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Structural studies of multilayered films composed of immiscible pairs Cu and Ta
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Journal of Magnetism and Magnetic Materials 126 (1993) 161-163 North-Holland
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Structural studies of multilayered films composed of immiscible pairs Cu and Ta T. Koyano a, C.H. Lee a, T. Fukunaga a, U. Mizutani M. Nishikawa c, E. Kita c and A. Tasaki c
a S. Ikeda b, y . Higuchi c,
a Department of Crystalline Materials Science, Nagoya Uniuersity, Aichi 464, Japan b National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan c Institute of Applied Physics, Uniuersity of Tsukuba, Ibaraki 305, Japan Multilayered films of Cu and Ta were prepared with the alternate deposition technique. It was found that Ta layers have considerably disordered structure in these films. Resonance detector spectroscopy using a pulsed neutron source revealed that the Debye temperature of this disordered Ta is about 130 K lower than that of bulk Ta.
Introduction The C u - T a binary system is characterized by a positive mixing heat (AHmix > 0) and neither primary solid solution nor intermetallic compound exists in the equilibrium phase diagram [1]. Nevertheless, it has been reported that an amorphous phase can be obtained by co-sputtering technique [2] and mechanical alloying (MA) of elemental powders [3]. It is claimed that the amorphization by MA proceeds only when Cu and Ta crystallites are reduced to less than 100 ,~ in size and that such fine Cu and Ta particles change their thermodynamic properties and behave as a system with AHmix < 0 [4]. When powders of Cu and Ta are repeatedly kneaded and sheared, a laminated structure of constituent elements is formed during mechanical alloying process. Therefore, studies on C u / T a multilayered films would provide further insight into the solid state reaction between the immiscible Cu and Ta metals. It has been reported that lSlTa nuclei can resonantly absorb neutrons with an energy of 4.28 eV, and, hence the resonance detector spectroscopy (RDS) would offer valuable information about the thermal vibration of Ta atoms [5]. Since the peak value of the capture cross section for neutrons is several orders of magnitude larger than the scattering cross section, the RDS spectrum can be measured even for a thin film with thickness less than a few 1000 .A. In the present experiment, we studied the structure and lattice dynamics of Ta atoms in C u / T a films by using the X-ray diffraction technique in combination with the RDS above mentioned. Correspondence to: Dr. T. Koyano, Department of Crystalline Materials Science, Nagoya University, Aichi 464, Japan. Fax + 81.52.7822129.
Experimental procedure The C u / T a films were prepared with the alternate deposition technique in a vacuum better than 5 x 10 -7 Torr. Cu and Ta were evaporated from two electron beam heating apparatus. The deposition rates for Cu and Ta were 0.5 A / s e c . The first layer deposited was Ta for all samples. Several different thicknesses were designed for the Ta layers. They were 20, 50, 100, 200 and 500 ,~ while keeping its total thickness of Ta layers to be 2000 A. The thickness of corresponding Cu layers were determined so as to adjust an average composition to Cu30Ta70. Sapphire (012) wafers were chosen as a substrate to reduce a background in the X-ray diffraction spectra and to avoid chemical reaction with the films at elevated temperatures. A Si (111)wafer at room temperature was also used for deposition of a pure Ta film. X-ray diffraction experiments with the 0-20 scanning mode were carried out with Cu K~ radiation using a two-circle goniometer equipped with a graphite (00.2) counter monochromator. The RDS measurements for lSlTa nuclei were made by using the Resonance Analyzer Time of flight spectrometer (RAT) installed at a pulsed neutron source KENS in National Laboratory for High Energy Physics, Tsukuba, Japan. Details for the RDS experiment and data analysis have been described in ref. [5]. o
Results and discussion X-ray diffraction patterns for various C u / T a films deposited on sapphire and Si substrates held at room temperature are shown in fig. 1. A C u / T a (14 ,~/50 A) film was subsequently annealed at 600°C for 12 h. The diffraction spectrum for this sample is incorporated in fig. 1. Two broad diffraction lines are observed around 20 = 38 ° and 67 ° for as-deposited films, and, in
