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Effect of O2 pressure on the synthesis of titanium oxide film by ion beam enhanced deposition
June 2000
Materials Letters 44 Ž2000. 105–109 www.elsevier.comrlocatermatlet
Effect of O 2 pressure on the synthesis of titanium oxide film by ion beam enhanced deposition Xianghui Wang ) , Feng Zhang, Zhihong Zheng, Changrong Li, Lizhi Chen, Huimin Wang, Xianghuai Lui Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Science, 865 Changning Road, Shanghai 200050, People’s Republic of China Received 5 February 1999; received in revised form 1 December 1999; accepted 17 January 2000
Abstract A series of titanium oxide films have been synthesized on silicon wafers by ion beam enhanced deposition at different O 2 pressures. X-ray Photoelectron Spectroscopy ŽXPS., X-ray Diffraction ŽXRD., Glancing Angle Diffraction and Rutherford Backscattering Spectroscopy ŽRBS. were used to analyze the composition, structure and orientation of the films. From the experimental results, it was found that: Ž1. the titanium oxide films synthesized at different O 2 pressures exhibit a polycrystal structure with preferred orientation; Ž2. when O 2 pressure is lower than 8.4 = 10y4 Pa, the main composition of the film is TiO whose preferred orientation changed from Ž220. to Ž031. with the increase of O 2 pressure; Ž3. when O 2 pressure is higher than 8.4 = 10y4 Pa, rutile-type TiO 2 with Ž200. preferred orientation was found to be the major composition of the film. The higher the O 2 pressure is, the more stable the composition and preferred orientation become. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide film; Oxygen pressure; Ion beam enhanced deposition
1. Introduction Titanium oxide has a wide range of application. Not only can it be used as pigment and catalyst, but it can also be used in capacitor fabrication and sensor technology. Now it is considered for introduction into biomaterial area because of its good hemocompatibility with the human body. Several methods have been used to synthesize titanium oxide film including metal-organic chemical vapor deposition, filtered arc deposition, DC mag)
Corresponding author. E-mail address: sarah – [email protected] ŽX. Wang..
netron reactive sputtering, cold plasma torch, ion beam enhanced deposition ŽIBED., and others w1–4x. Rizzo et al. w5x considered that reactive ion beam assistance appears to be a useful technique to fabricate optical coatings with the correct stoichiometry. Ultrathin Ž- 10 nm. titanium oxide films have been synthesized on the Mo Ž100. surface by Oh et al. w6x to be used for model catalysis studies. Rutile-type TiO 2 film with Ž100. preferred orientation, which showed a good hemocompatibility, has been successfully synthesized by Zhang et al. w7,8x, who have found that lower OrTi ratio may result in a better blood compatibility if the surface energy keeps the same. The OrTi ratio is related to the composition
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 1 0 - 0
106
X. Wang et al.r Materials Letters 44 (2000) 105–109
of the film. In order to investigate the relationship between the OrTi ratio and hemocompatibility, titanium oxide films with different compositions had to be formed. IBED is a good method to synthesize titanium oxide films because the composition and orientation of the film can be accurately controlled by process parameters. Besides, adhesion between the film and the substrate is firm and the thickness of the film can be controlled in the range of several angstrom to several microns. Bombardment ion, ion beam energy, ion beam current density, O 2 pressure, evaporation rate and substrate temperature are the main process parameters of IBED. In this paper, O 2 pressure was set up as a variable parameter. A series of titanium oxide films with different compositions and orientation were synthesized by IBED at different O 2 pressures, and the relationship between O 2 pressure and the structure of the film is discussed in detail.
