Crystallization properties of Ge-Sb and Ge-Bi-Te nanoparticles
by pulsed laser irradiation
Takashi MIHARA, Akio TSUCHINO, Shuji SATO, Kazuya HISADA,Rie KOJIMA, Noboru YAMADA, and Shigeru FURUMIYA
Advanced Technology Research Laboratories, Panasonic Corporation
3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan
Phone: +81-774-98-2575, FAX: +81-774-98-2406, E-mail: mihara.takashi@jp.panasonic.com
Abstract
Laser-induced crystallization times (tc) were investigated for Ge10Sb90 and Ge2Bi2Te5 in the forms of nanoparticles
less than 50nm in the diameters and 10nm-thick blanket films. tc of nanoparticle samples are longer than that of theblanket film samples. It is remarkable that tc of Ge10Sb90 nanoparticles (12s) is 500 times longer than that of theblanket film (24ns), while tc of Ge2Bi2Te5 nanoparticles (80ns) is comparable to that of the blanket film (56ns).
This result may have a close relationship with the difference of crystallization process between Ge10Sb90 andGe2Bi2Te5, which should be taken into account for the optical data storage composed of phase-change nanoparticles.
1. Introduction
Nanoparticle phase-change material is one of the promising candidates for the next generation optical data storage.
In order to increase the recording density of the phase-change optical memories far beyond the optical resolution
limit, it is inherently important suppressing the thermal interference between recording marks. One of the promising
methods for that is adoption of isolated nanoparticles. Several studies have been carried out on crystallization
properties of nano-sized phase-change materials; e.g., by pulsed electric current in PC-RAM devices1 or by heatingin the inactive gas2. However, there are no reports on laser-induced crystallization properties. In this study, weinvestigated the pulsed laser-induced crystallization times for isolated nanoparticles of Ge10Sb90 and Ge2Bi2Te5 bycontrast with those for blanket films. It is well-known that Ge10Sb90 and Ge2Bi2Te5 are typical high-speed
phase-change alloys and show rather different crystallization processes, especially the former is as the“growth-dominant” type3 and the latter is as the “nucleation-dominant” type4.
2. Experiments
2.1 Preparation of the isolated nanoparticles
Nanoparticles of the two phase-change materials were fabricated by nano-imprint lithography and plasma etching.
The upper figure in Fig. 1 shows the preparation procedure. Firstly, the film stack including the 10nm thickphase-change film (Ge10Sb90 or Ge2Bi2Te5) was formed on the SiO2 substrate by sputtering method. Secondly, theUV resin mask pattern of the 50nm-diameter dots at 100nm intervals was imprinted on the top of the film stack.
Thirdly, CF4 plasma etching of the Si-N film was performed, that was followed by O2 ashing process for removing
the residual UV resin. Fourthly, Ar etching of the phase-change film was performed. It is expected that the Ar etching
will cause less damage to the phase-change material comparing to CF4 etching. The AFM images obtained just after
the Ar etching are shown at the bottom in Fig. 1. The observed average diameter of Ge10Sb90 and Ge2Bi2Te5 particles
were 45nm and 47nm, for each. Finally, the obtained nanoparticles were sealed with 15nm-thick Zr-Cr-O protective
film. In addition to these nanoparticle samples, blanket film samples having the same stack were prepared asreferences.
2.2 Two lasers optical system
An optical experimental system shown in Fig. 2 was built for evaluating the crystallization times of the samples. Agreen laser beam (=532nm) and an infrared laser beam (=785nm) are coaxially introduced into an objective lenshaving 0.70-NA and focused onto a sample. The focused spot sizes of the green and the infrared laser beams are
1.0m and 1.6 m, respectively. The pulse duration of the green laser is 50ps (fixed), and the pulse energy level of it
can be adjusted to each sample using the transmission-variable ND filter. Both of the pulse duration and the power
can be changed for the infrared laser (pulse duration: 2nsDC). Two sets of an LED changer for illumination and aCCD camera are also equipped for observing the reflection and transmission microscopic images of the sample
surfaces. Here, five different wavelengths (white, 448nm, 530nm, 627nm, and 655nm) can be selected for obtaining

a high contrast image just by switching the LEDs.
2.3 Evaluation of crystallization times
In order to estimate crystallization times, the minimum pulse duration required for crystallization was studied. Fig.
3 shows the experimental procedure: i) each sample having many nanoparticles on it was uniformly crystallized by
DC infrared laser irradiation (initial-state), ii) a center area of the sample was amorphized by the green laser
irradiation (write-state), and iii) the area was re-crystallized by the infrared laser irradiation, again (erase-state). Thiserase process of iii) was achieved at various erase conditions while the write conditions of ii) was fixed to theaverage pulse energy for amorphization. A sufficiently large optical contrast according to the phase transitions wereobtained from the group of approximately 100 nanoparticles, though the change from individual each one
nanoparticle was too small for observation. Here, we defined the erase rate of (E) for amorphous nanoparticles asfollowing equation: E[%]=(BwBe)/(BwBi)100, where B denotes the grayscale brightness of the microscopic
image measured at a laser-irradiated area. The subscripts of w, e, i mean that the state of “write”, “erase”, and“initial”, respectively. Accordingly, the crystallization time of (tc) is defined as the minimum pulse duration forsatisfying E>90% in the process iii).
The above series of experiments were conducted also for the blanket film samples. Thus, the dependence of tc on
the compositions and the sample configurations were investigated.

