High Speed Crystal Growth in Phase-change Recording Materials for Practical Use
Toshiyuki Matsunaga a, Rie Kojima b, Noboru Yamada c, Kouichi Kifune d Yoshiki Kubota e, Sinji Kohara faDevice Solutions Center, R&D Division, Panasonic Corporation
3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan
Correspondence e-mail: matsunaga.toshiyuki@jp.panasonic.comb

AVC Networks Company, Panasonic Corporation / c Department of Materials Science & Engineering,
Kyoto University / d Faculty of Liberal Arts and Sciences, Osaka Prefecture University / e Graduate School of Science,
Osaka Prefecture University / f Japan Synchrotron Radiation Research Institute/SPring-8
ABSTRACT
Phase change recording is now extensively used for high density non-volatile memories. The typical phase-changematerials are GeTe-Sb2Te3 pseudo-binary alloys and Sb-Te based multinary alloys with some dopants such as
Ag-In-Sb-Te quadruple compounds. These crystals approximately have two kinds of crystalline, metastable and stablephases. The crystals of these metastable phases are very simple and can be approximated by cubic structures; on theother hand, those of stable phases have very complicated long-period layer structures. This structural feature, which isthat these materials have cubic-like crystalline phases, is one of major reasons for them to enable such high-speed
phase change. The crystal structures of these two phase-change recording materials surprisingly resemble each other;
all these crystals consist of 3+3 coordination structures in average irrespective of their phases. In addition, the atomic
configurations of these transient crystalline phases of both materials vary with temperature from such simple
cubic-like structures to very complicated long-period layer structures through atomic diffusion.
Key words: phase-change memory, GST, AIST, XRD, structural analysis, Rietveld method
1. Overview of structures and their transformations
Phase change recording is now extensively used for high density non-volatile memories [1]. Since 1970s, various
materials have been proposed for the purpose, and today we have obtained two superior materials of GeTe-Sb2Te3(GST) [2] and Sb-Te based alloys such as Ag-In-Sb-Te quadruple compounds (AIST) [3]. The crystals of thesematerials have two, stable and metastable phases; their structures are astonishingly similar to each other. When theiramorphous materials are annealed, metastable crystalline structures are first formed; after that, stable phases are built
up. The stable phases have long-period stacking structures with cubic close-packed periodicity; on the other hand, themetastable phases show six-layer structures. More precisely, that for GST metastable crystals is an NaCl-type
structure; on the other hand, AISTs exhibit A7-type structures. These two kinds of transient structures are, however,very similar to each other, although their structure types are different to each other. It is worth mentioning that allthese metastable and stable crystals can be also described as commensurately or incommensurately modulated
structures. We can find significant difference between the transformation of the metastable to stable crystalline phase,and that of the amorphous and metastable crystalline phase. It is presumed that the former is caused by atomic
diffusion; on the other hand, the latter is originated in interchanges of atomic bonds almost without the atomic
diffusion.
2. Structural details of the crystals
Two kinds of chalcogenide compounds, GeTe-Sb2Te3 and Sb-Te, have two kinds of crystalline phases, stable andmetastable phases. The stable phases have very complicated long period structures; on the other hand, the structures ofthe metastable phases are very simple and spatially isotropic like amorphous structures.

2-1) Stable crystalline phases
It has been found that in thermal equilibrium, these systems form
various intermetallic compounds represented by the chemical formulas(GeTe)n(Sb2Te3)m and (Sb2)n(Sb2Te3)m (n, m: integer), respectively. All
these compounds have trigonal structures with 2n + 5m cubic
close-packed periodicity without exception (more specifically, theresidual of (2n + 5m)/3 = 0 and ≠ 0 leads to the space groups, 31
and 3, respectively); and they are called homologous compounds
because of their similarity in structure. These two kinds of (pseudo-)binary crystals are isostructures to each other when n and m each arerespectively the same as those of the other. Their structures are
systematically characterized by the stacking of the (GeTe)n and(Sb2Te3)m or (Sb2)n and (Sb2Te3)m blocks along the  -axes, with very
long cell dimensions in the conventional three-dimensional (3-D)
structure description [4-13]. More generally and more precisely; it has
been assumed that these structures should be described ascommensurately or incommensurately modulated four-dimensional(4-D) structures characterized by modulation vectors q = γ·c*sub in thereciprocal space [14]. As seen in Fig. 1, in the real space, csub is thefundamental vector formed by three-layer cubic stacking. In an actual
structure modulated by another vector csub /γ along the c-axis, the shifts( ∆ ) of the atoms from their equivalent positions (k  ;  =±1, ±2, ⋯ ) can be written as∆  = ∆  + |

|/.

