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Temperature dependence of structural and optical properties of GeSbTe alloy thin films
Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
Temperature dependence of structural and optical properties of GeSbTe alloy thin films A. Chablia,*, C. Vergnauda, F. Bertina, V. Gehannob, B. Valonb, B. Hyotb, B. Bechevetb, M. Burdina, D. Muyarda a
LETI/CEA.G, Silicon Technology Department, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France b LETI/CEA.G, Optronic Department, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France
Abstract Ge2Sb2Te5 films sandwiched by ZnS–SiO2 layers were studied by spectroscopic ellipsometry from room temperature up to 8001C. An irreversible modification of both materials is pointed out. ZnS cubic phase precipitation occurs after heating at 6501C, shown by grazing incidence X-ray diffraction. Chemical modification in phase change material is observed above 3001C, revealed by a typical behavior of a transparent layer. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Phase change; Optical properties; Temperature dependence; Ellipsometry
The thermo-optical simulation of the recording process in phase change (PC) optical memory applications, requires a database of the optical constants of the materials involved in the devices, over a wide range of temperature. This is highlighted by the simulation results presented in Fig. 1 where it is shown that the temperature profile inside the PC material layer induced by laser irradiation is significantly dependent on the assumption made on the temperature evolution of its optical constants. This paper presents a study of the temperature dependence of the optical constants both of the dielectric and the PC materials generally used in PC devices, motivated by the fact that only few data are available [1,2]. The optical constants are investigated using spectroscopic ellipsometry performed from room *Corresponding author. Fax: +438-7845-94. E-mail address: [email protected] (A. Chabli).
temperature (RT) up to 8001C under argon atmosphere on an ES4G ellipsometer from SOPRA, suited with a homemade furnace. The incidence angle is 761 as determined after the furnace mechanical building up. This is sufficiently close to the Brewster angle of the silicon substrate. The spectral range covers 0.23–1.7 mm. The measurement temperatures were 181C, 1501C and increased by increments of 501C above 3001C. The spectrum acquisition took about 30 min at each temperature. The investigated samples reproduce a PC optical disk stack on a silicon substrate. They consist of a Ge2Sb2Te5 alloy film sandwiched by ZnS– 20 mol% SiO2 films (abbreviated GeSbTe and ZnS–SiO2), in order to avoid its oxidation. GeSbTe films were deposited by DC magnetron sputtering and ZnS–SiO2 films by RF magnetron sputtering, using composite targets and argon atmosphere in both cases. Single ZnS–SiO2 films
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 4 7 1 - 7
A. Chabli et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
510
1
1 2
600
0.5
RT Final
cos ∆
600°C
400
0 -2
0
300°C RT Initial
200
-0.5
-1
0 1 Position (µm)
2
Fig. 1. Simulation of the temperature profile in the PC material layer under laser irradiation: assuming (1) no change and (2) a reduction by a factor of 2 of the optical properties above melting temperature, as expected from K: Yasuda et al. [1] for example. The x-axis shows the position from the center of the laser spot (position zero).
deposited on silicon substrates were also studied. Indeed their optical constants must be established in order to reduce the ellipsometric spectra of the stack. The film thickness, about 30 nm for GeSbTe and 150 nm for ZnS–SiO2, were determined by X-ray specular reflectometry. Grazing incidence X-ray diffraction checked the film’s structural properties. Fig. 2 presents the recorded spectra of cosðDÞ at different temperatures. The RT spectra are different before and after the heating cycle for both types of samples, pointing out an irreversible transformation of the films. The diffraction diagrams obtained before and after heating cycle are given in Fig. 3. After heating at 6501C, the three most intense diffraction peaks of the ZnS cubic phase appear clearly showing that crystallization occurs in the dielectric layer. Also, the temperature dependence of the ellipsometric spectra indicates a significant evolution of the optical constants of the samples. Using the ZnS–SiO2 optical constants deduced from the ellipsometric measurements on the single layer samples, we could extract the optical constants of the PC material from the stack sample measurements. The ellipsometric spectra are reduced using a Tauc-Lorentz dispersion function [3] for both the
(a) -1
1 RT Final 0.5 RT Initial cos ∆
Temperature (°C)
800
0
800°C
-0.5
-1 0.3 (b)
0.5
0.7
0.9
Wavelength (µm)
Fig. 2. Ellipsometric spectra at different temperatures for (a) a 160 nm thick single layer of ZnS–SiO2 on an Si substrate and (b) a GeSbTe alloy film sandwiched by ZnS–SiO2 films reproducing a PC optical disk stack on an Si substrate.
