Veronique Sousa

Phase change memory as synapse for ultra-dense neuromorphic systems: Application to complex visual pattern extraction

We demonstrate a unique energy efficient methodology to use Phase Change Memory (PCM) as synapse in ultra-dense large scale neuromorphic systems. PCM devices with different chalcogenide materials were characterized to demonstrate synaptic behavior. Multi-physical simulations were used to interpret the results. We propose special circuit architecture (“the 2-PCM synapse”), read, write, and reset programming schemes suitable for the use of PCM in neural networks. A versatile behavioral model of PCM which can be used for simulating large scale neural systems is introduced. First demonstration of complex visual pattern extraction from real world data using PCM synapses in a 2-layer spiking neural network (SNN) is shown. System power analysis for different scaled PCM technologies is also provided.

Electrical Behavior of Phase-Change Memory Cells Based on GeTe

In this letter, we present a study on the electrical behavior of phase-change memories (PCMs) based on a GeTe active material. GeTe PCMs show, first, extremely rapid SET operation (yielding a gain of more than one decade in energy per bit with respect to standard GST PCMs), second, robust cycling, up to 1 × 105, with 30-ns SET and RESET stress time, and third, a better retention behavior at high temperature with respect to GST PCMs. These results, obtained on single cells, suggest GeTe as a promising alternative material to standard GST to improve PCM performance and reliability.

Thermal characterization and analysis of phase change random access memory

The cross-plane thermal conductivity of Ge2Sb2Te5, either in its amorphous state or fcc crystallized state, and titanium nitride (TiN) thin films has been measured at room temperature by the 3ω method. These materials are involved in the fabrication of phase change random access memory (PC-RAM), Ge2Sb2Te5 and TiN being the PC and pseudoelectrode materials, respectively. The thermal conductivity of insulating SiO2 and ZnS:SiO2 layers was determined too. Each thermal conductivity measurement was performed by the means of at least two strip widths in order to check both the measurement self-consistency and the measurement accuracy. The performance of PC-RAM cells, i.e., the time needed to reach the melting temperature of the PC material and the cooling speed, has been evaluated as a function of both the measured thermal conductivity of the PC material and the reset current intensity independently of the thermal properties of the pseudoelectrodes by the way of analytical formula. The influence of the thicknes...

Physical aspects of low power synapses based on phase change memory devices

In this work, we demonstrate how phase change memory (PCM) devices can be used to emulate biologically inspired synaptic functions in particular, potentiation and depression, important for implementing neuromorphic hardware. PCM devices with different chalcogenide materials are fabricated and characterized. The asymmetry between the potentiation and depression behaviors of the PCM is stressed. Detailed multi-physical simulations are performed to study the underlying physics of the synaptic behavior of PCM. A versatile behavioral model and a multi-level circuit-compatible model are developed for system and circuit-level neuromorphic simulations. We propose a unique low-power methodology named the 2-PCM Synapse, to use PCM devices as synapses in large scale neuromorphic systems. To show the strength of our proposed solution, we efficiently simulated fully connected feed-forward spiking neural network capable of complex visual pattern extraction from real world data.

A Novel Programming Technique to Boost Low-Resistance State Performance in Ge-Rich GST Phase Change Memory

In this paper, we examine the problem of the drift of the low-resistance state (LRS) in phase change memories based on C or N doped and undoped Ge-rich Ge2Sb2Te5. A novel procedure, named R-SET technique, is proposed to boost the SET speed of these innovative phase change materials by overcoming the decrease of crystallization speed caused by Ge enrichment. The R-SET technique allows, at the same time, an optimized SET programming of the memory cell and the reduction of the LRS drift with respect to standard SET procedures. A circuit that generates the desired R-SET pulse based on a time reference scheme is proposed and discussed.

Phase change memory for synaptic plasticity application in neuromorphic systems

In this paper, we show that Phase Change Memory (PCM) can be used to emulate specific functions of a biological synapse similar to Long Term Potentiation (LTP) and Long Term Depression (LTD) plasticity effects. The dependence of synaptic weight on programming pulse width and pulse amplitude is shown experimentally for the PCM devices. Different combinations of consecutive LTD and LTP events have been experimentally demonstrated and analyzed for the PCM synapse.

