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Structure transformation of Ti films deposited on SiC single crystal substrates
Materials Characterization 134 (2017) 64–68
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Structure transformation of Ti films deposited on SiC single crystal substrates
MARK
Lei Lia,⁎, Yan Liub, Xiaonan Maoa, Liying Zenga, Vincent JIc a b c
Northwest Institute for Nonferrous Metal Research, Xian, ShannXi 710016, China Northeastern Universities, Shenyang, LiaoNing 110819, China LEMHE/ICMMO, UMR 8182, University Paris-Sud11, Orsay 91405, France
A R T I C L E I N F O
A B S T R A C T
Keywords: Structure transformation Face centered cubic Ti Magnetron sputtering Epitaxial film
This paper reports the structure transformation of Ti films deposited on SiC(0001) single crystal substrates using DC magnetron sputtering. Film thickness, sputter power and temperature of deposition were changed to investigate structure transformation of Ti films. The structure characterization of Ti film was performed by means of X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscope (HRTEM). The results showed that the Ti film grew epitaxially with a face centered cubic (fcc) structure even the thickness is up to about 50 nm. High temperature and low sputter power are propitious to the formation of fcc-Ti. An interesting intermediate state of the fcc-hcp transformation was observed. This intermediate structure could help us to understand the mechanism of thickness-dependent fcc-hcp transformation in Ti thin films.
1. Introduction Titanium (Ti) and its alloy are widely used in aerospace, medical, and nuclear field where their high strength, light weight, corrosion resistance, and nuclear absorption cross section are of prime interest. The phase structures and structure transitions of titanium, which are of tremendous scientific and technological interest, have attracted a great deal of attention for many years [1–3]. Ti crystallizes in a hexagonal close packed (α phase) structure at ambient conditions but transforms to a body centered cubic (β phase) structure at about 882 °C. At room temperature, Ti transforms from α to ω phase under high pressure between 2 and 11 GPa [1,4–6]. This discrepancy is in part due to the difference in sample purity [5]. The other two kinds of structure of Ti have been found when the pressure increased to 118 GPa for γ phase and 140 GPa for δ phase, while the formation of β phase is not detected until 220 GPa [7,8]. Apart from those structures mentioned above, there is another face centered cubic (fcc) structure which had been reported in thin epitaxial films of Ti. The fcc-Ti was first reported in 1969 by Wawner and Lawless in thin films deposited on NaCl single crystals [9]. Since then, fcc-Ti thin films with different substrates had been often reported [10–17]. First-principles study had revealed that the total energy of the fcc-Ti is significantly higher than that of the α-Ti and ω-Ti even with a volume reduction of 15% [3]. With increasing thickness, the strain energy from lattice misfit could not maintain the fcc structure and the films
⁎
transform to hcp structure. Therefore, the fcc-Ti could only exist below certain critical thickness and undergo fcc-hcp transformation with the increase of film thickness. Although the thickness-dependent fcc-hcp transformation of Ti film had been reported by many researchers (Table 1), few papers focused on the process of this transformation and the mechanism is still not clear. In the present paper, we report the structure transformation of Ti films deposited on SiC(0001) single crystal substrates using DC magnetron sputtering. Film thickness, sputter power and temperature of deposition were changed to investigate structure transformation of Ti films. An interesting intermediate state of the fcc-hcp transformation was observed which could help us to understand the mechanism of thickness-dependent structure transformation in Ti thin films. 2. Sample Preparation and Measurements The samples were prepared on SiC(0001) single crystal substrate using DC magnetron sputtering from Ti (99.99%) target at 350 °C and 500 °C. The substrates were ultrasonically degreased, sequentially cleaned in acetone, alcohol, and de-ionized water, and dried by the purge of nitrogen gas. Then, the substrates were heated at 500 °C for 1 h in a high vacuum to prepare a clean surface. The base pressure before deposition was 5.0 × 10− 8 Torr. The Ar pressure during deposition was 4 mTorr. The sputter powers were 20 W and 50 W which correspond to the deposition rates of 0.01 nm/s and 0.03 nm/s respectively.
Corresponding author at: Northwest Institute for Nonferrous Metal Research, Xi'an 710016, China. E-mail addresses: [email protected] (L. Li), [email protected] (L. Zeng).
http://dx.doi.org/10.1016/j.matchar.2017.09.041 Received 31 July 2017; Received in revised form 17 August 2017; Accepted 18 September 2017 Available online 10 October 2017 1044-5803/ © 2017 Published by Elsevier Inc.
Materials Characterization 134 (2017) 64–68
L. Li et al.
Table 1 The reported experimental data of fcc-Ti. fcc-Ti sample
Preparation condition
Critical thickness
Lattice parameter
Reference
Epitaxial film
NaCl(001) (111) Al(001) Al(110) SiC(0001) MgO(001) Si(100) Vacuum ARC Cryogenic compression Mechanical attrition
20–30 nm
0.433 nm
[9]
~ 1.14 nm 0.5–0.6 nm > 80 nm 4–6 nm 144 nm 300 nm /
0.4146 nm 0.415 nm 0.438 nm 0.425 nm 0.41638 nm 0.420 nm 0.4302 nm
[10] [11] [12] [13] [14] [15] [16]
/
0.4327–0.4351 nm
[17]
Epitaxial film Epitaxial film Epitaxial film Epitaxial film Polycrystal Polycrystal Polycrystal Nanocrystal particle
Table 2 The deposition conditions of the Ti films. No.
