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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 17 — Aug. 13, 2012
  • pp: 18585–18590
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Femtosecond laser-induced crystallization of amorphous Ga-Sb-Se films and coherent phonon dynamics

Weiling Zhu, Yegang Lu, Simian Li, Zhitang Song, and Tianshu Lai  »View Author Affiliations


Optics Express, Vol. 20, Issue 17, pp. 18585-18590 (2012)
http://dx.doi.org/10.1364/OE.20.018585


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Abstract

Femtosecond laser-irradiation-induced phase change of new environment friendly Te-free amorphous Ga-Sb-Se films is studied by coherent phonon spectroscopy. New coherent optical phonons (COP) occur when laser irradiation power reaches some threshold, implying laser-induced phase change taken place. Pump power dependence of COP dynamics reveals the phase change as crystallization and crystallization quality is comparable to one of annealing crystallization, showing application potential of Ga-Sb-Se films in optical phase change memory. The laser-irradiated crystallization of different component Ga-Sb-Se films is studied. It is found crystallization threshold power depends on Sb content, implying Sb-content control of the crystallization temperature of Ga-Sb-Se films.

© 2012 OSA

1. Introduction

2. Sample and experiment system

Three samples with different composition, Ga1Sb6Se3, Ga3Sb4Se3, Ga1Sb3Se6, are studied. They are all about 10 nm thick, and grown on glass substrates by magnetron sputtering using separate Sb, GaSb, and Sb2Se3 targets. All depositions are performed at room temperature to ensure as-deposited films in an amorphous phase. The details on the conditions and procedures of the film preparation were described elsewhere [1

1. Y. G. Lu, S. N. Song, Y. F. Gong, Z. T. Song, F. Rao, L. C. Wu, B. Liu, and D. N. Yao, “Ga-Sb-Se material for low-power phase change memory,” Appl. Phys. Lett. 99(24), 243111 (2011). [CrossRef]

].

3. Laser-induced crystallization and its characterization by coherent phonon spectroscopy

3.1 In situ characterization of laser-induced crystallization of amorphous Ga1Sb6Se3 film

In our experiment, the irradiation laser is the same as pump laser. Its power is tuned by a neutral-density attenuator. Pump laser power is first increased up to a higher power and irradiates amorphous Ga1Sb6Se3 film for a few seconds to induce phase change. Then, pump power is decreased down to a lower level of 15 mW which does not lead to phase change. The irradiated area is measured in situ by transient reflectivity change under the lower pump power of 15 mW. Such measurement is repeated on a fresh spot after which is irradiated by a new higher irradiation power. All measurements are performed at room temperature and under a same low pump power of 15 mW. All transient photoreflectance changes are plotted in Fig. 1(a)
Fig. 1 (a) Transient photoreflectance changes taken on the amorphous Ga1Sb6Se3 film for different laser irradiation power but the same pump power of 15 mW. (b) Transient oscillation of COPs extracted from (a). (c) FFT spectra corresponding to (b). The bottom curves in (a, b, c) are taken from crystallized Ga1Sb6Se3 film by annealing.
for an increasing laser irradiation power (LIP) from 15 to 128 mW. It is obvious that the transient traces almost maintain unchanged when LIP is below 90 mW. However, they change markedly when LIP reaches 90 mW and higher. An oscillatory component occurs and is superimposed on a normal carrier dynamic profile, which is just so-called coherent optical phonon spectroscopy (COPS) [6

6. W. L. Zhu, C. Z. Wang, M. C. Sun, S. M. Li, J. W. Zhai, and T. S. Lai, “Characterization of femtosecond laser-irradiation crystallization and structure of multiple periodic Si/Sb80Te20 nanocomposite films by coherent phonon spectroscopy,” Opt. Express 19(23), 22684–22691 (2011). [CrossRef] [PubMed]

8

8. Y. W. Li, V. A. Stoica, L. Endicott, G. Y. Wang, C. Uher, and R. Clarke, “Coherent optical phonon spectroscopy studies of femtosecond-laser modified Sb2Te3 films,” Appl. Phys. Lett. 97(17), 171908 (2010). [CrossRef]

]. The amplitude of the oscillatory component is enhanced with increasing LIP. The appearance of coherent optical phonons (COP) shows that larger LIP results in microstructure changes of the amorphous film because all measurements are made under a same lower pump power of 15 mW, while such a lower power pump does not lead to any phase change, as shown by the top transient profile in Fig. 1(a).

