Preliminary study on gete - sbte and sm - b topological insulators

this respect, 
TI material having tetradymite structure is promised and it was first reported for Bi2Te2Se and 
Sb2Te2Se [10-12]. An important theme in the research of TIs is to reduce unintentionally-
doped bulk carrier that hinders observations of surface transport properties by using suitably 
Hong Duc University Journal of Science, E.3, Vol.8, P (5 - 12), 2017 
7 
doped elements [13-15]. In fact, another useful approach to reducing bulk carriers is to reduce 
the size of samples by making thin ribbons, films and nanowires [16-18]. Another candidate 
topological zero-gap semimetals are Heusler or half-Heusler compounds (LnAuPb, 
LnPtBi). Several experimental evidences of this have been reported so far [19-21]. 
Moreover, the rare earth containing crystal SmB6 is also predicted to be a TI due to strongly 
correlated heavy fermion material to exhibit topological surface states [22]. Recently, our 
collaborating research group in Japan has succeeded in fabricating high quality crystalline 
[(GeTe)2(Sb2Te3)1]n (GTST) topological superlattices, which lead to as much as a 95% 
reduction in the switching energy of electrical non-volatile phase-change random-access 
memory [23]. In this study, we investigated the effect of annealing process on the crystalline 
structures and electrical properties of the GTST multilayers, and influence of fabrication 
conditions on structure of SmB6 bulk samples prepared by the Aluminum-flux method. 
2. Experiment 
Most of the confirmed TI materials are chalcogenides. Since the chalcogen atoms are 
volatile, the syntheses of the TI materials can be done by using Bridgman method for bulk 
samples, molecular beam epitaxy (MBE) or sputtering system for thin films and chemical 
vapor transport for ribbon and nanowires samples. To avoid unexpected contaminations, the 
grown crystalline should be done in the vacuum or in Ar/I2 gas. 
The [(GeTe)2(Sb2Te3)1]n topological superlattices were fabricated at different substrate 
temperatures of 150 to 210 ºC on Si wafers using a helicon-wave sputtering system that has 
GeTe and Sb2Te3 composite targets (2-inch) and automated control shutters at pressures less 
than 0.5 Pa Ar. Thicknesses of each GeTe and Sb2Te3 sublayers were 0.85 nm and 1.0 nm, 
respectively. A 3 nm-thick Sb2Te3 layer was firstly deposited to ensure the strong crystalline 
orientation. Finally, a 20 nm-thick ZnS-SiO2 layer was deposited as a capping layer to protect 
the GTST multilayers from oxidation [24]. We then investigated the effect of annealing 
process on the crystalline structures and electrical properties of the GTST multilayers by using 
X-ray diffraction (XRD) and resistivity measurements, respectively. 
By reviewing several fabrication methods of topological insulator materials, we realized 
that Bridgman and Aluminum-flux methods can be realized on the research facilities in 
Vietnam. In addition, most components of TI materials contain toxic elements such as Pb, Bi, 
Sb, and Se. Therefore, the selection of investigational compounds would be selected to limit 
the effect on human and environment. In this work, SmB6 compound was chosen to synthesize 
by the aluminum-flux method with the starting materials of samarium ingot, boron powder 
and aluminum granules. Sm, B, and Al were weighted in an atomic ratio of 1:4:200. The 
mixture was placed in an alumina crucible and heated to 1150oC in the vacuum. During the 
reaction, the crucible was covered with an alumina lid to reduce Al evaporation. After 
maintaining at 1150oC for 2 hours, the furnace was slowly cooled to 25oC in 10 hours. The 
aluminum - flux was dissolved by a concentrated NaOH solution in a fume hood, and shiny 
single crystals of SmB6 of millimeter-size were picked out. 
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8 
Scanning/Transmission Electron Microscopy (SEM/TEM) and X-ray diffraction (XRD) 
measurements were used to study the structure of the SmB6 samples. 
3. Results and discussion 
The XRD patterns of the as-deposited GTST multilayers are shown in Figure 3a. For 
the higher depositing temperature of 210ºC multilayer has a better crystalline structure. 
