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Nitriding transformation of titanium thin films by nitrogen implantation
Thin Solid Films 464 – 465 (2004) 180 – 184 www.elsevier.com/locate/tsf
Nitriding transformation of titanium thin films by nitrogen implantation Y. Kasukabe a,*, J.J. Wang b, T. Yamamura b, S. Yamamoto c, Y. Fujino a a
International Student Center/Department of Electronic Engineering, Tohoku University, Kawauchi, Aoba, Sendai 980-8576, Japan b Department of Metallurgy, Tohoku University, Aramaki-Aza-Aoba 02, Aoba, Sendai 980-8579, Japan c Department of Material Development, Japan Atomic Energy Research Institute-Takasaki, 1233 Watanuki, Takasaki 370-1292, Japan Available online 23 July 2004
Abstract Changes of electronic structure during nitrogen implantation into heated Ti thin films, which were accompanied by crystallographic structural transformation, were studied by applying in situ electron energy loss spectroscopy (EELS) combined with self-consistent charge discrete variational (DV)-Xa molecular orbital (MO) calculations. Hydrogen atoms constituting TiHx were released from the films with heating at temperatures more than about 100 jC. The H-released unstable fcc-Ti was transformed into hcp-Ti. The energy loss peak observed by EELS for TiHx shifted to lower energies with the release of H atoms during heating. Nitrogen ions of N+2 with 62 keV were implanted into the hcp-Ti films held at 350 jC. The TiNy films were ‘‘epitaxially’’ formed by the transformation of hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral sites of the fcc-Ti sublattice by N atoms. The energy loss peak for hcp-Ti films observed by EELS during N implantation gradually shifted to higher energies with increasing the dose. The shift meant increase in the density of electrons occupying the hybridized valence band. The formation mechanism of TiNy was discussed with the results of the calculated overlap populations of Ti – Ti and Ti – N bonds. D 2004 Elsevier B.V. All rights reserved. PACS: 68.37.Lp.; 68.55.Jk.; 81.15.Aa.; 85.40.Ry Keywords: Ion implantation; TiN; Fcc-hcp transformation; In situ TEM; EELS
1. Introduction Titanium nitride (TiNy) of NaCl-type has widely been used for many purposes such as a hard wear-resistant coating in cutting tools, a diffusion barrier in microcircuits, a corrosion and abrasion resistant layer on optical components, etc. [1– 5]. Although its elastic and diffusion-barrier properties are highly anisotropic and depend strongly upon film orientation, the formation mechanisms of preferred orientation of epitaxial TiN film are not sufficiently understood. Thus, much interest has been focused on epitaxial film formation mechanisms with preferred orientation [6,7]. Recently, it was reported that NaCl-type (110)- and (001)oriented TiNy crystallites were ‘‘epitaxially’’ grown by nitrogen (N) implantation into CaF2-type (110)-oriented TiHx and (035)-oriented hcp-Ti in deposited Ti films held at room temperature (RT) [8,9]; here, the term ‘‘epitaxially’’ is used to denote the situation of partially inheriting the atomic arrangements of unimplanted TiHx and hcp-Ti, re* Corresponding author. Tel.: +81-22-217-7775; fax: +81-22-217-7978. E-mail address: [email protected] (Y. Kasukabe). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.082
spectively. However, the detailed ‘‘epitaxial’’ nitriding process of Ti films without the domains of TiHx, and the change of the bonding interaction of Ti atoms with ligand of N atoms during N implantation have not yet been understood. The purpose of this study is to throw light on the process of H release and to elucidate the nitriding process of pure hcp-Ti without TiHx. In this paper, the changes of the electronic structures induced by the heating of as-deposited Ti films and by N implantation were examined by applying in situ electron energy loss spectroscopy (EELS). The results of EELS, which were acquired in the imaging mode by using the Gatan model 666 PEELS spectrometer, were discussed along with the results of a self-consistent charge discrete variational (DV)Xa molecular orbital (MO) calculation [10] combined with transmission electron microscope (TEM) observations [11].