(1)

csub
csub /γqc*subFig. 1.
The crystal structure of Ge2Sb2Te5 andits diffraction pattern. The structure is shown inperspective, in which Ge, Sb, and Te are indicated bypurple, yellow and blue, respectively. The 0kℓ
diffraction plane is depicted in a square range of -1 ≤k ≤ 1 and 0 ≤ ℓ ≤ 15. The diameters of the reciprocal
points in the pattern are in proportion to the absolutevalues of their structure factors F(0kℓ). The atomic
layers along the cH -axis are arranged by two kindsof vectors in this structure: one is the fundamental
reciprocal vector formed by three-layer cubic
stacking and the other is a modulation vector*
expressed by q = γ ⋅ c sub . The former vector gives
fundamental reflections; on the other hand, the latterproduces superstructure reflections on the diffraction
patterns. The γ value of this crystal is a rational
number 5/3; which means that it has a commensurate
structure. In actual Ge2Sb2Te5 crystals, it is presumed
that the Ge and Sb sites are partially substituted by
Sb and Ge atoms, respectively [9].

This other vector csub/γ is called modulation vector. We can understandsuch modulation structures with a notion that the atomic positions witha base frequency are varied by another frequency, and can also regard
such a modulated structure as one kind of a distorted structure. When γs
are rational numbers, their structures correspond to commensurate structures; otherwise incommensurate structures.
For these (pseudo-) binary compounds, we can simply derive γ in terms of x as
γ = 2 - x/2,

(2)

when the chemical formula for these (pseudo-)binary systems are written as (GeTe) x (Sb2Te3) 1-x, or SbxTe1-x [14].
Therefore, only when γs are restricted to (discrete) rational numbers
written by 3(n+3m)/(2n+5m), they compose commensurate structures.
In the GeTe-Sb2Te3 pseudo-binary system, several intermetallic
compounds exclusively crystallized into commensurately modulated
structures, Ge3Sb2Te6 (n=3, m=1; 11R)[10], Ge2Sb2Te5 (n=2, m=1;
9P)[9], Ge1Sb2Te4 (n=1, m=1; 7R)[8], Ge1Sb4Te7 (n=1, m=2;12P)[12], and Ge1Sb6Te10 (n=1, m=3; 17R)[13], have been found up
to now to our knowledge; while it has been presumed that the Sb-Te
binary system contains not only such intermetallic compounds as the
GST materials but also a (wide) composition region seamlessly filled
Fig. 2.
Schematic view of frequency modulation.
with incommensurate SbxTe1-x compounds around Sb:Te=1:1. In
The frequency of the carrier signal (top) varies as thethese structures, all the constituent atoms have 3+3 coordination
voltage of the modulating signal (middle) changes to theatoms, which form distorted octahedral coordination same as in theradio frequency signal (bottom). The atoms seen in thismetastable structures. The central atom is more strongly bonded with
figure form a two-layer structure. They shift from thethe shorter three atoms than the other three.
equivalent positions owing to the frequency modulation.

