Structural investigation of Sb-based phase-change materials
Toshiyuki Matsunaga a, Rie Kojima b, Noboru Yamada b,Kouichi Kifune c, Yoshiki Kubotad, Sinji Kohara ea

Materials Science and Analysis Technology Center, Panasonic Corporation
3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan
Correspondence e-mail: matsunaga.toshiyuki@jp.panasonic.comb
Digital & Network Technology Development Center, Panasonic Corporation
3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japanc
Faculty of Liberal Arts and Sciences, Osaka Prefecture University
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, JapandGraduate School of Science, Osaka Prefecture University
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, JapaneJapan Synchrotron Radiation Research Institute/SPring-8,
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

Key words: phase-change memory, Ge10Sb80Te10, Ge15Sb70Te15, GeTe-Sb, XRD, structural analysis,
Rietveld method
ABSTRACTSb-based phase-change materials, Ge15Sb70Te15 and Ge10Sb80Te10, were scrutinized by using an X-ray
diffraction method. The sputtered films of these materials crystallize into A7-type single phases byinstantaneous laser annealing; however, Ge atoms incorporated into the Sb matrix readily precipitateout of the matrix to cohere into the diamond-type Ge crystallites, and then, these Ge crystallites
combine with Te in the matrix to form eventually NaCl-type GeTe crystallites. This decomposition was
found occurred at relatively low temperatures. On the contrary to these GeTe-Sb materials, Sb-Te
binary compounds keep their single phase structures up to near their melting temperatures; and besides,
just after the transformations from the amorphous to crystalline phases, the structures of these binarymaterials are expected to be A7-type structure irrespective of their compositions. It is therefore,
presumed that the amorphous and crystalline structures and material properties of Sb-Te binary alloys
are worth scrutinizing for us to develop future new phase-change materials.

1. INTRODUCTION
Phase change recording is now extensively used for high density non-volatile memories [1]. Since
1970s, various materials have been proposed, and today, we have obtained two kinds of superiormaterials, GeTe-Sb2Te3 (GST) pseudobinary and Sb-based multinary alloys such as AIST (Ag-In-Sb-Tequadruple compounds). Of the two, many investigations have been made on the GST psudobinarycompounds; however, it is supposed that investigations on the Sb-based materials are still insufficient.
To develop more superior materials for future high-density phase change memories, the examinations
for both materials will be indispensable. We accordingly investigated several kinds of Sb-based
multinary compounds by means of x-ray diffraction method. These investigations revealed, however,that all these alloys eventually separated into two crystalline phases: dopants such as Ge, Sn, Ag orIndium in the Sb-based materials are easy to come out of the Sb matrices, forming other tellurides. It isgenerally expected that single-phase crystalline compounds that robustly hold their phases are suitablefor us to use them stably as phase-change random-access-memory materials. In many Sb-based
phase-change materials, however, (Sb2Te3)-Sb, (Bi2Te3)-Bi, and (Bi2Se3)-Bi etc. can continuously form
single-phase crystals over a wide composition range on their binary lines. Our present investigation
pointed out that these pseudobinary-like multinary compounds have to be examined further in thefuture.

2. EXPERIMENTAL
Thin films of Ge15Sb70Te15 and Ge10Sb80Te10 with a thickness of approximately 300 nm were formedby sputtering on glass disks 120 mm in diameter. Each film was crystallized by laser irradiation andthen scraped off with a spatula to create first powder specimens. Besides these sputtered films, weprepared Ge15Sb70Te15 and Sn10Sb80Te10 ingots by melting stoichiometric mixtures of 99.999%-pure Ge,Sn, Sb, and Te in argon atmosphere and then quenching them in ice water. The second powder
specimens were produced by crushing the completed ingots. To prepare the two kinds of experimental
specimens for X-ray diffraction, these powders were packed into quartz capillary tubes with an internal
diameter of 0.3 mm. The openings of the capillaries were sealed by using an oxyacetylene flame toinsulate them against the atmosphere. The diffraction experiments were carried out using thelarge-diameter Debye-Scherrer camera with an imaging plate on the BL02B2 beam line at SPring-8 [2].
In order to confirm the radiation beam energy used, diffraction pattern of powdered CeO2 (a = 5.4111Å) was taken under the same condition. The experiments at low and high temperatures were carried out
while blowing nitrogen gas set at the specified temperatures onto the capillary tube. The crystal
structures were examined and refined using the Rietveld method [3]. The program JANA2000 [4] wasused for this purpose; intensity data in increments of 0.01° were obtained by reading the imaging platefor a pixel area of 50 µm2.

