Anomalous X-ray scattering reveals
the role of constituent elements in Ge2Sb2Te5
S. Koharaa, K. Oharaa, L. Temleitnera,b, K. Sugimotoa, T. Matsunagac, L. Pusztaib, M. Itoua, H.
Ohsumid, R. Kojimac, N. Yamadac, T. Usukie, A. Fujiwaraa, and M. Takataa,dJASRI/ SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, P. O.
Box 49, HungarycPanasonic Corporation, 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, JapandRIKEN, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japane
Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12
Koshirakawa, Yamagata 990-8560, Japanab

ABSTRACT
We succeeded, for the first time, in measuring anomalous X-ray diffraction data on amorphous
Ge2Sb2Te5 (GST), which allows us to distinguish Sb- and Te-related atomic correlations.
Furthermore, we have performed high-energy X-ray diffraction measurements on crystalline GST
and applied reverse Monte Carlo structural modeling, in combination with density functional
theory-molecular dynamics simulation, to both experimental data. We found that germanium andtellurium atoms form a “core” Ge-Te network associated with the formation of fourfold rings, whichrings can be recognized as crystalline nuclei. It is also suggested that the Ge-Te network canstabilize the amorphous phase at room temperature and that this network can persist in the crystalline
phase. On the other hand, antimony does not contribute to ring formation but constitutes a “pseudo”
network with tellurium, in which the characteristic Sb–Te distance is somewhat longer than thecovalent Sb–Te bond distance. This suggests that the Sb-Te pseudo network may act as a precursor
to forming critical nuclei during the crystallization process. The findings indicate that the Ge-Te core
network is responsible for the outstanding stability and rapid phase change of the amorphous phase
while the Sb-Te pseudo network is responsible for triggering critical nucleation.
INTRODUCTION
Chalcogenide phase-change (PC) materials have been used in rewritable storage media, digital
versatile disc – random access memory (DVD-RAM), DVD re-writable (DVD-RW), Blu-ray
disc–rewritable (BD-RE) and non-volatile memory such as phase-change random access memory(PC-RAM). Numerous studies on the structure of amorphous (a-) Ge2Sb2Te5 (GST) 1) have beenreported early in the last decade using experimental methods such as extended X-ray absorption
fine-structure spectroscopy (EXAFS), high-energy X-ray diffraction (HEXRD) with the aid ofreverse Monte Carlo (RMC) simulation, hard X-ray photoemission spectroscopy (HXPS) andtheoretical methods. Thus the structure of a-GST and its relationship with rapid phase change is
gradually being uncovered; the most recent results are reviewed in ref. 2. For understanding the roleof each element beyond the nearest coordination distance, with a special focus on the question of
why a-GST is stable at room temperature for a long time despite the fact that it can rapidly transformto the crystalline phase, we performed anomalous X-ray scattering (AXS) measurements at the Sband Te K edges. These experiments can provide information concerning the local environment of Sband Te atoms beyond the nearest neighbour distance, in contrast to EXAFS. Furthermore, RMC
modelling has been applied to AXS and HEXRD datasets on a-GST, while only to the latter for
crystal (c-) GST, with a special focus on the connectivity of Ge-Te units and Sb-Te units beyond thenearest coordination distance.3)

EXPERIMENTS
Details of sample preparation are described in ref. 4.
The high-energy X-ray diffraction experiment on c-GST was carried out at room temperature at theSPring-8 high-energy XRD beamline BL04B25) using a two-axis diffractometer. The incident X-ray
energy was 61.6 keV.
Room temperature anomalous X-ray scattering measurements on a-GST were carried out at theSPring-8 single crystal structure analysis beamline BL02B1 using a 4-circle diffractometer. TheAXS measurements were performed at 4 energies, 30.172 keV (Sb far edge), 30.422 keV (Sb nearedge), 31.500 keV (Te far edge), and 31.750 keV (Te near edge).
RMC simulation on a-GST was carried out by starting from an atomic configuration obtained by
RMC/density functional (DF)-molecular dynamics (MD) simulation (number of atoms: 460).6) The
experimental ΔSSb(Q) and ΔSTe(Q) obtained from AXS measurement and the S(Q) obtained from ahigh-energy X-ray diffraction measurement7) were fitted simultaneously by employing the RMC++
code.8) Modelling of c-GST was carried out using 7200 particles by the RMCPOW simulation
code.9)STe(Q)

