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Core level photoemission study of nitrided hafnium silicate thin films
Microelectronic Engineering 84 (2007) 2302–2305 www.elsevier.com/locate/mee
Core level photoemission study of nitrided hafnium silicate thin films N. Barrett a,*, O. Renault b, P. Besson b, Y. Le Tiec b, F. Martin b, F. Calvat a a
CEA-DSM/DRECAM-SPCSI, CEA-Saclay, 91191 Gif-sur-Yvette, France CEA-LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
b
Abstract We present a synchrotron radiation and Al KD core level study of ultra-thin nitrided hafnium silicate thins films, potential candidate for a high N oxide to replace SiO2. Nitriding the as deposited, uniform silicate results in the formation of Si-N and Hf-N bonds, with a nitrogen profile near the silicate/SiO2 interface. Controlled etching reveals that the nitridation has proceeded beyond the silicate into the SiO2 bottom oxide, without degrading the sharp silicate/SiO2 interface. The 1.2 nm Hf0.4Si0.6O2 nitrided silicate on 0.6 nm SiO2 has an estimated equivalent oxide thickness of 1.1 nm. Keywords: gate oxide; hafnium silicate; photoemission; synchrotron radiation
1. Introduction Among many potential high-Nҏoxides, one of the most promising candidates is nitrided hafnium silicate. HfSiON can grow without the formation of a significant interface layer on SiO2 buffer. Silicates are more resistant to cation diffusion and recrystallisation than HfO2 [1]. N acts as a diffusion barrier to B coming from the p-type Si substrate. Nitridation inhibits phase separation through the establishment of inter-cation Hf-N-Si bridging bonds.
Corresponding author. Tel.:+ 33169083272; fax: +33169088446. E-mail address:[email protected] (N. Barrett)
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.04.131
Finally, the addition of nitrogen to HfSiO improves the dielectric constant, attaining values between 12 and 15 [2-3]. Note that the Hf content plays a role in the final dielectric constant, and there is a trade off between crystallization temperature and N. The dielectric constant for a hafnium silicate scales with the Hf content, N ~ 13 for 40% Hf cations, and we may expect N ~ 15 for 70% Hf cation concentration. Core level photoemission is a valuable tool to investigate the chemical and electronic environment at the atomic scale. The local atomic structure is responsible for the chemical core level shifts. Visokay et al [2] examined sputter deposited HfSiON, and demonstrated clearly the existence of
N. Barrett et al. / Microelectronic Engineering 84 (2007) 2302–2305
Si-N bonding, not Si-O-N. The Si 2p core level shift was +2.3 eV for the Si-N environment, compared to +3.0 eV for the unnitrided silicate. Thus, the nitrogen occupies a bridging site between the silicon and hafnium cations. According to Shang et al. [4], adding N to oxide and silicate crystalline structures reduces the Hf coordination, thus making ionic diffusion more costly in terms of energy, tying the Hf to the amorphous inter-cation structure via the Hf-N bonds [5]. The presence of oxy-nitride O-N bonds is therefore critical; they will be less efficient in inhibiting cation diffusion than bridging N atoms. The core level signature of N-O bonding is very distinctive. Chang [6] estimated a 1.8 eV shift per O atom binding to N, whereas the formation of Si-N bonding gave a core level shift of +3.0 eV on the Si 2p level. Pant et al. [7] have studied the effect of nitridation on the Hf 4f core levels. They also observe a 2 eV shift for complete nitridation of the local Hf environment. They measure a 1.4 eV shift to higher binding energies per oxygen atom neighbor on the N 1s line. Similarly, Akbar et al. [8] showed that post-deposition 600 °C NH3 nitridation shifts the Hf 4f core level upwards towards the Fermi level by about 1 eV. We present a synchrotron radiation and laboratory Al KD photoemission study of the Hf 4f, Si 2p and N 1s core levels in a thin nitrided hafnium silicate film
Etching of the nitrided layer for the depth profile was done using diluted HF acid solution and checked by ellipsometry. The samples are shown in figure 1. Photoemission measurements were performed at the SuperACO storage ring (LURE, Orsay, France), on beamline SA73 using a photon energy of 160 eV. The overall energy resolution (monochromator and analyzer combined) was 75-100 meV. Laboratory XPS measurements were carried out using a Al KD source (hQ = 1486.6 eV). Depth sensitivity to the local chemical environment is obtained from the escape depth dependence of the photoelectrons on their kinetic energy for each of the samples illustrated in figure 1. The valence band results have already been published, and quantified the band bending at the oxide/substrate interface and the final state screening of the photoemission in the silicate overlayer. [9] The binding energy correction due to final state screening in the silicate is included in the synchrotron radiation results presented here. 3. Results and discussion Figure 2 shows the Hf core level spectra with photon energies of 160 eV and 1486.6 eV.
