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Crystallization Dynamics of As-Deposited Amorphous Aginsbte Thin Film Induced by Picosecond Laser Pulses

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Crystallization Dynamics of As-Deposited Amorphous Aginsbte Thin Film Induced by Picosecond Laser Pulses Home Search Collections Journals About Contact us My IOPscience Crystallization dynamics of as-deposited amorphous AgInSbTe thin film induced by picosecond laser pulses This content has been downloaded from IOPscience. Please scroll down to see the full text. 2010 J. Phys. D: Appl. Phys. 43 175401 (http://iopscience.iop.org/0022-3727/43/17/175401) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 130.237.29.138 This content was downloaded on 06/09/2015 at 03:40 Please note that terms and conditions apply. IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 43 (2010) 175401 (6pp) doi:10.1088/0022-3727/43/17/175401 Crystallization dynamics of as-deposited amorphous AgInSbTe thin film induced by picosecond laser pulses Huan Huang1, Fangyuan Zuo2, Fengxiao Zhai1, Yang Wang1,3, Tianshu Lai2,3, Yiqun Wu1 and Fuxi Gan1 1 Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China 2 State Key Laboratory of Optoelectronic Materials and Technology, Department of Physics, Sun Yet-Sen University, Guangzhou 510275, People’s Republic of China E-mail: [email protected] and [email protected] Received 21 January 2010, in final form 9 March 2010 Published 15 April 2010 Online at stacks.iop.org/JPhysD/43/175401 Abstract The time-resolved crystallization dynamics of as-deposited amorphous AgInSbTe thin films induced by single picosecond laser pulses has been studied. The crystallization process was shown to be a threshold-dependent multi-stage process. For the same film structure, the total crystallization time does not change significantly with different fluences in a broad fluence range. The total crystallization time can be effectively shortened by an additional thermally conductive silver underlayer. After the film has been primed with a low-fluence single ∼30 ps laser pulse, the crystallization process can be simplified to be a monotonic process with a markedly reduced crystallization time. (Some figures in this article are in colour only in the electronic version) 1. Introduction of the region (edge-growth crystallization), which is known as an erasing process in a rewritable disk system. The growth The unconventional properties of phase-change materials, rate is larger than the nucleation rate to cause a growth- i.e. phase changing with pronounced change of optical and dominated crystallization. Although the crystalline structure electrical properties in a very short timescale, make them [10, 13] and crystallization mechanism [12, 14] of AIST have useful in optical data storage and they become promising been reported, there has hardly been any report about the candidates for non-volatile electronic memory applications time-resolved crystallization process of amorphous AIST, [1–3]. Pseudo-binary (GeTe)x (Sb2Te3)y (GST) alloys in particular when induced by ultrashort laser pulses (such [1, 4, 5] and quaternary AgInSbTe (AIST) alloys [6–10], as picosecond (ps) and femtosecond (fs) [15] laser pulses). with best performances in terms of speed and stability, were No detailed experimental data about excitation-condition- widely studied. However, despite the increasing technological dependent crystallization dynamics have been reported to applications, many fundamental problems of classic phase- our knowledge. Because of the existence of a minimum change materials remain poorly understood [11]. time required for crystalline nucleation to form and grow As known, different from the nucleation-dominated [16], the crystallization process is obviously not favoured GST phase-change material, AIST is classified to be a by using ultrashort laser pulses [17]. The ultrashort laser growth-dominated phase-change material [7, 12]. When laser pulse driven crystallization for a phase-change material is of pulses irradiate an amorphous area, crystallization of growth- considerable scientific interest. On the other hand, control dominated phase-change material is characterized by crystal of the crystallization process under extra non-equilibrium growth proceeding from the crystalline rim towards the centre conditions is also essential for designing fast phase-change 3 Authors to whom any correspondence should be addressed. materials for practical applications. 