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Low-energy focused Si ion beam deposition under oxygen atmosphere
Nuclear Instruments and Methods in Physics Research B 148 (1999) 42±46
Low-energy focused Si ion beam deposition under oxygen atmosphere J. Yanagisawa
a,*
, Y. Wang b, T. Hada a, K. Murase
b,c
, K. Gamo
a,c
a
Department of Physical Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan b Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0031, Japan c Research Center for Materials Science at Extreme Conditions, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
Abstract Direct deposition of Si by a 200 eV Si2 focused ion beam (FIB) on an Au-evaporated substrate was performed in oxygen atmosphere under a pressure of 10ÿ5 Torr. From Auger electron spectroscopy (AES) measurement, it was found that a large amount of oxygen was incorporated in the deposited material, but clear chemical shift in Si AES signal was not observed. From Raman scattering measurement, two broad signals, as well as large amorphous-Si (a-Si) signals, were observed at about 1000 and 1600 cmÿ1 . A small and broad signal was also observed at about 1550 cmÿ1 , which indicates the existence of oxygen molecules inside the deposited material. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 68.55.Nq; 78.30.Am; 81.15.Jj Keywords: Focused ion beam (FIB); Deposition; Silicon; Oxygen; Silicon oxide; Raman spectroscopy; Auger electron spectroscopy
1. Introduction Formation of silicon and silicon dioxide ®lms on a laterally selected area is a very important process in the Si-based technology. This can be performed by a direct deposition using low-energy focused ion beams (FIBs). In this process, ®lm purity or inclusion of residual gas molecules such
* Corresponding author. Tel.: +81 6 6850 6302; fax: +81 6 6850 6341; e-mail: [email protected].
as oxygen and carbon depends on background pressure and beam scanning conditions. In our previous study [1], it was found that pure silicon ®lms were deposited when Si2 beam was continuously irradiated in a dry vacuum ambient at a pressure of 10ÿ8 Torr, while silicon oxide ®lms were formed when the beam was scanned repeatedly in oxygen atmosphere of 10ÿ6 ±10ÿ5 Torr. The amount of the incorporated oxygen was increased by increasing the scanning period of the FIB or the oxygen pressure. It was also found that chemical nature of the deposited Si ®lms depended on the
0168-583X/98/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 7 9 - 4
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
43
beam scanning condition under oxygen atmosphere. However, bonding structure between silicon and oxygen atoms (or molecules) was not clari®ed, especially in a case that the oxidation of the deposited material was not complete. Recently, Skuja and G uttler [2] detected interstitial oxygen molecules in SiO2 glass. Murakami et al. [3] observed hydrogen molecules in crystalline silicon treated with atomic hydrogen. In the present study, oxygen-incorporated silicon materials were fabricated using a direct deposition of low-energy Si2 FIB under an oxygenrich ambient. Oxygen nature in the deposited materials was investigated using Auger electron spectroscopy (AES) and Raman scattering spectroscopy. It was found that the chemical behavior of oxygen depends on the Si beam energy and the oxygen to silicon ¯ux ratio.
focused with a lens, dispersed with a triple grating monochrometer (Jobin-Yvon T64000), and detected with a CCD multi-channel detector. Wave number resolution of the present system was about 1 cmÿ1 . To avoid the detection of the Raman signal from oxygen molecules in the air near the focused point of the excitation light, the sample surface was blown by Ar gas using a small nozzle. After the Raman spectra measurement, the thickness of the deposited materials were measured using a stylus measurement (DEKTAK) before and after the AES measurement for sample #1-2 and samples #3-1 and #4-3. AES measurement was performed in energy regions between 50 and 2098 eV and 45 and 110 eV to investigate the chemical composition and the bonding nature in the deposited material, respectively, using 3 keV Ar ion sputtering.
