Crystallization and amorphization studies of a Ge(2)Sb(2.3)Te(5) thin-film sample under pulsed laser irradiation

We present the results of crystallization and amorphization studies on a thin-film sample of Ge(2)Sb(2.3)Te(5), encapsulated in a quadrilayer stack as in the media of phase-change optical disk data storage. The study was conducted on a two-laser static tester in which one laser, operating in pulsed mode, writes either amorphous marks on a crystalline film or crystalline marks on an amorphous film. The second laser, operating at low power in the cw mode, simultaneously monitors the progress of mark formation in terms of the variations of reflectivity both during the write pulse and in the subsequent cooling period. In addition to investigating some of the expected features associated with crystallization and amorphization, we noted certain curious phenomena during the mark-formation process. For example, at low-power pulsed illumination, which is insufficient to trigger the phase transition, there is a slight change in the reflectivity of the sample. This is believed to be caused by a reversible change in the complex refractive index of the Ge(2)Sb(2.3)Te(5) film in the course of heating above the ambient temperature. We also observed that the mark-formation process may continue for as long as 1 mus beyond the end of the write laser pulse. This effect is especially pronounced during amorphous mark formation under high-power, long-pulse illumination.     

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.     

Design of electrical probe memory with TiN capping layer

The concept of electrical probe memory using phase-change media has recently received considerable attention due to its promising potential for next-generation data storage device. However, the physical performances of the conventional electrical probe memory are strongly limited by its diamond-like carbon capping layer ascribed to its large contact resistance and sharp difference between the theoretically optimized properties values and the experimentally measured values. Therefore, the diamond-like carbon capping layer is replaced by a titanium nitride layer here, and the modified device architecture is re-optimized by a newly developed three-dimensional model, resulting in a media stack consisting of a 2-nm Ge₂Sb₂Te₅ layer sandwiched by 2-nm titanium nitride layer with an electrical conductivity of 2?×?10⁵ Ω^?1 m^?1 and a thermal conductivity of 12 W m^?1 K^?1, and a 40-nm titanium nitride bottom layer with an electrical conductivity of 2?×?10⁶ Ω^?1 m^?1 and a thermal conductivity of 12 W m^?1 K^?1. The advantageous features of such a device on the writing of both crystalline and amorphous bits are also demonstrated according to the developed model.     

Highly scalable non-volatile and ultra-low-power phase-change nanowire memory

The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.     

Evanescent coupling in magneto-optical and phase-change disk systems based on the solid immersion lens

Results of numerical computations pertaining to evanescent wave coupling for near-field magneto-optical and phase-change disks based on the concept of the solid immersion lens are presented. We investigated the relation between the coupling efficiency and the width of the air gap in terms of the throughput of the recording process and the resolution of the readout signal. The simulations show a drastic decrease with a widening air gap of the coupling efficiency by means of evanescent waves into the recording medium. In magneto-optical readout, loss of the signal may be attributed to the reduction of magneto-optical interaction, the rise of reflectance, and the variation of the relative phase between the two components of polarization. In the phase-change readout the reduced reflectivity contrast between crystalline and amorphous marks is the cause of signal reduction.     

Feasibility of ultra-dense spin-tunneling random access memory

Three-dimensional (3-D) finite element models have been utilized to simulate electromagnetic behaviors in spin-tunneling random access memory (STram). The most significant contributors have been identified. Compared with conventional current-in-plane (CIP) giant magneto-resistive (GMR) memory, whose signal level is inversely proportional to the square root of the storage density, these current-perpendicular-to-plane (CPP) STram elements provide an excellent readout property in that their signal level is independent of their cross-section area. This result is so attractive that the density of STram should not be limited by signal degradation. Moreover, a magnetic flux closure design was found to reduce the crossfeed by about a factor of five, compared with conventional keeperless design, which is the most favored approach for achieving 10⁹ bits/cm² areal density. Although the storage mechanism described in this paper is made of STram, the flux-closure design could be generally applicable to other magnetic solid state memories     

Ferroelectric Tunnel Junctions: Modulations on the Potential Barrier

Recently, ferroelectric tunnel junctions (FTJs) have attracted considerable attention for potential applications in next-generation memories, owing to attractive advantages such as high-density of data storage, nondestructive readout, fast write/read access, and low energy consumption. Herein, recent progress regarding FTJ devices is reviewed with an emphasis on the modulation of the potential barrier. Electronic and ionic approaches that modulate the ferroelectric barriers themselves and/or induce extra barriers in electrodes or at ferroelectric/electrode interfaces are discussed with the enhancement of memory performance. Emerging physics, such as nanoscale ferroelectricity, resonant tunneling, and interfacial metallization, and the applications of FTJs in nonvolatile data storage, neuromorphic synapse emulation, and electromagnetic multistate memory are summarized. Finally, challenges and perspectives of FTJ devices are underlined.     