3. Results and discussions
The maps in Fig. 4 show the erase rates of Ge2Bi2Te5 and Ge10Sb90 samples as a function of pulse duration(horizontal) and the power (vertical) for erasing process. Each lattice point in the maps corresponds to one irradiation
condition.
The determined crystallization times (tc) for each sample were listed in Table 1. tc of the nanoparticle samples
become longer than that of the blanket film samples. In the case of Ge2Bi2Te5, tc of the nanoparticles (80ns) is
comparable to that of the blanket film (56ns). In contrast, in the case of Ge10Sb90, tc of the nanoparticles (12s)
becomes 500 times longer than that of the blanket film (24ns). That is to say, tc of the Ge10Sb90 nanoparticles ismore than 100 times longer than that of the Ge2Bi2Te5 nanoparticles, while tcs of the blanket films of each material
are comparable. It is possible to explain this remarkable difference by the difference of the nucleation processes
between the two materials.
Possible re-crystallization models for the amorphous nanoparticles and the amorphous blanket film of eachphase-change material are shown in Fig. 5. Ge2Bi2Te5 is known as a nucleation-dominant material, so the isolated
Ge2Bi2Te5 nanoparticles can be crystallized as well as the blanket film. On the other hand, Ge10Sb90 is known as a
growth-dominant material. In the case of the blanket films,Blan ket fi lm Nano particles
crystallization starts at the interface between amorphousand crystalline, which works as the trigger ofGe 2 Bi 2Te 556 ns
8 0nscrystallization. Isolated amorphous Ge10Sb90 nanoparticles,
24 ns
12s
Ge10Sb90
however, have no such interfaces at all. The absence of theinterface will be a considerable barrier for crystallization.
Table 1. Determined crystallization times of each material
4. Conclusions
We investigated the potential of nanoparticle phase-change alloys for higher density optical data storage. Thelaser-induced crystallization times (tc) were examined for nanoparticles of Ge10Sb90 and Ge2Bi2Te5 by contrast withthose for blanket films. Ge10Sb90 nanoparticles around 50nm in the diameters sample showed 500 times longer laser
crystallization time (tc) (12s) compared to that of the blanket film (24ns). This remarkable difference between
Ge10Sb90 and Ge2Bi2Te5 can be explained by the crystallization processes of these two phase-change alloys: thegrowth-dominant type for the former, and the nucleation-dominant type for the latter. Thus it can be concluded that
nanoparticle phase-change material will be possible and effective; however, it is essentially important to take into
account the crystallization mechanisms. In the case of nanoparticle optical media, nucleation-dominant type such as
Ge2Bi2Te5 will be favorable than growth-dominant type such as Ge10Sb90.
References[1] J. Lee et al.: IEEE Electoron Device Letters, 32, (2011) 1113.
[2] Y. Zhang et al.: J. Appl. Phys., 104, (2008) 074312[3] L. van Pieterson et al.: J. Appl. Phys., 97, (2005) 083520[4] J. H. Coombs et al.: J. Appl. Phys. 78, (1995) 4918

MoldUV regin mask 30nm
(φ50nm, pich 100nm)

CF4 etchingO2 ashing

Imprint

Protective layerdeposition

Ar etching

Zr-Cr-O 15nm

Si-N 15nm
Ge10Sb90／Ge2Bi2Te5 10nm
Zr-Cr-O 8nm
silver-alloy 40nm
SiO2 substrate
AFM

φ45nm-Ge10Sb90

φ47nm-Ge2Bi2Te50.4 m
0.3
0.2
0.1

0.4 m0.3
0.2
0.1
0.8 m

0.2 0.4 0.6

0.2 0.4 0.6

0.8 mFig. 1. Preparation procedure of phase-change nanoparticles

Focused beam
2ns～DC (variable) Green Laser IR Laser
=785nm
Illumination LEDchanger

IR Laser

Illumination LEDchanger

1.0µm
BEX

1.6µm

Objective Lens
CCDcamera
NA：0.70

GreenLaser
50ps (fix)=532nm

Variable BEX Half Mirror
ND Filter

DichroicMirror

/4 sample

PBS

XYZ stage

Fig. 2. Two lasers optical system

Green laser

Amorphized nanoparticle(write-state)

Infrared laser

Re-crystallized nanoparticle(erase-state)

Crystalline nanoparticle(initial-state)～100 nanoparticles
are irradiated.

Amorphization

Re-crystallization

Fig. 3. Experimental procedure for evaluation of the crystallization timeBlanket film

Nanoparticles

56ns

27

282421

IR Laser power [mW]

IR Laser power [mW]24

Ge2Bi2Te51613
1090%100%
8.180%～90%

6.270%～80%

4.360%～70%

2.45n80ns

33

19151312119.8
9.2
8.1

0%60%

6.2
5n 10n 20n 50n 100n200n 500n 1 2 5

10n 20n 30n 40n 60n 80n 100n

IR Laser pulse duration [s]IR Laser pulse duration [s]
28

24

24

21

19

16
13
1090%100%

8.1

80%～90%6.270%～80%

4.3
2.45n

IR Laser power [mW]

Ge10Sb90IR Laser power [mW]

24ns
27

12s

15131211
9.8
9.2

60%～70%8.1

0%60%

6.2
100n 500n 1 2 5 10 20 50 100 200 500 1m

10n 20n 30n 40n 60n 80n 100n

IR Laser pulse duration [s]IR Laser pulse duration [s]Fig. 4. Erase rates of each sample as a function of pulse duration (horizontal) andthe power (vertical) for erasing process. Encircled lattice points mean the conditions
in which the phase-change materials can be heated above a melting point. The redarrows indicate determined crystallization times (tc) for each sample, whichcorresponds to Table 1.

Blanket film⊿tc

Re-crystallization model

Nanoparticles⊿tc

Re-crystallization model

Laser
Ge2Bi2Te556ns

cry80ns

amo

Ge10Sb90

24ns

12s

Fig. 5. Possible re-crystallization models of blanket films and nanoparticles of each material