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In situ X-ray diffraction study of crystallization process of GeSbTe thin films during heat treatment
Applied Surface Science 244 (2005) 281–284 www.elsevier.com/locate/apsusc
In situ X-ray diffraction study of crystallization process of GeSbTe thin films during heat treatment Naohiko Kato*, Ichiro Konomi, Yoshiki Seno, Tomoyoshi Motohiro Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan Received 31 May 2004; accepted 1 October 2004 Available online 8 January 2005
Abstract The crystallization processes of the Ge2Sb2Te5 thin film used for PD and DVD-RAM were studied in its realistic optical disk film configurations for the first time by X-ray diffraction using an intense X-ray beam of a synchrotron orbital radiation facility (SPring8) and in situ quick detection with a Position-Sensitive-Proportional-Counter. The dependence of the amorphous-to-fcc phasechange temperature T1 on the rate of temperature elevation Ret gave an activation energy Ea: 0.93 eV much less than previously reported 2.2 eVobtained from a model sample 25–45 times thicker than in the real optical disks. The similar measurement on the Ge4Sb1Te5 film whose large reflectance change attains the readability by CD-ROM drives gave Ea: 1.13 eV with larger T1 than Ge2Sb2Te5 thin films at any Ret implying a lower sensitivity in erasing as well as a better data stability of the phase-change disk. # 2004 Elsevier B.V. All rights reserved. PACS: 81.30.Hd; 68.55. a Keywords: Phase-change; Crystallization; Amorphous; Synchrotron radiation; X-ray diffraction; GeSbTe thin film
1. Introduction The Ge2Sb2Te5 ternary alloy is a key recording material for phase-change type optical disks such as PD and DVD-RAM. The recording and erasing are carried out by causing its amorphous-crystalline phase-change during momentary heating by a laser pulse. Several crystallographic analyses during temperature elevations have been reported for the GeSbTe * Corresponding author. Tel.: +81 561 63 5221; fax: +81 561 63 5443. E-mail address: [email protected] (N. Kato).
films [1–3]. Two stages of phase-change: (a) amorphous-to-fcc and (b) fcc-to-hexagonal have been reported. Typically, the phase-change (a) has been reported to take place at T1 142 8C at the rate of temperature elevation Ret 0.17 8C/s [1], where model samples with films much thicker (500– 800 nm) than 20 nm in real optical disks were prepared to get sufficient diffraction signals using laboratory X-ray sources. However, exact T2 of fcc-tohexagonal of Ge2Sb2Te5 film could not be measured. From a differential scanning calorimetry of the powder scratched off from the substrate, the activation energy Ea for the phase-change (a) has been derived to
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.145
282
N. Kato et al. / Applied Surface Science 244 (2005) 281–284
Fig. 1. X-ray diffraction apparatus on the beam-line BL16XU in SPring-8.
be 2.2 eV based on the Johnson–Mehl–Avrami equation (JMA) [2]. However, there remains still room for controversy because of the film sample 25–40 times thicker than the realistic one. Direct measurements of realistic samples had been anticipated for detailed thermal design of optical disks. Previously, the present authors have studied the dependence of optical constants and microstructure of the (GeTe)x–(Sb2Te3)1 x thin films on the pseudobinary composition x to determine the optimal composition for phase-change recording materials[3]. The maximal difference in optical constants between crystalline and amorphous phases and the minimal average crystalline grain size of the films were obtained at around x = 0.90: Ge4Sb1Te5. The crystallization properties such as T1, T2 and Ea around x = 0.90: Ge4Sb1Te5 have not been clarified yet, in contrast to hexagonal Ge2Sb2Te5 whose kinetic crystallization behavior has been studied well. In this paper, the crystallization process of Ge2Sb2Te5 and Ge4Sb1Te5 were studied in realistic film configurations of optical disks utilizing intense Xray beam of a synchrotron orbital radiation facility (SPring-8), and in situ quick detection with a PositionSensitive-Proportional-Counter (PSPC).