Especially, the intensity of the (001) crystalline direction gradually increases when the 
depositing temperature increases from 150 to 210ºC. These results can be explained by the 
self-organized van der Waals epitaxy model due to the reactive selectivity of the surface [25]. 
It has been reported that Te forms a compound with Si, while Sb does not. On the other hand, 
both the Sb and Te react with O which will form a mixed position of Sb and Te on the first 
Sb2Te3 sublayer. Therefore, it is expected that the surface oxide would be effectively removed 
at higher temperatures, resulting in a reactive selectivity and the surface would be covered 
with a monolayer of Tb preferentially. Then the second monolayer would be Sb as the layer 
by layer structure of Te and Sb atomics. 
The 150ºC as-deposited multilayer was annealed at 210ºC for 1 hour and its XRD 
patterns was measured again as shown in Figure 3b. It is clearly seen that the crystalline 
structure was significantly improved after the annealing process due to reconstruction at 
higher temperatures. 
Figure 3. X-ray diffraction patterns of (a) 150, 170, 190, and 210 ºC as-deposited GTST 
multilayers and (b) the 150ºC as-deposited multilayer was annealed at 210ºC for 1 hour 
The magnetic field dependences of the resistivity measured at room temperature for the 
150, 210ºC as-deposited and 210ºC annealed GTST multilayers are shown in figure 4. For the 
150ºC as-deposited sample, its resistivity is one order higher than those of the other ones. This 
high resistivity is comparable with that of the GeSbTe alloy. It means that the main part of the 
150ºC as-deposited sample is not in well crystalline structures as shown in the XRD patterns 
(figure 3) but in alloy compounds of GeTe and SbTe which have semiconductor behaviors at 
Hong Duc University Journal of Science, E.3, Vol.8, P (5 - 12), 2017 
9 
room temperature. On the other hand, the resistivity of the 210ºC as-deposited and 210ºC 
annealed GTST multilayers is as low as that of a good GeTe-SbTe superlatice. It suggested 
that the annealing process at high temperatures can improve the crystalline structure as well as 
the electrical property of the low temperature deposited GTST films due to the reconstruction 
of atoms. 
0
0.01
0.02
0.03
0.04
0.05
-10 -5 0 5 10
150C as-deposited
210C as-deposited
annealed at 210C
R
e
si
st
iv
ity
 (
o
h
m
.c
m
)
Magnetic Field (kOe) 
Figure 4. Magnetic field dependences of resistivity for 150ºC as-deposited (opened circles) 
and 210ºC as-deposited (opened squares) GTST multilayers, and the 150ºC as-deposited 
multilayer was annealed at 210ºC for 1 hour (solid circles) 
Figure 5. XRD pattern of SmB6 bulk sample 
synthesized at 11500C 
Figure 6. SEM image of SmB6 bulk sample 
synthesized at 11500C 
As shown in [22], the single crystal structure of SmB6 is cubic lattice with constant 
a = 4.1353Å. Figure 5 shows XRD pattern of SmB6 prepared at 1150oC using Sm, B, and Al 
as raw material. Aluminum here plays the role of reducing the melting temperature of 
elements in the component. It can be seen that all the diffraction peaks are not really the 
Hong Duc University Journal of Science, E.3, Vol.8, P (5 - 12), 2017 
10 
SmB6 single crystal structure. However, its XRD patterns are well indexed and assigned to 
the parallel crystal planes of (221), (220), (211), (002), (111) [26]. The impurity phases are 
identified to be Sm element. 
The SEM image is shown in Figure 6. It can be seen that the SmB6 bulk sample 
prepared at 1150oC is mainly composed of a great deal of aggregated particles without any 
regular shapes, there are some large grains with non-cubic morphology mixing together. In the 
next steps, we will continue to synthesize samples at different temperatures to obtain the 
better SmB6 single crystals. 