Temperature (°C)
Sputter power (W)
Thickness (nm)
S1 S2 S3 S4
500 500 500 350
20 20 50 20
50 100 100 50
The thicknesses of Ti films were about 50 nm and 100 nm. Table 2 shows the depositions conditions of the Ti films. The magnetron sputtering device is a commercially available Lab-18 system from Kurt J. Lesker Company. With the exact computer control system of Lab-18, we could deposit ultrathin film by control the sputtering time accurately. The structure of the films was studied by X-Ray diffraction (Rigaku International Corporation, D/Max-2500/PC). To investigate the crystallographic orientation, a θ-2θ scan was performed using Cu Kα1 radiation with the wavelength of 0.15406 nm. HighResolution Transmission Electron Microscope observations were performed using a JEM-2010 (JEOL Ltd., Tokyo, Japan) electron microscope. The chemical and phase composition of Ti films were determined by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250 XPS, VG-scientific sigma probe spectrometer employing a monochromated Al-Kα source and a hemispherical analyser. A pass energy/step of 50/ 0.1 eV was used for narrow scans.
Fig. 1. The XRD patterns of Ti films deposited on SiC substrates.
Table 3 The data of diffraction peaks of Ti films. Peak data
S1
S2
S3
S4
2θ (counts)
37.38°(7950) / 79.68°(763)
37.96°(69,102) 39.98°(1948) 81.16°(5401)
38.20°(142,317) / 81.84°(9265)
38.26°(37,153) / 81.90°(2595)
that the structure of S1 is face-centered cubic and S1 grows epitaxially with the orientation relationship (111)fcc-Ti//(0001)SiC. It is obvious that the diffraction peaks shift to high angle direction from S1 to S4 as shown in Fig. 1(b). The sample S2 was deposited in the same conditions as S1, but the thickness of S2 is 100 nm. Its main diffraction peak at 37.96° is 0.49° less than the (0002)α-Ti peak in bulk materials(2θ = 38.45°). The interplanar spacing associated with this peak is 0.2368 nm. The corresponding interplanar spacing of high angle peaks (2θ = 81.16°) is just the half of the value calculated with previously mentioned peaks. So, these two peaks correspond to the same crystallographic direction. The sample S2 shows the characteristic of a strong epitaxial film, even the Kβ peak at 34.18° is observed. The other peak of S2 at 39.98° corresponds to (10−11)α-Ti which only has a 0.22° shift. This observation of (10–11)α-Ti is abnormal, because the film could not grow epitaxially with two directions. Therefore, S2 must undergo a thickness-dependent fcc-hcp transformation during the deposition. The structure of S2 is no longer fcc as S1. The onset thickness of the fcc-hcp transformation is between 50 and 100 nm. The sample S3 was deposited with high sputter power (50 W). Its main diffraction peak has a 0.25° shift from (0002)α-Ti and the intensity (counts) is doubled relative to S2. The sample S4 was deposited at low deposition temperature (350 °C). Its main diffraction peak has a 0.19° shift from (0002)α-Ti and the intensity (counts) is almost 4.7 times
3. Results and Discussion 3.1. XRD Observation of Ti Film Deposited on SiC Substrate Fig. 1(a) shows the XRD patterns of SiC substrate and Ti films (S1S4). In order to obtain enough intensity of XRD patterns for thin films (≤ 100 nm), the Kβ filter is removed. The SiC(0001) substrate shows strong peaks of (0001) and (0002). In addition to the peaks of SiC substrate, the diffraction peaks of Ti film are observed. The data of diffraction peaks of all samples are listed in Table 3. Fig. 1(b) shows the zoomed diffraction patterns (2θ = 33.5°–40.7°) using the logarithm of counts as longitudinal coordinates. The two dash lines correspond to the 2θ positions of (111)fcc-Ti (a = 0.416 nm) and (0002)α-Ti (a = 0.295 nm, c = 0.468 nm) diffraction peaks respectively [14]. It is clear from Fig. 1(b) and Table 2 that the main diffraction peak of S1 is at 2θ = 37.38°. The interplanar spacing associated with this peak is 0.2404 nm. This diffractogram is consistent with the reported fcc-Ti [10,11]. Therefore, the peak at 2θ = 37.38° could be indexed as (111)fcc-Ti. The lattice parameter of S1 calculated from the diffraction peak is 0.4164 nm, which is in good agreement with the reported parameters of fcc-Ti [14]. We calculate the 2θ of (222)fcc-Ti with the lattice parameter a = 0.4164 nm. The value of 2θ of (222) fcc-Ti is 79.68°, which fits the observed peak in Table 2 very well. It indicates 65
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Fig. 3. The TEM pattern of sample S1. Fig. 2. High-resolution XPS-spectrum of Ti 2p of sample S1.
relative to S1. The high angle peaks (2θ = 81.84° for S3, 2θ = 81.90° for S4) correspond to (0004)α-Ti. Both the diffraction peaks correspond to the same crystallographic direction. So, the samples S3 and S4 are hexagonal close packed structure and grown epitaxially with c-axis perpendicular to the substrate surface. The results indicate that high temperature and low sputter power are propitious to the formation of fcc-Ti. 3.2. XPS Analysis XPS is capable of providing both qualitative and quantitative information about the presence of different elements at the surface. In the XPS spectrum, the Ti2P energy levels are broken into two energy levels due to the spin orbit coupling of electrons, Ti2P1/2 and Ti2P3/2. Fig. 2 shows the high-resolution XPS-spectrum of Ti2P of sample S1 prepared by DC sputtering. It is clear that the peaks in the spectrum are attributed to metallic Ti, corresponding with Ti2P3/2 at a binding energy (BE) of 454.0 eV and Ti2P1/2 at BE of 460 eV. These values of binding energies for metallic Ti are in good agreement with the data reported before [18,19]. The other Ti2P peaks for titanium nitrides, carbides, oxides and oxynitrides, with binding energies at 454.8, 458, 456 and 455 eV respectively, cannot be observed in the collected spectra [18,20]. This indicates the absence of any secondary phase on the surface of the thin film. These results obtained from XPS spectrum are in good agreement with XRD data.
Fig. 4. The HRTEM pattern of sample S1.