3. RESULTS & DISCUSSION
3-1. X-ray-diffraction-experiment results for several Ge(Sn)Te-Sb alloys
Sputtered Ge15Sb70Te15 film
The diffraction patterns obtained for the sputtered Ge15Sb70Te15 film are shown in Fig.1. The resultof a search-match analysis revealed the diffraction patterns taken at low temperatures in the figure to bealmost identical to that of As, Sb or Bi crystal with an A7-type structure [5]. This phase belongs to thespace group R3 m [6]. The three constituent elements, Ge, Sb, and Te, randomly occupy 6(c) site. Thechanges in diffraction lines with increasing temperature show that the single-phase A7-type structure is
kept up to 600 K; however the peaks for diamond-type Ge ( Fd 3 m [6], a = 5.65735 Å at 20 °C [7])appear at 651 K along with the peaks indicating an A7-type structure. The Rietveld analyses performed
with these diffraction patterns provided temperature dependence of the structural parameters of thismaterial as shown in Fig.2. As seen in this figure, the cell dimensions almost linearly vary withtemperature up to 360 K. However, above this temperature up to 600 K, a, c, and B irregularly vary withtemperature. This curious behaviour of these parameters between 360 K and 600 K strongly implies
that precipitation of Ge atoms begins to take place at around 360 K.
Sputtered Ge10Sb80Te10 film

For sputtered Ge10Sb80Te10 film, we have confirmed that it was crystallized into an A7-type single
phase just after laser initialization as well as the sputtered Ge15Sb70Te15 film. However, we found that,after keeping it at room temperature for half a year, diamond-type Ge was extracted out of the matrix
even at that low temperature.
Melt-quench Ge15Sb70Te15 alloy

Figure 3 shows the diffraction patterns obtained for Ge15Sb70Te15 prepared by the melt-quench
method. As seen in the figure, this specimen showed two-phase coexistence of A7-type Sb and
NaCl-type GeTe phases irrespective of measurement temperature. This clearly shows that the coolingrate produced by the melt quenching method was insufficient to obtain an alloy crystallized into anA7-type single phase. The lattice constant of this GeTe phase is 5.990 Å at room temperature, whichcorresponds to that of (GeTe)0.90(Sb2Te3)0.10 metastable cubic phase [8] (Here, we assume that theNaCl-type structures can be present only on the GeTe-Sb2Te3 pseudo-binary line in the Ge-Sb-Te
ternary system). On the other hand, those of the remaining phase are 4.251  11.297 Å, which areslightly smaller than those of pure Sb, 4.307  11.312 Å (from our measurement result). This meansthat the remaining phase is not a pure Sb material but contains a small number of Ge or Te atoms. Themolar ratios analyzed by the Rietveld method are shown in the same figure, which shows that a few Snand Te atoms still remain in the Sb matrix.
Melt-quench Sn10Sb80Te10 alloy

Figure 4 shows the diffraction patterns obtained for Sn10Sb80Te10 prepared by the melt-quench
method. As seen in the figure, this specimen showed two-phase coexistence of A7-type Sb andNaCl-type SnTe phases irrespective of measurement temperature. The molar ratios analyzed by theRietveld method are shown in the same figure, which shows that Sb can dissolve little SnTe.
3-2. Reaction products

Our investigation on sputtered GeTe-Sb films revealed that Ge atoms incorporated into the Sbmatrix precipitate out of the matrix to cohere into the diamond-type Ge crystallites first, and next, theseGe crystallites combines with Te in the matrix to form NaCl-type GeTe crystallites. These GeTe-Sb
have semi-metallic features. GeTe-Sb2Te3 pseudo-binary compounds are semiconductors with very
narrow band gaps. Semiconducting or semi-metallic feature gives a material low thermal conductivity,
which is convenient for rapid ascending heat to cause an instantaneous phase change. For the Sb-based
phase change materials, it is essential to add some other elements to pure Sb in order to ensure theamorphous stability, namely the long-term data retention. This investigation strongly suggests that, tomake these Sb-based materials more superior phase-change materials, in other words, to give themhigher cyclabilities, we have to carefully examine and adopt the precipitates in the Sb matrix. X-ray
diffraction analyses for various future candidates will give us convincing clues to develop new material
with better performance.
3-3. Similarity in crystal structure between GeTe-Sb2Te3 pseudo-binary and Sb-Te binary systems