2. Experiments The sample prepared were of the layered structure: Si substrate/ZnS–SiO2 (thickness: 155 nm)/Ge2Sb2Te5
(20 nm) or Ge4Sb1Te5 (20 nm)/ZnS–SiO2 (40 nm). This film configuration was almost the same as in the typical phase-change disk [4]. The each film was fabricated by sputter-deposition using a Ge2Sb2Te5 target or a Ge4Sb1Te5 target. The resultant film composition determined with Electron Probe X-ray Microanalyser was more specifically Ge2Sb2Te5 and Ge39Sb10Te51 rather than Ge4Sb1Te5, respectively. Fig. 1 shows the XRD apparatus of the beam-line BL16XU [5] in SPring-8. Ar 90%–CH4 10% gas flow for the PSPC was ⁄4 cc/min. The sample was heated from 28 8C to 400 8C at different Ret between 0.6 8C/s and 2.5 8C/s by the ceramics heater in the atmosphere. The PSPC was located on the 2u arm to cover 2u = 24– 338. The XRD patterns were measured at the following conditions; X-ray energy: 8.0 keV, incident angle: 1.08, four-jawed slit: horizontal 1 mm/vertical 0.1 mm. The configuration of lead sheet to shield scattered X-ray and the thickness of Al foil on the PSPC to control X-ray intensity into the PSPC was optimized to reduce the background noise and the error of T1 and T2 less than 2 8C. One XRD pattern was obtained every 2 s.
3. Results and discussion Fig. 2 shows typical XRD patterns of Ge2Sb2Te5 film during temperature elevation at Ret of 0.6 8C/s. The two diffraction peaks can be identified as a fcc phase [1,6], a hexagonal phase [1,7], respectively. The phase-change (a) at 160 8C and the phase-change (b)
N. Kato et al. / Applied Surface Science 244 (2005) 281–284
283
Table 1 Dependence of phase change temperatures on the rate of temperature elevation (Ret) of the Ge2Sb2Te5 and Ge39Sb10Te51 thin films
Fig. 2. X-ray diffraction patterns of Ge2Sb2Te5 (20 nm) film as the heater temperature (ht) is increased from 28 8C to 400 8C at the rate of temperature elevation (Ret) of 0.6 8C/s.
at 280 8C were observed. The hexagonal peak disappeared at 400 8C due to changes of the composition by interlayer reactions. Fig. 3 shows typical XRD patterns of Ge39Sb10Te51 film during temperature elevation at Ret of 0.6 8C/s. The phase-
Fig. 3. X-ray diffraction patterns of Ge39Sb10Te51 (20 nm) film as the heater temperature (ht) is increased from 28 8C to 400 8C at the rate of temperature elevation (Ret) of 0.6 8C/s.
Film composition
Ret (8C/s)
T1 (8C) (amorphous-to-fcc)
T2 (8C) (fcc-to-hexagonal)
Ge2Sb2Te5
0.6 1.1 1.6 2.5
151 158 162 174
260 292 297 302
Ge39Sb10Te51
0.6 1.1 1.4 2.1
170 174 182 186
– – – –
change (a) at heater temperature, ht = 180 8C was observed and the fcc phase maintained between 180 8C and 400 8C. Ge39Sb10Te51 showed the phasechange (a), only. The temperature at the X-ray beam spot was lower than ht. T1 and T2 of the sample were determined by correcting this difference. Table 1 shows the T1 and T2 in both Ge2Sb2Te5 and Ge39Sb10Te51 thin films at different Ret. T1 and T2 at Ret = 0.6 8C/s in Ge2Sb2Te5 were estimated to be 151 8C, 260 8C, respectively. T1 at Ret = 0.6 8C/s in Ge39Sb10Te51 was estimated to be 170 8C. T1 and T2 increased with increase in Ret in both Ge2Sb2Te5 and Ge39Sb10Te51 thin films. Fig. 4 shows a Kissinger’s plot of the data (T1, Ret) and (T2, Ret) based on JMA equation in both Ge2Sb2Te5 and Ge39Sb10Te51 thin films. The gradient of the fitting line for the four (T1, Ret) points in the Ge2Sb2Te5 thin film gave Ea: 0.93 eV for the phasechange (a). Fig. 4(a) also shows a point for (142 8C, 0.17 8C/s) in the Ge2Sb2Te5 thick film [1]. The dotted line is of a gradient consistent with 2.2 eV of Ea [1]. This result gave a much less Ea than previously reported one on a model sample 25–40 times thicker than real thickness in the optical disks. Further, the gradient of the fitting line of the four (T2, Ret) points in the Ge2Sb2Te5 thin film gave Ea ⁄ 0.69 eV for the phase-change (b). This indicates that the fcc-tohexagonal transition occurs more easily than the amorphous-to-fcc transition. The phase-change (a) in the Ge39Sb10Te51 film took place at higher T1 with a larger Ea than in the Ge2Sb2Te5 film. It was suggested that the amorphous Ge39Sb10Te51 film has a lower sensitivity in erasing as well as a better data stability of the phase-change disk than the amorphous Ge2Sb2Te5 film.