3.3. The TEM Analysis of fcc-Ti In order to confirm the structure of sample S1, TEM and HRTEM were performed. Typical cross-sectional TEM micrograph of S1 is shown in Fig. 3. The film is uniform and flat. The thickness is about 46–50 nm, which is very close to the nominal thickness of 50 nm. Fig. 4 shows cross-sectional high-resolution electron micrograph of Ti/SiC interface of S1. The abrupt interface indicates that there is no formation of silicide and carbide phases. The SiC substrate and the Ti layers have a commensurate interface arrangement, and no dislocations are observed at the interface. The insets are the Fourier transform patterns obtained from atomic arrangement in the areas labeled A and B. There are two sets of spots in the selected area diffraction (SAD) pattern from the Ti/SiC interface of sample S1 (Fig. 5), corresponding to the Fourier transform patterns of A and B. The outer spots correspond to Ti film, and the inner spots to the SiC substrate. We measured the distance and angles of the spots in SAD pattern, and compared with the standard diffraction pattern of fcc structure. From the detailed analysis of cross-sectional HRTEM images and SAD pattern, we could identify that the sample S1 is face-centered cubic structure with a Fm3m space
Fig. 5. Selected area diffraction pattern from the Ti/SiC interface of sample S1.
group. The lattice constant and nearest-neighbor atom spacing near the interface are 0.416 nm and 0.294 nm, respectively. The fcc structure is observed through the whole film from the interface to the film surface. The orientation relationship of Ti/SiC interface is (111)fcc-Ti// (0001)SiC. We calculated the average interplanar spacing (From the interface to the 50th layer) of (111)fcc-Ti from Fig. 4, d = 0.2406 nm, which is very close to the d(111) of fcc-Ti calculated from the XRD data (d = 0.2404 nm). 3.4. Phase Transformation Mechanism in Epitaxial Ti Film Fig. 6 shows cross-sectional high-resolution electron micrograph of Ti/SiC interface of S2. It is clear to observe the abrupt interface of Ti film and SiC substrate. The Fourier transform pattern inset in Fig. 6 66
Materials Characterization 134 (2017) 64–68
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Fig. 6. The HRTEM pattern of the Ti/SiC interface of sample S2.
corresponds to the selected square with dotted line near the Ti/SiC interface. The pattern indicates the film is hcp structure and c-axis of sample S2 is perpendicular to the substrate surface. That means the sample S2 has transformed from fcc to hcp in the area of Ti/SiC interface. The HRTEM pattern of the middle area of sample S2 (Fig. 7) shows two different atomic arrangements, labeled as areas A and B respectively. The Fourier transform patterns of areas A and B inset in Fig. 7 are obviously different. The atomic arrangement of areas A is the major structure which is the same as Fig. 6. This area corresponds to the (0002)α-Ti peak of XRD patterns. As to area B, the angle of the two atomic arrangement directions is Ψ = 118.6°, which corresponds to the angle between (0002) and (10–11) of hcp-Ti. Therefore, the atomic arrangement of area B corresponds to the (10–11)α-Ti peak of XRD patterns. The stability of fcc-Ti film is related to the thickness of the film and misfit of substrate. With the increase of thickness, the strain energy from misfit of substrate could not maintain the fcc structure, and lead to a structure transformation. According to the reports, the fcc-Ti film transforms to hcp-Ti with increasing thickness [9–13]. However, the structure transformation mechanism has not been well characterized and the fcc-hcp structure transformation process is still not clear. The observation of (10–11)α-Ti peak of XRD patterns reveals more information about the structure transformation. The formation of atomic arrangement in areas B as shown in Fig. 7 which corresponds to the (10–11)α-Ti peak may have two possibilities: 1) This area is an irregular arrangement resulting from lattice distortion during structure transformation by accident. 2) This atomic arrangement in areas B is an intermediate state of the structure transformation. In order to confirm the possibility, we redo the experiments of sample S2 for several times. The results show the same (0002)α-Ti and (10–11)α-Ti peaks in XRD
Fig. 8. The XRD patterns of S2 deposited on Al2O3(0001) (a) and MgO(111) (b) substrates.
patterns. Moreover, the sample S2 was deposited on Al2O3(0001) and MgO(111) substrates with the same condition. Fig. 8 shows the XRD patterns of S2 on different substrates. The (10–11)α-Ti peaks could be observed in all the patterns of Fig. 8. It is worth noting that the sample S2 deposited on Al2O3(0001) is fcc structure. So, the (10–11)α-Ti peak indicates the onset thickness of the structure transformation of S2 on Al2O3(0001) substrate is 100 nm. These results indicate that the atomic arrangement corresponds to the (10–11)α-Ti peak is not formed by accident. The second possibility could be confirmed. There are many models for explaining the fcc-hcp structure transformation [16,21–22]. As to the epitaxial films of Ti, the process of fcchcp structure transformation could be reasoned from the results above. During the deposition of S2, the Ti atoms stack with a high-energy fcc structure at the beginning because of the restriction of lattice mismatch. As the thickness increases, the interplanar spacing of (111)fcc-Ti decreases gradually from the interface to the surface of the film. The (111)fcc-Ti diffraction peak shifts to high angle direction. When the thickness reaches the critical value, the intermediate state corresponds to the (10–11)α-Ti peak forms at the film surface, and the majority of the film is still fcc structure (Fig. 8(a)). Then, on one hand, the (111)fcc-Ti epitaxial film transforms to (0002)α-Ti epitaxial film through the intermediate state of (10–11)α-Ti. On the other hand, the newly deposited atoms at the surface stack with (0002)α-Ti direction. When the structure transformation finished, the (10–11)α-Ti peak disappears. The intermediate state of (10–11)α-Ti peak had been observed in the hcp-fcc structure transformation of Ti. Manna et al. reported the formation of fcc-Ti by mechanical attrition [9]. After 30 h ball milling, the hcp-Ti particle transformed to fcc structure. From the XRD patterns of Fig.1 in reference 9, the (10–11)α-Ti peak is observed in the fcc-Ti particles, and the other peaks of hcp-Ti disappear after 30 h of mechanical attrition.