2. Experimental 3.5 nm thick HfxSi1-xO2 layers were deposited by CVD (Chemical Vapor Deposition) using metalloorganic precursors on a p doped (2x1015 cm-3 Si(100) substrate. As Dep
3.5 nm N
1.2 nm N
0 nm N
Fig. 1. The four types of sample on Si substrate used to profile the nitridation: 3.5 nm silicate; 3.5 nm nitrided silicate; 1.2 nm nitrided silicate; 0.6 nm bottom oxide.
The substrate underwent controlled thermal oxidation before deposition giving a 0.6 nm thick silica layer, measured by ellipsometry. Nitridation was carried under NH3 at 750 °C for 30 minutes.
Fig. 2. The Hf 4f core level spectra for 3.5 nm silicate; 3.5 nm nitrided silicate; 1.2 nm nitrided silicate using photon energies of 160 eV (left) and 1486.6 eV (right).
For the as deposited silicate, one 4f component (light grey) is visible at a binding energy of 17.8 eV, suggesting a homogeneous layer. A second component is observed shifted by 0.9 and 0.7 eV to lower binding energies for the 3.5 nm and 1.2 nm
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N. Barrett et al. / Microelectronic Engineering 84 (2007) 2302–2305
nitrided silicate films respectively, associated with Hf-N bonding (grey). In order to further probe the nitride chemistry profile we have measured the Hf 4f core levels at normal and 45° emission, see figure 3 (left). The 4f intensity does not change, but the weight of the Hf-N component decreases for normal emission, suggesting that there are less Hf-N environments close to the silica interface.
Fig. 3. Hf 4f spectra at 45° and normal detection for the 1.2 nm nitrided silicate (left); N 1s core level spectra (right).
The silicate stoichiometry is estimated to be Hf0.5Si0.5O2 from the XPS intensities, see table 1. Finally there is an overall shift in the XPS spectra to lower binding energy as etching proceeds, confirming the band bending already reported [9]. Table 1. Hf, Si and N stoichiometries determined by XPS. Sample
3.5nm silicate
3.5nm N
1.2nm silicate-N
Hf/(Hf+Si)
0.52
0.55
0.38
N/(N+Si)
-
0.56
0.42
0.6nm silica 0.16
The N 1s XPS spectra are also presented in Fig. 3 (right). The estimated N concentrations are reported in table 1. There are two main components, one at 396.8 eV, attributed to N-Hf environment [7], and another 1 eV higher at 397.9 eV. The latter is
assigned to N threefold coordinated by Si. A smaller, third component is observed 400.4 eV, close to that reported for an oxy-nitride environment [7,10]. After etching the N-Hf intensity decreases. This is strong confirmation that the nitrogen indeed occupies a bridging site between the two different cations. The decrease in nitrogen concentration approaching the silica interface is confirmed. However, most dramatically, after complete removal of the nitrided silicate, the N 1s signal persists at a binding energy of 398 eV. This indicates that nitrogen atoms coordinated in silicon nitride like environments persist either in the SiO2 bottom oxide or in the substrate. The N 1s intensity suggests 6-7% N in the 0.6 nm SiO2/Si sample, corresponding to an atom density of 3x1021 cm-3. Following Toyoda et al.