To understand the essence of the laser-induced phase change, a crystalline Ga1Sb6Se3 film crystallized by thermal annealing [1

1. Y. G. Lu, S. N. Song, Y. F. Gong, Z. T. Song, F. Rao, L. C. Wu, B. Liu, and D. N. Yao, “Ga-Sb-Se material for low-power phase change memory,” Appl. Phys. Lett. 99(24), 243111 (2011). [CrossRef]

] is also measured under a lower pump of 15 mW. Its transient photoreflectance change is also plotted in Fig. 1(a) at the bottom. Similar oscillatory component to one appeared in laser-induced phase-changed film occurs, which implies that the laser-induced phase change or microstructure change is just crystallization. Therefore, the Ga1Sb6Se3 film has applicable potential in optical phase change storage.

To clearly show laser-irradiation-induced crystallization, it is necessary quantitatively to analyze coherent phonon transient traces in Fig. 1(a). The oscillatory and non-oscillatory components are separated by digital low-pass filtering the transient data in Fig. 1(a) [9

9. Y. G. Wang, X. F. Xu, and R. Venkatasubramanian, “Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett. 93(11), 113114 (2008). [CrossRef]

]. The oscillating components are plotted in Fig. 1(b). It shows clearly the amplitude of oscillatory component first increases with LIP and reaches a saturation at high LIP. They are fast Fourier-transformed (FFT). Corresponding FFT spectra are plotted in Fig. 1(c). It is obviously seen that a COP peak starts to appear at 4.56THz as LIP rises up to 90 mW and its intensity is enhanced with the increase of LIP, directly showing the enhancement of laser-irradiation-induced the degree of crystallization. The frequency of 4.56 THz agrees very well with the reported one of A1g optical phonon mode (4.50 THz) of crystalline Sb at room temperature [10

10. G. A. Garrett, T. F. Albrecht, J. F. Whitaker, and R. Merlin, “Coherent THz phonons driven by light pulses and the Sb problem: what is the mechanism?,” Phys. Rev. Lett. 77(17), 3661–3664 (1996). [CrossRef] [PubMed]

,11

11. H. J. Zeiger, J. Vidal, T. K. Cheng, E. P. Ippen, G. Dresselhaus, and M. S. Dresselhaus, “Theory for displacive excitation of coherent phonons,” Phys. Rev. B Condens. Matter 45(2), 768–778 (1992). [CrossRef] [PubMed]

], implying laser-irradiation-crystallized Ga1Sb6Se3 film contains crystalline Sb. This also agrees very well with results of X-ray diffraction which shows amorphous Ga1Sb6Se3 could be crystallized at 250 oC and crystalline Ga1Sb6Se3 film is composed of rhombohedral Sb and orthorhombic Sb2Se3 crystallites [1

1. Y. G. Lu, S. N. Song, Y. F. Gong, Z. T. Song, F. Rao, L. C. Wu, B. Liu, and D. N. Yao, “Ga-Sb-Se material for low-power phase change memory,” Appl. Phys. Lett. 99(24), 243111 (2011). [CrossRef]

].

