P04

Theoretical and experimental research
for SiTe/Sb2Te3 phase change superlattice
Yuta Saito*, Junji Tominaga, Paul J. Fons and Alexander V. Kolobov
Nanoelectronics Research Institute, National Institute of Advanced Industrial Science & Technology (AIST)
Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, 305-8562, Japan*E-mail: yuta-saito@aist.go.jp
ABSTRACTIn this study, the fundamental properties of SiTe/Sb2Te3 superlattices were investigated theoretically andexperimentally. In the theoretical study, ab-initio density functional theory (DFT) codes were used. Several(SiTe)2(Sb2Te3)x superlattice models were designed with different numbers of the Sb2Te3 blocks and thecorresponding band structures were calculated with and without spin-orbit coupling (SOC). It was found that the band
gap of a (SiTe)2(Sb2Te3)1 superlattice closes only at the Γ point. This is the most basic feature of a Dirac semimetal.
Based on the simulations, the SiTe/Sb2Te3 superlattices were experimentally fabricated on a Si substrate by RFmagnetron sputtering at various deposition temperatures. The deposition temperature dependence of the crystal
growth and crystallinity of the SiTe/Sb2Te3 superlattices were studied by x-ray diffraction (XRD) analysis andtransmission electron microscopy (TEM).
Key words: Phase change superlattice, iPCM, Topological insulator
1. INTRODUCTION
Recently, interfacial phase-change memory (iPCM) was reported using GeTe/Sb2Te3 superlattice structure andshowed superior performance such as extremely low power switching with a much faster switching speed comparedwith a PCM using the conventional Ge2Sb2Te5 alloy film [1]. In iPCM, coherently aligned Ge atoms are reversibly
switched between octahedral and tetrahedral states based on theumbrella flip model proposed by Kolobov et al. [2], resulting in a
sufficient resistance difference. Furthermore, besides such the excellent
properties as memory application, it was reported that iPCM showedmagnetic properties, which is speculated to a relation to topologicalinsulators [3]. Although iPCM has been attracting considerable attention
as both electrical non-volatile memory and magnetic device, thematerial research has not yet been carried out sufficiently. In this work,
we report several properties of SiTe/Sb2Te3 superlattices as analternative candidate.
2. EXPERIMENTS
In order to investigate the structural stability and corresponding bandstructure, two kinds of ab-initio density functional theory (DFT) codes,called CASTEP and WIEN2K, were carried out. In both calculations, ageneralized gradient approximation (GGA) of Perdew-Burke-Ernzerhof(PBE) was used. In WIEN2K calculation, spin-orbit coupling (SOC) wasincluded to introduce the relativistic effects. [(SiTe)x/(Sb2Te3)y]z (x, yand z are integer) superlattice films were prepared by RF-magnetron
sputtering on a Si single crystal using SiTe and Sb2Te3 alloy targets.
The film thickness of each layer and the number of cycles werecontrolled by opening and closing the shutters in front of each
sputtering target. The deposition temperatures were controlled by aheater equipped under the substrate station in the sputter chamber. Xray diffraction (XRD) analyzes were carried out using Cu-Kα as an xray source to investigate the crystal orientation of the superlattice films.
Transmission electron microscope (TEM) was used to observe thecross-sectional microstructure of the superlattice films.

3. RESULTS AND DISCUSSION
Fig. 1(a) shows the stabilized (SiTe)2(Sb2Te3)1 superlattice structure byab-initio calculation. The structure has the same atomic layered structure
as that of the GeTe/Sb2Te3 superlattice reported by Sa et al [4]. The
calculated lattice constants are a = 0.4111 nm and c = 1.9447 nm,
respectively, which are similar to that corresponding GeTe/Sb2Te3
superlattice (a = 0.4206 nm and c = 1.9357 nm). The bulk band structure
including SOC in the (SiTe)2(Sb2Te3)1 superlattice is plotted in Fig. 1(b).
It can be seen that the band gap closes only at the Γ point. This is the mostbasic feature of a Dirac semimetal.
Fig. 2 shows the XRD patterns of the [(SiTe)2(Sb2Te3)1]20 superlattices
deposited at 200, 250 and 280oC. XRD indicates that the[(SiTe)2(Sb2Te3)1]20 film has highly oriented of to the 001 plane of Sb2Te3
normal to the substrate. The peaks, which are speculated to result in thesuperlattice structure, can be seen at the deposition temperature of above250oC.
A cross-sectional TEM image of a [(SiTe)2(Sb2Te3)4]8 superlattice
deposited at 250oC is shown in Fig. 3. The TEM also revealed thatobtained superlattice film had a highly oriented structure characterizedwith the lattice fringes parallel to the Si substrate.
4. CONCLUSION
We investigated the fundamental properties of SiTe/Sb2Te3 superlattices
theoretically and experimentally. Ab-initio first principle calculation
revealed that the (SiTe)2(Sb2Te3)n (n: 1, 2, 4 and 6) structures could stably
exist. The band structures of each superlattice structure were calculated with SOC. The (SiTe)2(Sb2Te3)1 superlattice
showed a Dirac semimetal-like band structure. Based on the theoretical calculation, we successfully fabricated the(SiTe)2(Sb2Te3)1,4 film on a Si single crystal by RF-magnetron sputtering. The XRD and TEM indicated that theobtained superlattice films had highly oriented to the 001 plane of Sb2Te3 normal to the substrate.
ACKOWLEADGMENT
This work has been done in “Ultra-Low Voltage Device Project” of LEAP (Low-power Electronics Association &Project) funded and supported by METI and NEDO. The authors thank R. Kondo for technical assistance.
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501[2] A. V. Kolobov, P. Fons, A. I. Frenkel, A. Ankudinov, J. Tominaga and T. Uruga, Nat. Mater. 3 (2004) 703[3] J. Tominaga, R. E. Simpson, P. Fons and A. V. Kolobov, Appl. Phys. Lett. 99 (2011) 152105[4] B. Sa, J. Zhou, Z. Sun, J. Tominaga and R. Ahuja, Phys. Rev. Lett. 109 (2012) 096802