An anomalous x-ray scattering study on the structure of the rapid phase-change material Ge2Sb2Te5

JASRI/SPring-8

Koji Ohara

¤  Our previous structural study on amorphous Ge2Sb2Te5 (a-GST)

¤  Anomalous X-ray scattering (AXS)

¤  Experimental set up at BL02B1 beamline of SPring-8

¤  Refined structure of a-GST derived from the combination of AXS and RMC

¤  Comparison between a- and crystalline (c-) GST

¤  Role of each element of a-GST in rapid phase-change process

Combination of synchrotron measurements and RMC-DFT/MD simulation1)

200

150

100

50

0

-50

-100-12 -8 -4 0

Energy (eV)

DO

S (1

/eV)

RMC-DF/MD

Expt.2)

1)  J. Akola et al., Phys. Rev. B 80 (2009) 020201(R). 2)  J. J. Kim et al., Phys Rev. B 76 (2007) 115124.

2

1

020151050

XRD RMC refined model

Q (Å-1)

S(Q

)

1.1

1.0

0.9201510

RMC-DFT/MD model1) shows good agreement with experimental HEXRD and XPS data simultaneously

Role of each element is still unclear

4R

4R

4R

6R

4R

9R

4R

Network formation of (Ge or Sb)–Te

: Te : Ge or Sb

Atomic structure Electron DOS

Large fraction of 4-fold rings

We need the measurement to identify the role of each element.

Anomalous X-ray scattering (AXS)

AXS measurement provides us with structural information up to intermediate-range which can not be obtained by XAFS measurement.

f(Q, E) = f0(Q) + f ’(E) + if ”(E)

-10

-5

32.031.531.030.530.0

f’

Sb K edgeTe K edge

Energy (KeV)

25

20

15

10

5

0151050

Q (Å-1)

I(Q)

(co

un

ts x

104

)

Near edge

Far edge

ΔE=300eV

ΔE=50eV

Atomic number Sb : 51 Te : 52

Structural information of the only Te related correlations (Te-Ge, Te-Sb, and Te-Te) �

Monochromator : Si (311) with a sagittal focusing system Sample : a-GST Scan mode : 2θ step-scan in a vertical scattering

plane with transmission geometry

Incident X-ray �

sample�

Powder Ge2Sb2Te5 encapsulated in a silica tube

AXS measurement setup @ BL02B1

AXS measurements were performed at 4 energies: 30.172 keV (Sb far edge) 30.422 keV (Sb near edge) 31.500 keV (Te far edge) 31.750 keV (Te near edge)

Incident X-ray �

Ge detector �

Slit �

5)  O. Gereben, P. Jóvári, L. Temleitner, and L. Pusztai, J. Optoelectron. Adv. Mater. 9 (2007) 3021. 6)  A. Mellergård and R. L. McGreevy, Acta Cryst., A55 (1999) 783.

Crystalline phase: Initial configuration : 10 × 10 × 10 supercell configuration of 7,200 particles Experimental data : total structure factor S(Q) measured at 61 keV Program code : RMCPOW 6) code was used

Amorphous phase: Initial configuration : RMC/DFT-MD simulation model1) of 460 atoms Experimental data : total structure factor S(Q) measured at 61 keV

differential structure factor for Sb, ΔSSb(Q) measured at Sb K edge differential structure factor for Te, ΔSTe(Q) measured at Te K edge

Program code : RMC++5) code

Condition of RMC modeling on a- and c-GST

1)  J. Akola et al., Phys. Rev. B 80 (2009) 020201(R).

5

0109876543210

Q (Å-1)

S(Q

)

Agreements with diffraction are excellent.

X-ray Structure factor S(Q) for a- and c-GST

c-GST�

a-GST�

○: Experimental data −: RMC model

4

3

2

1

0151050

: Experimental data : RMC

Q (Å-1)

ΔS

Sb(

Q)

ΔS

Te(Q

)

Sb-X

Te-X

（X= Ge, Sb, and Te）

Differential S(Q) of a-GST derived from AXS

Agreements between the AXS and the RMC model are good

9876543210

151050

9876543210

151050Δ

STe

(Q)

wij(Q

)·S

ij(Q)

Q (Å-1)

ΔS

Sb(

Q)

Q (Å-1)

wij(Q

)·S

ij(Q)

