Desmond Loke

Breaking the Speed Limits of Phase-Change Memory

Exploiting Defects in a Jam Phase-change materials that can readily switch between crystalline and amorphous states are increasingly finding use in nonvolatile memory devices (see the Perspective by Hewak and Gholipour). Using high-resolution transmission electron microscopy, Nam et al. (p. 1561) show that for Ge2Sb2Te5, the application of an electric field drives crystal dislocations in one direction, leading to their accumulation and eventual jamming, which causes the phase transition. Loke et al. (p. 1566) found that by applying a constant low voltage to Ge2Sb2Te5, they could accelerate its phase-switching speeds, without harming the long-term stability of the switched state. A constant applied voltage causes preordering and accelerates phase changes in Ge2Sb2Te5, leading to faster switching. Phase-change random-access memory (PCRAM) is one of the leading candidates for next-generation data-storage devices, but the trade-off between crystallization (writing) speed and amorphous-phase stability (data retention) presents a key challenge. We control the crystallization kinetics of a phase-change material by applying a constant low voltage via prestructural ordering (incubation) effects. A crystallization speed of 500 picoseconds was achieved, as well as high-speed reversible switching using 500-picosecond pulses. Ab initio molecular dynamics simulations reveal the phase-change kinetics in PCRAM devices and the structural origin of the incubation-assisted increase in crystallization speed. This paves the way for achieving a broadly applicable memory device, capable of nonvolatile operations beyond gigahertz data-transfer rates.

Enabling Universal Memory by Overcoming the Contradictory Speed and Stability Nature of Phase-Change Materials

The quest for universal memory is driving the rapid development of memories with superior all-round capabilities in non-volatility, high speed, high endurance and low power. Phase-change materials are highly promising in this respect. However, their contradictory speed and stability properties present a key challenge towards this ambition. We reveal that as the device size decreases, the phase-change mechanism changes from the material inherent crystallization mechanism (either nucleation- or growth-dominated), to the hetero-crystallization mechanism, which resulted in a significant increase in PCRAM speeds. Reducing the grain size can further increase the speed of phase-change. Such grain size effect on speed becomes increasingly significant at smaller device sizes. Together with the nano-thermal and electrical effects, fast phase-change, good stability and high endurance can be achieved. These findings lead to a feasible solution to achieve a universal memory.

Ultrafast switching in nanoscale phase-change random access memory with superlattice-like structures

Phase-change random access memory cells with superlattice-like (SLL) GeTe/Sb(2)Te(3) were demonstrated to have excellent scaling performance in terms of switching speed and operating voltage. In this study, the correlations between the cell size, switching speed and operating voltage of the SLL cells were identified and investigated. We found that small SLL cells can achieve faster switching speed and lower operating voltage compared to the large SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved in the 40 nm SLL cells, respectively, both significantly faster than those observed in the Ge(2)Sb(2)Te(5) (GST) cells of the same cell size. 40 nm SLL cells were found to switch with low amorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which is much lower than the 1.6 V required by the GST cells of the same cell size. These effects can be attributed to the fast heterogeneous crystallization, low thermal conductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLL structure promises applications in high speed and low power memory devices.

Ultrafast phase-change logic device driven by melting processes

Significance The ever-increasing demand for faster computers is tackled by reducing the size of devices, but this is becoming almost impossible to continue. To improve the speed of computers, a solution is to increase the number of operations performed per device. Numerous operations in phase-change–based “in-memory” logic devices have previously been achieved using crystallization, but they show slow speeds, mostly due to a trade-off between the crystallization speed and stability of the initialized-glassy states. Here, we instead control melting processes to perform logic operations. Ultrafast melting speeds and diverse operations were achieved. Computer simulations and electrical measurements show the origin and kinetics of melting. These advances open the doorway for developing computers that can perform calculations at well beyond current processing rates. The ultrahigh demand for faster computers is currently tackled by traditional methods such as size scaling (for increasing the number of devices), but this is rapidly becoming almost impossible, due to physical and lithographic limitations. To boost the speed of computers without increasing the number of logic devices, one of the most feasible solutions is to increase the number of operations performed by a device, which is largely impossible to achieve using current silicon-based logic devices. Multiple operations in phase-change–based logic devices have been achieved using crystallization; however, they can achieve mostly speeds of several hundreds of nanoseconds. A difficulty also arises from the trade-off between the speed of crystallization and long-term stability of the amorphous phase. We here instead control the process of melting through premelting disordering effects, while maintaining the superior advantage of phase-change–based logic devices over silicon-based logic devices. A melting speed of just 900 ps was achieved to perform multiple Boolean algebraic operations (e.g., NOR and NOT). Ab initio molecular-dynamics simulations and in situ electrical characterization revealed the origin (i.e., bond buckling of atoms) and kinetics (e.g., discontinuouslike behavior) of melting through premelting disordering, which were key to increasing the melting speeds. By a subtle investigation of the well-characterized phase-transition behavior, this simple method provides an elegant solution to boost significantly the speed of phase-change–based in-memory logic devices, thus paving the way for achieving computers that can perform computations approaching terahertz processing rates.

