Optical–Electrical Properties of Aginsbte Phase Change Thin Films

Journal of Non-Crystalline Solids 356 (2010) 889–892

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Journal of Non-Crystalline Solids

journal homepage: www.elsevier.com/locate/jnoncrysol

Optical–electrical properties of AgInSbTe phase change thin ﬁlms under single picosecond laser pulse irradiation

Fengxiao Zhai a, Fangyuan Zuo b, Huan Huang a, Yang Wang a,*, Tianshu Lai b,*, Yiqun Wu a, Fuxi Gan a a Key Laboratory of High Power Lasers Material, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China b State Key Laboratory of Optoelectronic Materials and Technology, Department of Physics, Sun Yat-Sen University, Guangzhou 510275, China article info abstract

Article history: Phase transition triggered by picosecond (ps) single-shot laser pulse in as-deposited AgInSbTe ﬁlms was Received 21 May 2009 achieved with appropriate laser ﬂuence. Structure transition is further conformed by micro-area X-ray Received in revised form 16 December 2009 diffraction measurement. Current–voltage curves of the amorphous and the picosecond laser-crystallized Available online 13 January 2010 area were measured by conductive atomic microscopy. Ultrafast laser pulse recording and electrically reading will be very promising for phase change data storage due to high data transfer rate and high read- Keywords: out signal-to-noise ratio. These results will be helpful to make ultrafast optical–electrical hybrid phase Amorphous semiconductors change data storage feasible. Crystallization Ó 2009 Elsevier B.V. All rights reserved. Laser–matter interactions

1. Introduction pulse with pulse width of 500 ns was used for optical recording

with Ge2Sb2Te5 phase change medium. The structure of phase change material can be switched be- As crystallization in amorphous background is more controlla- tween crystalline and amorphous states by optical or electrical ble and multi-state facile for optical writing in optical–electrical pulses. A focused laser or a current pulse is often employed as joule hybrid device [2,3], fast and huge resistance change during the la- heating source for reversible phase change data storage in optical ser-induced crystallization is the most important requirement for disk or phase change random access memory (PCRAM). The stored optical–electrical hybrid recording media. As well-known, pseu- information can be readout by probing the reflectivity or conduc- do-binary (GeTe)x(Sb2Te3)y (GST) alloys and quaternary AgInSbTe tivity of the amorphous or crystalline phase. In general, the optical (AIST) alloys [4–6], of the best performances in terms of speed reflectivity contrast is commonly about 10–30% between amor- and stability, were widely studied for applications in optical disk phous and crystalline state. However, corresponding contrast in storage and PCRAM. With different crystallization process and electrical resistance can be achieved to about 4–6 orders (not so crystal growth mechanism, the rapid crystallization of the smaller high for practical applications). On the other hand, phase change crystallites can be observed in AIST than GST under the same laser can be driven by ultra-short laser pulse, such as femtosecond or pulse irradiation [7]. In this work, fast crystallization driven by a picosecond laser pulses. However, until now, nanosecond or sub- single picosecond laser pulse was achieved in a proper fluence nanosecond (several hundred picoseconds) electrical pulses [1] range on AIST thin film. Electrical measurements show that the were ready for phase transitions in a phase change memory cell. resistivity contrast can be achieved to about 77 between the ps la- It will be very promising if the speed advantage of optical approach ser pulse crystallized area and the amorphous background. These combine with the signal contrast advantage of electrical approach. results will be helpful to make ultrafast optical–electrical hybrid Besides the potential high data transfer rate, ultra-short laser phase change memory feasible. pulse-induced phase change is also very promising due to enhancement of the cooling rate and suppression of the thermal diffusion effects. Recently, the feasibility of optical–electrical hy- 2. Experimental brid data storage, namely optical recording and electrical reading, was preliminarily demonstrated in our previous work [2]. A laser Ag8In14Sb55Te23 thin films with thickness of 200 nm were deposited on K9 glass substrate by DC magnetron sputtering. An additional 100 nm-thick Ag layer was deposited under the AIST layer as an electrode for electrical measurement samples. The ar- * Corresponding authors. E-mail addresses: [email protected] (F. Zhai), [email protected] (Y. Wang), gon gas flow rate of 80-sccm was used during sputtering, while [email protected] (T. Lai). the background pressure and work pressure were about

Fig. 1. Experimental setup of real time reﬂectivity measurements.

