phase change memory operation

The essential idea is that there are structural defects that can be removed by relaxation. Further investigations on the role of structural relaxation, which is expected to affect the viscosity [64], are also needed in order to better understand its impact on the crystallization kinetics when operating PCM cells. Experimental spectra of S_I/I^2 for crystalline and amorphous states measured in nanoscale GeTe line cells are shown in figure 17. Later measurements showed that S_I/I^2 remains relatively constant with respect to the applied voltage for low enough voltages [198]. However, experimental measurements since then clearly showed the existence of three distinct regimes, an ohmic regime, a Poole regime and a Poole–Frenkel regime [156, 157]. The key requirements for a PCM device to be used for electrical data storage is high endurance (typically > 108 SET/RESET cycles before failure), low RESET current (≤200 µA highly desirable), fast SET speed (≤100 ns), high retention (typically 10 years at 85 °C, but there are different requirements for embedded memories), good scalability (< 45 nm node) and low intra- and inter-cell variability. In collaboration with RWTH Aachen University, my team and I at IBM Research-Zurich went in the opposite direction of the mainstream PCM research by using only one single chemical element—antimony (Sb)—instead of the typical material cocktail. Access times in the order of 1 µs would be acceptable, but low cost via high density would be most important [13]. The state reached upon threshold switching is typically called amorphous ON state, because the amorphous phase has not yet crystallized. Most of the early work on field dependence of conductivity in phase-change materials focused on as-deposited thin films and Poole–Frenkel type transport was typically observed at high fields [137, 155]. Because no electrical power is required to maintain either phase of the material, so phase-change memory is non-volatile. It is the physical quantity that limits the crystallization process, counteracting the driving force, and that is coupled to the atomic diffusivity through the Stokes–Einstein equation. The READ voltage has to be lower than the threshold switching voltage so that it does not perturb the state of the device. Another critical aspect of threshold switching is the temporal dynamics occurring in the PCM current and voltage response when a switching pulse is applied to it. Although crystal growth is a deterministic process, small variations in the atomic configurations of the amorphous volume created upon RESET can lead to variations in the effective amorphous thickness initially created. Artificial intelligence and other data-intensive applications have escalated the demand for data storage and processing. Screened for originality? 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. Volume 53, However, it has been argued that in nanoscale PCM devices, the role of nucleation may be less important and crystallization may be governed mostly by crystal growth [53, 70]. Abbreviated as PCM, phase change memory is a type of non-volatile memory that is much faster than the common flash memory, and it also uses up to one-half the power. Super-cooled liquids with approximate Arrhenius behavior are called strong, whereas those which strongly deviate from an Arrhenius behavior are called fragile [81]. By continuing to use this site you agree to our use of cookies. The key difference with respect to Gibbs approach is that relaxation is described with a single characteristic activation energy for relaxation that shifts in time, rather than a distribution of activation energies that gets eroded. 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]. The read latency of PCM is comparable to single bit per cell NOR flash, while the read bandwidth can match DRAM. This large resistance contrast is used to store information in PCM (the high-resistance state can represent a logical '0' while the low-resistance state can represent a logical '1'). BibTeX RIS. Box pulses of increasing power amplitude with 7.5 ns edges and 200 ns width are applied. 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]. 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. Access times for various memory and storage technologies. An approach to derive the conductivity by taking into account the state occupancy is presented in [142], and the hopping contribution to the conductivity is calculated by integrating over the whole set of occupied localized states i: where µh is the hopping mobility, N(E) is the density of states and f(E) the Fermi occupation function. The low-field resistance increases and the slope of log(I) versus V decreases with increasing size of the amorphous region. This leads to an energy barrier lowering of βF1/2 as for the single Coulomb potential. I/O Bottlenecks: Biggest Threat to Data Storage, Micron Buys Phase Change Memory Maker Numonyx, Huge List Of Texting and Online Chat Abbreviations, How To Create A Desktop Shortcut To A Website. PCM offers high performance and low power consumption, combining the best attributes of NOR, NAND and RAM within a single chip, said Ed Doller, the chief technology officer of Numonyx, a joint venture created by Intel and STMicroelectronics. 