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Phase transformations induced in Ge1Sb2Te4 films by single femtosecond pulses
Materials Science and Engineering B 131 (2006) 88–93
Phase transformations induced in Ge1Sb2Te4 films by single femtosecond pulses S.M. Huang ∗ , Z. Sun, C.X. Jin, H.B. Zhu, Y. Yao, Y.W. Chen, Z.J. Zhao Department of Physics, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, PR China Received 25 January 2006; received in revised form 8 March 2006; accepted 30 March 2006
Abstract We have investigated the phase transformations induced in a Ge1 Sb2 Te4 system by a femtosecond laser exposure. The system has a multilayer structure of 10 nm ZnS–SiO2 /(10–100 nm) Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. The morphology and contrast of marks written in both amorphous and crystalline backgrounds by single fs pulses were characterized using an optical microscope. X-ray diffraction was applied to identify the crystal structures formed by single 108 fs shots. The characteristics and the conditions of crystalline → amorphous or amorphous → crystalline transitions in the multilayer structures with different Ge1 Sb2 Te4 layer thickness triggered by single shots were investigated. The pulse energy window for the crystallization or amorphization in the Ge1 Sb2 Te4 systems was established. The mechanism of phase changes triggered by femtosecond laser pulses is discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Femtosecond laser; Phase change optical recording; Amorphization; Crystallization
1. Introduction Optical data storage techniques based on ultrafast lasers have attracted considerable interest in recent years. There are numerous reports about three-dimensional (3D) optical data storage techniques based on multiphoton absorption [1–4]. On the other hand, short pulse width lasers such as the femto to picosecond pulse laser have been used to develop ultrafast phase change recording technologies. The promise of these technologies is that ultrafast lasers have potentials to resolve the heat diffusion limitation of the conventional laser recording, achieving high area recording densities, and increasing data transfer rates beyond current limits. The possibility of inducing crystallization or amorphization has been studied in some phase change materials with ultrashort pulses from 100 fs to 30 ps [5–11]. The work has mostly been concentrated on Sb-rich, GeSb films [7–11]. It was shown that reversible changes between the amorphous and crystalline states could be achieved in GeSb films on carbon coated mica upon irradiation with single 500 fs laser pulses [7], and the phase reversibility was also demonstrated in the GeSb
∗
Corresponding author. E-mail address: [email protected] (S.M. Huang).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.03.040
film on the substrate upon irradiation with single 30 ps laser pulses by designing the film configuration (film thickness and substrate) to control the heat flow conditions in the configuration [8,11]. One of the most important goals for any ultrafast phase change recording technology is to make it possible to record and retrieve data rapidly using compact and relatively inexpensive equipment with high repetition rates (such as non-amplified laser systems and non-immersion optics) and materials. Any technique that will also allow for data storage to be achieved under similar constraints will be all the more advantageous. To this end, the investigation of the mechanisms of ultrafast laserinduced phase changes to achieve reverse phase transformations in phase change materials with single fs shots and establish laser parameter windows for phase change systems is very important. The laser parameters should include ultrafast laser pulse duration, pulse energy or intensity, and wavelength because these factors play important roles in ultrafast phase changes of the systems. Chalcogenides are used in reversible optical information storage such as re-writable compact and gigital versatile discs with nanosecond pulsed lasers. This nanosecond laser recording technology is very mature. Stoichiometric compositions on the Sb2 Te3 –GeTe pseudobinary system, such as Ge1 Sb2 Te4 , Ge2 Sb2 Te5 and Ge1 Sb4 Te7 , are regarded as fast phase change
S.M. Huang et al. / Materials Science and Engineering B 131 (2006) 88–93
materials since both amorphization and crystallization can be triggered by laser irradiations of very short duration, 30–100 ns [12]. The study of GeSbTe system has mainly focused on the crystallization or amorphization triggered by nanosecond laser [12–14]. In this work, we present laser-induced crystallization or amorphization in Ge1 Sb2 Te4 films under the action of single femtosecond laser pulse. We used femtosecond laser pulses to investigate the phase transformations in a multilayer structure where the active Ge1 Sb2 Te4 layer is interfaced with a poor thermal conducting layer, e.g., a dielectric ZnS–SiO2 composite, on the polycabonator substrate. Crystalline to amorphous or amorphous to crystalline phase changes induced by the single femtosecond shots in the active Ge1 Sb2 Te4 layer was examined and characterized using optical microscopy and X-ray diffraction (XRD). The mechanism of the phase transformations is discussed. The conditions to achieve reverse phase transformations in the Ge1 Sb2 Te4 film within the conventional optical disk structure using single femtosecond laser pulse irradiation is investigated, and the related parameter window is provided. 2. Experimental details The disks were prepared on 0.6 mm-thick-polycarbonate substrates using an ULVCA multi chambers sputtering technology. The phase change material Ge1 Sb2 Te4 is sandwiched between two dielectric layers. Amorphous Ge1 Sb2 Te4 films with thickness of 10–100 nm were prepared by dc magnetron sputtering. The lower dielectric layer and the upper dielectric layer were deposited by RF magnetron sputtering. The thickness of the lower ZnS–SiO2 layer is 80 nm while the upper ZnS–SiO2 layer is 10 nm thick. The thickness of all the layers is in the conventional thickness range of optical disk structure design. The phase change recording layer in these disks fabricated by sputtering is in an amorphous state. To record any information in nanosecond laser recording technology, they need to be converted to a crystalline state, achieved by a process known as “initialization”. The fabricated disk was initialized in half a region of the whole disk by Shibasoku optical disc initializer. As a result, every disk provides both amorphous and previously crystallized regions for the fs laser irradiation. Laser writing and treatment was done by crossing both types of regions every time. The fs laser system consisting of a Ti:sapphire oscillator (Spectra-Physics Tsunami) and a regenerative amplifier (Spectra-Physics Spitfire) provides fs laser pulses for the study. A self-mode-locked Ti:sapphire laser oscillator produces about 80 fs pulses at a wavelength of 800 nm and a repetition rate of 80 MHz. The oscillator provides seed pulses into the regenerative amplifier, which is based on chirped pulse amplification (CPA) technique. The wavelength of output beam from amplifier is 800 nm. The repetition rate can be adjusted from 1 to 1000 Hz and the beam profile emitted from the regenerative amplifier is Gaussian shape. The fs laser beam was focused on the disk by a ×20 optical microscope objective. The disk was placed on a 3-axis, X–Y–Z, motion system. Z stage was used to adjust the vertical distance between the disk and microscope objective and the laser beam was focused in the phase change layer of the