2. Experimental Detailed descriptions of the preparation method of the deposited-Ti films were presented in an earlier paper [8]. The 100-nm-thick Ti films separated from NaCl substrates
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were heated up to 350 jC at a heating rate of 2 jC/min in a 400 kV analytical and high resolution TEM combined with ion accelerators at JAERI-Takasaki [12], in order to release H atoms from as-deposited Ti films. The implantations of N2+ ions with 62 keV into the Ti films held at 350 jC were performed in the TEM. According to a Monte Carlo simulation using the TRIM85 code, the mean projected range and full width at half maximum of the depth profile of N2+with 62 keV were 55 and 66 nm, respectively, and thus most of the implanted ions were to be retained inside the Ti films. The N concentrations in Ti films were evaluated from the implantation dose measured by a Faraday cage. The maximum dose in this experiment was 5.40 1017 ions/ cm2, which corresponded to the average atomic concentration value of 0.954 of N/Ti ratio in the Ti films.
3. Results and discussion In situ observations by TEM, the detailed results of which were shown in the other paper [11], elucidated that Ti films grown on NaCl (001) surfaces at RT consisted of hcp-Ti and CaF2-type TiHx (xi1.5), and that H atoms which constituted TiHx were released from the films with heating and were completely released at 350 jC. The Hreleased unstable fcc-Ti sublattice was transformed into hcpTi. Thus, nitrogen ions were implanted into the hcp-Ti films without TiHx, which were held at 350 jC in order to prevent the Ti films from absorbing the hydrogen atoms. The NaCltype TiNy is ‘‘epitaxially’’ formed in N-implanted hcp-Ti by
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the transformation of the hcp-Ti to fcc-Ti, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral (O) sites of the fcc-Ti by N atoms [11]. Here, the hcp-Ti lattice was expanded by N occupation and strain due to the lattice expansion was considered as a driving force for the hcp-fcc transformation of Ti sublattices, which will be discussed later. Fig. 1 shows the variation of energy loss spectra with the temperature of the films. The loss peak ( f 47.0 eV) denoted by the line of Ti– M2,3 corresponds to the loss peak due to the excitation from 3p 1/2 and/or 3/2 states of Ti atoms to the conduction band. It should be noted that the Ti– M2,3 peaks are almost invariant with temperature, which means that the energies of the core levels are almost invariant during this experiment. The energy loss of f 17.0 eV, indicated by a solid triangle for hcp-Ti at RT in Fig. 1(b), was found to agree well with the theoretical value of f 17.6 eV, calculated with the assumption that 3d and 4s electrons are all free [13]. For TiHx in Fig. 1(a), the energy loss peak of f 19.5 eV, indicated by a solid triangle at RT, is due to excitation of plasmons by electrons in the valence band consisting of bonding states formed by Ti 3d– 4p and H 1s orbitals, which is in agreement with the energy loss reported by Thomas [14]. The higher loss energy for TiHx than for hcp-Ti at RT can be considered to reflect the existence of additional electrons from H atoms, which result in an increase in the electron density of the hybridized band of Ti 3d– 4p and H 1s orbitals. The energy of the loss peaks indicated by solid triangles for TiHx in Fig. 1(a) shifts to lower energies with increasing film temperature. This means
Fig. 1. Variation of EELS spectra with the film temperature. Solid triangles indicate the loss peaks due to plasmon excitation. (a) Heating of TiHx grown in the deposited Ti films. (b) Heating of hcp-Ti grown in the deposited Ti films.
