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phase change memory operation

Phased Change Memory • Instead of using laser beam, electric current is used to heat the material to switch between amorphous and crystalline phases – High current, high temperature: amorphous phase, high resistance – Medium current, lower temperature: crystalline phase, low resistance • Low current to sense resistance Figure 9. The substantially smaller bottom electrode and hence higher current density ensure that most of the electric power is dissipated within the phase-change material close to the bottom electrode. In optical memory, the phase-change material is heated with a laser source and it is the contrast in optical reflectivity between the amorphous and crystalline phases that is used to store information. The phase-change memory technology is based on a chalcogenide compound able to reversibly switch between two stable states, namely, an amorphous high- resistive state and a crystalline low-resistive one, enabling the storage of the logical bit. (a) I–V characteristics of three different resistance states (low, intermediate, and high). The crystal growth velocity is highly temperature dependent and determined by the free energy difference between liquid and crystalline phases (increases growth velocity when it increases) and the viscosity (decreases growth velocity when it increases). Phase-change heterostructure enables ultralow noise and drift for memory operation | Science. where \Delta H_\text{m} is the heat of fusion. The origin of the threshold switching mechanism in PCM is a long standing debate which is still not resolved despite the fact that the phenomenon was first observed more than 50 years ago by Ovshinsky [93, 94]. Phase change memory (PCM) is one of the most promising candidates for next generation nonvolatile memory. Depending on the device structures, functional materials, or switching conditions, some mechanisms might be more prominent than others, and understanding how they interact will likely yield significant insight. Reprinted by permission from Springer Nature: Nature Materials [7], Copyright (2007). where κ is the thermal conductivity and σ the electrical conductivity of the phase-change material. The total generation rate for electrons and holes consists of the sum of a thermal generation Gtherm and a field-dependent generation G which is assumed to be proportional to the carrier concentration as well as to a monotonically increasing function of the electric field g(F). Figure 10. Storage technologies include NOR and NAND Flash, magnetic hard-drive disks (HDDs) and tape. So far, the cost of NAND flash technology has been driven to extremely low levels through the use of leading edge lithography, but it remains to be seen how much lower it can go. In the crystalline phase of GeTe, both Ge and Te atoms are threefold coordinated. This approach naturally gives rise to a logarithmic evolution of the relaxation without the need for unnatural requirements on the activation energy spectrum for the relaxation of defects. The right part of the programming curve is mostly bidirectional, with the melt-quench process dominating the phase transition (e.g. For the former, the resulting fragility values in the super-cooled liquid typically have to be very high (> 100) to explain the experimental data [53, 64, 88]. A long low current pulse (SET) is applied to bring the PCM device to the low-resistance crystalline state. We note, however, that such models considering tunneling only from a single defect state cannot quantitatively reproduce the experimental I–V characteristics measured on line cells of as-deposited amorphous phase-change materials [145]. A better understanding of the quantitative link between the state of relaxation of the material, the density of defects in the bandgap and electrical observables would shed further light on the relaxation processes. Ovshinsky noted possible commercial applications of using these materials as the active region of electronic switches and memory cells [5]. The aim is to perform machine learning tasks using a neural network system whereby the neurons and/or synapses composing the neural network are implemented with memristive devices. The crystallization process typically takes much longer than the amorphization process, around tens to hundreds of nanoseconds, and crystallization is realized at temperatures typically above ~500 − 600 K but below T_\mathrm{melt} [60]. The current traces shown in figure 10(b) indicate that the current increases slowly over the delay time duration until a sharp rise occurs [122]. Most of the direct experimental measurements have been performed in the low temperature regime, that is when temperature is lower than the temperature of maximum crystallization. A typical PCM cell is designed such that the volume of phase-change material that must be melted and quenched to the amorphous state to completely block the current path through the device is minimized. Indeed, for low-power pulses, the PCM temperature distribution might favor non-negligible nucleation in the center of the amorphous region. The driving force for such a relaxation is the difference between the local energy minima of two neighboring states. The READ operation in PCM typically consists of reading the resistance of the PCM device through the application of a low voltage pulse. The main consequence of the localized states for multiple-trapping transport is that the position of the Fermi level may change with respect to temperature because the number of bound holes and electrons in localized states will change according to the Fermi occupation function. It simply comes from the fact that the peak of the inter-center barrier is not at s/2 anymore at high electric field and the influence of the second center can be neglected. Over time, the atomic configuration of this state will relax towards an energetically more favorable 'ideal glass' configuration. They indicate that the low-field resistance increases and the slope of log(I) versus V decreases with increasing size of the amorphous region. The book also contains design engineering details on PCM cell architecture, PCM cell arrays (including electrical circuit … It has been shown that the effect of drift can be significantly mitigated by using the M-metric [164, 190–192]. If the regime of fast crystallization (see figure 6) is rapidly bypassed by fast quenching, the atomic mobility at temperatures below this regime becomes so small that the atoms cannot rearrange and find their most energetically favorable configuration during cooldown, and are thus frozen into a non-equilibrium (or 'glassy') amorphous state. Nonetheless, the changes in the electronic density of states resulting from structural relaxation are still being debated. Phase change memory is widely considered as the most promising candidate as storage class memory (SCM), bridging the performance gaps between dynamic random access memory and flash. Scalability: Scaling is another area where PCM offers a difference. \dot{q}_{\mathrm{loss}} represents the heat transported away from the phase-change material. Figure 2. A significant implication of this concept is that the clear-cut distinction between memory and computing is blurred, which may lead to entirely new computational models and algorithms that would take advantage of non-von Neumann architectures. Hence, PCM could compete well in terms of forward scaling for increasing main memory density and capacity due to challenges in making DRAM capacitors small and yet being able to store charge reliably.

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