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Nonvolatile memories (NVMs) are key devices in computers to save a user’s information. Besides flash memory, several types of NVMs that use magnetoresistance, resistance change of metal oxides, and phase change of chalcogenide alloys have been studied. Among these, phase-change random-access memory (PC-RAM) is competitive from the viewpoint of switching speed, high durability, and scalability. In 2017, Intel and Micron Technology shipped commercial devices named Optane that use a phase-change material as storage class memories. Condensed-matter physicists have recently been attracted to phase-change materials because of their functionality as topological insulators. If the topological phase state is controllable and applied to PC-RAM, electron spin transfer and storage effects will be further available in addition to electrical resistance switching.
Temperature-dependent variations in electric switching and transverse resistance of phase-change [(GeTe)2(Sb2Te3)]n (n=4 and 8) chalcogenide superlattice (CSL) films were studied using conductive scanning probe microscopy (SPM). Three temperature regions with different electric transport properties were recognized in point current-voltage (I-V) spectra and the surface potential maps measured with tantalum and platinum-coated SPM cantilevers. At around 80°C the switching voltage decreased abruptly from ∼2 V to 0.5 V and the thermal coefficient of resistance changes its sign, indicating different carrier transport mechanisms. The observed changes correlated with decrease in the surface potential by ∼150 meV from 25 to 150°C. The results were ascribed to an opening of the CSL electronic band gap near the Fermi energy caused by thermal stress, which led to the transition from a Dirac-like semimetal to a narrow-gap semiconductor.
Topological (GeTe)/(Sb2Te3) superlattices (SL) are of
practical interest for memory applications because of different mechanism of
electric conductance switching in the crystalline phase. In the work, electrical
switching behavior of individual SL grains was examined employing a multimode
scanning probe microscope (MSPM) in a lithography mode at room temperature.
Using programmed bias voltage with different amplitude and pulse duration, we
observed the position-dependent variations of the switching voltage and the
current injection delay for [(GeTe)2
(Sb2Te3)]4 SLs on Si(100). The results shed
a light on the role of electric field and hot-electron injection on the SL
We report on the study of single devices of phase-change (Ge2Sb2Te5) memory cells in line cell type devices. Devices were investigated employing an x-ray nanobeam of only about 150 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell. XANES spectra showing the device in the amorphous and crystalline state have been successfully collected after switching the device in situ at the synchrotron. By monitoring the fluorescence response of the sample constituent materials at a constant photon energy (corresponding to the Ge K-edge absorption edge) as a function of x-ray beam position on the sample 2D maps have been produced.
We introduce a technique to permit x-ray absorption spectroscopy studies focusing on individual phase-change (Ge2Sb2Te5) memory cells in fully integrated PC-RAM structures. Devices were investigated employing an x-ray nanobeam of only about 300 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell and enabled us to investigate individual devices without interference from non-switching material surrounding the area of interest. By monitoring the fluorescence signals of tungsten and germanium at a photon energy corresponding to the Ge K-edge absorption edge white line position, we were successful in producing 2D area maps of the active cell region, which clearly show the imbedded tungsten heater element and the switched region of the phase change material. Additionally, position dependent changes in the phase change material could be traced by taking an array of XANES spectra at the Ge K-edge on and in the vicinity of individual devices.
The origin of sub-diffraction-limit apertures in Sb-based thin films is discussed. Electromagnetic energy can be channeled by these apertures thus allowing near-field focussing- the Super-RENS effect. The aperture formation within Sb, Sb2Te3, Sb2Te, SbTe and Ge2Sb2Te5 is investigated by time resolved optical pump-probe techniques and found to occur without melting. Density functional calculations have shown that these materials exhibit a thresholdlike change in their optical properties below their melting temperatures. The threshold is shown to be a consequence of thermally induced misalignment of p-orbital bonds. It is the non-linearity of this process that leads to the formation of the sub-diffraction-limit apertures.