S(Q)

SSb(Q)

RESULTS
Figure 1(a) shows total scattering (a)(b) structure factors, S(Q), for4c-GST
anda-GST.
The0.5
Sb-X
diffraction pattern of c-GST
3exhibits sharp Bragg peaks and a0.0
0 1 2 3 4 55diffuse scattering pattern, whilec-GST
2
only the latter appears fora-GST. It is demonstrated thatTe-X
1the S(Q) of the RMC modela-GST
(solid line) agrees well with
0experimental data (open circles).
00 1 2 3 4 5 6 7 8 9 1005
1015Differential structure factors,Q (Å-1)
Q (Å-1)
ΔS(Q), for Sb and Te obtained
from AXS measurements areFig. 1 Comparison between the experimental structure factors
shown in Fig. 1(b). The(open circle) and results of RMC modelling (solid line). (a)differential structure factor of Sb,
Total scattering data for c-GST and a-GST obtained by HEXRD
ΔSSb(Q) exhibits a peak at Q=1
measurements. The low Q part of c-GST data is enlarged in theÅ-1. This peak is also observed ininset. (b) ΔS(Q) for a-GST obtained by AXS measurements atSb and Te K absorption edges.
HEXRD data for both phases(due to diffuse scattering present
in c-GST), as shown in Fig. 1(a).
For obtaining statistical structural features beyond the nearest coordination distance, ring statistics
were calculated using a shortest path analysis.10) As can be seen in Fig. 2(a), fourfold and sixfold
rings consisting of ‘–(Ge, Sb)–Te–’ units are dominant in c-GST. The existence of sixfold rings isdue to the 20% vacancies on Ge/Sb mixing sites. As discussed in ref. 4, small ‘–(Ge, Sb)–Te–’ rings
such as fourfold, fivefold, and sixfold rings are dominant in a-GST. The numbers of rings consistingof ‘–Ge–Te–’ units and of ‘–Sb–Te–’ units in a-GST are shown in Fig. 2(b). Although thedistribution of ‘–(Ge, Sb)–Te–’ units has already been discussed in earlier studies,2,4,6,7) we found forthe first time that Ge-Te bonds form large numbers of fourfold and sixfold rings, while thecontribution of Sb-Te bonds to the ring distribution is very small. Therefore it is suggested that thecore network constructed by Ge-Te covalent bonds in a-GST is similar to that in c-GST, and that thiscore network plays an important role in stabilizing the amorphous phase at room temperature forlong time.

Ratio of string
connection (%)Number of ringsFraction (arb. unit)

