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Laser pump and synchrotron radiation probe microdiffraction forinvestigation of optical recording process
Shigeru Kimura1, Nobuhiro Yasuda1, Yoshimitsu Fukuyama1, Kiminori Ito2, Yoshihito Tanaka2, Toshiyuki
Matsunaga3, Rie Kojima4, Kazuya Hisada4, Takashi Mihara4, Akio Tsuchino4, Norihito Fujinoki3,
Masahiro Birukawa5, Masaki Takata1,2 and Noboru Yamada6
1JASRI, 1-1-1 Kouto, Sayo-gun, Sayo-cho, Hyogo 679-5198, Japan, kimuras@spring8.or.jp,
RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-gun, Sayo-cho, Hyogo 679-5148, Japan, 3 R&D Division, Panasonic
Corporation, 3-1-1 Yagumo-naka-machi, Moriguchi, Osaka 570-8501, Japan, 4 AVC Networks Company, Panasonic
Corporation, 1-15 Matsuo, Kadoma, Osaka 571-8504, Japan, 5 On leave from Panasonic Corporation, 6 Kyoto
University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan
2ABSTRACTWe have developed a system of laser-pump and synchrotron radiation probe microdiffraction to investigate the phasechange process on a nanosecond time scale of phase-change thin layers and/or nanometer-sized dots embedded inmulti-layer structures,. The measurements were achieved by combining i) the pump-laser system with a pulse width of300 ps, ii) a highly brilliant focused microbeam with wide peak-energy width (ΔE/E ~ 2%) made by focusing helicalundulator radiation without monochromatization, and iii) a precise sample rotation stage to make repetitive
measurements. We successfully detected a very weak time-resolved diffraction signal by using this system from 100nm-thick Ge2Sb2Te5 phase-change thin layers and Ge10Sb90 nanometer-sized dots embedded in the thermally managed
multi-layered structure.
Key words: Phase Change, Synchrotron Radiation, Microdiffraction, Ge2Sb2Te5, Ge10Sb90, Pump and Probe

1. INTRODUCTION
Digital versatile disks (DVDs) and Blu-ray Discs (BDs) are two of the most convenient storage devices for large
amounts of information such as that in videos and digital photographs. These are due to extensive efforts anddevelopments that have been made in the progress of device materials. The developed phase-change materials canreportedly complete the phase change process with 20-ns laser irradiation. However, faster phase changes are stillrequired for next-generation optical recording media. To achieve this purpose, it is crucial to precisely observe thecrystallization process on a nanosecond time scale.
We therefore developed a new system of laser-pump and SR probe microdiffraction [Fig. 1(a)] to investigate thephase-change processes in a real optical recording media structure.1 We achieved this by combining i) a pump laser
system with a pulse width of 300 ps and a repetition rate of 1 kHz, ii) a highly brilliant focused microbeam with peakenergy width (ΔE/E~2%) accomplished by focusing helical undulator radiation without monochromatization, and iii)
a precise sample disk rotation stage with a system of position feedback for repetitive measurements.
Using this system, we successfully revealed the laser-induced crystallization process in real time both for 100-nmthick Ge2Sb2Te5 phase-change thin layers1 and Ge10Sb90 nanometer-sized dots2 embedded in the thermally managed
multi-layered structure.
2. INSTRUMENTATION
We improved the system of time-resolved microdiffraction we previously developed,3,4 installed at the SPring-8
BL40XU helical undulator beamline. The system mainly consisted of a subsystem for laser-pump and SR probe
measurements and a subsystem of x-ray microdiffraction with a sample rotation stage, as can be seen from theschematic in Fig. 1(a).
The previous system used a mode-locked Ti:sapphire laser oscillator equipped with a regenerative amplifier (SpectraPhysics, USA) that had a pulse width of 130 fs acting as a pump laser. However, Ge2Sb2Te5 phase-change layers with

thicknesses of less than 200 nm were not crystallized by pumplaser irradiation since the pulse width of 130 fs was too short tocrystallize thin phase-change layers. We therefore enlarged thepulse width to 300 ps by omitting the pulse compression processfrom the system of femtosecond laser amplification. The
modified laser system produced pulses with a pulse energy of 1mJ, a wavelength of 800 nm, and a repetition rate of 948.98 Hz,
which was 1/220 of the revolution frequency of the storage ring.
The laser was synchronized to SR pulses to measure the change
in time-dependent diffraction intensity due to the crystallizationof the phase-change layers. Figure 1(b) to (d) show time chartsof the laser and the SR pulses we used in the experiment.
Detailed synchronization procedure was described in ref. 1. The
probe SR pulses were detected by using an avalanche
photodiode (APD) with a time resolution of 1.6 ns and stored ina multi-channel scaler (MCS). To accumulate repetitive data,
we used the scattered pump laser pulses detected by using
another APD as trigger pulses for the MCS.
Fig. 1 Schematic of (a) set up for laser pump
A higher flux incident beam is required to detect very weaksynchrotron radiation (SR) probe microdiffraction
diffraction from the thin Ge2Sb2Te5 phase-change layers
and time charts of (b) SR pulses, (c) gate of x-ray
embedded in a real device structure. We also need topulse selector, and (d) laser pulses.
simultaneously make many repetitive measurements for one
sample disk to save the prepared sample volume. We adopted a wide energy width microbeam to satisfy bothconditions created by focusing helical undulator radiation without monochromatization. A high-photon-flux beam(~1015 photons/s) can be used for quasi-monochromatic SR x-rays because the higher harmonics of the helicalundulator are emitted toward an off-axial beam direction by extracting the central part of the radiation. The flux of thequasi-monochromatic SR x-rays is about 100 times that of monochromatized x-rays because of the peak energy widthdifference between that with (~0.02%) and that without (~2%) monochromatization. The quasi-monochromatic SR xrays were focused on the disk sample by using a phase zone plate. We used a Ta phase zone plate with a diameter of
100 μm, an innermost zone radius of 5.0 μm, an outermost zone width of 250 nm, a zone number of 100, and a zone
thickness of 2.5 μm, which was fabricated by NTT Advanced Technology Corporation, Japan.5