2. Experimental The titanium oxide films were deposited onto optically polished silicon Ž100. substrate by electron beam evaporation of titanium at a rate of 0.4 nmrs in an oxygen atmosphere. A beam of energetic xenon ions was used to bombard the growing titanium oxide film simultaneously with a fixed energy of 40 keV. The ion beam current density was maintained at 40 mArcm2 . The incidence angle of the titanium vapor stream and xenon ion beam to the substrate was 458, respectively. The base pressure in the deposition chamber was about 8.6 = 10y5 Pa, and the deposition chamber was backfilled with high purity oxygen gas during deposition. As a variable parameter, the filled oxygen pressure was set at 7.3, 8.0, 8.1, 8.2, 8.4 8.6 and 15 = 10y4 Pa, respectively. The chemical states of titanium, oxygen, carbon and silicon on the surface and near-surface of the synthesized titanium oxide films were studied by X-ray Photoelectron Spectroscopy ŽXPS.. The spectra were curve-fitted, using a computer assisted Gaussian–Lorentzian peak model. The binding energy of the C 1s line was taken as 284.6 " 0.4 eV for calibrating the obtained spectra. The structure of the film was investigated by X-ray diffraction ŽXRD. analysis. Since all diffrac-
tion beams received by counter recording of XRD are produced only by crystal planes that parallel the specimen surface, glancing angle diffraction, which is suitable for the analysis of polycrystal with free orientation, was used for an additional analysis. The glancing angle used here was fixed at 18. Rutherford backscattering spectroscopy ŽRBS. was performed using a 2 MeV Heq ion beam at a scattering angle of 1658 to investigate the OrTi ratio of synthesized titanium oxide film.
3. Results XPS results show that there is an amount of contaminating carbon on the surface of the titanium oxide films. After 10 min of argon ion sputtering, the counts of carbon decreased sharply. Fig. 1 shows the XPS spectrum of Ti 2p from the surface of titanium oxide film prepared at different O 2 pressures. The binding energy of Ti 2P3r2 and Ti 2P1r2 is observed to be at 458.8 and 464.5 eV, respectively, for all films assigned to Ti 4q in TiO 2 , with a peak separation of 5.7 eV between these two peaks. No difference was observed for titanium oxide films prepared at different O 2 pressures because these films have been exposed to air for some time and all of them have naturally oxidized to form a pure TiO 2 layer on the surface. Fig. 2 shows the high-resolution spectrum of Ti 2P of the titanium oxide film prepared at an O 2 pressure of 8.2 = 10y4 Pa after 10 min of argon ion sputtering. The spectrum is dominated by a peak at ; 458.9 eV, which is assigned to Ti 4q in TiO 2 . There are also other two peaks located according to the deconvolution procedure at ; 456.9 and ; 455 eV, which are the Ti 3q state and Ti 2q state in Ti 2 O 3 and TiO, respectively. The peak separation between Ti 2p1r2 and Ti 2p3r2 is about 5.7 " 0.1 eV. The XPS spectra of titanium oxide films that were prepared at different O 2 pressures had similar results, except that the Ti 4qrTi 3qrTi 2q ratio varied slightly. As O 2 pressure increased, the atom percentage of TiO decreased. On the basis of XPS analysis results, XRD and glancing angle diffraction were used to analyze the structure and orientation of the films. Fig. 3 shows the XRD pattern of titanium oxide films synthesized at different O 2 pressures. When O 2 pressure is only
X. Wang et al.r Materials Letters 44 (2000) 105–109
107
Fig. 1. Ti 2p XPS spectrum on the surface of titanium oxide film.
7.3 = 10y4 Pa, the film dominated by TiO with a highly Ž220. orientation whose diffraction peak located at 2 u s 43.18. As the O 2 pressure increased, it can be seen that the Ž220. diffraction peak dropped, and a new TiO diffraction peak located at 2 u s 37.18 appears and grows, which indicates that the orientation of TiO is changing from Ž220. to Ž031.. When O 2 pressure reached 8.2 = 10y4 Pa, the diffraction peaks of rutile-type TiO 2 Ž110. and Ž200. appeared besides TiO Ž031.. The peak intensity ratio, I Ž200.rI Ž110., is about 2.65, which suggests that crystallites grow preferentially with the Ž200. plane parallel to the substrate surface, for according to the
standard rutile phase of ASTM card Ž21-1276., the I Ž200.rI Ž110. is about 0.08 for random distribution. When O 2 pressure reached over 8.6 = 10y4 Pa, the peak of TiO Ž031. disappeared. Rutile-type TiO 2 , with Ž200. preferred orientation, becomes the major phase of the film. The orientation of Ti 2 O 3 cannot be determined by XRD although its existence has been verified by XPS analysis. The main reason is that the diffraction peak of Ti 2 O 3 Ž104. overlapped with Si Ž100. at
Fig. 2. Ti 2p XPS spectrum of titanium oxide film after 10 min of argon ion sputtering.
Fig. 3. XRD patterns of titanium oxide films synthesized at different O 2 pressure.