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that the electron density in the valence band consisting of Ti 3d –4p and H 1s bonding states decreases with the increasing in film temperature due to the release of H atoms from the TiHx. Thus, it is considered that the excitation of electrons of Ti 3d – 4p and H 1s bonding states to the antibonding states has been induced by heating the TiHx up to 100 –150 jC, and that the increase in the number of phonons by heating also contributes to make the excitation of electrons of the bonding states to the antibonding states easy. Fig. 2 shows the variation of energy loss spectra with the dose of N implantation into hcp-Ti at 350 jC. The energy loss peaks due to plasmon excitation indicated by solid triangles gradually shifted to higher energies with increasing dose. This means that the number of N atoms bonding to Ti atoms increases with the dose, and that the electron density in the hybridised N 2p/Ti 3d – 4p valence band also increases with the dose. The loss peak ( f 16.0 eV) denoted by the dotted line connected to the solid circle can be seen at N/Ti z 0.38. This peak is considered to be due to excitation of N 2s electrons to the conduction band [3]. To investigate changes in the electronic structures of asdeposited and N-implanted Ti films in detail, DV-Xa MO calculations have been performed for four cluster models shown in Fig. 3(a) – (d). The Ti19 cluster of Fig. 3(a) corresponds to a part of the hcp-Ti structure. The Ti –Ti distances are taken to be 0.29238 nm, corresponding to those of the ideal bulk crystal structure. Fig. 3(b) shows the
Fig. 2. Variation of EELS spectra with the dose of N ions. Solid triangles indicate the loss peaks due to plasmon excitation. The energy loss of f 17.0 eV was found to agree well with the theoretical value of f 17.6 eV calculated assuming the 3d and 4s electrons are all free [13].
Ti13H8 cluster of CaF2-type structure. The hydrogen atom as indicated by E occupies the central position (tetrahedral (T) site) of the tetrahedron as formed by A –D atoms in the fccTi sublattice. The Ti – Ti distances are assumed to be 0.31183 nm, corresponding to those of the observed TiHx in as-deposited Ti films. A Ti19N cluster is shown in Fig. 3(c). The nitrogen atom, indicated by G, occupies the central position (O site) of the octahedron as formed by A – F atoms in the hcp-Ti cluster of Fig. 3(a). The Ti– Ti distances are assumed to be 0.29238 nm, which is the same as those of the Ti19 cluster. A Ti14N13 cluster model for NaCl-type TiN is shown in Fig. 3(d). The nitrogen atom as indicated by G occupies the O sites of the octahedron as formed by A – F atoms in the fcc-Ti sublattice. The Ti – N distances are assumed to be 0.21200 nm, corresponding to those of the bulk crystal structure of TiN. Table 1 shows the calculated Mulliken bond overlap populations (OP) between Ti atoms, Ti and H atoms, and Ti and N atoms for the employed models. The values of OP refer to the strength of covalent bonds. The Ti –Ti OP for the Ti19 cluster is evaluated to be 0.231. The OP between Ti atoms such as B and C atoms bonding to the nitrogen G atom for the Ti19N cluster in Fig. 3(c) is evaluated to be 0.135, which is much smaller than that for the Ti19, irrespective of the same Ti– Ti bond length. The Ti– N OP for the Ti19N is larger than the Ti– Ti OP for the Ti19. The OP between the central B atom and the surrounding atom as A, C, and D atoms in Fig. 3(b) for the Ti13H8 cluster is evaluated to be 0.083, which is much smaller than that for the Ti19, while the OP value of Ti–H covalent bonds is 0.184, which is smaller than that of Ti– N covalent bonds for the Ti19N. Therefore, it is considered that the formation of Ti –H covalent bonds leads to weakening Ti –Ti bonds. In other words, the release of H atoms from the TiHx results in forming stronger Ti – Ti bonds and contracting the TiHx lattice. The OP between the face-centered A atom and the atoms B, C, D and E in Fig. 3(d) for the Ti14N13 cluster is evaluated to be 0.042, which is much smaller than that for the Ti19, while the OP value of Ti–N covalent bonds is 0.289, which is larger than that of Ti– Ti covalent bonds for the Ti19. Comparing the values of OP of Ti14N13 with those of Ti19 and Ti19N, it can be seen that the occupation of the O sites by N atoms leads to a decrease in the OP values of Ti– Ti bonds, whereas the OP values of Ti– N covalent bonds does not differ so much. This means that the occupation of the O site by N atoms gives rise to a weakening of Ti–Ti bonds and to an increase in the number of Ti– N bonds with the increase in implanted N atoms. An increase in the number of Ti– N bonds heralds the onset of nitriding of Ti films. The OP value of each bond between Ti atoms for Ti19 and Ti19N cluster models in Fig. 3(a) and (c) has been inspected in detail, and is shown in Table 2. Table 2 shows the existence of relatively large differences between the
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Fig. 3. Schematic illustrations of (a) Ti19 cluster, (b) Ti13H8 cluster, (c) Ti19N cluster and (d) Ti14N13 cluster models. Solid, large open and small open circles represent Ti, N and H atoms, respectively.
