Phase Change Memory

The thermal and electrical properties of phase change materials, mainly GeSbTe alloys, in the crystalline state strongly depend on their phase and on the associated degree of order. The switching of Ge atoms in superlattice structures with trigonal phase has been recently proposed to develop memories with reduced switching energy, in which two differently ordered crystalline phases are the logic states. A detailed knowledge of the stacking plane sequence, of the local composition and of the vacancy distribution is therefore crucial in order to understand the underlying mechanism of phase transformations in the crystalline state and to evaluate the retention properties. This information is provided, as reported in this paper, by scanning transmission electron microscopy analysis of polycrystalline and epitaxial Ge2Sb2Te5 thin samples, using the Z-contrast high-angle annular dark field method. Electron diffraction clearly confirms the presence of compositional mixing with stacking blocks of 11, 9 or 7 planes corresponding to Ge3Sb2Te6, Ge2Sb2Te5, and GeSb2Te4, alloys respectively in the same trigonal phase. By increasing the degree of order (according to the annealing temperature, the growth condition, etc) the spread in the statistical distribution of the blocks reduces and the distribution of the atoms in the cation planes also changes from a homogenous Ge/Sb mixing towards a Sb-enrichment in the planes closest to the van der Waals gaps. Therefore we show that the trigonal phase of Ge2Sb2Te5, the most studied chalcogenide for phase-change memories, is actually obtained in different configurations depending on the distribution of the stacking blocks (7–9–11 planes) and on the atomic occupation (Ge/Sb) at the cation planes. These results give an insight in the factors determining the stability of the trigonal phase and suggest a dynamic path evolution that could have a key role in the switching mechanism of interfacial phase change memories and in their data retention.

Phase Change Materials are solids which are characterized by a unique combination of properties. They exist in an amorphous and a crystalline phase with remarkably different optical and electrical properties caused by an unusual change of bonding when the amorphous phase is crystallized. It is possible to change the phase of such a material in very short times (nanoseconds) and repeatedly between the two phases which makes phase change materials ideal candidates for data storage. This paper reviews in detail the relationship between the bonding mechanisms and the resulting physical properties of phase change materials. It describes the change of bonding from ordinary covalent bonding in the amorphous phase to resonance bonding in the crystalline phase with additional disorder, resulting in unconventional physical properties of phase change materials. These properties lead to the development of phase change data storage applications. Phase change optical data storage, phase change random access memory, and emerging applications including neuromorphic computing are described with particular emphasis on material requirements and material engineering for phase change random access memory.

Study on thermal properties of organic phase change materials for energy storage

Advances in resource utilization of waste in phase change materials

The accurate measurement of thermal properties in phase change materials holds significant importance for engineering applications. This research introduces fuzzy inference methods to estimate the thermal properties of phase change materials. The solution to the coupled heat transfer involving radiation and conduction in material is achieved through a hybrid approach, which combines the finite volume method with the discrete ordinate method. The estimation process is structured as an inverse problem, where the temperature on the material surface acts as the measurement signal for conducting the inverse analysis. Both the fuzzy inference method and the decentralized fuzzy inference method are utilized to address the inverse heat transfer problem. This enables the determination of latent heat and thermal conductivities for both solid and liquid regions within the phase change material. Retrieval results demonstrate that the thermal properties of phase change materials can be accurately estimated using fuzzy inference methods. While both two fuzzy inference methods perform similarly in estimating a single parameter, the fuzzy inference method has limitations in multiparameter estimation tasks. Conversely, the decentralized fuzzy inference method yields accurate results in simultaneous estimation problems. Furthermore, this method proves robust in estimating the thermal properties of phase change materials, even in the presence of noisy data.

Abstract

5 - Phase change memory (PCM) materials and devices

Phase change materials such as pseudobinary GeTe-Sb2Te3 (GST) alloys are an essential part of existing and emerging technologies. Here, we investigate the electrical and optical properties of epitaxial phase change materials: α-GeTe, Ge2Sb2Te5 (GST225), and Sb2Te3. Temperature-dependent Hall measurements reveal a reduction of the hole concentration with increasing temperature in Sb2Te3 that is attributed to lattice expansion, resulting in a non-linear increase of the resistivity that is also observed in GST225. Fourier transform infrared spectroscopy at room temperature demonstrates the presence of electronic states within the energy gap for α-GeTe and GST225. We conclude that these electronic states are due to vacancy clusters inside these two materials. The obtained results shed new light on the fundamental properties of phase change materials such as the high dielectric constant and persistent photoconductivity and have the potential to be included in device simulations.

