surface of a thermally-oxidized film; (b) 2-nm depth from the surface for the thermally-oxidized film sputteretched using 500 V Ar+ ions; (c) 2-nm depth from the surface for the thermally-oxidized film sputter-etched using 10 kV C60+ ions; (d) as-grown surface of a CVD oxide film; (e) 2-nm depth from the surface for the CVD oxide film sputter-etched using 500 VAr+ ions; and (f) 2-nm depth from the surface for the CVD oxide film sputter-etched using 10 kV C60+ ions.
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ion gun with an integral Wien filter was used. Both thermallyoxidized and CVD-prepared SiO2 films with 25-nm thickness were investigated to examine the spectra of SiO2 films which depend on the preparation conditions as a reference for the SiOx spectra. In addition, two types of sputter etching using 500 V Ar+ and 10 kV C60+ ions were performed to investigate the influence of beam-induced damage; the damage can cause a reduction in the oxidation states of the films which can alter the XPS spectra. All spectra were analyzed after subtracting an integrated background [17,20]. TOF-SIMS measurements were performed using an ULVAC-PHI Trift III TOF-SIMS spectrometer. Sputter etching was accomplished by Ar+ ions at 1 kVand 200 nA rastered over
a 1000 mm 1000 mm area. This corresponds to an etching rate of approximately 0.04 nm/s for the SiO2 films. TOF-SIMS analyses were performed with positive polarity using a pulsed Au3+ primary ion beam operating at 22 kV and 2.0 nA. A bias voltage of +3 kV was applied to the sample holder in this instrument for measurements of positive secondary ions. The beam was rastered over a 100 mm 100 mm surface area. Secondary ions in the mass range from 0 to 1860 atomic mass units (amu) were collected for 300 s. Charge compensation using an electron flood gun was required. The working pressure was in the 10 7 Pa range. Identification of the peaks of the secondary ion species was carried out using the masses of the peaks with an accuracy of within 0.01 amu.
Fig. 2. XPS spectra from SiO1.94 films on a PET film: (a) as-deposited surface; (b) 2-nm and 10-nm depth surfaces sputter-etched using 500 VAr+ ions; (c) 2-nm and 10-nm depth surfaces sputter-etched using 10 kV C60+ ions; and (d) enlarged spectra of the 10-nm depth surface etched using 10 kV C60+ ions. The spectrum is decomposed into two spectra. Peak positions are marked by arrows. Binding energies (EB) of Si 2p1/2 and Si 2p3/2 in silicon are also shown by arrows.
K. Chiba, Y. Takenaka / Applied Surface Science 254 (2008) 2534–2539
MM2 calculations of the SiOx structure were carried out using the program package from Molecular Design Limited. 3. Results and discussion XPS analyses were carried out on the SiO2 and SiO1.94 films. Fig. 1 shows the Si 2p spectra in the SiO2 films for as-grown (before sputtering, Fig. 1a, d) and approximately 2 nm depth from the surfaces (after sputtering, Fig. 1b, c, e, f) of the films. All spectra appear with symmetric peak profiles, corresponding to Si 2p core spectra of silicon atoms surrounded by four oxygen atoms with a binding energy of 103.4 eV [17]. Small differences can be observed in the spectra between the thermally-oxidized and CVD-prepared SiO2 films. A slight increase in the full-width half-maximum (FWHM) of the spectra by about 10–15% can be seen after both Ar+ and C60+ sputtering, which is attributed to lattice damage induced by ion impact on the surfaces. It should be noted that the symmetry of the peak profiles is maintained. These peaks can be fitted with a Lorentzian band modified by the Gaussian shape [17]; they are best fitted by 88% Gaussian–12% Lorentzian bands. XPS analysis was carried out for the SiO1.94 films. Fig. 2 shows the
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Si 2p spectra in the SiO1.94 films for the as-deposited surfaces (before sputtering, Fig. 2a) and at approximately 2 nm and 10 nm depths from the surfaces (after sputtering with Ar+ and C60+ shown in Fig. 2b and c, respectively). Asymmetric features, with a tail on the low binding energy side, appeared in the spectra. A slight increase in the FWHM can also be noticed after Ar+ and C60+ sputtering, however, the increase was smaller with C60+ than with Ar+. The enlarged spectrum for approximately 10 nm depth from the surface after sputtering with C60+ is shown in Fig. 2d. This asymmetric Si 2p peak can be decomposed into two spectra using 88% Gaussian–12% Lorentzian bands; it consists of a major peak with a binding energy (EB) of 103.4 eVand a minor peak with a binding energy of 102.2 eV. The ratio of peak areas of the minor peak to the major peak was approximately 0.1. The peak with the binding energy of 103.4 eVand FWHM of 1.9 eV corresponds to silicon atoms surrounded by four oxygen atoms [17], due to the SiO2 matrix. The minor peak with the binding energy of 102.2 eV and FWHM of 2.86 eV can be attributed to the silicon suboxide. Nevertheless, the peak of the suboxide can appear as a single spectrum peak, and the broad FWHM suggests the influence of multiple components
