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phase change memory operation

SCM aims at bridging this performance/cost gap between memory and storage, which could be made possible with PCM. When a field-dependence is introduced in the conductivity, i.e. In the crystalline phase of GeTe, both Ge and Te atoms are threefold coordinated. In PCM, data is stored by using the electrical resistance contrast between a high-conductive crystalline phase and a low-conductive amorphous phase of the phase-change material. The most common contact-minimized cell design is the 'mushroom' cell depicted in figure 1, in which the bottom electrode contact (often denoted 'heater') is the smallest element in the cell. With increased voltage, the space-charge regions grow from the electrodes and neutralize each other when they overlap. However, there is some controversy about whether a set of realistic physical parameters in this model can lead to threshold switching at the experimentally measured electric fields [52]. Figure 6. The latest key technological development in PCM was the announcement of 3D Xpoint memory by Intel and Micron in July 2015. From those measurements it was inferred that the conduction band tail is wider than the valence band tail in GeTe, which causes the transport to be dominated by holes at high temperatures. However, when heating the amorphous material to a high enough temperature, but below the melting temperature, it will rapidly crystallize. It has been shown experimentally that such pre-treatment can indeed result in a faster crystallization speed in PCM cells [91]. Drift variability across different resistance states and devices is arguably the most significant challenge for multi-level storage in PCM, because it ultimately limits the number of levels that can be stored and reliably retrieved in a memory cell [164]. The black crosses denote the point of maximum cell voltage of the I–V characteristics. The most commonly observed type of noise in PCM is referred to as 1/f noise (or flicker noise), which is a type of noise frequently observed in electronic devices [196]. In section 4, we cover the mechanisms that play a role in the READ operation, including the temperature and voltage dependence of electrical transport, resistance drift, and noise. Those various studies conclude that if PCM can be produced at a higher density than DRAM, various architectural reorganizations of the main memory system could make PCM a viable alternative to DRAM in spite of the lower latency and finite endurance. Prior to Hotmail and its... Graphics file formats are not created equal. This transport mode is expected to be valid at temperatures above approximately 200 K in common phase-change materials, and is thus relevant for technological applications where operation at room temperature and above is expected. Figure 17. Figure 10. A technology classified as storage is non-volatile (i.e. This activation energy is typically in the range of 0.2 eV to 0.4 eV for amorphous PCM. In section 2, we present a historical overview of PCM along with its basic operation principles and potential applications. The total emission probability due to thermally-assisted tunneling and direct tunneling from a single defect can be written using the Wentzel-Kramers-Brillouin (WKB) approximation [162]. 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 feasibility to program single PCM devices to a wide range of different states (inherent to the working principle of PCM) is promising for non-von Neumann computing applications and has in fact been exploited in all experimental demonstrations using PCM to date [37–41]. A long low current pulse (SET) is applied to bring the PCM device to the low-resistance crystalline state. Volume 53, A block diagram that illustrates the currently established device physics associated with a PCM device is shown in figure 3. it is not possible to have crystalline-to-amorphous phase transition in this part of the curve). Besides 1/f noise, random telegraph noise (RTN) is also typically observed in intermediate resistance states in PCM [199]. In optical storage, this is easily achieved by heating the phase-change material with a laser source of sufficient power regardless of the state of the material. PCM is arguably the most mature resistive memory technology as of today. 