4. Conclusion 
We investigated the effect of annealing process on the crystalline structures and 
electrical properties of the GTST multilayers. It is found that the annealing process at high 
temperatures can improve the crystalline structure as well as the electrical property of the low 
temperature deposited GTST films due to the reconstruction of atoms. On the other hand, we 
also fabricated SmB6 single crystals by the Aluminum-flux method and investigated their 
structure. The results showed that the SmB6 bulk sample prepared at 1150oC is mainly 
composed of a great deal of aggregated particles without regular shapes. There are some large 
grains with non-cubic morphology mixing together. 
ACKNOWLEDGMENTS 
This work was supported by the basic project of Hong Duc university under grant 
number of ĐT-2016-42. A part of the work was done in the Key Laboratory of Electronic 
Materials and Devices, Institute of Materials Science (IMS), Vietnam Academy of Science 
and Technology (VAST), Vietnam. We are grateful to Assoc. Prof. Dr. Nguyen Huy Dan 
(IMS, VAST, Vietnam) for helpful discussions. 
References 
[1] D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava, and M. Z. Hasan (2008), 
A topological Dirac insulator in a quantum spin Hall phase, Nature: vol. 452, pp. 
970-974, 
[2] P. Roushan, J. Seo, C. V. Parker, Y. S. Hor, D. Hsieh, D. Qian, A. Richardella, M. Z. 
Hasan, R. J. Cava, and Yazdani (2009), Topological surface states protected from 
backscattering by chiral spin texture, Nature, vol. 460, pp. 1106-1109. 
[3] A. Nishide, A. A. Taskin, Y. Takeichi, T. Okuda, A. Kakizaki, T. Hirahara, K. 
Nakatsuji, F. Komori, Y. Ando, amd I. Matsuda (2010), Direct mapping of the spin-
filtered surface bands of a three-dimensional quantum spin Hall insulator, Phys. Rev. 
B, vol. 81, 041309(R). 
Hong Duc University Journal of Science, E.3, Vol.8, P (5 - 12), 2017 
11 
[4] H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang, and S.-C. Zhang (2009), Topological 
insulators in Bi, Nature Phys. vol. 5, pp. 438-442. 
[5] C.-X. Liu, X.-L. Qi, H. Zhang, X. Dai, Z. Fang, and S.-C. Zhang (2010), Model 
Hamitonian for topological insulators, Phys. Rev. B, vol. 82, 045122, 2010. 
[6] Y. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. 
J. Cava, and M. Z. Hasan (2009), Observation of a large-gap topological-insulator 
class with a single Dirac cone on the surface, Nature Phys., vol. 5, pp. 398-402. 
[7] Y. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D. H. 
Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain, and Z.-X. Shen (2009), 
Experimetal realization of a three-dimentional topological insulator, Bi2Te3, Science, 
vol. 325, pp. 178-181. 
[8] D. Hsieh, Y. Xia, D. Qian, L. Wray, F. Meier, J. H. Dil, J. Osterwalder, L. Patthey, A. 
V. Fedorov, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan 
(2009), Observation of time-reversal-protected single-Dirac-cone topological-insulator 
states in Bi2Te3 and Sb2Te3, Phys. Rev. Lett., vol. 103, 146-401. 
[9] Y. Jiang, Y. Wang, M. Chen, Z. Li, C. Song, K. He, L. Wang, X. Chen, X. Ma, and Q. 
K. Xue (2012), Landau quantization and the thickness limit of topological insulator 
thin films of Sb2Te3, Phys. Rev. Lett., vol. 108, 016401. 
[10] C. H. Li et al. (2014), Electrical detection of charge-current-induced spin polarization 
due to spin-momentum locking in Bi2Se3, Nature Nanotech., vol. 9, pp. 218-224. 
[11] Z. Ren, A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando (2010), Large bulk resistivity 
and surface quantum osillations in the topological insulator Bi2Te2Se, Phys. Rev. B, 
vol. 82, 241306(R). 
[12] J. Xiong, A. C. Petersen, D. Qu, Y. S. Hor, R. J. Cava, and N. P. Ong (2012), Quantum 
oscillations in a topological insulator Bi2Te2Se with large bulk resistivity, Physica E, 
vol. 44, pp. 917-920. 