We demonstrate, both experimentally and by computer simulation, that while the metastable face-centered cubic (fcc) phase of Ge-Sb-Te becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. We present evidences that the pressure-induced amorphisation phenomenon strongly depends on the concentration of vacancies included in the Ge/Sb sublattice, but is thermally insensitive. Upon higher compression, a body-centered cubic phase is obtained in both cases at around 30 GPa. Upon decompression, the amorphous phase is retained when starting with the fcc phase while the initial structure is recovered when starting with the trigonal phase. We argue that the presence of vacancies and the associated subsequent large atomic displacements lead to nanoscale phase separation and the loss of the initial structure memory in the fcc staring phase of Ge-Sb-Te. We futher compare the amorphous phase obtained via the pressure route with the melt quenched amorphous phase.
The influence of stress on the phase change behaviour of Ge2Sb2Te5 encapsulated by ZnS-SiO2 and TiN is investigated using temperature dependent Extended X-ray Asbsorption Fines Structure and Ellipsometry to determine the crystallisation temperature. The encapsulation material surrounding the Ge2Sb2Te5 has an increasingly dominant effect on the material's ability to change phase and can cause a profound increase in its crystallization temperature. We have experimentally shown that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress we have maintained the bulk crystallization temperature in Ge2Sb2Te5 films just 2 nm thick.
Chalcogenides, in particular germanium-antimony-tellurium (GeSbTe) and antimony-rich tellurium (R-SbTe) based alloys, are the most technologically significant alloys currently being applied to recordable optical storage as typified by rewritable digital versatile discs (DVD-RW), DVD random access memory, (DVD-RAM). The same alloys are also being applied to nonvolatile random access memory electrical memory in the form of phase change random access memory (PCRAM). In 2004, the phase transition mechanism of GeSbTe was first revealed, demonstrating that the amorphous state is not a random configurational network but is locally well-ordered with the crystalline to amorphous switching process being based upon Ge atoms moving between octahedral and tetrahedral symmetry positions. The kinetic barrier between these two states gives rise to the non-volatile nature of GeSbTe as a storage medium. In contrast, no theoretical analysis has been proposed for SbTe alloys because a Ge-free system. In this paper, the Sb2Te structure has been investigated using the local density approximation (LDA) using a plane-wave basis and compared with experimental results. The effect of external stress on the structure was also investigated. It was found that Sb2Te undergoes two phase-transitions at around 18 GPa (compressive) and −3 GPa (tensile). In the case of negative stress, the c-axis was found to expanded more than the other axes, giving rise a large refractive index change. We report on coherent (uniaxial) melting induced by the breaking a sigma bond between Sb2Te3 and Sb superlattices. We believe this to be the origin of the phase transition that induces a large change in physical properties.
In this paper, an optical biosensor based on the localized surface plasmon resonance (LSPR) of Ag nanostructured films is proposed and demonstrated. The Ag nanostructured films, which are fabricated by the reduction of AgOx thin films, exhibit a strong LSPR at wavelengths around 370 nm in an air environment. The reflectance spectra of the Ag nanostructured film represent that the shift in the LSPR wavelength follows a linear dependence on the refractive index of the surrounding medium. By varying the concentration of streptavidin solution, we demonstrate that the Ag nanostructured films functionalized with thiol and biotin molecules can sensitively detect a binding event between biotin and streptavidin molecules.
In addition to their wide-spread application in the re-writable optical memory markets, phase-change memory alloys are also poised to take a prominent role in future non-volatile memory applications due to their potential for low-energy usage and indefinite cyclability compared with their silicon-based flash memory counterparts. In contrast with their widespread use, however, the details of the crystalline to amorphous switching process utilized for memory storage remain an active research topic with many details still lacking. Considering the conflicting requirements for high-speed switching, yet long term data storage integrity, a deeper understanding of these materials is essential for insightful application development. We have used x-ray absorption fine structure spectroscopy (XAFS), a technique equally suitable for amorphous and crystalline phases to elaborate details in structural changes in the phase-change process for a variety of phase-change alloys in static measurements. As the kinetics of the switching process are the linchpin for optimizing switching characteristics, we have recently initialted dynamic measurements of light -induced structural changes in Ge-Sb-Te (GST) alloys. These measurements have been carried out synchronously using both femtosecond and nanosecond laser pump pulses in conjunction with 100~ps x-ray pulses generated by an electron storage ring. By synchronously triggering the laser with a variable sub-nanosecond delay, we have been able to use XAFS to probe details of the dynamics of the switching process. Preliminary results learned from this approach applied to GST alloys are presented.