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disk. X–Y motion system was used to move the disk relative to the laser beam on a horizontal plane to write data bits in the disk. During the fs laser processing, each data point was written only by a single pulse through adjusting the speeds of the X and Y stages as well as the repetition rate of the fs laser beam. In order to adjust the laser energy incident on the disk, external attenuators were used in the optics path before the optical objective. The pulse duration of the laser beam after external attenuators was 108 fs, measured by GRENOUILLE/VideoFROG, model 8-50, Newport. After the laser treatment, the morphologies and contrast of the written data bits in the disk were examined using an optical microscope. The crystal structure transformed by single fs shots was measured by a high resolution X-ray diffractometer (HRXRD) (Bruker D8 Discover) using a Cu K␣ radiation. 3. Results The morphology and optical characteristics of marks written in the crystallized or amorphous backgrounds of Ge1 Sb2 Te4 layers with various thicknesses by single fs pulses using different laser fluence were examined and studied. The results are shown in Figs. 1 and 2. The first sample reported is a disk with a multilayer structure of 10 nm ZnS–SiO2 /15 nm Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. One half of the disk was initialized to provide both amorphous and crystallized regions for the fs laser irradiation. Laser irradiation and treatment was done by crossing both types of regions every time. Fig. 1 shows an optical image of amorphous and crystallized backgrounds after initialization. The morphology and contrast of both amorphous and crystallized backgrounds can be clearly observed under the optical microscope by regulating its focused lens and filters. Amorphous and crystallized regions show differ-
Fig. 1. Optical image of crystallized and amorphous backgrounds before fs laser irradiation.
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Fig. 2. Optical image of marks formed in the crystallized or amorphous backgrounds of Ge1 Sb2 Te4 films with various thickness by single fs pulses using different average laser fluence. In the crystallized background of a Ge1 Sb2 Te4 15 nm thick film: (a) 22 mJ/cm2 , (b) 70 mJ/cm2 , and (c) 85 mJ/cm2 ; in the amorphous background of a Ge1 Sb2 Te4 50 nm thick film: (d) 75 mJ/cm2 , (e) 85 mJ/cm2 , and (f) 112 mJ/cm2 .
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ent contrast under the microscope. The amorphous background shows dark color under the microscope while the crystallized one displays bright color as shown in Fig. 1. After femtosecond single pulse exposure, there are marks, circular dark spots, formed in the crystallized background as shown in Fig. 2(a), but no phase change can be observed in the amorphous background. The contrast of marks written in the crystallized background is similar to that of the amorphous background shown in Fig. 1. The size of each mark is about 1.4 m in diameter. The average incident laser fluence is 22 mJ/cm2 . Fig. 2(b) shows a typical morphology and contrast of marks formed in the crystallized background with a higher average laser fluence of 70 mJ/cm2 . The contrast of dark marks is similar to that of the amorphous phase in Fig. 1. With this laser fluence, larger dark marks about 2.7 m in diameter appear in the background, still no phase change can be found, but damage is caused at center of the laser spot, in the amorphous background. When the laser fluence is increased further, the data spots obtained in the crystallized background become bigger. Their contrast is similar to that of the amorphous region. This trend keeps for marks formed in crystallized background until the average laser fluence is increased up to 85 mJ/cm2 . With an average laser fluence of 85 mJ/cm2 , damage is caused in the crystallized region as shown in Fig. 2(c). The second sample reported is a disk with a multilayer structure of 10 nm ZnS–SiO2 /50 nm Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. Its active layer thickness is greater than that of the first sample. One half of the disk was also initialized to provide both amorphous and crystallized regions for the fs laser irradiation. After a femtosecond single pulse exposure, there are very small marks, circular bright spots, formed in the amorphous background as shown in Fig. 2(d), but no change can be observed in the crystallized background. The contrast of marks written in the amorphous background is similar to that of the crystallized background shown in Fig. 1. The size of each mark is about 0.4 m in diameter. The average incident laser fluence is 75 mJ/cm2 . Fig. 2(e) shows a typical morphology and contrast of marks formed in the amorphous background with a higher average laser fluence of 85 mJ/cm2 . The contrast of bright marks is similar to that of the crystallized phase in Fig. 1. With this laser fluence, larger and clearer marks about 2.0 m in diameter appear in the amorphous background, still no phase change can be found in the crystallized background. When the laser fluence is increased further, the data spots obtained in the amorphous background become bigger and brighter. Their contrast is similar to that of the crystallized region. This trend keeps for marks formed in amorphous background until the average laser fluence is increased up to 112 mJ/cm2 . With an average laser fluence of 112 mJ/cm2 , damage is created at the centers of marks formed in amorphous background as shown in Fig. 2(f). Bright data bit arrays (10 m × 10 m) with a fluence of 85 mJ/cm2 were produced in an area of 1 cm × 1 cm for XRD analysis. Fig. 3 shows the XRD result. In the case of crystallized background, no change can be found in the crystallized background until the average
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Fig. 3. XRD diffraction pattern of the sample (1 cm × 1 cm) with date bit arrays (10 m × 10 m) formed in the amorphous background of a Ge1 Sb2 Te4 50 nm thick film with an average laser fluence of 85 mJ/cm2 .