OP value of A –B, A –C, A –D, A –E atoms and that of A – I, A – H atoms, and between the OP value of C – B, D – E atoms and that of C – I, D –H atoms for Ti19N, compared with those for Ti19. This indicates that bonds between Ti atoms of the octahedron occupied by N atom at the center, as the octahedron ABCDEF in Fig. 3(c), weaken, whereas bonds between Ti atoms of the octahedron not occupied by N atoms, as the octahedron ACDHIJ, are not changed, or rather strengthen a little. The weakening of bonds caused by the occupation of the O sites by N atoms promotes expansion of the spacing between (001)-planes, which results in the TEM-observed lattice expansion in the c-axis of hcp-Ti by N implantation [11]. On the other hand, the OP of Ti– N bonds as A –G and F – G bonds become relatively large compared with the OP of Ti– Ti bonds in the octahedron ABCDEF in Fig. 3(c). Thus, it can be considered that the strengthening of the A –G and F –G bonds promotes the shear in the FL < 010> direc-
tion on the (001)-plane including B, E, F atoms in Fig. 3(c). This shear direction corresponds to the direction of BS in Fig. 6(a) of the previous paper [8]. That is, in order to obtain an fcc sublattice by the hcp-fcc transformation, the atoms on the (001)-plane including B, E, F atoms in Fig. 3(c) have to be shifted. The direction of shift is the FL < 010> direction. After the shift, the F atom, e.g., has to be at the center of gravity of the BEF triangle. The projected line of line FA to the (001)-plane including B, E, F atoms in Fig. 3(c) is on the line FL. It should be noted that the force in the EF direction induced by strong E – G and C –G bonds, or that in the BF direction induced by strong B –G and D –G bonds, does not crystallographically contribute to the forming of fcc-Ti. Thus, the shift of the F atom to the center of gravity of the triangle BEF promoted by the forming of the strong A –G and F –G bonds and the weakening of the C – B and D – E bonds in
Table 1 Bond overlap populations between Ti atoms, Ti and H atoms, and Ti and N atoms for the employed models
Table 2 Overlap population of each bond between Ti atoms for the Ti19 and Ti19N cluster models shown in Fig. 3(a) and (c)
Cluster model
Ti – Ti OP
Ti – H OP
Ti19 Ti19N Ti13H8 Ti14N13
0.231 0.135 0.083 0.042
Cluster model
A – B, A – C, A – D, A – E atoms
A – I, A – H atoms
C – B, D – E atoms
C – I, D – H atoms
0.184
Ti19 Ti19N
0.231 0.135
0.231 0.254
0.319 0.200
0.319 0.348
Ti – N OP 0.277 0.289
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Fig. 3(c) can be considered to be the origin for the hcp-fcc transformation of Ti sublattices.
4. Conclusions Nitriding of epitaxially grown Ti films by N implantation were studied by applying in situ EELS combined with DV-Xa calculations. The Ti films at RT consisted of hcpTi and CaF2-type TiHx. H atoms constituting TiHx were released from the films with heating. The H-released unstable fcc-Ti sublattice was transformed into hcp-Ti. The loss peak by EELS for TiHx shifted to the lower energy side due to the release of H atoms from TiHx during heating. The TiNy is ‘‘epitaxially’’ formed by the N implantation into the hcp-Ti films held at 350 jC, through the transformation of the hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of O sites of the fcc-Ti by N atoms. The loss peak during N implantation into hcpTi films gradually shifted to the higher energy side with the increase in dose, due to the increase in the electron density in the hybridised valence band. The bonding interaction of Ti sublattices with N atoms gives rise to the forming of stronger covalent bonds, which promotes the weakening of Ti– Ti bonds. Thus, it is considered that the shear in the < 010> direction of hcp-Ti promoted by the forming of the strong Ti– N bonds and the weakening of the Ti– Ti bonds is the origin for the hcp-fcc transformation of Ti sublattices.
