Solid-state Memory Research Articles

A physics-based model of electrical conduction decrease with time in amorphous Ge2Sb2Te5

The time-stability of the electrical characteristics of chalcogenide materials is one of the most important issues for their use in nonvolatile solid state memory applications. In particular the electrical conduction of the glassy phase evolves with time due to two different physical phenomena: the crystallization and the so-called low conductivity drift. Despite the physics of crystallization having been extensively studied in literature, the latter is mainly described by phenomenological relationships, and its physical comprehension is still under discussion. In this paper we study the amorphous phase low-field conductivity drift and its dependence on the temperature experienced by the device. We developed an experimental procedure able to separate the reversible change in the electrical conductivity with temperature due to the material semiconductorlike behavior from the nonreversible one related to the drift mechanism. A drift model explaining such nonreversible conductivity change as a band diagram modification is also provided and calibrated on experimental data. The present work finally introduces alternative metrics for drift quantification that can be useful in order to compare different materials.

Industrial perspectives panel

This year, we have a different format for HPCA's industrial session. We will have an "industrial perspectives panel" with practitioners from industry presenting their perspectives on interesting future technical challenges and opportunities for research. We have panelists from IBM, Microsoft, NVIDIA, and Sun exploring a spectrum of interesting areas from consumer to enterprise markets including "hot" topics around multicores, graphics accelerators, and solid-state memory.

Conductance switching in Ag2S devices fabricated by in situ sulfurization

We report a simple and reproducible method to fabricate switchableAg2S devices.The α-Ag2S thin films are produced by a sulfurization process after silverdeposition on an Si substrate. Structure and composition of theAg2S are characterized using XRD and RBS. Our samples show semiconductor behaviour at lowbias voltages, whereas they exhibit reproducible bipolar resistance switching at higher biasvoltages. The transition between both types of behaviour is observed by hysteresis in theI–V curves, indicatingdecomposition of the Ag2S, increasing the Ag+ ionmobility. The as-fabricated Ag2S samples are a good candidate for future solid state memory devices, as they showreproducible memory resistive properties and they are fabricated by an accessible andreliable method.

A study on optical pickup actuator for small form-factor optical disc drives

Optical disc drives (ODDs) such as CD, DVD and HD-DVD are cheaper and more distributable than solid-state memories and hard disc drives (HDDs). In the era of mobile environment, ultra-slim ODDs have been studied and developed to show the feasibility of mobile application. This paper is a study on miniaturization of an optical pickup actuator, one of key components for ultra-slim ODDs. A newly configured actuator which is targeted to be installed in 5 mm height ODDs is presented in this work. In order to reduce the height of the optical pickup actuator, a novel structure of suspension is developed by arranging Ω-shaped plate-suspension at both sides of a moving part in same plane. Also, the reduction of unwanted resonance peak, which is one of main issues in optical pickup actuators, is attempted. The presented structure of suspension is optimized by finite element analysis in terms of linearity and damping characteristic that reduces the unwanted resonance such as the rolling, pitching modes. The electromagnetic circuits, which generate driving forces, are arranged for high sensitivity and the reduction of the unwanted resonances. Developed actuator is tested to confirm stable frequency response characteristics in focusing and tracking directions as well as low tilt characteristics. In addition, the actuator is mounted on a DVD pickup system and optical signal from this optical pickup is detected. This reveals the feasibility as an information storage for mobile devices.

Instrument Data Processing Unit for THEMIS

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is a NASA Medium-class Explorer (MIDEX) mission, launched on February 17, 2007. The mission employs five identical micro-satellites, or “probes,” which line-up along the Earth’s magnetotail every four days in conjunctions to determine the trigger and large-scale evolution of magnetic substorms. The probes are equipped with a comprehensive suite of instruments that measure and track the motion of thermal and super-thermal ions and electrons, and electric and magnetic fields, at key regions in the magnetosphere. Primary science objectives require high data rates at periods of scientific interest, large data volumes, and control of science data collection on suborbital time scales. A central Instrument Data Processing Unit (IDPU) is necessary to organize and prioritize the data from the large number of instruments into a 200 MB solid state memory. The large data volume produced by the instruments requires a flexible memory capable of both high resolution snapshots during conjunctions and coarser survey data collection throughout the orbit. Onboard triggering algorithms select and prioritize the snapshots based on data quality to optimize the science data that is returned to the ground. This paper presents a detailed discussion of the hardware and software design of the THEMIS IDPU, describing the heritage design that has been fundamental to the THEMIS mission success so far.

