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). An experiment showing the low-field resistance variation of a PCM device after RESET during the application of a time-varying temperature profile is presented in figure 16(b). To find out more, see our, Browse more than 100 science journal titles, Read the very best research published in IOP journals, Read open access proceedings from science conferences worldwide, © 2020 The Author(s). Moreover, experimental observation of bandgap widening upon drift has also been reported via Fourier transform infrared spectroscopy (FTIR) measurements [174]. In addition to modeling and experimental characterization efforts, several approaches have been tried in order to counteract the effect of resistance drift to retrieve the stored information in a PCM device. Investigations of the crystallization process are also being pursued using ab initio molecular dynamics simulations by several groups [85–87]. As the memory cell shrinks on flash, the number of electrons stored on the floating gate shrinks. 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 relaxation proceeds in a sequence of transitions between neighboring unrelaxed amorphous states. The general approach to arrive at a spectrum S(f) \propto 1/f is to assume that there are many fluctuation events, each with a relaxation time \tau = \tau_0 \exp(W/k_BT), where \tau_0^{-1} is the attempt frequency to surpass the barrier W. If it is then assumed that W is distributed uniformly, this approach yields a 1/f spectrum [201]. An in-depth comparison between the simulation methods used in the different works as well as the criteria used to define the bandgap will be required in order to understand the origin of this discrepancy. The most notable ones are the double-injection model by Mott [102] and Henisch [103] and the generation-recombination model of Adler [104]. 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. Moreover, the length of the pulse has to be long enough so that complete crystallization of any previously created amorphous region occurs. Even more recently, the geometric and electronic structures of the localized states in the band gap of Ge2Sb2Te5 involved in the resistance drift process have also been analyzed [172]. 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. One possible avenue could be to build devices with different thermal environments in order to vary the effective thermal resistance and capacitance, which should influence only the thermal processes leading to switching. When the anode is the bottom electrode, the Thomson effect heat drain will push the hotspot further down into the bottom electrode, which will increase the plugging power because some of the input power will be dissipated within the electrode instead of the phase-change material. ΔG(T) is the Gibbs energy difference between the liquid and the crystalline phase and serves as the driving force for crystallization. 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. Figure 4. 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. 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 transformation is accompanied by a strong change of electrical and optical properties. Experimental spectra of S_I/I^2 for crystalline and amorphous states measured in nanoscale GeTe line cells are shown in figure 17. Recent simulations confirming this statement have been performed by Bogoslovskiy and Tsendin [114]. It has been postulated that this superlattice stack switches between high resistance and low resistance states without melting the material [16]. To transform the material back to the amorphous phase, it needs to be heated above its melting temperature and then rapidly cooled down. Memory switching (total crystallization) occurs when the amorphous ON state I–V characteristic merges with that of the crystalline state. \dot{q}_{\mathrm{loss}} represents the heat transported away from the phase-change material. where R(t0) is the resistance measured at time t0. Reprinted figure with permission from [171], Copyright (2016) by the American Physical Society. For E_a < 4k_BT_{amb}, the negative differential behavior is absent. Many works have attempted to experimentally measure the growth velocity as a function of the temperature in different phase-change materials, both in the amorphous as-deposited state [61, 62, 65, 83], and in the melt-quenched state of memory cells [53, 64, 84]. An alternative modeling approach proposed recently is based on collective relaxation [169, 180, 181]. The PCM device is RESET in between each delay time measurement. What is also observed are significant fluctuations of the resistance over time for the higher resistance states. 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 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. 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]. In section 3, we cover the device physics related to the WRITE operation, including thermal characteristics, crystallization mechanism, threshold switching, and inherent WRITE stochasticity. 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). 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. Download PDF Abstract: We survey the current state of phase change memory (PCM), a non-volatile solid-state memory technology built around the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states in so-called phase change materials. As the relaxation proceeds, defects with lower activation energies will be removed first, followed by those with higher activation energies. Thus, a temperature dependence of s arising from the Fermi occupation function is expected [145, 146]. 