3.2 The excitation power dependence of coherent phonon dynamics of the crystallized Ga1Sb6Se3 film

To further understand crystalline statuses of laser-induced crystallized film, the excitation power dependence of coherent phonon dynamics is investigated on the crystallized Ga1Sb6Se3 film that has been irradiated by the laser power of 120 mW. The differential reflectivity transient traces are taken for an increasing pump power from 15 to 95 mW with an increment of 10 mW. The oscillatory components are extracted from the transient traces by the method mentioned above, and plotted in Fig. 2(a)
Fig. 2 (a) The excitation power dependence of coherent phonon dynamics of the crystallized Ga1Sb6Se3 film by laser irradiation of 120mW. The scattered filled circles denote the experimental data, while the solid lines are the best fittings to the experimental data. (b) FFT spectra corresponding to the experimental data in (a). (c) The excitation power dependence of the frequency of COP for both laser and annealing crystallization Ga1Sb6Se3 films. (d) The excitation power dependence of the lifetime of COP for both laser and annealing crystallization Ga1Sb6Se3 films.
by scattered filled circles. It is evident from Fig. 2(a) that the oscillatory amplitude of COP is enhanced with the increase in pump power, revealing more COP excited at higher pump power. The COP data are fast Fourier-transformed. The corresponding FFT spectra are plotted in Fig. 2(b). Each FFT spectrum shows an obvious single peak, and the intensity of the peak increases with the increase in pump power. It is noteworthy that the position of the peak exhibits an obvious redshift from 4.56 to 4.45 THz with the increase in pump power. This redshift phenomenon may be explained by the temperature dependence of COP frequency and is a typical character of COP in crystals [12

12. M. Hase, Y. Miyamoto, and J. Tominaga, “Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2Te3 superlattices,” Phys. Rev. B 79(17), 174112 (2009). [CrossRef]

], as found in crystalline GeTe/Sb2Te3 superlattices. As discussed in the last subsection, the peaks in Fig. 2(b) originate from COP of crystalline Sb. It has been reported that the frequency of COP in crystalline Sb was redshifted from 4.65 THz at 8 K [13

13. K. Ishioka, M. Kitajima, and O. V. Misochko, “Coherent A1g and Eg phonons of antimony,” J. Appl. Phys. 103(12), 123505 (2008). [CrossRef]

] to 4.50 THz at room temperature [10

10. G. A. Garrett, T. F. Albrecht, J. F. Whitaker, and R. Merlin, “Coherent THz phonons driven by light pulses and the Sb problem: what is the mechanism?,” Phys. Rev. Lett. 77(17), 3661–3664 (1996). [CrossRef] [PubMed]

,11

11. H. J. Zeiger, J. Vidal, T. K. Cheng, E. P. Ippen, G. Dresselhaus, and M. S. Dresselhaus, “Theory for displacive excitation of coherent phonons,” Phys. Rev. B Condens. Matter 45(2), 768–778 (1992). [CrossRef] [PubMed]

]. Similar temperature dependence was also observed in GeTe/Sb2Te3 superlattices [12

12. M. Hase, Y. Miyamoto, and J. Tominaga, “Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2Te3 superlattices,” Phys. Rev. B 79(17), 174112 (2009). [CrossRef]

]. Higher the pump power is, higher the lattice temperature rises up to by electron-phonon coupling exchange heating, thus leading to the redshift of COP’s frequency.

A single exponential damped oscillatory function, A exp(-t/τ) cos(2πνt + φ), is used to fit the oscillatory traces in Fig. 2(a) by least square fitting, where A, τ, ν and φ denote the amplitude, lifetime, frequency and initial phase of COP, respectively. The best fittings are also plotted in Fig. 2(a) by solid lines. The pump power dependence of the ν extracted by the fit is plotted Fig. 2(c) by the solid line with filled squares plus error bars and agrees very well with FFT spectra in Fig. 2(b). The pump power dependence of the lifetime τ extracted is plotted Fig. 2(d) by solid line with filled squared plus errors. Clearly, the τ decreases with the increase in pump power. This dependence may be explained by the enhancement of incoherent optical phonon emitting with pump power and is also a typical feature of COP in crystals [9

9. Y. G. Wang, X. F. Xu, and R. Venkatasubramanian, “Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett. 93(11), 113114 (2008). [CrossRef]

]. Similar dependence of lifetime or rate (1/τ) on excitation power was also reported in crystalline Bi2Te3, Sb2Te3 and their superlattices [9

