Multiple parameters of nanocomposite Si/Sb80Te20 multilayer films are possibly optimized simultaneously to satisfy the development of ideal phase-change memory devices by adjusting chemical composition and physical structure of multilayer films. The crystallization and structure of the films are studied by coherent phonon spectroscopy. Laser irradiation power dependence of coherent optical phonon spectroscopy reveals laser-induced crystallization of the amorphous multilayer film, while coherent acoustic phonon spectroscopy reveals the presence of folded acoustic phonons which suggests a good periodic structure of the multilayer films. Laser irradiation-induced crystallization shows applicable potentials of the multilayer films in optical phase change storage.
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Sb2Te3 films have potential applications in high-speed phase change (PC) random access memory devices due to its fast growth-dominated PC mechanism . However, it has the disadvantages of low crystallization temperature (Tc = 80 °C)  which leads to poor thermal stability of the memorized information, and a high melting temperature (TM = 617)  which results in a large energy consumption during the reset process of data storage. Youm et al found that increasing Sb content in Sb-Te alloy could raise Tc from 80 to 140 °C and reduce the TM from 615 to 543 °C . Lee et al found that the addition of Ag and In into Sb70T30 could lead to further the increase of Tc up to 168 °C and the reduction of TM down to 526 °C . Recently, Simpson et al  found that the addition of Bi into Sb8Te2 could decrease the crystallization time by an order of magnitude at the concentration of 8 at. % Bi, greatly increasing write rate of data recording in PC memory devices, but the Tc again reduced down to 115 °C from 165 °C. Therefore, it seems difficult to optimize all parameters (Tc, TM, crystallization time and resistivity in crystalline state etc.) of SbTe alloy only by the doping or adjustment of Sb content . Wang et al just reported on a multiple periodic nanocomposite Si/Sb80Te20 multilayer film , and found that the Tc and resistivity of the multilayer film might be adjusted by the thickness of Si and/or Sb80Te20 layers. They found Tc increased with increasing (reducing) Si (Sb80Te20) layer thickness, and could range largely from 120 to 220 °C. Meanwhile, they also found that the resistivity of the multilayer film in the crystallized state was increased at least by two orders of magnitude, leading to a driven current reduced at least by one order of magnitude during the reset process of data recording. Therefore, combining adjustments of SbTe alloy composition with of Si/SbTe multilayer structure, it is obvious possibly to optimize all parameters of the multilayer films for the development of ideal PC memory devices. Furthermore, Wang et al  also realized current-driven PC recording/erasing with 50 ns electrical pulses. However, it is unknown yet whether the nanocomposite films are suitable for optical PC storage because no data has been reported so far on laser-induced crystallization of the films.
In this article, femtosecond laser-irradiation-induced crystallization of the nanocomposite films is investigated in situ using coherent phonon spectroscopy. It is well known that the coherent phonon spectroscopy is a powerful spectroscopic tool and very sensitive to microstructure change induced by phase change  or even different doping type . It has been extensively used to characterize microstructure change by detecting characteristic phonon modes of various microstructures [6, 8]. Thus, laser-induced crystallization and coherent phonon dynamics of the Si/Sb80Te20 multilayer films are studied by coherent phonon spectroscopy. Coherent optical phonon spectra reveal the laser-induced crystallization of Sb80Te20 layer, while coherent acoustic phonon spectra, especially folded acoustic phonon spectra observed, expose good periodic structure of the multilayer films. It is shown that the multilayer films also have good potentials in the application of optical PC storage because it has a higher crystallization (170 °C)  and lower melting (543 °C)  temperatures than the crystallization (140 °C)  and melting (632 °C)  temperatures of the phase change material of Ge2Sb2Te5 which is used commercially, respectively.
2. Sample and experiment
Two samples studied consist of ten periods of Si/Sb80Te20 films, where the Si and Sb80Te20 layer films all are 5 nm thick, and are grown on glass substrates by radio frequency magnetron sputtering using Si and Sb80Te20 targets. All depositions are performed at room temperature to ensure as-deposited films in the amorphous phase. The details on the conditions and procedures of the multilayer film preparation were described elsewhere . One of the samples was heated in the protective Ar-atmosphere at a heating rate of 10 °C/min. Meanwhile, temperature dependence of the sheet resistance of the sample was measured, and showed that the phase change occurred at 170 °C . Therefore, the thermal stability of [Si/Sb80Te20]10 films is enhanced greatly. In the following, the sample heated is referred to as “crystallized”, while the as-deposited sample as “amorphous”.