Fig. 7. The HRTEM pattern of sample S2.
67
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work. References [1] J.C. Jamieson, Crystal structures of titanium, zirconium, and hafnium at high pressures, Science 140 (1963) 72–73. [2] H. Xia, G. Parthasarathy, H. Luo, Y.K. Vohra, A.L. Ruoff, Crystal structures of group IVa metals at ultrahigh pressures, Phys. Rev. B 42 (1990) 6736–6738. [3] A. Aguayo, G. Murrieta, R. de Coss, Elastic stability and electronic structure of fcc Ti, Zr, and Hf: a first-principles study, Phys. Rev. B 65 (2002) 092106. [4] D. Errandonea, Y. Meng, M. Somayazulu, D. Häusermann, Pressure-induced α → ω transition in titanium metal: a systematic study of the effects of uniaxial stress, Physica B 355 (2005) 116–125. [5] Y.K. Vohra, S.K. Sikka, S.N. Vaidya, R. Chidambaram, Impurity effects and reaction kinetics of the pressure-induced α → ω transformation in Ti, J. Phys. Chem. Solids 38 (1977) 1293–1296. [6] C.W. Greeff, D.R. Trinkle, R.C. Albers, Shock-induced α–ω transition in titanium, J. Appl. Phys. 90 (2001) 2221–2226. [7] Y.K. Vohra, P.T. Spencer, Novel gamma-phase of titanium metal at megabar pressures, Phys. Rev. Lett. 86 (2001) 3068–3071. [8] Y. Akahama, H. Kawamura, T. Lebihan, New δ (distorted-bcc)-titanium to 220 GPa, Phys. Rev. Lett. 87 (2001) 275503. [9] F.E. Wawner, K.R. Lawless, Epitaxial growth of titanium thin films, J. Vac. Sci. Technol. 6 (1969) 588–590. [10] A.A. Saleh, V. Shutthanandan, N.R. Shivaparan, R.J. Smith, Epitaxial growth of fcc Ti films on Al(001) surfaces, Phys. Rev. B 56 (1997) 9841–9847. [11] S.K. Kim, F. Jona, P.M. Marcus, Growth of face-centred-cubic titanium on aluminium, J. Phys. Condens. Matter 8 (1996) 25. [12] Y. Sugawara, N. Shibata, Interface structure of face-centered-cubic-Ti thin film grown on 6H-SiC substrate, J. Mater. Res. 15 (2000) 2121–2124. [13] T. Kado, Structure of Ti films deposited on MgO(001) substrates, Surf. Sci. 454 (2000) 783–789. [14] J. Chakraborty, K. Kumar, R. Ranjan, S.G. Chowdhury, S.R. Singh, Thickness-dependent fcc–hcp phase transformation in polycrystalline titanium thin films, Acta Mater. 59 (2011) 2615–2623. [15] M. Fazio, D. Vega, A. Kleiman, D. Colombo, L.M. Franco Arias, A. Márquez, Study of the structure of titanium thin films deposited with a vacuum arc as a function of the thickness, Thin Solid Films 593 (2015) 110–115. [16] D.H. Hong, T.W. Lee, S.H. Lim, W.Y. Kim, S.K. Hwang, Stress-induced hexagonal close-packed to face-centered cubic phase transformation in commercial-purity titanium under cryogenic plane-strain compression, Scr. Mater. 69 (2013) 405–408. [17] I. Manna, P.P. Chattopadhyay, P. Nandi, F. Banhart, H.-J. Fecht, Formation of facecentered-cubic titanium by mechanical attrition, J. Appl. Phys. 93 (2003) 1520–1524. [18] E. Galvanetto, F.P. Galliano, F. Borgioli, U. Bardi, A. Lavacchi, XRD and XPS study on reactive plasma sprayed titanium–titanium nitride coatings, Thin Solid Films 384 (2001) 223–229. [19] Y.L. Jeyachandran, S.K. Narayandass, D. Mangalaraj, S. Areva, J.A. Mielczarski, Properties of titanium nitride films prepared by direct current magnetron sputtering, Mater. Sci. Eng. A 445 (2007) 223–236. [20] N. Arshi, J. Lu, C.G. Lee, J.H. Yoon, B.H. Koo, F. Ahmed, Thickness effect on properties of titanium film deposited by d.c. magnetron sputtering and electron beam evaporation techniques, Bull. Mater. Sci. 36 (2013) 807–812. [21] H.C. Wu, A. Kumar, J. Wang, X.F. Bi, C.N. Tomé, Z. Zhang, S.X. Mao, Rollinginduced face centered cubic titanium in hexagonal close packed titanium at room temperature, Sci Rep 6 (2016) 24370. [22] H. Zhao, X. Hu, M. Song, S. Ni, Mechanisms for deformation induced hexagonal close-packed structure to face-centered cubic structure transformation in zirconium, Scr. Mater. 132 (2017) 63–67.
Fig. 9. The variation of location of C closed-packed atomic plane induced the atomic stack as AAB.