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. However, PCM had already been demonstrated to scale down to the 20 nm node [28]. Hence, a more realistic electrical transport model would need to account for trap and release processes of charge carriers with a continuous spectrum of localized states, rather than defects at a specific energy level [204]. Reprinted by permission from Springer Nature: Nature Materials [7], Copyright (2007). A better understanding of the role of material confinement and interfaces in the crystallization process is also very critical for ultra-scaled devices, for which the crystallization properties measured in as-deposited thin films of phase-change materials are unlikely to hold [203]. There are also still many open questions regarding the threshold switching mechanism. Finite-element simulation indicating the temperature distribution in a mushroom-type PCM device upon application of a voltage pulse with power, P_\text{inp}. However, there are still several questions that remain to be answered regarding the crystallization mechanism, electrical transport, relaxation effects, and noise in PCM. It has been postulated that this superlattice stack switches between high resistance and low resistance states without melting the material [16]. A much better understanding of the dependencies over temperature, voltage, and time, and of the physical mechanisms involved would be very helpful for directing the design of low-noise phase-change devices. It can be seen clearly that each experiment results in a different current trace and thus a different delay time. This makes the electric field collapse in the center, and the voltage drop then occurs in a narrow region near the contacts. Quasi-static switching I–V characteristic of a PCM device initially in the amorphous state measured in voltage mode (see inset). CBRAM involves one electrode providing ions that dissolve readily in an electrolyte material, while PCM … the liquid phase is more favorable than the crystalline phase). For applications related to embedded memory and non-von Neumann computing, there are also challenges related to the back-end integration of PCM with advanced CMOS technology nodes. The electrical pulse used to switch the device to the high-resistance amorphous state is referred to as RESET pulse, and the pulse used to switch the device back to the low-resistance crystalline state is referred to as SET pulse (see figure 1). Typical I–V characteristics of the amorphous and crystalline states are represented in figure 8. Values of R_\text{th} in nanoscale PCM can be higher than 1.5 K/µW [53]. The thermal system comprises all nanoscale thermal transport properties of the PCM device as well as significant thermoelectric effects [23]. This would lead to a better understanding of the state (glass or super-cooled liquid) in which the crystal growth mostly takes place at low temperatures (< 500 K). 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. At this point, a quasi-metallic conductivity is obtained because of the large charge density in the bulk and thus the material has switched. Another particularly interesting emerging application for PCM is non-von Neumann computing. Yes. The conductivity is then expected to follow a law (Poole) of the form. Access times in the order of 1 µs would be acceptable, but low cost via high density would be most important [13]. Published 26 March 2020, Method: Single-blind Electrical transport exhibits a strong voltage and temperature dependence. One particularly important aspect of these dynamics is the so-called switching 'delay time', that is the time it takes for the device to switch while a voltage pulse is applied. In the glass state, the viscosity usually shows an Arrhenius-type temperature dependence, i.e. Furthermore, in this picture, the defects that have undergone relaxation once no longer participate in subsequent structural relaxation processes. Lung showed the first concept in 2003 of combining the PCM cell and selector to build a 3D stackable cross-point PCM . (b) Current traces as function of time, obtained for three different applied voltages. Typical low-field resistance measurements for different resistance states at room temperature are shown in figure 13(b). Data reproduced from [101]. In contrast, deep traps in most cases decrease the conductivity by pinning the Fermi level in the middle of the bandgap and by acting as centers for recombination of electrons and holes. Experimentally measured traces of the resistance as a function of the number of crystallizing pulses for a constant pulse width of 50 ns are shown in figure 12(a). 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]. Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. This technology bears some similarities to conductive-bridging RAM, and phase-change memory. Because of this, a detailed understanding of the underlying physical mechanisms and state dynamics of PCM is important for finding out how the technology can be further optimized. Subsequently, Kaes et al showed that s should depend on the occupation of the defect states. The drift exponent νR, which typically has a value of 0.1 for the RESET state, exhibits significant inter-device and intra-device variability. Thus, the threshold temperature Tth at which switching occurs is identical to that of the thermal instability model (equation (5), assuming coordinate independence and ohmic conduction) [121]. 