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Si-multilayered films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e921–e922
Physical properties of as-deposited and post-treated Ni/Si-multilayered films Y.P. Leea,*, S.M. Kima, J.S. Parka, Y.V. Kudryavtsevb, K.W. Kimc a
Quantum Photonic Science Research Center and Department of Physics, Hanyang University, Seoul 133-791, South Korea b Institute of Metal Physics, National Academy of Sciences of Ukraine, Kiev 03142, Ukraine c Department of Physics and Advanced Materials, Sunmoon University, Asan 336-708, Choongnam, South Korea
Abstract Solid-state reactions in the Ni/Si-multilayered films (MLF) induced by ion-beam mixing (IBM) and thermal annealing are studied by using spectroscopic ellipsometry. Z-NiSi phase is spontaneously formed in the interface during the deposition of MLF, when the thickness is less than 6 nm, and IBM and thermal annealing after the deposition further continue the formation of Z-NiSi phase. r 2004 Elsevier B.V. All rights reserved. PACS: 68.65.Ac; 78.66.w Keywords: Multilayered films; Ion-beam mixing; Silicide; Optical conductivity
Interlayer solid-state reaction in 3d-transition-metal/ Si-multilayered films (MLF) during deposition is an intriguing problem. An amorphous phase of Z-NiSi may be formed owing to the interlayer mixing in Ni/Si MLF [1] and various other silicides can be formed. Due to a rather narrow composition range of silicide formation and high melting points, processing of these intermetallics by the conventional methods is extremely difficult. From a thermodynamical point of view, the nickel silicide formation caused by thermal annealing of Ni films deposited onto Si wafer is a consequence of the free-energy decrease in such a reaction and some external energy input can substantially accelerate the silicide formation by changing the thermodynamic energy of the system. Annealing of polycrystalline Ni film with 100–500 nm in thickness deposited onto a single-crystalline Si substrate leads to firstly a formation of Ni2Si phase after annealing at 473–600 K and to the formation of Z-NiSi phase after annealing at 873 K [2]. On the other hand, an employment of the ion-beam mixing (IBM) for the silicide formation implies that the *Corresponding author. Tel.: +82-2-2290-0916; fax: +82-22281-5573. E-mail address: [email protected] (Y.P. Lee).
two layers can be interacted by energetic incoming particles in such a way that it cannot be achievable by normal equilibrium techniques. This includes not only the activation of the interfacial phase transition but also the diffusion necessary to maintain the silicide growth [3]. It is well known that both optical and magnetooptical (MO) properties of metals depend strongly on their electron energy structures, which are correlated with the atomic and magnetic ordering. The interdiffusion between Ni and Si atoms in the Ni/Si MLF induced by IBM or thermal annealing should change its chemical and atomic ordering in the reactive zone as well as a decrease in the thickness of pure Ni. It was shown that the nucleation of new crystalline phase in the amorphous matrix of alloy could be detected by optical tools rather than by the conventional XRD methods. This fact creates the favorable conditions for the comparison of optical properties of the reacted Ni/Si MLF with those of various silicides. We prepared three Ni/Si MLF by DC sputtering onto single-crystalline Si substrates kept at room temperature (RT). They are (3 nm Ni/2.69 nm Si)40 (3 nm Ni/5.4 nm Si)50, and (3 nm Ni/10.6 nm Si)22 MLF. The individual sublayer thicknesses were chosen in order to make
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.234
ARTICLE IN PRESS Y.P. Lee et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e921–e922
e922
as-deposited
Ni Si
60
bulk (left scale)
IBM
30
o
Ni2Si
annealing @ 200 C/1h
Ni3Si
o
30
o
25
14
-1
25 40
σ (10 s )
14
-1
σ (10 s )
14
-1
σ (10 s )
annealing @ 200 C/1h & 400 C/1h
50
20
20
30
15
20
(3 nm Ni/5.37 nm Si)50 (3 nm Ni/10.7 nm Si)22 (3 nm Ni/2.69 nm Si)40 1
2
3
15
as deposited
4
(right scale) 4
3
5
Photon energy (eV) ε
1
2
Fig. 1. Optical conductivity spectra of Ni/Si MLF together with those of various bulk Ni silicides.
