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Kinetic crystallization behavior of SbOx thin films
ARTICLE IN PRESS
Physica B 352 (2004) 206–209 www.elsevier.com/locate/physb
Kinetic crystallization behavior of SbOx thin films Ming Fang, Qing-hui Li, Fu-xi Gan Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 31 October 2003; received in revised form 15 July 2004; accepted 16 July 2004
Abstract The crystallization behavior of SbOx thin films, prepared by reactive DC magnetron sputtering under different partial oxygen pressures, was examined. The crystal structures of the as-deposited and the annealed films were identified by Xray diffraction. The crystalline phase was identified as a rhombohedral structure with a c=a value of 2.6 (a ¼ 4:307 nm; c ¼ 11:273 nm). The temperatures of crystallization at different heating rates were determined by using a differential scanning calorimeter. Based on the Kissinger formula, the crystallization activation energies of these amorphous films were calculated. The experimental results showed that the activation energy increased with increasing partial oxygen pressure, while the enthalpy difference between the as-deposited and the crystalline states decreased. r 2004 Elsevier B.V. All rights reserved. Pacs: 81.15.Cd; 81.70.Pg; 81.05.Je; 61.10.Nz Keywords: SbOx thin films; Reactive sputtering; XRD; DSC; Crystallization kinetics
1. Introduction Non-stoichiometric thin films of SbOx, GeOx and TeOx were found to have a feasibility to be used as optical recording media [1]. SbOx film is particularly suitable for this use, since this kind of thin film is known to present a strong optical contrast upon phase change while retaining a stable oxygen content [2].
Corresponding author. Tel.: +86-21-69918577; fax: +86-
21-69918800. E-mail address: [email protected] (M. Fang).
The crystallization kinetics have been confirmed to be based on a thermal activation process. The behavior could be described by the Johnson–Mehl–Avrami equation [3]. By assuming that the crystallization rate reaches its maximum value in the transformation process, the activation energy can be derived from the Kissinger formula [4] ln
a E ¼ þ C; T 2c RT c
ð1Þ
where a is the heating rate, E is the activation energy, Tc is the crystallization temperature at heating rate a; R is the gas constant, and C is a constant. When a series of experiments at different
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.07.012
ARTICLE IN PRESS M. Fang et al. / Physica B 352 (2004) 206–209
SbOx films were deposited onto k9 glass substrates by reactive DC magnetron sputtering using a pure (99.999%) antimony target in a mixture of argon-oxygen environment. The background pressure in the system was 5.0 104 Pa and the sputtering pressure was 6.0 101 Pa, while the oxygen pressures were 4.0 102 , 7.5 102 and 1.3 101 Pa, respectively. The sputtering power used in the experiment was 150 W. A differential scanning calorimeter (DSC; Universal V2.4F TA Instruments) was used to analyze the thermal behavior of the sputtered material. The DSC measurement was scanned at different heating rates. The melting temperature of pure indium and lead was used to calibrate the instrument. During the experiment, the weight of each sample was about 8 mg and the powder scratched from the surface of the k9 glass substrate was sealed inside an aluminum pan in N2 atmosphere to prevent oxidation. Annealing was conducted under Ar atmosphere at 250 1C for 30 min. X-ray diffraction (XRD) was used to examine the structure of the as-deposited films and the annealed films.