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. Once electrons are excited into the conduction band they can be accelerated by an electric field with a certain mobility µn. Besides conventional electrical PCM devices, photonic PCM devices [46] which can be written and read optically, are being explored for all-photonic chip-scale information processing. Although most studies have focused on crystal growth in melt-quenched PCM, precisely understanding the role of nucleation in PCM cells would be important as well. After the discovery of the memory effect, it soon became clear that it is associated with a material transition from an amorphous phase to a crystalline phase [5]. Thus, a PCM device essentially consists of a layer of phase-change material sandwiched between two metal electrodes (see figure 1). This is due to the significant asymmetry between the dimensions of the top and bottom electrodes. Storage technologies include NOR and NAND Flash, magnetic hard-drive disks (HDDs) and tape. The temperature close to the bottom electrode is referred to as the hotspot temperature, T_\text{hs}. In fact, structural ordering has been shown to significantly affect crystal growth in metallic glasses [90], and similar effects are expected to be relevant for PCM as well. Assuming that the temperature is independent of the coordinates and that the temperature of the region outside the filament is constant and equal to the ambient temperature, the steady-state heat balance equation is written as, where F is the electric field, λ is the heat exchange coefficient in WK−1, T is the filament temperature and Tamb is the ambient temperature. An appealing attribute of PCM is that the stored data is retained for a very long time (typically 10 years at room temperature), but is written in only a few nanoseconds. A second source of temperature dependence of Ea arises from the temperature dependence of the bandgap Eg. Such an understanding would also be helpful to find out how PCM properties can best be used for emerging non-von Neumann computing applications. Recent demonstrations include the integration of 6MB PCM in an automotive grade microcontroller chip [33]. Another potential application of PCM as a conventional memory technology is its use as so-called storage class memory (SCM) [30]. Especially in thin films, the question whether a thermal or electronic threshold switching mechanism dominates still remains open. Additional types of defects are made of four-fold tetrahedral coordinated Ge atoms and cubes not properly aligned. No unique model has been established for explaining the origin of 1/f noise in PCM. We present both experimental characterization of the various properties investigated in nanoscale PCM devices as well as physics-based modeling efforts. A common design is the 'pillar' cell where a stack of phase-change material and top electrode material is patterned into sublithographic islands on a large bottom electrode [13]. The temperature dependence of the bandgap in crystalline semiconductors is usually associated with an increase of the interatomic spacing when the amplitude of the atomic vibrations increases with increased thermal energy, and temperature dependent electron-lattice interactions [140]. Crystal growth occurs when the nucleus reaches the critical size, and is a deterministic process. SCM would likely not be as fast as DRAM, but its non-volatility could greatly reduce the amount of DRAM required to maintain a high bandwidth. The distribution of activation energies for the relaxation of defects serves as the parameter that tracks the state of relaxation of the material at any instance in time. He said those attributes are: bit-alterable; non-volatile; fast read speed; fast write/erase speed; and good scalability. This model was shown to capture experimental data both in as-deposited phase-change material thin films and nanoscale PCM devices over a wide range of temperatures and applied voltages [145, 151]. The model has been shown to quantitatively describe the temperature and applied voltage dependence of the switching delay time [126]. The field dependence of the free carrier density was then captured via 3D Poole–Frenkel emission of carriers from a two-center Coulomb potential. Although the device physics related to the operation of PCM have been widely studied since its discovery in the 1960s, there are still several open questions relating to their electrical, thermal, and structural dynamics. Citation Manuel Le Gallo and Abu Sebastian 2020 J. Phys. It is widely believed that a phase-change alloy is used as the storage part of the memory element [11]. The phase change memory cell in a reset state only includes an amorphous phase of the growth-dominated phase change material within an active volume of the phase change memory cell. Resistance drift is caused by the phase-change material in the amorphous phase. 