2. Experimental
3. Results and discussion
200 eV Si2 FIB was obtained by applying a retarding potential on the target (substrate), 100 V less than the acceleration voltage. Si2 was irradiated on a 600-nm-thick-Au-evaporated Si substrate at room temperature in high vacuum (sample #4-3) or oxygen atmosphere (samples #12 and #3-1) of 1.4 10ÿ5 Torr, as described in detail elsewhere [1]. The beam was scanned repeatedly over a square region. It took about 0.12 s to scan one frame. The base area and the height of the deposited materials, as well as other deposition conditions, are summarized in Table 1. Stokes Raman spectra were measured at room temperature with the 514.5 nm line of an Ar ion laser as an excitation source. The incident light was focused to about 50 lm diameter with an energy of 50 mW. The Raman scattering light was
Fig. 1 shows the signal ratio of oxygen to silicon (KL2;3 L2;3 ) in the dierential AES spectra for samples #1-2 and #4-3 as a function of sputtering time, and those for Si wafer and thermally oxidized Si wafer (referred to ``thermal SiO2 '') are also shown for reference. The O/Si ratio of sample #1-2 was comparable to that of the thermal SiO2 . These oxygen should be incorporated during the deposition because only samples deposited in high oxygen pressure showed high O/Si ratio. The O/Si ratio of sample #4-3 was much smaller than that of sample #1-2, but was larger than that of the Si bulk wafer which was measured as a reference. To investigate the chemical state of the incorporated oxygen through the Si L2;3 VV signal, AES measurement was performed in detail in energy ranging from 45 to 110 eV. The result is shown in
Table 1 Parameters for the present 200 eV Si2 FIB deposition Sample #3-1 #1-2 #4-3
FIB current (nA) 3.2 1.5 1.0
Background pressure (Torr) 1.4 10 1.4 10 < 10ÿ7
ÿ5 ÿ5
Area of deposited material (lm2 )
Maximum height of deposited material
90 170 100 100 150 150
P 1lm 350 nm P 100 nm
44
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
Fig. 1. Dierential AES signal ratio of oxygen to silicon (KL2;3 L2;3 ) as a function of 3 keV Ar ion sputtering time. Results for pure Si and thermally oxidized Si wafers were also shown for reference.
Fig. 2. Pure Si wafer showed very sharp signal at an energy about 92 eV. In the silicon dioxide ®lm (thermal SiO2 ), this signal shifted to lower energy of around 78 eV. This is reported as the chemical shifts in AES spectra [4] from elemental Si to SiO2 . In sample #4-3, only one sharp peak was observed at about 92 eV and the shape of the spectrum was almost the same as that of the Si wafer. This indicates that there is no oxygen bonded chemically with Si in sample #4-3. In addition to the large signal at about 92 eV, very broad signal was observed around 80 eV in sample #1-2 and the shape of this signal was different from that of the 78-eV signal observed for thermal SiO2 . This indicates that the bonding nature among silicon and oxygen atoms in sample #1-2 is dierent from those of SiO2 and there are many intermediate bonding structures between silicon atoms and oxygen atoms (or molecules). In our previous work [1], Si deposited at an energy of 100 eV showed clear signal around the energy of 80 eV compared with sample #1-2, although the O/Si ratio was almost the same with sample #1-2. The dierence may be caused by a dierence in damage induced during the FIB irradiation or a time to
Fig. 2. Detailed AES spectra in the energy region from 45 to 110 eV for samples shown in Fig. 1 after 30 or 40 s sputtering.
dissociate physisorbed oxygen molecules may play an important role in the formation of Si±O chemical bonds in the present deposition process. To investigate the oxygen states in the deposited materials more clearly, Stokes Raman shifts were measured up to about 1920 cmÿ1 . Fig. 3 shows the Raman spectra for deposited materials listed in Table 1, as well as that for SiO2 glass for reference. Each spectrum was normalized at the maximum peak position. The present Raman spectra of SiO2 glass at the wave number smaller than 1300 cmÿ1 was very similar to the reported spectra [5]. In the spectra of sample #4-3, two strong and two weak signals were observed at the wave number of about 156, 311, 378, and 478 cmÿ1 . In addition to these signals, another two signals were
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
observed at 622 and 967 cmÿ1 , which were almost twice the value of 311 and 478 cmÿ1 . These peaks and the shape of this spectra are very similar to that of pure amorphous-Si (a-Si) [6], and the origin of these signals can be identi®ed to TA, LA, LO, and TO one-phonon processes, and 2LA and 2TO two-phonon processes, respectively. Because there
Fig. 3. Stokes Raman spectra for the deposited materials shown in Table 1. That for SiO2 glass was also shown for reference. Each spectrum was normalized at the maximum peak position.