[1], this translates into reductions of 1.2 eV in the gap. The analysis of the valence band data suggested a value of 1.6 eV in band gap reduction [9]. The Si 2p core level spectra are particularly rich because of their narrower line width, allowing one to better resolve components due to distinct chemical environments. The classical Si 2p soft X-ray spectrum for an ultra thin silica layer on a Si substrate has five components: Si0 (substrate), Si4+ (silica) and the three suboxides Si1+, Si2+, Si3+[11]. The Si 2p spectra are shown in figure 4 and summarized in table 2. For hQ = 160 eV the 2p spectrum of the as deposited silicate has a small contribution due to the SiO2 bottom oxide (light grey) and a main component (grey) at a binding energy ~ 3.1 eV greater than that expected for bulk Si. Following Giustino et al.’s [12] first principles calculations for zirconium silicate of the Si 2p chemical shift as a function of nearest neighbours species, and given the similarity between zirconium and hafnium chemistries, we can estimate the hafnium content to be x ~ 0.45-0.50, in good agreement with the value obtained from the XPS intensities. On nitridation a component, between 2.0 and 2.2 eV with respect to Si0 is observed (dark grey). The Si-O-N environment visible in the N 1s spectrum is too weak to be resolved. After complete etching we recover the expected spectrum for an ultra-thin silica layer with its sub-oxides (light grey). In principle, this means seven components with a mean separation 'E of ~ 0.7 eV. The sub-oxide positions are known, but natural line widths mean
N. Barrett et al. / Microelectronic Engineering 84 (2007) 2302–2305
that there will be some degeneracy between the silicate and nitride states. In some of the spectra it was not possible to include all the sub-oxide peaks notably in the case with the largest number of peaks. The Al KD spectra reveal simultaneously the silicate, the bottom oxide and the substrate allowing us to estimate 0.4 eV band bending at the SiO2/Si interface.
nitridation of a hafnium silicate layer leads to hafnium and silicon nitride bonding in the silicate and partly into the substrate. Oxy-nitride bonding is marginal. The silicate stoichiometry is Hf0.5Si0.5O2. Nitrogen in cation bridging sites, the permittivity of ~ 10, the band offset > 1 eV and small EOT ~ 1.1 nm make this an interesting high N gate oxide. Acknowledgements Some of the measurements were performed during the last beam time before shutdown of the LURE synchrotron. We thank all the staff over the years who made an excellent light source work. References
Fig. 4. The Si 2p core level spectra for 3.5 nm silicate; 3.5 nm nitrided silicate; 1.2 nm nitrided silicate; silica using photon energies of 160 eV (left) and 1486.6 eV (right). Table 2. Si 2p core level binding energies determined using hQ = 160 eV, with respect to the flat band Si0 peak. Si4+ 0 nm-N
3.9
1.2nm-N
3.7
Si*
Si3+
3.1
2.5
Si-N1
2.8
3.5nm-N
3.1
Silicate
3.0
Si2+ 1.8
2.0 2.1
Following Kato et al.’s [13] values of the permittivity as a function of Hf content we can estimate the k of the silicate film to be ~ 9-10, giving an equivalent oxide thickness (EOT) of 1.1 nm for the 1.2 nm Hf0.5Si0.5O2/0.6 nm SiO2. 4. Conclusions Using synchrotron radiation and laboratory core level photoelectron spectroscopy we demonstrate that