X. Wang et al.r Materials Letters 44 (2000) 105–109
108
2 u s 338 and the other diffraction peaks of Ti 2 O 3 is too weak to be identified. Fig. 4 shows the glancing angle diffraction pattern of the titanium oxide films that were synthesized at different O 2 pressures. From this figure, it can be seen that the synthesized titanium oxide films show a polycrystal structure with TiO, Ti 2 O 3 and TiO 2 coexisting in the film. As O 2 pressure increases, the diffraction peaks of TiO weaken while TiO 2 diffraction peaks enhance. The titanium oxide films prepared at an O 2 pressure of 7.3 = 10y4 Pa and above 8.4 = 10y4 Pa exhibit a highly preferred orientation, so no diffraction peak is detected by glancing angle diffraction. From XRD and glancing diffraction results, it can be confirmed that the titanium oxide films synthesized by IBED at different O 2 pressures showed a polycrystal structure with preferred orientation. When O 2 pressure reaches 7.3 = 10y4 Pa, the film is dominated by TiO Ž220.. As O 2 pressure increased, the diffraction pattern became complicated due to the creation of a new substance phase, as well as the alternation of the orientation of TiO. Fig. 5 shows the RBS date of titanium oxide film synthesized at an O 2 pressure of 8.2 = 10y4 Pa. The OrTi ratio can be calculated from it by using the following formula: CO C Ti
s
Fig. 5. RBS date of titanium oxide film synthesized by IBED.
can be obtained from the figure, and Z is the atomic number of the element. Here, the ratio of OrTi is calculated to be 1.754, just between that of the Ti 2 O 3 and TiO 2 , which implies that neither Ti 2 O 3 nor TiO 2 can individually exist in the film. When O 2 pressure is higher than 8.4 = 10y4 Pa, rutile-type TiO 2 with Ž110. and Ž220. preferred orientation becomes the predominant phase of the film. As the O 2 pressure was increased, the atom percentage of TiO and Ti 2 O 3 decreased or even disappeared, and the composition and orientation became stable.
2 HO Z Ti
H Ti ZO2
where C represents the atom percentage of the element, H is the counts height of the element which
Fig. 4. Glancing angle diffraction patterns of titanium oxide films synthesized at different O 2 pressure.
4. Discussion When O 2 pressure is as low as 7.3 = 10y4 Pa, the concentration of oxygen in the chamber is just enough to form TiO. The Ž220. plane of TiO has lower surface energy, and it will grow preferentially. Therefore, the film exhibits highly Ž220. preferred TiO structure. As O 2 pressure was increased, Ti 2 O 3 formed as impurity phase, which has been detected by glancing angle diffraction. These impurities blocked grain boundary migration and caused the Ž220. texture to decrease w9x. Meanwhile, more O 2 was adsorbed on the surface of the depositing film accompanied by the increase of oxygen content in the chamber. The adsorption of O 2 altered the surface energy of different crystal phases w10x. That made it possible for the formed texture in TiO phase to be transferred from Ž220. to Ž031.. When O 2 pressure exceeded 8.4 = 10y4 Pa, there is enough
X. Wang et al.r Materials Letters 44 (2000) 105–109
O 2 in the chamber to form pure TiO 2 phase. Due to the ‘‘Channel effect’’ w11x caused by ion beam, the formed TiO 2 phase exhibits highly Ž200. preferred orientation. As we know, Ž110. crystal plane of rutile-type TiO 2 has the lowest surface free energy. In general, the ratio of I Ž200.rI Ž110. is only 0.08 if the crystals of titanium oxide film grow randomly. However, under the bombard direction used here, the Ž200. crystallite is the so-called ‘‘channel’’ plane of the crystal. Thus, less damage would be produced in crystallites with Ž200. than with Ž110. and other crystallites. As a result, the crystallites with Ž200. orientation tend to serve as recrystallization centers so that it would grow preferentially with the Ž110. orientation. That is, the growth of Ž110. orientation competes with that of Ž200. orientation. The competition result is related to g , which represents the arrival ratio of the bombard ion ŽXeq. and the evaporate atom ŽTi. but has no business with O 2 pressure. Under the g used here, the competition result is that Ž200. orientation becomes the preferred orientation of rutile-type TiO 2 .
