Fast Phase Change Research Articles

High thermal stability and fast speed phase change memory by optimizing GeSbTe with Scandium doping

Phase change materials along the GeTe-Sb isoelectronic tie line are proved to be promising host material for optimization, based on which the well-known golden composition is developed. Here, Sc doped Ge2Sb1Te2 has been proposed for phase change memory (PCM) application, showing higher thermal stability and faster operation speed than those of the golden composition. The fast speed of 40 ns, high 10-year data retention of 160 °C, and good endurance of 6×105 cycles have made Sc0.2Ge2Sb1Te2 a promising candidate for PCM application. The impact of Sc on the microstructure is believed to be essential for those improvements in PCM.

Atomistic insights into the enhanced stability and phase change speed of Al–Sb phase change materials

In this work, ab-initio molecular dynamics simulations were performed for Al–Sb system with and without excessive Al. Our results demonstrated the population of low angle bond is apparently increased due to the addition of excessive Al. The necessary bond transition from low angle to high angle in crystallization process was significantly retarded, which might be the origin of enhanced thermal stability of AlSb with excessive Al. The weak Al–Sb bond in the low Al–Sb–Al bond angle accounts for the fast phase change speed of Al–Sb system. Our work clarified the physical origin of the superior properties of Al–Sb system at the atomic scale and provided some new insights toward PCMs design.

Characteristics of Sb6Te4/VO2 Multilayer Thin Films for Good Stability and Ultrafast Speed Applied in Phase Change Memory**Supported by the National Natural Science Foundation of China under Grant No 11774438, the Natural Science Foundation of Jiangsu Province under Grant No BK20151172, the Qing Lan Project, the Opening Project of State Key Laboratory of Silicon Materials under Grant No SKL2017-04, the Opening

The Sb6 Te4/VO2 multilayer thin films are prepared by magnetron sputtering and the potential application in phase change memory is investigated in detail. Compared with Sb6 Te4, Sb6 Te4/VO2 multilayer composite thin films have higher phase change temperature and crystallization resistance, indicating better thermal stability and less power consumption. Also, Sb6 Te4/VO2 has a broader energy band of 1.58 eV and better data retention (125°C for 10 y). The crystallization is suppressed by the multilayer interfaces in Sb6 Te4/VO2 thin film with a smaller rms surface roughness for Sb6 Te4/VO2 than monolayer Sb4 Te6. The picosecond laser technology is applied to study the phase change speed. A short crystallization time of 5.21 ns is realized for the Sb6 Te4(2 nm)/VO2 (8 nm) thin film. The Sb6 Te4/VO2 multilayer thin film is a potential and competitive phase change material for its good thermal stability and fast phase change speed.

GPS navigation data bit decoding error during strong equatorial scintillation

Strong equatorial scintillation is often characterized by simultaneous fast phase changes and deep amplitude fading. The combined effect poses a challenge for GNSS receiver carrier tracking performance. One of the consequences of the strong scintillation is increased navigation message data bit decoding error. Understanding the rate of the data bit decoding error under equatorial scintillation is essential for high accuracy and high integrity applications. We present the statistical relationship between the data bit decoding error occurrences and the intensity of amplitude scintillation based on the processing of intermediate frequency GPS scintillation data collected on Ascension Island in March 2013. A third-order phase lock loop (PLL) is implemented to process the data and to access the data bit error typically expected in conventional receivers. A Kalman filter-based PLL is also used to process the same data to demonstrate that the data bit decoding error can be reduced through advanced carrier tracking designs.

Investigation of Sb65Se35/Sb multilayer thin films for high speed and high thermal stability application in phase change memory

Sb65Se35/Sb multilayer composite thin films were prepared by depositing the Sb65Se35 and Sb layers alternately. In situ resistance vs. temperature was measured and the crystallization temperature increased with thickening the Sb65Se35 layer in Sb65Se35/Sb thin films. The data retention temperature of 10 years increased greatly from 14 °C of pure Sb to 103 °C of [Sb65Se35(3 nm)/Sb(7 nm)]3. Also, the band gap was broadened and the surface became smoother. X-ray diffraction patterns for the studied materials revealed that Sb and Sb2Se3 phases coexisted in Sb65Se35/Sb thin films. Absorbing the advantages of the fast phase change for Sb, the [Sb65Se35(1 nm)/Sb(9 nm)]5 multilayer thin film had an ultrafast amorphization speed of 1.6 ns. The results indicated that Sb65Se35/Sb multilayer thin film was a potential phase change material for fast speed and good stability.