Intensity (arb.unit)
2. Experimental details
heat treatment. Fig. 1 shows the X-ray spectra of the as-deposited (a) and the annealed SbOx films (b). From Fig. 1, it could be found that the asdeposited films were amorphous. After annealing, their state was crystalline. According to the Joint Committee on Power Diffraction Standards (JCPDS) cards, the annealed films appeared to have a rhombohedral structure with a c/a value of 2.6 (a ¼ 4:307 nm; c ¼ 11:273 nm). Fig. 2 shows the DSC curves of the SbOx films deposited at Po2 ¼ 4:0 102 Pa with the heating rates of 5, 10 and 15 1C/min, respectively. There is a clear exothermic peak in each curve in Fig. 2. Fig. 3 shows the relationship between the crystallization temperature, Tc, and the heating rate for three SbOx films. We could see that Tc of each film increased with increasing heating rate and also with increasing partial oxygen pressure. The
Po2 = 1.3×10−1Pa Po2 = 7.5×10−2Pa
Po2 = 4.0×10−2Pa
10
20
30
(a)
40 50 2θ / (°)
60
70
(012) (015) (110) (104)
Intensity (arb.unit)
heating rates are performed, a straight line is obtained by plotting ln ða=T 2c Þ versus 1/(RTc). The activation energy is calculated using Kissinger’s plot. The crystallization kinetics of the phase-change in alloys has been studied extensively in detail [5–7]. However, the literature on the crystallization behavior of SbOx thin films is relatively sparse. In this paper, we try to explain its crystallization behavior by using the method of differential scanning calorimeter (DSC) analysis and to provide some useful information for its application in optical recording.
207
(006)
(012) (110)
Po2 = 1.3×10−1Pa
(110)
Po2 = 7.5×10−2Pa
(012)
Po2 = 4.0×10−2Pa
3. Results and discussion XRD patterns were recorded and studied in order to understand the structure changes after
10 (b)
20
30
40 50 2θ / (°)
60
70
Fig. 1. X-ray diffraction spectra of the as-deposited (a) and annealed (b) SbOx thin films.
ARTICLE IN PRESS
10 °C/min 5 °C/min 50
100
150
200
250
−1 11.0 Po2 = 1.3×10 Pa 10.8 Po2 = 7.5×10−2Pa 10.6 Po2 = 4.0×10−2Pa 10.4 10.2 10.0 9.8 9.6 9.4 9.2 2.0 2.1 2.2 2.3 2.4 2.5 2.6
−1
T/°C
Tc
Fig. 2. DSC curves for SbOx thin films at different heating rate, sputtered at Po2=4.0 102 Pa.
(K
−1
)
Fig. 4. Plot of ln ða=T 2c Þ vs 1/Tc of SbOx thin films sputtered at three different oxygen pressures.
45
Po2 = 1.3×10−1Pa
200
−3
/10
4
Ea ¦ ∆H
40
Po2 = 7.5×10−2Pa
150
−2
Po2 = 4.0×10 Pa
¦H∆H (J/g)
Tc (˚C )
35
3
30 25
2
20 15
1
10 5 0
100 4
6
8 10 12 dT/dt (°C/min)
14
16
Fig. 3. Relationship between crystallizing temperature and heating rate for SbOx thin films sputtered at different oxygen pressures.
corresponding activation energies of the SbOx films were calculated using Kissinger’s method. They are 0.966, 1.517 and 3.243 eV, derived from Fig. 4, corresponding to the films deposited at partial oxygen pressures of 4.0 102, 7.5 102 and 1.3 101 Pa, respectively. The enthalpy differences between the as-deposited and the crystalline states decrease with oxygen pressure, while the activation energy increases, as shown in Fig. 5. The phase transformation process of the material is a thermally activated process. There is an
Ea (eV)
Heat Flow (arb.unit)
15 °C/min
Ln (Tc 2/ α)
M. Fang et al. / Physica B 352 (2004) 206–209
208
0 4
6
8
Po2
10 / (10-2 Pa )
12
14
Fig. 5. Activation energy and enthalpy differences between amorphous and crystalline states of SbOx thin film as a function of Po2.
energy barrier to be overcome when the material is transformed from the current phase to another one. The higher the energy barrier is, the more stable the primary phase is. In order to allow recorded data to be kept unchanged for a long time, it is important for an optical recording medium to possess a high activation energy. From this point, it is essential to enhance the oxygen content in SbOx films when the films are to be used in optical recording. However, besides stability, there are some properties such as the crystallization time, reflectivity contrast and carrier to noise ratio (CNR), etc. which must be considered
ARTICLE IN PRESS M. Fang et al. / Physica B 352 (2004) 206–209
209
when a material is used as an optical recording medium. So, in order to determine optimum oxygen content in SbOx films, further experiments are needed. Related work is in progress.
three different sputtering oxygen partial pressures, which are 4.0 102, 7.5 102 and 1.3 101 Pa, respectively.
