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Electric resistivity and conduction mechanismfor molten Sb2Te3-GeTe system
R Endo, S Maeda, S Kumamoto, M Kuwahara*, Y Kobayashi and M Susa
Dept of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo
152-8552, Japan*Photonics Research Institute, National Institutes of Advanced Industrial Science and Technology, 1-1-1 Higashi,Tsukuba-shi, Ibaraki 305-8562, Japan
E-mail: rie@mtl.titech.ac.jp
ABSTRACTThe aims of this study are to measure electric resistivitiy of the molten Sb2Te3-GeTe pseudo binary system and toinvestigate the compositional dependence of electric resistivity and the conduction mechanism. The samples usedwere Sb2Te3, GeSb4Te7, GeSb2Te4, Ge2Sb2Te5 and GeTe. The measurements were conducted using the four-terminal
method over temperature ranges between the respective melting temperatures of the samples and 1150 K during bothcooling and heating cycles. Electric resistivity for all the samples decreased with increasing temperature, suggestingthat the samples are liquid semiconductors. In the Sb-Te system, Sb2Te3 had the largest electric resistivity at a certain
temperature; on the other hand, significant compositional dependence was not found in the Sb2Te3-GeTe system. The
measurement of electro motive force suggested that the carrier of Sb2Te3 and Ge2Sb2Te3 were hole, i.e., the samples
are p-type semiconductors. The compositional dependence of electric resistivity is explained from the viewpoints ofenthalpy of mixing.
Keywords Sb-Te alloy, Phase Change Memory, Electric resistivity, liquid
1. INTRODUCTION
The chalcogenides in the Sb2Te3-GeTe pseudo binary system such as Ge2Sb2Te5 are used as phase change materials(PCM) in phase-change random access memory(PRAM)[1]. PRAM utilizes the electric resistance contrast associatedwith phase change for data storage. Accordingly, the electric resistivity of PCMs is essential to optimal operation ofPRAM devices. There is one report available about electric resistivity of liquid Ge2Sb2Te5[2], and the resistivity shows
negative temperature dependence, suggesting that Ge2Sb2Te5 is liquid semiconductor. Furthermore, to understand theconduction mechanism it is essential to determine the carrier type. The present work aims to measure electricresistivities of the Sb2Te3-GeTe pseudo binary system in the liquid state and to determine the carrier type of liquid
Sb2Te3 and Ge2Sb2Te5.

2. EXPERIMENTAL
Samples used in this study were Sb2Te3, GeSb4Te7, GeSb2Te4, Ge2Sb2Te5 and GeTe: all of these compositions arein Sb2Te3-GeTe pseudo binary system. Reagent grade Sb2Te3 and GeTe powders were weighed to the desired
compositions and mixed in an alumina mortar. Electric resistivities were measured using the four-terminal method[2]over temperature ranges between the respective melting temperatures of the samples and 1150 K during both coolingand heating cycles. The electromotive force of the cylindrical samples was also measured for Sb2Te3 and Ge2Sb2Te5
by applying temperature difference of 10 – 30 K to the sample.

3. RESULTS AND DISCUSSION
Figure 1 shows the electric resistivity of solid and liquid GeSb2Te4: the values for liquid are measured in this study
and those for solid are reported by Konstantinov et al[3]. The electric resistivity increases with increasing temperature
up to about 750 K, which temperature dependence is same as that for metal. However, above 750 K, the electricresistivity decreases with increasing temperature in both solid and liquid states, which temperature dependence issame as that for semiconducting Si. Furthermore, the electric resistivity decreases drastically associated with melting.
This temperature dependence of electric resistivity was also observed for other samples. Figure 2 shows measured
Seebeck coefficient of Sb2Te3 together with reported values. The Seebeck coefficient increases with increasing
temperature in the solid state and drastically decreases with melting. The Seebeck coefficient is positive. This suggeststhat the carrier of Sb2Te3 is hole, i.e., the sample is p-type liquid semiconductor. Ge2Sb2Te5 was also considered to be
p-type semiconductor in liquid state. Figure 3 shows GeTe concentration dependence of electric resistivity for
Sb2Te3-GeTe system at 1070 K. GeSb2Te4 shows the largest electric resistivity; however, the compositional
dependence is not significant compared with that for Sb-Te system. To investigate the compositional dependence forelectric resistivity for both systems, firstly, the pseudo gap energy(Ea), i.e., the energy between Fermi energy andmobility edge in the valence band, was calculated for all the samples in liquid states on the basis of the followingequation:⎛ Ea ⎞⎟⎝ kT ⎠σ = σ 0 exp⎜ −where σ, σ0 , k and T represent electric conductivity, i.e. the reciprocal of electric resistivity, a constant, Boltzmann’sconstant, and temperature. The calculated value of Ea showed correlation with electric resistivity: sample which haslarger electric resistivity showed greater activation energy. The activation energy would relate to bonding energy
between atoms, which can be evaluated form enthalpy of mixing. Figures 4 shows enthalpy of mixing for Sb2Te3GeTe systems[6]. The enthalpy of mixing shows no significant compositional dependence, which suggests that theinteraction between Sb and Ge is small in this system, and Sb and Ge atoms are mainly bonding with Te atoms. Thissuggests that the carriers, i.e., holes in this system, move thorough Te atoms to other atoms.

REFERENCES[1] M. Wuttig, N. Yamada: Nat. Mater., 6 (2007) 824[2] R. Endo, S. Maeda, Y. Jinnai, R. Lan, M. Kuwahara, Y. Kobayashi, M. Susa: Jpn. J. Appl. Phys., 49 (2010) 065802[3] P. P. Konstantinov, L. E. Shelimova, E. S. Avilov, M. A. Kretova, V.S. Zemskov: Inorg. Mater., 37 (2001) 662[4] J. Chen, T. Sun, D. Sim, H. Peng, H. Wang, S. Fan, H. H. Hng, J. Ma, F. Y. C. Boey, S. Li, M. K. Samani, G. C. K.
Chen, X. Chen, T. Wu, Q. Ya: Chem. Mater, 22 (2012) 3086
[5] H. Jeon, H. Ha, D. Yun, J.: J. Phys. Chem. Solid,s 52 (1991) 579[6] S.A.Al'fer, A.A. Vecher, O.A. Egorov, L.A. Mechkovskii : Russ. J. Phys. Chem. 55 (1981) 910

Seebeck coefficient, α/μVK-1

Electric resistivity, ρ/μΩm

20
Konstantinov et alThis study

1000200400 600 800
Temperature, T/K1501001000

500200400600800

1000

Temperature, T/KFig. 2 Temperature dependence of Seebeck

resistivity of SbGe2Te4

coefficient of Sb2Te3 with reported data[4, 5]

5Enthalpy of mixing/ Jmol-1

Electric resistivity, ρ /μΩmFig. 1 Temperature dependence of electric

430204060mol% GeTe

80

10020-2-40204060mol% GeTe

80100

Fig. 3 Electric resistivity of Sb2Te3-GeTe

Fig. 4 Enthalpy of mixing for Sb2Te3-GeTe systemsystem at 1070 K as function of GeTe

as a function of GeTe concentration [6]concentration