Luping Shi

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.

Fast phase transitions induced by picosecond electrical pulses on phase change memory cells

The reversible and fast phase transitions induced by picosecond electrical pulses are observed in the nanostructured GeSbTe materials, which provide opportunities in the application of high speed nonvolatile random access memory devices. The mechanisms for fast phase transition are discussed based on the investigation of the correlation between phase transition speed and material size. With the shrinkage of material dimensions, the size effects play increasingly important roles in enabling the ultrafast phase transition under electrical activation. The understanding of how the size effects contribute to the phase transition speed is of great importance for ultrafast phenomena and applications.

III–V Multiple-Gate Field-Effect Transistors With High-Mobility

We report an In_0.7Ga_0.3As n-channel multiple-gate field-effect transistor (MuGFET), featuring a lightly doped high-mobility channel with 70% indium and an epi-controlled retrograde-doped fin structure to suppress short-channel effects (SCEs). The retrograde well effectively reduces subsurface punch-through in the bulk MuGFET structure. The multiple-gate structure achieves good electrostatic control of the channel potential and SCEs in the In_0.7Ga_0.3As n-MuGFETs as compared with planar In_0.7Ga_0.3As MOSFETs. The In_0.7Ga_0.3As n-MuGFET with 130-nm channel length demonstrates a drain-induced barrier lowering of 135 mV/V and a drive current exceeding 840 μA/μm at V_DS = 1.5 V and V_GS - V_T = 3 V.

\hbox{In}_{0.7}\hbox{Ga}_{0.3}\hbox{As}

A light spot that is smaller than a half wavelength will subsequently diverge in all directions. In this letter, the authors model a subwavelength (0.42λ) super-resolution light beam which propagates over a long distance without any divergence. This can be achieved by placing a multibelt pure-phase-type binary optical element on the lens pupil. The authors also report a useful approach for designing the optical element, based on vector diffraction theory, which can be used in paraxial and nonparaxial focusing and imaging systems.

Channel and Epi-Controlled Retrograde-Doped Fin

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.

Subwavelength and super-resolution nondiffraction beam

A general macromodel of the phase change random access memory (PCRAM) elements for use in HSPICE-based computer simulator is proposed in this paper by introducing physical models of PCRAM elements. It can simulate the dc and transient behavior of PCRAM element. In this paper, the model was integrated with the standard R/W circuit and successfully simulated the R-I curve and dependence between amplitude and width of programming pulses. A comparison between experimental and simulation results were also given. Furthermore, by including the partial crystallization states, the macromodel was developed for simulating the multilevel storage.

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

Germanium–antimony–tellurite (GST) is a very attractive material not only for rewritable optical media but also for realizing solid state devices. Recently, the study of the switching mechanism between the amorphous and crystal states has actively been carried out experimentally and theoretically. Now, the role of the flip-flop transition of a Ge atom in a distorted simple-cubic unit cell is the center of discussion. Turning our viewpoint towards a much wider region beyond a unit cell, we can understand that GeSbTe consists of two units: one is a Sb2Te3 layer and the other is a Ge2Te2 layer. On the based of this simple model, we fabricated the superlattice of GST alloys and estimated their thermal properties by differential scanning calorimetry (DSC). In this paper, we discuss the proof of the Ge switch on the basis of thermo-histories.

HSPICE macromodel of PCRAM for binary and multilevel storage

Two-dimensional (2D) nanobump arrays were fabricated by laser irradiation of a regular lattice of absorptive polystyrene (PS) microspheres on an undoped (100) Si wafer. The experiments were performed with single-pulse 248 nm KrF laser radiation. The structure of the arrays fabricated by this method was characterized by field emission scanning electron microscope and atomic force microscope. The near-field effects under the absorptive particle are studied. The ablation and thermal processes induced by the optical near-field around the particles are investigated. The formation mechanism of nanobumps is discussed.

Role of Ge Switch in Phase Transition: Approach using Atomically Controlled GeTe/Sb2Te3 Superlattice

Sn-doped Ge–Sb–Te material was prepared by laser synthesis. It has a rocksalt crystal structure for Sn doping content less than 30at.%. A phase change temperature tester was developed to in situ measure crystallization temperature and melting point of Sn-doped Ge–Sb–Te. The crystalliza-tion temperature of Sn-doped Ge–Sb–Te is close to that of Ge2Sb2Te5 while its melting point is much lower than that of Ge2Sb2Te5. The melting points of Sn9.8Ge20.3Sb28.4Te41.5 and Sn18.8Ge19.5Sb25.3Te36.4 are 475 and 450°C, respectively. The crystallization speed was tested by an ultraviolet light at pulse duration of 30ns. It exhibits a high crystallization speed.

Nanobump arrays fabricated by laser irradiation of polystyrene particle layers on silicon

A series of superlatticelike (SLL) structure incorporated with two phase-change materials GeTe and Sb7Te3 was applied in lateral phase change memory. Power consumption and lifetime were used as two criteria to optimize the SLL structure. It was found that with the thickness ratio of GeTe to Sb7Te3 at 1.6, the RESET current could be as low as 1.5 mA and the endurance could reach as high as 5.3×106 cycles. By varying the thickness ratio of GeTe to Sb7Te3, the crystallization temperature of SLL structures and the performance of lateral phase change memory with these SLL structures can be controlled.