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]. Because of the inhomogeneous temperature distribution, small dimensions, and high temperatures reached during SET and RESET in PCM, large temperature gradients will occur in the device. 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. σ(F, T), the model is commonly referred to as electro-thermal because along with the thermal effects, electronic processes leading to a field dependence of the conductivity are considered. Resistance drift makes it difficult to reliably detect the different resistance states of PCM over time. Small amounts of expensive high-performance volatile memory sit near the CPU whereas vast amounts of low-cost yet slow storage are used to stock data. Normalized current spectral density S_I/I^2 of amorphous and crystalline states in a nanoscale GeTe line cell. 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. In the mid-1950s, the semiconducting properties of chalcogenide-based glasses were discovered by Kolomiets and Goryunova at the Ioffe Physical-Technical Institute [3]. A schematic illustration of a PCM device with mushroom-type device geometry is shown in Fig. At this voltage, the delay time increases asymptotically and the device will not switch for applied voltages somewhat below it. Phase change memory (PCM) is one of the most promising candidates for next generation nonvolatile memory. The ratio of S_I/I^2 between amorphous and crystalline states is usually observed to be comparable to the resistance ratio of the two states. Resistive memory devices (or memristive devices) that remember the history of the current that previously flowed through them, are promising candidates for this application. This is consistent with the microscopic picture of structural relaxation presented in the previous section, because the activation energy is expected to increase upon drift from the consumption of mid-gap defects and bandgap widening due to local reordering [167, 170, 171, 174]. 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). An Arrhenius-type temperature dependence has been commonly reported, spanning more than eight orders of magnitude of growth velocity, up to high temperatures typically above 500 K [53, 64, 65, 68, 84]. 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). The simulations shown along with the experimental data were performed with the thermally-assisted threshold switching model presented in [101]. The field dependence of the free carrier density was then captured via 3D Poole–Frenkel emission of carriers from a two-center Coulomb potential. It has been shown that the effect of drift can be significantly mitigated by using the M-metric [164, 190–192]. (a) Change in low-field resistance when applying progressively higher RESET currents. Similar to the double-injection model, injection via Fowler–Nordheim tunneling is believed to start from the electrode. Such electro-thermal models have been proposed to explain threshold switching in chalcogenide glasses in the 1970s by Boer [95], Warren [96], Kroll [97] and Shaw [98]. Data from [169]. The experiment was repeated 100 times and the device was RESET in between each experiment. 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. Thermally-initiated switching will occur when the temperature increase within the device due to Joule heating induces a significant conductivity increase due to thermal activation of carriers. IBM continues to innovate and drive advances in memory technology. Basic operation principles. The mushroom-type PCM device displayed consists of a layer of phase-change material sandwitched between a top electrode (TE) and a narrower bottom electrode (BE). The steady-state balance between generation and recombination in a homogeneous system can be written as, where τp is the characteristic hole capture time and p_0 = G_{therm} \tau_p. It can be seen that the threshold voltage monotonically increases with increasing RESET current, hence with increasing size of the amorphous region. 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. The term in the square brackets captures the thermally activated atomic transfer across the solid-liquid interface. The black crosses denote the point of maximum cell voltage of the I–V characteristics. 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 amplitude of the crystallizing pulses is substantially larger than the threshold switching voltage to avoid any delay time stochasticity as well as to provide sufficient current to induce Joule heating and crystal growth. At first, electrons recombine close to the cathode and holes close to the anode, creating a negative and positive space-charge at the cathode and anode, respectively. The high conductive on-state region (with excess electron concentration in the shallow trap state) eventually fills up the whole active volume of phase-change material. This enables the execution of code directly from the memory, without an intermediate copy to RAM. Webopedia is an online dictionary and Internet search engine for information technology and computing definitions. The induced melting erases any periodic atomic arrangement that was previously created. Memristive devices include PCM but also other emerging non-volatile memories such as resistive random access memory (RRAM), conductive bridge random access memory (CBRAM), or magnetic random access memory (MRAM) [36]. This activation energy is typically in the range of 0.2 eV to 0.4 eV for amorphous PCM. Reproduced from [151]. 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