You Yin

Characterization of nitrogen-doped Sb2Te3 films and their application to phase-change memory

In this study, sputtered undoped and nitrogen doped Sb2Te3 (ST and STN) films were systematically investigated by x-ray diffraction (XRD) and resistance measurements. Their application to lateral phase-change memory (PCM) is presented as well. The STN film sputtered at a flow rate ratio (N2∕Ar) of 0.07 proved to have both high stability and low power consumption, implying its high performance in PCM applications. In the STN films (N2∕Ar>0.15), the hexagonal Te phase first appeared at 160 °C, and then the orthorhombic SbN phase appeared at 290 °C. The phase separation made it very difficult for these films to switch reversibly between the crystalline and the amorphous phase.

Self-Assembled Incorporation of Modulated Block Copolymer Nanostructures in Phase-Change Memory for Switching Power Reduction

Phase change memory (PCM), which exploits the phase change behavior of chalcogenide materials, affords tremendous advantages over conventional solid-state memory due to its nonvolatility, high speed, and scalability. However, high power consumption of PCM poses a critical challenge and has been the most significant obstacle to its widespread commercialization. Here, we present a novel approach based on the self-assembly of a block copolymer (BCP) to form a thin nanostructured SiOx layer that locally blocks the contact between a heater electrode and a phase change material. The writing current is decreased 5-fold (corresponding to a power reduction by 1/20) as the occupying area fraction of SiOx nanostructures is increased from a fill factor of 9.1% to 63.6%. Simulation results theoretically explain the current reduction mechanism by localized switching of BCP-blocked phase change materials.

Associative memory realized by a reconfigurable memristive Hopfield neural network

Although synaptic behaviours of memristors have been widely demonstrated, implementation of an even simple artificial neural network is still a great challenge. In this work, we demonstrate the associative memory on the basis of a memristive Hopfield network. Different patterns can be stored into the memristive Hopfield network by tuning the resistance of the memristors, and the pre-stored patterns can be successfully retrieved directly or through some associative intermediate states, being analogous to the associative memory behaviour. Both single-associative memory and multi-associative memories can be realized with the memristive Hopfield network.

A Novel Lateral Phase-Change Random Access Memory Characterized by Ultra Low RESET Current and Power Consumption

We have fabricated and studied single lateral phase-change random-access-memory (PRAM), which has a confined Ge2Sb2Te5 (GST) channel connected by two wide TiN electrodes of relatively low resistivity. Its switching current for Reset operation could be as low as 4–20 µA, about one or two orders of magnitude lower than that of the conventional bottom contact PRAM cell. Its corresponding switching power for Reset operation is about 2–4 µW. The reason for such ultra low Reset current and power could be that Joule heating occurred mainly in the GST channel, instead of the resistive heater in the conventional PRAM cell.

Simulation of proposed confined-chalcogenide phase-change random access memory for low reset current by finite element modelling

A confined-chalcogenide (CC) cell structure for reducing the reset current of phase-change random access memory (PRAM) is proposed in this investigation. Both single normal-bottom-contact (NBC) (for reference) and proposed CC PRAM cells are simulated by two-dimensional finite element modelling. The simulated amorphous region of the NBC cell after reset operation is generally a semiellipse, which agrees very well with the reported experimental results. The CC cell has a rectangular amorphous region after reset operation. The reset operation current of the CC cell is much lower than that of the NBC cell. The CC cell structure needs a low reset current and a low power consumption and has a simple configuration.

Dependences of Electrical Properties of Thin GeSbTe and AgInSbTe Films on Annealing

We studied the electrical properties of 20- and 50-nm-thick Ge2Sb2Te5 and AgInSbTe films for nonvolatile lateral transistor memory devices. Both kinds of thin films were prepared as film samples and device samples which were then annealed at temperatures from 140 to 415°C. It is known that crystal size can be effectively reduced with film thickness on the basis of X-ray diffraction analysis. The resistances of all film samples annealed at 140–415°C decreased by approximately 5–6 orders of magnitude. In the case of device samples, however, the source-drain resistances of Ge2Sb2Te5 samples were first reduced and then reversely increased and it seemed that the resistances of AgInSbTe samples did not drop. The abnormal resistance increase above the crystallization temperature may be caused by phase change and thermal expansion, as we analyzed in this paper. Finally, the resistance changes of device samples with channel lengths in the range of 0.4–3 µm were discussed from the point of view of miniaturizing the phase change memory device.

Finite Element Analysis of Dependence of Programming Characteristics of Phase-Change Memory on Material Properties of Chalcogenides

The programming characteristics of a phase-change memory (PCM) cell with a chalcogenide layer contacted by a resistive heater are investigated by finite element modelling. As analyzed in this study, the characteristics are markedly affected by the resistivity of the phase-change chalcogenide material. A higher reset current of 1.6 mA is required for the as-fabricated virgin PCM than that of 1.3 mA for the cycled PCM because of the resistivity difference of the chalcogenides in the two cases. More importantly, a chalcogenide layer with a much higher resistivity than the resistive heater is necessarily adopted for a higher energy efficiency to markedly reduce reset current to 0.6 mA or even lower while slightly increasing reset voltage.

Emulating the paired-pulse facilitation of a biological synapse with a NiOx-based memristor

We study the paired-pulse-induced response of a NiOx-based memristor. The behavior of the memristor is surprisingly similar to the paired-pulse facilitation of a biological synapse. When the memristor is stimulated with a pair of electrical pulses, the current of the memristor induced by the second pulse is larger than that by the first pulse. In addition, the magnitude of the facilitation decreases with the pulse interval, while it increases with the pulse magnitude or pulse width.

Programming margin enlargement by material engineering for multilevel storage in phase-change memory

In this work, we investigate the effect of the material engineering on programming margin in the double-layered phase-change memory, which is the most important parameter for the stability of multilevel storage. Compared with the TiN/SbTeN cell, the TiSiN/GeSbTe double-layered cell exhibits the resistance ratio of the highest to lowest resistance levels up to two to three orders of magnitude, indicating much larger programming margin and thus higher stability and/or more available levels. Our calculation results show that the resistivities of the top heating layer and the phase-change layer have a significant effect on the programming margin.

Flexible One Diode-One Phase Change Memory Array Enabled by Block Copolymer Self-Assembly

Flexible memory is the fundamental component for data processing, storage, and radio frequency communication in flexible electronic systems. Among several emerging memory technologies, phase-change random-access memory (PRAM) is one of the strongest candidate for next-generation nonvolatile memories due to its remarkable merits of large cycling endurance, high speed, and excellent scalability. Although there are a few approaches for flexible phase-change memory (PCM), high reset current is the biggest obstacle for the practical operation of flexible PCM devices. In this paper, we report a flexible PCM realized by incorporating nanoinsulators derived from a Si-containing block copolymer (BCP) to significantly lower the operating current of the flexible memory formed on plastic substrate. The reduction of thermal stress by BCP nanostructures enables the reliable operation of flexible PCM devices integrated with ultrathin flexible diodes during more than 100 switching cycles and 1000 bending cycles.