9. Y. G. Wang, X. F. Xu, and R. Venkatasubramanian, “Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett. 93(11), 113114 (2008). [CrossRef]

]. The pump power dependence of ν and τ of COP agrees very well with behaviors in single crystals [9

9. Y. G. Wang, X. F. Xu, and R. Venkatasubramanian, “Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett. 93(11), 113114 (2008). [CrossRef]

,12

12. M. Hase, Y. Miyamoto, and J. Tominaga, “Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2Te3 superlattices,” Phys. Rev. B 79(17), 174112 (2009). [CrossRef]

], implying the crystalline status of laser-induced Ga1Sb6Se3 film crystallization is good, and crystalline quality is high. Otherwise, τ should increase with pump power, as shown in amorphous GeTe/Sb2Te3 superlattices [12

12. M. Hase, Y. Miyamoto, and J. Tominaga, “Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2Te3 superlattices,” Phys. Rev. B 79(17), 174112 (2009). [CrossRef]

].

To understand any difference between laser-induced and heating-induced crystallization, a pump power-dependent experiment is carried out on annealing crystallized Ga1Sb6Se3 film, but their transient traces are not shown out here. By fitting oscillatory components as mentioned afore, the pump power dependence of ν and τ of the COP of the annealing crystallized Ga1Sb6Se3 film is obtained and also plotted in Figs. 2(c) and 2(d), respectively, by dashed lines with filled squares and filled triangles plus errors. Obviously, for both laser-crystallized and heating-crystallized Ga1Sb6Se3 films, the pump power dependence of ν agrees very well, while the one of the τ does also well except for an offset in value, which further show the consistency of the laser-induced and annealing-induced crystallization. As for a offset difference appeared between COP lifetimes of laser-induced and annealing-induced crystallized films, it may be attributed to different environment coupling to the detection area. Annealing-induced crystallization is taken place in whole film, while laser irradiation only leads to crystallization of a small irradiated area. As a result, COP in laser-induced crystallized film is coupled to an amorphous environment which leads to a fast decay of COP oscillation, or a shorter lifetime.

4. Effect of composition on laser-induced crystallization of Ga-Sb-Se films

In order to understand the influence of the atomic content on the crystallization of the Ga-Sb-Se films, we measure the crystallization characteristics of another two amorphous films, Ga3Sb4Se3 and Ga1Sb3Se6. Figures 3(a)
Fig. 3 (a) and (c) Laser irradiation power dependence of transient photoreflectance changes taken on the amorphous Ga3Sb4Se3 and Ga1Sb3Se6 films, respectively, under a same pump power of 15 mW during all measurements. (b) FFT spectra corresponding to oscillatory components in (a). (d) Sheet resistance vs. temperature for three samples.
and 3(b) show the time-resolved transient reflectivity changes and the FFT spectrum of Ga3Sb4Se3 film at a same low pump power of 15 mW after a different LIP, respectively. They show that Ga3Sb4Se3 film starts to crystallize as LIP reaches about 112mW. The COP peak still occurs at 4.56 THz, as Fig. 3(b) shows. Figure 3(c) shows the time-resolved transient reflectivity change of Ga1Sb3Se6 film after different LIP. It is obvious that crystallization has not taken place until LIP reaches 128 mW which is the maximum available in our experiment. For all three samples studied, Ga1Sb3Se6, Ga3Sb4Se3 and Ga1Sb6Se3 (studied in last section), their crystallization threshold LIP decreases with the increase of Sb content. We conjecture the reason is the decrease of crystallization temperature with the increase of Sb content because similar phenomena were reported in Sb-contained phase change recording materials [14

14. L. van Pieterson, M. H. R. Lankhorst, M. van Schijndel, A. E. T. Kuiper, and J. H. J. Roosen, “Phase-change recording materials with a growth-dominated crystallization mechanism: a material review,” J. Appl. Phys. 97(8), 083520 (2005). [CrossRef]

16

16. M. J. Kang, S. Y. Choi, D. Wamwangi, K. Wang, C. Steimer, and M. Wuttig, “Structural transformation of SbxSe100−x thin films for phase change nonvolatile memory applications,” J. Appl. Phys. 98(1), 014904 (2005). [CrossRef]

].