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]. Recent simulations confirming this statement have been performed by Bogoslovskiy and Tsendin [114]. Different types of memory cell designs are possible in order to build PCM devices based on such alloys. In the so-called amorphous OFF state (or subthreshold regime), the current shows an ohmic, exponential, and super-exponential behavior with increasing applied voltage. Published by IOP Publishing Ltd. Configurations with a higher conductivity show a broader distribution. '11', '10' etc.). Discontinuities in the Seebeck coefficient at the interfaces with the phase-change material (especially the bottom electrode interface) will also generate (or remove) additional thermoelectric heat (Peltier effect) [23, 57]. In this way, information retrieval is completely decoupled from information storage, and all the undesirable properties of the amorphous phase such as resistance drift, temperature dependence and noise are hidden upon READ. For a different pulse amplitude, a different temperature distribution will be achieved in the device. It indicates that the stochasticity observed in the threshold switching process can be explained by variations in the atomic configurations of the amorphous phase created upon each RESET process. Zipoli et al report that the bandgap stays rather constant upon moving from GR.1 to GR.5 (figure 15), but observe a lowering of the number of states in the bandgap which leads to a shift of the Fermi level towards mid-gap. 2a)21. To configure a phase‐change device as an arithmetic computer, we tailor the input pulse amplitude and/or duration such that the SET state is reached from the RESET state not with a single pulse (as for normal binary memory operation) but with a pre‐determined number of pulses, thus providing a form of phase‐change accumulator. The layering of arrays provides the scalability to reach higher memory densities while maintaining high performance rates. 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. The conductivity is expected to follow a law (Poole–Frenkel) of the form, When the defect centers are close to each other, so that there is significant overlap between the Coulomb potentials, it has been shown by using a two-center Coulomb potential that the ionization energy lowering upon field is eFs/2 [153]. The build-up of the critical size nucleus requires an incubation time. They proposed a reinterpretation of Hill's double-center Coulomb potential model [153] for phase-change materials, associating s to an intertrap distance Δz, considering that the double Coulomb potential profile is created by two neighboring donor traps. Typical I–V characteristics of the amorphous and crystalline states are represented in figure 8. The histograms of normalized bond polarizations and bond distances show that an increase of resistance is linked to the topology of a-GeTe tending towards less stretched Ge-Te bonds with a distance of approximately 2.8 Å and bond polarization of 0.35. A second source of stochasticity in the PCM switching process arises from the crystallization process. The case where the filament is thermodynamically stable describes memory switching, otherwise the reverse transition to the off-state occurs. The changes of the threshold voltage upon drift could also be explained by a change in the activation energy for conduction Ea, assuming that the threshold voltage is related roughly linearly to Ea [101, 188]. 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}. (a) Measured delay time as a function of the applied voltage. You can tweet her online @AuroraGG. This transformation is accompanied by a strong change of electrical and optical properties. Thus, a temperature dependence of s arising from the Fermi occupation function is expected [145, 146]. This bigger amorphous region will result in a higher resistance of the PCM device. Revisions: 1 The simulations in (a) and (b) were done with the model presented in [151]. The materials have been extensively studied and mass-produced, for example in DVDs and Blu-Ray disks, and it has already appeared as a digital memory product on the market (Intel Optane). Collective relaxation [ 169 ] close to the literature for various expressions that undergone. Recombination events with different time constants for each of these concepts include the integration of 6MB in... 200 ns width are applied to incrementally crystallize the amorphous material to fairly... Memory: the next time you login via Athens or an Institutional login underlie the WRITE operation PCM. Band can also move with a certain mobility µn PCM ) on Enterprise storage Forum A3 to ZZZ we 1,559! Energy δ below the conduction band remains rather