基于GeSbTe膜的探针存储机制的研究

针对基于原子力显微镜(AFM)的探针相变存储研究中存储介质和存储方贩性的研究.比较了用直流磁控溅射部分不同工艺参数所制备的GeSb2Te4薄膜的表面性能,同时对探针诱导相变机理进行了初步探讨.试验观察的结果表明,利用AFM导电探针对相变化材料GeSb2Te4膜施加一定的直流电压,可以通过形貌和相结构的变化来获得存储的信息点,并且通过施加一定时间的反向电压可以实现信息点的消除.     

Multispeed rewritable optical-recording method with an initialization-free phase-change disk

A new method of multispeed rewritable optical recording is presented. An initialization-free phase-change optical disk is proposed as a candidate for multispeed rewritable optical recording. The simulated results of the initialization-free disk at different linear velocities show that the cooling rate increases from approximately 18.69% to 37.96%. A model that combines the crystallization acceleration effect due to the additional layers and the rapid cooling rate due to the initialization-free disk structure is proposed as the physical mechanism of the multispeed recording method with an initialization-free disk. The dynamic optical-recording properties of the initialization-free DVD-RAM disk at different recording speeds shows that the initialization-free phase-change optical-recording disk is compatible with a broad range of recording speeds from 3.49 to 12.21 m/s.     

Ultra-high-density phase-change storage and memory

Phase-change storage is widely used in optical information technologies (DVD, CD-ROM and so on), and recently it has also been considered for non-volatile memory applications. This work reports advances in thermal data recording of phase-change materials. Specifically, we show erasable thermal phase-change recording at a storage density of 3.3 Tb inch(-2), which is three orders of magnitude denser than that currently achievable with commercial optical storage technologies. We demonstrate the concept of a thin-film nanoheater to realize ultra-small heat spots with dimensions of less than 50 nm. Finally, we show in a proof-of-concept demonstration that an individual thin-film heater can write, erase and read the phase of these storage materials at competitive speeds. This work provides important stepping stones for a very-high-density storage or memory technology based on phase-change materials.     

Measurement of the thermal coefficients of rewritable phase-change optical recording media

We describe a method to estimate the heat capacity of the substrate, the dielectric layer, and the phase-change layer of phase-change optical recording media as well as the thermal conductivity of the phase-change layer in its crystalline state. Measurements were carried out on spinning disks with the beam of light focused and locked onto the groove track. The method relies on the identification of the solid-to-liquid phase transition that occurs in the phase-change layer and takes advantage of the dependence of thermal diffusion on track velocity and irradiation time.     

可擦重写光盘记录介质材料及其特性

可擦重写光盘因其能对已写入文件进行改写,因此比只读光盘、一次写入光盘具有更大的灵活性,从而具有很好的应用前景。论述了两种可擦重写光盘记录介质即磁光盘记录介质与相变光盘记录介质的记录机理,并详细阐述了两种记录介质的性能要求。     

Sources of noise in erasable optical disk data storage

Noise sources in the readback signal for phase-change and magneto-optical disks at red, green, and blue wavelengths are examined, and a simple model is presented to explain the observed noise spectra. For phase-change disks the media noise, which corresponds to ~0.4% fluctuation in the disk's amplitude reflection coefficient, is the limiting performance factor for the conventional detection scheme. In magneto-optical media the depolarization noise, whose fluctuations are ~0.05% of the disk's reflection coefficient, is the major contributor to the media noise in the differential detection scheme. In phase-change optical disks the main sources of noise are the roughness of the groove profiles and the graininess of the polycrystalline recording layer. In nongrooved regions of the disk the media noise measured with green light is found to be nearly the same as that obtained with the red light. In magneto-optical disks the scattering of light from the rough groove profiles, as well as media inhomogeneities, gives rise to depolarization. Measurements on nongrooved regions of a magneto-optical disk indicate that the media noise obtained with the green light is somewhat higher than that obtained with the red light.     