1.5 104 pa and 0.8 pa, respectively. The sputtering power is 3. Results 70 W for Ag and 150 W for AIST. A pump–probe system was employed to observe the phase change process by monitoring the Fig. 2(a–d) shows the optical microscope images of the as- reflectivity change in real time. The schematic setup of the deposited AIST phase change film upon picosecond laser pulse irra- pump–probe system was shown in Fig.1. The pump pulses, with diation with difference fluence. When the laser fluence is not high- duration of 30 ps at wavelength 532 nm, are delivered by a fre- er than 14.95 mJ/cm2, almost no change can be observed on the quency-doubled mode-locked Nd:YAG laser (Ekspla, PL2143B). film. With the increase of the laser fluence, the color of irradiated The probe beam was provided by a cw He–Ne laser (633 nm), part becomes brighter, and a little ablation will begin to occur as which was focused at incidence of about 45° to the center of the the fluence is higher than 27.61 mJ/cm2. The bright area in Fig. 2 pumped area. The detection system was a high-speed silicon ava- indicates the formation of a crystalline area in AIST film on the lanche photodiode connected to a 500 MHz digital oscilloscope as-deposited background using a single picosecond laser pulse with a total time resolution of about 2 ns. After each laser pump- with appropriate laser fluence. This phase transition from amor- ing, the sample should be moved to a fresh region for the next phous state to crystalline state is conformed by micro-area XRD measurement. analysis. Fig. 3 gives the X-ray diffraction patterns for as-deposited After irradiation, the sample surfaces are observed by optical background and irradiated area (corresponding to the bright spots microscopy. Micro-area X-ray diffraction (XRD) (Rigaku D/max- in Fig. 2b). The diffractograms of the non-irradiated area show no 2550) analysis was carried out to characterize the microstructure peaks confirming the amorphous nature of as-deposited thin films. change of the irradiated and non-irradiated areas of the samples. The emergence of diffraction peaks for irradiated area indicates A conductive atomic force microscope (C-AFM) (Veeco, Multi- crystal structure has been formed by single 30 ps laser irradiation. Mode™) was used to determinate the micro-area electrical charac- The dynamics of phase transition were studied by measuring teristic of phase change films. the evolution of the reflectivity in real time. Fig. 4 shows the

Fig. 2. Optical microscope images of the laser-crystallized area on as-deposited AIST ﬁlms with pump ﬂuence: (a) 14.95 mJ/cm2, (b) 20.27 mJ/cm2, (c) 27.61 mJ/cm2, and (d) 33.18 mJ/cm2. F. Zhai et al. / Journal of Non-Crystalline Solids 356 (2010) 889–892 891

80 70 (012) 60 irradiated area 50 irradiated area (110) 40 30 20 10 0 1.2 Intensity(a.u.) 0.9 Current (nA) as-deposited area non-irradiated area 0.6

0.3

0.0 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 2-Theta (degree) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 DC bias (V) Fig. 3. X-ray diffraction pattern for as-deposited and laser irradiated AIST ﬁlm. Fig. 5. I–V curves of as-deposited and picosecond laser pulse irradiated area of the AIST ﬁlms.

Ι ΙΙ a 2 ΙΙΙ 14.95mW/cm flow conditions. Steps I, II and III in curves of Fig. 4 are thought to correspond to melting, solidification after undercooling [9] and recalescence-based crystallization process [10–14], respectively. For the same film structure, the total crystallization time doesn’t change obviously with different fluences in a broad fluence range. 2 b 20.27mW/cm The total crystallization time can be effectively shortened by an additional thermally conductive underlayer or by pretreatment with low-fluence laser pulse. After primed with low-fluence laser pulse, the crystallization process can be simplified to be a mono- 2

Reflectivity (a.u.) c 27.61mW/cm tonic crystal growth process with a markedly reduced crystallization time. The incubation time of recalescence-involved crystallization process is much longer than and that of the priming process, which implies two different embryo seeding mechanisms. 2 The detailed crystallization dynamics, mechanism and acceleration d 33.18mW/cm strategy will be analyzed in a separate paper. -50 0 50 100 150 200 250 300 350 400 In order to analyze the simultaneous change of electric proper- Time (ns) ties between irradiated area (crystalline state) and as-deposited background (amorphous state), a C-AFM was used to scan the cur- Fig. 4. Real time reflectivity transients of as-deposited AIST films upon irradiation rent distribution of the irradiated and as-deposited area. The de- by a single picosecond laser pulse with different laser fluences. tailed description of experimental setup and sample preparation for electrical measurement can be found in Ref. [2]. The Pt/Ir tip of C-AFM and Ag underlayer sever as top and bottom electrode time-resolved reflectivity of as-deposited AIST film under picosec- respectively. Fig. 5 shows current–voltage (I–V) curves of as-depos- ond single laser pulse irradiation with difference laser fluence. As ited and picosecond laser pulse irradiated area of the AIST films. shown in Fig. 4(a), the real time reflectivity keeps roughly constant Corresponding to the high optical contrast as shown in Fig. 2 and with laser fluence of 14.95 mJ/cm2, corresponding to no obvious Fig. 4, huge difference in resistance between the irradiated and change in optical micrograph as shown in Fig. 2(a). The curves of non-irradiated area can be observed in Fig. 5. reflectivity with laser fluence of 20.27, 27.61 and 33.18 mJ/cm2 For data storage applications, the contrast between difference shown in Fig. 4(b–d) are similar. The reflectivity sharply increases phase states in electrical or optical characteristics is more worth- upon irradiation leading to a higher level (step I), which is followed while. So we definite the resistivity contrast as the resistivity ratio by a decrease to the minimum slightly higher than the initial one between amorphous and crystalline states. It can be described by: (step II) and a subsequent increase to a stable value (step III). The stable final reflectivity indicates the formation of the crystal- q q ¼ a ; ð1Þ line states [8] as shown in Fig. 2(b–d). The total crystallization time n q (when reflectivity increases from its low initial level to its high c stable final level) is about 210 ns. where qn is the resistivity contrast, qa and qc denotes resistivity of amorphous state (non-irradiated area) and crystalline state (irradi- 4. Discussion ated area) respectively. The relationship between the resistance and

