Set-reset Operation Research Articles

The improvement of endurance characteristics in a superlattice-like material-based phase change device

Improvement of endurance characteristics has been a hot area of phase-change memoryresearch. The properties of a phase-change material are believed to play an important role in device endurance. Repeated SET–RESET operation always leads to material failure problems, such as composition deviation and phase separation. Moreover, the quality of the electrode and the electrode contact also determine the endurance characteristics. In this study, C nanolayers were periodically inserted into the phase-change material Ge2Sb2Te5 (GST) to fabricate a superlattice-like (SLL) structure. Although some of C bonded with some of the Ge, Sb and Te atoms, more C atoms prefer nanometer-scale clusters at the grain boundary in the SLL film. The typical local configuration of GST was unchanged when artificial C nanolayers were inserted. Transmission electron microscopy experiments revealed that the bonded C atoms and nanometer-scale C clusters may occupy the spontaneously created holes and defects, preventing composition deviation of the phase-change material and prolonging the electrode service life. The contact surface between the phase-change material and the electrode is also improved. As a result, we found that the endurance cycle could be improved by up to 106 for a GST/C SLL film-based device.

A Fast Weight Transfer Method for Real-Time Online Learning in RRAM-Based Neuromorphic System

In this work, a synaptic weight transfer method for a neuromorphic system based on resistive-switching random-access memory (RRAM) is proposed and validated. To implement the on-chip trainable neuromorphic system which utilizes large-scale hardware synapse units, a fast and reliable write scheme needs to be established. Based on the experimental results, it is confirmed that the gradual set and full reset operation is the most suitable operation scheme for fast programming due to the fundamental reliability characteristics of the resistive-switching memory cell. Also, the superiority of this programming method using the proposed RRAM compact model is demonstrated. In addition, a one weight/one synaptic device structure is newly adopted for realizing high-density synapse arrays by using a nonnegative weight constraint in supervised learning. Finally, the pattern recognition accuracies obtained at the software and hardware levels are compared.

All-optical switching and flip-flop based on dynamically controlled bistability in a V-type atomic system

We have proposed a scheme for all-optical switching and flip-flop based on controllable optical bistability in a simple three-level V-type atomic system assisted by an incoherent pump field. Due to the auxiliary incoherent pump, the atomic population is redistributed in the three levels, and then the absorption-dispersion property and the Kerr nonlinearity of the medium are different from those of the conventional V-type electromagnetically induced transparency system. The results show that both the thresholds and the area (or shape) of the hysteresis loop can be controlled efficiently via the adjustment of either the incoherent pump field or the coupling field. As a result, with a constant cavity input, the output of the optical cavity can be switched between the low- and high-power states of different bistable curves by tuning the optical fields. Based on this effect, we realize all-optical steady-state switching and set-reset flip-flop operation by adding a pulse sequence (positive pulse or set-reset pulse) onto either the incoherent pump field or the coupling field. Such a simple and effective technique of optical switching and flip-flop can find application in all-optical signal processing systems.

Direct observation of Ag filament growth and unconventional SET-RESET operation in GeTe amorphous films

We report on the direct observation of Ag filament growth and a peculiar resistance switching in amorphous GeTe films with a lateral electrode geometry. The Ag filament growth was monitored by in-situ optical microscopy. The resistance switching was studied in three electrode pairs, Ag–Ag, Pt–Ag, and Pt–Ag/Pt (Ag electrode covered with Pt). In all the three electrode pairs, similar dendritic Ag filaments were clearly observed growing along both directions from one electrode to the other, according to the applied bias polarity. However, the SET and RESET processes are quite different. The Ag-Ag pair produces a unipolar clockwise switching. The Pt–Ag pair shows a bipolar counter-clockwise switching, as predicted in the basic electrochemical metallization theory, but the observed switching polarity is exactly opposite to the basic theory prediction. The Pt–Ag/Pt pair produces a unipolar counter-clockwise switching. The peculiar SET/RESET processes are explained on the basis of strong Ag diffusion into GeTe matrix resulting in an asymmetric effective electrode pair. The findings suggest that the SET/RESET processes are controlled by the amount of Ag and the electrode geometry.

Germanium Nitride as a Buffer Layer for Phase Change Memory

The results of adhesion improvement and SET-RESET operation voltage reduction for the GeN buffer layer are presented. It is found that the adhesive strength between the Ge2Sb2Te5(GST) layer and the layer below could be increased at least 20 times, which is beneficial for solving the phase change material peeling issue in the fabrication process of phase change memory (PCM). Meanwhile, the RESET voltage of the PCM cell with a 3-nm-thick GeN buffer layer can be reduced from 3.5 V to 2.2 V. The GeN buffer layer will play an important role in high density and low power consumption PCM applications.