Time-resolved pump-probe photo-reflectivity spectroscopy is used to study the coherent phonon dynamics of the amorphous and crystallized multilayer films. A train of 60 femtosecond pulse laser from a self-mode-locked Ti: sapphire laser oscillator with the central wavelength of 840 nm and a pulse repetition rate of 94 MHz is directed into a standard pump-probe setup, and split into parallel two beams of lasers, a strong pump and a weak probe beams with a pump-to-probe intensity ratio of >15. The emerged pump and probe transmit through a convex lens of 50 mm focal length and are focused nearly normally to a same area on the surface of the sample located at the back focal plane of the lens. The probe reflected from the sample surface is detected by a Si photodiode whose output electrical signal is measured by a lock-in amplifier which is referenced at the modulation frequency of an optical chopper that modulated the pump beam at 1.13 kHz. All optical experimental measurements are performed at room temperature and under a low pump power of 15 mW to avoid any pump-induced PC.
Raman scattering measurement is performed with a ReniShaw inVia Reflex Raman spectrometer at 514.5 nm laser excitation.
3. Results and discussion
3.1 In situ characterization of laser irradiating crystallization
Transient photoreflectance changes are first taken on the amorphous film and plotted in Fig. 1 for increasing laser-irradiation power (LIP) from 15 to 55 mW. It is noted that each transient trace is taken on a fresh spot which was irradiated for a few seconds by a given power laser. It is also worth emphasizing that the laser irradiation to a fresh spot on the amorphous sample film is also made by the pump pulses by first increasing pump power to some higher level and irradiating the fresh spot for a few seconds, and then decreasing pump power back to a low level of 15 mW used in all measurements. Then, in situ measurements are made. It is evidently seen that an oscillatory component is superimposed on a normal carrier dynamic profile, which is just so-called coherent phonon spectroscopy (CPS) [6–8], and reflects the vibration of coherent optical phonons (COP) excited by femtosecond pump pulses. It is obvious that the transient traces maintain unchanged almost when LIP is below 45 mW. However, they change markedly when LIP reaches 45 mW and higher, including not only dynamic behavior but also dephasing time of coherent phonons changed. It is shown that laser irradiation results in some changes of the amorphous film because all measurements are made under a same low pump power of 15 mW that does not lead to any change of the samples. We believe those changes just reflect laser-irradiation-induced microstructure change, that is the PC. To obtain direct evidence of laser-irradiation-induced PC, it is necessary quantitatively to analyze coherent phonon transient traces in Fig. 1. The oscillatory and non-oscillatory components are separated by digital low-pass filtering  on the transient data in Fig. 1. The oscillating components are plotted in Fig. 2(a) . They are fast Fourier-transformed (FFT). Cor-responding FFT spectra are plotted in Fig. 2(b). It is obviously seen that COP spectra change markedly when LIP rises up to 45 mW and higher. A strong peak appears at 4.52 THz, directly showing laser-irradiation-induced PC because the vibration frequency of COP corresponds closely to microstructures. The frequency of 4.52 THz agrees very well with one of A1g optical phonon mode (4.50 THz) of crystalline Sb at room temperature [10, 11], implying the excess Sb in Sb80Te20 layer is crystallized during laser-irradiation-induced PC, which agrees very well with previous report on crystallization of δ-phase SbTe binary thin films . Therefore, we believe the laser-irradiation-induced PC proceeds toward the crystallization. In other words, laser irradiation can lead to crystallization of amorphous Si/Sb80Te20 multilayer film, implying its applicable potential in optical PC storage. On the other hand, the dephasing time and relative intensity of COP modes at 3.90 and 4.52 THz can be obtained by fitting the oscillatory components in Fig. 2(a) to a double damped oscillatory exponential decay sum function, and are plotted in Fig. 2(c). It is obvious that the dephasing times, τ3.90 and τ4.52, of two COP modes at 3.90 and 4.52 THz are LIP-independent within experimental error, but the ratio of 4.52 THz to 3.90 THz modes in intensity increases first slowly and then sharply when LIP is larger than 35 mW, showing that both phonon modes are independent and correspond to different microstructures, and the mode at 4.52 THz enhances with increasing LIP as the intensity (I3.90) of the mode at 3.90 THz keeps almost constant. This further supports the viewpoint of laser-irradiation-induced crystallization.