45
was no other signals in the Raman spectra, sample #4-3 can be considered to be pure a-Si and it is consistent with the AES result mentioned above. In the spectrum of sample #1-2, two strong signals at about 156 and 478 cmÿ1 observed in sample #4-3 were reduced and broadened. On the other hand, very broad signals were observed near 1000 and 1600 cmÿ1 . These characteristics were dierent from that of SiO2 glass. In addition, a small signal was observed at about 1550 cmÿ1 . This indicates the existence of oxygen molecules. Sample #3-1 which was deposited under the same oxygen pressure with sample #1-2 showed similar Raman spectra to each other. To clarify the characteristics of the signal observed at about 1550 cmÿ1 in detail, Raman spectra between 1300 and 1900 cmÿ1 were compared with those of crystal-Si (c-Si) wafer, thermal SiO2 , and air (without any samples at the focused position of the exciting light), for reference. The result is shown in Fig. 4. These spectra are not normalized in this ®gure. In the air sample, very sharp signal originated from oxygen molecules in a gas phase was observed at 1555 cmÿ1 . This signal was also observed at the same wave number in both thermal SiO2 and SiO2 glass, as indicated as B in the ®gure. However, no such signal was observed in pure Si (both a-Si of sample #4-3 and c-Si wafer). Therefore, this signal originated from the oxygen molecules inside the SiO2 materials is not from the oxygen molecules in the atmosphere near the sample surface. In addition, the wave number at the peak position was not changed and the shape of the signal was rather sharp both in thermal SiO2 and SiO2 glass, indicating that oxygen molecules can exist without forming any chemical bond with other atoms in these materials. In fact, the existence of the interstitial oxygen molecules in SiO2 was reported by Skuja and G uttler [2] using photoluminescence, although it was not observed in the Raman spectrum. In the spectra of samples #1-2 and #3-1, a broad signal was observed at about 1550 cmÿ1 , indicated as A in the ®gure. The peak position was a little shifted to lower wave number and the width of the signal was broadened, in contrast to the SiO2 materials. From the AES result, it is possible that sample #1-2 has various kinds of bonding
46
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
4. Conclusion 200 eV Si2 FIB was irradiated on Au-evaporated substrate at room temperature under oxygen atmosphere at a pressure of 10ÿ5 Torr. From AES measurement, it was found that a large amount of oxygen was incorporated inside the deposited material. Instead of the chemical shift of the Si signal at about 78 eV which indicates the Si±O bonds in silicon dioxides, very broad signal was observed near the energy of 80 eV. From Raman spectroscopy measurement, two broad signals were observed at about 1000 and 1600 cmÿ1 in the deposited materials, as well as signals related to a-Si. A small and broad signal was also observed at about 1550 cmÿ1 , which is very near to the wave number of the Raman signal of oxygen molecules in a gas phase. Considering the AES results, oxygen can exist in a molecular form bonding with silicon atoms. This is in contrast with our previous observation where Si deposited at an energy of 100 eV showed a chemical shift of the Si signal. To understand the dierence, it may be necessary to investigate the dependence on beam energy and/or oxygen to Si incoming ¯ux ratio in detail. Acknowledgements
Fig. 4. Raman spectra in higher wave-number region obtained for each material shown in the ®gure. The position A and B indicate the observed signal position concerned to oxygen molecules.
structure. One possible assignment is oxygen molecules inside samples #1-2 and #3-1. Because of the short scanning period of the Si2 FIB irradiation during the deposition, large amount of oxygen can be incorporated inside the deposited material as a molecular form and partially bonded with silicon without dissociation, resulted in the peak shift and broadening.
The authors would like to thank T. Goto for a help of sample preparation and K. Mino for AES operation. References [1] J. Yanagisawa, H. Nakayama, O. Matsuda, K. Murase, K. Gamo, Nucl. Instr. and Meth. B 127/128 (1997) 893. [2] L. Skuja, B. G uttler, Phys. Rev. Lett. 77 (1996) 2093. [3] K. Murakami, N. Fukata, S. Sasaki, K. Ishioka, M. Kitajima, S. Fujimura, J. Kikuchi, H. Haneda, Phys. Rev. Lett. 77 (1996) 3161. [4] P.H. Holloway, Surface Sci. 54 (1976) 506. [5] B.O. Mysen, D. Virgo, Phys. Chem. Minerals 12 (1985) 77. [6] D. Bermejo, M. Cardona, J. Non-Cryst. Solids 32 (1979) 405.