0022-3727/10/175401+06$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA J. Phys. D: Appl. Phys. 43 (2010) 175401 H Huang et al Most typical growth-dominated phase-change materials 3. Results and discussion are Sb-based alloys. Morilla et al first observed the crystallization process using real-time reflectivity The time-resolved reflectivity measurements can provide measurements with nanosecond (ns) resolution time in growth- an indirect observation of the structural evolution process dominated Ge0.07Sb0.93 phase-change thin films by irradiation for phase-change materials. Figures 1(a) and (b) show with ps laser pulses of different fluences [18]. Siegel et al found the normalized reflectivity change (NRC) transients of the recalescence phenomenon after solidification in Ge0.07Sb0.93 AgInSbTe (100 nm)/glass substrate film structure and the thin films upon ps laser pulses irradiation [19]. In the AgInSbTe(100 nm)/Ag(100 nm)/glass substrate film structure strict sense, AIST is a SbTe-based alloy rather than a Sb- irradiated by ps laser pulses with different fluences, based alloy. The contents of Sb in AIST alloys are often respectively. between 50 and 75 at%. Ag and In are bonded to Te to From figure 1, we can observe similar NRC transients form a distorted local structure around Sb and Te. The as a multi-stage process for the same film structure irradiated − existence of Ag and In contributes oppositely, i.e. Ag to the by a ps laser pulse with different fluences in 17–34 mJ cm 2 thermal stability of amorphous marks and In to the high-speed range. The final stable reflectivity will drop when the fluence phase change [10]. Different from the thermal annealing of irradiation pulse is lower. The reflectivity transient with a based crystallization kinetics [20, 21], a special laser pulse, multi-stage process will disappear when the fluence is below −2 in particular an ultrashort laser pulse, driven crystallization about 15 mJ cm , while ablation on the sample will begin to −2 dynamics of AIST should be expected. occur when the laser fluence is above about 35 mJ cm . In this paper, crystallization dynamics of as-deposited The initial low and final high reflectivity in this multi- amorphous AgInSbTe thin films induced by single ps laser stage process is confirmed to correspond to the as-deposited pulses with different fluences was studied using time-resolved amorphous state and crystallized state, respectively. Figure 2 reflectivity measurements with ns resolution. The transient shows the micro-area XRD pattern of the two sample structures phase-change process during crystallization is discussed. before and after ps laser pulse irradiation. Two approaches for decreasing the total crystallization time As shown in figure 2, Sb, In3Te4 and AgSbTe2 crystalline are demonstrated and different mechanisms are analysed phases seem to be coexisting in the ps laser pulse crystallized qualitatively. films, which is basically consistent with that of thermally annealed AgInSbTe films with a similar chemical composition [13]. The broadening of diffraction peaks may be due to 2. Experimental the overlap of the peaks of different crystalline phases at slightly different positions. It should be pointed out that AgInSbTe single-layer thin films and AgInSbTe/Ag bi-layer for the AgInSbTe/Ag bi-layer film, only a silver crystalline thin films were prepared, respectively, on glass substrates by phase appeared in the as-deposited sample while AgInSbTe direct current magnetron sputtering using a AgInSbTe alloy was amorphous. target and a pure Ag target (99.999% in purity). The base For the AgInSbTe single-layer sample, the initial low pressure in the deposition chamber is typically about 1.5 × reflectivity (zone I in figure 1(a)) rises sharply to a relatively − 10 4 Pa. Sputtering is performed in an Ar atmosphere of about higher level within 2–3 ns (even maybe within a time smaller 0.6 Pa with an optimized power. The chemical composition of than the resolution of our experiment) after a ∼30 ps laser AgInSbTe films was confirmed to be Ag 2.4 at%, In 9.2 at%, pulse irradiation, and then decreases gradually (within 147 ns) Sb 60.5 at% and Te 27.9 at% by EDS measurement (equipped to a relatively lower reflectivity (zone II in figure 1(a)), with SEM, JSM-6360LA). The thicknesses of the AgInSbTe which is a little higher than the initial level. After that, the layer and the Ag layer are both 100 nm. A pump-probe system reflectivity gradually increases to the final stable level within was established for real-time reflectivity measurements. The 60 ns (zone III in figure 1(a)). The total crystallization time light source for pumping was a ps frequency-doubled mode- is about 207 ns. In fact, similar NRC transients have been locked neodymium yttrium aluminium garnet (Nd : YAG) laser widely observed in Sb-based GeSb series thin films upon with a pulse duration of ∼30 ps and a wavelength of 532 nm ps or ns laser pulse irradiation [18, 19, 22–25]. The most (EKSPLA PL2143B). The pulse energy stability is better than important characteristic is the existence of a transient minimum 5% at 532 nm. The laser beam was focused by a convex lens on as shown in the interface of zones II and III in figure 1. This the surface of the sample with a spot diameter of about 1.5 mm. special process is thought to have originated from the melting- A continuous wave probe light beam with a wavelength of involved recalescence phenomena, which have been reported 632.8 nm and a power of about 1 mW from a He–Ne laser to take place in Ge and GeSb films [19, 26].
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