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Structure of Ti films deposited on MgO(001) substrates
Surface Science 454–456 (2000) 783–789 www.elsevier.nl/locate/susc
Structure of Ti films deposited on MgO(001) substrates T. Kado * Chugoku National Industrial Research Institute, 2-2-2 Hiro-Suehiro, Kure, Hiroshima 737-01, Japan
Abstract The growth and structure of Ti thin films deposited by electron beam evaporation at 273 K in a vacuum of about 10−7 Pa on MgO(001) have been studied by means of transmission electron microscopy. It is found that a thin layer, whose thickness is up to about 4 nm, is fcc (or tetragonal ) structure. When the thickness increases to more than about 6 nm, the structure gradually changes to an hcp structure with two different orientations: one is (00.1) //(001) //(001) , and the other is (03.5) //(001) . The electron diffraction patterns from 60 nm hcpTi fccTi MgO hcpTi fccTi thick Ti film also show reflection spots at {110} points on fcc Ti reciprocal plane, which show metastable c-hydride precipitation takes place in the layer. The orientation relationships and growth mechanism of the epitaxial Ti films on MgO(001) are discussed using molecular dynamics simulations performed by Hara et al. for MBE growth of a Lennard-Jones system. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Magnesium oxides; Molecular dynamics; Titanium
1. Introduction The epitaxial layering of metal can often change its crystal structures from that of the bulk. These modified phases are called pseudomorphous [1], and recently epitaxially grown superlattices have been prepared using pseudomorphism. For example, Ti/MgO superlattices grown on MgO(001) substrates were composed of rock-salt type MgO and tetragonal Ti (a =0.4170 nm, 0 c =0.4062 nm) which is quite different from the 0 Ti bulk structure of hcp [2,3]. The tetragonal Ti can be explained by the fcc Ti which is distorted to be fct Ti in the epitaxially grown layers. Fcc Ti was first reported in 1969 by Wawner and Lawless in thin films of Ti deposited on NaCl single crystals [4] and since then c-phase Ti of fcc * Fax: +81 823 73 3284. E-mail address: [email protected] (T. Kado)
(or fcc) structure has been often reported [5–9]. However, the lattice constant of fcc Ti is reported to be of various values between 0.401 and 0.442 nm and recently instead of fcc Ti, titanium hydride (CaF structure, a =0.441 nm) and titanium oxide 2 0 (NaCl structure, a =0.418 nm) [10–12] have been 0 suggested to explain the electron diffraction patterns of cubic structure Ti. The cubic structure Ti is reported to transform to the bulk hcp structure (a-Ti, a =0.295–0.296 nm, c =0.471–0.468 nm) 0 0 with various orientations when the film is between several nm and 300 nm thick, and the critical value depends on the conditions and kinds of substrates [9–12]. The transformed structures are very complex and the growth mechanism has not been well characterized. In the present paper, the structural changes of Ti films evaporated on MgO buffer grown on MgO (001) substrates with an increase of film thickness are discussed, in order to improve the crystalline quality of the Ti/MgO superlattices. The orienta-
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tion relationships and growth mechanism of the epitaxial Ti films on MgO(001) are also discussed using molecular dynamics simulations for MBE growth of a Lennard-Jones system [13].
2. Experimental procedure The Ti/MgO superlattices and the Ti monolayers were grown on MgO buffer layers, which were epitaxially grown on MgO(001) substrates by the electron beam evaporation method in a stainless chamber which was evacuated by a turbomolecular pump and a titanium sublimation pump. The base pressure was about 2×10−8 Pa. Commercially supplied Ti rod and MgO pellets were used for the evaporation sources, the purities of which are stated to be 99.9% and 99.99%, respectively. After charging, the Ti source was heated for more than 5 h by an electron beam to remove surface oxide and absorbed gases in the 10−7 Pa range. MgO (001) single crystals were used as the substrates. The substrates were heated at 1123 K for 180 s in a high vacuum to prepare a clean surface and then cooled to 773 K to form a MgO (001) buffer layer about 20 nm thick [14]. After that, the Ti films were grown on the substrates at 273 or 173 K. The pressure during the evaporation of Ti was under 5×10−7 Pa and the deposition rates of Ti was 0.02 nm s−1 [14]. Surface analysis of the thinnest layer studied was performed by transmission electron diffraction and high-resolution electron microscopic techniques using JEOL, JEM-2000EX. The samples were thinned by an ordinary ion milling method for electron microscopy [15]. The accelerating voltage is 200 kV and the diameter of the restricted area for transmission electron diffraction is 50 nm.