dielectric and the PC materials. The optical constants of the silicon substrate are extrapolated from Jellison and Humlicek data [4,5]. Fig. 4 shows the dispersion curves obtained between RT and 3001C for the GeSbTe films. The variation observed between RT and 1501C is typical of the transition from amorphous to crystallized phase, as it can be deduced from the RT measurements on amorphous and crystalline samples [6]. This transition is known to take place between 1001C and 1501C. We present in Fig. 5 the variation of the extinction coefficient with the temperature at the two wavelengths usually used in the recording
A. Chabli et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
511
7
Refractive index
Diffraction intensity (a.u.)
6
2
5
300°C
4
RT
150°C
3 2
1
(a) 1
30
40 50 2 θ (degree)
60
70
80
Fig. 3. X-ray diffraction at 0.51 of incidence using the CuKa radiation before (1) and after (2) heating at 6501C. Diffraction peaks of the ZnS cubic phase as given by ASTM files are indicated by the vertical bars.
process. This variation indicates that the layer becomes progressively but irreversibly transparent above 3001C since the extinction coefficient drops down to 0.05 and remains at this low level when RT is recovered. A transition from cubic to hexagonal phase is expected around 4001C. But is assumed to be undetectable in standard optical measurements even if the hexagonal phase is birefringent since the layers are polycrystalline. Indeed, cubic or hexagonal phase layers have nearly the same reflectivity at room temperature. More probably the observed transformation is a chemical modification of the PC material, namely an oxidation through a chemical interaction with ZnS–SiO2 layers. In summary, we have developed spectroscopic ellipsometry at high temperature to study PC materials. Our results reveal the meta-stability of
Fig. 5. GeSbTe alloy extinction coefficient at two wavelengths versus temperature as deduced from ellipsometric measurements. The drop above 3001C indicates that the layer becomes transparent. The RT initial value is not recovered after the heating cycle.
300°C 150°C
3 2
RT 1 0 0.4
0.6
0.8
1.0
1.2
Wavelength (µm)
(b)
Fig. 4. GeSbTe alloy optical properties as deduced from ellipsometric measurements between RT and 3001C using a Tauc-Lorentz dispersion function.
4 Extinction coefficient
20
Extinction coefficient
4 10
640 400
3
2
wavelength (nm)
1
0
0
100
200 300 400 Temperature (°C)
500
600
512
A. Chabli et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
the optical device stack, which may be of great importance for the cyclability performances.
References [1] K. Yasuda, M. Ono, K. Aratani, A. Fukumoto, M. Kaneko, Jpn. J. Appl. Phys. 32 (1993) 5210. [2] J.R. Liu, P.Y. Liu, N.Y. Tang, H.P.D. Shieh, Appl. Opt. 37 (35) (1998) 8187.
[3] G. E. Jellison, Jr., F. A. Modine, Appl. Phys. Lett. 69 (1996) 371 and 2137 (Errata). [4] G.E. Jellison Jr., F.A. Modine, J. Appl. Phys. 76 (6) (1994) 3758. [5] J. Sik, J. Hora, J. Humlicek, J. Appl. Phys. 84 (11) (1998) 6291. [6] T. Ide, M. Suzuki, M. Okada, Jpn. J. Appl. Phys. 34 (1995) L529.