resistivity can be described by qn = RS/d, where R, S and d denotes As well known, the high quenching rates obtained after ultra- the resistance, surface area and ﬁlm thickness respectively. For short laser pulse-induced melting will make it more favor to re- the tip-contacting mode micro-area resistance measurements, the amorphize. The above mentioned crystallization process can only surface area can be simply treated as the contacting area of the be fulﬁlled under some well-chosen parameters by tailoring of heat tip [2,15]. 892 F. Zhai et al. / Journal of Non-Crystalline Solids 356 (2010) 889–892

The I–V curves in Fig. 5 can be treated approximately as linear measuring the change of electrical resistivity, thus making ultrafast and the resistance can be obtained from the average slope of them. optical–electrical hybrid data storage possible. The resistance of the amorphous background and crystallized dot are carried out to be about 5 108 and 6.5 106 X respectively, Acknowledgements which are near the typical resistance value for amorphous state and partially crystallized state. When the thickness change during This work is partially supported by National Natural Science crystallization process is ignored, the resistivity contrast can be ob- Foundation of China (Nos. 50872139, 10874247, 60644002, tained as about 77. In contrast with the optical contrast of reflectiv- 60678009, 50502036), National Basic Research Program of ity, the electrical resistivity contrast of about two orders in China (2007CB935402) and Chinese Academy of Sciences magnitude is huge enough for high signal-to-noise ratio (SNR) (KJCX2.YW.M06). readout in phase change memory cell. These results will be helpful to make ultrafast optical–electrical hybrid phase change memory References feasible. Recording by ultrafast laser and reading with electrical resistance measurement will be beneficial to improve the date [1] W.J. Wang, L.P. Shi, R. Zhao, et al., Appl. Phys. Lett. 93 (2008) 043121. [2] H.J. Sun, L.S. Hou, Y.Q. Wu, F.X. Zhai, Chin. Phys. Lett. 25 (2008) 2915. transfer rate and readout SNR. [3] E. Mytilineou, S.R. Ovshinsky, B. Pashmakov, et al., J. Non-Cryst. Solids 352 (2006) 1991. [4] M. Wuttig, N. Yamada, Nature Mater. 6 (2007) 824. 5. Conclusions [5] A.V. Kolobov, P. Fons, J. Tominaga, Thin Solid Films 515 (2007) 7534. [6] M. Wuttig, C. Steimer, Appl. Phys. A 87 (2007) 411. Fast phase transition triggered by single picosecond laser pulses [7] N. Fukuyama, N. Yasuda, J. Kim, et al., Appl. Phys. Exp. 1 (2008) 045001. [8] M.C. Morilla, J. Solis, C.N. Afonso, Jpn. J. Appl. Phys. 36 (1997) L1015. in as-deposited AIST films could be achieved in a proper laser flu- [9] S.R. Stiffler, M.O. Thompson, P.S. Peercy, Phys. Rev. B 43 (1991) 9851. ence range. The time-resolved crystallization dynamics shows a [10] J. Siegel, J. Solis, C.N. Afonso, J. Appl. Phys. 80 (1996) 6677. complex multi-step process. Electrical measurements show that [11] J. Siegel, J. Solis, C.N. Afonso, J. Appl. Phys. 82 (1997) 236. [12] J. Siegel, J. Solis, C.N. Afonso, Appl. Phys. Lett. 75 (1999) 1071. the resistivity change between the amorphous background and [13] J. Siegel, C.N. Afonso, J. Solis, Appl. Phys. Lett. 75 (1999) 3102. the crystallized dot can be about 77 times. All these results suggest [14] J. Siegel, A. Schropp, J. Solis, C.N. Afonso, Appl. Phys. Lett. 84 (2004) 2250. that the ultrafast laser-recorded bit can be readout electrically by [15] H.J. Sun, L.S. Hou, Y.Q. Wu, et al., J. Non-Cryst. Solids 354 (2008) 5563.