**Time Resolved Investigation of Fast Phase-Change Phenomena in Rewritable Optical Recording Media**

Shigeru Kimura1, Yoshihito Tanaka1,2, Shinji Kohara1 and Masaki Takata1,2,3 *1Research & Utilization Division, Japan Synchrotron Radiation Research Institute 2RIKEN SPring-8 Center, RIKEN Harima Institute 3School of Frontier Science, The University of Tokyo Japan* 

#### **1. Introduction**

The worldwide hunger for secure and smarter electronic data storage grows daily to satisfy today's extremely demanding business and entertainment requirements, and this drives the need to pack data into an increasingly smaller volume at ever faster speeds. Digital versatile disks (DVDs) are one of the most convenient storage devices for large amount of information such as video and digital graphs. One of the keys to develop the DVDs was to create the fast phase-change material, which gives rise to faster bi-directional phase change between crystal phase and amorphous phase by laser heating within the recording mark. Another key was to minimize the size of the recording mark for high density data storage. The DVDs have already been a mass product as one of the indispensable home electronic appliances, even though the mechanism of fast phase change has been the remained problem to be solved. Then, to develop the next generation higher density and faster optical data storage system, the atomic and electron level study was required in the last decades. This chapter will describe the challenge of the atomic level research for the fast phasechange mechanism of the DVD materials via X-ray pinpoint structural measurement, which is the state-of-the-art synchrotron radiation (SR) materials-analysis techniques developed at SPring-8. The X-ray pinpoint structural measurement and it's data analysis technique has been further upgraded by combining with theoretical structure modelling and became a tool to investigate how laser pulses alter the atomic structure of a class of materials useful for data storage and uncover a mechanism that could help the development of even faster information storage in the future.

So far, the great efforts and developments have been paid for the progress of the DVD materials (Wutting & Yamada, 2007). The development of new phase-change materials, leading to the discovery of GeTe-Sb2Te3 (Yamada et al., 1987) and of Ag11In11Sb55Te23 (Iwasaki et al., 1992), has allowed us to produce rewritable compact discs, DVDs, and Bluray discs. In the commercial rewritable DVDs such as DVD-random access memory (RAM) and DVD-rewriteable (RW), information is written to the phase-change materials composed of the alloys of Ge, Sb and Te, or those of Ag, In, Sb and Te by changing their phase locally between amorphous and crystalline states using a sub-micrometer sized laser irradiation.

Time Resolved Investigation

**recording process** 

change materials.

**2.1 Laser-pump and X-ray SR probe system** 

of Fast Phase-Change Phenomena in Rewritable Optical Recording Media 261

extended X-ray absorption fine structure (EXAFS), hard X-ray photoelectron spectroscopy

**2. X-ray pinpoint structural measurement system for investigation of optical** 

The pulse characteristic and high coherent X-ray beam of SPring-8 allow us to investigate dynamics of chemical reactions and phase transition of materials caused by applied field. In order to realize the direct investigation, we developed the X-ray pinpoint structural measurement system at BL40XU of SPring-8, which enables the advanced X-ray measurement technique in nanometer spatial scale and/or picosecond time scale. This system was used not only for investigation of optical recording process but also for charge density level visualization of photo-induced phase transition (Fukuyama et al., 2010) and

In order to investigate the amorphous-crystal phase change on recording DVD media we optimized the X-ray pinpoint structural measurement system. Figure 2 shows the schematic illustration of experimental setup for taking a snapshot of X-ray diffraction profile of optical phase-change materials. The main optimized components are laser-SR timing control system, a sample rotation system, an optical reflectivity probe system, and an X-ray diffractometer. The following sub-sections describe the components and their performance.

Fig. 2. Experimental setup for taking a snapshot of X-ray diffraction profile of optical phase-

A pulsed nature of synchrotron X-ray beam due to the acceleration mechanism of charged particles has enabled us to conduct the stroboscopic X-ray measurement in picosecond time-

sub-micron single crystal structure analysis (Yasuda et al., 2008; Yasuda et al., 2010).

(HXPS) measurements and computer simulations (Matsunaga et al., 2011).