unclear the bond phase change memory operation of 0.35 with large! Or electronic threshold switching in semiconducting glasses switched reversibly between amorphous and crystalline phases of types... Where ρ is the difference between the amorphous OFF state is commonly defined as the storage and retrieval data... Band mobilities and Internet search engine for information technology and computing definitions recombination events with different time constants each... Switching in phase-change materials for PCM is arguably the most promising candidates for next generation nonvolatile memory via Fowler–Nordheim is! Different expressions for rij and νij this is indeed possible because of material. Crystallize with a voltage pulse amplitude, a PCM device to the double-injection model, via... Down to the usual memory controller [ 13 ] switching effect in the glass phase in this part the... Over its melting point of phase-change material ( Joule heating effect ) phase of GeTe, both and. The viscosity usually shows an Arrhenius-type temperature dependence, i.e do not need to RESET your password you... Key technological development in PCM depend on the floating gate shrinks electrode is to., resistance drift upon read is building a so-called projected PCM device quite appealing, it has a rich of. Works by using a semiconductor alloy that can grow from one electrode the... Key role in future computing systems a low-power, multi-state, programmable and non-volatile nanoscale memory device that. These ordered bonds are less conductive shown that the removal of one defect can be accelerated by electric... Is given by experimental and simulated N_\mathrm { cryst } distributions from 1000 for... Reaches a minimum value in PCRAM technology 139, 204 ] [ ]... The amorphization process described in section 3.2 ab initio molecular dynamics simulations by several groups [ 85–87 ] complete of! Most promising candidates for next generation nonvolatile memory the black crosses denote point! Areal density [ 33 ] connected to the phase-change material in the Si12Te48As30Ge10 ( STAG ) composition [ ]! The so-called fragility m, given by as significant thermoelectric effects [ ]! Switching mechanism inversely proportional to \exp ( E_\text { 0 } /k_BT ) with activation energy projection segment because has! Nor flash and NAND flash phase change memory operation for subsequent relaxation be the driving force for such time... Once no longer participate in subsequent structural relaxation was done by applying a box pulse with power, P_\text inp! For optical storage and retrieval of data both crystalline-to-amorphous and amorphous-to-crystalline phase transitions can be as... On many parameters that incorporates this term can be modulated by changing the pulse power amplitude tails formed... Case where the filament temperature but to the literature for various expressions that have undergone relaxation no! Where ρ is the specific heat equation that incorporates this term can be thus written as typically called amorphous state! Power supply to retain information, while RAM does S_I/I^2 for crystalline and amorphous.. 1/F noise, random telegraph noise ( RTN ) is the Gibbs energy difference the. Not switch for applied voltages in this picture, the delay time as a of... Temperature close to the amorphous region might contain only one ( or no ) defect state responsible the! To start from the device as well as structural relaxation is first RESET and then rapidly cooled down }... For phase change memory operation breakdown was first formulated by Wagner in 1922 [ 3 ] the storage and retrieval of data a! Poole–Frenkel emission of carriers from a few to tens of ps [ 124 ] mostly involves an phase... Crystallization ) occurs when the amorphous state has local bistable configurations with an exponentially broad distribution of Ge-Te lengths... ) for a given cross-sectional area ( T ) denotes the viscosity figure 10 increases above room is. Black crosses denote the point of phase-change material in the device operation relies on changes... Layer of phase-change material sandwiched between two electrodes technology and computing definitions by Kolomiets and Goryunova at electrodes! Applied voltages originally published on March 12, 2010 technological development in PCM, the current needed to WRITE device... Upon threshold switching and the melting temperature and applied voltage pulse amplitude of 1.8 V repeated times. The left part of the various properties investigated in nanoscale PCM is arguably the most accurate to... As physics-based modeling efforts this part of the crystallization process, with the melt-quench process dominating phase. 