To understand the origin of peak at 3.90 THz, the laser irradiation crystallization of a 5 nm thick as-deposited Sb80Te20 film on a glass substrate is studied as a function of LIP by CPS. The FFT spectra of COP are obtained as described afore and plotted Fig. 3(a) for different LIP. The spectra look quite similar to those in Fig. 2(b), and contain two peaks at 3.90 and 4.52 THz. The latter peak enhances with increasing LIP, showing its laser-irradiation resultant. The coexistence of 3.90 THz modes in single layer Sb80Te20 and multilayer films shows that it originates from amorphous Sb80Te20 and does not relate to Si layer in the multilayer film. Therefore, we conjecture the broad peak at 3.90 THz originates from a characteristic phonon mode of amorphous Sb80Te20 films. A Raman scattering measurement on amorphous 5 nm-thick single layer Sb80Te20 and multilayer films is performed. Raman spectra are plotted in Fig.3(b). Obviously, a broad peak appears at 4.14 THz for both amorphous films. It should correspond to the broad peak at 3.90 THz in COP spectra, while small difference between 3.90 and 4.14 THz may be ascribed to systematic and experimental errors between two different measurement methods. Therefore, Raman peak at 4.14 THz provides strong evidence for the assignment of COP mode at 3.90 THz to the phonon mode of amorphous Sb80Te20 film. The phenomenon of which amorphous phase has itself characteristic phonon mode different from the mode of crystalline phase was also observed in amorphous Ge2Sb2Te5 film . Meanwhile, the observation of COP mode in the amorphous film also shows higher detection sensitivity of CPS to microstructures than nano-beam electron diffraction which did not show observable diffraction patterns in the amorphous multilayer film . On the other hand, Fig. 3(a) shows a crystallization threshold power occurs at ~85 mW for single layer film, whereas Fig. 2(b) does a lower one of ~45 mW for the multilayer film. The falling of crystallization threshold power is helpful to reduce writing power of optical storage devices.
To directly understand the laser irradiation-induced PC, a contrast experiment is carried out on the crystallized multilayer film. The transient photoreflectance change is taken. The oscillatory component of COPs is extracted by the method mentioned afore and plotted in Fig. 4(a) by top transient. For comparison conveniently, the transient oscillation of the amorphous film irradiated by 55 mW laser (top curve in Fig. 2(a)) is also re-plotted in Fig. 4(a) (bottom curve). Their FFT spectra are plotted in Fig. 4(b) and look almost the same except the peak amplitude scale at 4.52 THz. Both FFT spectra present a sharp peak at 4.52 THz, showing two sample films containing the same phases, while the annealed film was confirmed in the crystallized state by the sheet resistance measurement and nano-beam electron diffraction . Therefore, it is directly proven that femtosecond laser irradiation can lead to the crystallization of the amorphous multilayer film.
3.2 Characterization of multilayer film structure
If one views the transient traces more carefully in Fig. 1, one can find there are some low frequency oscillations besides a high frequency oscillation from COPs. Then, what do the low frequency oscillations reflect? To understand the origin of the low frequency oscillations, large time scale transient photoreflectance changes are taken, and plotted in Fig. 5 for the amorphous film and crystallized samples films by annealing and laser irradiation, respectively.
Three transient traces look very similar except the dynamic behavior in first ten picoseconds. More importantly, the low frequency oscillation looks not harmonic. Therefore, it may be synthesized by multiple low frequency harmonic oscillations. Thus, it is very necessary to obtain their spectra.