laser fluence is increased up to150 mJ/cm2 . When the average laser fluence is as high as 150 mJ/cm2 , damage is made in the crystallized background. 4. Discussion For the first sample with a 15 nm thick Ge1 Sb2 Te4 layer, from Figs. 1 and 2(a)–(c), the morphology and contrast of marks formed by the fs single pulse exposure in the crystallized background is very clear and similar to that of the amorphous region of the disk. This result indicates that re-amorphization has been achieved upon irradiation with single 108 fs laser pulses when the average laser fluence is greater than 22 mJ/cm2 , but less than 85 mJ/cm2 . A single pulse at a high fluence of 85 mJ/cm2 induces the re-amorphization of a larger region and partial ablation at its center as shown in Fig. 2(c). This can be attributed to the higher energy density at the center part of the fs spot than at its edge because of the Gaussian-like intensity distribution of the fs laser beam. The portions at the centers of the fs spots have been ablated as a consequence of the fs irradiation. For the first sample, however, the inverse transformation, i.e., crystallization of the amorphized phase, has not been observed using a single 108 fs laser pulse by changing the laser fluence between the surface melting and damage thresholds. Amorphization was also reported to occur in Ge2 Sb2 Te5 system by a 120 fs laser exposure [5]. For the second sample with a 50 nm thick Ge1 Sb2 Te4 layer, as shown in Fig. 2(d)–(f), marks produced in the amorphous background are with morphology and contrast very similar to that of the crystallized region shown in Fig. 1. The result can be attributed to the transformation from the amorphous to crystalline phase. This crystallization has been achieved upon irradiation with single fs laser pulses when the average laser fluence is greater than 75 mJ/cm2 , but less than 112 mJ/cm2 . A single pulse at a very high fluence of 112 mJ/cm2 induces the crystallization of a larger region and partial ablation at its center as shown in
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Fig. 2(f). This can also be attributed to the higher energy density at the center part of the fs spot than at its edge because of the Gaussian-like intensity distribution of the fs laser beam. The portions at the centers of the fs spots have been ablated as a consequence of the fs irradiation. XRD result shown in Fig. 3 provides a further evidence for the crystallization. The result confirms the face-centered cubic structure for the second sample subjected to single 108 fs laser irradiation [15]. Therefore, crystallization of the amorphized phase has been demonstrated by single 108 fs laser irradiation of the second sample. However, the inverse transformation, i.e., re-amorphization of the pre-crystallized phase has not succeeded with single 108 fs laser irradiation only by adjusting fluence between the surface melting and damage thresholds in the second sample. On excitation with an ultrashort pulse, the system undergoes several stages of relaxation before returning to equilibrium. The energy is transferred first to the electrons and then to the lattice. The interaction includes several regimes, carrier excitation, thermalization, carrier removal and thermal and structural effects, of carrier excitation and relaxation [16]. The various processes do not occur sequentially. They overlap in time, forming a continuous chain of events spanning the entire range from femtoseconds to microseconds. Excited with an ultrashort pulse, the pulse energy is deposited within a very short time in a very thin surface layer, leading to the high-reflectivity, disordered phase (sometimes called a “molten” phase) observed in a few picoseconds after excitation, then melting or ablation (for strong excitation) and resolidification. The inability of the first sample to crystallize or the second sample to amorphize upon single pulse irradiation is related to the ultrashort laser pulse duration and the undercooling achieved prior to solidification. As we know, the nanosecond-induced amorphization of GeSbTe films is produced at quenching rates >3.4 × 109 K/s [17]. The quenching rates expected for irradiation with fs and ps are in both cases >1012 K/s and therefore the formation of amorphous phases may be expected for the two samples. Nevertheless the undercooling achieved prior to solidification is another important factor involved in the formation of amorphous phases by rapid solidification processes. During the initial solidification process into an amorphous one, the solidification enthalpy releases, resulting in lowering the supercooling and promoting the nucleation and growth of the crystalline phase. The amount of solidification enthalpy released depends on both the film thickness and the nature of the solid phase which nucleates homogeneously (amorphous or crystalline). Both the effects of relaxation excess and recalescence could be prevented by using thinner films on low-thermal-conductivity substrates. The bulk solidification and recalescence had been prevented in the first sample due to its very thin phase change layer. As a result, the 15 nm thick Ge1 Sb2 Te4 sample is able to amorphize upon single fs pulse irradiation. But the inverse transformation, i.e., crystallization of the amorphized phase, frustrated using single fs pulse in this sample. On the other hand, effects of recalescence (undercooling decreases) took place in the second sample due to its thick phase change layer. These effects have impeded the re-amorphization in the second sample. Previous research on optical media operated in ns laser pulse showed that in most of stoichiometric systems