Chalcogenide glasses as prospective materials for optical memories and optical data storage

This paper first reviews recent progress in the understanding of the nanometer-scale mechanism of reversible photostructural changes in chalcogenide glasses, and its relevance to various photo-induced phenomena. Then the principles of phase-change optical recording are described. Finally, a novel technique for overcoming the diffraction limit in optical recording is considered, namely the super-resolution near-field structure. This technique, used in combination with multicomponent Te-based chalcogenides as a recording medium, is believed to be a prototype for future optical-storage devices.     

A numerical simulation for two-dimensional moving boundary problems with a mushy zone

For materials such as alloy, organic phase-change materials and many others, the change of phases may take place over a temperature range. This leads to phase-change problems with the mushy zone in which the solid and liquid phases coexist. The present study introduces a numerical method combining the Laplace transform technique and the control volume method to solve two-dimensional phase-change problems with the mushy zone. The hybrid numerical method involves the control volume formulation for the space domain and the Laplace transform technique for the time domain. The Taylor's series approximation is applied to linearize nonlinear terms in the governing equation. The transfinite mapping method is used to generate control-volume meshes in each region. The growth of the mushy zone is unknown a priori and is predicted by using the least-square iteration scheme. It will be found that the present hybrid numerical method can be efficiently applied to solve two-dimensional phase-change problems with a mushy zone.     

Phase-change optical recording: Past, present, future

We discuss specific features of Te-based compounds that made them the best materials for the phase-change data storage. It is demonstrated that the phase-change recording is due to a switch of Ge atoms between octahedral and tetrahedral symmetry positions within the Te face-centered cubic lattice. It is this nature of the transition that makes the Te-based media fast and stable. The driving force for this transition is also discussed. The chapter is concluded by introduction of a concept of the super-resolution near-field structure (super-RENS) disc that allows to reduce a bit size well below the diffraction limit and makes 100 GB/disc storage a reality.     

Measurement of the thermal coefficients of nonreversible phase-change optical recording films

We have developed a procedure to obtain the critical temperature for the amorphous-to-crystalline phase transition as well as the thermal conductivity and the specific heat of the phase-change media of optical recording. The procedure involves estimating the thermal conductivity from the data obtained by measuring the threshold cw laser power required for inducing phase transition. Then, from the data obtained in short-pulse measurements, we estimate the specific heat. In principle this method can yield the thermal parameters of any number of layers, so long as one of the layers is made of a phase-change material having a well-defined transition temperature.     

Mirror hologram recording with a phase-change medium

We propose mirror hologram recording with a phase-change material, which has a large refractive-index difference between its amorphous and crystalline states. It offers excellent diffraction efficiency and is erasable and nonvolatile. We designed an optimum multilayer structure for high diffraction efficiency by simulating the effect of each film thickness on diffraction efficiency. Experiments with a germanium tellurium alloy as the phase-change material show a high diffraction efficiency close to the calculated value. This medium can also be used for directly drawing computer-generated holograms. Lee-type computer-generated holograms were drawn on this medium with optical-disk technology.     

Optical characterization of multilayer stacks used as phase-change media of optical disk data storage

We report results of measurements of the optical constants of the dielectric layer (ZnS-SiO2), reflecting layer (aluminum-chromium alloy), and phase-change layer (GeSbTe, AgInSbTe) used as the media of phase-change optical recording. The refractive index n and the absorption coefficient k of these materials vary to some extent with the film thickness and with the film deposition environment. We report the observed variations of optical constants among samples of differing structure and among samples fabricated in different laboratories.     

Thermal properties and crystallization dynamics of a phase-change alloy for write-once optical data storage

Using a two-laser static tester, we measured the crystallization temperature and the thermal conductivity of a phase-change alloy thin film used in write-once-read-many media of optical data storage. The experimental technique, in general, and the calibration procedures, in particular, are described. The measurement results are used as entry points into numerical calculations that ultimately yield estimates of the material parameters. Valuable information about the dynamics of mark formation (i.e., localized crystallization) in amorphous phase-change alloy films is obtained from the observed variations of the sample reflectance under short-pulse and long-pulse recording conditions. The dependence of these reflectance variations on the laser pulse power has also been investigated.     