2-2) Metastable crystalline phases
Just after the phase transformation from the amorphous phase, whichcorresponds to deleting recorded marks in optical disks, GST and AIST
amorphous compounds both crystallize into six-layer homologous structures.
The formers are B1-type (NaCl) structures; on the other hand, the latters are
A7-type (α-As) structures. The A7-type crystals hold 3+3 coordination
structures as well as other homologous crystals mentioned above. The
GeTe
Sboctahedra formed by these coordination atoms are oriented along the c-axis
directions. Recently, it has been found that GeTe also maintains 3+3
Fig. 3.
The structures of the GeTe
coordination structure even in its high-temperature cubic phase; however, the low-temperature phase ( R3m ) and Sb ( R 3 m )octahedra of this crystal are randomly distributed different from the case of shown in the hexagonal (thin lines) unit cells. The
structure of the high-temperature phase
AIST [18] (Not only in this binary compound but also in GST pseudo-binary NaCl-type
for GeTe drawn with thick lines is deformed by
cubic compounds; we believe that such coordination structures remain even the phase transition to the low-temperature phase.
in their cubic phases). These two structures are naturally very similar to each (Pseudo-) cubic cell is depicted with thick lines aswell also in the Sb structure model. This large
other as seen in Fig. 3, because they are ones of homologous structures in cubic cell is composed of eight small, pseudocommon. If Ge and Te atoms are randomly located at any atomic sites of the simple cubic cells. As seen in these drawings, both
crystals are six-layer structures, any constituent
GeTe crystal, its structure coincides with that of Sb. These structures are the atom has six (3+3) coordination atoms (three
simplest (γ=3/2) of all the homologous structures, and they are spatially atoms at upper layer + three atoms at lower layer).
isotropic, which can be well approximated by cubic structures. We have beenasserting that the materials that can take such simple and spatially isotropic structures just after the recrystallization
are suitable for high-speed phase-change recording materials; it is presumed that such a structural property is one of
the essential features for them because topological resemblance is required for reduction of atomic switch lengths
through the phase change.
2-3) Structural transformations in the crystalline phases
As mentioned in the introduction, NaCl-type GST (metastable) crystals transform to the stable trigonal structuresby giving sufficient heat treatments. These structural transformations are first-order phase transition phenomena. The
former crystals suddenly change to the latters when raising the temperature. These transition temperatures have been
examined by the differential scanning calorimetry [2]. For instance, metastable Ge2Sb2Te5 maintains its NaCl-type
structure up to 230 ºC; above this transition temperature, the crystal abruptly changes to 9-layer trigonal structure (seeFig. 1) with showing an exothermic peak. The γ values characterizing these structures are 3/2 and 5/3, respectively;
the above means that γ discretely changes from the formerz
to the latter at its specific transition temperature. In thezcases of Ge1Sb2Te4, it falls into 21-layer structure (γ=12/7)
Te
at 210 ºC. The metastable structures transform to the stablestructures through atomic diffusion as shown in Fig.4. OnTeGe
the other hand, different from these GST compounds, theyxatomic rearrangements in Sb-Te crystals show second
Te
Ge, Sb
Ge Sb
25% vacancy
order-like phase transformations.
Ge Sb
Te
Te

25 Ge75Sb

Te
Te

25 Ge75Sb

Te

50 Ge50Sb

12.5

Te

25 Ge75Sb

12.5

Te
Te

25

25 Ge75Sb

12.5

25

Te

50 Ge50Sb
25Ge50Sb

Te
25Ge50Sb

Te
12.5Ge25 Ge75Sb

Te

Te
Te
25 Ge75Sb
Te
50 Ge50Sb

Te
25Ge50Sb

Tex

Fig. 4.
The schematic drawing of the atomic diffusion in themetastable GeSb2Te4 crystal. The diffusion causes the structuretransition from the metastable phase (on the left) to the stable phase (onthe right). The Ge/Sb layer in the metastable phase includes
vacancies up to 25 at.%. It is logical to assume that the atomic
diffusion of Ge and Sb towards the vacancies, as shown in this figure,generates the structure transformation from the former to the latter.

Sb-Te materials are used in practice as phase-change
recording materials by doping third and/or fourth elements
such as Ag and/or In to improve the endurance of theamorphous phase and raise the transition temperature to the
crystalline phase. The diffraction patterns in a heatingprocess
obtainedforthesputtered
AIST(Ag3.4In3.7Sb76.4Te16.5) amorphous film are shown in Fig. 5[15]. The amorphous phase transformed into an A7-type
crystalline single phase, as seen in this graph, at (around)
416 K. This six-layer crystalline phase is kept up to around545 K; however, phase separation written by the following equation
takes place above this temperature.
Ag3.4In3.7Sb76.4Te16.5 → Ag3.4In3.4Te6.8 + In0.3Sb76.4Te9.7.

(3)

The first decomposition product has a CuFeS2-type structure, which
was tightly held up to the melting temperature around 870 K. The
second product, which can virtually be regarded as an Sb-Te binary
compound written as Sb89Te11 (when expressed as a percentage),
gradually changes its atomic arrangement with increasing
temperature until obtaining the final stable crystal structure. The
atomic configuration determined for the latter is shown in Fig. 6 as
an approximated 3-D structure with γ = 45/29. As seen in Fig. 7, γ
maintained a constant value of 1.5 up to a temperature of around 591
K, at which AgInTe2 (CuFeS2-type structure) came out. However,
above this temperature, γ continuously grew larger with increasing
temperature and reached a value of around 1.55 at high temperaturesnear the melting point (~870K) of Sb89Te11. This γ value
corresponded well with the value of 1.5565 expected from thecomposition of Sb88.7Te11.3 [see Eq. (2)].