Fig. 3. Positive ion TOF-SIMS spectra from SiO1.94 films on a PET film and SiO2 films on Si. (a) Spectra from SiO1.94 films in the mass range of 0–200 amu. Sputter etching is performed with 1 kVAr+ ions to approximately 10-nm depth from the surface. The inset shows the spectra using an intensity scale enlarged by a factor of 20 in the mass range of 90–200 amu. (b) Spectra from SiO1.94 films with an intensity scale enlarged by a factor of 120. Possible fragment structures are shown. Note that the ratio of the intensities of the ion species marked by an asterisk is used for the analysis of the film structure. (c) Spectra from thermally-oxidized SiO2 films as a reference. Possible fragment structures are shown.
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attributable to silicon atoms surrounded by different numbers of oxygen atoms. It has been identified that silicon atoms can have up to five different valence states: Si0+, Si1+, Si2+, Si3+, and Si4+, where these valence states correspond to silicon atoms in the matrix surrounded by 0, 1, 2, 3, and 4 oxygen atoms, respectively [21]. A number of authors [21–25] have reported the chemical shift of Si 2p with these valence states by analyses of Si-very thin (<3-nm thick) SiO2 film interfaces and CVDprepared SiOx (0 < x < 0.8) using both synchrotron radiation and XPS. The values of the chemical shift are mostly obtained as 0 eV, 0.9–1.0 eV, 1.7–1.85 eV, 2.5–2.6 eV, and 3.5–3.7 eV for Si0+, Si1+, Si2+, Si3+, and Si4+, respectively. The variations in these values are small. Taking the values of the chemical shift reported by Shallenberger [24] into consideration and their similarity to the present work, for thick SiOx films using XPS, the binding energies correspond to 102.4 eV and 101.7 eV for Si3+and Si2+ relative to 103.4 eV for Si4+ in this study, respectively. This indicates that the suboxide peak with the binding energy of 102.2 eV is located at a position between Si3+and Si2+, and close to Si3+. It is also to be noted that atomic photo-ionization cross sections for XPS and the escape depths multiplied by the density of Si atoms in silicon oxides [22] are nearly the same for Si3+and Si2+. These data suggest that silicon atoms in the suboxide species in the SiO1.94 films are predominantly surrounded by two and three oxygen atoms, and exist with nearly twice the number of Si3+as Si2+; this can be deduced from the decomposed curves from the peak with the binding energy of 102.2 eV shown with dotted lines in Fig. 2d. Further addition of the Si1+ element does not significantly improve the curve fitting of the spectra. This corresponds to good agreement with a model for SiO1.5 described by Philipp [13], suggesting a mixture of Si–Si and Si–O bonds on an atomic scale in the structures. Si0+, that is, elementary silicon attributable to silicon clusters was found to a negligible extent in the SiO1.94 films. Fig. 3 shows TOF-SIMS spectra for the SiO1.94 films on PET films and thermally-oxidized SiO2 films on Si at a depth of approximately 10 nm after sputtering with 1 kV Ar+ ions. For the SiO1.94 films, the spectrum in the mass range from 0 to 200 is shown in Fig. 3a. Strong Si+ and Si2+ peaks can be observed, whereas the intensities of ions rapidly decreased with increasing mass number. The expansion of the intensity axis of the spectra in the mass range from 90 to 200 is shown in the inset. Notably, a strong peak around 100 amu appeared in this spectrum. The 96 amu peak arises from Ti2+ due to the boat material being made of Ti in the evaporation. The ion species in Fig. 3b were determined based on data analyzed for thermally grown SiO2 on Si and the interface [26]. This indicates that a large number of Si3O+ species, likely attributable to Si–Si–Si– O+, can be found in the film in addition to the species originating from the SiO2 matrix. For the SiO2 films, the spectrum in the mass range from 90 to 160 is shown in Fig. 3c. The intensity of Si3O+ is seen to be negligibly small. Relative intensities of the Si3O+, Si3O2+, and Si3O3+ peaks normalized to the intensity of the Si3O4+ peak were calculated, and the results are listed in Table 1. The Si3O4+ peak identified is likely attributable to Si–O–Si(O)–O–Si(O)+ due to the SiO2 matrix
Fig. 4. Molecular modelling of the structure of SiO1.94 films. Silicon monoxide is embedded in SiO2 matrix as a predominant form of Si3O4. This structure was calculated and energetically minimized using the MM2 package.
