319-326 Conference-ISSS-4-Carbonizing Processes of Titanium Thin Films by Carbon-Implantation

Observations of the crystallographic structural changes of deposited Ti films (100 nm thick) during Cimplantation were made by using transmission electron microscope (TEM) for the purpose to elucidate the carbonizing processes of Ti films. The Ti films were deposited by an electron beam heating method onto thermally cleaned NaCl (001) surfaces held at room temperature (RT). Both the 200 kV conventional TEM and the 400 kV analytical high resolution TEM combined with ion accelerators were used. The deposited Ti films consisted of mainly (03 · 5)-oriented hcp-Ti and (110)-oriented CaF2 type TiHx (x ∼ 1.5). Carbon ions (C) with 26 keV were implanted into the Ti films held at RT in the 400 kV TEM. The H atoms which constituted TiHx were released from the as-deposited Ti films during the early stage of C-implantation. In the C-implanted Ti film (C/Ti=0.54), there coexisted (001)and (110)oriented NaCl-type TiCy and a small amount of hcp-Ti. The results of TEM observation indicated that (001)and (110)-oriented TiCy were epitaxially formed by the transformation of (03 ·5)oriented hcp-Ti and (110)-oriented TiHx, respectively. In the cases, the hcp-Ti sublattice is transformed into fcc-Ti sublattice of TiCy with inheritance of square atomic arrangement of hcp-Ti. The hcp-fcc transformation was discussed with the results of a self-consistent charge discrete variational (DV)-Xα molecular orbital (MO) calculation. [DOI: 10.1380/ejssnt.2005.319]


I. INTRODUCTION
Titanium carbide (TiC) films, prepared by various techniques such as pulsed laser deposition (PLD) [1], chemical vapor deposition (CVD) [2,3], evaporation [4] and sputtering [5][6][7][8], are a class of materials with many desirable physical properties, including extreme hardness, high melting point, excellent electrical conductivity, magnetic susceptibility and so on, which make them outstanding candidates for many applications in various environments [9][10][11][12].Recently, it was reported that these properties were related to crystallographic textures and/or orientations of TiC films [13][14][15][16].Therefore, much interest has been focused on crystallographic orientation and epitaxial growth of TiC.As for epitaxial growth of TiC, especially the atomistic mechanism of carbonizing processes by Cimplantation is not understood clearly.
The purpose of this work is first, to examine the TiC y films prepared by C-implantation, applying transmission electron microscope (TEM) observation and Rutherford backscattering spectrometry (RBS), and then to discuss the formation mechanism of TiC y films.We focus our attention on the transformation process from the oriented hcp-Ti to preferentially oriented TiC y in the film with the dose of C, taking account of the variation of the lattice constants of hcp-Ti and TiC y .The results were also discussed with those of a self-consistent charge discrete variational (DV)-Xα molecular orbital (MO) calculation [17] and the epitaxial hcp-fcc transformation mechanism of hcp-Ti by N-implantation [18,19].