[1] S. Toyoda, J. Okabayashi, H. Kumigashira, M. Oshima, K. Ono, M. Niwa, K. Usuda, G. L. Liu, Appl. Phys. Lett. 84 (2004) 2328-2330. [2] M. R. Visokay, J. J. Chambers, A. L. P. Rotondaro, A. Shanware, L. Colombo, Appl. Phys. Lett. 80 (2002) 3183-3185. [3] M. Koyama, A. Kaneko, T. Io, M. Koike, Y. Kamata, R. Iijima, Y. Kamimuta, A. Takashima, M. Suzuki, C. Hongo, S. Inumiya, M. Takayanagi, A. Nishiyama, IEEE IEDM 2002, 849-852. [4] G. Shang, P. W. Peacock, J. Robertson, Appl. Phy. Lett. 84 (2004) 106. [5] J. Morais, L. Miotti, K. P. Bastos, S. R. Teixeira, I. J. R. Baumvol, , A. L. P. Rotondaro, J. J. Chambers, M. R. Visokay, L. Colombo, M. C. Matrins Alves, Appl. Phys. Lett. 86 (2005) 212906. [6] J. P. Chang, M. L. Green, V. M. Donnelly, R. L. Opila, J. Eng Jr., J. Sapjeta, P. J. Silverman, B. Weir, H. C. Lu, T. Gustafsson, E. Garfunkel, J. Appl. Phys. 87 (2000) 4449. [7] G. Pant, P. Punchaipetch, M. J. Kim, R. M. Wallace, B. E. Gnade, Thin Solid Films 460 (2004) 242-246. [8] M. S. Akbar, S. Gopalan, H.-J. Cho, K. Onishi, R. Choi, R. Nieh, C. S. Kang, Y. H. Kim, J. Han, S. Krishnan, J. C. Lee, Appl. Phys. Lett. 82 (2003) 17571759. [9] N. T. Barrett, O. Renault, P. Besson, Y. Le Tiec, F. Martin, Appl. Phys. Lett. 88 (2006) 162906. [10] M. Bhat, J. Ahn, D. L. Kwong, M. Arendt, J. M. White, Appl. Phys. Lett. 64 (1994) 1168-1170. [11] F. J. Himpsel, F. R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Phys. Rev. B 38 (1988) 60846096. [12] F. Giustino, A. Buongiorno, A. Pasquarello, Appl. Phys. Lett. 81 (2002) 4233-4235. [13] H. Kato, T. Nango, T. Miyagawa, T. Katagiri, K. S. Seol, Y. Ohki, J. Appl. Phys. 92 (2002) 1106-1111.
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Core level photoemission study of nitrided hafnium silicate thin films N. Barrett a,*, O. Renault b, P. Besson b, Y. Le Tiec b, F. Martin b, F. Calvat a a
CEA-DSM/DRECAM-SPCSI, CEA-Saclay, 91191 Gif-sur-Yvette, France CEA-LETI, MINATEC, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
b
Abstract We present a synchrotron radiation and Al KD core level study of ultra-thin nitrided hafnium silicate thins films, potential candidate for a high N oxide to replace SiO2. Nitriding the as deposited, uniform silicate results in the formation of Si-N and Hf-N bonds, with a nitrogen profile near the silicate/SiO2 interface. Controlled etching reveals that the nitridation has proceeded beyond the silicate into the SiO2 bottom oxide, without degrading the sharp silicate/SiO2 interface. The 1.2 nm Hf0.4Si0.6O2 nitrided silicate on 0.6 nm SiO2 has an estimated equivalent oxide thickness of 1.1 nm. Keywords: gate oxide; hafnium silicate; photoemission; synchrotron radiation
1. Introduction Among many potential high-Nҏoxides, one of the most promising candidates is nitrided hafnium silicate. HfSiON can grow without the formation of a significant interface layer on SiO2 buffer. Silicates are more resistant to cation diffusion and recrystallisation than HfO2 [1]. N acts as a diffusion barrier to B coming from the p-type Si substrate. Nitridation inhibits phase separation through the establishment of inter-cation Hf-N-Si bridging bonds.