Acknowledgements This work was partially supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
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Nitriding transformation of titanium thin films by nitrogen implantation Y. Kasukabe a,*, J.J. Wang b, T. Yamamura b, S. Yamamoto c, Y. Fujino a a
International Student Center/Department of Electronic Engineering, Tohoku University, Kawauchi, Aoba, Sendai 980-8576, Japan b Department of Metallurgy, Tohoku University, Aramaki-Aza-Aoba 02, Aoba, Sendai 980-8579, Japan c Department of Material Development, Japan Atomic Energy Research Institute-Takasaki, 1233 Watanuki, Takasaki 370-1292, Japan Available online 23 July 2004
Abstract Changes of electronic structure during nitrogen implantation into heated Ti thin films, which were accompanied by crystallographic structural transformation, were studied by applying in situ electron energy loss spectroscopy (EELS) combined with self-consistent charge discrete variational (DV)-Xa molecular orbital (MO) calculations. Hydrogen atoms constituting TiHx were released from the films with heating at temperatures more than about 100 jC. The H-released unstable fcc-Ti was transformed into hcp-Ti. The energy loss peak observed by EELS for TiHx shifted to lower energies with the release of H atoms during heating. Nitrogen ions of N+2 with 62 keV were implanted into the hcp-Ti films held at 350 jC. The TiNy films were ‘‘epitaxially’’ formed by the transformation of hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral sites of the fcc-Ti sublattice by N atoms. The energy loss peak for hcp-Ti films observed by EELS during N implantation gradually shifted to higher energies with increasing the dose. The shift meant increase in the density of electrons occupying the hybridized valence band. The formation mechanism of TiNy was discussed with the results of the calculated overlap populations of Ti – Ti and Ti – N bonds. D 2004 Elsevier B.V. All rights reserved. PACS: 68.37.Lp.; 68.55.Jk.; 81.15.Aa.; 85.40.Ry Keywords: Ion implantation; TiN; Fcc-hcp transformation; In situ TEM; EELS
1. Introduction Titanium nitride (TiNy) of NaCl-type has widely been used for many purposes such as a hard wear-resistant coating in cutting tools, a diffusion barrier in microcircuits, a corrosion and abrasion resistant layer on optical components, etc. [1– 5]. Although its elastic and diffusion-barrier properties are highly anisotropic and depend strongly upon film orientation, the formation mechanisms of preferred orientation of epitaxial TiN film are not sufficiently understood. Thus, much interest has been focused on epitaxial film formation mechanisms with preferred orientation [6,7]. Recently, it was reported that NaCl-type (110)- and (001)oriented TiNy crystallites were ‘‘epitaxially’’ grown by nitrogen (N) implantation into CaF2-type (110)-oriented TiHx and (035)-oriented hcp-Ti in deposited Ti films held at room temperature (RT) [8,9]; here, the term ‘‘epitaxially’’ is used to denote the situation of partially inheriting the atomic arrangements of unimplanted TiHx and hcp-Ti, re* Corresponding author. Tel.: +81-22-217-7775; fax: +81-22-217-7978. E-mail address: [email protected] (Y. Kasukabe). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.082
spectively. However, the detailed ‘‘epitaxial’’ nitriding process of Ti films without the domains of TiHx, and the change of the bonding interaction of Ti atoms with ligand of N atoms during N implantation have not yet been understood. The purpose of this study is to throw light on the process of H release and to elucidate the nitriding process of pure hcp-Ti without TiHx. In this paper, the changes of the electronic structures induced by the heating of as-deposited Ti films and by N implantation were examined by applying in situ electron energy loss spectroscopy (EELS). The results of EELS, which were acquired in the imaging mode by using the Gatan model 666 PEELS spectrometer, were discussed along with the results of a self-consistent charge discrete variational (DV)Xa molecular orbital (MO) calculation [10] combined with transmission electron microscope (TEM) observations [11].