5. Conclusion The titanium oxide films synthesized by IBED at different O 2 pressures are polycrystal films that show a preferred orientation. TiO, Ti 2 O 3 and TiO 2 coexisted in the films although their percentage varied with O 2 pressure. When O 2 pressure is lower than 8.4 = 10y4 Pa, the main composition of the films is TiO, as well as
109
Ti 2 O 3 , because there is a lack of O 2 . The preferred orientation of TiO transferred from Ž220. to Ž031. as the O 2 pressure was increased. When O 2 pressure is higher than 8.4 = 10y4 Pa, rutile-type TiO 2 with Ž200. preferred orientation becomes the major composition of the film. The preferential growth of Ž200. crystallite is due to the socalled ‘‘Channel effect’’.
References w1x R.W. Sprague, C.F. Hickey, I.J. Hodgkinson, J. Appl. Phys. 71 Ž7. Ž1992. 3201, 1 April. w2x D. Wicaksana, A. Kobayashi, A. Kinbara, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. 1479, Jul.rAug. w3x F. Zhang, Z. Zheng, Y. Chen, Y. Mao, X. Liu, Thin Solid Films 312 Ž1998. 1. w4x E.C. Plspprty, K.H. Dahmen, R. Hanert, K.H. Erst, Chem. Vap. Deposition 5 Ž2. Ž1999. 79. w5x A. Rizzo, L. Mirenghi, A. Puirini, S. Scaglione, Surf. Coat. Technol. 91 Ž1997. 153. w6x W.S. Oh, C. Xu, D.Y. Kim, D.W. Goodman, J. Vac. Sci. Technol. A 15 Ž3. Ž1997. 1710, MayrJun. w7x F. Zhang, Z. Zheng, X. Ding, Y. Mao, Y. Chen, Z. Zhou, S. Yang, X. Liu, J. Vac. Sci. Technol. A 15 Ž4. Ž1997. 1824, Jul.rAug. w8x F. Zhang, Z. Zheng, Y. Chen, X. Liu, A. Chen, Z. Jiang, J. Biomed. Mater. Res. 42 Ž1. Ž1998. 128. w9x J.F. Pocza, A. Barna, P.B. Barna, I. Pozsgai, G. Radnoczi, ´ Jpn. J. Appl. Phys. Suppl. 2 Ž1974. 525, part 1. w10x M. Adamik, P.B. Barna, I. Tomov, Thin Solid Films 317 Ž1998. 64. w11x F. Zhang, Z. Zheng, Y. Chen, D. Liu, X. Liu, J. Appl. Phys. 15 Ž1998. 4101, April.
Materials Letters 44 Ž2000. 105–109 www.elsevier.comrlocatermatlet
Effect of O 2 pressure on the synthesis of titanium oxide film by ion beam enhanced deposition Xianghui Wang ) , Feng Zhang, Zhihong Zheng, Changrong Li, Lizhi Chen, Huimin Wang, Xianghuai Lui Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Science, 865 Changning Road, Shanghai 200050, People’s Republic of China Received 5 February 1999; received in revised form 1 December 1999; accepted 17 January 2000
Abstract A series of titanium oxide films have been synthesized on silicon wafers by ion beam enhanced deposition at different O 2 pressures. X-ray Photoelectron Spectroscopy ŽXPS., X-ray Diffraction ŽXRD., Glancing Angle Diffraction and Rutherford Backscattering Spectroscopy ŽRBS. were used to analyze the composition, structure and orientation of the films. From the experimental results, it was found that: Ž1. the titanium oxide films synthesized at different O 2 pressures exhibit a polycrystal structure with preferred orientation; Ž2. when O 2 pressure is lower than 8.4 = 10y4 Pa, the main composition of the film is TiO whose preferred orientation changed from Ž220. to Ž031. with the increase of O 2 pressure; Ž3. when O 2 pressure is higher than 8.4 = 10y4 Pa, rutile-type TiO 2 with Ž200. preferred orientation was found to be the major composition of the film. The higher the O 2 pressure is, the more stable the composition and preferred orientation become. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide film; Oxygen pressure; Ion beam enhanced deposition
1. Introduction Titanium oxide has a wide range of application. Not only can it be used as pigment and catalyst, but it can also be used in capacitor fabrication and sensor technology. Now it is considered for introduction into biomaterial area because of its good hemocompatibility with the human body. Several methods have been used to synthesize titanium oxide film including metal-organic chemical vapor deposition, filtered arc deposition, DC mag)
Corresponding author. E-mail address: sarah – [email protected] ŽX. Wang..