Fig. 5.
Temperature dependence of the X-ray
powderdiffraction
proﬁlesforasputteredAg3.4In3.7Sb76.4Te16.5 amorphous film in a heating process.
The amorphous halo patterns are observed at low
temperatures from 295 K to 416 K; however at around
416 K, the Bragg peaks of A7-type structure appear in thehalo pattern. As temperature is raised further, the A7-typesingle phase separates into two phases, AgInTe2 and an
Sb-Te binary compound, at around 591 K. At lower 2θ
angles than 5º, Bragg peaks were hardly observed at any
measurement temperatures.

As has so far been shown, the structural transformations withtemperature in the two materials are notably different from each other.
In the GST pseudo-binary materials, γ abruptly changes withtemperature at first-order phase transition; on the other hand, in the
Sb-Te binary materials, it continuously varies. The latter variation
behaves like a second-order phase transition phenomenon. As formaterials with Ge atoms, on the other hand, it is conceivable that such obvious first-order phase transitions observedin these materials will probably result from simultaneous breaking of sp3-electronic configurations of Ge atoms at the
specific temperatures.
cFig. 6.
Structural model of Sb26Te3 (820 K), which is approximated by 3D commensurately modulated structure. The atomic positions of Sb and Te
are shown by yellow and blue, respectively. The light purple and chartreuse green inserted in the figure respectively show csub and csub /γ which characterize
this (long period) modulated structure.

3. Structural details of the amorphous phases
In GST amorphous materials, three constituent atoms are strongly
connected with a few specific neighboring atoms. Coordination
numbers for three constituent atoms approximately follow the 8-N rule,different from those of the crystalline phase; for instance, those of
a-GST225 are ca 4, 3, and 2.5 around Ge, Sb, and Te atoms respectively(only for Te atoms, about half of them have two coordination atoms andthe other half three). The bond lengths are also shorter than in the
crystalline phase. These bonding schemes show the characteristics ofcovalent bonds often observed in chalcogenide amorphous materials.
The atomic configuration is maintained almost unchanged up to thetransition temperature to the crystalline phase [16], [17]. It has been
considered that such a strong covalent bonding nature gives theseamorphous materials sufficient endurance, namely, long-term data
preservation. In these GST amorphous materials, if three kinds of atoms

Fig. 7.
Temperature dependences of modulation
period (○○) obtained from 4-D structural analyses for
Sb89Te11. That for corresponding GST material(x=0.89) is, on the other hand, expected to vary
stepwise with temperature as shown by dotted line [2].

ABAB squareconnect with each other with equal probability, six kinds of atomic pairs should befound at the ratio provided by a random covalent network model. However, GST
amorphous materials comprise a far larger number of Ge-Te and Sb-Te atomic
pairs than the mean values given by a random covalent network model. This isvery interesting because the crystalline phase is formed only by these two pairs. In
a-GST225, 40% of the rings are fourfold or six-fold and a considerable amount of
the relict GeTe4 tetrahedra and SbTe3 pyramids whose bond angles are distributed
around 90º as same as those of the crystalline phase [18], [19]; we can find even
NaCl-type crystalline fragments themselves in the amorphous atomic network as
seen in Fig. 8. These fragments are innumerably embedded in the amorphous
atomic network; then, once sufficient heat energy is supplied to the amorphousmaterial, those crystalline fragments act as first nuclei to crystallize theirsurroundings all at once.

Fragment of
NaCl lattice

It was revealed in AIST materials that both a-AIST (left panel) and c-AIST(right panel) assembled (distorted) 3 + 3 octahedra into their structures as shown in
Fig. 8.
The amorphous structure Fig. 9. In the amorphous state, interatomic distance between the central atom and(section of 460-atom) solved by a DF-MD
method model of a-GST (24 Å × 24 Å × 12 three shorter coordination atoms is considerably shorter than in the crystallineÅ) [*]. Ge, red; Sb, blue; Te, yellow; large state; they are connected to each other by a stronger covalent force than those incavity, pink. A large cavity is present the crystal to form isolated molecule by themselves. From a topological point of
among the atomic arrangement in thisstructure. Enlarged configuration shows view, one coordination polyhedron (in this case, an octahedron) composes twothat a-GST has many structural units tetrahedra; of the two, the one formed by shorter bonds is a basic molecular unit in(ABAB-squares) with alternating Te andGe (or Sb) atoms. We can observe even a the amorphous structure. These molecules are weakly connected by vertices orfragment of NaCl lattice.
edges; a spatially periodic arrangement is not required of them. For this reason, theorientation vectors made by green arrows are randomly oriented as seen in the left
bottom panel of Fig. 9. However, once sufficient heat energy is given to the amorphous material, it causes interchangeof one bond in a 3+3 coordination octahedron to rearrange, parallel neighboring two vectors. This rearrangement
swiftly propagates from neighbor to neighbor forming the crystalline preferred orientation. This amorphous material
contains few crystalline embryos (atomic rings or clusters) as
observed in amorphous GST; then crystal growth is much more
dominant in the structural transformation from the amorphousto the crystalline phase.
4. basic structure maintainedtransformation