3. Experimental results 3.1. Transmission electron diffraction of thin Ti layer In the case of the Ti monolayers grown on MgO (001) buffer layer epitaxially at 173 K, RHEED patterns from the Ti layer showed streak
patterns almost the same as those from the MgO layer with fourfold symmetry, when the thickness of Ti layers are less than 4–6 nm [2]. The streak pattern changed to be a curved line pattern when the thickness was increased. This indicates the crystal structure of fcc Ti changed to other structures at the thickness of about 6 nm. Fig. 1a shows the selected area diffraction pattern from a 5 nm thick Ti layer. Bragg reflections slightly broaden in the ring direction showing an fcc (001) pattern. Fig. 1b shows the diffraction patterns from the cross-sectional region of a 5 nm thick Ti layer including an MgO buffer layer. Bragg reflections from the Ti layer and MgO buffer layer overlap each other, indicating that the electron diffraction pattern from the Ti layer is an fcc pattern. These results indicate the Ti has fcc structure, the lattice constant of which is estimated to be 0.425 nm. The orientation relationships between the Ti layer and the MgO buffer layer (MgO substrate) are considered to be (001) //(001) and [100] // Ti MgO Ti [100] . MgO 3.2. Transmission electron diffraction of a thick Ti layer The selected area diffraction patterns from various areas of a 60 nm thick Ti layer grown at 273 K are shown in Fig. 2. In Fig. 2a, many spots without halo rings are formed in lines in hh0 directions, which are considered to cause the curved lines of the RHEED pattern during the deposition. These spots are observed partially to form hexagonal or rhombic symmetry as shown Fig. 2a∞. These results indicate that, when the thickness of the Ti layer on MgO(001) increase to more than 4–6 nm, the crystal structure of fcc Ti changes to other structures, owing to the reduction of stress in the layer, partially conserving relationships with the atomic spacing of fcc Ti. As shown in Fig. 2(a∞), the many reflection spots on the reciprocal Ti (001) plane can be identified to be from hcp Ti with two kinds of orientations and fcc Ti: one is from (00.1) hcp Ti (marked as 1 and 1∞), another is from (03.5) hcp Ti (marked as 2 and 2∞) considering double diffraction with an underline and the other is from (001) fcc Ti (marked as F ). Because of the good crystallinity of the crystallites, many double
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Fig. 1. Selected area diffraction patterns from a 5 nm thick Ti layer. (a) Diffraction pattern obtained at the layer plane. (b) Diffraction pattern obtained in the cross-sectional region of the Ti layer including MgO buffer layer.
diffractions are observed. The reflections of 1 and 1∞, and 2 and 2∞ indicate two kinds of orientations of (00.1) hcp planes rotated by 90° with respect to each other and two kinds of (03.5) hcp planes rotated by 90° with respect to each other, respectively. The two kinds of orientation relationships between hcp Ti and fcc Ti may be (00.1) // hcpTi (001) //(001) , and (03.5) //(001) (or fccTi MgO hcpTi fccTi [12.1] ([011: 1] )//[001] ), which agree hcpTi hcpTi fccTi with the relationships proposed by Kasukabe [10] and Harada [11,12]. The transmission electron diffraction patterns from other areas are shown in Fig. 2b and c. Fig. 2b shows slightly broadened reflections from the (001) fcc Ti plane. Fig. 2c shows reflection spots from the (001) fcc Ti and from the (03.5) hcp Ti plane including other spots, which are observed at {110} points of fcc Ti reciprocal plane. It is noteworthy that {110} reflections are forbidden as reflections from an fcc structure. 3.3. High-resolution lattice images of a thick Ti layer High-resolution lattice images from two different areas of the same 60 nm thick Ti sample
are shown in Fig. 3. Dark and bright spots correspond to the atomic arrangement of Ti and the images indicate many areas with different orientations. Fig. 3a shows the lattice image from fcc Ti area. The inset is the Fourier transform obtained from atomic arrangement in the area labeled A, and this shows that the area labeled A consisted of a (001) fcc Ti crystallite. Fig. 3b shows the lattice image from an hcp Ti area. The insets, upper left and lower right, are the Fourier transforms obtained from atomic arrangements in the areas labeled B and C, and these show that the areas labeled B and C consisted of (00.1) and (03.5) hcp Ti crystallites, respectively.