netron reactive sputtering, cold plasma torch, ion beam enhanced deposition ŽIBED., and others w1–4x. Rizzo et al. w5x considered that reactive ion beam assistance appears to be a useful technique to fabricate optical coatings with the correct stoichiometry. Ultrathin Ž- 10 nm. titanium oxide films have been synthesized on the Mo Ž100. surface by Oh et al. w6x to be used for model catalysis studies. Rutile-type TiO 2 film with Ž100. preferred orientation, which showed a good hemocompatibility, has been successfully synthesized by Zhang et al. w7,8x, who have found that lower OrTi ratio may result in a better blood compatibility if the surface energy keeps the same. The OrTi ratio is related to the composition
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 1 0 - 0
106
X. Wang et al.r Materials Letters 44 (2000) 105–109
of the film. In order to investigate the relationship between the OrTi ratio and hemocompatibility, titanium oxide films with different compositions had to be formed. IBED is a good method to synthesize titanium oxide films because the composition and orientation of the film can be accurately controlled by process parameters. Besides, adhesion between the film and the substrate is firm and the thickness of the film can be controlled in the range of several angstrom to several microns. Bombardment ion, ion beam energy, ion beam current density, O 2 pressure, evaporation rate and substrate temperature are the main process parameters of IBED. In this paper, O 2 pressure was set up as a variable parameter. A series of titanium oxide films with different compositions and orientation were synthesized by IBED at different O 2 pressures, and the relationship between O 2 pressure and the structure of the film is discussed in detail.
2. Experimental The titanium oxide films were deposited onto optically polished silicon Ž100. substrate by electron beam evaporation of titanium at a rate of 0.4 nmrs in an oxygen atmosphere. A beam of energetic xenon ions was used to bombard the growing titanium oxide film simultaneously with a fixed energy of 40 keV. The ion beam current density was maintained at 40 mArcm2 . The incidence angle of the titanium vapor stream and xenon ion beam to the substrate was 458, respectively. The base pressure in the deposition chamber was about 8.6 = 10y5 Pa, and the deposition chamber was backfilled with high purity oxygen gas during deposition. As a variable parameter, the filled oxygen pressure was set at 7.3, 8.0, 8.1, 8.2, 8.4 8.6 and 15 = 10y4 Pa, respectively. The chemical states of titanium, oxygen, carbon and silicon on the surface and near-surface of the synthesized titanium oxide films were studied by X-ray Photoelectron Spectroscopy ŽXPS.. The spectra were curve-fitted, using a computer assisted Gaussian–Lorentzian peak model. The binding energy of the C 1s line was taken as 284.6 " 0.4 eV for calibrating the obtained spectra. The structure of the film was investigated by X-ray diffraction ŽXRD. analysis. Since all diffrac-
tion beams received by counter recording of XRD are produced only by crystal planes that parallel the specimen surface, glancing angle diffraction, which is suitable for the analysis of polycrystal with free orientation, was used for an additional analysis. The glancing angle used here was fixed at 18. Rutherford backscattering spectroscopy ŽRBS. was performed using a 2 MeV Heq ion beam at a scattering angle of 1658 to investigate the OrTi ratio of synthesized titanium oxide film.
3. Results XPS results show that there is an amount of contaminating carbon on the surface of the titanium oxide films. After 10 min of argon ion sputtering, the counts of carbon decreased sharply. Fig. 1 shows the XPS spectrum of Ti 2p from the surface of titanium oxide film prepared at different O 2 pressures. The binding energy of Ti 2P3r2 and Ti 2P1r2 is observed to be at 458.8 and 464.5 eV, respectively, for all films assigned to Ti 4q in TiO 2 , with a peak separation of 5.7 eV between these two peaks. No difference was observed for titanium oxide films prepared at different O 2 pressures because these films have been exposed to air for some time and all of them have naturally oxidized to form a pure TiO 2 layer on the surface. Fig. 2 shows the high-resolution spectrum of Ti 2P of the titanium oxide film prepared at an O 2 pressure of 8.2 = 10y4 Pa after 10 min of argon ion sputtering. The spectrum is dominated by a peak at ; 458.9 eV, which is assigned to Ti 4q in TiO 2 . There are also other two peaks located according to the deconvolution procedure at ; 456.9 and ; 455 eV, which are the Ti 3q state and Ti 2q state in Ti 2 O 3 and TiO, respectively. The peak separation between Ti 2p1r2 and Ti 2p3r2 is about 5.7 " 0.1 eV. The XPS spectra of titanium oxide films that were prepared at different O 2 pressures had similar results, except that the Ti 4qrTi 3qrTi 2q ratio varied slightly. As O 2 pressure increased, the atom percentage of TiO decreased. On the basis of XPS analysis results, XRD and glancing angle diffraction were used to analyze the structure and orientation of the films. Fig. 3 shows the XRD pattern of titanium oxide films synthesized at different O 2 pressures. When O 2 pressure is only
X. Wang et al.r Materials Letters 44 (2000) 105–109
107
Fig. 1. Ti 2p XPS spectrum on the surface of titanium oxide film.
