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Femtosecond laser-irradiation-induced phase change of a new amorphous Si2Sb2Te3 film with a good thermal stability and low reset current is studied by coherent phonon spectroscopy. New coherent optical phonons (COP) occur as laser irradiation fluence reaches some threshold, implying laser-induced phase change emerged. The compositions in phase-changed area revealed by COP modes agree well with ones in reported annealed crystallized film, implying laser-induced phase change as crystallization. Pump fluence dependence of COP dynamics reveals good crystallization quality of the phase-changed film, exhibiting promising application of Si2Sb2Te3 films in optical phase change memory. Acoustic phonons are also found and identified.
©2013 Optical Society of America
1. Introduction
The exploration of new phase-change materials is very important for the development of next generation of phase-change storage devices. Amorphous Si-Sb-Te films have been studied intensively in recent years as a new kind of electrically driven phase-change material [1–6]. It was found that Si-Sb-Te film had high thermal stability and crystallization temperature over 180 °C [1]. It was also demonstrated that Si18Sb52Te30-based phase-change memory cell had a short crystallization time of 40 ns and the capability of multilevel storage [1], while Si3Sb2Te3-based phase-change memory cell even showed a shorter crystallization time of 20 ns [4] and a low reset current of ~1.3 mA [6]. The composition dependence of crystallization behaviors of amorphous Si2Sb2Tex films showed that Si2Sb2Te3 has the best phase stability and data retention as well as a high crystallization speed [5]. Those results suggest that Si2Sb2Te3 or Si2Sb2Te3-like amorphous films are a new excellent phase-change material with promising applications in electrically driven phase-change memory devices. However, unfortunately we still have known nothing on their light-driven phase change because no researches on optical phase change were reported yet. Are those materials suitable for optical phase change storage? It is well known that optical phase-change memory is as important as electrical phase-change memory. Therefore, the exploration of optical phase-change behaviors of Si-Sb-Te films is surely important and necessary for the development of next generation of optical phase-change memory devices.
In this article, femtosecond laser-irradiated crystallization behaviors of Si2Sb2Te3 amorphous film are studied to evaluate its potential applications in optical phase-change memory. Coherent phonon spectroscopy is used in situ to characterize the crystallization behaviors due to its high sensitivity to microstructure change [7–9]. It is found that laser-induced crystallization can be taken place when laser-irradiation fluence exceeds some threshold, which corresponds to the appearance of new coherent optical phonon modes. The crystalline quality of laser-irradiated crystallized Si2Sb2Te3 film is also checked by coherent phonon dynamics and found good, implying the promising applications of Si2Sb2Te3 film in optical phase-change memory.
2. Sample and experiment
Si2Sb2Te3 alloy film under study is about 11 nm thick and grown on a glass substrate by co-sputtering Si, Sb and Te targets at room temperature to ensure as-deposited films in an amorphous phase, showing a high crystallization temperature of ~540 K [5]. The details on the conditions and procedures of the film preparation were described elsewhere [5].
Time-resolved pump-probe differential transmission spectroscopy is used to study the coherent phonon dynamics of the sample after which is irradiated by different laser fluence, monitoring crystallization process. A train of 60 femtosecond laser pulses is from a self-mode-locked Ti: sapphire laser oscillator with the central wavelength of 830 nm and a pulse repetition rate of 94 MHz, and directed into a standard pump-probe setup. The emerging two parallel beams, a strong pump and a weak probe with a pump-to-probe intensity ratio of >15 are focused to a same area on the surface of the sample located at the back focal plane of a convex lens of 31 mm focal length. The transient transmission change of the probe 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.
3. Laser-induced crystallization and its in situ characterization by coherent phonon spectroscopy
In our experiment, the irradiation laser is the same as pump laser. Its power is tuned by a neutral-density attenuator. Pump laser is first increased up to a higher fluence and irradiates amorphous Si2Sb2Te3 film for few tens of seconds to induce phase change. Then, pump fluence is decreased down to a lower level of 0.08 mJ/cm2 which does not lead to phase change. The irradiated area is measured in situ by transient transmission change under the lower pump fluence of 0.08 mJ/cm2. Such measurement is repeated on a fresh spot after which is irradiated by a new higher irradiation fluence. All measurements are performed at room temperature and under a same low pump level of 0.08 mJ/cm2, and all energy fluence mentioned in this paper means peak value. All transient differential transmission traces are plotted in Fig. 1 for increasing laser irradiation fluence (LIF) from 0.08 to 0.59 mJ/cm2. It is obvious that the two transient traces for LIF below 0.27 mJ/cm2 only reflect normal carrierdynamics. However, the traces are changed markedly when LIF reaches 0.27 mJ/cm2 and higher. An oscillatory component occurs and is superimposed on a normal carrier dynamic profile, which is just so-called coherent optical phonon spectroscopy [7–9]. The oscillatory amplitude is enhanced with increasing LIF, implying the increase of crystallized area in the probe spot with LIF [10]. The emergence of coherent optical phonons (COP) shows that larger LIF results in microstructure changes (phase change) of the sample because all measurements are made under a same lower pump fluence of 0.08 mJ/cm2, while such a lower pump fluence itself does not lead to any phase change, as shown by the bottom transient profile in Fig. 1.