Theory and design consideration for cross-talk-free quasi-distributed sensing of fast fiber phase variation

It is the first time, to the best of the author’s knowledge, that a method for accurately detecting quasi-continuously distributed dynamic phase change in a fiber over a long range has been treated theoretically as an inverse problem. For interrogation, we have proposed a special architecture that performs a common spatiotemporal optical trajectory interferometric reflectometry. We have theoretically demonstrated that, by means of the architecture, fast phase changes occurring in a great number of fiber segments constituted by cascaded low-reflectance mirrors can be interrogated without intersegment cross-talk errors. A practical sensor system employing frequency modulated continuous wave data acquisition is designed and analyzed. The potential sensing performance is discussed.

A Semi-Open Loop GNSS Carrier Tracking Algorithm for Monitoring Strong Equatorial Scintillation

Strong equatorial ionospheric scintillation of radio signals is often associated with simultaneous deep amplitude fading and rapid random carrier phase fluctuations. It poses a challenge for satellite navigation receiver carrier phase tracking loop operation. This paper presents a semi-open loop algorithm that utilizes the known position of a stationary receiver and satellite orbit information to estimate carrier phase variations during deep signal fading. Strong GPS scintillation data collected on Ascension Island in March 2013 are processed to reveal fast phase changes of half and full cycles over tens of milliseconds on all three GPS frequencies. Simulated signals are used to validate the algorithm and confirm that the fast phase changes are true scintillation effects rather than receiver processing artifacts. The algorithm performance is demonstrated through comparison with several closed-loop implementations. Potential errors associated with receiver clock bias and other environmental factors are also analyzed. A statistical summary of the concurrent fast phase changes and deep fades based on the processing results of the Ascension Island data is presented.

Simultaneously Good Stability and High Speed Based on Oxygen-Doped Zn15Sb85 Material

In this study, the amorphous-to-crystalline transitions of oxygen-doped Zn15Sb85 materials were investigated by in situ film resistance measurements. After oxygen doping, the thermal stability, resistance and bandgap of the Zn15Sb85 material increased significantly. The data retention temperature for 10 years increased from 58°C of pure Zn15Sb85 to 200°C of oxygen-doped Zn15Sb85. The X-ray diffraction pattern showed that lots of Sb phases existed in O-doped Zn15Sb85, which induced the fast phase change. The formation of some Zn and Sb oxides were confirmed by X-ray photoelectron spectroscopy. The surface morphology observed by atomic force microscopy demonstrated that oxygen doping refined the grain size and restrained the crystallization. An ultra-short switching time of 2.40 ns was achieved for oxygen-doped Zn15Sb85 material. The results showed that appropriate oxygen doping could improve the performance of the Zn15Sb85 material in phase change memory.

Fast phase change with no preconditions

Phase-Change Memory Random access memory (RAM) devices that rely on phase changes are primarily limited by the speed of crystallization. Rao et al. combined theory with a simple set of selection criteria to isolate a scandium-doped antimony telluride (SST) with a subnanosecond crystallization speed (see the Perspective by Akola and Jones). They synthesized SST and constructed a RAM device with a 700-picosecond writing speed. This is an order of magnitude faster than previous phase-change memory devices and competitive with consumer dynamic access, static random access, and flash memory. Science , this issue p. [1423][1]; see also p. [1386][2] [1]: /lookup/doi/10.1126/science.aao3212 [2]: /lookup/doi/10.1126/science.aaq0476

The improvement of phase-change properties on Ge2Sb2Te5 using the superlattice-like structure

Ge2 Sb2 Te5 (GST)/Ge superlattice-like (SLL) films are investigated in the study. Reflection spectra measurements with polarized excitations and atomic force microscopy are carried out to investigate phase-change properties. Because of the Ge cladding films, GST phase-change properties could be improved. The thicker Ge cladding film is believed could lead to a relatively larger grain size. GST phase-change behaviors in the films can be adjusted by changing the layer thickness. We found the [GST(5 nm)/Ge(5 nm)]10 SLL film has the higher crystallization temperature, faster phase change speed, better thermal stability and relatively smooth surface. The GST/Ge SLL films may have the potential for commercial use.

Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing.

Operation speed is a key challenge in phase-change random-access memory (PCRAM) technology, especially for achieving subnanosecond high-speed cache memory. Commercialized PCRAM products are limited by the tens of nanoseconds writing speed, originating from the stochastic crystal nucleation during the crystallization of amorphous germanium antimony telluride (Ge2Sb2Te5). Here, we demonstrate an alloying strategy to speed up the crystallization kinetics. The scandium antimony telluride (Sc0.2Sb2Te3) compound that we designed allows a writing speed of only 700 picoseconds without preprogramming in a large conventional PCRAM device. This ultrafast crystallization stems from the reduced stochasticity of nucleation through geometrically matched and robust scandium telluride (ScTe) chemical bonds that stabilize crystal precursors in the amorphous state. Controlling nucleation through alloy design paves the way for the development of cache-type PCRAM technology to boost the working efficiency of computing systems.