II. EXPERIMENTAL
The 100-nm-thick Ti films were deposited by an electron-beam heating method in an ultrahigh vacuum onto thermally cleaned NaCl substrates held at room temperature (RT), with a deposition rate of 10 nm/min.The detailed descriptions of the preparation of Ti films were presented in earlier papers [18,19].Carbon ions (C + ) of 26 keV were implanted into the deposited Ti films at an angle of 30 • to the surface normal in the 400 kV analytical and high resolution TEM combined with ion accelerators at JAERI-Takasaki [20].In this paper, the average concentration ratio, C/Ti, is estimated from the dose measured by a Faraday cage.The maximum dose in this experiment was 3.05 ×10 17 ions/cm 2 , which corresponded to the C/Ti ratio of 0.54.This C/Ti ratio is not equal to the value of y in TiC y , because both hcp-Ti and TiC y crystallites exist in C-implanted Ti films and it is conceivable that the concentration of C in hcp-Ti crystallites is less than that in TiC y crystallites.Therefore, in the present experiment, C/Ti ratios indicate the average concentration of implanted C atoms compared to Ti atoms ISSN 1348-0391 c 2005 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) in the films.The average concentration of C in Ti films could also be evaluated from the RBS spectrum in this experiment.
Figures 1(a) and 1(b) show, respectively, a typical electron diffraction (ED) pattern and a bright field (BF) image taken from the Ti film deposited on the NaCl substrate held at RT.The BF image of Ti film shows bandlike contrast elongated in the < 110 > direction of the substrate, as indicated by a white arrow in Fig. 1(b).An analysis of Fig. 1(a) indicates that hcp-Ti (lattice constants: a = 0.296 nm, c = 0.471 nm) and CaF 2 -type TiH x (x ∼ 1.5 [21]; lattice constant: a = 0.441 nm) grow in this film.The reflections indicated by the fourindex system and three-index system with an asterisk are obtained from the hcp-Ti and TiH x , respectively.The results of growth of both hcp-Ti and TiH x agree with those of previous papers [19,22].The orientation relationships between the hcp-Ti and the NaCl substrate are  from the hcp-Ti and TiC y , respectively.An analysis of Fig. 3(a) indicates that NaCl-type TiC y (lattice constant: a = 0.430 nm) and hcp-Ti (lattice constants: a = 0.299 nm, c = 0.478 nm) coexist in this film, and that no reflections of TiH x can be seen.This means that H atoms are gradually released from TiH x during C-implantation.http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp)  reflection of (110)-oriented TiC y and from the 020 reflection of (001)-oriented TiC y , respectively.It can be seen from Fig. 5(a) that hcp-Ti (lattice constants: a = 0.299 nm, c = 0.479 nm) and NaCl-type TiC y (lattice constant: a = 0.432 nm) coexist in this film.Note that reflections of TiH x cannot be seen.Furthermore, it is revealed from Fig. 5   Typical RBS spectra taken from the unimplanted Ti film and the C-implanted Ti film with C/Ti=0.54 are shown in Fig. 6.It can be seen that after the Cimplantation the peak of C appears clearly throughout the depth of the film, whereas the height of the Ti peak decreases slightly owing to the existence of C in the Ti film.The small peak of oxygen (O) may be due to CO, CO 2 , and/or H 2 O molecules adsorbed on the film surface during the transfer of the sample to the RBS measurement chamber.The C/Ti value evaluated from the RBS spectrum was 0.53, which was in agreement with the C/Ti value of 0.54 estimated from the implantation dose measured by a Faraday cage.This fact means that implanted C atoms did not escape from the Ti films during the implantation at least up to about C/Ti=0.54.
To investigate the electronic structures of as-deposited and C-implanted Ti films and changes of bonding natures by C-implantation, DV-Xα MO calculations have been performed for four cluster models shown in Figs.7(a)-(d).The Ti 19 cluster of Fig. 7(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. 7(b) shows the Ti 13 H 8 cluster of CaF 2 -type structure.The hydrogen atom as indicated by E occupies the central position of the tetrahedron as formed by A-D atoms in the fcc-Ti sublattice.The Ti-Ti distances are assumed to be 0.31183 nm, corresponding to those of the observed TiH x in as-deposited Ti films.A Ti 19 C cluster is shown in Fig. 7(c).The carbon 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. 7(a).The Ti-Ti distances are assumed to be 0.29238 nm, which is the same as those of the Ti 19 cluster.A Ti 14 C 13 cluster model for NaCl-type TiC is shown in Fig. 7(d).The carbon 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-C distances are assumed to be 0.21600 nm, corresponding to those of the bulk crystal structure of TiC.
Table I shows the calculated Mulliken bond overlap populations (OP) between Ti atoms, Ti and H atoms, and Ti and C atoms for the employed models.The values of OP refer to the strength of covalent bonds.The Ti-Ti OP value for the Ti 19 cluster is evaluated to be 0.231.The OP value between Ti atoms such as B and C atoms bonding to the carbon G atom for the Ti 19 C cluster in Fig. 7(c) is evaluated 0.143, which is much smaller than that for the Ti 19 , irrespective of the same Ti-Ti bond length.The Ti-C OP value for the Ti 19 C is larger than the Ti-Ti OP value for the Ti 19 .
In Fig. 7(b) for the Ti 13 H 8 cluster, the OP value between the central B atom and the surrounding atom such as A, C and D atoms is evaluated to be 0.083, which is much smaller than that for the Ti 19 , while the OP value of Ti-H covalent bond is 0.184, which is smaller than that of Ti-C covalent bonds for the Ti 19 C. Therefore, it is considered that the release of H atoms from the TiH x and the occupation of O-sites by C atoms result in forming TiC compounds with high hardness and strength.
The OP value between the face-centered A atom and the atoms B, C, D and E in Fig. 7(d) for the Ti 14 C 13 cluster is evaluated to be 0.046, which is much smaller than that for the Ti 19 , while the OP value of Ti-C covalent bonds is 0.358, which is larger than that of Ti-Ti covalent bonds for the Ti 19 .Comparing the values of OP of Ti 14 C 13 with those of Ti 19 and Ti 19 C, it can be seen that the occupation of the O-sites by C atoms leads to a decrease in the OP values of Ti-Ti bonds, whereas the OP values of Ti-C covalent bonds do not differ so much.This means that the occupation of the O-sites by C atoms gives rise to a weakening of Ti-Ti bonds and to an increase in the number of Ti-C bonds with the increase in the number of implanted C atoms.Thus, the increase in the number of Ti-C bonds heralds the onset of carbonizing of Ti films.