[13] Z. Ren, A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando (2011), Observations of two-
dimensional quantum oscillation and ambipolar transport in the topological insulator 
Bi2Se3 achieved by Cd doping, Phys. Rev. B, vol. 84, 075316. 
[14] H. Ji, J. M. Allred, M. K. Fuccillo, M. E. Charles, M. Neupane, L. A. Wray, M. Z. 
Hasan, and R. J. Cava (2012), A topological insulator in the tetradymite family, Phys. 
Rev. B, vol. 85, 201103. 
[15] P. Gehring, H.M.Benia, Y.Weng, R. Dinnebier, C. R. Ast, M.Burghard, and K. Kern 
(2013), A natural topological insulator, Nano Lett., vol. 13, 1179. 
[16] D. Kong, J. C. Randel, H. Peng, J. J. Cha, S. Meister, K. Lai, Y. Chen, Z.-X. Shen, H. 
C. Manoharan, and Y. Cui (2010), Topological insulator nanowires and nanoribbons, 
Nano Lett., vol. 10, pp. 329-333. 
[17] D. Kong, Y. Chen, J. J. Cha, Q. Zhang, J. G. Analytis, K. Lai, Z. Liu, S. S. Hong, K. J. 
Koski, S.-K. Mo, Z. Hussain, I. R. Fisher, Z.-X. Shen, and Y. Cui (2011), Ambipolar 
field effect in topological insulator nanoplates of (BixSb1-x)2Te3, Nature Nanotechnol., 
vol. 6, pp. 705-709. 
Hong Duc University Journal of Science, E.3, Vol.8, P (5 - 12), 2017 
12 
[18] S. S. Hong, J. J. Cha, D. Kong, and Y. Cui (2012), Ultra-low carrier concentration and 
surface-dominant transport in antimony-doped Bi2Se3 topological insulator 
nanoribbons, Nature Commun., vol. 3, pp. 757-763. 
[19] S. Chadov, X.-L. Qi, J.K¨ubler, G. H. Fecher, C. Felser, and S.-C. Zhang (2010), 
Tunable multifunctional topological insulators in ternary Heusler compounds, Nature 
Mater, vol. 9, pp. 541-545. 
[20] H. Lin, L. A. Wray, Y. Xia, S. Xu, S. Jia, R. J. Cava, A. Bansil, and M. Z. Hasan 
(2010), Half-Heusler ternary compounds as new multifunctional experimental 
platforms for topological quantum phenomena, Nature Mater., vol. 9, 546-549. 
[21] D. Xiao, Y. Yao, W. Feng, J. Wen, W. Zhu, X.-Q. Chen, G. M. Stocks, and Z. Zhang 
(2010), Half-Heusler compounds as a new class of three dimensional topological 
insulators, Phys. Rev. Lett., vol. 105, 096404. 
[22] M. Ciomaga Hatnean, M. R. Lees, D. McK. Paul & G. Balakrishnan (2013), Large, 
high quality single-crystals of the new Topological Kondo Insulator, SmB6, Nat. Sci. 
Rep., vol.3, 3071, 2013. 
[23] Tominaga J et al. (2015), Giant multiferroic effects in topological GeTe-Sb2Te3 
superlattices, Sci. Technol. Adv. Master, vol. 16, 014402. 
[24] D. Bang et al. (2014), Mirror-symmetric Magneto-optical Kerr Rotation using Visible 
Light in [(GeTe)2(Sb2Te3)1]n Topological Superlattices, Nat. Sci. Rep., vol. 4, 5727; 
DOI:10.1038/srep05727. 
[25] Y. Saito et al. (2015), Self-organized van der Waals epitaxy of layered chalcogenide 
structures, Phys. Status Solidi B,vol. 252, 2151-2158. 
[26] Lihong Bao et al. (2015), SmB6 nanoparticals: Synthesis, valence states, and magnetic 
properties, Journal of Alloys and Compounds, vol. 651, pp. 19-23.