Ge-Ge

Ge-Sb

Ge-Te

Sb-Sb

Sb-Te

Te-Te

Ge-Ge

Ge-Sb

Ge-Te

Sb-Sb

Sb-Te

Te-Te

Sb-X Te-X

X-ray-weighted partial structure factor wij(Q)･Sij(Q) of a-GST

wij(Q)·Sij(Q) for ΔSSb(Q) wij(Q)･Sij(Q) for ΔSTe(Q)

Contribution of Sb-Te is dominant in ΔSSb(Q) and that of Te-Te is dominant in ΔSTe(Q)

The peak observed at Q = 1 Å-1 in ΔSSb(Q) can be assigned to the contribution of Sb-Te correlation

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8765432

Black: c-GST, Blue: a-GST

g ij(r

)

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87654326543210

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Ge-Ge Ge-Sb Ge-Te

Sb-Sb Sb-Te Te-Te

Both the first Ge-Te and Sb-Te correlation lengths in a-GST are shorter than those in c-GST, due to the formation of covalent bond in a-GST

Real-space function obtained from the RMC models

Difference in periodicity is clear between both phases

r (Å)

a-GST7) a-GST8) a-GST1)" a-GST"(this work) 8-N rule c-GST

NGe 3.9±0.7 3.85 3.82 3.83 4 6.0

NSb 2.8±0.5 3.12 3.31 3.06 3 6.0

NTe 2.4±0.6 1.99 2.49 2.45 2 4.8

1) J. Akola et al., Phys. Rev. B 80 (2009) 020201(R). 7)  D. A. Baker et al., Phys. Rev. Lett. 96 (2006) 255501. 8)  P. Jóvári et al., Phys. Rev. B 77 (2008) 035202.

Both Ge and Sb fulfill the “8-N rule” and only Te is overcoordinated

Coordination number

Several models do not fulfill the 8-N rule! �

40

30

20

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01211109876543

706050403020100

n-fold ring

Frac

tion

(arb

. uni

t)

a-GST Ge(Sb)-Te

c-GST Ge(Sb)-Te ring

Ring statistics in a- and c-GST

✔ Core network is constructed by mainly Ge-Te correlation in a-GST ✔ This core network plays an important role in stabilizing the amorphous phase

4R 4R

4R

4R

Ge Te Sb

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10

01211109876543

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n-fold ring

Num

ber o

f rin

gsa-GST

Sb-Te ring

a-GST Ge-Te ring

Connectivity analysis on Ge-Te and Sb-Te correlations in a-GST

Search string connectivity of bonds within given distances to analyze the connection of rings which is hidden in gij(r) �

or

4R 4R

4R

4R

4R 4R

4R

4R

Ge Te Sb

Sb-Te bonds up to 3.5 Å form a psuedo network �

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05.04.54.03.53.0

Ge-Te Sb-Te

r (Å)

Rat

io o

f stri

ng"

conn

ectio

n (%

)Ge-Te bonds up to 3.2 Å form a core network �

Sb-Te bonds up to 3.2 Å do NOT form network

Connectivities of bonds were searched with varying maximum distance �

Connectivity on Ge-Te and Sb-Te correlation in a-GST

Antimony maintains an unique atomic ordering with tellurium beyond the nearest coordination covalent bond distance (dotted line)

Since Sb-Te correlations beyond the nearest coordination distance form the “pseudo” network, it can transform rapidly into Sb-Te bonds by a laser irradiation.

Role of antimony in a-GST

Up to 3.2Å Up to 3.5Å Ge Te Sb

 　 　 Amorphous phase 　               　 　 　 　 　 　   Crystalline phase

Network formation in phase-change process

✔Sb-Te bonds form a pseudo network triggers critical nucleation

✔Ge-Te bonds form a core (ring) network stabilizes amorphous phase

4R

4R

4R

4R

in crystallization process9) K. Ohara, L. Temleitner, K. Sugimoto, S. Kohara et al., Adv. Funct. Mater., 22 (2012) 2251.

u  To investigate the role of germanium and antimony in rapid phase-

change material, synchrotron radiation anomalous X-ray scattering technique was used for structural modelling.

u  It is found for the first time that Ge-Te bonds up to the nearest

coordination distance form the core network with ring formation to stabilize amorphous phase, while Sb-Te correlations beyond the nearest coordination distance form the pseudo network triggers critical nucleation of rapid phase change process.

u  The network formation is important to understand the origin of rapid phase change at atomic level.