A. Growth of (110)-oriented TiCy
As is apparent from Fig. 1, the (110)-oriented TiH x grows mainly in the band-like contrast region elongated in the < 110 > direction of the (110)-oriented TiH x and of the NaCl substrate.Moreover, the analysis of Figs. 3 and  5 elucidates that the (110)-oriented TiC y also exists in the band-like contrast region elongated in the < 110 > direction of the (110)-oriented TiC y and of the NaCl substrate.Therefore, the (110)-oriented TiC y in the band-like contrast region is considered to be formed by carbonizing the (110)-oriented TiH x in the as-deposited Ti film, accompanied by the release of H atoms from the TiH x and by the occupation of O-sites of the H-escaped fcc-Ti sublattice by C atoms.It is noteworthy that this process does not change the orientation of the fcc-Ti sublattice.

B. Growth of (001)-oriented TiCy
Careful comparison of Fig. 1 with Figs. 3 and 5 indicates that the (03 • 5)-oriented hcp-Ti is preferentially grown outside the band-like contrast region in the deposited Ti film, whereas that of the (001)-oriented TiC y http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp)The orientation relationship between (03 • 5)-oriented hcp-Ti and (001)-oriented fcc-Ti, which is the Ti sublattice of TiC y , is shown in Fig. 8.The closed circles represent Ti atoms.The (001)-oriented TiC y are epitaxially formed by the transformation of (03 • 5)-oriented hcp-Ti sublattice to (001)-oriented fcc-Ti sublattice during the C-implantation, inheriting the square EFJK and the octahedron GEFJKL in Fig. 8(a) as well as by the occupation of O-sites of the (001)-oriented fcc-Ti sublattice by C atoms.The shaded square EFJK in Fig. 8(a) is the (03 • 4) plane.A dihedral angle of the (03 • 4) plane and (03 • 5) plane which is parallel to the substrate surface is about 6.2 • .Therefore, the former also is nearly parallel to the substrate surface.In order to obtain an fcc-Ti structure as shown in Fig. 8(b), the B atom in Fig. 8 rather strengthen a little.The weakening of bonds caused by the occupation of the O-sites by C atoms promotes expansion of the spacing between (00 • 1)-planes, which results in the TEM-observed lattice expansion in the caxis of hcp-Ti by C-implantation.On the other hand, the OP value of Ti-C 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. 7(c).Thus, it can be considered that the strengthening of the A-G and F-G bonds promotes the shear in the FL < 01 • 0 > direction on the (00 • 1)-plane including B, E and F atoms in Fig. 7(c).This shear direction corresponds to the direction of BS in Fig. 8(a).Then, in order to obtain an fcc sublattice by the hcp-fcc transformation, the atoms on the (00 • 1)plane including B, E and F atoms in Fig. 7(c) have to be shifted in the FL < 01 • 0 > direction.After the shift, the F atom, e.g., has to be at the center of gravity of the triangle BEF.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 Fig. 7(c) can be considered to be the origin for the hcp-fcc transformation of Ti sublattices.

V. CONCLUSIONS
Carbon ions (C + ) with 26 keV were implanted into the deposited Ti film, which consisted of mainly (110)oriented TiH x and (03 • 5)-oriented hcp-Ti grown on NaCl substrates held at RT.The C-implantation results in the epitaxial formation of (110)-oriented and (001)-oriented TiC y .The epitaxial formation processes of these oriented TiC y are as follows: ( 3 ) The hcp-fcc transformation of Ti sublattices is induced by the shear in the < 01 • 0 > direction of hcp-Ti promoted by the weakening of Ti-Ti bonds and the forming of strong Ti-C bonds.

FIG. 1 :
FIG. 1: ED pattern (a) and BF image (b) and DF images (c) and (d) taken from a 100-nm-thick Ti film grown on a NaCl substrate held at RT. (c) and (d) were taken with the 002 * reflection of TiHx and the 11 • 1 reflection of hcp-Ti, respectively.Band-like contrast region elongated in the < 110 > direction of the substrate is indicated by a white arrow in (b)-(d).