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N. Kato et al. / Applied Surface Science 244 (2005) 281–284
for the first time by the XRD using an intense X-ray beam of synchrotron radiation and in situ quick detection with PSPC. The dependence of T1 on Ret gave a much less Ea: 0.93 eV than previously reported one obtained from a model sample 25–40 times thicker than real films in the disks. The exact T1 and Ea of thin film rather than conventional data from thick film will be available for the accurate thermal design of the disks. The crystallization process of Ge39Sb10Te51 film whose large reflectance change attains the readability by CD-ROM drives was also clarified by this measurement. Ge39Sb10Te51 gave Ea: 1.13 eV with larger T1 than Ge2Sb2Te5 indicating a lower sensitivity in erasing as well as a better data stability of the phasechange disk.
References
Fig. 4. Kissinger’s plots of the Ge2Sb2Te5 thin films and the Ge39Sb10Te51 thin films and the activation energy Ea for the phase-change: (a) amorphous-to-fcc and (b) fcc-to-hexagonal.
4. Conclusion The crystallization process of Ge2Sb2Te5 thin film in the realistic optical disk could be observed directly
[1] N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, J. Appl. Phys. 69 (1991) 2849. [2] D. Chiang, T.-R. Jeng, D.-R. Huang, Y.-Y. Chang, C.-P. Liu, Jpn. J. Appl. Phys. 38 (1999) 1649. [3] N. Kato, Y. Takeda, T. Fukano, T. Motohiro, S. Kawai, H. Kuno, Jpn. J. Appl. Phys. 38 (1999) 1707. [4] R. Kojima, S. Okabayashi, T. Kashihara, K. Horai, T. Matsunaga, E. Ohno, N. Yamada, T. Ohta, Jpn. J. Appl. Phys. 37 (1998) 2098. [5] N. Kato, I. Konomi, Y. Seno, Spring-8 User Exp. Rep. 5 (2000) 387. [6] T.H. Jeong, M.R. Kim, H. Seo, J.W. Park, C. Yeon, Jpn. J. Appl. Phys. 39 (2000) 2775. [7] I.I. Petrov, R.M. Imamov, Z.G. Pinsker, Kristallografiya 13 (1968) 417.
In situ X-ray diffraction study of crystallization process of GeSbTe thin films during heat treatment Naohiko Kato*, Ichiro Konomi, Yoshiki Seno, Tomoyoshi Motohiro Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan Received 31 May 2004; accepted 1 October 2004 Available online 8 January 2005
Abstract The crystallization processes of the Ge2Sb2Te5 thin film used for PD and DVD-RAM were studied in its realistic optical disk film configurations for the first time by X-ray diffraction using an intense X-ray beam of a synchrotron orbital radiation facility (SPring8) and in situ quick detection with a Position-Sensitive-Proportional-Counter. The dependence of the amorphous-to-fcc phasechange temperature T1 on the rate of temperature elevation Ret gave an activation energy Ea: 0.93 eV much less than previously reported 2.2 eVobtained from a model sample 25–45 times thicker than in the real optical disks. The similar measurement on the Ge4Sb1Te5 film whose large reflectance change attains the readability by CD-ROM drives gave Ea: 1.13 eV with larger T1 than Ge2Sb2Te5 thin films at any Ret implying a lower sensitivity in erasing as well as a better data stability of the phase-change disk. # 2004 Elsevier B.V. All rights reserved. PACS: 81.30.Hd; 68.55. a Keywords: Phase-change; Crystallization; Amorphous; Synchrotron radiation; X-ray diffraction; GeSbTe thin film
1. Introduction The Ge2Sb2Te5 ternary alloy is a key recording material for phase-change type optical disks such as PD and DVD-RAM. The recording and erasing are carried out by causing its amorphous-crystalline phase-change during momentary heating by a laser pulse. Several crystallographic analyses during temperature elevations have been reported for the GeSbTe * Corresponding author. Tel.: +81 561 63 5221; fax: +81 561 63 5443. E-mail address: [email protected] (N. Kato).