4. Conclusion
Acknowledgement
SbOx films were deposited onto k9 glass substrates by reactive DC magnetron sputtering from a pure (99.999%) antimony target in an oxygen environment. The crystal structures of the films were identified by XRD. The structure of the as-deposited film is amorphous, while the films changed from amorphous to crystalline states due to heat treatment. There is an exothermal peak observed in the DSC curve for SbOx sputtered film. The crystallization temperature increases with increasing heating rate. Under the same heating rate, the crystallization temperature increases with increasing sputtered oxygen pressure. The activation energies calculated from Kissinger’s formula are 0.966, 1.517 and 3.243 eV corresponding to the
This work was supported by the National Natural Science Foundation of China (60207005).
References [1] T. Ohta, M. Takenaga, N. Akahira, T. Yamashita, J. Appl. Phys. 53 (1983) 8497. [2] F. Vega, C.N. Afonso, Appl. Phys. B. 62 (1996) 235. [3] N. Ohshima, J. Appl. Phys. 79 (1996) 8357. [4] H.E. Kissinger, Anal. Chem. 29 (1959) 1702. [5] N. Yamada, E. Ohno, N. Akahira, K. Nishiuchi, K. Nagata, M. Takao, Jpn. J. Appl. Phys. 26 (Suppl. 26–4) (1987) 61. [6] M. Okuda, J.C. Ree, J. Fusong, H. Naito, T. Matsushita, Jpn. J. Appl. Phys. 26 (Suppl. 26–4) (1987) 71. [7] M. Mehdi, G. Brun, J.C. Jumas, J.C. Tedenac, J. Mater. Sci. 30 (1995) 5732.
Physica B 352 (2004) 206–209 www.elsevier.com/locate/physb
Kinetic crystallization behavior of SbOx thin films Ming Fang, Qing-hui Li, Fu-xi Gan Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 31 October 2003; received in revised form 15 July 2004; accepted 16 July 2004
Abstract The crystallization behavior of SbOx thin films, prepared by reactive DC magnetron sputtering under different partial oxygen pressures, was examined. The crystal structures of the as-deposited and the annealed films were identified by Xray diffraction. The crystalline phase was identified as a rhombohedral structure with a c=a value of 2.6 (a ¼ 4:307 nm; c ¼ 11:273 nm). The temperatures of crystallization at different heating rates were determined by using a differential scanning calorimeter. Based on the Kissinger formula, the crystallization activation energies of these amorphous films were calculated. The experimental results showed that the activation energy increased with increasing partial oxygen pressure, while the enthalpy difference between the as-deposited and the crystalline states decreased. r 2004 Elsevier B.V. All rights reserved. Pacs: 81.15.Cd; 81.70.Pg; 81.05.Je; 61.10.Nz Keywords: SbOx thin films; Reactive sputtering; XRD; DSC; Crystallization kinetics
1. Introduction Non-stoichiometric thin films of SbOx, GeOx and TeOx were found to have a feasibility to be used as optical recording media [1]. SbOx film is particularly suitable for this use, since this kind of thin film is known to present a strong optical contrast upon phase change while retaining a stable oxygen content [2].
Corresponding author. Tel.: +86-21-69918577; fax: +86-
21-69918800. E-mail address: [email protected] (M. Fang).