(03 • 5 )
Figures1(a) and 1(b) show, respectively, a typical electron diffraction (ED) pattern and a bright field (BF) image taken from the Ti film deposited on the NaCl substrate held at RT.The BF image of Ti film shows bandlike contrast elongated in the < 110 > direction of the substrate, as indicated by a white arrow in Fig.1(b).An analysis of Fig.1(a) indicates that hcp-Ti (lattice constants: a = 0.296 nm, c = 0.471 nm) and CaF 2 -type TiH x (x ∼ 1.5[21]; lattice constant: a = 0.441 nm) grow in this film.The reflections indicated by the fourindex system and three-index system with an asterisk are obtained from the hcp-Ti and TiH x , respectively.The results of growth of both hcp-Ti and TiH x agree with those of previous papers[19,22].The orientation relationships between the hcp-Ti and the NaCl substrate are (03 • 5)Ti//(001)NaCl and [21 • 0]Ti//[110]NaCl, hereafter called (03 • 5)-oriented hcp-Ti, and (21 • 0)Ti//(001)NaCl and [00 • 1]Ti//[100]NaCl, hereafter called (21 • 0)-oriented hcp-Ti.The (03 • 5)-oriented hcp-Ti gives rise to the 11 • 1 reflection, for example, whereas the 00 • 2 reflection stems from (21 • 0)-oriented hcp-Ti.As can be seen from the ED intensity of Fig. 1(a), the growth of the (03 • 5)-oriented hcp-Ti is preferred.Schematic representation and indexing of ED patterns of these oriented hcp-Ti and TiH x crystallites are given in Fig. 2. Solid and open circles represent reflections of (03 • 5)-oriented hcp-Ti, whereas solid and open triangles represent reflections of (21 • 0)-oriented FIG. 4: Schematic representation and indexing of ED patterns of TiCy in Fig. 3(a).The reflections marked by solid circles are due to (001)-oriented TiCy, whereas those marked by solid and open squares are due to (110)-oriented TiCy.Rotation of the set of solid squares about the origin by 90 • gives rise to the reflections marked by open squares.
FIG. 5: ED pattern (a) and BF image (b) and DF images (c) and (d) taken from a C-implanted Ti film with C/Ti = 0.54.(c) and (d) were taken with 002 and 020 reflections in (a), respectively.
FIG. 8: Schematic representation of the carbonizing process of hcp-Ti.In (a), the shaded square EFJK of (03 • 5)-oriented hcp-Ti is nearly parallel to the substrate surface.In order to obtain a fcc-Ti structure as shown in (b), the B atom in (a) has to be shifted to the center of gravity of the triangle ABQ.Solid circles labeled as alphabetical character with an asterisk in (b) represent Ti atoms on the (111) plane of the fcc-structure.Redrawing of (b) in view of the cubic symmetry leads to the well-known fcc-structure in (c).It should be noted that the square EFJK and the octahedron GEFJKL in (a) correspond to those in (c) and preserved in this hcp-fcc transformation.

( 1 )
The (110)-oriented TiC y in the band-like contrast region as shown in Fig. 3(b) is formed by carbonizing the (110)-oriented TiH x in the as-deposited Ti film without changing the orientation of the fcc-Ti sublattice, accompanied by the release of H atoms from the TiH x and by the occupation of its O-sites of H-escaped fcc-Ti sublattice by C atoms.( 2 ) The (001)-oriented TiC y is epitaxially formed by the transformation of (03 • 5)-oriented hcp-Ti to (001)oriented fcc-Ti during the C-implantation, inheriting the square atomic arrangement such as EFJK and/or the octahedron such as GEFJKL in Fig. http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp) Volume 3 (2005) Nishida, et al. 8(a), occupying O-sites of the fcc-Ti by C atoms.

TABLE I :
Bond overlap populations between Ti atoms, Ti and H atoms and Ti and C atoms for the employed models.TiC y crystallites grow preferentially outside the band-like contrast region.Therefore, it can be considered that (110)-oriented TiC y grown in the band-like contrast region is formed by carbonizing (110)oriented TiH x in the unimplanted Ti film, accompanied by the release of H atoms. On the other hand, (001)-oriented TiC y grown outside the band-like contrast region should be transformed from (03 • 5)-oriented hcp-Ti in Ti films by C-implantation.Moreover, the intensity of reflections of TiC y increases with C dose.This means that a higher dose of C-implantation leads to an increase in the areas of TiC y .