The (Sb2)n(Sb2Te3)m, (Bi2)n(Bi2Te3)m, (GeTe)n(Sb2Te3)m, and (GeTe)n(Bi2Te3)m homologous series are
generally expected to have (stable) structures with the same stacking period in the thermal equilibrium
when each series has the same values of n and m. In other words, all these homologous structures canbe characterized as trigonal structures with 2n+5m cubic close-packed periodicity. The residual of(2n+5m)/3 = 0 and ≠ 0 lead to the formation of crystals having structures with primitive (P) andrhombohedral (R) unit cells, respectively. In the former structures, a unit cell is identical to a basic
structural unit specific to the homologous structures mentioned above; on the other hand, in the latter
case, a unit cell comprises three cubic close-packed units [9]. The binary compounds can be obtainedby replacing Ge and Te layers in the pseudobinary compounds with Sb or Bi layers, and vice versa, i. e.,(GeTe)n(Sb2Te3)m ↔ (Sb2)n(Sb2Te3)m or (GeTe)n(Bi2Te3)m ↔ (Bi2)n(Bi2Te3)m.On the other hand, thestructure of the metastable phases of the GST materials is very similar to that of the A7 type structureof Sb or Bi. If Ge(/Sb) and Te randomly occupy the two kinds of the atomic sites, namely Cl and Nasites, the structure with the resultant atomic configuration corresponds to the latter structure. Thus GST

and Sb-Te compounds are very similar in structure with each other.
4. CONCLUSION

The crystalline and electronic structures of the GeTe-Sb materials are very similar to those of GSTmaterials; i.e., the atoms are orthogonally connected by p-p bond in common with both materials [8]. Itwas found that the sputtered films of these GeTe-Sb compounds crystallize into single phases with theA7 structure of the same type as Sb by an instantaneous laser annealing; however, these compounds
tend to readily separate into two phases. One is the diamond-type Ge (or NaCl-type GeTe) cubic phaseand the other is the A7-type Sb(-Te) trigonal phase. Interestingly, the sputtered and laser crystallized
Ge15Sb70Te15 film showed a completely different thermal response to an ingot Ge15Sb70Te15 made by amelt-quench method. The former showed phase separation at around 600K with increasing temperature,on the other hand, the latter had already shown a diffraction pattern of the two-phase coexistence asprepared. A Ge10Sb80Te10 sputtered film, which had formed a single-phase A7-type crystalline phasejust after laser initialization, gave the Bragg peaks of the diamond-Ge phase in the A7-type diffraction
pattern a few months later by keeping it at room temperature. On the contrary to these GeTe-Sbmaterials, Sb-Te binary compounds keep their single phase structures up to near their melting
temperatures; and besides, just after the transformations from the amorphous to crystalline phases, thestructures of these binary materials are expected to be A7-type structure irrespective of theircompositions. Thus Sb-Te compounds are very similar in structure with GST compounds; therefore, theamorphous and crystalline structures and material properties of Sb-Te binary alloys are worth
scrutinizing for us to develop future new phase-change materials.

REFERENCES[1] M. Wuttig and N.Yamada: Nat. Mater. 6, 824 (2007).
[2] E. Nishibori, M. Takata, K. Kato, M. Sakata, Y. Kubota, S. Aoyagi, Y. Kuroiwa, M. Yamakata,and N. Ikeda, Nucl. Instrum. Methods A 467−468, 1045 (2001).
[3] H. M. Rietveld, J. Appl. Cryst. 2, 65 (1969).
[4] V. Petříček, and M. Dušek: Jana2000 Crystallographic Computing Program, Institute of Physics,
Academy of Sciences of the Czech Republic, Praha (2000).
[5] G. L. Clark: Applied X-Rays (McGraw-Hill, New York, 1955)[6] Edited by T. Hahn: International Tables for Crystallography, Vol. A (Kluwer, Dordrecht, 1995).
[7] R W. G. Wyckoff: Crystal Structures, Vol. 1 (Interscience, New York, 1963).
[8] T. Matsunaga, R. Kojima, N. Yamada, K. Kifune, Y. Kubota, Y. Tabata, and M. Takata: Inorg.
Chem. 45, 2235 (2006).
[9] T. Matsunaga, N. Yamada, K. Kifune, Y. Kubota: JOURNAL OF CRYSTALLOGRAPHIC
SOCIETY OF JAPAN 51, 292 (2009). [in Japanese]

Intensity (arb. units)

40000

sputtered Ge15Sb70Te15 film
<dT/dt>=10º/min
5min for each measurement

30000

20000

D-Ge(111)

D-Ge(202)

D-Ge(113)

D-Ge(313)

D-Ge(224)

768K
651K
592K
475K
358K
289K

10000

05

1015

2025

2-Theta (deg.)

