S32

Crystallographic Analysis of SnXTe100-X/Sb2Te3 Superlattice andProposal of its Low Switching Power Mechanism
Susumu Soeya,1 Toshimichi Shintani,1 Takahiro Odaka,1 Reiko Kondou,1, 2 and Junji Tominaga1, 2
1Collaborative Research Team Green Nanoelectronics Center,
National Institute of Advanced Indusrtial Science and Technology (AIST)West 7A, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
2
Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japansusumu-soeya@aist.go.jp
We performed XRD measurements on previously discovered SnXTe100-X/Sb2Te3 superlattice(SL) with ultra-low switching power (X=10, 20, 35, and 50 at.%), and then analyzed its crystalline
structure for proposal of the ultra-low switching power mechanism. XRD showed that only a little
SnTe/Sb2Te3 SL and major NaCl-type SnSbTe phases coexist for X=50, and that only a little
SnTe/Sb2Te3 SL, major Bi2Te3-type SnSbTe, and Te phases coexist for X=10, 20, and 35. The
detailed crystallographic analysis showed as follows. There was a high possibility that both theNaCl-type and Bi2Te3-type SnSbTe-alloy phases have SL structures themself: “Self-assembly SL”,
with the vacancy layer in the specific (Sn, Sb) and Te layer, and that something new phenomenonsuch as Sn switching in the SnSbTe self-assembly SL leads to the ultra-low switching power.
Ⅰ. INTRODUCTION
Recently, our group discovered SnXTe100 － X/Sb2Te3 diluted SL device which showed theultra-low switching power.1-3 The consumed power was approximately 1/2250th (X=10 at.%)～
1/15th (X=50 at.%) that of the GST225 device.1-3 The XRD results showed that only a littleSnTe/Sb2Te3 SL and major NaCl-type SnSbTe-alloy (only a little SnTe/Sb2Te3 SL, major Bi2Te3-type
SnSbTe-alloy, and Te) phases coexist for X=50 (X=10, 20, and 35) (later Figs. 1 and 3 references).
Since the SnTe/Sb2Te3 SL phase was only a little, the cause of the ultra-low switching power may beso-called Sn switching in the SL phase1 seemed to be unreasonable.
The purpose of this current study is, therefore, to carry out the crystallographic analysis of theSnSbTe phase, and afterward to propose the ultra-low switching power mechanism.
Ⅱ．EXPERIMENTAL
Sb2Te3 (10 nm)/[SnXTe100－X (1 nm)/Sb2Te3 (4 nm)]9 SL film (X＝10, 20, 35, and 50 at.%) was
deposited at substrate temperature of 200℃ by using a dc magnetron sputtering apparatus. XRD
measurement (Cu Kα) was carried out to analyze the crystalline structure of the SnSbTe phase.
The structure of the SnSbTe phase with NaCl-type for X=50 (Bi2Te3-type for X=10, 20, and 35)
was examined by using the following procedure (a)－(d): 6(a)Experimental integral peak-intensity ratio of (I111/I222)Obs. ((I006/I009)Obs.) was estimatedfrom the SnSbTe(111) and (222) peaks (SnSbTe(006) and (009) peaks) obtained in the XRD
experiments. (I111 and I222 (I006 and I009): integral peak-intensity of SnSbTe(111) and (222) peaks(SnSbTe (006) and (009) peaks)).
(b)
Conceivable vacancies were taken in the SnSbTe structure, and modeled.
(c)
Theoretical integral peak-intensity ratios of (I111/I222)Cal. ((I006/I009)Cal.) were estimated for
all the modeled SnSbTe structures by using Eqs. (1) and (2).

2

2I hkl ≈ Fhkl × LZ × P = Fhkl ×

1 + cos 2 2θ×P
sin 2 θ cos θ(1).

NFhkl = ∑ f n e 2πi (hx (n )+ ky (n )+lz (n ))(2).

nFhkl, P, and LZ are the structure factor, multiplicity factor, Lorenz polarization factor, respectively. θis the Bragg angle, f n is the atomic scattering factor for atomic sort n, and hkl and xyz are the mirror
index and the fractional coordinates, respectively.4(d)From the obtained (I111/I222)Obs. and (I111/I222)Cal. ((I006/I009)Obs. and (I006/I009)Cal.), the orderparameter S was estimated using the following equation. 5[I 111 I 222 ]obs
[I111 I 222 ]cal
[I I ]
S 2 = 006 009 obs[I 006 I 009 ]calS2 =for X=50 (3).
for X=10, 20, and 35(4).