Tailoring Transient‐Amorphous States: Towards Fast and Power‐Efficient Phase‐Change Memory and Neuromorphic Computing

A new methodology for manipulating transient-amorphous states of phase-change memory (PCM) materials is reported as a viable means to boost the speed, yet reduce the power consumption of PC memories, and is applicable to new forms of PCM-based neuromorphic devices. Controlling multiple-pulse interactions with PC materials may provide an opportunity toward developing a new paradigm for ultra-fast neuromorphic computing.

Microscopic Mechanism of Doping‐Induced Kinetically Constrained Crystallization in Phase‐Change Materials

A comprehensive microscopic mechanism of doping-induced kinetically constrained crystallization in phase-change materials is provided by investigating structural and dynamical dopant characteristics via ab initio molecular dynamics simulations. The information gained from this study may provide a basis for a fast screening of dopant species for electronic memory devices, or for understanding the general physics involved in the crystallization of doped glasses.

Superlatticelike dielectric as a thermal insulator for phase-change random access memory

Superlatticelike (SLL) dielectric comprising of Ge2Sb2Te5 and SiO2 was employed to reduce the power and increase the speed of phase-change random access memories (PCRAMs). In this study, we found that PCRAM cells with SLL dielectric require lower currents and shorter pulse-widths to switch compared to the cells with SiO2 dielectric. As the thickness of the SLL period reduces, the power and speed of the cells improved further due to the better thermal confinement of the SLL dielectric. Fast phase-change in 5 ns was observed in large cells of 1 μm, showing the effectiveness of SLL dielectric for advanced memory applications.

Understanding the multistate SET process in Ge-Sb-Te-based phase-change memory

Multilevel operation is a topic of much current research in the field of phase-change memory materials, representing the most feasible method for increasing memory density beyond the ultimate scaling limits of the cell size. In this work, we present a combined experimental and ab initio molecular dynamics study of the formation of intermediate states during the crystallisation of Ge2Sb2Te5 (GST). A single intermediate resistance level is formed within a narrow voltage window, and simulations suggest this consists of microscopic crystallites embedded in a bulk amorphous phase. These findings are interpreted within the framework of classical nucleation theory, and a mechanism is proposed to explain the formation of the intermediate state. Our findings suggest that it may be difficult to obtain multiple intermediate states reliably during the crystallisation of Ge2Sb2Te5 and shed light on the fundamental limitations of using this method for multilevel programming.

Engineering grains of Ge 2 Sb 2 Te 5 for realizing fast-speed, low-power, and low-drift phase-change memories with further multilevel capabilities

Phase-change memory (PCM) represents one of the best candidates for a “universal memory”. However, its slow SET speed, high RESET power, and high resistance drift present key challenges towards this ambition. Here, grain-engineered Ge2Sb2Te5 is exploited to control the crystallization kinetics, and electrical properties of PCM. We report 120 % higher SET speeds with respect to conventional scaling. Good stability (140°C), 30 % RESET power reduction, and 2X lower resistance drift were also achieved. A 4-state/2-bit multilevel cell was further demonstrated. This provides a route to making high-density PCM devices.

Ab initio molecular-dynamics simulation of neuromorphic computing in phase-change memory materials

We present an in silico study of the neuromorphic-computing behavior of the prototypical phase-change material, Ge2Sb2Te5, using ab initio molecular-dynamics simulations. Stepwise changes in structural order in response to temperature pulses of varying length and duration are observed, and a good reproduction of the spike-timing-dependent plasticity observed in nanoelectronic synapses is demonstrated. Short above-melting pulses lead to instantaneous loss of structural and chemical order, followed by delayed partial recovery upon structural relaxation. We also investigate the link between structural order and electrical and optical properties. These results pave the way toward a first-principles understanding of phase-change physics beyond binary switching.