The low frequency oscillatory component can be obtained by best fitting a transient trace in Fig. 5 to an exponential decay function which is then subtracted from the fitted transient trace. The oscillatory components obtained are plotted in Fig. 6(a) , and look complex and unordered. They are fast Fourier-transformed. The FFT spectra are plotted in Fig. 6(b). It is somewhat surprising that three FFT spectra look very similar and simple in structure, and all contain three main peaks located at ~0.035, ~0.261 and ~0.335 THz. The three frequencies are below 0.5 THz and in the range of acoustic phonon frequency. The fact, the amorphous and crystallized multilayer films have similar coherent acoustic phonon (CAP) spectra, strongly implies that CAP spectra mainly are related to or reflect the periodic structure of the multilayer films because the amorphous and crystallized multilayer films are similar only in macrostructure, but not at all in microstructure. Actually, acoustic phonons reflect long-range order. Similar acoustic phonon modes were observed extensively in semiconductor superlattices, such as GaAs/AlAs [13, 14], and Si/Si1-xGex  superlattices, and called as folded acoustic phonons (FAP) [13–15]. Our multiple periodic nanocomposite films actuallybelong to the superlattice. Therefore, we conjecture three peaks in Fig. 6(b) should originate from FAPs. As shown in Fig. 3(b), in the range of low frequency Raman shift, the increasing speed of Raman signal with decreasing shift frequency is much faster in the multilayer film than in single layer film, which also provides the indirect evidence to the presence of FAPs. A quantitative calculation is necessary exactly to assign the three peaks. For Si and Sb80Te20 layers, referred to as layer 1 and 2, respectively, their parameters are taken as thickness d1 = d2 = 5 nm, medium density ρ1 = 2.23 kg/m3 , and ρ2 = 6.80 × 103 kg/m3 [16, 17], the sound velocity v1 = 4400 m/s , and v2 = 2900 m/s , as well as the index of refraction n1 = 1.5 , and n2 = 6.0 , at the wavelength of 840 nm. The ratio of sound impedance, K = ρ1v1/ ρ2v2, between layer 1 and layer 2, can be calculated as K = 0.4976. The effective sound velocity in the periodic structure can be calculated by the formula , v = (d1 + d2)/[(d1/v1)2 + (d2/v2)2 + (K2 + 1)/K*d1d2 /(v1v2)]1/2 = 3301 m/s. The effective index of refraction in the periodic structure can be computed by = 4.373. The wave vector of CAP probed in backscattering geometry (reflective geometry) should be equal to q = 4πn/λ = 6.539 × 107 m−1. The angular frequency of FAPs can be calculated by ,
Where the m denotes the folded index. For the given folded index m = 0, −1 and + 1, one can obtained f0 = ω0/2π = 0.034 THz, f-1 = ω-1 /2π = 0.296 THz, and f+1 = ω+1/ 2π = 0.364 THz by Eq. (1). Evidently, these calculated frequency values (0.034, 0.296, 0.364) agree well with the experimental values (0.035, 0.261, 0.335) in Fig. 6(b) with considerations of uncertain errors of multiple parameters used in the calculations. Therefore, we can assign the three frequencies of 0.035, 0.261, and 0.335 THz to 0-, −1-, and + 1-order FAP modes of the periodic multilayer films, respectively. As for the peak at ~0.133 THz, it may be one of satellite lines originating from the standing acoustic phonon waves in finite-size superlattice . Actually, one can indeed see fine structures or satellite peaks between m = 0 and m = −1 peaks. The observation of FAPs shows good periodic structure of the multilayer films.
The laser-induced crystallization and structure of the multilayer films have been studied by CPS. COP and CAP are observed, and mainly reflect local microstructure and periodicity of the multilayer structure, respectively. Laser irradiation power dependence of COP reveals laser-induced phase change. The similarity between COP spectra of the films phase-changed by laser irradiation and annealing reveals that laser irradiation indeed leads to the crystallization of amorphous film. The observation of FAPs shows good periodicity of the multilayer films. In addition, a new characteristic phonon mode at 3.90 THz of 5 nm thick amorphous Sb80Te20 films is observed and identified by CPS and Raman, respectively.
This work is partially supported by National Natural Science Foundation of China under grant Nos. 10874247, 61078027, National Basic Research and High Technology Development Programs of China under grant Nos. 2010CB923200, 2008AA031402, and Natural Science Foundation of Guangdong Province under grant No. 9151027501000077 as well as doctoral specialized fund of MOE of China under grant No. 20090171110005.
