Search

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature. The structure of an ideal glass, crystallisation mechanism, nanoscale effects, and even parallels to organic chemistry can be drawn by conceptualizing the amorphous GeTe structure as a superposition of Te and Ge lattices with atom-specific dynamics. [ABSTRACT FROM AUTHOR]

An attractive trend in the further progress of nanoelectronics is the development of a new generation of non-volatile memory devices, namely electric phase memory or PC-RAM (Phase Change Random Access Memory). However, there are a number of related unsolved problems, such as the stability of the amorphous phase, high power consumption, long information storage time, etc. In order to solve these problems, a new approach has been proposed, which consists in using Ag–Cu binary alloy nanoparticles as PC-RAM cells. To this end, the molecular dynamics method was used to study the processes of structuring these alloy nanoparticles (NPs) with a size of D = 2–10 nm of different compositions when the heat energy removal rate was varied. The stability criteria of the amorphous and crystalline structure were evaluated and conclusions were drawn about the target composition and size of NPs, suitable for the fabrication of phase change memory cells. It has been shown that using binary Ag–Cu alloy NPs, it is possible to reduce the size of a single cell to 6–8 nm and the information recording time to 2.5 ns and, for the first time, based on the eutectic approach, to achieve the stability of the amorphous and crystalline structure at different rates of heat energy removal. [ABSTRACT FROM AUTHOR]

Sb2Te thin films were deposited on SiO2/Si (100) substrates by magnetron sputtering. We investigated the crystallization behavior of Sb2Te thin films for phase-change memory applications. The experimental results show that the optical reflectivity is increased by more than 30% before and after crystallization. The resistance is decreased at least 102 orders of magnitude in the crystallization process, which suggests the application potential of Sb2Te thin film as optoelectronic storage material. The computational results show that the local crystallization activation energy is 1.72 eV and the crystal growth velocity is 4.96 m s−1, confirming the strong crystallization tendency of Sb2Te thin film. We examined the features of amorphous local bonding and found that the local octahedral geometry is the structural origin of optoelectronic contrast and crystallization tendency. Furthermore, the presence of weak Sb-Sb, Sb-Te, and Te-Te bonds and high mobility of Sb atoms facilitate the high-speed and low-activation-energy crystallization. Hence, we suggested that reducing the number of octahedrons and introducing the strong chemical bonds could promote stable optoelectronic memory applications of Sb2Te thin film. [ABSTRACT FROM AUTHOR]

In this paper, the Mg35Sb65/Sn15Sb85 superlattice-like films were prepared by magnetron sputtering and the comprehensive properties of were systematically studied. The amorphous-to-crystalline transformation was researched with a situ resistance–temperature measurement system to evaluate the thermal stability. X-ray diffraction, Raman and transmission electron microscopy were employed to explore the change of phase structure at different annealing temperatures. The reversible resistance switching was achieved for [MgSb(7 nm)/SnSb(3 nm)]5-based devices. The results showed that Mg35Sb65/Sn15Sb85 superlattice-like film was a promising phase change material with good thermal stability, fast phase transition and low power consumption. [ABSTRACT FROM AUTHOR]

The phase change behavior of Ga40Sb60/Sn15Sb85 multiple interfacial film was studied systematically for its application in phase change memory. The thermal stability of amorphous state was evaluated using the Arrhenius formulas. The evolution of phase structure indicates that the Sb phase exists in the Sb-rich Ga40Sb60/Sn15Sb85, Ga40Sb60 and Sn15Sb85 films. The change in density and thickness during crystallization was obtained by X-ray reflectance. The nanoscale change in roughness of the Ga40Sb60/Sn15Sb85 film was confirmed by atomic force microscopy. The rate for crystallization and amorphization transformation was measured by a picosecond laser pump-probe system. The phase change memory device based on [Ga40Sb60 (7 nm)/Sn15Sb85 (3 nm)]5 thin film was fabricated. An ultra-fast switching and low power were achieved for Ga40Sb60/Sn15Sb85 multiple interfacial film. [ABSTRACT FROM AUTHOR]