The vacancy layer and/or site which satisfy 0.7 (±0.1)≦S≦1.0 (±0.1) “as a judgment whether theSnSbTe phase is ordered or not” were searched.6 The resultant vacancy configuration is judged to be
the most likely.6
Ⅲ. RESULTS
A. Crystalline structure of X=50 at.%Figure 1 shows an XRD profile for the SnTe/Sb2Te3 SL film. Only a little SnTe(111)/Sb2Te3(001) SL phase and a major SnSbTe phase with the NaCl-type structure coexisted.
The theoretical relatively integral peak-intensity (I111, Cal.) of the SnSbTe phase is expressed asfollows: 42I 111, Cal . ≈ 4 f ( Sn , Sb )－ 4 f Te × LZ × Pf Sn ≈ f Sb ≈ f Te
∴ I 111 ≈ 0(5).
So, the SnSbTe(111) peak of Fig. 1 should not be observed. The structure that can allow this peak
was thus searched according to the aforementioned procedure (a)－(d).
The resultant configuration: crystalline structure, was shown in Fig. 2 (a). In Fig. 2 (b), that ofGeTe layer in GeTe(111)/Sb2Te3(001) SL film was also shown for comparison.6－9 The -(Sn, Sb)(layer)-Te-Vacancy-Te-(Sn, Sb)- stacks in order along the SnSbTe[111] for Fig. 2 (a) wellcorresponded to the –Ge-Te-Vacancy-Te-Ge- stacks for Fig. 2 (b). The structure of the SnSbTe phase
was all the same as the GeTe layer in the GeTe(111)/Sb2Te3(001) SL film.
SnSbTe[111](a)GeTe[111]
(b)

TeTe

(Sn, Sb) layer

SnSbTe(222)

SnSbTe(220)
Sb2Te3(00 15)

200

SnSbTe(200)400

SnSbTe(111)600

Sb2Te3(006)This peak should not be observed !

Sb2Te3(003)

Peak intensity (cps )800

Ge layer

Te layer

Te layer

Vacancy Layer

Vacancy Layer

Te layer
05

1015

20

SnTe(111)

2530

352θ (deg.)

4045

5055

SnTe(222)

Fig. 1 XRD
of ofSnTe/Sb2Te32SL
Fig.1
XRDprofile
profileSnTe/SbTe3film.
SL film.

Te layer

Te (Sn, Sb) layer
cbaTecIn-planeba

Ge layer
In-planeFig.
2 The moststructure(a) the SnSbTe-alloy
the SnTe/Sb2Te3 SLfilm andFig.2
Thelikely
mostlikelyforstructurefor (a) phase
the inSnSbTe-alloy
phasein(b) the GeTe layer in the GeTe/Sb2Te3 SL film.

the SnTe/Sb2Te3 SL film and (b) the GeTe layer in the GeTe/Sb2Te3
SL film.

Sb2Te3(01 14) with Sn

Te(102)100

Sb2Te3(00 18) with Sn

200

Sb2Te3(00 15) with Sn300

Sb2Te3(015) with Sn

400

Sb2Te3(009) with Sn

Sb2Te3(003) with Sn

Peak intensity (cps)500

Sb2Te3(006) with Sn

These peaks were used for analysis !
600101520SnTe(111)

2530

35

2θ (deg. )40455055

1 block

600
400
200

Vacancy layer(Non Te layer)

a[110]
projection

Te(Sn, Sb)
Te

c

5

10 15 20 25 30 35 40 45 50 55

2θ (deg. )Te layer(Sn, Sb) layer
Te layerb(Sn, Sb) layer
a1 block

Te layer
Vacancy layer

Teb (Sn, Sb)
TeTe layer(Sn, Sb) layer
Te layerbFig.Fig.
4 4Thelikelystructure
structure
Themostmost likely
SnSbTe-alloy phase
phase infor forthetheSnSbTe-alloy
inthe
theSnXTe100-X/Sb2Te3 SL film

0

Vacancy Layer(Sn, Sb)c

Sb2Te3(205)
SnSbTe(222)800

Te layer(Sn, Sb)Te

SnTe(222)

SnSbTe(220)1000

SnSbTe(200)1200

SnSbTe(111)

Peak intensity (cps)Like hallow !