4. Discussion The above high-resolution lattice images and the transmission electron diffraction of thin Ti film evaporated on MgO (001) have provided evidence that in the initial stage of the deposition Ti had a cubic structure, and at more than 6 nm of film thickness the cubic structure changed to an hcp structure with [00.1] or [12.1] orientation. As a result of the diffraction patterns shown in Fig. 2,
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Fig. 2. Selected area diffraction patterns from various area of a 60 nm thick Ti layer. (a) Area of an hcp Ti crystallite, (b) area of an fcc Ti crystallite, and (c) area of fcc Ti and hcp Ti including c-hydride precipitates. (a∞) The same area as shown in (a), showing (00.1) hcp Ti (marked as 1 and 1∞), (03.5) hcp Ti (marked as 2 and 2∞) considering double diffractions with underlines and the other is from (001) fcc Ti (marked as F ). 1 and 1∞ (2 and 2∞) indicate two kinds of orientations of (00.1) hcp planes ((03.5) hcp planes) rotating by 90° with respect to each other.
almost all spots except for the {110} spots can be explained by hcp Ti, and the {110} spots are considered to be from metastable a c-hydride precipitate which has an fcc structure and has often been observed in a-Ti specimens containing a small amount of hydrogen, 0.5–3 at.% [16,17]. We could not get clear evidence of TiO or of other
titanium hydrides. Since hydrides were observed only in specimens grown immediately after a fresh Ti charge was placed in the electron beam evaporation vessel [16 ] and only a few diffraction patterns showed {110} spots, the Ti layer contained a very small amount of hydride precipitate. It is considered that pseudomorphous fcc and/or
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Fig. 3. High-resolution lattice images from two different areas of the same 60 nm thick Ti layer. (a) The lattice image from fcc Ti area. The inset is the Fourier transform obtained from atomic arrangement in the area labeled A. (b) The lattice image from hcp Ti area. The insets upper left and lower right are the Fourier transforms obtained from atomic arrangement in the areas labeled B and C, and show that the areas labeled B and C consisted of (00.1) and (03.5) hcp Ti crystallites, respectively.
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Fig. 4. (a–c) Schematic illustrations of the top view of the layer deposited by molecular beam epitaxy for a two-component Lennard-Jones system, simulated by Hara et al. Lattice mismatch caused the introduction of composite discommensulation lines (DCLs) on the surface (a), and the DCL splits into two parallel DCLs with an increase of layering (b) and (c). (d ) Relationship between the (00.1), (03.2) and (03.4) planes of hcp Ti. The direction of the incident electron beam is from A to B, which is the [12.1] direction corresponding to the normal direction of the hcp (03.5) plane or the normal direction of the fcc (001) plane.