The advantages of latent heat storage systems are heat storage materials capable of providing large energy storage densities and are capable of storing heat at almost constant temperature according to the transition temperature at each phase change. The thermophysical properties of phase change material (PCM) materials are important to know before they are applied in a variety of uses. The thermophysical properties of PCM materials are very important to note i.e. melting point, super cooling temperature, latent heat, heat type and thermal conductivity of the PCM material. Methods of determining the thermophysical properties of PCM materials that have been widely used are differential thermal analysis (DTA), differential scanning calorimetry (DSC), and calorimetry. These three methods have weaknesses. DTA and DSC methods that have very small test samples (110 mg) cause the materials thermophysical properties to be different when used in larger quantities. Another disadvantage is that DTA and DSC measurement equipment is complex and expensive, and cannot measure latent heat, specific heat and thermal conductivity of multiple PCM samples simultaneously. The weakness of the calorimetry method is the process of changing the two phases of PCM that is difficult to observe. T-history method that uses simple equipment has been widely used at this time. In this research, we will use T-History method to measure the thermophysical properties of phase change materials that will be used as heat storage material in solar water heating system. The material is: paraffin, beeswax, cow fat, and mixture of the material with a certain ratio. From the test, results can be concluded that the use of T-History method gives good results with a difference of 7% with the measurement results using DSC.

Part I: Fundamentals of Electron Theory: Introduction. Wave Properties of Electrons. The Schroedinger Equation. Solution of the Schroedinger Equation for Four Specific Problems. Energy Bands in Crystals. Electrons in a Crystal.- Part II: Electrical Properties of Materials: Electrical Conduction in Metals and Alloys. Semiconductors. Electrical Properties of Polymers, Ceramics, Dielectrics and Amorphous Materials.- Part III: Optical Properties of Materials: The Optical Constants. Atomistic Theory of the Optical Properties. Quantum Mechanical Treatment of the Optical Properties. Applications.- Part IV: Magnetic Properties of Materials: Foundations of Magnetism. Magnetic Phenomena and Their Interpretation - Classical Approach. Quantum Mechanical Considerations. Applications.- Part V: Thermal Properties of Materials: Introduction. Fundamentals of Thermal Properties. Heat Capacity. Thermal Conduction. Thermal Expansion.- Appendices.- Index.

Phase change materials are widely used in our lives, and it is of great practical significance to learn and study the thermal properties of phase change. However, at present, most of the calorimetric devices on the market to measure the thermal properties of phase change materials are carried out in the sealed interval, and students cannot observe the complete phase change process. At the same time, it is rarely covered in teaching experiments because the instrumentation is too expensive and complex. In this paper, the relationship between the latent heat of phase change and temperature difference curves of phase change materials and the peak area enclosed by the baseline is obtained through theoretical derivation, and a simple open calorimetric device is constructed. The device uses the Peltier plate as the heat source, measures the temperature using the thermoelectric effect of the semiconductor zinc oxide film, and completely records the temperature change of the phase transition process through the data acquisition card and the LABVIEW program. In this paper, gallium metal was used as a sample, and the parameters <i>h_T</i>=0.52(J/(K·S)) and the latent heat of phase change of 10 groups of gallium metal of different masses between 0.206~0.692g were calibrated. The device has simple structure and convenient operation, which can provide technical reference for studying the thermal properties of phase change materials and university physics experiments.

Study on thermal properties of phase change material by an optical DSC system

Energy and daylighting performance of a building containing an innovative glazing window with solid-solid phase change material and silica aerogel integration

Phase change materials are a technologically important materials class and are used for data storage in rewritable DVDs and in phase change random access memory. Furthermore, new applications for phase change materials are emerging. Phase change materials with a high structural quality, such as offered by epitaxial films, are needed in order to study the fundamental properties of phase change materials and to improve our understanding of this materials class. Here, we review the progress made in the growth of crystalline phase change materials by physical methods, such as molecular beam epitaxy, sputtering, and pulsed laser deposition. First, we discuss the difference and similarities between these physical deposition methods and the crystal structures of Ge2Sb2Te5, the prototype phase change material. Next, we focus on the growth of epitiaxial GST films on (0 0 1)- and (1 1 1)-oriented substrates, leading to the conclusion that (1 1 1)-oriented substrates are preferred for the growth of phase change materials. Finally, the growth of GeTe/Sb2Te3 superlattices on amorphous and single crystalline substrates is discussed.

Chalcogenide based compound, especially Sb2Te3 and related compounds, exhibit pronounced structural and optical contrast with rapid phase transition from amorphous and crystalline phase. This makes them suitable candidates for rewritable optical storage media and phase change random access memory. Simultaneously, Sb2Te3 have reported that one of the best p-type thermoelectric materials characteristics at room temperature, and topological insulators property. As electron and phonon plays a crucial role in determining the performances of any real devices, a better understanding of the amorphous phase and crystal phase, and the transition mechanism between them, is imperative. Recent, Optical pump THz probe spectroscopy (OPTP) is a powerful tool to study the ultrafast carrier dynamics of structural phase transition occurring on ultrafast time scales, as has been applied to a variety of materials, such as semimetal and semiconductor, and Mott insulators examined by using a femtosecond pump-probe technique and found that the appearance of the coherent vibrational modes was significantly modified upon the phase change.In this study, we investigate ultrafast carrier dynamics in {Sb(3)Te(9)}n thin film that the phase change from amorphous into crystalline states can be explained by THz-TDS and OPTP, which is the relationship between structural phase transition and optical properties transition in THz range. We observed that OPTP spectroscopy was closely dependent on phase of the Sb2Te3 film, indicating that decay time is a crucial carrier dynamics mechanism to the phase transformation process.