We have demonstrated that certain chalcogenide layers within a spinning super-RENS optical disc allow to squeeze the 650 nm laser beam to a spot size as fine as 50 nm using a 15-nm chalcogenide film. The near-field light was focused at a depth of just over 30 nm after passing through a chalcogenide film. Finite-difference time-domain (FDTD) simulations also reproduced these results. We suggest that a conductive ring aperture generated in the chalcogenide layers plays an important role in the localized light focusing.
Surface-enhanced Raman scattering (SERS) efficiency of silver nanoparticles formed by laser irradiation or thermal annealing in sputtered silver oxide layers was examined. Silver nanoparticles formed by irradiation of He-Ne laser light (632.8 nm) to a sputtered silver oxide thin film thermally annealed at 300°C show good SERS, while silver nanoparticles formed by thermal annealing at 600°C scarcely show SERS. From these results, it is deduced that thermal annealing at a proper temperature results in formation of silver nuclei that can be precursors of silver nanoparticles with desirable sizes to induce the SERS, while thermal annealing at a higher temperature results in the formation of large silver particles that no longer cause the SERS.
RF-magnetron sputtered thin films of silver oxide (AgOx) were recently applied to ultra-high density optical data storage. It has been elucidated that the AgOx film sandwiched by protection layers shows very attractive characteristics in strong light-scattering, local plasmon generation and super-resolution by focussing a laser beam on it. Especially, the combination with an active recording film (optical phase change or magneto-optical) used in the currently recordable optical disks improves the storage density and overcomes the diffraction limit. In this paper, we describe the basic characteristics of nano-scale light scattering centers generated in the AgOx films and the interaction with ultra-high density recorded mark patterns in a near-field region. In addition, we provide the structural transition of the AgOx film by thermal and laser annealing treatment.
Thin silver oxide films used as mask layers in super-Resolution Nearfield Structure (super-RENS) disks for high density optical data storage were reactively sputter-deposited and their composition was determined by spectroscopic means. We found that the stoichiometry of the films changed with the oxygen content in the sputtering gas atmosphere. With a stepwise increase in the percentage of O2 from 0 - 100%, the corresponding layers consist of Ag, mixtures of Ag and Ag2O, Ag2O, mixtures of Ag2O and AgO and AgO. Laser activation of such oxidic phase containing deposits results in the decomposition of the material and excitation of strong local plasmons in the remaining silver clusters. This was confirmed by acquiring surface enhanced Raman spectra (SERS) of benzoic acid (BA), copper phthalocyanine (CP) and internal carbon impurities on silver oxide substrates. From this data, we conclude that the sub-wavelength resolution obtained in super-RENS disks is mediated by local surface plasmons on small silver particles forming in the mask layer.
Super-resolution near-field structure (Super-RENS) was prepared by a heliconwave-plasma sputtering method to improve the disk property that is combined with a magneto-optical (MO) recording disk. Antimony and silver-oxide mask layers were prepared by the method and refractive indices were measured. Recording and retrieving of signals beyond the resolution limit (<370 nm) were achieved for both mask cases. Attempts to optimize the disk structure were also made using a conventional sputtering method. The smallest mark size was around 200 nm and the highest carrier-to-noise ratio (CNR) was 30 dB for 300-nm mark and 22 dB for 250-nm, when using a laser wavelength of 780 nm and a numerical aperture of 0.53. We have found that there is a competing super-resolutional mechanism besides Super-RENS that appears when high readout laser power is applied. This mechanism played rather an important role at least in the mark-size range of 200-370 nm.
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