(b)
To understand the atomic ordering (a)7030
by Ge-Te and Sb-Te bonds in60
c-GST
a-GST
detail, connectivities of atoms were50(Ge, Sb)-Te ring Ge-Te ring
2040
calculated,byvaryingthe30
10maximum distance within which
20
10atomic pairs were considered to be00
connected. As can be seen in Fig.
3 4 5 6 7 8 9 10 11 1240
a-GST
a-GST
203(a), about 60% of Ge-Te bonds up(Ge,Sb)-Tering
Sb-Te ring30
to 3.2 Å form a core network
2010whereas Sb-Te bonds up to 3.2 Å10
do not form such a network. This00
3 4 5 6 7 8 9 10 11 12
3 4 5 6 7 8 9 10 11 12
distance corresponds to the covalentn-fold ringn-fold ring
bond distance determined by6)
AtomicDF-MD
simulations.
Fig. 2 Ring statistics for c-GST and a-GST calculated from
configurations of Ge-Te and Sb-Te
the RMC models. (a) Normalized fraction of ‘–(Ge,with bonds considered up to 3.2 ÅSb)–Te–’rings. (b) Numbers of ‘–Ge–Te– rings’ andare shown in Figs. 3(b) and 3(c),‘–Sb–Te–rings’ in a-GST. The total number of ‘–(Ge,respectively. It can be clearlySb)–Te–’ rings is normalized since the number of particles indistinguished that Ge-Te bonds
the simulation box is not same for c-GST and a-GST.
form a core network, which
stabilizes the amorphous phase, whereas Sb-Te bonds do not. Furthermore it is evident that Ge-Te
bonds form large fractions of fourfold (highlighted by blue) and sixfold (highlighted by light blue)
rings, which can be recognized as crystalline nuclei as shown in Fig. 2d of ref. 4. On the other hand,about 70 % of the Sb-Te pairs form a pseudo network, as can be seen in Figs. 3(a) and 3(d), whenbonds are considered up to 3.5 Å. In other words, the pseudo Sb-Te network becomes visible when
we extend the correlation distance up to 3.5 Å, while such a feature cannot be observed for Ge-Te
connectivities.
On the basis of structural features in a-GST found above, we illustrate the phase-change scheme inFig. 4. As highlighted by solid lines in (a)(b)
100Fig. 4(a), Ge-Te bonds significantly
80
contribute to the network manifested
60
by fourfold rings. Therefore Ge andGe-Te
Sb-Te
40
Te can be recognized as network
Ge-Te
(3.2Å)
20
forming elements that stabilize theamorphous phase by strong Ge-Te
0
3.0
3.5
4.0
4.5
5.0
covalent bonds11) at room temperaturer (Å)for long time. The core Ge-Te(d)
network may remain in the (c)crystalline-like phase, which featureexplains rapid crystallization. As
highlighted by dotted lines in Fig.
Sb-Te
Sb-Te
(3.2Å)
(3.5Å)
4(b), the Sb-Te correlations beyond
the nearest coordination distance forma pseudo Sb-Te network. This unusual
atomic ordering in terms of Sb-TeFig. 3 Atomic configuration and connectivity of Ge-Te
correlations can be ascribed to theand Sb-Te in a-GST obtained by the RMC model. (a)combination of two positively
Connectivity of Ge-Te and Sb-Te with different rmax. (b)charged atoms (Ge, -0.22 electrons;
Atomic configuration of Ge-Te (connectivity is consideredSb, 0.32 electrons; Te, 0.22
up to 3.2 Å). (c) Atomic configuration of Sb-Te
andallowstheelectrons12))(connectivity is considered up to 3.2 Å). (d) Atomic
amorphous phase to form criticalconfigurationof Sb-Te (connectivity considered up to 3.5
nuclei via forming Sb-Te bonds byThe
theeefold,fourfold, fivefold, and sixfold rings areÅ).
small atomic displacements ofhighlighted
byred,blue,green, and light blue, respectively.
antimony and tellurium atoms in thecrystallization process. Thus our finding canreasonably explain the reason why thecrystallization of a-GST is faster than that ofamorphous GeTe in DVD.13)
CONCLUSION
We found that germanium forms a covalent core
network with tellurium to stabilize the amorphous
phase at room temperature, which core network canremain in the crystalline phase. Antimony
maintains an atomic ordering with tellurium
beyond the Sb-Te covalent bond distance in a-GST;
these atomic pairs can rapidly fit into the Ge-Tecore network in the crystalline phase at higher
temperature induced by a laser irradiation. Thisbehavior is similar to the Sb local environment ofdistorted 3 + 3 octahedron4) found in another phasechange material, Ag3.5In3.8Sb75.0Te17.7 (AIST), inwhich Sb possesses locally 3 extra neighbours
beyond the nearest coordination covalent bonddistance. Our finding can reasonably explain theroles of germanium and antimony in DVD withrespect to rapid phase change and durability. Thus
it is demonstrated that the observation of network
formation is important to reveal the origin of rapid
phase change at atomic level. This finding is a
crucial new key concept for designing new PC
materials with excellent durability.

(a)(b) SbTe

Amorphous phaseGe

Crystal phaseFig. 4
Schematic drawing showing thephase-change process in a-GST. (a) highlights
the Ge-Te core network; (b) highlights theSb-Te pseudo network. Black colored sticks
show Ge-Te bonds up to 3.2 Å. Gray coloredsticks show Sb-Te bonds up to 3.2 Å and dottedlines show Sb-Te correlations between 3.2 to3.5 Å.

ACKNOWLEDGEMENTS
This work was supported by the Japan Science and Technology Agency via the Strategic
Japanese-Finland Cooperative Program on “Functional Materials”. LP and LT were partiallysupported by the Hungarian National Development Agency (NFÜ), via grant No.
TÉT_10-1-2011-0004.
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