[26]. For the SiO1.94 films, the ratio of the intensity of Si3O+ to that of Si3O4+ is 20.3, which is much larger than that observed for the SiO2 films. The data obtained with XPS and TOF-SIMS suggest that silicon atoms are surrounded by both two and three oxygen atoms and partial structures of Si–Si–Si–O may exist in the suboxides in the SiO1.94 films. We can thus postulate the chemical formula of the suboxide species in the SiO1.94 films to be SiO1.5–(Si–O)n–SiO1.5. The silicon monoxide structure (Si– O)n is linked to the SiO2 matrix via oxygen atom bridges. Considering the intensity factor in the spectra of the silicon suboxides in the XPS data, n seems most likely to be 1.0, but is distributed as greater or smaller numbers in the structure corresponding to peak broadening. A simple calculation suggests that SiO1.94 corresponds to approximately 9 at.% of Si3O4 embedded in the SiO2 matrix. It is important to note that this value is consistent with the value of approximately 9.2% of silicon suboxides in the SiO2 matrix obtained by XPS analysis, as shown in Fig. 2d. Fig. 4 illustrates a molecular model of the structure with the nearest neighbours of the matrix structures. This is calculated Table 1 Comparison between relative intensities of Si3On+ (n = 1, 2, 3, 4) ions emitted from SiO2 film (10-nm depth surface) and SiO1.94 film (10-nm depth surface) Source
SiO2 SiO1.94 a b
Ion species 28
Si3O+ (100)a
28
Si3O2+ (116)
28
Si3O3+ (132)
28
–b 20.3
0.20 2.9
0.28 1.3
1.0 1.0
The values in parentheses represent mass number m/z. Negligibly small.
Si3O4+ (148)
K. Chiba, Y. Takenaka / Applied Surface Science 254 (2008) 2534–2539
and energetically minimized using the MM2 force field. This model indicates that the structure of the SiO1.94 films consists of silicon monoxide embedded in SiO2, and most likely exists as a predominant form of Si3O4. The existence of Si–Si structures in the SiO2 matrix will give rise to dense parts in loose glass networks. This model may provide a more preferable explanation for the data obtained than the model wherein various amounts of Si1+ are postulated to exist in the films [27].
References
4. Conclusions
[9] [10]
The structure of SiOx (x = 1.94) films has been investigated using both X-ray photoemission spectroscopy and time-offlight secondary ion mass spectrometry. The SiOx films were deposited by vacuum evaporation. XPS spectra show that SiO1.94 films are composed of silicon suboxides and the SiO2 matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si3O+ ion species were found in the TOFSIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO2, and this structure most likely exists as a predominant form of Si3O4. The embedded structure of silicon monoxide in SiO2 films is presented in this study. This will expand the scope of nanocrystals, for instance, of Si and Ge embedded in a matrix that exhibits unique and attractive behaviours and properties [28,29]. Furthermore, the existence of silicon monoxide in equilibrium can be inferred, not only for a single atomic layer in the Si–SiO2 interface [30], but also for thick materials. Acknowledgements We thank Tohoku Semiconductor Co. and Oike & Co. for supplying silicon dioxide films on silicon wafers and PET/SiOx films, respectively. We are indebted to ULVAC-PHI, Inc. for measurements of XPS spectra, and to Mr. M. Kawamura of Tokushima Bunri University for his help in TOF-SIMS measurements.
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