the amorphous phase cannot be crystallized upon irradiation with pulses shorter than a few tens of ns [12]. But, our work once again confirms that both crystallization and amorphization can be achieved in the Ge1 Sb2 Te4 system upon irradiation with single 108 fs laser pulses. The final structure induced upon irradiation and thus the reverse phase transformation of the process is strongly conditioned by the thermal properties of dielectric layer/the film/substrate system no matter the initial film state, amorphous or crystalline. In this work, phase reversibility was not able to achieved upon the single 108 fs laser irradiation of the Ge1 Sb2 Te4 layer with different thickness (10–100 nm) in our designed disks by only adjusting the fluence, however, we think reversible phase cycling by a single fs pulse of a fix laser in a Ge1 Sb2 Te4 system for both amorphization and crystallization could be obtained by changing the laser pulse duration, carefully adjusting the fluence and accurately designing the configuration (dielectric layer/film thickness/dielectric layer substrate). Conventional optical disk recording is performed by laser spot on irradiation a rotational disk. In this case, the laser irradiation time on the portion of the disk is around 10–100 ns, rather long time compared to femtosecond laser spot irradiation. The recording process includes heat diffusion in the layers. The temperature increased area is wider than the laser spot size, which means the mark size and the position of mark are determined not only by the beam factor but also the disk thermal characteristics and the pulse duration. The amorphous marks formed by nanosecond laser pulses are surrounded by a large crystalline band edge while amorphous marks created by fs pulses are without the crystalline edge band [5]. Heat diffusion of conventional laser recording limits the performance of future high-density and high-data rate optical disk. Therefore, the femtosecond laser has provided a method to resolve the heat diffusion limitation of the conventional laser recording. As reported in the above, the characteristic fluence ranges in which re-amorphization and crystallization occur in our Ge1 Sb2 Te4 disks: re-amorphization is induced for fluences typically about 22–85 mJ/cm2 , while crystallization requires higher values, above 75–112 mJ/cm2 . The corresponding single shot energy range is 5.6–21.0 nJ for re-amorphization, and 18.5–22.3 nJ for crystallization. Our Ti:sapphire laser oscillator has a repetition rate of 82 MHz, and its single pulse energy is high up to 13 nJ. Thus, for this model oscillator, the single shot of Ti:sapphire laser oscillator with a repetition rate of 40 MHz can provide enough energy to achieve crystalline to amorphous or amorphous to crystalline phase change in our Ge1 Sb2 Te4 disk. A repetition rate of 40 MHz is still very high compared to that of the nanosecond lasers used in the optical recording. The former is three orders of magnitude higher than the latter. On the other hand, an important consequence of this behavior is the fact that the extrapolation of the fluences required for crystallization or amorphization under sub-ps laser pulses to a diffraction-limited spot size are consistent with pulse absolute energies in the sub-nJ range. The laser requirements for the application of the material as a sub-ps laser pulse-driven optical recording medium are thus compatible with those provided by compact diode-pumped ps/fs laser oscillators. Recording or retrieving data rapidly is possible using the oscillator as a laser source because of its very high rep-
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etition rate. Therefore, it is promising to record and retrieve data rapidly in the Ge1 Sb2 Te4 film within the conventional optical disk structure using a non-amplified laser system. 5. Conclusions The phase transformations induced in a Ge1 Sb2 Te4 system has been investigated by a single femtosecond laser exposure. The system has a multilayer structure of 10 nm ZnS–SiO2 /(15–100 nm) Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. The morphology and contrast of marks written in both amorphous and crystalline backgrounds by single fs pulses were characterized. The detailed results of optical microscopy evaluation of the morphology and contrast of marks written in both amorphous and crystalline backgrounds by single 108 fs pulses are present. X-ray diffraction was applied to identify the crystal structures formed by single fs shots. A well-defined laser fluence and pulse energy window for achieving crystalline to amorphous or amorphous to crystalline phase change in the Ge1 Sb4 Te7 systems using single 108 fs pulses is provided. The mechanism of phase transformations triggered by single femtosecond laser pulses is discussed. Acknowledgements This work was supported by Foundations of Pujiang Talented Person Plans of Shanghai Science & Technology (no. 05PJ14037), Shanghai-Applied Materials Research and Development fund (no. 0519) and Shanghai Nanotechnology (no. 0552nm042).