fct Ti is not stable when the film thickness increased by more than 6 nm and changed to bulk hcp structure. Concerning the relationships between cubic Ti and hcp Ti, Kasukabe et al. discussed in detail from the viewpoint of the relationships between bcc, fcc and hcp structures. However, since the structural change is caused by the increasing film thickness during the deposition, it is reasonable to discuss this using a dynamical model. Fig. 4d shows the relationship between the (00.1), (03.2) and (03.4) planes of hcp Ti. In electron diffraction, the direction of the incident electron beam is shown from A to B, which is the [12.1] direction, corresponding to the normal direction of the hcp (03.5) plane or the normal direction of the fcc (001) plane. The dihedral angle of the (03.4) and the (03.5) plane is only 6.2°. It is noteworthy that a part of (03.2) plane consisted of triangular lattices making a line and a part of (03.4) plane consisted of square lattices making a line. The line of the (03.2) plane
and the line of the (03.4) are arranged parallel and alternately. In order to simulate the process of molecular beam epitaxy for a two-component Lennard-Jones system, molecular dynamics simulations have been performed by Hara et al. [13]. They calculated the system where the atomic size of the incident beam is different from that of the substrate with a square lattice plane. In Fig. 4(a– c), schematic illustrations of the top view of the deposited layer are shown, which are some results of the simulation when the atomic size of the overlayer is larger than that of the substrate and the size mismatch (j) between the substrate and the overlayer atoms is 0.05%. Lattice mismatch results in the introduction of composite discommensulation lines (DCLs) with triangular lattices on a surface plane [13] as shown in Fig. 4a because of the reduction of the overall strain energy by sacrificing the local stabilization energy along the DCL. The atoms on the DCL shift slightly in the growth direction and form triangle lattices. With the increased layering, it is shown by the simulation that the DCL splits into two parallel elemental DCLs as shown in Fig. 4b and c. In the case of the layering of Ti on the MgO(001) plane, j= −0.013, and in spite of the small and negative value of j, the DCL may be expected to be introduced softly with the layering. The dihedral angle of the triangle lattice plane and the square lattice plane is expected to be small. If we assume that the arrangement in rows of triangular lattice planes and square lattice planes plays an important role in the formation of the hcp structure, once (a) type DCL or (b) type DCL is introduced during Ti deposition, hcp Ti may be grown with [00.1] orientation or with [12.1] orientation which is nearly parallel to the normal direction of the (03.4) plane, respectively. Since fcc Ti is metastable pseudomorphous and hcp Ti is stable, it is considered that the transformation can occur easily to reduce the accumulated strain energy in the fcc Ti layer during the deposition. It is also considered that (a)-type DCLs and (b)-type DCLs on a surface are hcp embryos for fcc–hcp transformation. As the DCLs extend to two kinds of 100 directions of fcc Ti, two kinds of (00.1) hcp planes rotated by 90° with respect to each other and two
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kinds of (03.5) hcp planes rotated by 90° with respect to each other may appear.
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Acknowledgements The author is thankful to T. Yoshioka and K. Shibatomi of JEOL for the assistance of TEM work.
5. Conclusion Epitaxial Ti layers have been prepared on MgO(001) substrates by electron beam evaporation in an ultrahigh vacuum. The crystal structures of Ti layers on MgO(001) were determined by transmission electron diffraction. It is found that the thin layer, whose thickness is up to about 4– 6 nm, is fcc (or tetragonal ) structure. When the thickness increases to more than about 6 nm, the structure is changed gradually to an hcp structure with two different orientations: one is (00.1) //(001) //(001) , and the other is hcpTi fccTi MgO (03.5) //(001) . The electron diffraction pathcpTi fccTi terns from a 60 nm thick Ti film also show reflection spots at {110} points on the fcc Ti reciprocal plane, which are considered to be from a c-titanium hydride precipitate. Considering the results of the molecular dynamics simulations by Hara et al., it is concluded that the lattice mismatch between Ti and the MgO substrate introduced composite DCLs. It is considered that the two kinds of DCLs, (a)-type DCLs and (b)-type DCLs, on a surface shown in Fig. 4 may become embryos, embedded in an fcc Ti matrix, for formation of hcp Ti crystallites with [00.1] orientation and [12.1] orientation, respectively.