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)
T. Koyano et al. / Structural studies of Cu / Ta films
162
contrast, the annealed film exhibits sharp diffraction lines of bcc Ta and fcc Cu as expected from the equilibrium phase diagram [1]. In the small scattering angle region (20 < 5°), many peaks due to the composition modulation are observed for as-deposited films though they are not periodic in the Q-space. This implies that the interfaces between Cu and Ta layers are not smooth. Because of roughness, the X-ray diffraction profile in a wide scattering angle region can be analyzed without taking into account possible coherent scattering [6]. The intensity of Cu ( l i d line increases with increasing the thickness of Cu and Ta layers, but signals still remain broad. In order to identify these broad signals, we deposited a pure Ta film with 500 A in thickness onto a Si (111) wafer at room temperature. Surprisingly, the profile is turned out to be broad as shown in fig. l(e). Hence, we learned that Ta layers deposited on a substrate at room temperature are always quite disordered in the absence of adjacent Cu layers. A comparison of fig. 1 (e) with (d) immediately tells us that the (200) line of bcc Ta is absent in the former, and the (211) line of bcc Ta (20 = 69.7 °) is replaced by a broad peak located at 20 = 67 ° in the as-deposited film. Fukunaga et al. have pointed out that the (200) line of a bcc structure preferentially disappears in the course of the amor-
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Fig. 1. X-ray diffraction spectra for films deposited on substrates at room temperature. (a) Cu/Ta (5.5 ,~/20 A); (b) Cu/Ta (14 A/50 ~,); (c) Cu/Ta (28 A/100 A) and (d) Cu/Ta (14 A/50 A) film annealed at 600°C for 12 h, respectively. (e) is a profile for a Ta film on Si (111) with 500 A in thickness. Sharp peaks at 20 = 28.47° (out of scale) and 58.93° originate in the substrate.
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Fig. 2. RDS spectra for lSlTa recorded at 20 K. Symbols are as follows: (A) Cu/Ta (5.5 A/20 A), (*) Cu/Ta (56 A/200 A) films and (o) Ta foil, respectively.
phization process when a bcc metal is involved as a major component in MA [7]. This implies that the degree of disorder in the bcc lattice may well be judged from the reduction in the intensity of the (200) line. Ta atoms in the as-deposited layer are, therefore, considerably disordered and no longer posses a rigid bcc structure. The RDS spectra were measured at 20 K for C u / T a (5.5 A / 2 0 A), C u / T a (56 A / 2 0 0 A) films and a Ta foil with 7 ixm in thickness• They are plotted as a function of time of flight of incoming neutrons in fig. 2. The line widths for both C u / T a films are the same (2.8 }xs) and sharper than that for the Ta foil (3.6 ITS). these line widths correspond to the effective temperatures [5] of about 50 and 100 K, respectively. We analyzed the data using the Debye model, where the dispersion curvc of phonons is approximated to a linear function of the wave number q. The Debye temperature 0 D for the Ta foil turned out to be about 250 K, in good agreement with that of bulk material. In contrast, that of C u / T a films is found to be about 120 K regardless of the thickness of Ta layers. A combination of the results of RDS spectroscopy and X-ray diffraction experiments led us to conclude that an extremely low 0 D results from the disordered Ta. X-ray diffraction profiles for films deposited on sapphire substrates at 270°C are shown in fig. 3. A Ta film with 500 A in thickness exhibits a sharp diffraction pattern of the bcc structure, while C u / T a (14 A / 5 0 A) film shows broad Ta signal and sharp lines of fcc Cu. No diffraction lines in the small scattering angle region are observed. A diffraction spectrum for C u / T a (139 A / 5 0 0 A) film (fig. 3(c)) is composed of diffraction lines of sharp and broad Ta, and fcc Cu. It is considered that the sharp and broad Ta lines originate in the
T. Koyano et al. / Structural studies of Cu / Ta films I
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the Cu and Ta are deposited at high temperatures. This particular role of Cu atoms would be reflected in the characteristic feature of the C u - T a amorphous phase, where Cu atoms of only 5 at% are necessary to amorphize Ta atoms, and its crystallization temperature exceeds 700°C [2]. Acknowledgments: We would like to thank to Dr. Y. Nakai of University of Tsukuba and Mr. H. Akatsuka of Nagoya University for the R D S experiments.