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References [1] E.N. Glezer, M. Milosavljevic, L. Huang, R.J. Finlay, T.-H. Her, J.P. Callan, E. Mazur, Opt. Lett. 21 (1996) 2023. [2] M. Watanabe, H. Sun, S. Juodkazis, T. Takahashi, S. Matsuo, Y. Suzuki, J. Nishii, H. Misawa, Jpn. J. Appl. Phys. 37 (1998) L1527. [3] B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y.S. Lee, D. McCordMaughon, J. Qin, H. Rockel, M. Rumi, X.-L. Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51. [4] S. Gu, L. Hou, Opt. Lett. 24 (1999) 948. [5] T. Ohta, N. Yamada, H. Yamamoto, T. Mitsuyu, T. Kozaki, J. Qiu, K. Hirao, in: M. Wuttig, L. Hesselink, H.J. Borg (Eds.), Applications of Ferromagnetic and Optical Materials, Storage and Magnetoelectronics, Materials Research Society Symposium Proceedings, vol. 674, 2001, p. V1.1.1. [6] J. Siegel, A. Schropp, J. Solis, C.N. Afonso, M. Wuttig, Appl. Phys. Lett. 84 (2004) 2250. [7] C.N. Afonso, J. Solis, F. Catalina, C. Kalpouzos, Appl. Phys. Lett. 60 (1992) 3123. [8] M.C. Morilla, J. Solis, C.N. Afonso, Jpn. J. Appl. Phys. Part 1 36 (1997) 1015. [9] J. Siegel, C.N. Afonso, J. Solis, Appl. Phys. Lett. 75 (1999) 3102. [10] J.P. Callan, A.M.-T. Kim, C.A.D. Roeser, E. Mazur, J. Solis, J. Siegel, C.N. Afonso, J.C.G. de Sande, Phys. Rev. Lett. 86 (2001) 3650. [11] J. Solis, C.N. Afonso, Appl. Phys. A: Mater. Sci. Process. 76 (2003) 331. [12] N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, M. Takao, J. Appl. Phys. 69 (1991) 2849. [13] V. Weidenhof, N. Pirch, I. Friedrich, J. Appl. Phys. 86 (1999) 5879. [14] V. Weidenhof, I. Friedrich, S. Ziegler, M. Wuttig, J. Appl. Phys. 89 (2001) 3168. [15] Z.L. Mao, H. Chen, A.-L. Jung, J. Appl. Phys. 78 (1995) 2338. [16] S.K. Sundaram, E. Mazur, Nature Mater. 1 (217) (2002) 217. [17] T. Ohta, J. Optoelectron. Adv. Mater. 3 (2001) 609.
Phase transformations induced in Ge1Sb2Te4 films by single femtosecond pulses S.M. Huang ∗ , Z. Sun, C.X. Jin, H.B. Zhu, Y. Yao, Y.W. Chen, Z.J. Zhao Department of Physics, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, PR China Received 25 January 2006; received in revised form 8 March 2006; accepted 30 March 2006
Abstract We have investigated the phase transformations induced in a Ge1 Sb2 Te4 system by a femtosecond laser exposure. The system has a multilayer structure of 10 nm ZnS–SiO2 /(10–100 nm) Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. The morphology and contrast of marks written in both amorphous and crystalline backgrounds by single fs pulses were characterized using an optical microscope. X-ray diffraction was applied to identify the crystal structures formed by single 108 fs shots. The characteristics and the conditions of crystalline → amorphous or amorphous → crystalline transitions in the multilayer structures with different Ge1 Sb2 Te4 layer thickness triggered by single shots were investigated. The pulse energy window for the crystallization or amorphization in the Ge1 Sb2 Te4 systems was established. The mechanism of phase changes triggered by femtosecond laser pulses is discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Femtosecond laser; Phase change optical recording; Amorphization; Crystallization
1. Introduction Optical data storage techniques based on ultrafast lasers have attracted considerable interest in recent years. There are numerous reports about three-dimensional (3D) optical data storage techniques based on multiphoton absorption [1–4]. On the other hand, short pulse width lasers such as the femto to picosecond pulse laser have been used to develop ultrafast phase change recording technologies. The promise of these technologies is that ultrafast lasers have potentials to resolve the heat diffusion limitation of the conventional laser recording, achieving high area recording densities, and increasing data transfer rates beyond current limits. The possibility of inducing crystallization or amorphization has been studied in some phase change materials with ultrashort pulses from 100 fs to 30 ps [5–11]. The work has mostly been concentrated on Sb-rich, GeSb films [7–11]. It was shown that reversible changes between the amorphous and crystalline states could be achieved in GeSb films on carbon coated mica upon irradiation with single 500 fs laser pulses [7], and the phase reversibility was also demonstrated in the GeSb
∗
Corresponding author. E-mail address: [email protected] (S.M. Huang).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.03.040
film on the substrate upon irradiation with single 30 ps laser pulses by designing the film configuration (film thickness and substrate) to control the heat flow conditions in the configuration [8,11]. One of the most important goals for any ultrafast phase change recording technology is to make it possible to record and retrieve data rapidly using compact and relatively inexpensive equipment with high repetition rates (such as non-amplified laser systems and non-immersion optics) and materials. Any technique that will also allow for data storage to be achieved under similar constraints will be all the more advantageous. To this end, the investigation of the mechanisms of ultrafast laserinduced phase changes to achieve reverse phase transformations in phase change materials with single fs shots and establish laser parameter windows for phase change systems is very important. The laser parameters should include ultrafast laser pulse duration, pulse energy or intensity, and wavelength because these factors play important roles in ultrafast phase changes of the systems. Chalcogenides are used in reversible optical information storage such as re-writable compact and gigital versatile discs with nanosecond pulsed lasers. This nanosecond laser recording technology is very mature. Stoichiometric compositions on the Sb2 Te3 –GeTe pseudobinary system, such as Ge1 Sb2 Te4 , Ge2 Sb2 Te5 and Ge1 Sb4 Te7 , are regarded as fast phase change