References [1] [2] [3] [4]
Y.T. Cheng, W.J. Meng, Phys. Rev. Lett. 76 (1996) 3999. T. Kado, Mater. Sci. Forum 287–288 (1998) 531. T. Kado, Appl. Phys. Lett. 69 (1996) 2208. F.E. Wawner Jr., K.R. Lawless, J. Vac. Sci. Technol. 6 (1969) 588. [5] C.G. Rhodes, J.C. Williams, Metall. Trans. A 6A (1975) 1670. [6 ] Y. Song, S.P. Tang, J.H. Xu, O.N. Mryasov, A.J. Freeman, Philos. Mag. B 70 (1994) 987. [7] M. Asta, F. Fontaine, M.V. Schlifgaarde, M. Sluiter, M. Methfessel, Phys. Rev. B 46 (1992) 5055. [8] S.K. Kim, F. Jona, P.M. Marcus, J. Phys.: Condens. Matter 8 (1996) 25. [9] D.V. Heerden, D. Josell, D. Shechtman, Acta Mater. 44 (1996) 297. [10] Y. Kasukabe, Y. Yamada, L.A. Bursill, Philos. Mag. A 67 (1993) 73. [11] T. Harada, H. Ohkoshi, J. Cryst. Growth 171 (1997) 433. [12] T. Harada, H. Ohkoshi, J. Cryst. Growth 173 (1997) 109. [13] K. Hara, M. Ikeda, O. Ohtsuki, K. Terakura, M. Mikami, Y. Tago, T. Oguchi, Phys. Rev. B 39 (1989) 9476. [14] T. Kado, T. Yamada, J. Appl. Phys. 77 (1995) 6651. [15] D.B. Williams, C.B. Carter, Transmission Microscopy, Plenum, New York, 1996. [16 ] S.R. Peddada, I.M. Robertson, H.K. Birnbaum, J. Mater. Res. 8 (1993) 291. [17] H. Nakamura, M. Koiwa, Acta Metall. 32 (1984) 1799.
Structure of Ti films deposited on MgO(001) substrates T. Kado * Chugoku National Industrial Research Institute, 2-2-2 Hiro-Suehiro, Kure, Hiroshima 737-01, Japan
Abstract The growth and structure of Ti thin films deposited by electron beam evaporation at 273 K in a vacuum of about 10−7 Pa on MgO(001) have been studied by means of transmission electron microscopy. It is found that a thin layer, whose thickness is up to about 4 nm, is fcc (or tetragonal ) structure. When the thickness increases to more than about 6 nm, the structure gradually changes to an hcp structure with two different orientations: one is (00.1) //(001) //(001) , and the other is (03.5) //(001) . The electron diffraction patterns from 60 nm hcpTi fccTi MgO hcpTi fccTi thick Ti film also show reflection spots at {110} points on fcc Ti reciprocal plane, which show metastable c-hydride precipitation takes place in the layer. The orientation relationships and growth mechanism of the epitaxial Ti films on MgO(001) are discussed using molecular dynamics simulations performed by Hara et al. for MBE growth of a Lennard-Jones system. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Magnesium oxides; Molecular dynamics; Titanium
1. Introduction The epitaxial layering of metal can often change its crystal structures from that of the bulk. These modified phases are called pseudomorphous [1], and recently epitaxially grown superlattices have been prepared using pseudomorphism. For example, Ti/MgO superlattices grown on MgO(001) substrates were composed of rock-salt type MgO and tetragonal Ti (a =0.4170 nm, 0 c =0.4062 nm) which is quite different from the 0 Ti bulk structure of hcp [2,3]. The tetragonal Ti can be explained by the fcc Ti which is distorted to be fct Ti in the epitaxially grown layers. Fcc Ti was first reported in 1969 by Wawner and Lawless in thin films of Ti deposited on NaCl single crystals [4] and since then c-phase Ti of fcc * Fax: +81 823 73 3284. E-mail address: [email protected] (T. Kado)
(or fcc) structure has been often reported [5–9]. However, the lattice constant of fcc Ti is reported to be of various values between 0.401 and 0.442 nm and recently instead of fcc Ti, titanium hydride (CaF structure, a =0.441 nm) and titanium oxide 2 0 (NaCl structure, a =0.418 nm) [10–12] have been 0 suggested to explain the electron diffraction patterns of cubic structure Ti. The cubic structure Ti is reported to transform to the bulk hcp structure (a-Ti, a =0.295–0.296 nm, c =0.471–0.468 nm) 0 0 with various orientations when the film is between several nm and 300 nm thick, and the critical value depends on the conditions and kinds of substrates [9–12]. The transformed structures are very complex and the growth mechanism has not been well characterized. In the present paper, the structural changes of Ti films evaporated on MgO buffer grown on MgO (001) substrates with an increase of film thickness are discussed, in order to improve the crystalline quality of the Ti/MgO superlattices. The orienta-
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tion relationships and growth mechanism of the epitaxial Ti films on MgO(001) are also discussed using molecular dynamics simulations for MBE growth of a Lennard-Jones system [13].