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Fig. 3. X-ray diffraction spectra for C u / T a films deposited on sapphire (012) wafers at 270°C. (a) Ta film with 500 A in thickness; (b) Cu/Ta (14 A / 5 0 A); (c) Cu/Ta (139 * / 5 0 0 A), respectively. first Ta layer and those sandwiched between Cu layers, respectively. This indicates that Cu atoms should be responsible for stabilizing the disordered Ta, though
[1] Binary Phase Diagrams (2nd ed.), eds. T.B. Massalski et al. (ASM International, Ohio, 1990), p. 1486. [2] J.E. Oh, J.A. Woollam, K.D. Aylesworth, D.J. Sellmyer and J.J. Pouch, J. Appl. Phys. 60 (1986) 4281. [3] C.H. Lee, M. Mori, T. Fukunaga and U. Mizutani, Mat. Sci. Forum 88-90 (1992) 399. [4] C.H. Lee, T. Fukunaga, Y. Yamada, H. Okamoto and U. Mizutani, J. Phase Equilibria 14 (1993) 167. [5] H. Rauh and N. Watanabe, Nucl. Inst. and Meth. 222 (1984) 507. [6] T. Koyano, E. Kita, K. Ohshima and A. Tasaki, J. Phys. Condensed Matter 3 (1991) 5921. [7] T. Fukunaga, Y. Homma, M. Misawa and K. Suzuki, J. Non-Cryst. Solids 117/118 (1990) 721.
Journal of Magnetism and Magnetic Materials 126 (1993) 161-163 North-Holland
Ad"
Structural studies of multilayered films composed of immiscible pairs Cu and Ta T. Koyano a, C.H. Lee a, T. Fukunaga a, U. Mizutani M. Nishikawa c, E. Kita c and A. Tasaki c
a S. Ikeda b, y . Higuchi c,
a Department of Crystalline Materials Science, Nagoya Uniuersity, Aichi 464, Japan b National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan c Institute of Applied Physics, Uniuersity of Tsukuba, Ibaraki 305, Japan Multilayered films of Cu and Ta were prepared with the alternate deposition technique. It was found that Ta layers have considerably disordered structure in these films. Resonance detector spectroscopy using a pulsed neutron source revealed that the Debye temperature of this disordered Ta is about 130 K lower than that of bulk Ta.
Introduction The C u - T a binary system is characterized by a positive mixing heat (AHmix > 0) and neither primary solid solution nor intermetallic compound exists in the equilibrium phase diagram [1]. Nevertheless, it has been reported that an amorphous phase can be obtained by co-sputtering technique [2] and mechanical alloying (MA) of elemental powders [3]. It is claimed that the amorphization by MA proceeds only when Cu and Ta crystallites are reduced to less than 100 ,~ in size and that such fine Cu and Ta particles change their thermodynamic properties and behave as a system with AHmix < 0 [4]. When powders of Cu and Ta are repeatedly kneaded and sheared, a laminated structure of constituent elements is formed during mechanical alloying process. Therefore, studies on C u / T a multilayered films would provide further insight into the solid state reaction between the immiscible Cu and Ta metals. It has been reported that lSlTa nuclei can resonantly absorb neutrons with an energy of 4.28 eV, and, hence the resonance detector spectroscopy (RDS) would offer valuable information about the thermal vibration of Ta atoms [5]. Since the peak value of the capture cross section for neutrons is several orders of magnitude larger than the scattering cross section, the RDS spectrum can be measured even for a thin film with thickness less than a few 1000 .A. In the present experiment, we studied the structure and lattice dynamics of Ta atoms in C u / T a films by using the X-ray diffraction technique in combination with the RDS above mentioned. Correspondence to: Dr. T. Koyano, Department of Crystalline Materials Science, Nagoya University, Aichi 464, Japan. Fax + 81.52.7822129.