Fig. 1 Transient differential transmission traces measured under a same low pump fluence of 0.08 mJ/cm2 for different laser irradiation fluence indicated in figure. The traces are shifted upward for clarity. The “amorphous” is equivalent to a laser irradiation fluence of 0.08 mJ/cm2.
It is necessary to analyze the oscillatory components in Fig. 1 to further understand laser-irradiation-induced phase change. The oscillatory and non-oscillatory components are separated by digital low-pass filter. The separated oscillating components are plotted in Fig. 2(a). Fast Fourier transformed (FFT) spectra corresponding to Fig. 2(a) are plotted in Fig. 2(b). It is obviously seen from FFT spectra in Fig. 2(b) that a COP mode first occurs at 3.67 THz as LIF reaches 0.27 mJ/cm2. Then the mode is enhanced with increasing LIF. Meanwhile, another COP mode appears at 2.0 THz as LIF is increased up to 0.59 mJ/cm2. The appearance of those COP modes suggests the generation of new ordered microstructures, that is so-called phase change. The COP mode at 3.67 THz agrees very well with A1g optical phonon mode of crystalline Te at room temperature [9]. The COP mode at 2.0 THz accords well with A1g mode of crystalline Sb2Te3 [9, 11]. Therefore, laser-irradiation-induced phase change leads to the emergence of new crystalline phases, Sb2Te3 and Te, which agrees very well with heating crystallization of amorphous Si2Sb2Tex films [5], where X-ray diffraction, electron diffraction and transmission electron microscopy reveal that annealed Si2Sb2Te6 consisted of amorpghous Si, crystalline Sb2Te3 and Te. The accordance between the results of the heating and laser irradiation shows that femtosecond laser irradiation indeed leads to the crystallization of amorphous Si2Sb2Te3 films. Therefore, we are sure that Si2Sb2Te3 alloy film is suitable for optical phase-change storage.
Fig. 2 (a) Transient oscillation of COP separated from Fig. 1 for corresponding laser irradiation power; (b) FFT spectra corresponding to (a). Each trace is shifted upward for clarity.
To understand crystalline quality of laser irradiation crystallized Si2Sb2Te3 alloy film, we study the pump fluence dependence of COP dynamics. Amorphous Si2Sb2Te3 alloy film is first irradiated for few tens of seconds by a high LIF of 0.59 mJ/cm2 to obtain crystalline Si2Sb2Te3 film, as shown before. Then, transient differential transmission is measured on the irradiated spot for different pump fluence but identical low probe fluence. All transient differential transmission traces are plotted in Fig. 3. One can see obvious oscillation even for lower pump fluence of 0.08 mJ/cm2, which implies the irradiated spot is indeed crystallized because such a low pump fluence itself can lead to crystallization, as shown in Fig. 1. The oscillatory amplitude becomes larger with increasing pump fluence, but lasting time of the oscillation seems to become shorter with increasing pump fluence. Those features are just the characteristics of COP in single crystal [11, 12]. To characterize the dynamics of COP quantitatively, the oscillatory component is separated out as described before and plotted in Fig. 4(a). One can see they are almost harmonic damped oscillation because the strength of the mode at 2.0 THz is much weaker than one of the mode at 3.67 THz, as shown in Fig. 2(b).
Fig. 3 Transient differential transmission traces of laser-irradiation crystallized Si2Sb2Te3 film for different laser pump fluence. All traces are shifted upward for clarity.
Fig. 4 (a) Transient oscillation of COP separated from Fig. 3. The traces are shifted upward for clarity. (b) The lifetime of COP as a function of pump fluence. (c) The frequency of COP vs. pump fluence. The solid line in (b) and (c) is the guide for the eye.