Compared with pure Antimony (Sb) materials, Cerium (Ce) doped Sb alloys is proved to be a promising candidate with better thermal stability for phase change memory application. In this paper, the Ce doped Sb films were synthesized by magnetron sputtering. The crystallization temperature (Tc), activation energy (Ea) and the temperature for 10 years data retention (Tten) all increased significantly by increasing Ce dopants, meaning a much better thermal stability. These results may ascribe to disturbing crystallization process from the existence of Ce-Sb bond. [ABSTRACT FROM AUTHOR]

We propose a method for evaluating kinetic parameters for the crystallization of thin films of phase change materials. Its basic principle is to jointly use model-free and model isoconversional methods in analyzing differential scanning calorimetry results. Using this method, we have identified the reaction model and evaluated the activation energy for crystallization and pre-exponential factor as a function of the degree of conversion for GeSbTe-based thin films. [ABSTRACT FROM AUTHOR]

SbSe alloy is used as a dopant for Ti-Sb-Te phase change material aiming to improve the thermal stability of Ti-Sb-Te based phase change memory cell. In this paper, the thermal and electrical properties of SbSe-doped TiSbTe are respectively investigated. Compared with TiSbTe material, (SbSe)(TiSbTe) has better thermal stability with crystallization temperature of 203 °C and estimated 10-year data retention of 102 °C. For the (SbSe)(TiSbTe)-based PCM cell, it inherits the advantage of fast switching speed and good endurance of TST based one with wider SET/RESET window. The effects of SbSe doping on the structure of TiSbTe are also systemically studied. In crystalline (SbSe)(TiSbTe) film structure, Se is prone to substitute Te and bond with Sb and Te in the SbTe lattice structure which may decrease the crystal lattice constant. [ABSTRACT FROM AUTHOR]

The amorphous to crystalline phase change of nitrogen-doped GeSb thin films were investigated by in␣situ film resistance measurements. The thermal stability and data retention increased with the increase of N doping concentration. Compared with GeSb, a higher crystalline resistance of N-doped GeSb thin films was obtained, which is beneficial for the reduction of RESET operation consumption of phase change memory. The analysis of x-ray diffractomer indicated that nitrogen doping can refine the grain size. The measurement of atomic force microscopy revealed that the crystallization was inhibited and the surface of thin films became smoother after N doping. A reversible phase transition was realized by the picosecond laser pulses and the switching speed of crystallization was measured. [ABSTRACT FROM AUTHOR]

We report the crystallization and phase-transition behavior of GeSbTe thin films doped with indium phosphorus (InP). Pure GeSbTe thin films and InP-doped GeSbTe thin films were prepared by use of an rf magnetron sputtering method. After thermal annealing, electrical and optical changes in the thin films were observed. Sheet resistance and reflectance measurements revealed that InP doping suppresses crystallization of GeSbTe. X-ray diffraction analysis confirmed that addition of In and P atoms inhibits the phase transition from face-centered cubic to hexagonal closed-packed. Nucleation of the doped GeSbTe thin films was delayed at an annealing temperature of 100°C; after thermal annealing, neither segregation nor formation of a secondary phase occurred. These results indicate that InP doping improves the amorphous stability of GeSbTe thin films. It is believed this enhanced amorphous stability is a result of the formation of multiple, strong crosslinks by the In and P atoms. [ABSTRACT FROM AUTHOR]

Oxygen-doped germanium telluride phase change materials are proposed for high temperature applications. Up to 8 at.% oxygen is readily incorporated into GeTe, causing an increased crystallisation temperature and activation energy. The rhombohedral structure of the GeTe crystal is preserved in the oxygen doped films. For higher oxygen concentrations the material is found to phase separate into GeO2 and TeO2, which inhibits the technologically useful abrupt change in properties. Increasing the oxygen content in GeTe-O reduces the difference in film thickness and mass density between the amorphous and crystalline states. For oxygen concentrations between 5 and 6 at.%, the amorphous material and the crystalline material have the same density. Above 6 at.% O doping, crystallisation exhibits an anomalous density change, where the volume of the crystalline state is larger than that of the amorphous. The high thermal stability and zero-density change characteristic of Oxygen-incorporated GeTe, is recommended for efficient and low stress phase change memory devices that may operate at elevated temperatures. [ABSTRACT FROM AUTHOR]