Experimental procedure The C u / T a films were prepared with the alternate deposition technique in a vacuum better than 5 x 10 -7 Torr. Cu and Ta were evaporated from two electron beam heating apparatus. The deposition rates for Cu and Ta were 0.5 A / s e c . The first layer deposited was Ta for all samples. Several different thicknesses were designed for the Ta layers. They were 20, 50, 100, 200 and 500 ,~ while keeping its total thickness of Ta layers to be 2000 A. The thickness of corresponding Cu layers were determined so as to adjust an average composition to Cu30Ta70. Sapphire (012) wafers were chosen as a substrate to reduce a background in the X-ray diffraction spectra and to avoid chemical reaction with the films at elevated temperatures. A Si (111)wafer at room temperature was also used for deposition of a pure Ta film. X-ray diffraction experiments with the 0-20 scanning mode were carried out with Cu K~ radiation using a two-circle goniometer equipped with a graphite (00.2) counter monochromator. The RDS measurements for lSlTa nuclei were made by using the Resonance Analyzer Time of flight spectrometer (RAT) installed at a pulsed neutron source KENS in National Laboratory for High Energy Physics, Tsukuba, Japan. Details for the RDS experiment and data analysis have been described in ref. [5]. o
Results and discussion X-ray diffraction patterns for various C u / T a films deposited on sapphire and Si substrates held at room temperature are shown in fig. 1. A C u / T a (14 ,~/50 A) film was subsequently annealed at 600°C for 12 h. The diffraction spectrum for this sample is incorporated in fig. 1. Two broad diffraction lines are observed around 20 = 38 ° and 67 ° for as-deposited films, and, in
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)
T. Koyano et al. / Structural studies of Cu / Ta films
162
contrast, the annealed film exhibits sharp diffraction lines of bcc Ta and fcc Cu as expected from the equilibrium phase diagram [1]. In the small scattering angle region (20 < 5°), many peaks due to the composition modulation are observed for as-deposited films though they are not periodic in the Q-space. This implies that the interfaces between Cu and Ta layers are not smooth. Because of roughness, the X-ray diffraction profile in a wide scattering angle region can be analyzed without taking into account possible coherent scattering [6]. The intensity of Cu ( l i d line increases with increasing the thickness of Cu and Ta layers, but signals still remain broad. In order to identify these broad signals, we deposited a pure Ta film with 500 A in thickness onto a Si (111) wafer at room temperature. Surprisingly, the profile is turned out to be broad as shown in fig. l(e). Hence, we learned that Ta layers deposited on a substrate at room temperature are always quite disordered in the absence of adjacent Cu layers. A comparison of fig. 1 (e) with (d) immediately tells us that the (200) line of bcc Ta is absent in the former, and the (211) line of bcc Ta (20 = 69.7 °) is replaced by a broad peak located at 20 = 67 ° in the as-deposited film. Fukunaga et al. have pointed out that the (200) line of a bcc structure preferentially disappears in the course of the amor-
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o,1
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•
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...:
.
,_ J i . ..Q
i • ,~
b)
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k
/% 30
40
× 2 50
60
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70
80
Fig. 1. X-ray diffraction spectra for films deposited on substrates at room temperature. (a) Cu/Ta (5.5 ,~/20 A); (b) Cu/Ta (14 A/50 ~,); (c) Cu/Ta (28 A/100 A) and (d) Cu/Ta (14 A/50 A) film annealed at 600°C for 12 h, respectively. (e) is a profile for a Ta film on Si (111) with 500 A in thickness. Sharp peaks at 20 = 28.47° (out of scale) and 58.93° originate in the substrate.