The fcc-hcp structure transformation corresponds to the atomic stack style variation from ABCABC to ABABAB. So, at least four of each six closed-packed atomic planes should change their locations during the structure transformation. The atomic stack as AAB or ABB may appears when the atomic planes change their position as shown in Fig. 9. This position variation may cause the formation of the intermediate state. 4. Conclusion The structure of Ti films prepared by magnetron sputtered on SiC(0001) single crystal substrate were investigated. The Ti film grew epitaxial with fcc structure even the thickness is up to about 50 nm. With the increase of thickness, an intermediate state of (10–11) hcp-Ti was observed. This intermediate structure indicated the film undergoes fcc-hcp structure transformation. The onset thickness of fcc-hcp structure transformation of Ti film on SiC(0001) substrate is about 50100 nm. The critical thickness of structure transformation is related to the deposition conditions. Acknowledgments The authors thank the Natural Science Foundation of China (grant no 51201138) and Industrial Science and Technology Research Program of Shaanxi Province (2016GY-228) for the support of this
68
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Structure transformation of Ti films deposited on SiC single crystal substrates
MARK
Lei Lia,⁎, Yan Liub, Xiaonan Maoa, Liying Zenga, Vincent JIc a b c
Northwest Institute for Nonferrous Metal Research, Xian, ShannXi 710016, China Northeastern Universities, Shenyang, LiaoNing 110819, China LEMHE/ICMMO, UMR 8182, University Paris-Sud11, Orsay 91405, France
A R T I C L E I N F O
A B S T R A C T
Keywords: Structure transformation Face centered cubic Ti Magnetron sputtering Epitaxial film
This paper reports the structure transformation of Ti films deposited on SiC(0001) single crystal substrates using DC magnetron sputtering. Film thickness, sputter power and temperature of deposition were changed to investigate structure transformation of Ti films. The structure characterization of Ti film was performed by means of X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscope (HRTEM). The results showed that the Ti film grew epitaxially with a face centered cubic (fcc) structure even the thickness is up to about 50 nm. High temperature and low sputter power are propitious to the formation of fcc-Ti. An interesting intermediate state of the fcc-hcp transformation was observed. This intermediate structure could help us to understand the mechanism of thickness-dependent fcc-hcp transformation in Ti thin films.
1. Introduction Titanium (Ti) and its alloy are widely used in aerospace, medical, and nuclear field where their high strength, light weight, corrosion resistance, and nuclear absorption cross section are of prime interest. The phase structures and structure transitions of titanium, which are of tremendous scientific and technological interest, have attracted a great deal of attention for many years [1–3]. Ti crystallizes in a hexagonal close packed (α phase) structure at ambient conditions but transforms to a body centered cubic (β phase) structure at about 882 °C. At room temperature, Ti transforms from α to ω phase under high pressure between 2 and 11 GPa [1,4–6]. This discrepancy is in part due to the difference in sample purity [5]. The other two kinds of structure of Ti have been found when the pressure increased to 118 GPa for γ phase and 140 GPa for δ phase, while the formation of β phase is not detected until 220 GPa [7,8]. Apart from those structures mentioned above, there is another face centered cubic (fcc) structure which had been reported in thin epitaxial films of Ti. The fcc-Ti was first reported in 1969 by Wawner and Lawless in thin films deposited on NaCl single crystals [9]. Since then, fcc-Ti thin films with different substrates had been often reported [10–17]. First-principles study had revealed that the total energy of the fcc-Ti is significantly higher than that of the α-Ti and ω-Ti even with a volume reduction of 15% [3]. With increasing thickness, the strain energy from lattice misfit could not maintain the fcc structure and the films
⁎
transform to hcp structure. Therefore, the fcc-Ti could only exist below certain critical thickness and undergo fcc-hcp transformation with the increase of film thickness. Although the thickness-dependent fcc-hcp transformation of Ti film had been reported by many researchers (Table 1), few papers focused on the process of this transformation and the mechanism is still not clear. In the present paper, we report the structure transformation of Ti films deposited on SiC(0001) single crystal substrates using DC magnetron sputtering. Film thickness, sputter power and temperature of deposition were changed to investigate structure transformation of Ti films. An interesting intermediate state of the fcc-hcp transformation was observed which could help us to understand the mechanism of thickness-dependent structure transformation in Ti thin films. 2. Sample Preparation and Measurements The samples were prepared on SiC(0001) single crystal substrate using DC magnetron sputtering from Ti (99.99%) target at 350 °C and 500 °C. The substrates were ultrasonically degreased, sequentially cleaned in acetone, alcohol, and de-ionized water, and dried by the purge of nitrogen gas. Then, the substrates were heated at 500 °C for 1 h in a high vacuum to prepare a clean surface. The base pressure before deposition was 5.0 × 10− 8 Torr. The Ar pressure during deposition was 4 mTorr. The sputter powers were 20 W and 50 W which correspond to the deposition rates of 0.01 nm/s and 0.03 nm/s respectively.
Corresponding author at: Northwest Institute for Nonferrous Metal Research, Xi'an 710016, China. E-mail addresses: [email protected] (L. Li), [email protected] (L. Zeng).
http://dx.doi.org/10.1016/j.matchar.2017.09.041 Received 31 July 2017; Received in revised form 17 August 2017; Accepted 18 September 2017 Available online 10 October 2017 1044-5803/ © 2017 Published by Elsevier Inc.
Materials Characterization 134 (2017) 64–68
L. Li et al.
Table 1 The reported experimental data of fcc-Ti. fcc-Ti sample
Preparation condition
Critical thickness
Lattice parameter
Reference
Epitaxial film
NaCl(001) (111) Al(001) Al(110) SiC(0001) MgO(001) Si(100) Vacuum ARC Cryogenic compression Mechanical attrition
20–30 nm
0.433 nm
[9]
~ 1.14 nm 0.5–0.6 nm > 80 nm 4–6 nm 144 nm 300 nm /
0.4146 nm 0.415 nm 0.438 nm 0.425 nm 0.41638 nm 0.420 nm 0.4302 nm
[10] [11] [12] [13] [14] [15] [16]
/
0.4327–0.4351 nm
[17]
Epitaxial film Epitaxial film Epitaxial film Epitaxial film Polycrystal Polycrystal Polycrystal Nanocrystal particle
Table 2 The deposition conditions of the Ti films. No.