Corresponding author. Tel.:+ 33169083272; fax: +33169088446. E-mail address:[email protected] (N. Barrett)
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.04.131
Finally, the addition of nitrogen to HfSiO improves the dielectric constant, attaining values between 12 and 15 [2-3]. Note that the Hf content plays a role in the final dielectric constant, and there is a trade off between crystallization temperature and N. The dielectric constant for a hafnium silicate scales with the Hf content, N ~ 13 for 40% Hf cations, and we may expect N ~ 15 for 70% Hf cation concentration. Core level photoemission is a valuable tool to investigate the chemical and electronic environment at the atomic scale. The local atomic structure is responsible for the chemical core level shifts. Visokay et al [2] examined sputter deposited HfSiON, and demonstrated clearly the existence of
N. Barrett et al. / Microelectronic Engineering 84 (2007) 2302–2305
Si-N bonding, not Si-O-N. The Si 2p core level shift was +2.3 eV for the Si-N environment, compared to +3.0 eV for the unnitrided silicate. Thus, the nitrogen occupies a bridging site between the silicon and hafnium cations. According to Shang et al. [4], adding N to oxide and silicate crystalline structures reduces the Hf coordination, thus making ionic diffusion more costly in terms of energy, tying the Hf to the amorphous inter-cation structure via the Hf-N bonds [5]. The presence of oxy-nitride O-N bonds is therefore critical; they will be less efficient in inhibiting cation diffusion than bridging N atoms. The core level signature of N-O bonding is very distinctive. Chang [6] estimated a 1.8 eV shift per O atom binding to N, whereas the formation of Si-N bonding gave a core level shift of +3.0 eV on the Si 2p level. Pant et al. [7] have studied the effect of nitridation on the Hf 4f core levels. They also observe a 2 eV shift for complete nitridation of the local Hf environment. They measure a 1.4 eV shift to higher binding energies per oxygen atom neighbor on the N 1s line. Similarly, Akbar et al. [8] showed that post-deposition 600 °C NH3 nitridation shifts the Hf 4f core level upwards towards the Fermi level by about 1 eV. We present a synchrotron radiation and laboratory Al KD photoemission study of the Hf 4f, Si 2p and N 1s core levels in a thin nitrided hafnium silicate film
Etching of the nitrided layer for the depth profile was done using diluted HF acid solution and checked by ellipsometry. The samples are shown in figure 1. Photoemission measurements were performed at the SuperACO storage ring (LURE, Orsay, France), on beamline SA73 using a photon energy of 160 eV. The overall energy resolution (monochromator and analyzer combined) was 75-100 meV. Laboratory XPS measurements were carried out using a Al KD source (hQ = 1486.6 eV). Depth sensitivity to the local chemical environment is obtained from the escape depth dependence of the photoelectrons on their kinetic energy for each of the samples illustrated in figure 1. The valence band results have already been published, and quantified the band bending at the oxide/substrate interface and the final state screening of the photoemission in the silicate overlayer. [9] The binding energy correction due to final state screening in the silicate is included in the synchrotron radiation results presented here. 3. Results and discussion Figure 2 shows the Hf core level spectra with photon energies of 160 eV and 1486.6 eV.
2. Experimental 3.5 nm thick HfxSi1-xO2 layers were deposited by CVD (Chemical Vapor Deposition) using metalloorganic precursors on a p doped (2x1015 cm-3 Si(100) substrate. As Dep
3.5 nm N
1.2 nm N
0 nm N
Fig. 1. The four types of sample on Si substrate used to profile the nitridation: 3.5 nm silicate; 3.5 nm nitrided silicate; 1.2 nm nitrided silicate; 0.6 nm bottom oxide.
The substrate underwent controlled thermal oxidation before deposition giving a 0.6 nm thick silica layer, measured by ellipsometry. Nitridation was carried under NH3 at 750 °C for 30 minutes.
Fig. 2. The Hf 4f core level spectra for 3.5 nm silicate; 3.5 nm nitrided silicate; 1.2 nm nitrided silicate using photon energies of 160 eV (left) and 1486.6 eV (right).