197Au irradiation study of phase‐change memory cell with GeSbTe alloy

AbstractA 197Au ion source was used to irradiate a Ge2Sb2Te5‐alloy‐based phase‐change memory (PCM) cell to study the ion‐irradiation effect on the properties of the cell. The PCM devices with the tungsten (W) heating electrode of 260 nm diameter were fabricated by 0.18 µm complementary metal‐oxide‐semiconductor (CMOS) technology. Four different doses (1010, 1011, 1012, and 5 × 1012 ions/cm2, respectively) were applied to irradiate the PCM cell. The samples before and after irradiation were characterized by current–voltage and resistance measurements at room temperature. It is found that the cell properties (resistance value of the amorphous and crystalline states, threshold voltage, and current for phase transition, etc.) have hardly changed, even for the sample irradiated up to 1012 ions/cm2 dose, and the cell still has good set–reset operation ability (above 105 cycles). Furthermore, the resistance ratio remains at 1000 even after 105 cycles of the set–reset operation. The results show the PCM cell with Ge2Sb2Te5 alloy has a strong ion‐irradiation tolerance.

Design for Delay Fault Testability of 2-Rail Logic Circuits

This paper presents a scan design for delay fault testability of 2-rail logic circuits. The flip flops used in the scan design are based on master-slave ones. The proposed scan design provides complete fault coverage in delay fault testing of 2-rail logic circuits. In two-pattern testing with the proposed scan design, initial vectors are set using the set-reset operation, and the scan-in operation for initial vectors is not required. Hence, the test application time is reduced to about half that of the enhanced scan design. Because the additional function is only the set-reset operation of the slave latch, the area overhead is small. The evaluation shows that the differences in the area overhead of the proposed scan design from those of the standard scan design and the enhanced scan design are 2.1 and -14.5 percent on average, respectively.

All-optical set-reset operation of a bistable semiconductor laser intracavity-coupled to a vertical-cavity surface-emitting laser

We experimentally demonstrate a bistable laser diode (BLD) formed by a vertical-cavity surface-emitting laser (VCSEL) coupled to an in-plane laser (IPL) that includes a saturable absorber section. The device can be switched between its two states (corresponding to either the in-plane laser or the vertical-cavity laser being on) by injection of optical pulses. We also discuss possible applications for this device.

GaInAsP-InP distributed feedback waveguides for all-optical switching

All-optical switching characteristics of waveguide devices having a nonlinear distributed feedback structure are reported theoretically and experimentally. These devices were composed of a strip-loaded GaInAsP-InP waveguide and a Bragg reflector, which were suitable for optical integrated circuit. In the experiments, all-optical and gate operation and set-reset operation for two output ports were successfully demonstrated.

All-optical switching in a distributed-feedback GaInAsP waveguide

The characteristics of all-optical switching in a waveguide device with a distributed-feedback structure were experimentally investigated. The device was composed of a strip-loaded GaInAsP/InP waveguide and a distributed-feedback structure, which was fabricated by a combination of reactive-ion etching and electron-beam exposure. In the experiments, several optical switching operations were demonstrated. In particular, the all-optical set-reset operation and threshold operation were obtained.

All-optical set-reset operation in a distributed feedback GaInAsP waveguide

An optical bistable device is realized by an optical feedback structure and the optical Kerr effect. We fabricated an optical distributed feedback waveguide in a GaInAsP-InP wafer. All-optical set-reset operation has been observed in the GaInAsP-InP waveguide at a wavelength of 1.56 /spl mu/m.

Optical bistability and set-reset operation of a Fabry-Perot semiconductor laser amplifier with two detuned light injections

Optical bistability in a Fabry-Perot semiconductor laser amplifier when two optical inputs detuned from the resonant wavelength of a semiconductor laser amplifier are injected is discussed. A split branch is found in the optical output versus input characteristics in addition to conventional optical bistability behavior. It is shown analytically and experimentally that set and reset can be achieved in the optical output of the semiconductor laser amplifier for each wavelength by applying two detuned optical pulses and using the split branch. >

Undershooting and set-reset operation in bistable laser diodes with inhomogeneous excitation

The effect of undershoot switching, downward switching from the upper to the lower state by undershooting caused by the rapid diminishing of the applied optical pulse, is analyzed for bistable laser diodes with inhomogeneous current injection. The height and width of the optical input pulse superposed on the optical bias are shown to play an important role in undershoot switching as well as the level of the optical bias. It is also shown that set-reset and the generation of the optical monostable pulse can be done by using undershoot switching. >