Temperature (°C)
Sputter power (W)
Thickness (nm)
S1 S2 S3 S4
500 500 500 350
20 20 50 20
50 100 100 50
The thicknesses of Ti films were about 50 nm and 100 nm. Table 2 shows the depositions conditions of the Ti films. The magnetron sputtering device is a commercially available Lab-18 system from Kurt J. Lesker Company. With the exact computer control system of Lab-18, we could deposit ultrathin film by control the sputtering time accurately. The structure of the films was studied by X-Ray diffraction (Rigaku International Corporation, D/Max-2500/PC). To investigate the crystallographic orientation, a θ-2θ scan was performed using Cu Kα1 radiation with the wavelength of 0.15406 nm. HighResolution Transmission Electron Microscope observations were performed using a JEM-2010 (JEOL Ltd., Tokyo, Japan) electron microscope. The chemical and phase composition of Ti films were determined by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250 XPS, VG-scientific sigma probe spectrometer employing a monochromated Al-Kα source and a hemispherical analyser. A pass energy/step of 50/ 0.1 eV was used for narrow scans.
Fig. 1. The XRD patterns of Ti films deposited on SiC substrates.
Table 3 The data of diffraction peaks of Ti films. Peak data
S1
S2
S3
S4
2θ (counts)
37.38°(7950) / 79.68°(763)
37.96°(69,102) 39.98°(1948) 81.16°(5401)
38.20°(142,317) / 81.84°(9265)
38.26°(37,153) / 81.90°(2595)
that the structure of S1 is face-centered cubic and S1 grows epitaxially with the orientation relationship (111)fcc-Ti//(0001)SiC. It is obvious that the diffraction peaks shift to high angle direction from S1 to S4 as shown in Fig. 1(b). The sample S2 was deposited in the same conditions as S1, but the thickness of S2 is 100 nm. Its main diffraction peak at 37.96° is 0.49° less than the (0002)α-Ti peak in bulk materials(2θ = 38.45°). The interplanar spacing associated with this peak is 0.2368 nm. The corresponding interplanar spacing of high angle peaks (2θ = 81.16°) is just the half of the value calculated with previously mentioned peaks. So, these two peaks correspond to the same crystallographic direction. The sample S2 shows the characteristic of a strong epitaxial film, even the Kβ peak at 34.18° is observed. The other peak of S2 at 39.98° corresponds to (10−11)α-Ti which only has a 0.22° shift. This observation of (10–11)α-Ti is abnormal, because the film could not grow epitaxially with two directions. Therefore, S2 must undergo a thickness-dependent fcc-hcp transformation during the deposition. The structure of S2 is no longer fcc as S1. The onset thickness of the fcc-hcp transformation is between 50 and 100 nm. The sample S3 was deposited with high sputter power (50 W). Its main diffraction peak has a 0.25° shift from (0002)α-Ti and the intensity (counts) is doubled relative to S2. The sample S4 was deposited at low deposition temperature (350 °C). Its main diffraction peak has a 0.19° shift from (0002)α-Ti and the intensity (counts) is almost 4.7 times
3. Results and Discussion 3.1. XRD Observation of Ti Film Deposited on SiC Substrate Fig. 1(a) shows the XRD patterns of SiC substrate and Ti films (S1S4). In order to obtain enough intensity of XRD patterns for thin films (≤ 100 nm), the Kβ filter is removed. The SiC(0001) substrate shows strong peaks of (0001) and (0002). In addition to the peaks of SiC substrate, the diffraction peaks of Ti film are observed. The data of diffraction peaks of all samples are listed in Table 3. Fig. 1(b) shows the zoomed diffraction patterns (2θ = 33.5°–40.7°) using the logarithm of counts as longitudinal coordinates. The two dash lines correspond to the 2θ positions of (111)fcc-Ti (a = 0.416 nm) and (0002)α-Ti (a = 0.295 nm, c = 0.468 nm) diffraction peaks respectively [14]. It is clear from Fig. 1(b) and Table 2 that the main diffraction peak of S1 is at 2θ = 37.38°. The interplanar spacing associated with this peak is 0.2404 nm. This diffractogram is consistent with the reported fcc-Ti [10,11]. Therefore, the peak at 2θ = 37.38° could be indexed as (111)fcc-Ti. The lattice parameter of S1 calculated from the diffraction peak is 0.4164 nm, which is in good agreement with the reported parameters of fcc-Ti [14]. We calculate the 2θ of (222)fcc-Ti with the lattice parameter a = 0.4164 nm. The value of 2θ of (222) fcc-Ti is 79.68°, which fits the observed peak in Table 2 very well. It indicates 65
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Fig. 3. The TEM pattern of sample S1. Fig. 2. High-resolution XPS-spectrum of Ti 2p of sample S1.
relative to S1. The high angle peaks (2θ = 81.84° for S3, 2θ = 81.90° for S4) correspond to (0004)α-Ti. Both the diffraction peaks correspond to the same crystallographic direction. So, the samples S3 and S4 are hexagonal close packed structure and grown epitaxially with c-axis perpendicular to the substrate surface. The results indicate that high temperature and low sputter power are propitious to the formation of fcc-Ti. 3.2. XPS Analysis XPS is capable of providing both qualitative and quantitative information about the presence of different elements at the surface. In the XPS spectrum, the Ti2P energy levels are broken into two energy levels due to the spin orbit coupling of electrons, Ti2P1/2 and Ti2P3/2. Fig. 2 shows the high-resolution XPS-spectrum of Ti2P of sample S1 prepared by DC sputtering. It is clear that the peaks in the spectrum are attributed to metallic Ti, corresponding with Ti2P3/2 at a binding energy (BE) of 454.0 eV and Ti2P1/2 at BE of 460 eV. These values of binding energies for metallic Ti are in good agreement with the data reported before [18,19]. The other Ti2P peaks for titanium nitrides, carbides, oxides and oxynitrides, with binding energies at 454.8, 458, 456 and 455 eV respectively, cannot be observed in the collected spectra [18,20]. This indicates the absence of any secondary phase on the surface of the thin film. These results obtained from XPS spectrum are in good agreement with XRD data.
Fig. 4. The HRTEM pattern of sample S1.
