Engineering Meta-materials for Future Phase-Change Memory*Junji TOMINAGA, Robert SIMPSON, Paul FONS and Alexander KOLOBOVFunctional Phase-Change Device Group (FunPC), Nanodevice Innovation Research Center (NIRC), National Instituteof Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, 305-8562, Japan

Abstract
Phase-change materials have already been installed as a recoding film in rewritable optical storages: DVD-RAM, -RW,and Blu-ray. GeSbTe alloys have still been the best materials in the optical storage media as well as phase-change
nonvolatile solid state memories. The alloys are pseudo-binaries composed of GeTe and Sb2Te3 compounds. Up tonow, the phase-change films have usually been deposited as a single composite layer by physical deposition techniques,
and all the physical & chemical properties reported are of the films. In the requirements of faster switching speeds,
longer cyclabilities, and more reliabilities in the amorphous and crystalline states, the fine control and advancement ofthe alloys are now urgent issues, especially in nonvolatile solid state memory using the phase-change mechanism.
From the requirement, our group has engaged in fabricating atomically controlled phase-change film (now we call“Phase-Change Meta-material, PCMM”) of GeSbTe, based on the first principle computer simulation since 2007.
PCMM is unlike a “superlattice-like” film reported by DSI Singapore [1], is fabricated as a crystal at a high temperature
under a strongly oriented crystalline growth condition. Here we report the computer models and real films fabricatedbased on the simulations, and compared the electrical properties on a real PCRAM device with the same composition
single film.
Keywords: phase-change film, Ge-Sb-Te, nonvolatile solid-state memory, PCRAM

1. Introduction
Phase-chnage solid state memory (PCRAM) has recently attracted a great attention as a new class memory on thepoint of view of nergy saving and green technology. The switching mechanism is the same as that in DVD-RAM,
except for the difference of a method to introduce input energy: laser pulse or electrical pulse current. In both cases,
energy is required for melting and crystallization from an amorphous state. As suspected, however, the input energy
especially for Reset (crystal to amorhous switching) is relatively higher than that of competitors like MRAM andReRAM unfortunately. The high energy for Reset has been one of the fatal issues to be overcome and to win thecompetition with the other memories. Is it possible to decrease the swithing energy in phase-change memory? The
answer is probably yes on the pojnt of view of thermodynamics. Once succeeded in reducing entropy due to phasechange, the swithing energy can greatly be decreased.
According to our hexagonal model [2, 3], the Ge2Sb2Te5 alloy may be thought of as alternately layered sheets ofGe2Te2 and Sb2Te3. We simulated both the crystalline [Ge-6 fold coordination] and amorphous [Ge-4 fold coordination]
structures consisting of [-Te-Sb-Te-Sb-Te-Ge-Te-v-Te-Ge-]n and[-Te-Sb-Te-Sb-Te-Te-Ge-v-Ge-Te-]n stacks
respectively using ab-initio density functional computation: wherev is an imaginary (vacancy) layer. The
refractive indices of these modelled structure was then compared with those experimentally measured. We havepreviously shown that two hexagonal structures are bistable with an energy difference of 40.6 meV/atom according tolocal density approximations (LDA) sumulation. The former has a cell volume of 256.13 Å3, while the latter has avolume of 261.21 Å3; the difference is 2.0%, which is smaller than that of the NaCl-like rundom composite material: 67% [4]. The lattice parameter c of the P3-m crystal with 6-fold Ge atoms is 17.786 Å, while that with 4-fold Ge atoms
is 18.255 Å. Thus, the volume change during the phase transition is a result of the c parameter shortening, and theactivation energy of 2.4 eV is dissipated by the Ge flip-flop transition [5]. The refractive indices experimentallymeasured and theoretically calculated are consistent with each other for the crystalline, 6-fold coordinated, Ge atoms
over the visible spectral range. Moreover, the imaginary index of the crystal with 4-fold Ge atoms was approximately
half that of the crystal with 6-fold Ge atoms; comparable with experimentally observed reflectivity changes in digital
versatile disc random access memory (DVD-RAM) using the same, Ge2Sb2Te5, composition although the material waspoly-crystalline. During crystallization, the Ge and Te atoms change position with each other along the c-axis, resulting
in the change from 4- to 6- fold, Te bonded, Ge atoms and vice versa in the amorphisation process. In the swithing
procedure, it should be noticed that he Sb2Te3 blocks are unchanged and serve as a skeleton for the whole structure.
This model is similar to the host-guest phenomena in intercalation chemistry. Thus, Sb2Te3 layers are the host, while
Ge2Te2 layers are the guest. The host layers hold the reactions or transitions of the guest molecules sandwiched between
them. Several hexagonal models have been reported [6,7], however, none of them are based on the aforementioned
Ge-flipping transition. Realizing the above atomic layer structure can greatly save entropy in the phase-change,resulting in reducing the input energy to PCRAM.