Low-energy focused Si ion beam deposition under oxygen atmosphere J. Yanagisawa
a,*
, Y. Wang b, T. Hada a, K. Murase
b,c
, K. Gamo
a,c
a
Department of Physical Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan b Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0031, Japan c Research Center for Materials Science at Extreme Conditions, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
Abstract Direct deposition of Si by a 200 eV Si2 focused ion beam (FIB) on an Au-evaporated substrate was performed in oxygen atmosphere under a pressure of 10ÿ5 Torr. From Auger electron spectroscopy (AES) measurement, it was found that a large amount of oxygen was incorporated in the deposited material, but clear chemical shift in Si AES signal was not observed. From Raman scattering measurement, two broad signals, as well as large amorphous-Si (a-Si) signals, were observed at about 1000 and 1600 cmÿ1 . A small and broad signal was also observed at about 1550 cmÿ1 , which indicates the existence of oxygen molecules inside the deposited material. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 68.55.Nq; 78.30.Am; 81.15.Jj Keywords: Focused ion beam (FIB); Deposition; Silicon; Oxygen; Silicon oxide; Raman spectroscopy; Auger electron spectroscopy
1. Introduction Formation of silicon and silicon dioxide ®lms on a laterally selected area is a very important process in the Si-based technology. This can be performed by a direct deposition using low-energy focused ion beams (FIBs). In this process, ®lm purity or inclusion of residual gas molecules such
* Corresponding author. Tel.: +81 6 6850 6302; fax: +81 6 6850 6341; e-mail: [email protected].
as oxygen and carbon depends on background pressure and beam scanning conditions. In our previous study [1], it was found that pure silicon ®lms were deposited when Si2 beam was continuously irradiated in a dry vacuum ambient at a pressure of 10ÿ8 Torr, while silicon oxide ®lms were formed when the beam was scanned repeatedly in oxygen atmosphere of 10ÿ6 ±10ÿ5 Torr. The amount of the incorporated oxygen was increased by increasing the scanning period of the FIB or the oxygen pressure. It was also found that chemical nature of the deposited Si ®lms depended on the
0168-583X/98/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 7 9 - 4
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
43
beam scanning condition under oxygen atmosphere. However, bonding structure between silicon and oxygen atoms (or molecules) was not clari®ed, especially in a case that the oxidation of the deposited material was not complete. Recently, Skuja and G uttler [2] detected interstitial oxygen molecules in SiO2 glass. Murakami et al. [3] observed hydrogen molecules in crystalline silicon treated with atomic hydrogen. In the present study, oxygen-incorporated silicon materials were fabricated using a direct deposition of low-energy Si2 FIB under an oxygenrich ambient. Oxygen nature in the deposited materials was investigated using Auger electron spectroscopy (AES) and Raman scattering spectroscopy. It was found that the chemical behavior of oxygen depends on the Si beam energy and the oxygen to silicon ¯ux ratio.
focused with a lens, dispersed with a triple grating monochrometer (Jobin-Yvon T64000), and detected with a CCD multi-channel detector. Wave number resolution of the present system was about 1 cmÿ1 . To avoid the detection of the Raman signal from oxygen molecules in the air near the focused point of the excitation light, the sample surface was blown by Ar gas using a small nozzle. After the Raman spectra measurement, the thickness of the deposited materials were measured using a stylus measurement (DEKTAK) before and after the AES measurement for sample #1-2 and samples #3-1 and #4-3. AES measurement was performed in energy regions between 50 and 2098 eV and 45 and 110 eV to investigate the chemical composition and the bonding nature in the deposited material, respectively, using 3 keV Ar ion sputtering.