3.3. The TEM Analysis of fcc-Ti In order to confirm the structure of sample S1, TEM and HRTEM were performed. Typical cross-sectional TEM micrograph of S1 is shown in Fig. 3. The film is uniform and flat. The thickness is about 46–50 nm, which is very close to the nominal thickness of 50 nm. Fig. 4 shows cross-sectional high-resolution electron micrograph of Ti/SiC interface of S1. The abrupt interface indicates that there is no formation of silicide and carbide phases. The SiC substrate and the Ti layers have a commensurate interface arrangement, and no dislocations are observed at the interface. The insets are the Fourier transform patterns obtained from atomic arrangement in the areas labeled A and B. There are two sets of spots in the selected area diffraction (SAD) pattern from the Ti/SiC interface of sample S1 (Fig. 5), corresponding to the Fourier transform patterns of A and B. The outer spots correspond to Ti film, and the inner spots to the SiC substrate. We measured the distance and angles of the spots in SAD pattern, and compared with the standard diffraction pattern of fcc structure. From the detailed analysis of cross-sectional HRTEM images and SAD pattern, we could identify that the sample S1 is face-centered cubic structure with a Fm3m space
Fig. 5. Selected area diffraction pattern from the Ti/SiC interface of sample S1.
group. The lattice constant and nearest-neighbor atom spacing near the interface are 0.416 nm and 0.294 nm, respectively. The fcc structure is observed through the whole film from the interface to the film surface. The orientation relationship of Ti/SiC interface is (111)fcc-Ti// (0001)SiC. We calculated the average interplanar spacing (From the interface to the 50th layer) of (111)fcc-Ti from Fig. 4, d = 0.2406 nm, which is very close to the d(111) of fcc-Ti calculated from the XRD data (d = 0.2404 nm). 3.4. Phase Transformation Mechanism in Epitaxial Ti Film Fig. 6 shows cross-sectional high-resolution electron micrograph of Ti/SiC interface of S2. It is clear to observe the abrupt interface of Ti film and SiC substrate. The Fourier transform pattern inset in Fig. 6 66
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Fig. 6. The HRTEM pattern of the Ti/SiC interface of sample S2.
corresponds to the selected square with dotted line near the Ti/SiC interface. The pattern indicates the film is hcp structure and c-axis of sample S2 is perpendicular to the substrate surface. That means the sample S2 has transformed from fcc to hcp in the area of Ti/SiC interface. The HRTEM pattern of the middle area of sample S2 (Fig. 7) shows two different atomic arrangements, labeled as areas A and B respectively. The Fourier transform patterns of areas A and B inset in Fig. 7 are obviously different. The atomic arrangement of areas A is the major structure which is the same as Fig. 6. This area corresponds to the (0002)α-Ti peak of XRD patterns. As to area B, the angle of the two atomic arrangement directions is Ψ = 118.6°, which corresponds to the angle between (0002) and (10–11) of hcp-Ti. Therefore, the atomic arrangement of area B corresponds to the (10–11)α-Ti peak of XRD patterns. The stability of fcc-Ti film is related to the thickness of the film and misfit of substrate. With the increase of thickness, the strain energy from misfit of substrate could not maintain the fcc structure, and lead to a structure transformation. According to the reports, the fcc-Ti film transforms to hcp-Ti with increasing thickness [9–13]. However, the structure transformation mechanism has not been well characterized and the fcc-hcp structure transformation process is still not clear. The observation of (10–11)α-Ti peak of XRD patterns reveals more information about the structure transformation. The formation of atomic arrangement in areas B as shown in Fig. 7 which corresponds to the (10–11)α-Ti peak may have two possibilities: 1) This area is an irregular arrangement resulting from lattice distortion during structure transformation by accident. 2) This atomic arrangement in areas B is an intermediate state of the structure transformation. In order to confirm the possibility, we redo the experiments of sample S2 for several times. The results show the same (0002)α-Ti and (10–11)α-Ti peaks in XRD
Fig. 8. The XRD patterns of S2 deposited on Al2O3(0001) (a) and MgO(111) (b) substrates.
patterns. Moreover, the sample S2 was deposited on Al2O3(0001) and MgO(111) substrates with the same condition. Fig. 8 shows the XRD patterns of S2 on different substrates. The (10–11)α-Ti peaks could be observed in all the patterns of Fig. 8. It is worth noting that the sample S2 deposited on Al2O3(0001) is fcc structure. So, the (10–11)α-Ti peak indicates the onset thickness of the structure transformation of S2 on Al2O3(0001) substrate is 100 nm. These results indicate that the atomic arrangement corresponds to the (10–11)α-Ti peak is not formed by accident. The second possibility could be confirmed. There are many models for explaining the fcc-hcp structure transformation [16,21–22]. As to the epitaxial films of Ti, the process of fcchcp structure transformation could be reasoned from the results above. During the deposition of S2, the Ti atoms stack with a high-energy fcc structure at the beginning because of the restriction of lattice mismatch. As the thickness increases, the interplanar spacing of (111)fcc-Ti decreases gradually from the interface to the surface of the film. The (111)fcc-Ti diffraction peak shifts to high angle direction. When the thickness reaches the critical value, the intermediate state corresponds to the (10–11)α-Ti peak forms at the film surface, and the majority of the film is still fcc structure (Fig. 8(a)). Then, on one hand, the (111)fcc-Ti epitaxial film transforms to (0002)α-Ti epitaxial film through the intermediate state of (10–11)α-Ti. On the other hand, the newly deposited atoms at the surface stack with (0002)α-Ti direction. When the structure transformation finished, the (10–11)α-Ti peak disappears. The intermediate state of (10–11)α-Ti peak had been observed in the hcp-fcc structure transformation of Ti. Manna et al. reported the formation of fcc-Ti by mechanical attrition [9]. After 30 h ball milling, the hcp-Ti particle transformed to fcc structure. From the XRD patterns of Fig.1 in reference 9, the (10–11)α-Ti peak is observed in the fcc-Ti particles, and the other peaks of hcp-Ti disappear after 30 h of mechanical attrition.
Fig. 7. The HRTEM pattern of sample S2.