The quasi-monochromatic SR x-rays with a peak photon energy of 12 keV were pre-focused by two bend mirrors,
whose focal point was located 8900 mm upstream of the phase zone plate. Only bunch train pulses then passed
through the x-ray pulse selector (Jülich, Germany) with a repetition rate of 948.98 Hz, and they were focused on thesample position by using the phase zone plate [Fig. 1(a)]. The focal length of the zone plate was 240 mm. The
horizontal and vertical beam sizes at the focal position (i.e., the sample position) were 2.8 μm (vertically) × 3.5 μm(horizontally), which were determined by the knife edge scan method. The photon flux was 1.5 × 109 photons/s, evenwhen only 0.38% of the SR x-rays passed through the x-ray pulse selector. We noted that the pulse selector also
effectively prevented the zone plate from being destroyed due to radiation damage.
Diffraction was measured with a high precision diffractometer equipped with a sample disk rotation stage. Therotation stage maintained the overlap of the x-ray microbeam and the pump laser beam on the sample surface when
the disk was rotating, which was achieved by using a low-eccentric spindle motor and a system of position feedback
control.1 This enabled us to use an x-ray microbeam for the repetitive measurements of x-ray diffraction.
3. CRYSTALLIZATION OF 100-nm-THICK Ge2Sb2Te5 PHASE-CHANGE THIN LAYERS
We investigated the dependence of the crystallization process of a 100-nm-thick Ge2Sb2Te5 thin layer embedded in a
multilayered structure on laser power to demonstrate the capabilities of the system we developed. The layered
structure of the disk sample is as follows. Both the top and bottom surfaces of the 100-nm-thick Ge2Sb2Te5 layer werecovered by 10-nm-thick ZnS (80 mol %)-SiO2 (20 mol %) layers for passivation. A 40-nm-thick Ag layer was formed
under the bottom ZnS-SiO2 layer as a laser reflection layer, which is a typical layered structure in phase-change
optical recording media. Commercial recording media generally have a polycarbonate (PC) substrate on the top ZnSSiO2 layer because the laser is irradiated through the PC substrate. However, we laid out an SiO2 glass substrate (120mm diameter and 1.1 mm thick) under the Ag layer to keep the surface flat when the disk was rotating. This is very

effective to keep the x-ray microbeam and the pump laser beam
overlapping in the Ge2Sb2Te5 layer even when using the feedback system.
We believe this substrate change did not affect the phase-change process.
The sample disk was mounted on the rotation stage and the glancing angle
for the x-ray beam was set to 12.0°. Because we confirmed that x-ray
diffraction pattern of the Ge2Sb2Te5 thin films showed NaCl type crystal
structure as reported in ref. 6, the most intense 200 diffraction peak wasdetected with the APD, which was mounted on the 2θ-arm of thediffractometer and fixed to the diffraction angle of 19.5°.
Figure 2 shows the dependence of the time-resolved diffraction peak
profiles on laser power due to the crystallization of the Ge2Sb2Te5 layer inthe sample disk with irradiation laser powers of (a) 476, (b) 420, (c) 355
and (d) 313 nJ. These were plotted by averaging the adjacent five channelsof the MCS, meaning that the time resolution was 8 ns and the time zero
was set to the time when the pump laser hits the sample disk. The
measurement time was only 900 seconds (~ 9.0 × 105 shots) for eachprofile. All four profiles could be obtained with one sample disk. This isadvantageous to investigate and compare the crystallization processes
precisely by changing various conditions such as the irradiation laser
power, wavelength, and pulse width.

Fig. 2 Dependence of time-resolveddiffraction peak profiles on laser
power due to crystallization of 100
nm-thick Ge2Sb2Te5 layer in sample
disk with irradiation laser powers of(a) 476, (b) 420, (c) 355, and (d) 313
nJ. Note that profiles have been
vertically displaced for clarity.