2. Experimental
3. Results and discussion
200 eV Si2 FIB was obtained by applying a retarding potential on the target (substrate), 100 V less than the acceleration voltage. Si2 was irradiated on a 600-nm-thick-Au-evaporated Si substrate at room temperature in high vacuum (sample #4-3) or oxygen atmosphere (samples #12 and #3-1) of 1.4 10ÿ5 Torr, as described in detail elsewhere [1]. The beam was scanned repeatedly over a square region. It took about 0.12 s to scan one frame. The base area and the height of the deposited materials, as well as other deposition conditions, are summarized in Table 1. Stokes Raman spectra were measured at room temperature with the 514.5 nm line of an Ar ion laser as an excitation source. The incident light was focused to about 50 lm diameter with an energy of 50 mW. The Raman scattering light was
Fig. 1 shows the signal ratio of oxygen to silicon (KL2;3 L2;3 ) in the dierential AES spectra for samples #1-2 and #4-3 as a function of sputtering time, and those for Si wafer and thermally oxidized Si wafer (referred to ``thermal SiO2 '') are also shown for reference. The O/Si ratio of sample #1-2 was comparable to that of the thermal SiO2 . These oxygen should be incorporated during the deposition because only samples deposited in high oxygen pressure showed high O/Si ratio. The O/Si ratio of sample #4-3 was much smaller than that of sample #1-2, but was larger than that of the Si bulk wafer which was measured as a reference. To investigate the chemical state of the incorporated oxygen through the Si L2;3 VV signal, AES measurement was performed in detail in energy ranging from 45 to 110 eV. The result is shown in
Table 1 Parameters for the present 200 eV Si2 FIB deposition Sample #3-1 #1-2 #4-3
FIB current (nA) 3.2 1.5 1.0
Background pressure (Torr) 1.4 10 1.4 10 < 10ÿ7
ÿ5 ÿ5
Area of deposited material (lm2 )
Maximum height of deposited material
90 170 100 100 150 150
P 1lm 350 nm P 100 nm
44
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
Fig. 1. Dierential AES signal ratio of oxygen to silicon (KL2;3 L2;3 ) as a function of 3 keV Ar ion sputtering time. Results for pure Si and thermally oxidized Si wafers were also shown for reference.
Fig. 2. Pure Si wafer showed very sharp signal at an energy about 92 eV. In the silicon dioxide ®lm (thermal SiO2 ), this signal shifted to lower energy of around 78 eV. This is reported as the chemical shifts in AES spectra [4] from elemental Si to SiO2 . In sample #4-3, only one sharp peak was observed at about 92 eV and the shape of the spectrum was almost the same as that of the Si wafer. This indicates that there is no oxygen bonded chemically with Si in sample #4-3. In addition to the large signal at about 92 eV, very broad signal was observed around 80 eV in sample #1-2 and the shape of this signal was different from that of the 78-eV signal observed for thermal SiO2 . This indicates that the bonding nature among silicon and oxygen atoms in sample #1-2 is dierent from those of SiO2 and there are many intermediate bonding structures between silicon atoms and oxygen atoms (or molecules). In our previous work [1], Si deposited at an energy of 100 eV showed clear signal around the energy of 80 eV compared with sample #1-2, although the O/Si ratio was almost the same with sample #1-2. The dierence may be caused by a dierence in damage induced during the FIB irradiation or a time to
Fig. 2. Detailed AES spectra in the energy region from 45 to 110 eV for samples shown in Fig. 1 after 30 or 40 s sputtering.
dissociate physisorbed oxygen molecules may play an important role in the formation of Si±O chemical bonds in the present deposition process. To investigate the oxygen states in the deposited materials more clearly, Stokes Raman shifts were measured up to about 1920 cmÿ1 . Fig. 3 shows the Raman spectra for deposited materials listed in Table 1, as well as that for SiO2 glass for reference. Each spectrum was normalized at the maximum peak position. The present Raman spectra of SiO2 glass at the wave number smaller than 1300 cmÿ1 was very similar to the reported spectra [5]. In the spectra of sample #4-3, two strong and two weak signals were observed at the wave number of about 156, 311, 378, and 478 cmÿ1 . In addition to these signals, another two signals were
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
observed at 622 and 967 cmÿ1 , which were almost twice the value of 311 and 478 cmÿ1 . These peaks and the shape of this spectra are very similar to that of pure amorphous-Si (a-Si) [6], and the origin of these signals can be identi®ed to TA, LA, LO, and TO one-phonon processes, and 2LA and 2TO two-phonon processes, respectively. Because there
Fig. 3. Stokes Raman spectra for the deposited materials shown in Table 1. That for SiO2 glass was also shown for reference. Each spectrum was normalized at the maximum peak position.