2. Experimental Detailed descriptions of the preparation method of the deposited-Ti films were presented in an earlier paper [8]. The 100-nm-thick Ti films separated from NaCl substrates
Y. Kasukabe et al. / Thin Solid Films 464 – 465 (2004) 180–184
were heated up to 350 jC at a heating rate of 2 jC/min in a 400 kV analytical and high resolution TEM combined with ion accelerators at JAERI-Takasaki [12], in order to release H atoms from as-deposited Ti films. The implantations of N2+ ions with 62 keV into the Ti films held at 350 jC were performed in the TEM. According to a Monte Carlo simulation using the TRIM85 code, the mean projected range and full width at half maximum of the depth profile of N2+with 62 keV were 55 and 66 nm, respectively, and thus most of the implanted ions were to be retained inside the Ti films. The N concentrations in Ti films were evaluated from the implantation dose measured by a Faraday cage. The maximum dose in this experiment was 5.40 1017 ions/ cm2, which corresponded to the average atomic concentration value of 0.954 of N/Ti ratio in the Ti films.
3. Results and discussion In situ observations by TEM, the detailed results of which were shown in the other paper [11], elucidated that Ti films grown on NaCl (001) surfaces at RT consisted of hcp-Ti and CaF2-type TiHx (xi1.5), and that H atoms which constituted TiHx were released from the films with heating and were completely released at 350 jC. The Hreleased unstable fcc-Ti sublattice was transformed into hcpTi. Thus, nitrogen ions were implanted into the hcp-Ti films without TiHx, which were held at 350 jC in order to prevent the Ti films from absorbing the hydrogen atoms. The NaCltype TiNy is ‘‘epitaxially’’ formed in N-implanted hcp-Ti by
181
the transformation of the hcp-Ti to fcc-Ti, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral (O) sites of the fcc-Ti by N atoms [11]. Here, the hcp-Ti lattice was expanded by N occupation and strain due to the lattice expansion was considered as a driving force for the hcp-fcc transformation of Ti sublattices, which will be discussed later. Fig. 1 shows the variation of energy loss spectra with the temperature of the films. The loss peak ( f 47.0 eV) denoted by the line of Ti– M2,3 corresponds to the loss peak due to the excitation from 3p 1/2 and/or 3/2 states of Ti atoms to the conduction band. It should be noted that the Ti– M2,3 peaks are almost invariant with temperature, which means that the energies of the core levels are almost invariant during this experiment. The energy loss of f 17.0 eV, indicated by a solid triangle for hcp-Ti at RT in Fig. 1(b), was found to agree well with the theoretical value of f 17.6 eV, calculated with the assumption that 3d and 4s electrons are all free [13]. For TiHx in Fig. 1(a), the energy loss peak of f 19.5 eV, indicated by a solid triangle at RT, is due to excitation of plasmons by electrons in the valence band consisting of bonding states formed by Ti 3d– 4p and H 1s orbitals, which is in agreement with the energy loss reported by Thomas [14]. The higher loss energy for TiHx than for hcp-Ti at RT can be considered to reflect the existence of additional electrons from H atoms, which result in an increase in the electron density of the hybridized band of Ti 3d– 4p and H 1s orbitals. The energy of the loss peaks indicated by solid triangles for TiHx in Fig. 1(a) shifts to lower energies with increasing film temperature. This means
Fig. 1. Variation of EELS spectra with the film temperature. Solid triangles indicate the loss peaks due to plasmon excitation. (a) Heating of TiHx grown in the deposited Ti films. (b) Heating of hcp-Ti grown in the deposited Ti films.
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that the electron density in the valence band consisting of Ti 3d –4p and H 1s bonding states decreases with the increasing in film temperature due to the release of H atoms from the TiHx. Thus, it is considered that the excitation of electrons of Ti 3d – 4p and H 1s bonding states to the antibonding states has been induced by heating the TiHx up to 100 –150 jC, and that the increase in the number of phonons by heating also contributes to make the excitation of electrons of the bonding states to the antibonding states easy. Fig. 2 shows the variation of energy loss spectra with the dose of N implantation into hcp-Ti at 350 jC. The energy loss peaks due to plasmon excitation indicated by solid triangles gradually shifted to higher energies with increasing dose. This means that the number of N atoms bonding to Ti atoms increases with the dose, and that the electron density in the hybridised N 2p/Ti 3d – 4p valence band also increases with the dose. The loss peak ( f 16.0 eV) denoted by the dotted line connected to the solid circle can be seen at N/Ti z 0.38. This peak is considered to be due to excitation of N 2s electrons to the conduction band [3]. To investigate changes in the electronic structures of asdeposited and N-implanted Ti films in detail, DV-Xa MO calculations have been performed for four cluster models shown in Fig. 3(a) – (d). The Ti19 cluster of Fig. 3(a) corresponds to a part of the hcp-Ti structure. The Ti –Ti distances are taken to be 0.29238 nm, corresponding to those of the ideal bulk crystal structure. Fig. 3(b) shows the
Fig. 2. Variation of EELS spectra with the dose of N ions. Solid triangles indicate the loss peaks due to plasmon excitation. The energy loss of f 17.0 eV was found to agree well with the theoretical value of f 17.6 eV calculated assuming the 3d and 4s electrons are all free [13].