Polarization holographic high-density optical data storage in bacteriorhodopsin film

Optical films containing the genetic variant bacteriorhodopsin BR-D96N were experimentally studied in view of their properties as media for holographic storage. Different polarization recording schemes were tested and compared. The influence of the polarization states of the recording and readout waves on the retrieved diffractive image's intensity and its signal-to-noise ratio were analyzed. The experimental results showed that, compared with the other tested polarization relations during holographic recording, the discrimination between the polarization states of diffracted and scattered light is optimized with orthogonal circular polarization of the recording beams, and thus a high signal-to-noise ratio and a high diffraction efficiency are obtained. Using a He-Ne laser (633 nm, 3 mW) for recording and readout, a spatial light modulator as a data input element, and a 2D-CCD sensor for data capture in a Fourier transform holographic setup, a storage density of 2 x 10(8) bits/cm2 was obtained on a 60 x 42 microm2 area in the BR-D96N film. The readout of encoded binary data was possible with a zero-error rate at the tested storage density.     

Ultrafast electrochemical lithiation of individual Si nanowire anodes

Using advanced in situ transmission electron microscopy, we show that the addition of a carbon coating combined with heavy doping leads to record-high charging rates in silicon nanowires. The carbon coating and phosphorus doping each resulted in a 2 to 3 orders of magnitude increase in electrical conductivity of the nanowires that, in turn, resulted in a 1 order of magnitude increase in charging rate. In addition, electrochemical solid-state amorphization (ESA) and inverse ESA were directly observed and characterized during a two-step phase transformation process during lithiation: crystalline silicon (Si) transforming to amorphous lithium-silicon (Li(x)Si) which transforms to crystalline Li(15)Si(4) (capacity 3579 mAh·g(-1)). The ultrafast charging rate is attributed to the nanoscale diffusion length and the improved electron and ion transport. These results provide important insight in how to use Si as a high energy density and high power density anode in lithium ion batteries for electrical vehicle and other electronic power source applications.     

有机电荷转移体系用于超高密度信息存储研究

采用有机电荷转移复合材料和共轭Ｃｃｈｉｆｆ碱作为存储介质。通过ＳＴＭ脉冲电压存储实验实现了存储密度大于１０＾１２ｂｉｔｓ／ｃｍ＾２的超高密度信息存储。用ＵＶ－Ｖｉｓ、Ｘ射线四圆衍射分析等方法对材料结构进行了表征,并用量子化学计算讨论了可能的存储机制。     

Force modulation for enhanced nanoscale electrical sensing

Scanning probe microscopy employing conductive probes is a powerful tool for the investigation and modification of electrical properties at the nanoscale. Application areas include semiconductor metrology, probe-based data storage and materials research. Conductive probes can also be used to emulate nanoscale electrical contacts. However, unreliable electrical contact and tip wear have severely hampered the widespread usage of conductive probes for these applications. In this paper we introduce a force modulation technique for enhanced nanoscale electrical sensing using conductive probes. This technique results in lower friction, reduced tip wear and enhanced electrical contact quality. Experimental results using phase-change material stacks and platinum silicide conductive probes clearly demonstrate the efficacy of the proposed technique. Furthermore, conductive-mode imaging experiments on specially prepared platinum/carbon samples are presented to demonstrate the widespread applicability of this technique.     

Thermal effect limits in ultrahigh-density magnetic recording

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in². This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at “stability ratios” (K_uV/K_BT)₀ ≃35 ± 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in ² conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits     

Non-linear optical functions of crystalline-Si resulting from nanoscale layered systems

New non-linear optoelectronic and photovoltaic behavior of crystalline silicon (c-Si) has been obtained with a strained nanoscale Si-layered system. We have found c-Si absorptances that even exceed values of amorphous silicon (a-Si) thin films. The present investigation exploits charge carrier and photon flux transformations at the so-called carrier collection limit. A correlation between free carrier density and the absorption coefficient could be established by combining reflectivity and transmissivity measurements on samples having different surface free carrier reservoirs. In summary, Si modifications implemented on the nanoscale lead to new effects that can widen applications of conventional Si devices.     

Solid-state memories based on ferroelectric tunnel junctions

Ferroic-order parameters are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1?×?10(6)?A?cm(-2)). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ～1?×?10(4)?A?cm(-2) at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories, and have the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism.     

Pulse-read on erasable thermal phase-change superresolution disks

New erasable thermal phase-change superresolution (EPSR) disks composed of mask and recording layers can increase recording density by the detection of the below-diffraction-limited marks within the readout spot. The formation of the aperture and the readout signal on the EPSR disk were analyzed. The feasibility of optically designed EPSR disks was evaluated by thermal simulation. A carrier-to-noise ratio of 32 dB at a mark size of 0.4 mum, 8 dB higher than that of a conventional disk, was obtained by application of a pulse-read method to the EPSR disks at a wavelength of 780 nm and a numerical aperture of 0.55.     