films [1–3]. Two stages of phase-change: (a) amorphous-to-fcc and (b) fcc-to-hexagonal have been reported. Typically, the phase-change (a) has been reported to take place at T1 142 8C at the rate of temperature elevation Ret 0.17 8C/s [1], where model samples with films much thicker (500– 800 nm) than 20 nm in real optical disks were prepared to get sufficient diffraction signals using laboratory X-ray sources. However, exact T2 of fcc-tohexagonal of Ge2Sb2Te5 film could not be measured. From a differential scanning calorimetry of the powder scratched off from the substrate, the activation energy Ea for the phase-change (a) has been derived to
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.145
282
N. Kato et al. / Applied Surface Science 244 (2005) 281–284
Fig. 1. X-ray diffraction apparatus on the beam-line BL16XU in SPring-8.
be 2.2 eV based on the Johnson–Mehl–Avrami equation (JMA) [2]. However, there remains still room for controversy because of the film sample 25–40 times thicker than the realistic one. Direct measurements of realistic samples had been anticipated for detailed thermal design of optical disks. Previously, the present authors have studied the dependence of optical constants and microstructure of the (GeTe)x–(Sb2Te3)1 x thin films on the pseudobinary composition x to determine the optimal composition for phase-change recording materials[3]. The maximal difference in optical constants between crystalline and amorphous phases and the minimal average crystalline grain size of the films were obtained at around x = 0.90: Ge4Sb1Te5. The crystallization properties such as T1, T2 and Ea around x = 0.90: Ge4Sb1Te5 have not been clarified yet, in contrast to hexagonal Ge2Sb2Te5 whose kinetic crystallization behavior has been studied well. In this paper, the crystallization process of Ge2Sb2Te5 and Ge4Sb1Te5 were studied in realistic film configurations of optical disks utilizing intense Xray beam of a synchrotron orbital radiation facility (SPring-8), and in situ quick detection with a PositionSensitive-Proportional-Counter (PSPC).
2. Experiments The sample prepared were of the layered structure: Si substrate/ZnS–SiO2 (thickness: 155 nm)/Ge2Sb2Te5
(20 nm) or Ge4Sb1Te5 (20 nm)/ZnS–SiO2 (40 nm). This film configuration was almost the same as in the typical phase-change disk [4]. The each film was fabricated by sputter-deposition using a Ge2Sb2Te5 target or a Ge4Sb1Te5 target. The resultant film composition determined with Electron Probe X-ray Microanalyser was more specifically Ge2Sb2Te5 and Ge39Sb10Te51 rather than Ge4Sb1Te5, respectively. Fig. 1 shows the XRD apparatus of the beam-line BL16XU [5] in SPring-8. Ar 90%–CH4 10% gas flow for the PSPC was ⁄4 cc/min. The sample was heated from 28 8C to 400 8C at different Ret between 0.6 8C/s and 2.5 8C/s by the ceramics heater in the atmosphere. The PSPC was located on the 2u arm to cover 2u = 24– 338. The XRD patterns were measured at the following conditions; X-ray energy: 8.0 keV, incident angle: 1.08, four-jawed slit: horizontal 1 mm/vertical 0.1 mm. The configuration of lead sheet to shield scattered X-ray and the thickness of Al foil on the PSPC to control X-ray intensity into the PSPC was optimized to reduce the background noise and the error of T1 and T2 less than 2 8C. One XRD pattern was obtained every 2 s.
3. Results and discussion Fig. 2 shows typical XRD patterns of Ge2Sb2Te5 film during temperature elevation at Ret of 0.6 8C/s. The two diffraction peaks can be identified as a fcc phase [1,6], a hexagonal phase [1,7], respectively. The phase-change (a) at 160 8C and the phase-change (b)
N. Kato et al. / Applied Surface Science 244 (2005) 281–284
283
Table 1 Dependence of phase change temperatures on the rate of temperature elevation (Ret) of the Ge2Sb2Te5 and Ge39Sb10Te51 thin films
Fig. 2. X-ray diffraction patterns of Ge2Sb2Te5 (20 nm) film as the heater temperature (ht) is increased from 28 8C to 400 8C at the rate of temperature elevation (Ret) of 0.6 8C/s.
at 280 8C were observed. The hexagonal peak disappeared at 400 8C due to changes of the composition by interlayer reactions. Fig. 3 shows typical XRD patterns of Ge39Sb10Te51 film during temperature elevation at Ret of 0.6 8C/s. The phase-
Fig. 3. X-ray diffraction patterns of Ge39Sb10Te51 (20 nm) film as the heater temperature (ht) is increased from 28 8C to 400 8C at the rate of temperature elevation (Ret) of 0.6 8C/s.