45
was no other signals in the Raman spectra, sample #4-3 can be considered to be pure a-Si and it is consistent with the AES result mentioned above. In the spectrum of sample #1-2, two strong signals at about 156 and 478 cmÿ1 observed in sample #4-3 were reduced and broadened. On the other hand, very broad signals were observed near 1000 and 1600 cmÿ1 . These characteristics were dierent from that of SiO2 glass. In addition, a small signal was observed at about 1550 cmÿ1 . This indicates the existence of oxygen molecules. Sample #3-1 which was deposited under the same oxygen pressure with sample #1-2 showed similar Raman spectra to each other. To clarify the characteristics of the signal observed at about 1550 cmÿ1 in detail, Raman spectra between 1300 and 1900 cmÿ1 were compared with those of crystal-Si (c-Si) wafer, thermal SiO2 , and air (without any samples at the focused position of the exciting light), for reference. The result is shown in Fig. 4. These spectra are not normalized in this ®gure. In the air sample, very sharp signal originated from oxygen molecules in a gas phase was observed at 1555 cmÿ1 . This signal was also observed at the same wave number in both thermal SiO2 and SiO2 glass, as indicated as B in the ®gure. However, no such signal was observed in pure Si (both a-Si of sample #4-3 and c-Si wafer). Therefore, this signal originated from the oxygen molecules inside the SiO2 materials is not from the oxygen molecules in the atmosphere near the sample surface. In addition, the wave number at the peak position was not changed and the shape of the signal was rather sharp both in thermal SiO2 and SiO2 glass, indicating that oxygen molecules can exist without forming any chemical bond with other atoms in these materials. In fact, the existence of the interstitial oxygen molecules in SiO2 was reported by Skuja and G uttler [2] using photoluminescence, although it was not observed in the Raman spectrum. In the spectra of samples #1-2 and #3-1, a broad signal was observed at about 1550 cmÿ1 , indicated as A in the ®gure. The peak position was a little shifted to lower wave number and the width of the signal was broadened, in contrast to the SiO2 materials. From the AES result, it is possible that sample #1-2 has various kinds of bonding
46
J. Yanagisawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 42±46
4. Conclusion 200 eV Si2 FIB was irradiated on Au-evaporated substrate at room temperature under oxygen atmosphere at a pressure of 10ÿ5 Torr. From AES measurement, it was found that a large amount of oxygen was incorporated inside the deposited material. Instead of the chemical shift of the Si signal at about 78 eV which indicates the Si±O bonds in silicon dioxides, very broad signal was observed near the energy of 80 eV. From Raman spectroscopy measurement, two broad signals were observed at about 1000 and 1600 cmÿ1 in the deposited materials, as well as signals related to a-Si. A small and broad signal was also observed at about 1550 cmÿ1 , which is very near to the wave number of the Raman signal of oxygen molecules in a gas phase. Considering the AES results, oxygen can exist in a molecular form bonding with silicon atoms. This is in contrast with our previous observation where Si deposited at an energy of 100 eV showed a chemical shift of the Si signal. To understand the dierence, it may be necessary to investigate the dependence on beam energy and/or oxygen to Si incoming ¯ux ratio in detail. Acknowledgements
Fig. 4. Raman spectra in higher wave-number region obtained for each material shown in the ®gure. The position A and B indicate the observed signal position concerned to oxygen molecules.
structure. One possible assignment is oxygen molecules inside samples #1-2 and #3-1. Because of the short scanning period of the Si2 FIB irradiation during the deposition, large amount of oxygen can be incorporated inside the deposited material as a molecular form and partially bonded with silicon without dissociation, resulted in the peak shift and broadening.
The authors would like to thank T. Goto for a help of sample preparation and K. Mino for AES operation. References [1] J. Yanagisawa, H. Nakayama, O. Matsuda, K. Murase, K. Gamo, Nucl. Instr. and Meth. B 127/128 (1997) 893. [2] L. Skuja, B. G uttler, Phys. Rev. Lett. 77 (1996) 2093. [3] K. Murakami, N. Fukata, S. Sasaki, K. Ishioka, M. Kitajima, S. Fujimura, J. Kikuchi, H. Haneda, Phys. Rev. Lett. 77 (1996) 3161. [4] P.H. Holloway, Surface Sci. 54 (1976) 506. [5] B.O. Mysen, D. Virgo, Phys. Chem. Minerals 12 (1985) 77. [6] D. Bermejo, M. Cardona, J. Non-Cryst. Solids 32 (1979) 405.