on the structure of the rapid phase-change material Ge

An anomalous x-ray scattering study

on the structure of the rapid phase-change material Ge

Sb

Te

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)

DOS (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

00 5 10 15 20

XRD

RMC refined model

Q (Å^-1)

S(Q)

1.1

1.0

0.9 10 15 20

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) = f

(Q) + f ’(E) + if ”(E)

-10 -5

32.0 31.5

31.0 30.5

30.0

f’

Sb K edge

Te K edge

Energy (KeV)

25 20 15 10 5 0

15 10

5 0

Q (Å^-1) I(Q) (counts 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, ΔS_Sb(Q) measured at Sb K edge differential structure factor for Te, ΔS_Te(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

00 1 2 3 4 5 6 7 8 9 10

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

0

15 10

5 0

 : 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

9 8 7 6 5 4 3 2 1 0

15 10

5 0

9 8 7 6 5 4 3 2 1 0

15 10

5 0

ΔS Te(Q)w ij(Q)·S ij(Q) Q (Å^-1)

ΔS Sb(Q)

Q (Å^-1) w ij(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 w_ij(Q)･S_ij(Q) of a-GST

w_ij(Q)·S_ij(Q) for ΔS_Sb(Q) w_ij(Q)･S_ij(Q) for ΔS_Te(Q)

Contribution of Sb-Te is dominant in ΔS_Sb(Q) and that of Te-Te is dominant in ΔS_Te(Q) The peak observed at Q = 1 Å^-1 in ΔS_Sb(Q) can be

assigned to the contribution of Sb-Te correlation

6 5 4 3 2 1

0 2 3 4 5 6 7 8

Black: c-GST, Blue: a-GST

g ij(r) 6 5 4 3 2 1

0 2 3 4 5 6 7 8 6 5 4 3 2 1

0 2 3 4 5 6 7 8

6 5 4 3 2 1

0 2 3 4 5 6 7 8 6 5 4 3 2 1

0 2 3 4 5 6 7 8 6 5 4 3 2 1

0 2 3 4 5 6 7 8

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-GST^1)" a-GST"

(this work) 8-N rule c-GST

N_Ge 3.9±0.7 3.85 3.82 3.83 4 6.0

N_Sb 2.8±0.5 3.12 3.31 3.06 3 6.0

N_Te 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 10 0

12 11 10 9 8 7 6 5 4 3 70 60 50 40 30 20 10 0

n-fold ring

Fraction (arb. unit)

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

20 10

0

12 11 10 9 8 7 6 5 4 3 30 20 10 0

n-fold ring

Number of rings

a-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 g_ij(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 100

80 60 40 20 0

5.0 4.5

4.0 3.5

3.0

Ge-Te Sb-Te

r (Å) Ratio of string" connection (%)

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 process

9) 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.