7.3 = 10y4 Pa, the film dominated by TiO with a highly Ž220. orientation whose diffraction peak located at 2 u s 43.18. As the O 2 pressure increased, it can be seen that the Ž220. diffraction peak dropped, and a new TiO diffraction peak located at 2 u s 37.18 appears and grows, which indicates that the orientation of TiO is changing from Ž220. to Ž031.. When O 2 pressure reached 8.2 = 10y4 Pa, the diffraction peaks of rutile-type TiO 2 Ž110. and Ž200. appeared besides TiO Ž031.. The peak intensity ratio, I Ž200.rI Ž110., is about 2.65, which suggests that crystallites grow preferentially with the Ž200. plane parallel to the substrate surface, for according to the
standard rutile phase of ASTM card Ž21-1276., the I Ž200.rI Ž110. is about 0.08 for random distribution. When O 2 pressure reached over 8.6 = 10y4 Pa, the peak of TiO Ž031. disappeared. Rutile-type TiO 2 , with Ž200. preferred orientation, becomes the major phase of the film. The orientation of Ti 2 O 3 cannot be determined by XRD although its existence has been verified by XPS analysis. The main reason is that the diffraction peak of Ti 2 O 3 Ž104. overlapped with Si Ž100. at
Fig. 2. Ti 2p XPS spectrum of titanium oxide film after 10 min of argon ion sputtering.
Fig. 3. XRD patterns of titanium oxide films synthesized at different O 2 pressure.
X. Wang et al.r Materials Letters 44 (2000) 105–109
108
2 u s 338 and the other diffraction peaks of Ti 2 O 3 is too weak to be identified. Fig. 4 shows the glancing angle diffraction pattern of the titanium oxide films that were synthesized at different O 2 pressures. From this figure, it can be seen that the synthesized titanium oxide films show a polycrystal structure with TiO, Ti 2 O 3 and TiO 2 coexisting in the film. As O 2 pressure increases, the diffraction peaks of TiO weaken while TiO 2 diffraction peaks enhance. The titanium oxide films prepared at an O 2 pressure of 7.3 = 10y4 Pa and above 8.4 = 10y4 Pa exhibit a highly preferred orientation, so no diffraction peak is detected by glancing angle diffraction. From XRD and glancing diffraction results, it can be confirmed that the titanium oxide films synthesized by IBED at different O 2 pressures showed a polycrystal structure with preferred orientation. When O 2 pressure reaches 7.3 = 10y4 Pa, the film is dominated by TiO Ž220.. As O 2 pressure increased, the diffraction pattern became complicated due to the creation of a new substance phase, as well as the alternation of the orientation of TiO. Fig. 5 shows the RBS date of titanium oxide film synthesized at an O 2 pressure of 8.2 = 10y4 Pa. The OrTi ratio can be calculated from it by using the following formula: CO C Ti
s
Fig. 5. RBS date of titanium oxide film synthesized by IBED.
can be obtained from the figure, and Z is the atomic number of the element. Here, the ratio of OrTi is calculated to be 1.754, just between that of the Ti 2 O 3 and TiO 2 , which implies that neither Ti 2 O 3 nor TiO 2 can individually exist in the film. When O 2 pressure is higher than 8.4 = 10y4 Pa, rutile-type TiO 2 with Ž110. and Ž220. preferred orientation becomes the predominant phase of the film. As the O 2 pressure was increased, the atom percentage of TiO and Ti 2 O 3 decreased or even disappeared, and the composition and orientation became stable.
2 HO Z Ti
H Ti ZO2
where C represents the atom percentage of the element, H is the counts height of the element which
Fig. 4. Glancing angle diffraction patterns of titanium oxide films synthesized at different O 2 pressure.
