Thermophysical Properties and Measuring Technique of Ge-Sb-Te Alloys for Phase Change Memory

Inhaltsverzeichnis

1. Frontmatter

2. Chapter 1. Introduction

   Rui Lan

   Abstract

   Since the first general-purpose electronic computer ENIAC came to the world in 1946, information and communication technology (ICT) is progressing with every passing day. The influence of information on the political, economic, social and cultural activities has been elevated to an absolutely important position. Human society has entered the information age.

3. Chapter 2. Establishment of the Hot-Strip Method for Thermal Conductivity Measurements of Ge–Sb–Te Alloys

   Rui Lan

   Abstract

   In this chapter, the possible problems for applying the hot-strip method to the thermal conductivity measurements of Ge–Sb–Te alloys have been discussed based on the practical measurement conditions, and the measurement setup and parameters have been modified. After the modification, the thermal conductivities of titanium and fused silica have been measured from 298 K up to about 800 K which is the temperature just below the melting point of Sb₂Te₃ alloy using the hot-strip method since they have many reported data. The results of titanium and fused silica have verified that the hot-strip method is able to give a reliable thermal conductivity data. Consequently, the thermal conductivities of Sb₂Te₃ alloy have been measured from 298 K up to about 800 K to confirm the applicability of the hot-strip method for the measurements of Ge–Sb–Te alloys since these alloys are easily oxidized and have high evaporation. By analyzing the characteristics of Sb₂Te₃ sample before and after the measurements and comparing the thermal conductivity results with the reported data, it is approved that the hot-strip method can be applied to the thermal conductivity measurements of Ge–Sb–Te alloys.

4. Chapter 3. Thermal Conductivities of Ge–Sb–Te Alloys

   Rui Lan

   Abstract

   In this chapter, the thermal conductivities of Sb–Te binary and Sb₂Te₃–GeTe pseudobinary alloys have been measured as functions of temperature and composition from room temperature up to below the respective melting temperature using the hot-strip method. The structures of Sb-rich Sb–Te single-phase alloys have shown similarity by the structure analysis as well as the thermal conductivities. The Te-rich Sb–Te alloys are two-phase alloys and the values of the thermal conductivity are close to those of Sb₂Te₃ alloy. The Sb₂Te₃–GeTe pseudobinary alloys also show similarity on the structure and thermal conductivity. All the single-phase alloys in Ge–Sb–Te alloy system have an increase of the thermal conductivity above 600 K except GeTe alloy, the thermal conductivity of which decreases monotonically with increasing temperature. The thermal conductivities of Sb₂Te₃–GeTe pseudobinary alloys have also been measured as a function of time at 773 K to confirm whether the phase transformation occurs. The results show there is no change on the thermal conductivity with time. By analyzing the uncertainty of the measurements and comparing with the reported data, the thermal conductivity results are thought reliable and able to be used to explain the thermal conduction mechanisms.

5. Chapter 4. Electrical Resistivities of Ge–Sb–Te Alloys

   Rui Lan

   Abstract

   In this chapter, the four-terminal method has been introduced for electrical resistivity measurements. The electrical resistivities of Sb₂Te₃–GeTe pseudobinary alloys have been measured as functions of temperature and composition from room temperature up to below the respective melting temperature. The electrical resistivities of all alloys increase with increasing temperature in the temperature range investigated. The electrical resistivity results of three ternary alloys show similarity as well as the structure. The measurements have also been carried out as a function of time at 773 K to confirm whether the phase transformation occurs. The results show the electrical resistivities of ternary Sb₂Te₃–GeTe pseudobinary alloys do not change while that of GeTe alloy increases slightly with time. This finding indicates that there are no phase transformation in the ternary alloys and the phase transformation of GeTe alloy progresses slowly even at high temperature. By analyzing the uncertainty of the measurements and comparing with the reported data, the electrical resistivity results are thought reliable and able to be used to explain the thermal conduction mechanisms.

6. Chapter 5. Thermal Conduction Mechanisms and Prediction Equations of Thermal Conductivity for Ge–Sb–Te Alloys

   Rui Lan

   Abstract

   In this chapter, based on the thermal conductivity and electrical resistivity data obtained in Chaps. 3 and 4, the thermal conduction mechanisms of Sb–Te binary and Sb₂Te₃–GeTe pseudobinary alloys have been discussed. Both the thermal conductivity and electrical resistivity of Sb₂Te₃–GeTe pseudobinary alloys show similarity, which is directly related to the structure similarity. According to the structure analysis in this work and research in the literature, the Sb₂Te₃–GeTe pseudobinary alloys can be considered as solid solution. Therefore, the thermal conduction mechanisms of Sb₂Te₃–GeTe pseudobinary alloys should be the same. The same case is for Sb-rich Sb–Te alloys. The Wiedemann–Franz (WF) law has been used to predict the electrical thermal conductivity part using the electrical resistivity data. The results show that the free electrons contribute to the most part of the thermal conductivity and the phonon contribution can be neglected. Bipolar diffusion plays an important role at high temperature and accounts for the increase of the thermal conductivity. The exception can be seen in GeTe alloy, which shows metallic properties in the temperature range investigated. The prediction equations have been proposed for the thermal conductivities of Sb–Te binary and Sb₂Te₃–GeTe pseudobinary alloys. The comparison between the predicted data and experimental data proves that the prediction equations can be used for the industrial applications.

7. Chapter 6. Densities of Ge–Sb–Te Alloys

   Rui Lan

   Abstract

   Ge–Sb–Te chalcogenide alloy has been widely investigated due to its applications in phase change random access memory (PCRAM). The density of Ge–Sb–Te alloy is a key factor for PCRAM applications. In this chapter, the densities of Sb₂Te₃ and Ge₂Sb₂Te₅ chalcogenide alloys in solid and molten states have been determined as a function of temperature by the sessile drop method. The density at room temperature was also determined by the Archimedean method to verify the reliability of data obtained by the sessile drop method. The density of solid alloys decreases linearly with increasing temperature and there is a discontinuous density decrease at the melting temperature due to the phase change. In the molten state, the density continues to decrease with increasing temperature. The molar volume of both chalcogenide alloys increases by about 4% from room temperature to just below the melting temperature and by 6% at the melting temperature from the solid to molten state, which may cause large stress for PCRAM devices.

8. Chapter 7. Summary and Conclusions

   Rui Lan

   Abstract

   The thermophysical properties including thermal and electrical conductivity and density of Sb–Te binary and Sb₂Te₃–GeTe pseudobinary chalcogenide alloys have great significance for the phase change random access memory (PCRAM) applications and scientific comprehension. The objectives of this book have been to provide the measuring technique and values of these properties for these alloys and to clarify the thermal conduction mechanisms. For the industrial applications, the prediction equations for thermal conductivities of these alloy systems have been proposed based on the thermal conductivity data and conduction mechanisms.