2. Entropy due to Phase Change
Using a phase-change meta-material (PC-MM) structure enables the phase-transition at a lower power than using
a single film of a PC alloy because of the suppression of the entropy dissipated in the switching cycle: Set (amorphousto crystal switching) and Reset (crystal to amorphous swithing). In thermodynamics, the input electrical energy must betransfered into the internal energy of the cell (∆E), the work for the phase-change (Wpc), and the entropy part of the totalenergy (T∆S). T∆S can be roughly estimated using the Sackur-Tetrode equation for an independent atomic canonicalensemble system [8]. Although the equation is for an ideal gas system and for a regular solution, it may give the upper
limit to an entropy of a real system including covalent bonds.
The partition function q is given by the average free space in a lattice volume, V.
q = 1/9・{2πM/(h2β)}3/2V
Where M is the atomic (molecular) weight, β=1/kT, h is Plank’s constant and k is the Boltzman constant.
partition function for the canonical ensemble Q is given by:
Q = qN/n1!/n2!/n3!, N= n1 + n2 + n3

(1)
The total(2)

N is the total number of atoms in the ensemble, and n1,2,3 are the partial numbers of independent atoms. Using equations(1) and (2) at 300oC and assuming a phase changed volume forms a hemispherical dome with a 75-nm diameter. Inaddition, ignoring the energy to heat a heat element (rod) contacted to a phase-change layer. In actual devices, a heatelement is inevitable to heat up a phase-change film by the melting temperature.
Applying equations (1) and (2) to a ternary condition, for the case of using a single film of Ge2Sb2Te5, T∆Ssingle isestimated to be 4.20 x 10-12 J. On the other hand, the dome requires 5.20x10-14 J as internal energy gain ∆E, assuming
the heat capacity Cp(GeSbTe) to be 0.25 J/g/K, a value measured in our laboratory. Therefore, almost 99% of the energy iswasted or released in the form of entropy in the case of a conventional device. Hence, Wpc is assumed to be theactivation energy of 2.34 eV, which corresponds to 1.53x10-13 J [3]. By modifying the Sackur-Tetrode equation for
the PC-MM structure, the entropies of multilayer structures and PC-MM cells can be estimated. Assuming a
superlattice-like structure and the PC-MM cells that can be divided into 10-stacked layers with equal thickness, in the
superlattice-like cell, Ge, Te and Sb atoms are allowed to move freely in three dimensions within each layer. For thisstructure, the entropy energy T∆Ssuperlattice-like is reduced to 4.26x10-13 J, which is ~11% of T∆Ssingle. In the PC-MM cell,on the other hand, the Ge and Te atoms in the GeTe layers are limited to movement in one dimension (along the c-axis)
based on the Ge flip-flop model; the motion of Te and Sb atoms in Sb2Te3 layers are fixed [2,3]. Leading to the changeof the superscript 3/2 into 1/2 in equation (1), the estimated entropy energy T∆SMM is 4.53x10-14 J, and is decreased by~1% of T∆Ssingle. That is, using of the PC-MM film allows for a 90% energy saving for the Set operation in comparison
to a conventional PCRAM device. As the partition function in the calculations reduces logarithmically with thenumber of stacked layers, our simple calculations can reproduce well trend in the experimental results. For the Reset
operation, the same calculation is applicable (see Table 1). In conclusion, use of a PC-MM film allows the input energy
to be transfered more efficiently to cell heating ∆E and Wpc by depressing T∆S. Conversely speaking, currently released
PCRAM devices only use one or two percent energy from the imput energy. All the computer simulations reported inPCRAM have only dealed with an equation of theat diffusion, and estimated temperature profiles in the memory cell.
However, such a large entropy dissipation of the imput energy was not discussed so far. Once based on the entropy
supression, the priority of our PC-MM structure is soon understood.

3. PC-MM Design &Fabrication
It is very much intriguing to actually fabricate, to realize, and to estimate the devise characteristics of the PC-MMstructures: [-Te-Sb-Te-Sb-Te-Ge-Te-v-Te-Ge-]n and [-Te-Sb-Te-Sb-Te-Te-Ge-v-Ge-Te-]n. Our thermal analysis studyof [(GeTe)2/(Sb2Te3)1]n stacks showed a strong crystallization temperature dependence on the Sb2Te3 layer thickness[12].
The Sb2Te3 crystallization temperature, Tc(Sb2Te3), rose from 90-100oC to 203oC, as the Sb2Te3 layer became thinner andapproached that of a monolayer. On the other hand, the GeTe crystallisation temperature, Tc(GeTe), marginallydecreased from 226oC to 208oC as a function of thickness between 0.5 nm and 5.0 nm. Further, the first small
endothermic peak, indicating a melting or amorphisation event, Tendo, was independent of layer thicknesses andoccurred at 399 ± 3 oC. The results show that both sub-layers are put under increasing tensile stresses as the layer
thickness is reduced. After the phase transition, the multilayer is further exposed by a stronger tensile stress against
the substrate. Therefore, the endothermic peak which, most likely, corresponds to the melting point of the Te-rich
GeTe phase, does not change due to the energetically favourable volume expansion of the melt. According to theseresults, the ideal PC-MM must be fabricated at a temperature between Tc(GeTe) and Tendo, otherwise internal stresses
generated during the deposition produces dislocations or a multi-crystalline film. Figure 1 shows a large high resolution
transmission electron microscope (TEM) image of the [(GeTe)2/(Sb2Te3)1]20 PC-MM (corresponding to Ge2Sb2Te5
composite alloy) deposited upon a Si (100) surface. The PC-MM was deposited by a helicon-wave magnetron
sputtering system at 250oC. A very smooth and laminar film was obtained with few defects. The electron diffraction
showed a strongly 111 oriented structure normal to the substrate surface.