S.M. Huang et al. / Materials Science and Engineering B 131 (2006) 88–93
materials since both amorphization and crystallization can be triggered by laser irradiations of very short duration, 30–100 ns [12]. The study of GeSbTe system has mainly focused on the crystallization or amorphization triggered by nanosecond laser [12–14]. In this work, we present laser-induced crystallization or amorphization in Ge1 Sb2 Te4 films under the action of single femtosecond laser pulse. We used femtosecond laser pulses to investigate the phase transformations in a multilayer structure where the active Ge1 Sb2 Te4 layer is interfaced with a poor thermal conducting layer, e.g., a dielectric ZnS–SiO2 composite, on the polycabonator substrate. Crystalline to amorphous or amorphous to crystalline phase changes induced by the single femtosecond shots in the active Ge1 Sb2 Te4 layer was examined and characterized using optical microscopy and X-ray diffraction (XRD). The mechanism of the phase transformations is discussed. The conditions to achieve reverse phase transformations in the Ge1 Sb2 Te4 film within the conventional optical disk structure using single femtosecond laser pulse irradiation is investigated, and the related parameter window is provided. 2. Experimental details The disks were prepared on 0.6 mm-thick-polycarbonate substrates using an ULVCA multi chambers sputtering technology. The phase change material Ge1 Sb2 Te4 is sandwiched between two dielectric layers. Amorphous Ge1 Sb2 Te4 films with thickness of 10–100 nm were prepared by dc magnetron sputtering. The lower dielectric layer and the upper dielectric layer were deposited by RF magnetron sputtering. The thickness of the lower ZnS–SiO2 layer is 80 nm while the upper ZnS–SiO2 layer is 10 nm thick. The thickness of all the layers is in the conventional thickness range of optical disk structure design. The phase change recording layer in these disks fabricated by sputtering is in an amorphous state. To record any information in nanosecond laser recording technology, they need to be converted to a crystalline state, achieved by a process known as “initialization”. The fabricated disk was initialized in half a region of the whole disk by Shibasoku optical disc initializer. As a result, every disk provides both amorphous and previously crystallized regions for the fs laser irradiation. Laser writing and treatment was done by crossing both types of regions every time. The fs laser system consisting of a Ti:sapphire oscillator (Spectra-Physics Tsunami) and a regenerative amplifier (Spectra-Physics Spitfire) provides fs laser pulses for the study. A self-mode-locked Ti:sapphire laser oscillator produces about 80 fs pulses at a wavelength of 800 nm and a repetition rate of 80 MHz. The oscillator provides seed pulses into the regenerative amplifier, which is based on chirped pulse amplification (CPA) technique. The wavelength of output beam from amplifier is 800 nm. The repetition rate can be adjusted from 1 to 1000 Hz and the beam profile emitted from the regenerative amplifier is Gaussian shape. The fs laser beam was focused on the disk by a ×20 optical microscope objective. The disk was placed on a 3-axis, X–Y–Z, motion system. Z stage was used to adjust the vertical distance between the disk and microscope objective and the laser beam was focused in the phase change layer of the
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disk. X–Y motion system was used to move the disk relative to the laser beam on a horizontal plane to write data bits in the disk. During the fs laser processing, each data point was written only by a single pulse through adjusting the speeds of the X and Y stages as well as the repetition rate of the fs laser beam. In order to adjust the laser energy incident on the disk, external attenuators were used in the optics path before the optical objective. The pulse duration of the laser beam after external attenuators was 108 fs, measured by GRENOUILLE/VideoFROG, model 8-50, Newport. After the laser treatment, the morphologies and contrast of the written data bits in the disk were examined using an optical microscope. The crystal structure transformed by single fs shots was measured by a high resolution X-ray diffractometer (HRXRD) (Bruker D8 Discover) using a Cu K␣ radiation. 3. Results The morphology and optical characteristics of marks written in the crystallized or amorphous backgrounds of Ge1 Sb2 Te4 layers with various thicknesses by single fs pulses using different laser fluence were examined and studied. The results are shown in Figs. 1 and 2. The first sample reported is a disk with a multilayer structure of 10 nm ZnS–SiO2 /15 nm Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. One half of the disk was initialized to provide both amorphous and crystallized regions for the fs laser irradiation. Laser irradiation and treatment was done by crossing both types of regions every time. Fig. 1 shows an optical image of amorphous and crystallized backgrounds after initialization. The morphology and contrast of both amorphous and crystallized backgrounds can be clearly observed under the optical microscope by regulating its focused lens and filters. Amorphous and crystallized regions show differ-
Fig. 1. Optical image of crystallized and amorphous backgrounds before fs laser irradiation.
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Fig. 2. Optical image of marks formed in the crystallized or amorphous backgrounds of Ge1 Sb2 Te4 films with various thickness by single fs pulses using different average laser fluence. In the crystallized background of a Ge1 Sb2 Te4 15 nm thick film: (a) 22 mJ/cm2 , (b) 70 mJ/cm2 , and (c) 85 mJ/cm2 ; in the amorphous background of a Ge1 Sb2 Te4 50 nm thick film: (d) 75 mJ/cm2 , (e) 85 mJ/cm2 , and (f) 112 mJ/cm2 .