For the as deposited silicate, one 4f component (light grey) is visible at a binding energy of 17.8 eV, suggesting a homogeneous layer. A second component is observed shifted by 0.9 and 0.7 eV to lower binding energies for the 3.5 nm and 1.2 nm
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N. Barrett et al. / Microelectronic Engineering 84 (2007) 2302–2305
nitrided silicate films respectively, associated with Hf-N bonding (grey). In order to further probe the nitride chemistry profile we have measured the Hf 4f core levels at normal and 45° emission, see figure 3 (left). The 4f intensity does not change, but the weight of the Hf-N component decreases for normal emission, suggesting that there are less Hf-N environments close to the silica interface.
Fig. 3. Hf 4f spectra at 45° and normal detection for the 1.2 nm nitrided silicate (left); N 1s core level spectra (right).
The silicate stoichiometry is estimated to be Hf0.5Si0.5O2 from the XPS intensities, see table 1. Finally there is an overall shift in the XPS spectra to lower binding energy as etching proceeds, confirming the band bending already reported [9]. Table 1. Hf, Si and N stoichiometries determined by XPS. Sample
3.5nm silicate
3.5nm N
1.2nm silicate-N
Hf/(Hf+Si)
0.52
0.55
0.38
N/(N+Si)
-
0.56
0.42
0.6nm silica 0.16
The N 1s XPS spectra are also presented in Fig. 3 (right). The estimated N concentrations are reported in table 1. There are two main components, one at 396.8 eV, attributed to N-Hf environment [7], and another 1 eV higher at 397.9 eV. The latter is
assigned to N threefold coordinated by Si. A smaller, third component is observed 400.4 eV, close to that reported for an oxy-nitride environment [7,10]. After etching the N-Hf intensity decreases. This is strong confirmation that the nitrogen indeed occupies a bridging site between the two different cations. The decrease in nitrogen concentration approaching the silica interface is confirmed. However, most dramatically, after complete removal of the nitrided silicate, the N 1s signal persists at a binding energy of 398 eV. This indicates that nitrogen atoms coordinated in silicon nitride like environments persist either in the SiO2 bottom oxide or in the substrate. The N 1s intensity suggests 6-7% N in the 0.6 nm SiO2/Si sample, corresponding to an atom density of 3x1021 cm-3. Following Toyoda et al.[1], this translates into reductions of 1.2 eV in the gap. The analysis of the valence band data suggested a value of 1.6 eV in band gap reduction [9]. The Si 2p core level spectra are particularly rich because of their narrower line width, allowing one to better resolve components due to distinct chemical environments. The classical Si 2p soft X-ray spectrum for an ultra thin silica layer on a Si substrate has five components: Si0 (substrate), Si4+ (silica) and the three suboxides Si1+, Si2+, Si3+[11]. The Si 2p spectra are shown in figure 4 and summarized in table 2. For hQ = 160 eV the 2p spectrum of the as deposited silicate has a small contribution due to the SiO2 bottom oxide (light grey) and a main component (grey) at a binding energy ~ 3.1 eV greater than that expected for bulk Si. Following Giustino et al.’s [12] first principles calculations for zirconium silicate of the Si 2p chemical shift as a function of nearest neighbours species, and given the similarity between zirconium and hafnium chemistries, we can estimate the hafnium content to be x ~ 0.45-0.50, in good agreement with the value obtained from the XPS intensities. On nitridation a component, between 2.0 and 2.2 eV with respect to Si0 is observed (dark grey). The Si-O-N environment visible in the N 1s spectrum is too weak to be resolved. After complete etching we recover the expected spectrum for an ultra-thin silica layer with its sub-oxides (light grey). In principle, this means seven components with a mean separation 'E of ~ 0.7 eV. The sub-oxide positions are known, but natural line widths mean
N. Barrett et al. / Microelectronic Engineering 84 (2007) 2302–2305
that there will be some degeneracy between the silicate and nitride states. In some of the spectra it was not possible to include all the sub-oxide peaks notably in the case with the largest number of peaks. The Al KD spectra reveal simultaneously the silicate, the bottom oxide and the substrate allowing us to estimate 0.4 eV band bending at the SiO2/Si interface.
