Thermal Conductivities of Phase Change Material
Sb-Te Alloys in solid states
R. Lan, R. Endo, 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
E-mail: lan.r.aa@m.titech.ac.jp* Photonics Research Institute, National Institutes of Advanced Industrial Science and Technology,
1-1-1 Higashi, Tsukuba-shi, Ibaraki 305-8562, Japan
ABSTRACTSb-Te alloys have drawn much attention due to the application in phase change random access memory. Thedesign and optimization of phase change random access memory device structures require thermophysicalproperties such as thermal conductivities of Sb-Te alloys. In this work, the thermal conductivities of Sb-x mol%Te
alloys (x = 14, 25, 44, 60, 70 and 90) have been measured by the hot strip method from room temperature up totemperature just below the respective melting points. The thermal conduction mechanism was discussed based on
the temperature dependence. It is proposed that free electron dominates the heat transport below 600 K andWiedemann-Franz law can be applied to predict the thermal conductivities of alloys using the electricalconductivities. Ambipolar diffusion is thought to contribute to the increase in the thermal conductivity at highertemperatures.
Keywords Sb-x at%Te alloys, Thermal conductivity, phase change materials

1. INTRODUCTION
Phase change random access memory (PCRAM) is a promising nonvolatile data storage technology for next
generation memory owing to higher performance characteristics such as faster operation and higher scalability 1). Sb-Te
binary alloys are the important candidates for phase change materials (PCMs) in PCRAM devices 2-4). In PCRAM
devices, the Joule heating induces a transformation of PCMs between amorphous and crystalline states reversibly,
which provides a dramatic change in electrical resistivity, and this resistivity change is the basis for data storage 5). The
design and optimization of PCRAM device structures require thermophysical properties such as thermal conductivities
of Sb-Te alloys. There have been some investigations about thermal conductivities of Sb-Te alloys; however, most ofthem focused on the properties of thin films6, 7). It has been proved that the thermal conductivity of thin film is sensitive
to the microstructure, and the heat conduction mechanisms should be discussed based upon the thermal conductivity ofbulk materials8). However, there are no enough thermal conductivity data for bulk materials of Sb-Te alloys available,
especially for high temperature. Consequently, the aims of the present work are to measure the thermal conductivities of
Sb-Te alloys as functions of temperature and chemical composition and to discuss the heat conduction mechanisms of
Sb-Te alloys based on the temperature and compositional dependences.

2. EXPERIMENTAL

Samples used were Sb-x at% Te (x = 14, 25, 44, 60, 70, and 80). Thermal conductivity was measured by the hot strip
method as shown in Figure 19). Cylindrical samples of Sb-Te (20mm diameter and 40-50mm length) were preparedfrom Sb and Te powders with 99.9 mass% purity. A powder mixture (100 g) was melted in a quartz crucible (20mminner diameter) at 973K for 4 h in vacuum, followed by furnace cooling. Subsequently, the sample was taken from thetube and cut into two along the vertical axis, and the cross sections were mechanically polished using emery papers up
to #2000. Thermal conductivity measurements were conducted in argon atmosphere from 298 K up to temperatures justbelow the respective melting points. Prior to measurements of Sb-Te alloys, the thermal conductivity of fused silica wasmeasured to confirm the reliability of the hot strip method because the thermal conductivity of fused silica is close tothat of Sb-Te binary alloys. Two pieces of fused SiO2 block (40×20×10 mm) were used as the samples.

3. RESULTS AND DISCUSSION
Fig. 2 shows typical temperature rises of the heater as a function of logarithm of time obtained during measurements
at 293K and 773 K for Sb75Te25 by the hot strip method. It can be seen that there is good linearity between T and lnt at
each temperature, which supports the reliability of the present measurements.
The thermal conductivities of Sb-x mol% Te alloys at room temperature are shown as a function of the Teconcentration in Fig. 3, together with the data for thin films reported by Kuwahara et al10) and the data for Sb and Te
recommended by Touloukian et al11). The thermal conductivities of the Te-rich alloys at room temperature are close to
those of Sb2Te3 and Te. The thermal conductivities of the Sb-rich alloys at room temperature increase remarkably withdecreasing Te concentration. The present data seem close to those obtained by Kuwahara et al. for thin films10) except
the composition of Sb-33 mol% Te.
Fig. 4 shows the temperature dependence of Sb-rich alloys together with the data for Sb obtained by Konno et al 12). It
can be seen that the thermal conductivities of all Sb-rich alloys keep roughly constant below approximately 600 K and,above 600 K, increase with increasing temperature. The thermal conductivities of the Sb-rich alloys decrease withincreasing Te concentration. Fig. 5 shows the temperature dependence of thermal conductivities for the Te-rich alloys
together with the data for Sb2Te3. It can be seen that the thermal conductivity of Sb2Te3 decreases with increasing
temperature up to approximately 600 K and then increases. The thermal conductivities of the Te-rich alloys decrease
with temperature increase until their melting points, and this behavior is similar to that of Sb2Te3 below 600 K. The
same dependence of the thermal and electrical conductivities13) of the Sb-Te single phase alloys at room temperature on
the Te concentration suggests that free electrons are predominant carriers for both the heat and electrical transport in theSb-Te single phase alloys. At higher temperature, the ambipolar diffusion is thought to be more effective and contributeto the increase of thermal conductivity.

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 / Wm-1K-1

20This work(bulk)
Kuwahara et al10)(thin film)
Touloukian 11)

293 K

10Fig. 1 Schematic diagram for hot strip method
Schematic diagram for hot strip method

0020406080100xTe / mol%Fig.3 Thermal conductivities of Sb-Te alloys atroom temperature as function of Te concentration

Fig. 2 Temperature rises of heater with timeat 293 K and 773 K for Sb75Te25

SbTeOSiFig.4 Thermal conductivities of Sb-rich samples
as function of temperature

Fig.5 Thermal conductivities of Te-rich samples
as function of temperature