1400(Sn, Sb) layer

Te(Sn, Sb)
TeFig.
Fig.33 XRD
XRDprofile
profileof SnofXTe
Sn100-X/Sb/Sbfilm
XTe
100-X
2Te
3 SL
2Te
3 SL
when X=10 at.%, representatively.
film when
X=10 at.%, representatively.
1600

Te layer(Sn, Sb)
Te05SnSbTe[111]Te/Sb220,Te3andSL 35film.
SnXTe100-X(X=10,at.%)(X=10, 20, and 35 at.%)ba

In-planeFig.5 5TwoTwo
stackedSnSbTe-lattices
Fig.
stacked-up
SnSbTe-lattices
of Fig. 2of(b).
Fig.2 (a).

Fig. 6 XRD profile of SnSbTe-alloy film by co-sputtering
Fig. 6 XRD profile of SnSbTe-alloy filmusing the SnTe and the Sb2Te3 targets.

prepared by co-sputtering.

B. Crystalline structure of X= 10, 20, and 35 at.%Figure 3 shows an XRD profile of SnXTe100-X/Sb2Te3 SL film when X＝10, representatively.
Only a little SnTe(111)/Sb2Te3(00l) SL phase, a major Sb2Te3 phase with Sn: Bi2Te3-type SnSbTe
phase, and a Te phase coexisted. The strong SnSbTe(00l) peaks mean that this phase mainly exists
the SL film.
The structure of SnSbTe phase was thus searched according to the aforementioned procedure(a)－(d). The resultant configuration: crystalline structure, was shown in Fig. 4. The configuration
was as follows: -Te-(Sn, Sb)-Te-Te-(Sn, Sb)-Te-(Sn, Sb)-Te-Te-(Sn, Sb)-Vacancy-(Sn, Sb)-Te-Te-(Sn,Sb)-Te- stacks, in order along the SnSbTe[001] per its one lattice.
Ⅳ. DISCUSSION
A. Switching mechanism in case of X=50 at.%
The mechanism on the low switching power was still unclear.1 But, figure 2 (a) offered the key
to consider it.
We stacked up two SnSbTe lattices of Fig. 2 (a) in its [111] (Fig. 5). Figuratively speaking, it
was possible that the upper (lower) lattice corresponds to the GeTe (Sb2Te3) layer in theGeTe(111)/Sb2Te3(001) SL. And that the vacancy layer of the upper (lower) lattice corresponds to avacancy layer (a Te－Te weak bond layer) of the GeTe (Sb2Te3) layer.7－9 Therefore, it was possiblethat the SnSbTe phase has a SL structure itself, i.e., “self-assembly SL structure”. And that something
new phenomenon such as Sn switching occurs in such a self-assembly SL phase, which leads to thelow switching power.1 (Consumed power: ～1/15th that of GST225 device.)
Figure 6 shows an XRD profile of SnSbTe film prepared by co-sputtering using the SnTe andSb2Te3 targets. The SnSbTe(111) peak was hallow pattern, which means that the NaCl-type SnSbTe
lattice has no vacancy layer. Only when the layer by layer sputtering between the SnTe and Sb2Te3layers was performed, the SnSbTe can have the self-assembly SL structure.