Multilayer SnSb4-SbSe Thin Films for Phase Change Materials Possessing Ultrafast Phase Change Speed and Enhanced Stability.

A multilayer thin film, comprising two different phase change material (PCM) components alternatively deposited, provides an effective means to tune and leverage good properties of its components, promising a new route toward high-performance PCMs. The present study systematically investigated the SnSb4-SbSe multilayer thin film as a potential PCM, combining experiments and first-principles calculations, and demonstrated that these multilayer thin films exhibit good electrical resistivity, robust thermal stability, and superior phase change speed. In particular, the potential operating temperature for 10 years is shown to be 122.0 °C and the phase change speed reaches 5 ns in the device test. The good thermal stability of the multilayer thin film is shown to come from the formation of the Sb2Se3 phase, whereas the fast phase change speed can be attributed to the formation of vacancies and a SbSe metastable phase. It is also demonstrated that the SbSe metastable phase contributes to further enhancing the electrical resistivity of the crystalline state and the thermal stability of the amorphous state, being vital to determining the properties of the multilayer SnSb4-SbSe thin film.

Linear permeability evolution of expanding conduits due to feedback between flow and fast phase change

AbstractConduits are ubiquitous and critical pathways for many fluids relevant for geophysical processes such as magma, water, and gases. Predicting flow through conduits is challenging when the conduit geometry coevolves with the flow. We theoretically show that the permeability (k) of a conduit whose walls are eroding due to fast phase change increases linearly with time because of a self‐reinforcing mechanism. This simple result is surprising given complex feedbacks between flow, transport, and phase change. The theory is congruent with previous experimental observations of fracture dissolution in calcite. Supporting computational fracture dissolution experiments showed that k only slightly increases until the dissolution front reaches the narrowest conduit constriction, after which the linear evolution of k manifests. The theory holds across multiple scales and a broad range of Peclet and Damkohler numbers and thus advances the prediction of dynamic mass fluxes through expanding conduits in various geologic and environmental settings.

Investigation of Cu–Sn–Se material for high-speed phase-change memory applications

Cu–Sn–Se material was prepared by magnetron sputtering to investigate its potential application in phase change memory. The amorphous-to-crystalline transition was studied by in situ film resistance measurements. Cu–Sn–Se material had lower activation energy for crystallization (1.60 eV) and higher crystallization resistance than SnSe. The amorphous Cu–Sn–Se had more narrow band gap compared to SnSe. After the adding of Cu, the crystallization of Cu–Sn–Se material was inhibited and the grain structure became more compact. The picosecond laser pulse measurement indicated that Cu–Sn–Se material had a fast phase change speed (3.36 ns). The results demonstrated that Cu–Sn–Se material was a promising phase change material which had low power and high speed application in phase change memory.

Numerical solution of one-dimensional transient, two-phase flows with temporal fully implicit high order schemes: Subcooled boiling in pipes

An extension for 1-D transient two-phase flows of the SIMPLE-ESDIRK method, initially developed for incompressible viscous flows by Ijaz is presented. This extension is motivated by the high temporal order of accuracy demanded to cope with fast phase change events. This methodology is suitable for boiling heat exchangers, solar thermal receivers, etc. The methodology of the solution consist in a finite volume staggered grid discretization of the governing equations in which the transient terms are treated with the explicit first stage singly diagonally implicit Runge-Kutta (ESDIRK) method. It is suitable for stiff differential equations, present in instant boiling or condensation processes. It is combined with the semi-implicit pressure linked equations algorithm (SIMPLE) for the calculation of the pressure field. The case of study consists of the numerical reproduction of the Bartolomei upward boiling pipe flow experiment. The steady-state validation of the numerical algorithm is made against these experimental results and well known numerical results for that experiment. In addition, a detailed study reveals the benefits over the first order Euler Backward method when applying 3rd and 4th order schemes, making emphasis in the behaviour when the system is subjected to periodic square wave wall heat function disturbances, concluding that the use of the ESDIRK method in two-phase calculations presents remarkable accuracy and computational advantages.

Improved thermal and optical properties of Al-doped Sn2Se3 phase change material

Al-doped Sn2Se3 phase change material has higher crystallization temperature and lower conductivity than Ge2Sb2Te5 material. A series of optical constants of Al-doped Sn2Se3 materials are measured by spectroscopic ellipsometry measurement. The energy band gap is broadened after Al doping and the grain size becomes smaller. The optical reflectivity is reduced while the reflectivity contrast is enhanced. The phase transition speed between the amorphous and crystalline states is investigated by the picosecond laser pulses. A fast phase change speed is observed for Al-doped Sn2Se3 material.