Amorphization induced by subnanosecond laser pulses in phase-change optical recording media

We have investigated the dynamics of amorphization induced in phase-change optical recording media by focused laser pulses of subnanosecond duration. We initiated localized amorphism by using a focused laser beam to melt the phase-change material and completed the change by rapid cooling by means of thermal diffusion. These studies were conducted by use of real-time reflectivity measurements with a pump-and-probe technique in which both pump and probe pulses had a duration of approximately 510 ps. Our transient-reflectivity measurements indicate that the process that leads to amorphism has three distinct stages, namely, rapid melting, solidification, and slow relaxation.     

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.     

Use of PCM Boards for Solar Cell Cooling

This contribution explains the use of fractal theory to describe thermal properties of materials. The basic idea is rooted in the theory of fractal fields defined in E-dimensional Euclidian space. Generic equations describing heat distribution are then specialized to describe the changes in heat transfer as a response to step-wise increases in the amount of heat added to the system. This model was then applied to the study of properties of a systems consisting of solar cells attached on a phase-change material (PCM) back sheet board. The aim of this study is to evaluate the ability of PCM boards to decrease the working temperature of solar cells and thus to increase the efficiency of the cells. Regression of experimental data was used to obtain model parameters. The parameters obtained this way were the thermal diffusivity, thermal conductivity, and specific heat, as well as parameters of the heat source and parameters related to the heat losses of the system. The method was then verified against parameters of the system based on poly-methyl-methacrylate and then applied to a PCM with a phase-change temperature of 25 °C. The values of the thermal parameters were determined at temperatures where both components of the PCM composite (Micronal^® and gypsum wall) were solid and below the phase-change temperature and then again at temperatures where one of the components (gypsum) was still solid, while the other one was already liquid (wax). The attempt to determine the parameters during the phase change was not made due the physicochemical processes taking place which would alter the measured data.     

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.     

Study on Heat Transfer Characteristics of Phase-Change Energy Storage Unit for Thermal Management

The objective of the study was to investigate the heat transfer characteristics of a phase-change energy storage unit for thermal management. Considering the conduction in the solid and natural convection in the liquid, a physical and mathematical model for heat transfer was formulated. The governing conservation equations were solved using the finite-volume method on fixed grids. An enthalpy-porosity method was used for modeling the melting phenomenon of a phase-change energy storage unit. The time and space movement of the phase front, the temperature distribution, and the heat dissipation rate have been analyzed based on the model. The influence of the unit geometry, heat source location, and types of phase-change materials on the thermal performance of the energy storage unit were investigated. The model and numerical method were evaluated by comparing the numerical predictions with the experimental results. There was found to be excellent agreement between the calculation and experiment, indicating that the numerical method for heat transfer simulation of a phase-change energy storage unit is accurate. The results from the analysis elucidate the thermal performance of the phase-change energy storage unit and will provide the basis for the design and optimization of thermal management systems.     

Optical-disk media fabrication for high data-transfer rate

Two media fabrication methods are developed in order to improve the data-transfer rate of optical disks. One is a beam-scanning method for multispiral pregroove recording. A sixfold spiral pregroove with a 1.6-μm spacing can be recorded with this method. The other is a method for fabrication of phase-change optical-disk media that provides signal reading at 670 nm, writing at 830 nm, and high erasability at high linear velocity. The linear recording density of disks fabricated in this way is 1.2 times higher than that of optical disks read at 830 nm, and the erasability is more than 25 dB at 40 m/s. These methods offer the possibility of a data transfer rate 10 times higher than with conventional methods.     

Measurement of the thermal conductivity of erasable phase-change optical recording media

We describe a method to estimate the thermal conductivity of the substrate, the dielectric layer, the phase-change (PC) layer, and the reflective layer of PC optical recording media. The method relies on the amorphous-to-crystalline phase transition that occurs in the PC layer and takes advantage of the difference in the thermal diffusion behavior under different-sized focused spots. All the results obtained here are reliable with better than ?5% accuracy, which is within the margin of our experimental error.