To prove our conjecture, the resistance-temperature (R-T) curves of the three amorphous film samples are measured and plotted in Fig. 3(d), giving out crystallization temperature of ~250, ~310 and above 400 °C, respectively, for Ga1Sb6Se3, Ga3Sb4Se3 and Ga1Sb3Se6 films. Therefore, our conjecture is proven, showing the crystallization temperature controllable by the control of composition in Ga-Sb-Se alloy.

5. Conclusion

Optical phase change characteristics of new environment friendly Te-free amorphous Ga-Sb-Se films, reported as electrical drive phase change materials, have been studied in this article by sensitive coherent phonon spectroscopy. Femtosecond laser-irradiation-induced phase change is revealed by occurrences of new coherent optical phonons when laser irradiation power reaches some threshold. Pump power-dependent dynamics of the coherent optical phonons shows that the frequency and lifetime of the coherent optical phonons decreases with the increase in pump power, agreeing well with the pump power dependence of COP dynamics in crystals. Consequently, laser-irradiated phase change is revealed as crystallization. A contrast experiment is also performed on annealing crystallized film, revealing both laser-irradiated and annealing crystallization identical in crystallization quality. The laser crystallization of different composition Ga-Sb-Se films is also studied. It is found crystallization threshold of LIP depends sensitively on the composition of Ga-Sb-Se films. The higher the Sb content in the Ga-Sb-Se films is, the lower crystallization threshold is, which agrees well with the Sb-content dependence of crystallization temperature revealed by resistance-temperature curves, and also shows the controllablity of crystallization threshold power of the Ga-Sb-Se films by composition. These results show Ga-Sb-Se films can also serve as an optical phase change recording material.

Acknowledgments

This work is partially supported by National Natural Science Foundation of China under grant Nos. 60906003, 60906004, 61006087, 61076121 and 61078027, National Basic Research of China under grant Nos. 2010CB923200, 2010CB934300, 2011CB932800, and doctoral specialized fund of MOE of China under grant No. 20090171110005.

References and links

1.

Y. G. Lu, S. N. Song, Y. F. Gong, Z. T. Song, F. Rao, L. C. Wu, B. Liu, and D. N. Yao, “Ga-Sb-Se material for low-power phase change memory,” Appl. Phys. Lett. 99(24), 243111 (2011). [CrossRef]

2.

S. Fujimori, S. Yagi, H. Yamazaki, and N. Funakoshi, “Crystallization process of Sb-Te alloy films for optical storage,” J. Appl. Phys. 64(3), 1000–1004 (1988). [CrossRef]

3.

C. B. Peng, L. Cheng, and M. Mansuripur, “Experimental and theoretical investigations of laser-induced crystallization and amorphization in phase-change optical recording media,” J. Appl. Phys. 82(9), 4183–4191 (1997). [CrossRef]

4.

L. P. Shi, T. C. Chong, X. Hu, and H. B. Yao, “Study of the dynamic crystallization behavior of GeSbTe phase change optical disk,” Jpn. J. Appl. Phys. 42(Part 1, No. 2B), 841–847 (2003). [CrossRef]

5.

M. L. Tseng, B. H. Chen, C. H. Chu, C. M. Chang, W. C. Lin, N. N. Chu, M. Mansuripur, A. Q. Liu, and D. P. Tsai, “Fabrication of phase-change chalcogenide Ge2Sb2Te5 patterns by laser-induced forward transfer,” Opt. Express 19(18), 16975–16984 (2011). [CrossRef] [PubMed]

6.