O-doped Sb materials for improved thermal stability and high-speed phase change memory application

The O-doped Sb materials were proved to have higher crystallization temperature (∼205 °C), larger crystallization activation energy (3.95 eV) and better data retention ability (143 °C for 10 years). The band gap was broadened by O-doping and the grain size was refined. The formation of Sb oxide increased the binding energy. The fast phase change speed was obtained for O-doped Sb materials by picosecond laser technology. After O-doping, the phase change film had a smaller surface roughness (1.05 nm).

Variable Stiffness Fiber with Self-Healing Capability.

A variable stiffness fiber made of silicone and low melting point alloys quickly becomes >700 times softer and >400 times more deformable when heated above 62 °C. It shows remarkable self-healing properties and can be clamped, knitted, and bonded, as shown in a foldable multi-purpose drone, a wearable cast for bone injuries, and a soft multi-directional actuator.

(Invited) Ge-Cu-Te Phase Change Material for Pcram Application

Phase Change Random Access Memory (PCRAM) is a promising candidate as next generation non-volatile memory. The principle of PCRAM relies on the resistance contrast between crystalline and amorphous states of phase change material (PCM). Generally, the crystalline phase shows a low electrical resistance (set state), while the amorphous phase shows a high electrical resistance (reset state). The phase transition of PCM is achieved by way of Joule heating induced by an electrical pulse. Currently, Ge-Sb-Te films, namely Ge2Sb2Te5 (GST) compound film, are widely studied for PCRAM application because they show fast phase change speed and excellent reversibility of phase transition. However, GST has a low crystallization temperature (~160°C), which limits high temperature data retention capability. In addition, GST possesses a relatively high melting point (~630°C), which leads to a high power consumption for reset operation. Moreover, with further scaling down of PCRAM memory cell, the thermal disturbance between cells becomes serious problem. To overcome those weakness and problem, it is desired to develop new PCMs with high crystallization temperature and low melting point. From the above background, we have studied the effect of doping element, X on the crystallization temperature in Ge-Te-based PCMs and found that Al, Si and Cu are very effective to improve the thermal stability of Ge-Te amorphous film, based on the total bonding enthalpy of amorphous Ge-Te-X. Among them, Ge-Cu-Te ternary film has been found to have various advantages for PCRAM application. According to Ge-Cu-Te ternary phase diagram, there is a GeCu2Te3 (GCT) ternary compound which shows a melting point of around 500°C. Moreover, Cu2Te-Ge33.3Te66.7 pseudobinary phase diagram indicates that the liquidus line deeply decreases toward the compound composition like eutectic-type phase diagram, which suggests that the amorphous phase can be easily obtained around the compound composition. In this study, the phase transition characteristics of GeTe-CuTe pseudobinary films were investigated. GeTe-CuTe pseudobinary films, including GeCu2Te3 compound film, with a thickness of 200 nm were fabricated by co-sputtering of GeTe and CuTe targets on SiO2/Si substrate. Moreover, to evaluate the memory operation characteristics, simple memory devices with GCT layer were fabricated using a conventional photolithography technique. It was found that in GeTe-CuTe pseudobinary film, the crystallization temperature increases with increasing Cu content in the range of Cu < 15 at.% and the maximum value of about 250°C is obtained, and then the crystallization temperature decreases with further increasing Cu content. The GCT amorphous was found to show the crystallization temperature of about 230°C. It was confirmed that the GCT memory cell exhibits a reversible phase transition between amorphous and crystalline states and shows a lower power consumption for reset operation than GST memory cell. In addition, it was also found by static laser testing that the phase change speed of the GCT film is as fast as that of GST film. Furthermore, it was found that the GCT shows unique phase transition characteristics, namely, a small volume expansion and reflectance decrease by crystallization. Such changes upon phase transition for the GCT are opposite to those for conventional PCMs. It was suggested from AIMD simulation that such unique phase transition characteristics may be caused by unusual structural features of GCT amorphous, such as short Cu bond lengths, threefold rings, and dense Cu-rich regions. In this presentation, the phase transition characteristics of Ge-Cu-Te ternary films and the memory device characteristics will be discussed.

Multimode Coherent Pattern in Bistatic Scattering From Randomly Corrugated Surfaces With Irregular Grooves at L-Band

This paper investigates the bistatic scattering from randomly corrugated surfaces with irregular grooves. For a given incident wave, the surface fields and the scattered field were computed by the method of moments in which the rooftop basis function was used to account for fast phase changes due to steep surface slopes. The total scattered field is decomposed into coherent and incoherent components to analyze their respective contributions. We found that, for randomly corrugated surfaces with irregular ridges or grooves, the coherent scattering is profound at several scattering angles with strong main lobes, whose beamwidths are strongly correlated with the ridge density. The numerical simulation has shown that the lobe angular shift away from the specular direction is quasi-linearly dependent on the ridge density, suggesting that the coherent scattering pattern substantially contains the surface geometric information. We expect this paper to offer deeper understanding of coherent imaging of rough surface and to help in designing a novel imaging system.