References and links
1. 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]
2. 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 Appl. Mater. Sci. 205(7), 1636–1640 (2008). [CrossRef]
3. M. L. Lee, L. P. Shi, Y. T. Tian, C. L. Gan, and X. S. Miao, “Crystallization behavior of Sb70Te30 and Ag3In5Sb60Te32 chalcogenide materials for optical media applications,” Phys. Status Solidi., A Appl. Mater. Sci. 205(2), 340–344 (2008). [CrossRef]
4. R. E. Simpson, D. W. Hewak, P. Fons, J. Tominaga, S. Guerin, and B. E. Hayden, “Reduction in crystallization time of Sb:Te films through addition og Bi,” Appl. Phys. Lett. 92(14), 141921 (2008). [CrossRef]
5. C. Wang, J. Zhai, Z. Song, F. Shang, and X. Yao, “Phase-change behavior in Si/Sb80Te20 nanocomposite multilayer films,” Appl. Phys., A Mater. Sci. Process. 103(1), 193–198 (2011). [CrossRef]
6. 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 (2000). [CrossRef]
7. K. Kato, K. Oguri, A. Ishizawa, K. Tateno, T. Tawara, H. Gotoh, M. Kitajima, H. Nakano, and T. Sogawa, “Doping-type dependence of phonon dephasing dynamics in Si,” Appl. Phys. Lett. 98(14), 141904 (2011). [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. J. B. Renucci, W. Richter, M. Cardona, and E. Schönherr, “Resonance Raman scattering in group Vb semimetals: As, Sb, and Bi,” Phys. Status Solidi, B Basic Res. 60(1), 299–308 (1973). [CrossRef]
12. J. Hernandez-Rueda, A. Savoia, W. Gawelda, J. Solis, B. Mansart, D. Boschetto, and J. Siegel, “Coherent optical phonons in different phases of Ge2Sb2Te5 upon strong laser excitation,” Appl. Phys. Lett. 98(25), 251906 (2011). [CrossRef]
13. A. Bartels, T. Dekorsy, H. Kurz, and K. Köhler, “Coherent zone-folded longitudinal acoustic phonons in semiconductor superlattices: excitation and detection,” Phys. Rev. Lett. 82(5), 1044–1047 (1999). [CrossRef]
14. C. Colvard, R. Merlin, M. V. Klein, and A. C. Gossard, “Observation of folded acoustic phonons in semiconductor superlattice,” Phys. Rev. Lett. 45(4), 298–301 (1980). [CrossRef]
15. P. X. Zhang, D. J. Lockwood, and J.-M. Baribeau, “Acoustic phonon peak splitting and satellite lines in Raman spectra of semiconductor superlattices,” Appl. Phys. Lett. 62(3), 267–269 (1993). [CrossRef]
16. N. Shimidzu, T. Nagatsuka, Y. Magara, N. Ishii, N. Kinoshita, and K. Sato, “Dynamic observation study of crystallization process in Sb-based phase-change materials,” Jpn. J. Appl. Phys. 46(16), L385–L387 (2007). [CrossRef]
17. K. J. Singh, R. Satoh, and Y. Tsuchiya, “Structure changes and compound forming effects in the molten Sb-Te system investigated by molar volume and sound velocity measurements,” J. Phys. Soc. Jpn. 72(10), 2546–2550 (2003). [CrossRef]
18. L. R. Testardi and J. J. Hauser, “Sound velocity in amorphous Ge and Si,” Sol. Phys. Comm. 21(11), 1039–1041 (1977). [CrossRef]
19. Y. G. Wang, C. Leibig, X. F. Xu, and R. Venkatasubramanian, “Acoustic phonon scattering in Bi2Te3/Sb2Te3 superlattices,” Appl. Phys. Lett. 97(8), 083103 (2010). [CrossRef]
20. S. K. Kim, Y. S. Kim, M. A. Kang, J. M. Sohn, and K. No, “Optical properties of a-Si films for 157 nm lithography,” Proc. SPIE 5130, 127–135 (2003). [CrossRef]
21. Y.-C. Her, H. Chen, and Y.-S. Hsu, “Effects of Ag and In addition on the optical properties and crystallization kinetics of eutectic Sb70Te30 phase-change recording film,” J. Appl. Phys. 93(12), 10097–10103 (2003). [CrossRef]