3. Device Performances
Until now the PC-MM device characteristics have not been compared against those of a device based on a single
composite Ge2Sb2Te5 film. Chong et al. Reported the characteristics of superlattice-like phase-change films, whichwere deposited at room temperature with layer thicknesses between 4 nm and 20 nm [1,9]. The films composed of

randomly-oriented, superlattice-like fragments, high-temperature annealing at 200oC for 5 min, except for small
periodic cycles [1].
We prepared base devices consisting of a TiN heating rod with a diameter ranging from 50-nm to 75-nm. The
thickness of the MM film was fixed at 35 nm, and was capped by a 40-nm thick TiN electrode. Three different PCMMs: [(GeTe)4/(Sb2Te3)2]10 and [(GeTe)2/(Sb2Te3)4]8, and a composite single layer of Ge2Sb2Te5 as a reference wereprepared. The resistance-current (R-I) and cycling characteristics of the three devices are summarized in Table 2. Inaddition, a FFT image of electron diffraction pattern observed just above the heater element was similar to a spot
pattern simulated from the PC-MM of [(GeTe)2/(Sb2Te3)4]8 model with Ge-4 coordinations, after a cycle test terminatedat Reset condition. The Set and Reset pulse lengths were fixed at 500 ns and 100 ns, respectively. Comparing the ratioof the switching energy required for the [(GeTe)2/(Sb2Te3)4]8 device with that of the Ge2Sb2Te5 single-layer device, one
finds only 0.25 and 0.34 of the energy is used for setting and resetting respectively, assuming the switching speed wasconstant in each device. The reset energies in Table 2 are reasonable values in comparison with Table 1, although theinternal energy gain from the heater element is underestimated. However, a more interesting matter is that the reset
energy becomes smaller as the (GeTe) block thickness is thinner. If the GeTe layer is thick, Ge flip-flop switch has
more degrees of freedom in three dimensions based on our entropy model, tending to the same value of the superlatticelike or finally the composite film. The device results using the MM films provide a strong evidence that the Ge flipflop switch is realized in a PCRAM device and a substantial energy saving is possible as a result. PC-MM willprobably revolutionise the phase-change memory device in the near future, with high-speed and low power switching
performances beyond any other nonvolatile memory devices.

5. Summary
We have succeeded in fabricating PC-MM films composed of [(GeTe)2/(Sb2Te3)1]20, [(GeTe)4/(Sb2Te3)2]10 and[(GeTe)2/(Sb2Te3)4]8 blocks and confirmed the device characteristics. Calculations have also shown that a significant
switching energy saving can be achieved by suppressing entropy, using PC-MM films. We argue that the PC-MM
device is no longer operated by the commonly accepted mechanism depending on amorphous to crystal phase transition,
but by a Ge flip-flop switch intercalated between Sb2Te3 blocks without any melting transition.

Acknowledgement
The work was supported by a project :“Research and Development of Nanoelectronic Device Technology” ofNEDO.

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Table 1 Switching energy required in Set & Reset, and entropy dissipations in the processes. In the model, theinternal energy gains of a heater element and of a crystal area surrounding the hemispherical dome (adiameter of 75-nm) were both ignored.
PC film structureT∆S (Set at300oC) (J)Total energy required in
ideal Set (J)∆HSet=∆E+Wpc+T∆S

T∆S (Reset at1000oC) (J)Total energy required in
ideal Reset (J)∆HReset=∆E+Wpc+T∆SSingle film

4.20 x10-12

4.41 x10-12

9.40 x10-12

9.81 x10-12superlattice-like [1]4.26 x10-13

6.31 x10-13

9.55 x10-13

1.37 x10-12

PC-MM

4.53 x10-14

2.50 x10-13

1.02 x10-13

5.15x10-13

Figure 1

PC-MM film composed of [(GeTe)2/(Sb2Te3)1]20 blocks deposited on Si wafer at a stress-free condition

Table 2

Device properties of three devices operated at 100 ns for Set and 500 ns for Reset.
Set (V)

Iset at 106 cycle(mA)

Reset(V)

Ireset (mA) at 106cycle

Reset energy (J)Ge2Sb2Te5 composite

3.0

6.0

0.30

1.25

7.5x10-10[(GeTe)4/(Sb2Te3)2]10

2.5

5.0

0.15

0.90

4.5x10-10[(GeTe)2/(Sb2Te3)4]8

1.5

3.5

0.15

0.73

2.6x10-10