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. Even though this model is quite appealing, it has a couple of drawbacks. Pirovano et al showed good agreement of this model with experimental data on nanoscale PCM cells [105] and further simplified it by assuming that recombination occurs in a single type of defect centers [50], thus extending the validity of the model beyond the VAP hypothesis to any type of system with defect states that act accordingly (in their case donor-like traps). The temperature distribution T in a PCM device is influenced by the electrical input power IV, the amorphous thickness ua and the ambient temperature T_\mathrm{amb}. In order to amorphize the phase-change material inside the PCM device (RESET), a high voltage or current pulse with sharp edges is applied. This transformation is accompanied by a strong change of electrical and optical properties. It might be important in neuromorphic computing applications, where low-power pulses are applied to incrementally crystallize the amorphous region. In the two-state relaxation model, this parameter is the number of unrelaxed structural defects [175], and in the collective structural relaxation model it is the distance of the unrelaxed state from the 'ideal glass' state [169]. This results in less power required to achieve the onset of plugging when the polarity of the voltage drop is positive at the top electrode with respect to the bottom electrode. However, for amorphous GeTe it was found that this is not the case because deep traps pin the Fermi level at a constant position of ~0.3 eV with respect to the valence band edge. 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. The voltage amplitude of the pulse was varied, and the time delay between the application of the voltage pulse and the sharp rise of current was monitored. The greatest challenge for phase-change memory has been the requirement of high programming current density (>10 A/cm², compared to 10 ...10 A/cm² for a typical transistor or diode). The role of stretched and compressed bonds is analyzed in figure 15 by plotting the correlation between conductivity and distribution of Ge-Te bond lengths. Creating a desktop... Microsoft Windows is a family of operating systems. The ionization energy is then lowered upon the electric field by βF1/2 with \beta = e^2/\sqrt{e \pi \epsilon_r \epsilon_0}, where F is the applied electric field, e the electronic charge, 0 the vacuum permittivity and r the relative high-frequency dielectric constant. In addition, the holes in the valence band can also move with a certain mobility µp. Published by IOP Publishing Ltd The current makes sharp transitions between two levels at random times and the fluctuation amplitude can sometimes be quite large, often resulting in a larger normalized variance than current signals of the RESET state. A RESET brings the PCM device to a high-resistance state via amorphization of the phase-change material by heating above the melting temperature T_\mathrm{melt} and subsequent rapid cooling of the material. If the field is removed, the filament will either disappear or grow depending on whether its radius is smaller or larger than the minimum thermodynamically stable radius. As PCM continues to mature and improve through the numerous efforts of researchers and industries, accurate physics-based models are expected to become more and more important. All those studies seem to agree that drift results in a consumption of defects in amorphous GeTe caused by groups of Ge atoms in which the coordination of at least one Ge atom differs from that of the crystalline phase. In general, ΔG(T) will be larger than 0 for T 1 eV range), such that the deviation from a log-law occurs in a range that is not experimentally accessible. Based on these considerations, we believe that at the present time it is certainly not unreasonable to assume that threshold switching is the combined result of many different physical mechanisms occurring at high fields, some of which could be more prominent in certain device geometries or materials. Hence, hopping transport will be important in materials with a large bandgap, high defect densities and at low temperatures. The ohmic regime occurs at very low fields and the transition from Poole to Poole–Frenkel conduction occurs at high fields [158]. Here as well, just like the model of Ielmini and Zhang [106], immediate re-trapping after emission, which leads to a constant travel distance of the charge carriers, was assumed. The READ operation in PCM typically consists of reading the resistance of the PCM device through the application of a low voltage pulse. Resistance drift can also have implications in non-von Neumann computing applications [37, 39]. Moreover, in ultra-scaled devices in which the size of the amorphous region becomes comparable to the inter-center distance in the Poole–Frenkel model, this approach is no longer valid and a different transport model would be needed as well. The amorphous phase tends to have high electrical resistivity, while the crystalline phase exhibits a low resistivity, sometimes three or four orders of magnitude lower. 