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work. References [1] J.C. Jamieson, Crystal structures of titanium, zirconium, and hafnium at high pressures, Science 140 (1963) 72–73. [2] H. Xia, G. Parthasarathy, H. Luo, Y.K. Vohra, A.L. Ruoff, Crystal structures of group IVa metals at ultrahigh pressures, Phys. Rev. B 42 (1990) 6736–6738. [3] A. Aguayo, G. Murrieta, R. de Coss, Elastic stability and electronic structure of fcc Ti, Zr, and Hf: a first-principles study, Phys. Rev. B 65 (2002) 092106. [4] D. Errandonea, Y. Meng, M. Somayazulu, D. Häusermann, Pressure-induced α → ω transition in titanium metal: a systematic study of the effects of uniaxial stress, Physica B 355 (2005) 116–125. [5] Y.K. Vohra, S.K. Sikka, S.N. Vaidya, R. Chidambaram, Impurity effects and reaction kinetics of the pressure-induced α → ω transformation in Ti, J. Phys. Chem. Solids 38 (1977) 1293–1296. [6] C.W. Greeff, D.R. Trinkle, R.C. Albers, Shock-induced α–ω transition in titanium, J. Appl. Phys. 90 (2001) 2221–2226. [7] Y.K. Vohra, P.T. Spencer, Novel gamma-phase of titanium metal at megabar pressures, Phys. Rev. Lett. 86 (2001) 3068–3071. [8] Y. Akahama, H. Kawamura, T. Lebihan, New δ (distorted-bcc)-titanium to 220 GPa, Phys. Rev. Lett. 87 (2001) 275503. [9] F.E. Wawner, K.R. Lawless, Epitaxial growth of titanium thin films, J. Vac. Sci. Technol. 6 (1969) 588–590. [10] A.A. Saleh, V. Shutthanandan, N.R. Shivaparan, R.J. Smith, Epitaxial growth of fcc Ti films on Al(001) surfaces, Phys. Rev. B 56 (1997) 9841–9847. [11] S.K. Kim, F. Jona, P.M. Marcus, Growth of face-centred-cubic titanium on aluminium, J. Phys. Condens. Matter 8 (1996) 25. [12] Y. Sugawara, N. Shibata, Interface structure of face-centered-cubic-Ti thin film grown on 6H-SiC substrate, J. Mater. Res. 15 (2000) 2121–2124. [13] T. Kado, Structure of Ti films deposited on MgO(001) substrates, Surf. Sci. 454 (2000) 783–789. [14] J. Chakraborty, K. Kumar, R. Ranjan, S.G. Chowdhury, S.R. Singh, Thickness-dependent fcc–hcp phase transformation in polycrystalline titanium thin films, Acta Mater. 59 (2011) 2615–2623. [15] M. Fazio, D. Vega, A. Kleiman, D. Colombo, L.M. Franco Arias, A. Márquez, Study of the structure of titanium thin films deposited with a vacuum arc as a function of the thickness, Thin Solid Films 593 (2015) 110–115. [16] D.H. Hong, T.W. Lee, S.H. Lim, W.Y. Kim, S.K. Hwang, Stress-induced hexagonal close-packed to face-centered cubic phase transformation in commercial-purity titanium under cryogenic plane-strain compression, Scr. Mater. 69 (2013) 405–408. [17] I. Manna, P.P. Chattopadhyay, P. Nandi, F. Banhart, H.-J. Fecht, Formation of facecentered-cubic titanium by mechanical attrition, J. Appl. Phys. 93 (2003) 1520–1524. [18] E. Galvanetto, F.P. Galliano, F. Borgioli, U. Bardi, A. Lavacchi, XRD and XPS study on reactive plasma sprayed titanium–titanium nitride coatings, Thin Solid Films 384 (2001) 223–229. [19] Y.L. Jeyachandran, S.K. Narayandass, D. Mangalaraj, S. Areva, J.A. Mielczarski, Properties of titanium nitride films prepared by direct current magnetron sputtering, Mater. Sci. Eng. A 445 (2007) 223–236. [20] N. Arshi, J. Lu, C.G. Lee, J.H. Yoon, B.H. Koo, F. Ahmed, Thickness effect on properties of titanium film deposited by d.c. magnetron sputtering and electron beam evaporation techniques, Bull. Mater. Sci. 36 (2013) 807–812. [21] H.C. Wu, A. Kumar, J. Wang, X.F. Bi, C.N. Tomé, Z. Zhang, S.X. Mao, Rollinginduced face centered cubic titanium in hexagonal close packed titanium at room temperature, Sci Rep 6 (2016) 24370. [22] H. Zhao, X. Hu, M. Song, S. Ni, Mechanisms for deformation induced hexagonal close-packed structure to face-centered cubic structure transformation in zirconium, Scr. Mater. 132 (2017) 63–67.
Fig. 9. The variation of location of C closed-packed atomic plane induced the atomic stack as AAB.
The fcc-hcp structure transformation corresponds to the atomic stack style variation from ABCABC to ABABAB. So, at least four of each six closed-packed atomic planes should change their locations during the structure transformation. The atomic stack as AAB or ABB may appears when the atomic planes change their position as shown in Fig. 9. This position variation may cause the formation of the intermediate state. 4. Conclusion The structure of Ti films prepared by magnetron sputtered on SiC(0001) single crystal substrate were investigated. The Ti film grew epitaxial with fcc structure even the thickness is up to about 50 nm. With the increase of thickness, an intermediate state of (10–11) hcp-Ti was observed. This intermediate structure indicated the film undergoes fcc-hcp structure transformation. The onset thickness of fcc-hcp structure transformation of Ti film on SiC(0001) substrate is about 50100 nm. The critical thickness of structure transformation is related to the deposition conditions. Acknowledgments The authors thank the Natural Science Foundation of China (grant no 51201138) and Industrial Science and Technology Research Program of Shaanxi Province (2016GY-228) for the support of this
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