1 as-deposited
thinner than skin-penetration depth. The structural characterization of Ni/Si MLF was performed by lowangle XRD. The magnetic state of the as-deposited Ni/ Si MLF was checked by studying the in-plane and outof-plane magnetization hysteresis loop, MðHÞ; at RT by using a vibrating sample magnetometer. The optical properties were studied at RT by spectroscopic ellipsometry. The MLF were ion-beam mixed in a high vacuum (B1 106 Torr) by Ar+ ions directed normally to the film surface with the following conditions: ion energy of 80 keV, ion flux of 1.5 106 A/cm2, and ion does of 1.5 1016 Ar+/cm2. Additionally, the as-deposited Ni/Si MLF were annealed in series at 453, 683, 823 and 1073 K and physical properties were also studied after each step of annealing. The magnetic properties of the as-deposited Ni/Si MLF look rather unusual. It is seen that the in-plane and out-of-plane MðHÞ loops (not shown) exhibit some features of ferromagnetic behavior, but they coincide with each other. Such a behavior reflects the lack of the easy axis for magnetization and may be explained by the formation of ferromagnetic granular structures with spherical symmetry in Ni sublayers. Fig. 1 shows the optical conductivity (OC) spectra of MLF together with three bulk silicides (NiSi, Ni2Si and Ni3Si). Only the OC spectra of as-deposited MLF with the Si sublayer thickness less than 6 nm resemble to that of NiSi; deep minimum at B1 eV and two peaks at B2 and B4.3 eV. This fact indicates that there is interlayer mixing during the deposition, forming almost equiatomic NiSi. This argument agrees well with the statement of Clevenger and Thompson [4] that in the as-deposited Ni/Si MLF an amorphous NiSi phase of about 4 nm in thickness is formed between polycrystalline Ni and amorphous Si sublayers. As shown in Fig. 2,
IBM
0
o
annealing @ 200 C/1h o
-1
o
annealing @ 200 C/1h & 400 C/1h
-2 1
2
3
4
5
Photon energy (eV)
Fig. 2. Optical conductivity spectra of (3 nm Ni/5.4 nm Si)50 MLF after an ion beam mixing or an annealing.
the OC spectrum of (3 nm Ni/5.4 nm Si)50 MLF is slightly changed after IBM or annealing, but closer to that of bulk NiSi. We applied the optical tools to investigate the solidstate reaction in the Ni/Si MLF. The comparison of the experimental OC spectra of the Ni/Si MLF, as-deposited or post-treated, with those of bulk silicides enables us to conclude that an amorphous regions with a stoichiometry close to NiSi are spontaneously formed during the deposition of the Ni/Si MLF. This work was supported by the KOSEF through Quantum Photonic Science Research Center, by a KRF Grant (2001-015-DS0015), and by MOST, Korea.
References [1] M.A. Hollander, B.J. Thijsse, E.J. Mittemeijer, Phys. Rev. B 42 (1990) 5481. [2] P.Gas Mangelinck, A. Grob, B. Pichaud, O. Thomas, J. Appl. Phys. 79 (1996) 4078. [3] Y.V. Kudryavtsev, Y.P. Lee, J. Dubowik, B. Szymanski, J.Y. Rhee, Phys. Rev. B 65 (2002) 104417. [4] L.A. Clevenger, C.V. Thompson, J. Appl. Phys. 68 (1990) 1325.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e921–e922
Physical properties of as-deposited and post-treated Ni/Si-multilayered films Y.P. Leea,*, S.M. Kima, J.S. Parka, Y.V. Kudryavtsevb, K.W. Kimc a
Quantum Photonic Science Research Center and Department of Physics, Hanyang University, Seoul 133-791, South Korea b Institute of Metal Physics, National Academy of Sciences of Ukraine, Kiev 03142, Ukraine c Department of Physics and Advanced Materials, Sunmoon University, Asan 336-708, Choongnam, South Korea
Abstract Solid-state reactions in the Ni/Si-multilayered films (MLF) induced by ion-beam mixing (IBM) and thermal annealing are studied by using spectroscopic ellipsometry. Z-NiSi phase is spontaneously formed in the interface during the deposition of MLF, when the thickness is less than 6 nm, and IBM and thermal annealing after the deposition further continue the formation of Z-NiSi phase. r 2004 Elsevier B.V. All rights reserved. PACS: 68.65.Ac; 78.66.w Keywords: Multilayered films; Ion-beam mixing; Silicide; Optical conductivity
Interlayer solid-state reaction in 3d-transition-metal/ Si-multilayered films (MLF) during deposition is an intriguing problem. An amorphous phase of Z-NiSi may be formed owing to the interlayer mixing in Ni/Si MLF [1] and various other silicides can be formed. Due to a rather narrow composition range of silicide formation and high melting points, processing of these intermetallics by the conventional methods is extremely difficult. From a thermodynamical point of view, the nickel silicide formation caused by thermal annealing of Ni films deposited onto Si wafer is a consequence of the free-energy decrease in such a reaction and some external energy input can substantially accelerate the silicide formation by changing the thermodynamic energy of the system. Annealing of polycrystalline Ni film with 100–500 nm in thickness deposited onto a single-crystalline Si substrate leads to firstly a formation of Ni2Si phase after annealing at 473–600 K and to the formation of Z-NiSi phase after annealing at 873 K [2]. On the other hand, an employment of the ion-beam mixing (IBM) for the silicide formation implies that the *Corresponding author. Tel.: +82-2-2290-0916; fax: +82-22281-5573. E-mail address: [email protected] (Y.P. Lee).