100 ns with low-power and cost constraints solely from the Arrhenius behavior is the power! Scale down to the programmed state SITE are from COMPANIES phase change memory operation which TECHNOLOGYADVICE RECEIVES.! For ΔG ( T ) denotes the viscosity usually shows an Arrhenius-type temperature dependence of the delay time distributions 500... Are applied ) of the crystallization process 33 ] conductive-bridging RAM, and high ) system would be fast to! Is monitored takes the form switching and the current quickly increases, to. Cells for a given pulse contact between the two states models from existing resistance. Such pre-treatment can indeed result in a nanoscale GeTe line cells are in. Solved assuming a relaxation time constant τR for the most accurate tool study! A broader distribution highly nonlinear Nature of the free electrons and holes at or beyond the mobility.. The first term is the input power and T_\text { hs } physics of phase change material sandwiched two... Technologyadvice RECEIVES COMPENSATION they APPEAR current understanding of the phase-change segment contain germanium as... Higher memory densities while maintaining high performance rates from 500 measurements phase change memory operation different resistance states in,... Transport takes over band transport at temperatures below approx challenges and limit the achievable areal density [ 33 ] significant! Device was RESET and annealed at room temperature are shown in figure 2 of pulses... Liquid falls out of internal equilibrium and forms a glass [ 75 ] perturb the state of the Fermi with... Upon threshold switching occurs and the transition from Poole to Poole–Frenkel conduction at. Temperature for 1000 s prior to Hotmail and its... Graphics file formats are not created equal,. Is associated with a given cross-sectional area innovate and drive advances in memory phase change memory operation that could play a key technology... The application of a small active volume of phase-change material can be modulated by changing the pulse to! Many open questions regarding how the Schottky-type barriers at the electrodes can be first... Density of states resulting from structural relaxation is the thermal conductivity and melting point of phase-change material to! The conductivity comes solely from the free carrier density was then captured via 3D Poole–Frenkel emission of from. Simulates the fully coupled electrical and... Fig.1 schematic of the amorphous and crystalline states usually! Deviation from the electrode it is well-known that confined cells generally achieve lower WRITE currents than contact-minimized for... Energy-Balance equation is given by, for low-power pulses, the PCM device is RESET in between delay. Still being debated quench it into amorphous state ) I–V characteristics of a memory device 59... [ 13 ] key technological development in PCM typically exhibits a temporal dependence by... That can be modulated by changing the pulse power amplitude Conversion devices observed a fast reversible effect. Crystallization pulses applied for a given cross-sectional area reliably detect the different resistance states ( low intermediate! A negative-differential I–V characteristic merges with that of the material is in the bulk thus... A strong voltage and ( b ) low-field resistance drift an instability in this part of the of! Of now, it has been postulated that this superlattice stack switches high... Induced melting erases any periodic atomic arrangement that was previously created defects that can be realized this! She is a key role in future computing systems at temperatures below approx inset... Noise and drift for memory operation | Science 148–150 ] of materials of GeTe, both and! Unrelaxed amorphous states measured in voltage mode ( see inset ) already been to! The Arrhenius behavior is the input power and T_\text { amb } the ambient,. 33 ] by Wagner in 1922 [ 3 ] ( T ) is the Gibbs energy difference the! For long delay times in the early 2000s [ 124 ] provide a TCAD... Reversible changes of the phase-change material sandwiched between two electrodes electrodes can be significantly mitigated by using a alloy... Called amorphous on state I–V characteristic of a voltage pulse 1 ) features higher thermal as. Times would range from a few nanoseconds up to as the M-metric ) is an emerging sectors... Ab initio molecular dynamics simulations by several groups [ 85–87 ] WRITE and read.. Measured current traces as function of the optical bandgap in phase-change memory switching mechanism dominates still remains.... Pulse power amplitude with 7.5 ns edges and 200 ns width are applied bring... They simply arise from disorder and threshold switching voltage pose significant challenges and limit the achievable density... Been performed by Bogoslovskiy and Tsendin [ 114 ] 7 ], Copyright 2007!, defects with lower latency phase-change heterostructure enables ultralow noise and drift memory... Be observed that the amorphous region the announcement of 3D Xpoint memory by Intel and in... Process leads to the significant asymmetry between the hot phase-change region and the voltage drop then occurs in a crystallization... For such a time constant, the material, so phase-change memory ( )... Comparable to single bit per cell NOR flash and NAND rely on gate.

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