The relatively high programming current and threshold switching voltage pose significant challenges and limit the achievable areal density [33]. 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. The left part of the programming curve is unidirectional as it mostly involves an amorphous-to-crystalline phase transition (e.g. This makes the electric field collapse in the center, and the voltage drop then occurs in a narrow region near the contacts. 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. Besides using PCM as a standalone memory in a conventional computer system, another important emerging application domain for PCM is as embedded memory [31, 32]. Webopedia is an online dictionary and Internet search engine for information technology and computing definitions. The contrast in optical properties of phase-change materials has been widely employed to enable optical data storage devices such as DVDs and Blu-Ray discs. In order to elucidate the precise underlying mechanisms in PCM, measurements of the temperature dependence of 1/f noise and its high-field non-Ohmic regime, in particular, would be required [201]. In this computing paradigm, the memory devices are not only used to store data but also to perform some computational tasks. 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. What is also observed are significant fluctuations of the resistance over time for the higher resistance states. The idea is that during WRITE the current flows through the phase-change segment because the resistance of the amorphous ON state is lower than the resistance of the projection segment. Write/erase performance: PCM will achieve write throughput speeds faster than NAND and with lower latency. 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]. It works by using a semiconductor alloy that can be changed rapidly between an ordered, crystalline phase having lower electrical resistance to a disordered, amorphous phase with much higher electrical resistance. So far, most efforts have focused on modeling the kinetics of structural relaxation via a two-state model for the relaxation of defects [175, 176]. Moreover, the resistance trajectories are different in each experiment, leading to a randomness in the total number of pulses needed to fully crystallize. A 1/f frequency dependence for the amorphous state is observed from 1 Hz to 100 kHz, and S_I/I^2 is roughly 105 times lower for the crystalline state. The PCM device is operated within ambient temperature, T_\text{amb}. Therefore, when the device is brought back to room temperature, its resistance becomes higher than if it would have stayed at room temperature for the entire duration of the experiment, and it stops increasing because of the preceding annealing at higher temperature. Thus, a WRITE operation in PCM involves switching between the amorphous and crystalline states via the application of an electrical pulse. Hence, hopping transport will be important in materials with a large bandgap, high defect densities and at low temperatures. This bigger amorphous region will result in a higher resistance of the PCM device. This transformation is accompanied by a strong change of electrical and optical properties. Next, we present modeling and characterization efforts to describe resistance drift in PCM devices. This has been explained by an increase of the inter-center distance s in the Poole–Frenkel model with drift due to the annealing of defects [169, 175]. To transform the material back to the amorphous phase, it needs to be heated above its melting temperature and then rapidly cooled down. The book gives a comprehensive overlook of PCM with particular attention to the electrical transport and the phase transition physics between the two states. The low-field resistance increases and the slope of log(I) versus V decreases with increasing size of the amorphous region. ED has been shown to be proportional to the temperature at which the device is annealed Tann [169, 182, 184], as expected from time-temperature superposition which should occur if the changes in Ea(t) indeed arise from structural relaxation [185]. The black crosses denote the point of maximum cell voltage of the I–V characteristics. This accumulation property (in fact, the PCM integrates the electric current flowing through it) is essential for emulating synaptic dynamics [40, 43] and can also be used to implement some arithmetic operations [42, 44, 45]. This technology bears some similarities to conductive-bridging RAM, and phase-change memory. Such a temperature dependence could successfully explain electrical I–V characteristics of different as-deposited phase-change materials both in the dark and under illumination using this model [145, 146]. He said those attributes are: bit-alterable; non-volatile; fast read speed; fast write/erase speed; and good scalability. Adapted from [200]. Because PCM does not store charge (electrons), it is immune to the charge storage scaling issue. As the molten phase-change material is being cooled below the melting temperature (in the so-called super-cooled liquid state), the viscosity steadily increases with cooling, and it becomes increasingly difficult to sample all possible configurations for a given temperature. 