Ti13H8 cluster of CaF2-type structure. The hydrogen atom as indicated by E occupies the central position (tetrahedral (T) site) of the tetrahedron as formed by A –D atoms in the fccTi sublattice. The Ti – Ti distances are assumed to be 0.31183 nm, corresponding to those of the observed TiHx in as-deposited Ti films. A Ti19N cluster is shown in Fig. 3(c). The nitrogen atom, indicated by G, occupies the central position (O site) of the octahedron as formed by A – F atoms in the hcp-Ti cluster of Fig. 3(a). The Ti– Ti distances are assumed to be 0.29238 nm, which is the same as those of the Ti19 cluster. A Ti14N13 cluster model for NaCl-type TiN is shown in Fig. 3(d). The nitrogen atom as indicated by G occupies the O sites of the octahedron as formed by A – F atoms in the fcc-Ti sublattice. The Ti – N distances are assumed to be 0.21200 nm, corresponding to those of the bulk crystal structure of TiN. Table 1 shows the calculated Mulliken bond overlap populations (OP) between Ti atoms, Ti and H atoms, and Ti and N atoms for the employed models. The values of OP refer to the strength of covalent bonds. The Ti –Ti OP for the Ti19 cluster is evaluated to be 0.231. The OP between Ti atoms such as B and C atoms bonding to the nitrogen G atom for the Ti19N cluster in Fig. 3(c) is evaluated to be 0.135, which is much smaller than that for the Ti19, irrespective of the same Ti– Ti bond length. The Ti– N OP for the Ti19N is larger than the Ti– Ti OP for the Ti19. The OP between the central B atom and the surrounding atom as A, C, and D atoms in Fig. 3(b) for the Ti13H8 cluster is evaluated to be 0.083, which is much smaller than that for the Ti19, while the OP value of Ti–H covalent bonds is 0.184, which is smaller than that of Ti– N covalent bonds for the Ti19N. Therefore, it is considered that the formation of Ti –H covalent bonds leads to weakening Ti –Ti bonds. In other words, the release of H atoms from the TiHx results in forming stronger Ti – Ti bonds and contracting the TiHx lattice. The OP between the face-centered A atom and the atoms B, C, D and E in Fig. 3(d) for the Ti14N13 cluster is evaluated to be 0.042, which is much smaller than that for the Ti19, while the OP value of Ti–N covalent bonds is 0.289, which is larger than that of Ti– Ti covalent bonds for the Ti19. Comparing the values of OP of Ti14N13 with those of Ti19 and Ti19N, it can be seen that the occupation of the O sites by N atoms leads to a decrease in the OP values of Ti– Ti bonds, whereas the OP values of Ti– N covalent bonds does not differ so much. This means that the occupation of the O site by N atoms gives rise to a weakening of Ti–Ti bonds and to an increase in the number of Ti– N bonds with the increase in implanted N atoms. An increase in the number of Ti– N bonds heralds the onset of nitriding of Ti films. The OP value of each bond between Ti atoms for Ti19 and Ti19N cluster models in Fig. 3(a) and (c) has been inspected in detail, and is shown in Table 2. Table 2 shows the existence of relatively large differences between the
Y. Kasukabe et al. / Thin Solid Films 464 – 465 (2004) 180–184
183
Fig. 3. Schematic illustrations of (a) Ti19 cluster, (b) Ti13H8 cluster, (c) Ti19N cluster and (d) Ti14N13 cluster models. Solid, large open and small open circles represent Ti, N and H atoms, respectively.
