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
Recently, Lu et al  reported Te-free environmental friendly Ga-Sb-Se material as a new electrically drive phase-change storage medium, and studied its basic properties related to phase-change memory (PCM), such as crystallization temperature and electrical driven switching rate etc . It was found that Ga1Sb6Se3 exhibited a better thermal stability, faster switching speed and lower power consumption with respect to conventional Ge2Sb2Te5 (GST) phase-change storage medium. Corresponding to electrical drive PCM, optical drive PCM is another active research subject in data storage field. There are many researches reported on it [2–5]. Therefore, to study basic properties related to optical drive PCM of new Ga1Sb6Se3 and potential PCM application is also interesting and important. However, no such data are available yet. In this article, femtosecond laser is used to irradiate Ga-Sb-Se films to explore possible laser-induced crystallization which is in situ monitored by highly sensitive coherent phonon spectroscopy [6–8]. It is found that femtosecond laser irradiation is indeed able to lead to crystallization of amorphous Ga-Sb-Se films as laser irradiation power exceeds some threshold. The crystalline quality of laser-induced crystallized Ga-Sb-Se films is also characterized by coherent phonon dynamics and found almost to be comparable to that of heating-crystallized Ga-Sb-Se films, implying the application potential of Ga-Sb-Se films in optical PCM.
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 .
Time-resolved pump-probe photo-reflectivity spectroscopy is used to study the coherent phonon dynamics of the Ga-Sb-Se films. The details on pump-probe experiment system have been described in previous reports . A train of 60 femtosecond laser pulses is from a self-mode-locked Ti: sapphire laser oscillator with the central wavelength of 840 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 sample surface by a convex lens of 50 mm focal length. 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 so that transient reflectivity change is measured. In our experimental geometry, a pump power of 10 mW corresponds to a pump fluence of 0.02 mJ/cm2.
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) 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) [6–8]. 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  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) . 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 [10,11], 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 .
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) 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 , 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  to 4.50 THz at room temperature [10,11]. Similar temperature dependence was also observed in GeTe/Sb2Te3 superlattices . 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 . Similar dependence of lifetime or rate (1/τ) on excitation power was also reported in crystalline Bi2Te3, Sb2Te3 and their superlattices . The pump power dependence of ν and τ of COP agrees very well with behaviors in single crystals [9,12], 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 .
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) 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 [14–16].
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.
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.
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.
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