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ent contrast under the microscope. The amorphous background shows dark color under the microscope while the crystallized one displays bright color as shown in Fig. 1. After femtosecond single pulse exposure, there are marks, circular dark spots, formed in the crystallized background as shown in Fig. 2(a), but no phase change can be observed in the amorphous background. The contrast of marks written in the crystallized background is similar to that of the amorphous background shown in Fig. 1. The size of each mark is about 1.4 m in diameter. The average incident laser fluence is 22 mJ/cm2 . Fig. 2(b) shows a typical morphology and contrast of marks formed in the crystallized background with a higher average laser fluence of 70 mJ/cm2 . The contrast of dark marks is similar to that of the amorphous phase in Fig. 1. With this laser fluence, larger dark marks about 2.7 m in diameter appear in the background, still no phase change can be found, but damage is caused at center of the laser spot, in the amorphous background. When the laser fluence is increased further, the data spots obtained in the crystallized background become bigger. Their contrast is similar to that of the amorphous region. This trend keeps for marks formed in crystallized background until the average laser fluence is increased up to 85 mJ/cm2 . With an average laser fluence of 85 mJ/cm2 , damage is caused in the crystallized region as shown in Fig. 2(c). The second sample reported is a disk with a multilayer structure of 10 nm ZnS–SiO2 /50 nm Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. Its active layer thickness is greater than that of the first sample. One half of the disk was also initialized to provide both amorphous and crystallized regions for the fs laser irradiation. After a femtosecond single pulse exposure, there are very small marks, circular bright spots, formed in the amorphous background as shown in Fig. 2(d), but no change can be observed in the crystallized background. The contrast of marks written in the amorphous background is similar to that of the crystallized background shown in Fig. 1. The size of each mark is about 0.4 m in diameter. The average incident laser fluence is 75 mJ/cm2 . Fig. 2(e) shows a typical morphology and contrast of marks formed in the amorphous background with a higher average laser fluence of 85 mJ/cm2 . The contrast of bright marks is similar to that of the crystallized phase in Fig. 1. With this laser fluence, larger and clearer marks about 2.0 m in diameter appear in the amorphous background, still no phase change can be found in the crystallized background. When the laser fluence is increased further, the data spots obtained in the amorphous background become bigger and brighter. Their contrast is similar to that of the crystallized region. This trend keeps for marks formed in amorphous background until the average laser fluence is increased up to 112 mJ/cm2 . With an average laser fluence of 112 mJ/cm2 , damage is created at the centers of marks formed in amorphous background as shown in Fig. 2(f). Bright data bit arrays (10 m × 10 m) with a fluence of 85 mJ/cm2 were produced in an area of 1 cm × 1 cm for XRD analysis. Fig. 3 shows the XRD result. In the case of crystallized background, no change can be found in the crystallized background until the average
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Fig. 3. XRD diffraction pattern of the sample (1 cm × 1 cm) with date bit arrays (10 m × 10 m) formed in the amorphous background of a Ge1 Sb2 Te4 50 nm thick film with an average laser fluence of 85 mJ/cm2 .
laser fluence is increased up to150 mJ/cm2 . When the average laser fluence is as high as 150 mJ/cm2 , damage is made in the crystallized background. 4. Discussion For the first sample with a 15 nm thick Ge1 Sb2 Te4 layer, from Figs. 1 and 2(a)–(c), the morphology and contrast of marks formed by the fs single pulse exposure in the crystallized background is very clear and similar to that of the amorphous region of the disk. This result indicates that re-amorphization has been achieved upon irradiation with single 108 fs laser pulses when the average laser fluence is greater than 22 mJ/cm2 , but less than 85 mJ/cm2 . A single pulse at a high fluence of 85 mJ/cm2 induces the re-amorphization of a larger region and partial ablation at its center as shown in Fig. 2(c). This can be attributed to the higher energy density at the center part of the fs spot than at its edge because of the Gaussian-like intensity distribution of the fs laser beam. The portions at the centers of the fs spots have been ablated as a consequence of the fs irradiation. For the first sample, however, the inverse transformation, i.e., crystallization of the amorphized phase, has not been observed using a single 108 fs laser pulse by changing the laser fluence between the surface melting and damage thresholds. Amorphization was also reported to occur in Ge2 Sb2 Te5 system by a 120 fs laser exposure [5]. For the second sample with a 50 nm thick Ge1 Sb2 Te4 layer, as shown in Fig. 2(d)–(f), marks produced in the amorphous background are with morphology and contrast very similar to that of the crystallized region shown in Fig. 1. The result can be attributed to the transformation from the amorphous to crystalline phase. This crystallization has been achieved upon irradiation with single fs laser pulses when the average laser fluence is greater than 75 mJ/cm2 , but less than 112 mJ/cm2 . A single pulse at a very high fluence of 112 mJ/cm2 induces the crystallization of a larger region and partial ablation at its center as shown in
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Fig. 2(f). This can also be attributed to the higher energy density at the center part of the fs spot than at its edge because of the Gaussian-like intensity distribution of the fs laser beam. The portions at the centers of the fs spots have been ablated as a consequence of the fs irradiation. XRD result shown in Fig. 3 provides a further evidence for the crystallization. The result confirms the face-centered cubic structure for the second sample subjected to single 108 fs laser irradiation [15]. Therefore, crystallization of the amorphized phase has been demonstrated by single 108 fs laser irradiation of the second sample. However, the inverse transformation, i.e., re-amorphization of the pre-crystallized phase has not succeeded with single 108 fs laser irradiation only by adjusting fluence between the surface melting and damage thresholds in the second sample. On excitation with an ultrashort pulse, the system undergoes several stages of relaxation before returning to equilibrium. The energy is transferred first to the electrons and then to the lattice. The interaction includes several regimes, carrier excitation, thermalization, carrier removal and thermal and structural effects, of carrier excitation and relaxation [16]. The various processes do not occur sequentially. They overlap in time, forming a continuous chain of events spanning the entire range from femtoseconds to microseconds. Excited with an ultrashort pulse, the pulse energy is deposited within a very short