Quasistatic shared libraries and XIP for memory footprint reduction in MMU-less embedded systems

Despite a rapid decrease in the price of solid state memory devices, system memory is still a very precious resource in embedded systems. The use of shared libraries and execution-in-place (XIP) is known to be effective in significantly reducing memory usage. Unfortunately, many resource-constrained embedded systems lack an MMU, making it extremely difficult to support these techniques. To address this problem, we propose a novel shared library technique called a quasi-static shared library and an XIP, both based on our enhanced position independent code technique. In our quasistatic shared libraries, global symbols are bound to pseudoaddresses at linking time and actual physical addresses are bound at loading time. Unlike conventional shared libraries, they do not require symbol tables that take up valuable memory space and, therefore, allow for expedited address translation at runtime. Our XIP technique is facilitated by our enhanced position independent code where a data section can be arbitrarily located. Both the shared library and XIP techniques are made possible by emulating an MMU's memory mapping feature with a data section base register (DSBR) and a data section base table (DSBT). We have implemented these proposed techniques in a commercial ADSL (Asymmetric Digital Subscriber Line) home network gateway equipped with an MMU-less ARM7TDMI processor core, 2MB flash memory, and 16MB RAM. We measured its memory usage and evaluated its performance overhead by conducting a series of experiments. These experiments clearly demonstrate the effectiveness of our techniques in reducing memory usage. The results are impressive: 35% reduction in flash memory usage when using only the shared library and 30% reduction in RAM usage when using the shared library and XIP together. These results were achieved with only a negligible performance penalty of less than 4%. Even though these techniques were applied to uClinux-based embedded systems, they can be used for any MMU-less real-time operating system.

Solid-state quantum memory using the 31P nuclear spin

The transfer of information between the entities that do the processing and memory is crucial — and problematic — for quantum computation. In classical systems the information transfer can include a copying step, where errors can be spotted and corrected, but in quantum systems copying is fundamentally precluded. Morton et al. demonstrate a technology that could solve the problem: the coherent storage and readout of information between electron-spin processing elements and memory elements based on a nuclear spin. The system utilizes phosphorus-31 spin donors in a silicon-28 crystal. The nuclear spin acts as a memory element that can faithfully store the full state of the electron spin for more than a second, then transfer it back to the electron spin with about 90% efficiency. The transfer of information between processing entities and memory is crucial for quantum computation; it is challenging because the process must remain coherent at all times to preserve the quantum nature of the information. This paper demonstrates coherent storage and readout of information between electron-spin processing elements and memory elements based on a nuclear spin. The transfer of information between different physical forms—for example processing entities and memory—is a central theme in communication and computation. This is crucial in quantum computation1, where great effort2 must be taken to protect the integrity of a fragile quantum bit (qubit). However, transfer of quantum information is particularly challenging, as the process must remain coherent at all times to preserve the quantum nature of the information3. Here we demonstrate the coherent transfer of a superposition state in an electron-spin ‘processing’ qubit to a nuclear-spin ‘memory’ qubit, using a combination of microwave and radio-frequency pulses applied to 31P donors in an isotopically pure 28Si crystal4,5. The state is left in the nuclear spin on a timescale that is long compared with the electron decoherence time, and is then coherently transferred back to the electron spin, thus demonstrating the 31P nuclear spin as a solid-state quantum memory. The overall store–readout fidelity is about 90 per cent, with the loss attributed to imperfect rotations, and can be improved through the use of composite pulses6. The coherence lifetime of the quantum memory element at 5.5 K exceeds 1 s.

Phase change materials and their application to random access memory technology

Phase change materials can exist in two different phases, the amorphous and the crystalline phase, which exhibit distinctly different physical properties. It is possible to repeatedly switch the state of these materials, from the amorphous phase to the crystalline phase by heating the material above its crystallization temperature, and from the crystalline to the amorphous phase by melt-quenching. Phase change materials have been utilized very successfully in all modern optical re-writable storage media such as CDs, DVDs and Blu-ray disks. Recently, they have also been applied to solid-state memory devices where their large difference in electrical resistivity is used to store information. This paper reviews the unique properties of phase change materials in particular as they are important for their application to these devices.