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E 275
280 Time of Flight (/~ sec)
285
Fig. 2. RDS spectra for lSlTa recorded at 20 K. Symbols are as follows: (A) Cu/Ta (5.5 A/20 A), (*) Cu/Ta (56 A/200 A) films and (o) Ta foil, respectively.
phization process when a bcc metal is involved as a major component in MA [7]. This implies that the degree of disorder in the bcc lattice may well be judged from the reduction in the intensity of the (200) line. Ta atoms in the as-deposited layer are, therefore, considerably disordered and no longer posses a rigid bcc structure. The RDS spectra were measured at 20 K for C u / T a (5.5 A / 2 0 A), C u / T a (56 A / 2 0 0 A) films and a Ta foil with 7 ixm in thickness• They are plotted as a function of time of flight of incoming neutrons in fig. 2. The line widths for both C u / T a films are the same (2.8 }xs) and sharper than that for the Ta foil (3.6 ITS). these line widths correspond to the effective temperatures [5] of about 50 and 100 K, respectively. We analyzed the data using the Debye model, where the dispersion curvc of phonons is approximated to a linear function of the wave number q. The Debye temperature 0 D for the Ta foil turned out to be about 250 K, in good agreement with that of bulk material. In contrast, that of C u / T a films is found to be about 120 K regardless of the thickness of Ta layers. A combination of the results of RDS spectroscopy and X-ray diffraction experiments led us to conclude that an extremely low 0 D results from the disordered Ta. X-ray diffraction profiles for films deposited on sapphire substrates at 270°C are shown in fig. 3. A Ta film with 500 A in thickness exhibits a sharp diffraction pattern of the bcc structure, while C u / T a (14 A / 5 0 A) film shows broad Ta signal and sharp lines of fcc Cu. No diffraction lines in the small scattering angle region are observed. A diffraction spectrum for C u / T a (139 A / 5 0 0 A) film (fig. 3(c)) is composed of diffraction lines of sharp and broad Ta, and fcc Cu. It is considered that the sharp and broad Ta lines originate in the
T. Koyano et al. / Structural studies of Cu / Ta films I
I
I
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i
i
(a)
163
the Cu and Ta are deposited at high temperatures. This particular role of Cu atoms would be reflected in the characteristic feature of the C u - T a amorphous phase, where Cu atoms of only 5 at% are necessary to amorphize Ta atoms, and its crystallization temperature exceeds 700°C [2]. Acknowledgments: We would like to thank to Dr. Y. Nakai of University of Tsukuba and Mr. H. Akatsuka of Nagoya University for the R D S experiments.
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References
. ' o
610
% 710
80
(deg.)
Fig. 3. X-ray diffraction spectra for C u / T a films deposited on sapphire (012) wafers at 270°C. (a) Ta film with 500 A in thickness; (b) Cu/Ta (14 A / 5 0 A); (c) Cu/Ta (139 * / 5 0 0 A), respectively. first Ta layer and those sandwiched between Cu layers, respectively. This indicates that Cu atoms should be responsible for stabilizing the disordered Ta, though
[1] Binary Phase Diagrams (2nd ed.), eds. T.B. Massalski et al. (ASM International, Ohio, 1990), p. 1486. [2] J.E. Oh, J.A. Woollam, K.D. Aylesworth, D.J. Sellmyer and J.J. Pouch, J. Appl. Phys. 60 (1986) 4281. [3] C.H. Lee, M. Mori, T. Fukunaga and U. Mizutani, Mat. Sci. Forum 88-90 (1992) 399. [4] C.H. Lee, T. Fukunaga, Y. Yamada, H. Okamoto and U. Mizutani, J. Phase Equilibria 14 (1993) 167. [5] H. Rauh and N. Watanabe, Nucl. Inst. and Meth. 222 (1984) 507. [6] T. Koyano, E. Kita, K. Ohshima and A. Tasaki, J. Phys. Condensed Matter 3 (1991) 5921. [7] T. Fukunaga, Y. Homma, M. Misawa and K. Suzuki, J. Non-Cryst. Solids 117/118 (1990) 721.