Temperature dependence of structural and optical properties of GeSbTe alloy thin films A. Chablia,*, C. Vergnauda, F. Bertina, V. Gehannob, B. Valonb, B. Hyotb, B. Bechevetb, M. Burdina, D. Muyarda a
LETI/CEA.G, Silicon Technology Department, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France b LETI/CEA.G, Optronic Department, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France
Abstract Ge2Sb2Te5 films sandwiched by ZnS–SiO2 layers were studied by spectroscopic ellipsometry from room temperature up to 8001C. An irreversible modification of both materials is pointed out. ZnS cubic phase precipitation occurs after heating at 6501C, shown by grazing incidence X-ray diffraction. Chemical modification in phase change material is observed above 3001C, revealed by a typical behavior of a transparent layer. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Phase change; Optical properties; Temperature dependence; Ellipsometry
The thermo-optical simulation of the recording process in phase change (PC) optical memory applications, requires a database of the optical constants of the materials involved in the devices, over a wide range of temperature. This is highlighted by the simulation results presented in Fig. 1 where it is shown that the temperature profile inside the PC material layer induced by laser irradiation is significantly dependent on the assumption made on the temperature evolution of its optical constants. This paper presents a study of the temperature dependence of the optical constants both of the dielectric and the PC materials generally used in PC devices, motivated by the fact that only few data are available [1,2]. The optical constants are investigated using spectroscopic ellipsometry performed from room *Corresponding author. Fax: +438-7845-94. E-mail address: [email protected] (A. Chabli).
temperature (RT) up to 8001C under argon atmosphere on an ES4G ellipsometer from SOPRA, suited with a homemade furnace. The incidence angle is 761 as determined after the furnace mechanical building up. This is sufficiently close to the Brewster angle of the silicon substrate. The spectral range covers 0.23–1.7 mm. The measurement temperatures were 181C, 1501C and increased by increments of 501C above 3001C. The spectrum acquisition took about 30 min at each temperature. The investigated samples reproduce a PC optical disk stack on a silicon substrate. They consist of a Ge2Sb2Te5 alloy film sandwiched by ZnS– 20 mol% SiO2 films (abbreviated GeSbTe and ZnS–SiO2), in order to avoid its oxidation. GeSbTe films were deposited by DC magnetron sputtering and ZnS–SiO2 films by RF magnetron sputtering, using composite targets and argon atmosphere in both cases. Single ZnS–SiO2 films
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 4 7 1 - 7
A. Chabli et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
510
1
1 2
600
0.5
RT Final
cos ∆
600°C
400
0 -2
0
300°C RT Initial
200
-0.5
-1
0 1 Position (µm)
2
Fig. 1. Simulation of the temperature profile in the PC material layer under laser irradiation: assuming (1) no change and (2) a reduction by a factor of 2 of the optical properties above melting temperature, as expected from K: Yasuda et al. [1] for example. The x-axis shows the position from the center of the laser spot (position zero).
deposited on silicon substrates were also studied. Indeed their optical constants must be established in order to reduce the ellipsometric spectra of the stack. The film thickness, about 30 nm for GeSbTe and 150 nm for ZnS–SiO2, were determined by X-ray specular reflectometry. Grazing incidence X-ray diffraction checked the film’s structural properties. Fig. 2 presents the recorded spectra of cosðDÞ at different temperatures. The RT spectra are different before and after the heating cycle for both types of samples, pointing out an irreversible transformation of the films. The diffraction diagrams obtained before and after heating cycle are given in Fig. 3. After heating at 6501C, the three most intense diffraction peaks of the ZnS cubic phase appear clearly showing that crystallization occurs in the dielectric layer. Also, the temperature dependence of the ellipsometric spectra indicates a significant evolution of the optical constants of the samples. Using the ZnS–SiO2 optical constants deduced from the ellipsometric measurements on the single layer samples, we could extract the optical constants of the PC material from the stack sample measurements. The ellipsometric spectra are reduced using a Tauc-Lorentz dispersion function [3] for both the
(a) -1
1 RT Final 0.5 RT Initial cos ∆
Temperature (°C)
800
0
800°C
-0.5
-1 0.3 (b)
0.5
0.7
0.9
Wavelength (µm)
Fig. 2. Ellipsometric spectra at different temperatures for (a) a 160 nm thick single layer of ZnS–SiO2 on an Si substrate and (b) a GeSbTe alloy film sandwiched by ZnS–SiO2 films reproducing a PC optical disk stack on an Si substrate.