Cellular automata modelling of phase-change memories

A novel approach to modelling phase-transition processes in phase change materials used for optical and electrical data storage applications is presented. The model is based on a cellular automaton (CA) approach to predict crystallization behaviour that is linked to thermal and electrical simulations to enable the study of the data writing and erasing processes. The CA approach is shown to be able to predict the evolution of the microstructure during the rapid heating and cooling cycles pertinent to data storage technology, and maps crystallization behaviour on the nanoscale. A simple example based on possible future nonvolatile phase-change random access solid-state memory is presented.

Overview of candidate device technologies for storage-class memory

Storage-class memory (SCM) combines the benefits of a solid-state memory, such as high performance and robustness, with the archival capabilities and low cost of conventional hard-disk magnetic storage. Such a device would require a solid-state nonvolatile memory technology that could be manufactured at an extremely high effective areal density using some combination of sublithographic patterning techniques, multiple bits per cell, and multiple layers of devices. We review the candidate solid-state nonvolatile memory technologies that potentially could be used to construct such an SCM. We discuss evolutionary extensions of conventional flash memory, such as SONOS (silicon-oxide-nitride-oxide-silicon) and nanotraps, as well as a number of revolutionary new memory technologies. We review the capabilities of ferroelectric, magnetic, phase-change, and resistive random-access memories, including perovskites and solid electrolytes, and finally organic and polymeric memory. The potential for practical scaling to ultrahigh effective areal density for each of these candidate technologies is then compared.

Crystallization properties of ultrathin phase change films

The crystallization behavior of ultrathin phase change films was studied using time-resolved x-ray diffraction (XRD). Thin films of variable thickness between 1 and 50nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST, Ge15Sb85, Sb2Te, and Ag- and In-doped Sb2Te were heated in a He atmosphere, and the intensity of the diffracted x-ray peaks was recorded. It was found for all materials that the crystallization temperature increases as the film thickness is reduced below 10nm. The increase depends on the material and can be as high as 200°C for the thinnest films. The thinnest films that show XRD peaks are 2nm for GST and N-GST, 1.5nm for Sb2Te and AgIn-Sb2Te, and 1.3nm for GeSb. This scaling behavior is very promising for the application of phase change materials to solid-state memory technology.

Magnetic Domain-Wall Racetrack Memory

Recent developments in the controlled movement of domain walls in magnetic nanowires by short pulses of spin-polarized current give promise of a nonvolatile memory device with the high performance and reliability of conventional solid-state memory but at the low cost of conventional magnetic disk drive storage. The racetrack memory described in this review comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip. Individual spintronic reading and writing nanodevices are used to modify or read a train of approximately 10 to 100 domain walls, which store a series of data bits in each nanowire. This racetrack memory is an example of the move toward innately three-dimensional microelectronic devices.

Three-levels conductance switching in an organic memory cell

A solid-state memory based on a hindered phenol substituted bithiophene is reported, showing conductance switching behaviour between three discrete states among which it is possible to cycle repeatedly on the basis of three characteristic threshold voltages. The experimental measurements show that the addressing of the three states is independent of the programming time, that the retention time for each state is longer than 15 h and that the devices endure current–voltage cycles well in excess of 50. Considerations based on density functional theory calculations are made to provide hints on the mechanism behind this memory effect.

THE TYPICAL PHOTOGRAPHIC MATERIALS AND THE SOLID-STATE MEMORY ELEMENT CHARACTERISTICS AND PARAMETERS COMPARATIVE ANALYSIS

The memory elements on the basis of nonideal heterojunction photoelectric characteristics have been researched. The sensitometry concepts application to the solid-state memory elements has allowed receiving universal expression for a sensor characteristic curve which coordinate well with the experimental data and can be used at similar devices sensitometric characteristics research. The conclusion that the sensor on the basis of nonideal heterojunction can be characterized with the help of classical sensitometric characteristics developed for photographic materials is made.