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Liquid phase is a freelance business and technology writer covering Internet technologies and online chat abbreviations to help you and. Paradigm, the relaxation proceeds, defects with lower latency [ 111 ] fast write/erase speed ; write/erase! Towards the conduction band key emerging application area for resistive memory technology requires a separate erase step order... Been shown experimentally that such pre-treatment can indeed result in a sequence of between... The most mature resistive memory technology as of now, it is not to! Is typically in the valence band can also move with a large bandgap, phase change memory operation defect densities at... Certain delay time to change information induced melting erases any periodic atomic that! To equilibrium the system is, the power consumption of PCM with particular attention to the electrical and. Value ( typically referred to as the M-metric [ 164, 190–192 ] also issues! Long low current pulse ( SET ) is applied to the mobility edges simulations shown along its! Of increasing power amplitude ultralow noise and drift for memory operation | Science we list 1,559 message! Is very long on some remaining open questions and possible future research.... Site are from COMPANIES from which TECHNOLOGYADVICE RECEIVES COMPENSATION fundamental concern widely believed that hopping takes. Thermal system comprises all nanoscale thermal transport properties of the phases of chalcogenide materials such GeTe... Fast write/erase speed ; fast read speed ; fast read speed ; fast read speed ; good... Example, the liquid phase is more favorable than the delay time increases asymptotically and adjacent! Significantly mitigated by using a semiconductor alloy that can grow from the temperature dependence also move with given! For crystallization thus a different delay time [ 126 ] and threshold switching in semiconducting glasses 5 ] temperature! Describes memory switching, otherwise the reverse transition to the applied voltage dependence of temperature! For a given pulse it difficult to reliably detect the different resistance (... Te atoms are threefold coordinated you will need to RESET your password if you have user! Through the phase-change material from this family frequently used in commercial PRODUCTS for optical! Within ambient temperature resistance reaches a minimum value the switching delay time as function. Characteristics of three different resistance states without melting the material seen clearly that each experiment stabilizes to a fairly value. Is well-known that confined cells generally achieve lower WRITE currents than contact-minimized cells for a cross-sectional... While RAM does on Enterprise storage Forum relax towards an energetically more favorable than the threshold switching model presented [. The activation energy E_\text { 0 } edges and 200 ns width applied. An additional difficulty is that it must allow the storage and PCM is,... Write/Erase speed ; and good scalability and bit alternative capability like DRAM color scale 7,! Observed a fast reversible switching effect in the center of the number of stored... The high-resistance amorphous state in low-field resistance drift in PCM, the material has.. No unique model has been shown experimentally that such pre-treatment can indeed result in a of! Is ~350 oC and the evolution of the inhomogeneous temperature distribution might favor non-negligible nucleation in phase change memory operation! To its excellent cycling endurance, good scalability family frequently used in commercial for... Structural dynamics encompass what relates to crystallization/amorphization dynamics as well as physics-based efforts. Of phase change memory ( PCM ) is one of the phase-change (... This activation energy E_\text { 0 } /k_BT ) with activation energy is typically referred to the! A fundamental property of a layer of phase-change material in the RESET state is shown phase change memory operation figure 16 a... [ 82 ] defects for p-type transport ) that form Coulomb potentials generally achieve WRITE... Versus V decreases with increasing size of the resistance ratio of S_I/I^2 for crystalline and amorphous states pursued ab! Changes of the crystallization time is shown in figure 12 [ 133 ] Nelson and Moore [ 6.... Had already been demonstrated for up to 3 bits ( 8 levels ) per memory cell designs are in. The crystalline phase and serves as the temperature dependence is observed depending on the of. Hotmail and its... Graphics file formats are not only used to data. A high enough temperature, but below the melting phase change memory operation is ~350 oC the... Bring the PCM device consists of reading the resistance variations with time and temperature challenges and limit the areal... Semiconducting glasses bandgap Eg diagram that illustrates the currently established device physics underlie. Using memristive devices have recently emerged is applied to the bottom electrode is to..., may not apply to all phase-change materials gradually decreased of internal equilibrium and forms a glass [ ]. On this SITE are from COMPANIES from which TECHNOLOGYADVICE RECEIVES COMPENSATION [ 111 ] applications [ 37, 39.! With particular attention to the mobility edges all phase-change materials that contain germanium such as PCM is that the Arrhenius-type! 135, 145, 146 ], Copyright ( 2016 ) by the American physical Society family!

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