We observed distinct variations in the diffraction intensity profiles on a nanosecond time scale in this series ofexperiments as the irradiation laser power was changed from 313 to 476 nJ, as can be seen from Fig. 2. Except for (a),the diffraction intensity profiles begin to increase with a delay of approximately 100 to 120 ns after 300 ps laserirradiation. These are very similar to that of the previously measured 300-nm-thick Ge2Sb2Te5 film.1 On the other
hand, the profile (a) shows a gradual increase of the intensity with shorter time delay. We think the laser power of 476
nJ causes the melt of the material film. This is the possible reason why the crystallization speed of (a) is slower thanthose of (b) to (d) which will be crystallized via solid process. Comparing (b) to (d), the time delay to start
crystallization is a little different. As the laser power decreases from (b) to (d), the delay time becomes shorter.
Because the crystallization process of the Ge2Sb2Te5 is reported to be nucleation-dominant type,7 we guess the delay
time is the time for nucleation. This indicates precise control of the temperature profile due to laser irradiation is very
important to realize faster phase-change.
4. CRYSTALLIZATION OF Ge10Sb90 NANOMETER-SIZED DOTS
Nanodot samples were formed with an amorphous Ge10Sb90 alloy with approximately 50-nm diameter.2 Thesesamples were fabricated by successive sputtering of multilayers on a patterned glass disk substrate with a diameter of
64- mm (HOYA Corp., Japan); on the glass substrate, UV-resin nanopillars with a diameter of 50- nm and a height of
100-nm were formed with a 100-nm pitch using the nano-imprinting technique.8 The layer stack was designed toimitate the structure of a commercial phase-change optical disk; it was optically optimized to exhibit a large change inreflectivity corresponding to phase transition from the amorphous to crystalline state. It was also designed to suppress
heat flow from Ge10Sb90 to the adjacent layers. According to previous reports, Sb-based phase-change alloys,including Ge10Sb90, were characterized by “growth-dominant crystallization” processes owing to their relatively low
nucleation rates but high growth rates.9 To overcome the low nucleation rates, we inserted pure Sb layers as seedlayers to shorten the crystallization time because the Sb layer would be crystallized during or just after deposition andserve as a precursor for the Ge10Sb90 layer in each stack. A silver layer with thickness of 40-nm was provided toincrease optical absorption in the Ge10Sb90 layer and also protect the resin pillars from thermal damages. Themultilayers were deposited almost exclusively on the tops of the nanopillars to successfully form nanodot-recording
elements. Approximately 2.4 × 1010 nanodots were formed in a zone (ϕ 33.0–35.4-mm) on a sample disk.
The size of the pump laser and probe SR beams on the sample surface were optimized according to the sample shape:
approximately 10 × 100 µm2 (laser) and 1.98 × 16.6 µm2 (SR). Because the size of the x-ray beam was rather large,
compared to that of the nanodots, we observed diffractions from approximately 1200 to 1300 nanodots within one
shot. Moreover, because x-ray diffraction from one shot was very weak, we compiled cumulative diffraction data forevery case by changing the position of the sample for every shot. Once we confirmed that the x-ray diffraction peaks

of the Ge10Sb90 nanodots used in this study exhibited rhombohedral symmetry
R-3m resembling pure Sb,10 we observed the change in the diffraction intensityof the 012 signal, the most intense peak of the rhombohedral structure.

(b) 212 nJ

Intensity (counts)

Figure 3 shows the experimental results of time-resolved diffraction intensityof the 012 peak,. Each curve was obtained by accumulating data fromapproximately 100,000 shots and plotted by averaging the adjacent ten data,
meaning that the time resolution was 16 ns, to smooth the curves. All
diffraction intensity profiles begin to increase with a delay of approximately 70
to 100 ns and saturated at approximately 150 ns after 300 ps laser irradiation.
Comparing (a) to (d), the time delay to start crystallization is a little different.
As the laser power decreases from (a) to (d), the delay time becomes a shorter.
This indicates that it may be effective to set the pumping laser power to slightly
above the threshold of crystallization for minimizing the crystallization time.

(a) 238 nJ(c) 186 nJ(d) 160 nJ

5. CONCLUTION
We successfully revealed the laser-induced crystallization processes in realtime for 100-nm-thick Ge2Sb2Te5 phase-change thin layers and Ge10Sb90
nanometer-sized dots embedded in the thermally managed multi-layered
structure using the system of laser-pump and SR probe microdiffraction we
newly developed. The system will be a very effective tool for exploring thefaster phase-change materials and device structures for the next generation.
ACKOWLEADGMENT

Time (ns)Fig. 3 Dependence of time-resolved
diffraction intensity on laser power
due to crystallization of the Ge10Sb90
nanodots with irradiation laser
powers of (a) 238, (b) 212, (c) 186,and (d) 160 nJ.

We are deeply grateful to Koji Sekiguchi, Kazuhiko Fujiie, and Osamu Kawakubo of Sony Corporation for their huge
efforts in the preparation of the nanodot samples. SR experiments were carried out at BL40XU of SPring-8 (Proposal
Nos. 2010A0030, 2010B0030, 2011A0030, 2011B0030, 2012A0030, and 2012B0030).
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