MAGNETIC RACE-TRACK — A NOVEL STORAGE CLASS SPINTRONIC MEMORY

A proposal for a novel storage-class memory is described in which magnetic domains are used to store information in a "magnetic race-track".1 The magnetic race-track shift register storage memory promises a solid state memory with storage capacities and cost rivaling that of magnetic disk drives but with much improved performance and reliability. The magnetic race track is comprised of tall columns of magnetic material arranged perpendicularly to the surface of a silicon wafer. The domains are moved up and down the race-track by nanosecond long current pulses using the phenomenon of spin momentum transfer. The domain walls in the magnetic race-track are read using magnetic tunnel junction magnetoresistive sensing devices arranged in the silicon substrate. Recent progress in developing magnetic tunnel junction devices with giant tunneling magnetoresistance exceeding 350% at room temperature will be mentioned.2 Experiments exploring the current induced motion and depinning of domain walls in magnetic nano-wires with artificial pinning sites will be discussed. The domain wall structure, whether vortex or transverse, and the magnitude of the pinning potential is shown to have surprisingly little effect on the current driven dynamics of the domain wall motion.3 By contrast the motion of DWs under nanosecond long current pulses is surprisingly sensitive to their length.4 In particular, we find that the probability of dislodging a DW, confined to a pinning site in a permalloy nanowire, oscillates with the length of the current pulse, with a period of just a few nanoseconds. Using an analytical model and micromagnetic simulations we show that this behaviour is connected to a current induced oscillatory motion of the DW. The period is determined by the DW mass and the curvature of the confining potential. When the current is turned off during phases of the DW motion when the DW has enough momentum, there is a boomerang effect that can drive the DW out of the confining potential in the opposite direction to the flow of spin angular momentum. Note from Publisher: This article contains the abstract only.

In search of time lost

During the past few years, the price of solid-state digital memory has dropped considerably, while storage capacity has increased. As a result, it is now possible to take thousands of high-resolution pictures with a digital camera or carry around hundreds of hours of music or podcasts. Such advances in digital storage mean that a portable personal digital archive of emails, photographs, recordings of phone conversations, GPS locations, is technically possible. Furthermore, in the not-too-distant future, prosthetic memories, which could help patients with memory impairments, might become a reality.

Detection and Classification of Single-Electron Jumps in Si Nanocrystal Memories

In this paper, we report a purposely designed instrumentation and a jump detection procedure for the measurement of single-electron phenomena in solid-state nonvolatile memories based on a silicon nanocrystal floating gate metal-oxide-semiconductor field-effect transistor. The stepwise evolution of the drain current of a memory cell after a ldquowriterdquo operation is monitored by means of a purposely designed low-noise acquisition system with a bandwidth of up to 10 kHz. The advantage of the measurement system background noise and bandwidth over a traditional semiconductor parameter analyzer performance is evident in the detection and classification of single-electron events.

Reproducible resistive switching in nonvolatile organic memories

Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 109V∕m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.

Photonic Polymer Replicas from Distributed Bragg Reflectors Structured Porous Silicon

In this paper we present experimental evidence for single-electron phenomena in solid-state memories based on silicon nanocrystals as storage elements. The stepwise evolution of the channel current of a written memory cell biased in the subthreshold regime is monitored by means of a purposely designed low noise acquisition system with a bandwidth of 1 kHz. Each channel current step-up is ascribed to a single-electron emission from the silicon nanocrystal to the silicon substrate and each current step-down is ascribed to a single-electron capture from the silicon substrate into the silicon nanocrystal. The effect of the measurement system bandwidth on the detection of single-electron events is discussed and a procedure for extracting the threshold voltage shift associated to these events is proposed. It is shown that single-electron charging and discharging events in a memory cell with an area of 4.5 x 10(-10) cm2 can cause threshold voltage shift at room-temperature of the order of several millivolts. Qualitative explanation for the observed threshold voltage shift distribution is given.

Direct observation of amorphous to crystalline phase transitions in nanoparticle arrays of phase change materials

We have used time-resolved x-ray diffraction to study the amorphous-crystalline phase transition in 20–80nm particles of the phase change materials Ge2Sb2Te5, nitrogen-doped Ge2Sb2Te5, Ge15Sb85, Sb2Te, and Sb2Te doped with Ag and In. We find that all samples undergo the phase transition with crystallization temperatures close to those of similarly prepared blanket films of the same materials with the exception of Sb2Te that shows the transition at a temperature that is about 40°C higher than that of blanket films. Some of the nanoparticles show a difference in crystallographic texture compared to thick films. Large area arrays of these nanoparticles were fabricated using electron-beam lithography, keeping the sample temperatures well below the crystallization temperatures so as to produce particles that were entirely in the amorphous phase. The observation that particles with diameters as small as 20nm can still undergo this phase transition indicates that phase change solid-state memory technology should scale to these dimensions.