High thermal stability and low power dissipation PCM with nanoscale oxygen‐doped SS thin film

To improve thermal stability and reduce power dissipation of phase‐change memory (PCM), the oxygen‐doped Sn₁₅ Sb₈₅ (SS) thin film is proposed by magnetron sputtering in this study. Comparing to undoped Sn15Sb85(SS), the oxygen‐doped‐SS thin film has superior thermal stability and better data retention. Meanwhile, the electrical conductivity of crystallisation oxygen‐doped‐SS thin film is also lower than that of SS, which means its less power consuming in PCM. The electrical conductivity ratio between amorphous and crystalline states for oxygen‐doped SS reaches up to two orders of magnitude. After oxygen doping, the root‐mean‐square surface roughness from amorphous (0.29 nm) to crystalline (0.46 nm) state for oxygen‐doped‐SS thin films becomes smaller. The switching time of amorphisation process for the oxygen‐doped‐SS thin film (∼2.07 ns) is shorter than Ge₂ Sb₂ Te₅ (GST) (∼3.05 ns). X‐ray diffractometer is recorded to investigate the change of crystalline structure. Thus, the authors infer that oxygen‐doped SS is a promising phase‐change thin film for PCM.Inspec keywords: sputter deposition, antimony compounds, X‐ray diffraction, phase change memories, thin films, surface roughness, doping, electrical conductivity, amorphisation, crystallisation, thermal stability, amorphous state, crystal structure, nanostructured materials, nanofabrication, oxygenOther keywords: oxygen doping, low power dissipation, high thermal stability, phase‐change memory, magnetron sputtering, nanoscale oxygen‐doped Sn15Sb85 thin film, electrical conductivity, crystallisation, crystalline state, amorphous state, root‐mean‐square surface roughness, amorphisation process, X‐ray diffractometry, crystalline structure, Sn₁₅ Sb₈₅      

Nanofountain‐Probe‐Based High‐Resolution Patterning and Single‐Cell Injection of Functionalized Nanodiamonds

Nanodiamonds are rapidly emerging as promising carriers for next‐generation therapeutics and drug delivery. However, developing future nanoscale devices and arrays that harness these nanoparticles will require unrealized spatial control. Furthermore, single‐cell in vitro transfection methods lack an instrument that simultaneously offers the advantages of having nanoscale dimensions and control and continuous delivery via microfluidic components. To address this, two modes of controlled delivery of functionalized diamond nanoparticles are demonstrated using a broadly applicable nanofountain probe, a tool for direct‐write nanopatterning with sub‐100‐nm resolution and direct in vitro single‐cell injection. This study demonstrates the versatility of the nanofountain probe as a tool for high‐fidelity delivery of functionalized nanodiamonds and other agents in nanomanufacturing and single‐cell biological studies. These initial demonstrations of controlled delivery open the door to future studies examining the nanofountain probe's potential in delivering specific doses of DNA, viruses, and other therapeutically relevant biomolecules.     

The effect of processing on the properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers

Semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers are biodegradable systems with potential as substrates for use in tissue regeneration. Previous studies have shown that severe embrittlement occurs on storage at room temperature restricting their application possibilities. Concepts such as secondary, advancing crystallisation causing changes in the amorphous/crystalline ratio have been mooted as the cause of the embrittlement. Using films prepared by extrusion and compression moulding procedures we have attempted to probe not only the pure amorphous and crystalline phases but also the interfacial region. Interpretation of dynamic mechanical and dielectric data highlights the changes in the nature of the interfacial region on processing. Moreover, the use of the Thermally Stimulated Discharge technique is a powerful probe for highlighting the morphological changes induced in multiphase systems by the processing step.     

Effect of the Crystallization Process and Solid State Storage on the Physico-Chemical Properties of Scale-up Lots of CI-936

N-(2,2-diphenylethyl)adenosine, designated as CI-936, is a novel, orally active antipsychotic agent. Depending on the manufacturing process, the drug substance exists in more than one crystalline form. Three lots of the drug were characterized by thermal analysis, scanning electron microscopy, X-ray diffraction, and dissolution. Two of these lots were found to be crystalline while the third was amorphous. The physical properties of the crystalline forms appear to change during storage under ambient conditions. The amorphous form, inspite of being in a high energy state, was not affected by storage. The absolute bioavailability of the amorphous form in dogs is more than 90%. In contrast, the other two crystalline lots demonstrated lower and unpredictable oral absorption profiles.     

Effect of the Crystallization Process and Solid State Storage on the Physico-Chemical Properties of Scale-Up Lots of CI-936

N-(2,2-diphenylethyl)adenosine, designated as CI-936, is a novel, orally active antipsychotic agent. Depending on the manufacturing process, the drug substance exists in more than one crystalline form. Three lots of the drug were characterized by thermal analysis, scanning electron microscopy, X-ray diffraction, and dissolution. Two of these lots were found to be crystalline while the third was amorphous. The physical properties of the crystalline forms appear to change during storage under ambient conditions. The amorphous form, inspite of being in a high energy state, was not affected by storage. The absolute bioavailability of the amorphous form in dogs is more than 90%. In contrast, the other two crystalline lots demonstrated lower and unpredictable oral absorption profiles.