W. L. Zhu, C. Z. Wang, M. C. Sun, S. M. Li, J. W. Zhai, and T. S. Lai, “Characterization of femtosecond laser-irradiation crystallization and structure of multiple periodic Si/Sb80Te20 nanocomposite films by coherent phonon spectroscopy,” Opt. Express 19(23), 22684–22691 (2011). [CrossRef] [PubMed]

7.

M. Först, T. Dekorsy, C. Trappe, M. Laurenzis, H. Kurz, and B. Béchevet, “Phase change in Ge2Sb2Te5 films investigated by coherent phonon spectroscopy,” Appl. Phys. Lett. 77(13), 1964–1966 (2000). [CrossRef]

8.

Y. W. Li, V. A. Stoica, L. Endicott, G. Y. Wang, C. Uher, and R. Clarke, “Coherent optical phonon spectroscopy studies of femtosecond-laser modified Sb2Te3 films,” Appl. Phys. Lett. 97(17), 171908 (2010). [CrossRef]

9.

Y. G. Wang, X. F. Xu, and R. Venkatasubramanian, “Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett. 93(11), 113114 (2008). [CrossRef]

10.

G. A. Garrett, T. F. Albrecht, J. F. Whitaker, and R. Merlin, “Coherent THz phonons driven by light pulses and the Sb problem: what is the mechanism?,” Phys. Rev. Lett. 77(17), 3661–3664 (1996). [CrossRef] [PubMed]

11.

H. J. Zeiger, J. Vidal, T. K. Cheng, E. P. Ippen, G. Dresselhaus, and M. S. Dresselhaus, “Theory for displacive excitation of coherent phonons,” Phys. Rev. B Condens. Matter 45(2), 768–778 (1992). [CrossRef] [PubMed]

12.

M. Hase, Y. Miyamoto, and J. Tominaga, “Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2Te3 superlattices,” Phys. Rev. B 79(17), 174112 (2009). [CrossRef]

13.

K. Ishioka, M. Kitajima, and O. V. Misochko, “Coherent A1g and Eg phonons of antimony,” J. Appl. Phys. 103(12), 123505 (2008). [CrossRef]

14.

L. van Pieterson, M. H. R. Lankhorst, M. van Schijndel, A. E. T. Kuiper, and J. H. J. Roosen, “Phase-change recording materials with a growth-dominated crystallization mechanism: a material review,” J. Appl. Phys. 97(8), 083520 (2005). [CrossRef]

15.

M. S. Youm, Y. T. Kim, Y. H. Kim, and M. Y. Sung, “Effects of excess Sb on crystallization of δ-phase SbTe binary thin films,” Phys. Status Solidi A 205(7), 1636–1640 (2008). [CrossRef]

16.

M. J. Kang, S. Y. Choi, D. Wamwangi, K. Wang, C. Steimer, and M. Wuttig, “Structural transformation of SbxSe100−x thin films for phase change nonvolatile memory applications,” J. Appl. Phys. 98(1), 014904 (2005). [CrossRef]

OCIS Codes
(160.2900) Materials : Optical storage materials
(210.4770) Optical data storage : Optical recording
(300.6500) Spectroscopy : Spectroscopy, time-resolved
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors

ToC Category:
Materials

History
Original Manuscript: June 11, 2012
Revised Manuscript: July 11, 2012
Manuscript Accepted: July 20, 2012
Published: July 30, 2012

Citation
Weiling Zhu, Yegang Lu, Simian Li, Zhitang Song, and Tianshu Lai, "Femtosecond laser-induced crystallization of amorphous Ga-Sb-Se films and coherent phonon dynamics," Opt. Express 20, 18585-18590 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-17-18585