dielectric and the PC materials. The optical constants of the silicon substrate are extrapolated from Jellison and Humlicek data [4,5]. Fig. 4 shows the dispersion curves obtained between RT and 3001C for the GeSbTe films. The variation observed between RT and 1501C is typical of the transition from amorphous to crystallized phase, as it can be deduced from the RT measurements on amorphous and crystalline samples [6]. This transition is known to take place between 1001C and 1501C. We present in Fig. 5 the variation of the extinction coefficient with the temperature at the two wavelengths usually used in the recording
A. Chabli et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
511
7
Refractive index
Diffraction intensity (a.u.)
6
2
5
300°C
4
RT
150°C
3 2
1
(a) 1
30
40 50 2 θ (degree)
60
70
80
Fig. 3. X-ray diffraction at 0.51 of incidence using the CuKa radiation before (1) and after (2) heating at 6501C. Diffraction peaks of the ZnS cubic phase as given by ASTM files are indicated by the vertical bars.
process. This variation indicates that the layer becomes progressively but irreversibly transparent above 3001C since the extinction coefficient drops down to 0.05 and remains at this low level when RT is recovered. A transition from cubic to hexagonal phase is expected around 4001C. But is assumed to be undetectable in standard optical measurements even if the hexagonal phase is birefringent since the layers are polycrystalline. Indeed, cubic or hexagonal phase layers have nearly the same reflectivity at room temperature. More probably the observed transformation is a chemical modification of the PC material, namely an oxidation through a chemical interaction with ZnS–SiO2 layers. In summary, we have developed spectroscopic ellipsometry at high temperature to study PC materials. Our results reveal the meta-stability of
Fig. 5. GeSbTe alloy extinction coefficient at two wavelengths versus temperature as deduced from ellipsometric measurements. The drop above 3001C indicates that the layer becomes transparent. The RT initial value is not recovered after the heating cycle.
300°C 150°C
3 2
RT 1 0 0.4
0.6
0.8
1.0
1.2
Wavelength (µm)
(b)
Fig. 4. GeSbTe alloy optical properties as deduced from ellipsometric measurements between RT and 3001C using a Tauc-Lorentz dispersion function.
4 Extinction coefficient
20
Extinction coefficient
4 10
640 400
3
2
wavelength (nm)
1
0
0
100
200 300 400 Temperature (°C)
500
600
512
A. Chabli et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 509–512
the optical device stack, which may be of great importance for the cyclability performances.
References [1] K. Yasuda, M. Ono, K. Aratani, A. Fukumoto, M. Kaneko, Jpn. J. Appl. Phys. 32 (1993) 5210. [2] J.R. Liu, P.Y. Liu, N.Y. Tang, H.P.D. Shieh, Appl. Opt. 37 (35) (1998) 8187.
[3] G. E. Jellison, Jr., F. A. Modine, Appl. Phys. Lett. 69 (1996) 371 and 2137 (Errata). [4] G.E. Jellison Jr., F.A. Modine, J. Appl. Phys. 76 (6) (1994) 3758. [5] J. Sik, J. Hora, J. Humlicek, J. Appl. Phys. 84 (11) (1998) 6291. [6] T. Ide, M. Suzuki, M. Okada, Jpn. J. Appl. Phys. 34 (1995) L529.