The crystallization kinetics have been confirmed to be based on a thermal activation process. The behavior could be described by the Johnson–Mehl–Avrami equation [3]. By assuming that the crystallization rate reaches its maximum value in the transformation process, the activation energy can be derived from the Kissinger formula [4] ln
a E ¼ þ C; T 2c RT c
ð1Þ
where a is the heating rate, E is the activation energy, Tc is the crystallization temperature at heating rate a; R is the gas constant, and C is a constant. When a series of experiments at different
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.07.012
ARTICLE IN PRESS M. Fang et al. / Physica B 352 (2004) 206–209
SbOx films were deposited onto k9 glass substrates by reactive DC magnetron sputtering using a pure (99.999%) antimony target in a mixture of argon-oxygen environment. The background pressure in the system was 5.0 104 Pa and the sputtering pressure was 6.0 101 Pa, while the oxygen pressures were 4.0 102 , 7.5 102 and 1.3 101 Pa, respectively. The sputtering power used in the experiment was 150 W. A differential scanning calorimeter (DSC; Universal V2.4F TA Instruments) was used to analyze the thermal behavior of the sputtered material. The DSC measurement was scanned at different heating rates. The melting temperature of pure indium and lead was used to calibrate the instrument. During the experiment, the weight of each sample was about 8 mg and the powder scratched from the surface of the k9 glass substrate was sealed inside an aluminum pan in N2 atmosphere to prevent oxidation. Annealing was conducted under Ar atmosphere at 250 1C for 30 min. X-ray diffraction (XRD) was used to examine the structure of the as-deposited films and the annealed films.
Intensity (arb.unit)
2. Experimental details
heat treatment. Fig. 1 shows the X-ray spectra of the as-deposited (a) and the annealed SbOx films (b). From Fig. 1, it could be found that the asdeposited films were amorphous. After annealing, their state was crystalline. According to the Joint Committee on Power Diffraction Standards (JCPDS) cards, the annealed films appeared to have a rhombohedral structure with a c/a value of 2.6 (a ¼ 4:307 nm; c ¼ 11:273 nm). Fig. 2 shows the DSC curves of the SbOx films deposited at Po2 ¼ 4:0 102 Pa with the heating rates of 5, 10 and 15 1C/min, respectively. There is a clear exothermic peak in each curve in Fig. 2. Fig. 3 shows the relationship between the crystallization temperature, Tc, and the heating rate for three SbOx films. We could see that Tc of each film increased with increasing heating rate and also with increasing partial oxygen pressure. The
Po2 = 1.3×10−1Pa Po2 = 7.5×10−2Pa
Po2 = 4.0×10−2Pa
10
20
30
(a)
40 50 2θ / (°)
60
70
(012) (015) (110) (104)
Intensity (arb.unit)
heating rates are performed, a straight line is obtained by plotting ln ða=T 2c Þ versus 1/(RTc). The activation energy is calculated using Kissinger’s plot. The crystallization kinetics of the phase-change in alloys has been studied extensively in detail [5–7]. However, the literature on the crystallization behavior of SbOx thin films is relatively sparse. In this paper, we try to explain its crystallization behavior by using the method of differential scanning calorimeter (DSC) analysis and to provide some useful information for its application in optical recording.
207
(006)
(012) (110)
Po2 = 1.3×10−1Pa
(110)
Po2 = 7.5×10−2Pa
(012)
Po2 = 4.0×10−2Pa
3. Results and discussion XRD patterns were recorded and studied in order to understand the structure changes after
10 (b)
20
30
40 50 2θ / (°)
60
70
Fig. 1. X-ray diffraction spectra of the as-deposited (a) and annealed (b) SbOx thin films.
ARTICLE IN PRESS
10 °C/min 5 °C/min 50
100
150
200
250
−1 11.0 Po2 = 1.3×10 Pa 10.8 Po2 = 7.5×10−2Pa 10.6 Po2 = 4.0×10−2Pa 10.4 10.2 10.0 9.8 9.6 9.4 9.2 2.0 2.1 2.2 2.3 2.4 2.5 2.6
−1
T/°C
Tc
Fig. 2. DSC curves for SbOx thin films at different heating rate, sputtered at Po2=4.0 102 Pa.