B. Switching mechanism in case of X= 10, 20, and 35 at.%
The mechanism on the ultra-low switching power was also unclear. Figure 4, however, offered
the key to consider it.
The vacancy layer of GeTe layer in the GeTe(111)/Sb2Te3(001) SL is the essence of Geswitching.7, 9 The Refs. 7 and 9 also mentioned that the Te－Te weak bond layer of Sb2Te3 layer inthat SL is also essential. And that the following partial arrangement along theGeTe[111]/Sb2Te3[001] SL, for example, is also essential:7,9-Te-Sb-Te-Te-Sb-Te-Sb-Te-Ge-Te-Vacancy-Te-Ge-Te-Sb-Te-.
As shown in Fig.4, the SnSbTe phase had the vacancy and the Te－Te weak bond layers. Andits phase also had the following partial arrangement along the SnSbTe[001]: -Te-(Sn, Sb)-Te-Te-(Sn,Sb)-Te-(Sn, Sb)-Te-Te-(Sn, Sb)-Vacancy-(Sn, Sb)-Te-Te-Sb-Te-. The latter arrangement in theSnSbTe[001] resembles the former one in the GeTe[111]/Sb2Te3[001] SL.7, 9 Thus, there was apossibility that the vacancy layer (the Te－Te weak bond layer) of the SnSbTe plays a similar role tothat of the GeTe (Sb2Te3) layer in the GeTe(111)/Sb2Te3(001) SL.7, 9 Therefore, it was possible thatthe SnSbTe phase has a SL structure itself, i.e., “self-assembly SL structure”.
Something new phenomenon concerning the numbers of Sn atoms such as Sn switching1 occurs
in the its self-assembly SL, the Sn atoms ought to operate for the resistance switching is decreased
with decreasing X, as it turned out, the maximum conservation of electric power, ～1/2250th that ofGST225 device (X=10 at.%), is considered to be obtained.3
In the SnXTe100-X/Sb2Te3 diluted SL also, the SnSbTe can have the self-assembly SL structure
only when the layer by layer sputtering between the SnXTe100-X and Sb2Te3 layers was performed.
Ⅴ. CONCLUSION
In summary, it was possible that the SnSbTe-alloy phase mainly existed the SnXTe100-X/Sb2Te3
SL film has a self-assembly SL structure, and that something new phenomenon in its self-assembly
SL leads to the ultra-low switching power.
ACKNOWLEDGEMENT
This research is granted by the JSPS through the First Program, initiated by the CSTP. This
study was also supported by NIMS and AIST-NPF Nanofabrication Platform, Japan.
REFERENCES
1S. Soeya, T. Odaka, T. Morikawa, T. Shintani, in Proceeding of the Phase Change OrientedScience (PCOS2012), Hamamatsu (2012), pp. 49-52.
2
T. Shintani, S. Soeya, and T. Odaka, in Proceeding of the GNC Symposium 2013, Tokyo (2013),
pp. 101-104.
3
S. Soeya, T. Shintani, T. Odaka, Reiko Kondou, and J. Tominaga, Appl. Phys. Lett. 103, 053103(2013).
4
B. D. Cullity, X-ray Diffraction, transrated by G. Matsumura (Agune, Tokyo, 1961), pp. 126-127.
5
B. D. Cullity, X-ray Diffraction, transrated by G. Matsumura (Agune, Tokyo, 1961), pp. 351-361.
6
S. Soeya and T. Shintani, J. Appl. Phys. 112, 034301 (2012).
7J. Tominaga, R. Simpson, P. Fons, and A. Kolobov, in Proceeding of the IEEE International
Electron Devices Meeting, San Francisco (2010), pp. 22.3.1-22.3.4.
8
R. E. Simpson, P. Fons, A. V. Kolobov, T. Fukaya, M. Krbal, T. Yagi, and J. Tominaga, Nat.
Nanotechnol. 6, 501 (2011).
9J. Tominaga, R. Simpson, P. Fons, and A. Kolobov, in Proceeding of the European Phase Change
Ovonics Symposium (E＼PCOS), Milano (2010), pp. 54-59.

APPENDIX
Thinning of SnTe/Sb2Te3 superlatticeFigure 7 shows the electrical properties of one unit SL composed of Sb2Te3(4 nm)/[SnTe(1nm)/Sb2Te3(4 nm)]1. The set (reset) voltage was ～0.2 V (0.39 V), and the set (reset) current was ～1.0μA (～2.27μA). Multiplying the reset voltage by the reset current yields switching power of ～0.89μW. Since the switching power of GST225 device with the same prober was ～6080μW, thispower is ～1/6800th that of GST225. As a result of crystallographic analysis by using theaforementioned procedure (a)－(d), the one unit SL also had the self-assembly SL structure shown in
Fig. 5. Therefore, thinning down to 5nm might thus be possible.
1.E+06

Reset

Read resistance (Ω)

Read resistance (Ω)

1.E+06

1.E+05

1.E+04

1.E+03
0.0

Set

0.2

0.4

0.6

0.8

1.0

1.2

Reset

1.E+05

1.E+04Set
1.E+0305

10

Pulse voltage (V)Dynamic current (μA)
Figure 7 Electrical properties of one unit SL composed of Sb2Te3(4 nm)/[SnTe(1 nm)/Sb2Te3(4 nm)]1.