Global and local structures of the Ge-Sb-Te ternary alloy system for a phase-change
memory device
Jae-Hyeon Eom,
1
Young-Gui Yoon,
2
Changwon Park,
1
Hoonkyung Lee,
1
Jino Im,
1
Dong-Seok Suh,
3
Jin-Seo Noh,
3
Yoonho Khang,
3
and Jisoon Ihm
1
1
School of Physics, Seoul National University, Seoul 151-747, Korea
2
Department of Physics, Chung-Ang University, Seoul, Korea
3
Materials Center, Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-Ri, Giheung-Eup, Yongin-Si, Gyeonggi-Do,
449-712, Korea
Received 14 October 2005; revised manuscript received 27 March 2006; published 6 June 2006
A detailed theoretical investigation on the global and local structures of the Ge-Sb-TeGST ternary alloy
system for the phase-change memory is presented. We examine the cohesive energy of the GeTe
n
Sb
2
Te
3
m
homologous series as well as the dependence of the energy on the atomic distribution. We show that the
cohesive energy decreases with increasing vacancy concentration and the vacancies repel each other to mini-
mize the number of dangling bonds. In Ge
2
Sb
2
Te
5
, Sb and Ge atoms favor two-dimensional layered and
three-dimensional agglomerated arrangements, respectively. In Ge
1
Sb
2
Te
4
, on the other hand, Ge atoms tend
to form a two-dimensional layered structure. Possible structural building blocks of the GST system are pro-
posed based on the density-functional theory total energy calculations.
DOI: 10.1103/PhysRevB.73.214202 PACS numbers: 71.15.Nc, 77.84.Bw
I. INTRODUCTION
Recently, the rapid growth of the multimedia technology
has demanded fast and small rewritable nonvolatile memo-
ryNVM devices as essential components. Various concepts
for the next-generation NVM have been proposed so far.
1
Among the candidates, the phase-change memoryPCM uti-
lizes the resistivity difference between crystalline and amor-
phous phases.
2
It is noted that a typical resistivity ratio be-
tween the amorphous and the crystalline phase is more than
1000.
3,4
This huge difference in resistivity establishes the
PCM as a strong candidate for the next-generation NVM.
For the PCM, stable amorphous and crystalline phases real-
izable in a single material are required. Ge-Sb-Te GST ter-
nary alloys are so far known as the best for the PCM due to
their stability 10 years and the fast switching time
100 ns. The GST is already used for rewritable compact
disks and digital video disks, taking advantage of the differ-
ence in reflectivity between two phases. However, knowl-
edge of the structure and basic physical properties of the
GST is still primitive, and the fact that the GST is a ternary
alloy makes the experimental and theoretical analysis very
difficult.
Admitting that the GST has a complex composition and
structure, some experimental data still exist which can guide
us into a partial understanding of the system. First, the phase
transition between the crystalline and amorphous phase is
very fast and reversible, which implies that the structure of
the amorphous phase may not be totally different from that
of the crystal. Therefore, the crystalline phase of the GST
could presumably provide some essential information on the
amorphous phase. The GST is known to show two kinds of
crystal structures; namely, the rocksaltNaCl-like and
hexagonal.
3,5,6
When the amorphous GST is heated, transi-
tion to the rocksalt-like structure occurs around 400 K, and
subsequent transition to the hexagonal structure around
500 K. The hexagonal structure is stable at low temperature
6
and the rocksalt-like structure is a metastable phase. In prac-
tice, only the metastable phases occurs in fast transition to
the crystalline phase. Since the phase transition between the
rocksalt-like metastable and the amorphous phase is used in
the operation of the PCM, we will now concentrate on the
metastable rocksalt-like structure.
X-ray diffraction study shows that 4a sites constituting
one of the two sublattices of the NaCl structure are fully
occupied by Te, and 4b sites are occupied by Ge, Sb, and
the vacancy. Second, GST ternary systems are composed of
GeTe and Sb
2
Te
3
binary alloys. This means that composition
of the GST system is given by GeTe
n
Sb
2
Te
3
m
with inte-
gers n and m. Moreover, a recent study on the phonon mode
of the GST system indicates that there is a local order in
units of GeTe and Sb
2
Te
3
.
7
We may expect that the distribu-
tion of Ge and Sb atoms, and vacancies on 4b sites are not
completely random, and the metastable phase of the GST
system is a somewhat distorted rocksalt structure maintain-
ing some local order of the structural subunits of GeTe and
Sb
2
Te
3
. It is noted that recent studies on the local structure of
the GST show that the crystal structure is distorted, and there
exist long and short bonds.
8–10
A building block of the
Ge
2
Sb
2
Te
5
unit has been proposed in the literature based on
the extended x-ray absorption fine structure data.
8
However,
their model is two-dimensional and does not include the
structure of the subunits GeTe and Sb
2
Te
3
; a more realistic
structure of the GST with a three-dimensional building block
originating from GeTe and Sb
2
Te
3
is desirable.
In this work, we calculate the cohesive energy of the
GeTe
n
Sb
2
Te
3
m
homologous series. We also investigate
the dependence of the energy on the distribution of the Ge
and Sb atoms and vacancies for Ge
2
Sb
2
Te
5
and Ge
1
Sb
2
Te
4
,
and propose a possible local structure based on the ab initio
supercell electronic structure calculations.
II. COMPUTATIONAL METHODS
Electronic structure calculations were performed using the
self-consistent ab initio pseudo-potential method with the
PHYSICAL REVIEW B 73, 214202 2006
1098-0121/2006/7321/2142025 ©2006 The American Physical Society 214202-1