(K
−1
)
Fig. 4. Plot of ln ða=T 2c Þ vs 1/Tc of SbOx thin films sputtered at three different oxygen pressures.
45
Po2 = 1.3×10−1Pa
200
−3
/10
4
Ea ¦ ∆H
40
Po2 = 7.5×10−2Pa
150
−2
Po2 = 4.0×10 Pa
¦H∆H (J/g)
Tc (˚C )
35
3
30 25
2
20 15
1
10 5 0
100 4
6
8 10 12 dT/dt (°C/min)
14
16
Fig. 3. Relationship between crystallizing temperature and heating rate for SbOx thin films sputtered at different oxygen pressures.
corresponding activation energies of the SbOx films were calculated using Kissinger’s method. They are 0.966, 1.517 and 3.243 eV, derived from Fig. 4, corresponding to the films deposited at partial oxygen pressures of 4.0 102, 7.5 102 and 1.3 101 Pa, respectively. The enthalpy differences between the as-deposited and the crystalline states decrease with oxygen pressure, while the activation energy increases, as shown in Fig. 5. The phase transformation process of the material is a thermally activated process. There is an
Ea (eV)
Heat Flow (arb.unit)
15 °C/min
Ln (Tc 2/ α)
M. Fang et al. / Physica B 352 (2004) 206–209
208
0 4
6
8
Po2
10 / (10-2 Pa )
12
14
Fig. 5. Activation energy and enthalpy differences between amorphous and crystalline states of SbOx thin film as a function of Po2.
energy barrier to be overcome when the material is transformed from the current phase to another one. The higher the energy barrier is, the more stable the primary phase is. In order to allow recorded data to be kept unchanged for a long time, it is important for an optical recording medium to possess a high activation energy. From this point, it is essential to enhance the oxygen content in SbOx films when the films are to be used in optical recording. However, besides stability, there are some properties such as the crystallization time, reflectivity contrast and carrier to noise ratio (CNR), etc. which must be considered
ARTICLE IN PRESS M. Fang et al. / Physica B 352 (2004) 206–209
209
when a material is used as an optical recording medium. So, in order to determine optimum oxygen content in SbOx films, further experiments are needed. Related work is in progress.
three different sputtering oxygen partial pressures, which are 4.0 102, 7.5 102 and 1.3 101 Pa, respectively.
4. Conclusion
Acknowledgement
SbOx films were deposited onto k9 glass substrates by reactive DC magnetron sputtering from a pure (99.999%) antimony target in an oxygen environment. The crystal structures of the films were identified by XRD. The structure of the as-deposited film is amorphous, while the films changed from amorphous to crystalline states due to heat treatment. There is an exothermal peak observed in the DSC curve for SbOx sputtered film. The crystallization temperature increases with increasing heating rate. Under the same heating rate, the crystallization temperature increases with increasing sputtered oxygen pressure. The activation energies calculated from Kissinger’s formula are 0.966, 1.517 and 3.243 eV corresponding to the
This work was supported by the National Natural Science Foundation of China (60207005).
References [1] T. Ohta, M. Takenaga, N. Akahira, T. Yamashita, J. Appl. Phys. 53 (1983) 8497. [2] F. Vega, C.N. Afonso, Appl. Phys. B. 62 (1996) 235. [3] N. Ohshima, J. Appl. Phys. 79 (1996) 8357. [4] H.E. Kissinger, Anal. Chem. 29 (1959) 1702. [5] N. Yamada, E. Ohno, N. Akahira, K. Nishiuchi, K. Nagata, M. Takao, Jpn. J. Appl. Phys. 26 (Suppl. 26–4) (1987) 61. [6] M. Okuda, J.C. Ree, J. Fusong, H. Naito, T. Matsushita, Jpn. J. Appl. Phys. 26 (Suppl. 26–4) (1987) 71. [7] M. Mehdi, G. Brun, J.C. Jumas, J.C. Tedenac, J. Mater. Sci. 30 (1995) 5732.