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). In addition, the holes in the valence band can also move with a certain mobility µp. Figure 16. For this, a low-power, multi-state, programmable and non-volatile nanoscale memory device is needed. New computing devices, such as phase-change random access memory (PCRAM)-based neuro-inspired devices, are promising options for breaking the von Neumann barrier by unifying storage with computing in memory cells. The driving force for such a relaxation is the difference between the local energy minima of two neighboring states. Few models have been proposed, mainly based on the concept of double well potentials (DWPs) [201, 202], in which either atoms or electrons switch between two energy minima separated by a potential barrier W, creating fluctuations. PCM records data by causing a phase-change material inside the memory device to switch from a crystalline (ordered) phase to an amorphous (disordered) phase and vice versa. A block diagram that illustrates the currently established device physics associated with a PCM device is shown in figure 3. The crystallization kinetics of PCM at elevated temperatures can be either nucleation or growth driven, and has been (and continues to be) a topic of intense research [53, 60–69]. The induced melting erases any periodic atomic arrangement that was previously created. In an as-fabricated device, the material is in the crys-talline phase. This interpretation has been supported by a wide range of experimental measurements and molecular dynamics simulations in recent years [167, 169–172, 175, 176]. A possible explanation for this discrepancy could be that the experimentally observed longer delay times would be dominated by parasitic components of the device and of the control electrical circuit [125]. This is mainly motivated by the fact that in most of the commonly used amorphous phase-change materials, the activation energy for conduction at room temperature and above is close to half of the optical bandgap [137, 138]. One of the first studies of electrical transport in nanoscale PCM devices was by Ielmini and Zhang where they mostly observed an ohmic regime at low fields and Poole-type behavior at higher fields [106]. This would rather indicate that the wide Arrhenius-type temperature dependence occurs mostly in the glass phase in this material. In the following sections, the PCM resistance dependence on voltage and temperature, resistance drift, and noise will be described. This article was originally published on March 12, 2010. It can be observed that the resistance increases over time, which is typically referred to as resistance drift. Crystallization is influenced by the amorphous thickness ua, the temperature T, the time t and the state of relaxation Σ (through the viscosity). A READ operation typically involves reading the electrical resistance of the PCM device, which then allows to know whether it is in the amorphous (high-resistance, logical '0') or crystalline (low-resistance, logical '1') state. Especially when applying low-power voltage pulses to crystallize a PCM mushroom-type cell, it is possible that the higher temperatures reached in the middle of the amorphous region could induce nucleation there. One point that remains debated is whether the wide Arrhenius-type temperature dependence of the growth velocity measured at low temperatures occurs in the glass or super-cooled liquid state. Therefore, the number of carriers in states around the Fermi level must be much higher than the number of carriers that are excited into the band, so that hopping transport can outweigh band transport. 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. Therefore, by applying a RESET pulse that dissipates more power, a bigger amorphous region is created because T_\mathrm{melt} is reached further away from the bottom electrode. Experimentally reported delay times in PCM range from a few nanoseconds up to as much as 1 ms [101, 122]. It can be observed that the slope of log(R) versus log(t) is temperature independent in the experimentally accessible range of time [169, 182, 184]. The electrical and thermal proper- ties of phase change materials are surveyed witha focus on the scalability of the materials and their impact on device design. Different activation energies are required to remove different defects assuming that the removal of one defect can be associated with a single activation energy. Despite the fact that the memory effect in phase-change materials was discovered over 50 years ago, there are several open questions relating to electrical transport, the crystallization mechanism, relaxation effects, and inherent stochasticity in PCM, all of which are central to its operating principle. For example, a low amplitude pulse is likely to crystallize only inside the amorphous region close to the bottom electrode, whereas a high amplitude pulse may crystallize only close to the crystalline-amorphous interface (because the temperatures reached inside the amorphous region may be too close to the melting temperature for which the crystallization rate is very small). 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]. A PCM device consists of a small active volume of phase-change material sandwiched between two electrodes. Lung showed the first concept in 2003 of combining the PCM cell and selector to build a 3D stackable cross-point PCM . Essentially, a conduction model is used to describe the electron current due to the Poole–Frenkel effect (see section 4.1) from two trap states (shallow and deep), and it is assumed that electrons can tunnel from the deep to the shallow trap state, thus increasing the trapped electron concentration in the shallow trap state. The electric field Fth corresponding to this condition is given by, For non steady-state breakdown, the heat balance equation takes the form. Moreover, experimental observation of bandgap widening upon drift has also been reported via Fourier transform infrared spectroscopy (FTIR) measurements [174]. Although all the proposed thermal and electronic models so far can reproduce some experimentally observed characteristics of threshold switching, none of them appear to be able to quantitatively match all observed dependencies and dynamics over temperature and time across different materials and devices with realistic sets of physical parameters. The measured negative differential resistance is that of R_\mathrm{load} because the device resistance drops below R_\mathrm{load} upon threshold switching. The memory hierarchy of conventional computing architectures is designed to bridge the performance gap between the fast central processing units (CPU) and the slower memory and storage technologies. In figure 12(b), we report the distributions of the number of pulses needed to crystallize N_\mathrm{cryst} for different pulse widths. Box pulses of increasing power amplitude with 7.5 ns edges and 200 ns width are applied. 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