T. C. Chong

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.

Low resistance, high dynamic range reconfigurable phase change switch for radio frequency applications

A GeTe reconfigurable phase change switch for radio frequency applications is presented. Low ON state resistance (180u2002Ω) and large dynamic range (7×103u2002X) were achieved through low resistance electrode design and high current. A partial crystallization and partial reamorphization model is proposed to explain the differences between the measured and calculated device ON (set) and OFF (reset) state resistances, respectively. The dependency between ON state resistance and reset current was estimated using a first order thermal design in steady state which suggests lower reset current by choosing materials of lower melting temperature and structures with better thermal isolation.

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.

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 1u2002μm, showing the effectiveness of SLL dielectric for advanced memory applications.

Improved Switching Uniformity and Low-Voltage Operation in

Significant improvements in the spatial and temporal uniformities of device switching parameters are successfully demonstrated in Ge/TaOx bilayer-based resistive switching devices, as compared with non-Ge devices. In addition, the reported Ge/TaOx devices also show significant reductions in the operation voltages. Influence of the Ge layer on the resistive switching of TaOx-based resistive random access memory is investigated by X-ray spectroscopy and the theory of Gibbs free energy. Higher uniformity is attributed to the confinement of the filamentary switching process. The presence of a larger number of interface traps, which will create a beneficial electric field to facilitate the drift of oxygen vacancies, is believed to be responsible for the lower operation voltages in the Ge/TaOx devices.

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The high programming current density of phase change memory (PCM) is an obstacle for its scaling and high density chip development. In this paper, an elevated-confined PCM (e-PCM) with self-aligned oxidation heater was proposed to reduce the programming current density by increasing the Joule heat and reducing heat loss simultaneously. 200u2009nm diameter size e-PCM with self-aligned TiWOx heater was fabricated and tested. The RESET current is 350u2009μA with 100u2009ns pulse and the corresponding programming current density is 1.12 MA/cm2. The low current density indicates this structure as a promising candidate for high density PCM chip applications.

-Based RRAM Using Ge Reactive Layer

GeTe materials were characterized using x-ray photoelectron spectroscopy in both the amorphous and crystalline states. Valence and conduction band alignments relative to a SiO2 reference were measured to allow the GeTe band diagram, work function, and electron affinity to be inferred. Hole barrier heights was also studied for several metal/GeTe systems (metal=Al,Ni,W) to extract the charge neutrality level of these interfaces for GeTe in both the crystalline and amorphous states. Near perfect Fermi-level pinning was observed for crystalline GeTe in contact with all of the metals with much less pinning observed for amorphous GeTe.

Programming current density reduction for elevated-confined phase change memory with a self-aligned oxidation TiWOx heater

The energy band alignment between stoichiometric phase change alloys residing along the pseudobinary line of GeTe–Sb2Te3[(GeTe)x(Sb2Te3)1−x] and SiO2 was obtained employing high-resolution x-ray photoelectron spectroscopy. The valence band offsets were determined using both the core-level spectra and valence band spectra in the analysis. The results obtained show that the band offsets vary with the composition of the (GeTe)x(Sb2Te3)1−x alloy, exhibiting a parabolic dependence on the amount of GeTe in the alloy. Increasing the proportion of GeTe in the (GeTe)x(Sb2Te3)1−x alloy was generally found to increase (decrease) the valence band (conduction band) offsets, while the binary alloys (GeTe, Sb2Te3) have similar band offset values. This information could be useful for phase change memory device design and optimization.