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References

  1. Y. G. Lu, S. N. Song, Y. F. Gong, Z. T. Song, F. Rao, L. C. Wu, B. Liu, and D. N. Yao, “Ga-Sb-Se material for low-power phase change memory,” Appl. Phys. Lett.99(24), 243111 (2011). [CrossRef]
  2. S. Fujimori, S. Yagi, H. Yamazaki, and N. Funakoshi, “Crystallization process of Sb-Te alloy films for optical storage,” J. Appl. Phys.64(3), 1000–1004 (1988). [CrossRef]
  3. C. B. Peng, L. Cheng, and M. Mansuripur, “Experimental and theoretical investigations of laser-induced crystallization and amorphization in phase-change optical recording media,” J. Appl. Phys.82(9), 4183–4191 (1997). [CrossRef]
  4. L. P. Shi, T. C. Chong, X. Hu, and H. B. Yao, “Study of the dynamic crystallization behavior of GeSbTe phase change optical disk,” Jpn. J. Appl. Phys.42(Part 1, No. 2B), 841–847 (2003). [CrossRef]
  5. M. L. Tseng, B. H. Chen, C. H. Chu, C. M. Chang, W. C. Lin, N. N. Chu, M. Mansuripur, A. Q. Liu, and D. P. Tsai, “Fabrication of phase-change chalcogenide Ge2Sb2Te5 patterns by laser-induced forward transfer,” Opt. Express19(18), 16975–16984 (2011). [CrossRef] [PubMed]
  6. W. L. Zhu, C. Z. Wang, M. C. Sun, S. M. Li, J. W. Zhai, and T. S. Lai, “Characterization of femtosecond laser-irradiation crystallization and structure of multiple periodic Si/Sb80Te20 nanocomposite films by coherent phonon spectroscopy,” Opt. Express19(23), 22684–22691 (2011). [CrossRef] [PubMed]
  7. M. Först, T. Dekorsy, C. Trappe, M. Laurenzis, H. Kurz, and B. Béchevet, “Phase change in Ge2Sb2Te5 films investigated by coherent phonon spectroscopy,” Appl. Phys. Lett.77(13), 1964–1966 (2000). [CrossRef]
  8. Y. W. Li, V. A. Stoica, L. Endicott, G. Y. Wang, C. Uher, and R. Clarke, “Coherent optical phonon spectroscopy studies of femtosecond-laser modified Sb2Te3 films,” Appl. Phys. Lett.97(17), 171908 (2010). [CrossRef]
  9. Y. G. Wang, X. F. Xu, and R. Venkatasubramanian, “Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett.93(11), 113114 (2008). [CrossRef]
  10. G. A. Garrett, T. F. Albrecht, J. F. Whitaker, and R. Merlin, “Coherent THz phonons driven by light pulses and the Sb problem: what is the mechanism?,” Phys. Rev. Lett.77(17), 3661–3664 (1996). [CrossRef] [PubMed]
  11. H. J. Zeiger, J. Vidal, T. K. Cheng, E. P. Ippen, G. Dresselhaus, and M. S. Dresselhaus, “Theory for displacive excitation of coherent phonons,” Phys. Rev. B Condens. Matter45(2), 768–778 (1992). [CrossRef] [PubMed]
  12. M. Hase, Y. Miyamoto, and J. Tominaga, “Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2Te3 superlattices,” Phys. Rev. B79(17), 174112 (2009). [CrossRef]
  13. K. Ishioka, M. Kitajima, and O. V. Misochko, “Coherent A1g and Eg phonons of antimony,” J. Appl. Phys.103(12), 123505 (2008). [CrossRef]
  14. L. van Pieterson, M. H. R. Lankhorst, M. van Schijndel, A. E. T. Kuiper, and J. H. J. Roosen, “Phase-change recording materials with a growth-dominated crystallization mechanism: a material review,” J. Appl. Phys.97(8), 083520 (2005). [CrossRef]
  15. M. S. Youm, Y. T. Kim, Y. H. Kim, and M. Y. Sung, “Effects of excess Sb on crystallization of δ-phase SbTe binary thin films,” Phys. Status Solidi A205(7), 1636–1640 (2008). [CrossRef]
  16. M. J. Kang, S. Y. Choi, D. Wamwangi, K. Wang, C. Steimer, and M. Wuttig, “Structural transformation of SbxSe100−x thin films for phase change nonvolatile memory applications,” J. Appl. Phys.98(1), 014904 (2005). [CrossRef]

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