2. Experimental procedure The Ti/MgO superlattices and the Ti monolayers were grown on MgO buffer layers, which were epitaxially grown on MgO(001) substrates by the electron beam evaporation method in a stainless chamber which was evacuated by a turbomolecular pump and a titanium sublimation pump. The base pressure was about 2×10−8 Pa. Commercially supplied Ti rod and MgO pellets were used for the evaporation sources, the purities of which are stated to be 99.9% and 99.99%, respectively. After charging, the Ti source was heated for more than 5 h by an electron beam to remove surface oxide and absorbed gases in the 10−7 Pa range. MgO (001) single crystals were used as the substrates. The substrates were heated at 1123 K for 180 s in a high vacuum to prepare a clean surface and then cooled to 773 K to form a MgO (001) buffer layer about 20 nm thick [14]. After that, the Ti films were grown on the substrates at 273 or 173 K. The pressure during the evaporation of Ti was under 5×10−7 Pa and the deposition rates of Ti was 0.02 nm s−1 [14]. Surface analysis of the thinnest layer studied was performed by transmission electron diffraction and high-resolution electron microscopic techniques using JEOL, JEM-2000EX. The samples were thinned by an ordinary ion milling method for electron microscopy [15]. The accelerating voltage is 200 kV and the diameter of the restricted area for transmission electron diffraction is 50 nm.
3. Experimental results 3.1. Transmission electron diffraction of thin Ti layer In the case of the Ti monolayers grown on MgO (001) buffer layer epitaxially at 173 K, RHEED patterns from the Ti layer showed streak
patterns almost the same as those from the MgO layer with fourfold symmetry, when the thickness of Ti layers are less than 4–6 nm [2]. The streak pattern changed to be a curved line pattern when the thickness was increased. This indicates the crystal structure of fcc Ti changed to other structures at the thickness of about 6 nm. Fig. 1a shows the selected area diffraction pattern from a 5 nm thick Ti layer. Bragg reflections slightly broaden in the ring direction showing an fcc (001) pattern. Fig. 1b shows the diffraction patterns from the cross-sectional region of a 5 nm thick Ti layer including an MgO buffer layer. Bragg reflections from the Ti layer and MgO buffer layer overlap each other, indicating that the electron diffraction pattern from the Ti layer is an fcc pattern. These results indicate the Ti has fcc structure, the lattice constant of which is estimated to be 0.425 nm. The orientation relationships between the Ti layer and the MgO buffer layer (MgO substrate) are considered to be (001) //(001) and [100] // Ti MgO Ti [100] . MgO 3.2. Transmission electron diffraction of a thick Ti layer The selected area diffraction patterns from various areas of a 60 nm thick Ti layer grown at 273 K are shown in Fig. 2. In Fig. 2a, many spots without halo rings are formed in lines in hh0 directions, which are considered to cause the curved lines of the RHEED pattern during the deposition. These spots are observed partially to form hexagonal or rhombic symmetry as shown Fig. 2a∞. These results indicate that, when the thickness of the Ti layer on MgO(001) increase to more than 4–6 nm, the crystal structure of fcc Ti changes to other structures, owing to the reduction of stress in the layer, partially conserving relationships with the atomic spacing of fcc Ti. As shown in Fig. 2(a∞), the many reflection spots on the reciprocal Ti (001) plane can be identified to be from hcp Ti with two kinds of orientations and fcc Ti: one is from (00.1) hcp Ti (marked as 1 and 1∞), another is from (03.5) hcp Ti (marked as 2 and 2∞) considering double diffraction with an underline and the other is from (001) fcc Ti (marked as F ). Because of the good crystallinity of the crystallites, many double
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Fig. 1. Selected area diffraction patterns from a 5 nm thick Ti layer. (a) Diffraction pattern obtained at the layer plane. (b) Diffraction pattern obtained in the cross-sectional region of the Ti layer including MgO buffer layer.