Film composition
Ret (8C/s)
T1 (8C) (amorphous-to-fcc)
T2 (8C) (fcc-to-hexagonal)
Ge2Sb2Te5
0.6 1.1 1.6 2.5
151 158 162 174
260 292 297 302
Ge39Sb10Te51
0.6 1.1 1.4 2.1
170 174 182 186
– – – –
change (a) at heater temperature, ht = 180 8C was observed and the fcc phase maintained between 180 8C and 400 8C. Ge39Sb10Te51 showed the phasechange (a), only. The temperature at the X-ray beam spot was lower than ht. T1 and T2 of the sample were determined by correcting this difference. Table 1 shows the T1 and T2 in both Ge2Sb2Te5 and Ge39Sb10Te51 thin films at different Ret. T1 and T2 at Ret = 0.6 8C/s in Ge2Sb2Te5 were estimated to be 151 8C, 260 8C, respectively. T1 at Ret = 0.6 8C/s in Ge39Sb10Te51 was estimated to be 170 8C. T1 and T2 increased with increase in Ret in both Ge2Sb2Te5 and Ge39Sb10Te51 thin films. Fig. 4 shows a Kissinger’s plot of the data (T1, Ret) and (T2, Ret) based on JMA equation in both Ge2Sb2Te5 and Ge39Sb10Te51 thin films. The gradient of the fitting line for the four (T1, Ret) points in the Ge2Sb2Te5 thin film gave Ea: 0.93 eV for the phasechange (a). Fig. 4(a) also shows a point for (142 8C, 0.17 8C/s) in the Ge2Sb2Te5 thick film [1]. The dotted line is of a gradient consistent with 2.2 eV of Ea [1]. This result gave a much less Ea than previously reported one on a model sample 25–40 times thicker than real thickness in the optical disks. Further, the gradient of the fitting line of the four (T2, Ret) points in the Ge2Sb2Te5 thin film gave Ea ⁄ 0.69 eV for the phase-change (b). This indicates that the fcc-tohexagonal transition occurs more easily than the amorphous-to-fcc transition. The phase-change (a) in the Ge39Sb10Te51 film took place at higher T1 with a larger Ea than in the Ge2Sb2Te5 film. It was suggested that the amorphous Ge39Sb10Te51 film has a lower sensitivity in erasing as well as a better data stability of the phase-change disk than the amorphous Ge2Sb2Te5 film.
284
N. Kato et al. / Applied Surface Science 244 (2005) 281–284
for the first time by the XRD using an intense X-ray beam of synchrotron radiation and in situ quick detection with PSPC. The dependence of T1 on Ret gave a much less Ea: 0.93 eV than previously reported one obtained from a model sample 25–40 times thicker than real films in the disks. The exact T1 and Ea of thin film rather than conventional data from thick film will be available for the accurate thermal design of the disks. The crystallization process of Ge39Sb10Te51 film whose large reflectance change attains the readability by CD-ROM drives was also clarified by this measurement. Ge39Sb10Te51 gave Ea: 1.13 eV with larger T1 than Ge2Sb2Te5 indicating a lower sensitivity in erasing as well as a better data stability of the phasechange disk.
References
Fig. 4. Kissinger’s plots of the Ge2Sb2Te5 thin films and the Ge39Sb10Te51 thin films and the activation energy Ea for the phase-change: (a) amorphous-to-fcc and (b) fcc-to-hexagonal.
4. Conclusion The crystallization process of Ge2Sb2Te5 thin film in the realistic optical disk could be observed directly
[1] N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, J. Appl. Phys. 69 (1991) 2849. [2] D. Chiang, T.-R. Jeng, D.-R. Huang, Y.-Y. Chang, C.-P. Liu, Jpn. J. Appl. Phys. 38 (1999) 1649. [3] N. Kato, Y. Takeda, T. Fukano, T. Motohiro, S. Kawai, H. Kuno, Jpn. J. Appl. Phys. 38 (1999) 1707. [4] R. Kojima, S. Okabayashi, T. Kashihara, K. Horai, T. Matsunaga, E. Ohno, N. Yamada, T. Ohta, Jpn. J. Appl. Phys. 37 (1998) 2098. [5] N. Kato, I. Konomi, Y. Seno, Spring-8 User Exp. Rep. 5 (2000) 387. [6] T.H. Jeong, M.R. Kim, H. Seo, J.W. Park, C. Yeon, Jpn. J. Appl. Phys. 39 (2000) 2775. [7] I.I. Petrov, R.M. Imamov, Z.G. Pinsker, Kristallografiya 13 (1968) 417.