time in a very thin surface layer, leading to the high-reflectivity, disordered phase (sometimes called a “molten” phase) observed in a few picoseconds after excitation, then melting or ablation (for strong excitation) and resolidification. The inability of the first sample to crystallize or the second sample to amorphize upon single pulse irradiation is related to the ultrashort laser pulse duration and the undercooling achieved prior to solidification. As we know, the nanosecond-induced amorphization of GeSbTe films is produced at quenching rates >3.4 × 109 K/s [17]. The quenching rates expected for irradiation with fs and ps are in both cases >1012 K/s and therefore the formation of amorphous phases may be expected for the two samples. Nevertheless the undercooling achieved prior to solidification is another important factor involved in the formation of amorphous phases by rapid solidification processes. During the initial solidification process into an amorphous one, the solidification enthalpy releases, resulting in lowering the supercooling and promoting the nucleation and growth of the crystalline phase. The amount of solidification enthalpy released depends on both the film thickness and the nature of the solid phase which nucleates homogeneously (amorphous or crystalline). Both the effects of relaxation excess and recalescence could be prevented by using thinner films on low-thermal-conductivity substrates. The bulk solidification and recalescence had been prevented in the first sample due to its very thin phase change layer. As a result, the 15 nm thick Ge1 Sb2 Te4 sample is able to amorphize upon single fs pulse irradiation. But the inverse transformation, i.e., crystallization of the amorphized phase, frustrated using single fs pulse in this sample. On the other hand, effects of recalescence (undercooling decreases) took place in the second sample due to its thick phase change layer. These effects have impeded the re-amorphization in the second sample. Previous research on optical media operated in ns laser pulse showed that in most of stoichiometric systems
the amorphous phase cannot be crystallized upon irradiation with pulses shorter than a few tens of ns [12]. But, our work once again confirms that both crystallization and amorphization can be achieved in the Ge1 Sb2 Te4 system upon irradiation with single 108 fs laser pulses. The final structure induced upon irradiation and thus the reverse phase transformation of the process is strongly conditioned by the thermal properties of dielectric layer/the film/substrate system no matter the initial film state, amorphous or crystalline. In this work, phase reversibility was not able to achieved upon the single 108 fs laser irradiation of the Ge1 Sb2 Te4 layer with different thickness (10–100 nm) in our designed disks by only adjusting the fluence, however, we think reversible phase cycling by a single fs pulse of a fix laser in a Ge1 Sb2 Te4 system for both amorphization and crystallization could be obtained by changing the laser pulse duration, carefully adjusting the fluence and accurately designing the configuration (dielectric layer/film thickness/dielectric layer substrate). Conventional optical disk recording is performed by laser spot on irradiation a rotational disk. In this case, the laser irradiation time on the portion of the disk is around 10–100 ns, rather long time compared to femtosecond laser spot irradiation. The recording process includes heat diffusion in the layers. The temperature increased area is wider than the laser spot size, which means the mark size and the position of mark are determined not only by the beam factor but also the disk thermal characteristics and the pulse duration. The amorphous marks formed by nanosecond laser pulses are surrounded by a large crystalline band edge while amorphous marks created by fs pulses are without the crystalline edge band [5]. Heat diffusion of conventional laser recording limits the performance of future high-density and high-data rate optical disk. Therefore, the femtosecond laser has provided a method to resolve the heat diffusion limitation of the conventional laser recording. As reported in the above, the characteristic fluence ranges in which re-amorphization and crystallization occur in our Ge1 Sb2 Te4 disks: re-amorphization is induced for fluences typically about 22–85 mJ/cm2 , while crystallization requires higher values, above 75–112 mJ/cm2 . The corresponding single shot energy range is 5.6–21.0 nJ for re-amorphization, and 18.5–22.3 nJ for crystallization. Our Ti:sapphire laser oscillator has a repetition rate of 82 MHz, and its single pulse energy is high up to 13 nJ. Thus, for this model oscillator, the single shot of Ti:sapphire laser oscillator with a repetition rate of 40 MHz can provide enough energy to achieve crystalline to amorphous or amorphous to crystalline phase change in our Ge1 Sb2 Te4 disk. A repetition rate of 40 MHz is still very high compared to that of the nanosecond lasers used in the optical recording. The former is three orders of magnitude higher than the latter. On the other hand, an important consequence of this behavior is the fact that the extrapolation of the fluences required for crystallization or amorphization under sub-ps laser pulses to a diffraction-limited spot size are consistent with pulse absolute energies in the sub-nJ range. The laser requirements for the application of the material as a sub-ps laser pulse-driven optical recording medium are thus compatible with those provided by compact diode-pumped ps/fs laser oscillators. Recording or retrieving data rapidly is possible using the oscillator as a laser source because of its very high rep-
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etition rate. Therefore, it is promising to record and retrieve data rapidly in the Ge1 Sb2 Te4 film within the conventional optical disk structure using a non-amplified laser system. 5. Conclusions The phase transformations induced in a Ge1 Sb2 Te4 system has been investigated by a single femtosecond laser exposure. The system has a multilayer structure of 10 nm ZnS–SiO2 /(15–100 nm) Ge1 Sb2 Te4 /80 nm ZnS–SiO2 /0.6 mm polycarbonate substrate. The morphology and contrast of marks written in both amorphous and crystalline backgrounds by single fs pulses were characterized. The detailed results of optical microscopy evaluation of the morphology and contrast of marks written in both amorphous and crystalline backgrounds by single 108 fs pulses are present. X-ray diffraction was applied to identify the crystal structures formed by single fs shots. A well-defined laser fluence and pulse energy window for achieving crystalline to amorphous or amorphous to crystalline phase change in the Ge1 Sb4 Te7 systems using single 108 fs pulses is provided. The mechanism of phase transformations triggered by single femtosecond laser pulses is discussed. Acknowledgements This work was supported by Foundations of Pujiang Talented Person Plans of Shanghai Science & Technology (no. 05PJ14037), Shanghai-Applied Materials Research and Development fund (no. 0519) and Shanghai Nanotechnology (no. 0552nm042).
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