diffractions are observed. The reflections of 1 and 1∞, and 2 and 2∞ indicate two kinds of orientations of (00.1) hcp planes rotated by 90° with respect to each other and two kinds of (03.5) hcp planes rotated by 90° with respect to each other, respectively. The two kinds of orientation relationships between hcp Ti and fcc Ti may be (00.1) // hcpTi (001) //(001) , and (03.5) //(001) (or fccTi MgO hcpTi fccTi [12.1] ([011: 1] )//[001] ), which agree hcpTi hcpTi fccTi with the relationships proposed by Kasukabe [10] and Harada [11,12]. The transmission electron diffraction patterns from other areas are shown in Fig. 2b and c. Fig. 2b shows slightly broadened reflections from the (001) fcc Ti plane. Fig. 2c shows reflection spots from the (001) fcc Ti and from the (03.5) hcp Ti plane including other spots, which are observed at {110} points of fcc Ti reciprocal plane. It is noteworthy that {110} reflections are forbidden as reflections from an fcc structure. 3.3. High-resolution lattice images of a thick Ti layer High-resolution lattice images from two different areas of the same 60 nm thick Ti sample
are shown in Fig. 3. Dark and bright spots correspond to the atomic arrangement of Ti and the images indicate many areas with different orientations. Fig. 3a shows the lattice image from fcc Ti area. The inset is the Fourier transform obtained from atomic arrangement in the area labeled A, and this shows that the area labeled A consisted of a (001) fcc Ti crystallite. Fig. 3b shows the lattice image from an hcp Ti area. The insets, upper left and lower right, are the Fourier transforms obtained from atomic arrangements in the areas labeled B and C, and these show that the areas labeled B and C consisted of (00.1) and (03.5) hcp Ti crystallites, respectively.
4. Discussion The above high-resolution lattice images and the transmission electron diffraction of thin Ti film evaporated on MgO (001) have provided evidence that in the initial stage of the deposition Ti had a cubic structure, and at more than 6 nm of film thickness the cubic structure changed to an hcp structure with [00.1] or [12.1] orientation. As a result of the diffraction patterns shown in Fig. 2,
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Fig. 2. Selected area diffraction patterns from various area of a 60 nm thick Ti layer. (a) Area of an hcp Ti crystallite, (b) area of an fcc Ti crystallite, and (c) area of fcc Ti and hcp Ti including c-hydride precipitates. (a∞) The same area as shown in (a), showing (00.1) hcp Ti (marked as 1 and 1∞), (03.5) hcp Ti (marked as 2 and 2∞) considering double diffractions with underlines and the other is from (001) fcc Ti (marked as F ). 1 and 1∞ (2 and 2∞) indicate two kinds of orientations of (00.1) hcp planes ((03.5) hcp planes) rotating by 90° with respect to each other.
almost all spots except for the {110} spots can be explained by hcp Ti, and the {110} spots are considered to be from metastable a c-hydride precipitate which has an fcc structure and has often been observed in a-Ti specimens containing a small amount of hydrogen, 0.5–3 at.% [16,17]. We could not get clear evidence of TiO or of other
titanium hydrides. Since hydrides were observed only in specimens grown immediately after a fresh Ti charge was placed in the electron beam evaporation vessel [16 ] and only a few diffraction patterns showed {110} spots, the Ti layer contained a very small amount of hydride precipitate. It is considered that pseudomorphous fcc and/or
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Fig. 3. High-resolution lattice images from two different areas of the same 60 nm thick Ti layer. (a) The lattice image from fcc Ti area. The inset is the Fourier transform obtained from atomic arrangement in the area labeled A. (b) The lattice image from hcp Ti area. The insets upper left and lower right are the Fourier transforms obtained from atomic arrangement in the areas labeled B and C, and show that the areas labeled B and C consisted of (00.1) and (03.5) hcp Ti crystallites, respectively.
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Fig. 4. (a–c) Schematic illustrations of the top view of the layer deposited by molecular beam epitaxy for a two-component Lennard-Jones system, simulated by Hara et al. Lattice mismatch caused the introduction of composite discommensulation lines (DCLs) on the surface (a), and the DCL splits into two parallel DCLs with an increase of layering (b) and (c). (d ) Relationship between the (00.1), (03.2) and (03.4) planes of hcp Ti. The direction of the incident electron beam is from A to B, which is the [12.1] direction corresponding to the normal direction of the hcp (03.5) plane or the normal direction of the fcc (001) plane.