OP value of A –B, A –C, A –D, A –E atoms and that of A – I, A – H atoms, and between the OP value of C – B, D – E atoms and that of C – I, D –H atoms for Ti19N, compared with those for Ti19. This indicates that bonds between Ti atoms of the octahedron occupied by N atom at the center, as the octahedron ABCDEF in Fig. 3(c), weaken, whereas bonds between Ti atoms of the octahedron not occupied by N atoms, as the octahedron ACDHIJ, are not changed, or rather strengthen a little. The weakening of bonds caused by the occupation of the O sites by N atoms promotes expansion of the spacing between (001)-planes, which results in the TEM-observed lattice expansion in the c-axis of hcp-Ti by N implantation [11]. On the other hand, the OP of Ti– N bonds as A –G and F – G bonds become relatively large compared with the OP of Ti– Ti bonds in the octahedron ABCDEF in Fig. 3(c). Thus, it can be considered that the strengthening of the A –G and F –G bonds promotes the shear in the FL < 010> direc-
tion on the (001)-plane including B, E, F atoms in Fig. 3(c). This shear direction corresponds to the direction of BS in Fig. 6(a) of the previous paper [8]. That is, in order to obtain an fcc sublattice by the hcp-fcc transformation, the atoms on the (001)-plane including B, E, F atoms in Fig. 3(c) have to be shifted. The direction of shift is the FL < 010> direction. After the shift, the F atom, e.g., has to be at the center of gravity of the BEF triangle. The projected line of line FA to the (001)-plane including B, E, F atoms in Fig. 3(c) is on the line FL. It should be noted that the force in the EF direction induced by strong E – G and C –G bonds, or that in the BF direction induced by strong B –G and D –G bonds, does not crystallographically contribute to the forming of fcc-Ti. Thus, the shift of the F atom to the center of gravity of the triangle BEF promoted by the forming of the strong A –G and F –G bonds and the weakening of the C – B and D – E bonds in
Table 1 Bond overlap populations between Ti atoms, Ti and H atoms, and Ti and N atoms for the employed models
Table 2 Overlap population of each bond between Ti atoms for the Ti19 and Ti19N cluster models shown in Fig. 3(a) and (c)
Cluster model
Ti – Ti OP
Ti – H OP
Ti19 Ti19N Ti13H8 Ti14N13
0.231 0.135 0.083 0.042
Cluster model
A – B, A – C, A – D, A – E atoms
A – I, A – H atoms
C – B, D – E atoms
C – I, D – H atoms
0.184
Ti19 Ti19N
0.231 0.135
0.231 0.254
0.319 0.200
0.319 0.348
Ti – N OP 0.277 0.289
184
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Fig. 3(c) can be considered to be the origin for the hcp-fcc transformation of Ti sublattices.
4. Conclusions Nitriding of epitaxially grown Ti films by N implantation were studied by applying in situ EELS combined with DV-Xa calculations. The Ti films at RT consisted of hcpTi and CaF2-type TiHx. H atoms constituting TiHx were released from the films with heating. The H-released unstable fcc-Ti sublattice was transformed into hcp-Ti. The loss peak by EELS for TiHx shifted to the lower energy side due to the release of H atoms from TiHx during heating. The TiNy is ‘‘epitaxially’’ formed by the N implantation into the hcp-Ti films held at 350 jC, through the transformation of the hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of O sites of the fcc-Ti by N atoms. The loss peak during N implantation into hcpTi films gradually shifted to the higher energy side with the increase in dose, due to the increase in the electron density in the hybridised valence band. The bonding interaction of Ti sublattices with N atoms gives rise to the forming of stronger covalent bonds, which promotes the weakening of Ti– Ti bonds. Thus, it is considered that the shear in the < 010> direction of hcp-Ti promoted by the forming of the strong Ti– N bonds and the weakening of the Ti– Ti bonds is the origin for the hcp-fcc transformation of Ti sublattices.
Acknowledgements This work was partially supported by a Grant-in Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
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