Figure 1. X-ray diffraction patterns for the sputtered Ge15Sb70Te15 crystalline film. No Bragg peaks are detected
at lower angles than 2θ=5° (not shown).

4.35

11.6

a (A7)

5.75

c (A7)

4.0

a (diamond)

0.25

0.24
11.4

3.0

5.70

0.23
2B (Å )

4.25

5.65

11.2

2.0z

Lattice constant (Å)

4.30

0.22

4.20

11.0

5.60

1.00.21

4.15

10.8
0500

1000

5.55
0500

1000

0.005001000

0.200500

1000

0500

1000

Temperature (K)Figure 2. Temperature dependences of the structural parameters and the atomic displacement parameters(isotropic temperature factors) B. In the structural parameters, a and c, show the unit cell edge lengths and z
represents the atomic position. The marks, ○ and ▲, in these figures represent those for A7-type SbTe, anddiamond-type Ge, respectively. Error bars are omitted for data with errors smaller than the marks.

GeTe
(202)

Intensity (arb. units)

40000

melt-quenched Ge15Sb70Te15<dT/dt>=10º/min
5min for each measurement

GeTe
(002)

30000

GeTe
(113)
GeTe
(004)

20000
GeTe
(111)

GeTe
(204)

689K
643K
551K
458K
365K
298K

10000

05

1015

20

25

2-Theta (deg.)

Figure 3. X-ray diffraction patterns for Ge15Sb70Te15 prepared by the melt-quench method. No Bragg peaks aredetected at lower angles than 2θ=5° (not shown).

Intensity (arb. units)

50000

melt-quenched Sn10Sb80Te10<dT/dt>=10º/min
5min for each measurement

SnTe
(002)

40000

SnTe(202)

30000

SnTe
(222)

SnTe
(111)

20000

SnTe
(004)

SnTe
(224)
866K
819K
726K
634K
542K
451K
361K
277K

10000

05101520252-Theta (deg.)

Figure 4. X-ray diffraction patterns for Sn10Sb80Te10 prepared by the melt-quench method. No Bragg peaks aredetected at lower angles than 2θ=5° (not shown).

Ge1Sb6Te10
49/51
33.0
32.0

RT
1:2

30.0
29.0

3:1

: Te

5/95
3/97(GeTe)n(Sb2Te3)m

28.0

18/82

27.0

75/25

3/97
5/95

stable phase

2:11:0
26.0
0.0

75/25

1:3: Sb

1:1

3V / atom (Å )Ge3Sb2Te6

0:1

metastable phase

31.0

: Ge

18/82

49/51
0.1

0.2

GeTe

0.3

0.4

0.5

0.6

0.7

0.8

0.9m/(n+m)

1.0

18/82

Sb2Te3

35/65

3/97
5/95

35/65

Ge1Sb2Te4

75/25
75/25

50/50

75/25

25/75

5/95
3/97

49/51

25/75

Ge1Sb4Te7

50/50

35/65
75/25

Ge2Sb2Te5

75/25
75/25

18/82

0/100

3/97

15/85

0/100

5/95

0/100

15/85

50/50

44/56

35/65

25/75

25/75

56/44

35/65

5/95

40/60

25/75

0/100

3/9715/85

0/100

44/56

75/25

25/75

56/44

75/25

n:m

Sb2Te3

18/82

35/65

3:1

2:1

18/82

15/85

50/50

1:1

0/100

49/51

1:2

1:3

0:1

Figure 5. Crystal structures of the GeTe-Sb2Te3 stable (homologous) phases. The inserted graph shows
composition dependence of the cubic root unit cell volume (V1/3) for the metastable and stable structures. In thesestructures, by appropriately replacing Ge and Te with 2xSb, we can obtain (Sb2)n(Sb2Te3)m homologous structuresinstead.