Therefore, a harmonic damped decay function is used best to fit those transient traces in Fig. 4(a). The frequency and lifetime of COP are obtained as a function of pump fluence. They are plotted in Fig. 4(b) and 4(c). One can see that both are almost decreased linearly with increasing pump fluence in the low fluence regime below 0.4 mJ/cm2, which agree well with the behaviors of COP in crystal [11–14]. The reduction in the lifetime is explained by the enhancement of incoherent phonon emission with increasing pump fluence [11, 12], while the decrease in the frequency is interpreted as the softening of crystal bonds [14]. The consistency or similarity between our pump fluence dependence of COP dynamics and one reported in single crystals implies high crystalline degree of laser-irradiated Si2Sb2Te3 film. As for the nonlinear effects or almost keeping constant, of the lifetime and frequency of COP with increasing pump fluence in the regime of > 0.4 mJ/cm2, they originate from the saturation effect of pump excitation. One can see clearly from Fig. 3 that the three transient traces are almost identical in either total reflectivity change or oscillatory amplitude for the three pump fluence of 0.4, 0.48 and 0.56 mJ/cm2, which implies the actual excitation of the sample is no longer enhanced with increasing pump fluence. Similar saturation phenomenon was also observed in single crystal of Bi [14].
4. Identification of acoustic phonons in transient transmission spectroscopy
It is easy to see some low frequency oscillations occur in transient transmission traces in Figs. 1 and 3 besides the high frequency oscillations analyzed before. To understand their origin, we record long delay scanning traces for different laser irradiation fluence but an identical low pump fluence of 0.08 mJ/cm2. All transient traces are plotted in Fig. 5(a). One can see from them that the low frequency oscillation occurs in all traces, which implies that it does not closely relate to the degree of crystallization of Si2Sb2Te3 film because different LIF corresponds to different the degree of crystallization. It shows that the low frequency oscillation should not originate from optical phonons which closely depend on the degree of crystallization, as shown in Figs. 1 and 2. As a result, the low frequency oscillation should befrom acoustic phonons. To identify the acoustic phonons, it is necessary to analyze the transient traces quantitatively. The oscillatory components are separated out as described before and plotted in Fig. 5(b). Their FFT spectra are plotted in Fig. 5(c). One can see the low frequency oscillation contains two obvious frequency components at 0.12 and 0.236 THz. It was reported the stress-induced acoustic wave traveling in glass substrate had a frequency of 42 GHz [15], much lower than 0.12 THz. Furthermore, acoustic traveling wave usually has only one frequency in one medium. These differences allow us to exclude the acoustic traveling wave origin of the low frequency oscillation. On the other hand, resonant acoustic phonons or acoustic standing wave may be generated in ultrathin film and have been reported in Ref. 14. It has the following frequency [15],
where d and v are the thickness and acoustic velocity of the film, respectively, m is a index number of resonant phonon modes and an integer. If the acoustic velocity of crystalline Sb2Te3 film, v = 2900 m/s [16], is used to estimate the frequency of resonant phonon modes due to the lack of acoustic velocity of Si2Sb2Te3 film, we can get f1 = 0.132 THz and f2 = 0.264 THz, which agree well with the experimental values of 0.12 and 0.236 THz in Fig. 5(c), respectively with the consideration of the uncertainty of the acoustic velocity and film thickness. Therefore, the acoustic phonons are identified as resonant acoustic phonons within the film, instead of acoustic wave traveling in the substrate.Fig. 5 (a) Transient transmissivity traces for different laser irradiation power but the same pump power of 15 mW; (b) Transient oscillation of COP separated from (a); (c) FFT spectra corresponding to (b). Each trace is shifted upward for clarity.
5. Conclusion
Femtosecond laser-induced phase-change behaviors of amorphous Si2Sb2Te3 alloy film have been investigated by coherent phonon spectroscopy. It is found the film can be crystallized well by femtosecond laser pulses. It is shown that the new film is also a new-type optical phase-change memory material besides its good electrically-driven phase-change performance. It has potential applications in both optical and electrical phase-change memory devices.
Acknowledgments
This work is partially supported by National Natural Science Foundation of China under grant Nos. 11274399, 61078027, 61076121 and 61006087, and National Basic Research under grant Nos. 2010CB923200, 2013CB922403, 2010CB934300, and 2011CB932800, as well as doctoral specialized fund of MOE of China under grant No. 20090171110005.
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