4. Discussion When O 2 pressure is as low as 7.3 = 10y4 Pa, the concentration of oxygen in the chamber is just enough to form TiO. The Ž220. plane of TiO has lower surface energy, and it will grow preferentially. Therefore, the film exhibits highly Ž220. preferred TiO structure. As O 2 pressure was increased, Ti 2 O 3 formed as impurity phase, which has been detected by glancing angle diffraction. These impurities blocked grain boundary migration and caused the Ž220. texture to decrease w9x. Meanwhile, more O 2 was adsorbed on the surface of the depositing film accompanied by the increase of oxygen content in the chamber. The adsorption of O 2 altered the surface energy of different crystal phases w10x. That made it possible for the formed texture in TiO phase to be transferred from Ž220. to Ž031.. When O 2 pressure exceeded 8.4 = 10y4 Pa, there is enough
X. Wang et al.r Materials Letters 44 (2000) 105–109
O 2 in the chamber to form pure TiO 2 phase. Due to the ‘‘Channel effect’’ w11x caused by ion beam, the formed TiO 2 phase exhibits highly Ž200. preferred orientation. As we know, Ž110. crystal plane of rutile-type TiO 2 has the lowest surface free energy. In general, the ratio of I Ž200.rI Ž110. is only 0.08 if the crystals of titanium oxide film grow randomly. However, under the bombard direction used here, the Ž200. crystallite is the so-called ‘‘channel’’ plane of the crystal. Thus, less damage would be produced in crystallites with Ž200. than with Ž110. and other crystallites. As a result, the crystallites with Ž200. orientation tend to serve as recrystallization centers so that it would grow preferentially with the Ž110. orientation. That is, the growth of Ž110. orientation competes with that of Ž200. orientation. The competition result is related to g , which represents the arrival ratio of the bombard ion ŽXeq. and the evaporate atom ŽTi. but has no business with O 2 pressure. Under the g used here, the competition result is that Ž200. orientation becomes the preferred orientation of rutile-type TiO 2 .
5. Conclusion The titanium oxide films synthesized by IBED at different O 2 pressures are polycrystal films that show a preferred orientation. TiO, Ti 2 O 3 and TiO 2 coexisted in the films although their percentage varied with O 2 pressure. When O 2 pressure is lower than 8.4 = 10y4 Pa, the main composition of the films is TiO, as well as
109
Ti 2 O 3 , because there is a lack of O 2 . The preferred orientation of TiO transferred from Ž220. to Ž031. as the O 2 pressure was increased. When O 2 pressure is higher than 8.4 = 10y4 Pa, rutile-type TiO 2 with Ž200. preferred orientation becomes the major composition of the film. The preferential growth of Ž200. crystallite is due to the socalled ‘‘Channel effect’’.
References w1x R.W. Sprague, C.F. Hickey, I.J. Hodgkinson, J. Appl. Phys. 71 Ž7. Ž1992. 3201, 1 April. w2x D. Wicaksana, A. Kobayashi, A. Kinbara, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. 1479, Jul.rAug. w3x F. Zhang, Z. Zheng, Y. Chen, Y. Mao, X. Liu, Thin Solid Films 312 Ž1998. 1. w4x E.C. Plspprty, K.H. Dahmen, R. Hanert, K.H. Erst, Chem. Vap. Deposition 5 Ž2. Ž1999. 79. w5x A. Rizzo, L. Mirenghi, A. Puirini, S. Scaglione, Surf. Coat. Technol. 91 Ž1997. 153. w6x W.S. Oh, C. Xu, D.Y. Kim, D.W. Goodman, J. Vac. Sci. Technol. A 15 Ž3. Ž1997. 1710, MayrJun. w7x F. Zhang, Z. Zheng, X. Ding, Y. Mao, Y. Chen, Z. Zhou, S. Yang, X. Liu, J. Vac. Sci. Technol. A 15 Ž4. Ž1997. 1824, Jul.rAug. w8x F. Zhang, Z. Zheng, Y. Chen, X. Liu, A. Chen, Z. Jiang, J. Biomed. Mater. Res. 42 Ž1. Ž1998. 128. w9x J.F. Pocza, A. Barna, P.B. Barna, I. Pozsgai, G. Radnoczi, ´ Jpn. J. Appl. Phys. Suppl. 2 Ž1974. 525, part 1. w10x M. Adamik, P.B. Barna, I. Tomov, Thin Solid Films 317 Ž1998. 64. w11x F. Zhang, Z. Zheng, Y. Chen, D. Liu, X. Liu, J. Appl. Phys. 15 Ž1998. 4101, April.