nitridation of a hafnium silicate layer leads to hafnium and silicon nitride bonding in the silicate and partly into the substrate. Oxy-nitride bonding is marginal. The silicate stoichiometry is Hf0.5Si0.5O2. Nitrogen in cation bridging sites, the permittivity of ~ 10, the band offset > 1 eV and small EOT ~ 1.1 nm make this an interesting high N gate oxide. Acknowledgements Some of the measurements were performed during the last beam time before shutdown of the LURE synchrotron. We thank all the staff over the years who made an excellent light source work. References
Fig. 4. The Si 2p core level spectra for 3.5 nm silicate; 3.5 nm nitrided silicate; 1.2 nm nitrided silicate; silica using photon energies of 160 eV (left) and 1486.6 eV (right). Table 2. Si 2p core level binding energies determined using hQ = 160 eV, with respect to the flat band Si0 peak. Si4+ 0 nm-N
3.9
1.2nm-N
3.7
Si*
Si3+
3.1
2.5
Si-N1
2.8
3.5nm-N
3.1
Silicate
3.0
Si2+ 1.8
2.0 2.1
Following Kato et al.’s [13] values of the permittivity as a function of Hf content we can estimate the k of the silicate film to be ~ 9-10, giving an equivalent oxide thickness (EOT) of 1.1 nm for the 1.2 nm Hf0.5Si0.5O2/0.6 nm SiO2. 4. Conclusions Using synchrotron radiation and laboratory core level photoelectron spectroscopy we demonstrate that
[1] S. Toyoda, J. Okabayashi, H. Kumigashira, M. Oshima, K. Ono, M. Niwa, K. Usuda, G. L. Liu, Appl. Phys. Lett. 84 (2004) 2328-2330. [2] M. R. Visokay, J. J. Chambers, A. L. P. Rotondaro, A. Shanware, L. Colombo, Appl. Phys. Lett. 80 (2002) 3183-3185. [3] M. Koyama, A. Kaneko, T. Io, M. Koike, Y. Kamata, R. Iijima, Y. Kamimuta, A. Takashima, M. Suzuki, C. Hongo, S. Inumiya, M. Takayanagi, A. Nishiyama, IEEE IEDM 2002, 849-852. [4] G. Shang, P. W. Peacock, J. Robertson, Appl. Phy. Lett. 84 (2004) 106. [5] J. Morais, L. Miotti, K. P. Bastos, S. R. Teixeira, I. J. R. Baumvol, , A. L. P. Rotondaro, J. J. Chambers, M. R. Visokay, L. Colombo, M. C. Matrins Alves, Appl. Phys. Lett. 86 (2005) 212906. [6] J. P. Chang, M. L. Green, V. M. Donnelly, R. L. Opila, J. Eng Jr., J. Sapjeta, P. J. Silverman, B. Weir, H. C. Lu, T. Gustafsson, E. Garfunkel, J. Appl. Phys. 87 (2000) 4449. [7] G. Pant, P. Punchaipetch, M. J. Kim, R. M. Wallace, B. E. Gnade, Thin Solid Films 460 (2004) 242-246. [8] M. S. Akbar, S. Gopalan, H.-J. Cho, K. Onishi, R. Choi, R. Nieh, C. S. Kang, Y. H. Kim, J. Han, S. Krishnan, J. C. Lee, Appl. Phys. Lett. 82 (2003) 17571759. [9] N. T. Barrett, O. Renault, P. Besson, Y. Le Tiec, F. Martin, Appl. Phys. Lett. 88 (2006) 162906. [10] M. Bhat, J. Ahn, D. L. Kwong, M. Arendt, J. M. White, Appl. Phys. Lett. 64 (1994) 1168-1170. [11] F. J. Himpsel, F. R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Phys. Rev. B 38 (1988) 60846096. [12] F. Giustino, A. Buongiorno, A. Pasquarello, Appl. Phys. Lett. 81 (2002) 4233-4235. [13] H. Kato, T. Nango, T. Miyagawa, T. Katagiri, K. S. Seol, Y. Ohki, J. Appl. Phys. 92 (2002) 1106-1111.
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