The recording and erasing are performed by controlling the laser intensity for irradiation, yielding a difference in the transient heating and cooling rates of the materials. A higher cooling rate from a temperature above the melting point generates the amorphous phase; a lower rate produces the crystalline phase from the amorphous phase. Because the optical reflectivity of the two states is different, this information can be easily read from the disk by measuring the reflectance with a low power laser. Recently developed DVD materials can complete the phase change with a 20 ns laser irradiation (Fig. 1).

Fig. 1. Schematic illustration of recording and erasing process in rapid phase-change materials.

In contrast to the reliability and high-speed-performance of DVD media, the mechanism of rapid phase change, especially crystallization from the amorphous phase is still not fully understood, in spite of a lot of investigation using transmission electron microscopy (Park et al., 1999; Naito et al., 2004), fluctuation electron microscopy (Kwon et al., 2007), optical (Wei & Gan, 2003), electronic (Lee et al., 2005), and structural (Yamada & Matsunaga, 2000; Kolobov et al., 2004; Kohara et al., 2006) studies. We therefore developed the X-ray pinpoint structural measurement system (Kimura et al., 2006), which enables the 40 picosecond time resolved pump and probe X-ray diffraction experiment using 100 nm scale SR beam. Using the system, we achieved the real-time *in situ* structural observation of the amorphous-crystal phase change process in nanosecond time-scale (Fukuyama et al., 2008a). The obtained results using the time-resolved X-ray diffraction apparatus coupled with *in situ* optical reflectivity monitor showed the strong correlation between the crystal growth and optical reflectivity, and the difference in the crystal growth process between Ge2Sb2Te5 (GST) and Ag3.5In3.8Sb75.0Te17.7 (AIST).

In the following section, we describe the detail of the X-ray pinpoint structural measurement system for investigation of optical recording process and its performance (Tanaka et al., 2009). Moreover, we describe the recent progress for fully understanding the atomic structure of AIST and compare it to GST through the research with X-ray diffraction,

The recording and erasing are performed by controlling the laser intensity for irradiation, yielding a difference in the transient heating and cooling rates of the materials. A higher cooling rate from a temperature above the melting point generates the amorphous phase; a lower rate produces the crystalline phase from the amorphous phase. Because the optical reflectivity of the two states is different, this information can be easily read from the disk by measuring the reflectance with a low power laser. Recently developed DVD materials can

Fig. 1. Schematic illustration of recording and erasing process in rapid phase-change

In contrast to the reliability and high-speed-performance of DVD media, the mechanism of rapid phase change, especially crystallization from the amorphous phase is still not fully understood, in spite of a lot of investigation using transmission electron microscopy (Park et al., 1999; Naito et al., 2004), fluctuation electron microscopy (Kwon et al., 2007), optical (Wei & Gan, 2003), electronic (Lee et al., 2005), and structural (Yamada & Matsunaga, 2000; Kolobov et al., 2004; Kohara et al., 2006) studies. We therefore developed the X-ray pinpoint structural measurement system (Kimura et al., 2006), which enables the 40 picosecond time resolved pump and probe X-ray diffraction experiment using 100 nm scale SR beam. Using the system, we achieved the real-time *in situ* structural observation of the amorphous-crystal phase change process in nanosecond time-scale (Fukuyama et al., 2008a). The obtained results using the time-resolved X-ray diffraction apparatus coupled with *in situ* optical reflectivity monitor showed the strong correlation between the crystal growth and optical reflectivity, and the difference in the crystal growth process between Ge2Sb2Te5 (GST) and

In the following section, we describe the detail of the X-ray pinpoint structural measurement system for investigation of optical recording process and its performance (Tanaka et al., 2009). Moreover, we describe the recent progress for fully understanding the atomic structure of AIST and compare it to GST through the research with X-ray diffraction,

materials.

Ag3.5In3.8Sb75.0Te17.7 (AIST).

complete the phase change with a 20 ns laser irradiation (Fig. 1).

extended X-ray absorption fine structure (EXAFS), hard X-ray photoelectron spectroscopy (HXPS) measurements and computer simulations (Matsunaga et al., 2011).