fct Ti is not stable when the film thickness increased by more than 6 nm and changed to bulk hcp structure. Concerning the relationships between cubic Ti and hcp Ti, Kasukabe et al. discussed in detail from the viewpoint of the relationships between bcc, fcc and hcp structures. However, since the structural change is caused by the increasing film thickness during the deposition, it is reasonable to discuss this using a dynamical model. Fig. 4d shows the relationship between the (00.1), (03.2) and (03.4) planes of hcp Ti. In electron diffraction, the direction of the incident electron beam is shown from A to B, which is the [12.1] direction, corresponding to the normal direction of the hcp (03.5) plane or the normal direction of the fcc (001) plane. The dihedral angle of the (03.4) and the (03.5) plane is only 6.2°. It is noteworthy that a part of (03.2) plane consisted of triangular lattices making a line and a part of (03.4) plane consisted of square lattices making a line. The line of the (03.2) plane
and the line of the (03.4) are arranged parallel and alternately. In order to simulate the process of molecular beam epitaxy for a two-component Lennard-Jones system, molecular dynamics simulations have been performed by Hara et al. [13]. They calculated the system where the atomic size of the incident beam is different from that of the substrate with a square lattice plane. In Fig. 4(a– c), schematic illustrations of the top view of the deposited layer are shown, which are some results of the simulation when the atomic size of the overlayer is larger than that of the substrate and the size mismatch (j) between the substrate and the overlayer atoms is 0.05%. Lattice mismatch results in the introduction of composite discommensulation lines (DCLs) with triangular lattices on a surface plane [13] as shown in Fig. 4a because of the reduction of the overall strain energy by sacrificing the local stabilization energy along the DCL. The atoms on the DCL shift slightly in the growth direction and form triangle lattices. With the increased layering, it is shown by the simulation that the DCL splits into two parallel elemental DCLs as shown in Fig. 4b and c. In the case of the layering of Ti on the MgO(001) plane, j= −0.013, and in spite of the small and negative value of j, the DCL may be expected to be introduced softly with the layering. The dihedral angle of the triangle lattice plane and the square lattice plane is expected to be small. If we assume that the arrangement in rows of triangular lattice planes and square lattice planes plays an important role in the formation of the hcp structure, once (a) type DCL or (b) type DCL is introduced during Ti deposition, hcp Ti may be grown with [00.1] orientation or with [12.1] orientation which is nearly parallel to the normal direction of the (03.4) plane, respectively. Since fcc Ti is metastable pseudomorphous and hcp Ti is stable, it is considered that the transformation can occur easily to reduce the accumulated strain energy in the fcc Ti layer during the deposition. It is also considered that (a)-type DCLs and (b)-type DCLs on a surface are hcp embryos for fcc–hcp transformation. As the DCLs extend to two kinds of 100 directions of fcc Ti, two kinds of (00.1) hcp planes rotated by 90° with respect to each other and two
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kinds of (03.5) hcp planes rotated by 90° with respect to each other may appear.
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Acknowledgements The author is thankful to T. Yoshioka and K. Shibatomi of JEOL for the assistance of TEM work.
5. Conclusion Epitaxial Ti layers have been prepared on MgO(001) substrates by electron beam evaporation in an ultrahigh vacuum. The crystal structures of Ti layers on MgO(001) were determined by transmission electron diffraction. It is found that the thin layer, whose thickness is up to about 4– 6 nm, is fcc (or tetragonal ) structure. When the thickness increases to more than about 6 nm, the structure is changed gradually to an hcp structure with two different orientations: one is (00.1) //(001) //(001) , and the other is hcpTi fccTi MgO (03.5) //(001) . The electron diffraction pathcpTi fccTi terns from a 60 nm thick Ti film also show reflection spots at {110} points on the fcc Ti reciprocal plane, which are considered to be from a c-titanium hydride precipitate. Considering the results of the molecular dynamics simulations by Hara et al., it is concluded that the lattice mismatch between Ti and the MgO substrate introduced composite DCLs. It is considered that the two kinds of DCLs, (a)-type DCLs and (b)-type DCLs, on a surface shown in Fig. 4 may become embryos, embedded in an fcc Ti matrix, for formation of hcp Ti crystallites with [00.1] orientation and [12.1] orientation, respectively.
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