during whole phase

The atomic configuration in the AIST amorphous material,
which has already been revealed by Matsunaga et al. [20], isheavily disordered, and has a spatially isotropic symmetry.
However, it has also been revealed that it serves 3+3
coordination structures even in such a disordered atomic
arrangement, as well as that of the crystalline phase: that is, thecoordination structures of both phases are very alike, i.e., they
have very similar local atomic arrangements (it is well known
that an A7-type crystal has a 3+3 coordination structure [21],[22]). This is one of major reasons that this material achieves asufficiently high phase-change speed by locally minimal bondinterchanges. As for the dopants such as Ag or In, it has beenpresumed that the presence of these atoms, separately or
together, serves to raise the crystallization temperature of theamorphous phase to obtain a sufficient endurance for long-term
data preservation. As well as the AIST crystal, it has been

①

③

②

④

(a) amorphous⑤(b) crystalline

Fig. 9.
The central atom with three short (red) and threelong (dashed) bonds crosses the center of the distorted
octahedron, interchanging a short and a long bond. Green:
resultant vector of short bonds. The bonding electrons are
excited by laser light (hν) to cause an interchange of bonding
electrons, for example, between the two bonds pointed by theyellow arch, which moves the atoms in the amorphous phase toform locally crystalline atomic configurations; as shown in thelower panels, the local crystalline atomic networks finally form
resonant bonding between periodic short and long bonds, whichleads to the crystalline A7 network. The shade plane shows adomain wall between crystalline phase and amorphous phase.

revealed that GeTe or GeTe-rich GeTe-Sb2Te3 NaCl-type (metastable) crystals also have 3 + 3 coordination structureeven in its high-temperature cubic phase [23], and that the three kinds of constituent atoms of these amorphousmaterials have the coordination numbers of approximately four for Ge, three for Sb, and two or three coordination
atoms for Te, fairly well satisfying the 8-N formation of Mott’s rule [24]. Then, also in these amorphous materials, theaverage coordination number corresponds to three. These amorphous materials intrinsically contain an obvious
number of crystal-like fragments in their atomic configurations [25], [26]. It is then very likely that, once sufficientenergy is given to them by laser irradiation or ohmic heating, crystallization starts from these crystalline embryos all
together; and whole amorphous mark are instantaneously crystallized by slight atomic shifts with bond interchanges
around the embryos in the same manner of the crystal growth process revealed in AIST.
5. Summary: Physics for high-speed phase change revealed by the structural analysis
In the amorphous phases of the chalcogenide phase-change materials, the atoms are connected by relatively strong
covalent bonds; the bonding electrons are localized between them. These atoms locally cohere to form molecules inthe system. The energy of the whole system and the configurational entropy are higher than those of the crystalline
phases. To cause the transformation from the amorphous to crystalline phase, activation energies are required beyond
the thresholds. In other words, sufficient energy is given to the system; the amorphous state is excited, and the
localized bonding electrons are used as resonance bondings in the crystalline state. As the results of the phasetransition, the free energy of the system decreases. Fortunately, these chalcogenide materials are generally susceptible
to visible lights; because their band gaps are sufficiently narrow not only for the crystalline states but also for theamorphous states, and they contain high-density electrons just below the Fermi energies. The narrow-band gap meansthat bond interchange from covalent bond to resonance bond can easily take place by the incident photons and thermal
phonons. In other words, these materials can be instantaneously heated up to the temperatures to cause rapid phasechanges to their crystalline phases by laser irradiation. Furthermore, the thermal conductivities of these chalcogenide
compounds are surprisingly low even in their crystalline phases, which means that local area of them can be quickly
heated up by less energy. Both compounds crystallize to very long-period layer structures in the thermal equilibrium;
however, by a happy chance, they have simple cubic or cubic-like structures in common. In addition, their atomic
configurations resemble well those of their amorphous phases. This structural feature for these materials enables
high-speed phase change from the amorphous to crystalline phase. The amorphous structures can be changed simply
by bond interchanges, without atomic diffusion.
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