two layers can be interacted by energetic incoming particles in such a way that it cannot be achievable by normal equilibrium techniques. This includes not only the activation of the interfacial phase transition but also the diffusion necessary to maintain the silicide growth [3]. It is well known that both optical and magnetooptical (MO) properties of metals depend strongly on their electron energy structures, which are correlated with the atomic and magnetic ordering. The interdiffusion between Ni and Si atoms in the Ni/Si MLF induced by IBM or thermal annealing should change its chemical and atomic ordering in the reactive zone as well as a decrease in the thickness of pure Ni. It was shown that the nucleation of new crystalline phase in the amorphous matrix of alloy could be detected by optical tools rather than by the conventional XRD methods. This fact creates the favorable conditions for the comparison of optical properties of the reacted Ni/Si MLF with those of various silicides. We prepared three Ni/Si MLF by DC sputtering onto single-crystalline Si substrates kept at room temperature (RT). They are (3 nm Ni/2.69 nm Si)40 (3 nm Ni/5.4 nm Si)50, and (3 nm Ni/10.6 nm Si)22 MLF. The individual sublayer thicknesses were chosen in order to make
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.234
ARTICLE IN PRESS Y.P. Lee et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e921–e922
e922
as-deposited
Ni Si
60
bulk (left scale)
IBM
30
o
Ni2Si
annealing @ 200 C/1h
Ni3Si
o
30
o
25
14
-1
25 40
σ (10 s )
14
-1
σ (10 s )
14
-1
σ (10 s )
annealing @ 200 C/1h & 400 C/1h
50
20
20
30
15
20
(3 nm Ni/5.37 nm Si)50 (3 nm Ni/10.7 nm Si)22 (3 nm Ni/2.69 nm Si)40 1
2
3
15
as deposited
4
(right scale) 4
3
5
Photon energy (eV) ε
1
2
Fig. 1. Optical conductivity spectra of Ni/Si MLF together with those of various bulk Ni silicides.
1 as-deposited
thinner than skin-penetration depth. The structural characterization of Ni/Si MLF was performed by lowangle XRD. The magnetic state of the as-deposited Ni/ Si MLF was checked by studying the in-plane and outof-plane magnetization hysteresis loop, MðHÞ; at RT by using a vibrating sample magnetometer. The optical properties were studied at RT by spectroscopic ellipsometry. The MLF were ion-beam mixed in a high vacuum (B1 106 Torr) by Ar+ ions directed normally to the film surface with the following conditions: ion energy of 80 keV, ion flux of 1.5 106 A/cm2, and ion does of 1.5 1016 Ar+/cm2. Additionally, the as-deposited Ni/Si MLF were annealed in series at 453, 683, 823 and 1073 K and physical properties were also studied after each step of annealing. The magnetic properties of the as-deposited Ni/Si MLF look rather unusual. It is seen that the in-plane and out-of-plane MðHÞ loops (not shown) exhibit some features of ferromagnetic behavior, but they coincide with each other. Such a behavior reflects the lack of the easy axis for magnetization and may be explained by the formation of ferromagnetic granular structures with spherical symmetry in Ni sublayers. Fig. 1 shows the optical conductivity (OC) spectra of MLF together with three bulk silicides (NiSi, Ni2Si and Ni3Si). Only the OC spectra of as-deposited MLF with the Si sublayer thickness less than 6 nm resemble to that of NiSi; deep minimum at B1 eV and two peaks at B2 and B4.3 eV. This fact indicates that there is interlayer mixing during the deposition, forming almost equiatomic NiSi. This argument agrees well with the statement of Clevenger and Thompson [4] that in the as-deposited Ni/Si MLF an amorphous NiSi phase of about 4 nm in thickness is formed between polycrystalline Ni and amorphous Si sublayers. As shown in Fig. 2,
IBM
0
o
annealing @ 200 C/1h o
-1
o
annealing @ 200 C/1h & 400 C/1h
-2 1
2
3
4
5
Photon energy (eV)
Fig. 2. Optical conductivity spectra of (3 nm Ni/5.4 nm Si)50 MLF after an ion beam mixing or an annealing.
the OC spectrum of (3 nm Ni/5.4 nm Si)50 MLF is slightly changed after IBM or annealing, but closer to that of bulk NiSi. We applied the optical tools to investigate the solidstate reaction in the Ni/Si MLF. The comparison of the experimental OC spectra of the Ni/Si MLF, as-deposited or post-treated, with those of bulk silicides enables us to conclude that an amorphous regions with a stoichiometry close to NiSi are spontaneously formed during the deposition of the Ni/Si MLF. This work was supported by the KOSEF through Quantum Photonic Science Research Center, by a KRF Grant (2001-015-DS0015), and by MOST, Korea.
References [1] M.A. Hollander, B.J. Thijsse, E.J. Mittemeijer, Phys. Rev. B 42 (1990) 5481. [2] P.Gas Mangelinck, A. Grob, B. Pichaud, O. Thomas, J. Appl. Phys. 79 (1996) 4078. [3] Y.V. Kudryavtsev, Y.P. Lee, J. Dubowik, B. Szymanski, J.Y. Rhee, Phys. Rev. B 65 (2002) 104417. [4] L.A. Clevenger, C.V. Thompson, J. Appl. Phys. 68 (1990) 1325.