Structural change with the resistance drift phenomenon in amorphous GeTe phase change materials’ thin films

Ageing of the amorphous phase of chalcogenide phase change materials is characterized by a large increase of their resistivity with time. This phenomenon, known as resistance drift and commonly attributed to structural relaxation, the nature of which remains unknown, has until now hindered the development of ultra-high multilevel storage devices. The origin of the resistance drift of amorphous GeTe thin films is studied here by resistivity measurements and grazing incidence x-ray absorption spectroscopy (GIXAS). The local order around Ge atoms is investigated at the Ge K-edge on a-GeTe samples previously set at different resistance drift levels by thermal annealing. In all samples, Ge–Ge and Ge–Te bonds coexist. This study demonstrates that the drift phenomenon is concomitant with structural changes linked to Ge–Ge homopolar bonds.

Phase Change and Magnetic Memories for Solid-State Drive Applications

The state-of-the-art solid-state drives (SSDs) now heterogeneously integrate NAND Flash and dynamic random access memories (DRAMs) to partially hide the limitation of the nonvolatile memory technology. However, due to the increased request for storage density coupled with performance that positions the storage tier closer to the latency of the processing elements, NAND Flash are becoming a serious bottleneck. DRAM as well are a limitation in the SSD reliability due to their vulnerability to the power loss events. Several emerging memory technologies are candidate to replace them, namely the storage class memories. Phase change memories and magnetic memories fall into this category. In this work, we review both technologies from the perspective of their possible application in future disk drives, opening up new computation paradigms as well as improving the storage characteristics in terms of latency and reliability.

Carbon-doped Ge 2 Sb 2 Te 5 phase-change memory devices featuring reduced RESET current and power consumption

In this paper, carbon-doped Ge₂Sb₂Te₅, integrating from 5% to 15% of carbon content, is studied as an alternative phase-change material. Accurate electrical characterizations were performed both on large and shrinked PCM devices. Compared to pure Ge₂Sb₂Te₅ based reference devices, a wide decrease of about 50% of the RESET current, which translates into a RESET power reduction of about 25%, is observed when 5% of carbon is added to Ge₂Sb₂Te₅. Moreover, an improved endurance up to 10⁸ cycles is obtained while maintaining a programming window higher than 2 orders of magnitude. An increase of about 30% of the activation energy for the crystallization process is also observed. Therefore, this paper suggests that Ge₂Sb₂Te₅ doped with 5% of carbon is a promising phase-change material for future PCM technology.

Thermal conductivity of carbon doped GeTe thin films in amorphous and crystalline state measured by modulated photo thermal radiometry

The thermal conductivity and thermal boundary resistance of GeTe and carbon doped GeTe thin films, designed for phase change memory (PCM) applications, were investigated by modulated photo thermal radiometry. It was found that C doping has no significant effect on the thermal conductivity of these chalcogenides in amorphous state. The thermal boundary resistance between the amorphous films and SiO2 substrate is also not affected by C doping. The films were then crystallized by an annealing at 450°C as confirmed by optical reflectivity analysis. The thermal conductivity of non-doped GeTe significantly increases after crystallization annealing. But, surprisingly the thermal conductivity of the crystallized C doped GeTe was found to be similar from that of the amorphous state and independent of C concentration. As for the amorphous phase, C doping does not affect the thermal boundary resistance between the crystalline GeTe films and SiO2 substrate. This behaviour is discussed thanks to XRD and FTIR analysis. In particular, XRD shows a decrease of crystalline grain size in crystalline films as C concentration is increased. FTIR analysis of the film before and after crystallization evidenced that this evolution could be attributed to the disappearing of Ge-C bonds and migration of C atoms out of the GeTe phase upon crystallization, limiting then the growth of GeTe crystallites in C-doped films.