Phase-change Recording Research Articles

GeBiTe相変化膜におけるBi L<sub>III</sub>吸収端のXAFS解析

The crystallization of GeBiTe (GBT) is faster than that of the well-known phase-change recording material GeSbTe (GST). Therefore, the investigation of GBT structure as well as its crystallization process is attractive. Accordingly, the high-speed crystallization of GBT is due to Bi. Thus, it was necessary to know the local structure around Bi within the amorphous GBT. Note that an interface layer, which is a very thin dielectric film adjacent to recording film, assists the crystallization of phase-change material in the optical recording media. The local structure around Bi in GBT within an actual media was analyzed using XAFS. Bi LIII-edge XAFS spectra of crystalline and amorphous GBT respectively, both with and without interface layer in the media, were obtained. As result of this analysis on GBT, nearest neighbor atom of Bi was found to be Ge. Moreover, within the amorphous GBT, the interatomic distance around Bi is larger than that around Ge. These are the differences between GST and GBT. We speculate that these factors contribute to the improvement of the GBT crystallization speed. On the other hand, the interface layer doesn’t influence the local structure of GBT; however, it does have an electric effect on the recording layer.

Amorphous-to-crystalline transition in Ge8Sb(2-x)BixTe11 phase-change materials for data recording

Structural and thermokinetic analyses were used to study the crystallization behavior of Ge8Sb(2-x)BixTe11thin films, promising materials for phase-change memory recording applications. By exploring the full compositional range, it was found that the Sb→Bi substitution leads to a decrease of crystallization enthalpy and activation energy of the main crystallization phase-change process. These trends were explained in terms of the changing structural ordering within the recently proposed new phase-change atomic switching mechanism. All of the compositions exhibited very similar transformation kinetics, confirming the uniformity of the phase-change mechanisms involved. It was further shown that rapid energy delivery achieved during heating, in the case of all investigated materials, leads to a transition from the classical nucleation/growth-based formation of 3D crystallites towards an autocatalytic phase-change process with an enormously increased speed of crystallization. Rapidity of the crystallization process was quantified for all of the studied compositions based on a novel Index of Crystallization Rapidity criterion – the results provided by this criterion showed that the highest crystallization speed was produced by the Ge8Sb0.8Bi1.2Te11 composition, which therefore from this point of view appears to be a suitable candidate for the new generation of phase-change memory recording devices.

Time-resolved phase-change recording mark formation with zinc oxide near-field optical active layer

In this paper, an optical active thin film of zinc oxide (ZnOx) nano-composites exploited for the enhancement of optical signals in an ultra-high density recording scheme has been demonstrated. Via the electron microscope investigation, the results display randomly distributed crystalline nanograins in the ZnOx thin films. Optical disks with the ZnOx nanostructured thin films show that the carrier-to-noise ratio (CNR) above 25 dB can be obtained at the mark trains of 100 nm, while the optimal writing power is reduced as a function of the increasing thickness of the ZnOx films. Furthermore, by conducting a series of the optical pump–probe experiments, the optical responses of recording marks on as-deposited phase-change Ge2Sb2Te5 (as-GST) recording layers present that the highly contrast bright recording bits can be acquired with the existence of the ZnOx nanostructured thin films, providing prospective potentials in future data storage and optoelectronic devices.

Shape effects of GeSbTe nanodots on the near-field interaction with a silver triangle antenna

We investigated the shape effects of GeSbTe nanodots on the near-field interaction with a silver triangle antenna using the three-dimensional finite-difference time-domain method, avoiding the difficulty of detecting near-field signals from a single dot that occurs in current measurements. The surface plasmon resonance of silver strengthens the near-field around nanodots made of GeSbTe, commonly used in phase-change recording. Using GeSbTe spheres and pillar dots with various top plane shapes, we investigated the relationship between the inner electric field concentration of GeSbTe nanodots and the radius of curvature of the corners facing the antenna tip. Reducing the radius of curvature strengthens the inner electric field of the dots, enhancing the near-field difference in intensity for the GeSbTe phase change. GeSbTe diamond pillars with a radius of curvature of 1 nm exhibit a near-field difference in intensity of 28% for the phase change. Using the antenna and the GeSbTe nanodot array, optical write-once recording is realized. The preliminary research in this study is expected to realize future optical disk storage using GeSbTe nanodots with diameters below 10 nm.

Phase-selective fluorescence of doped Ge2Sb2Te5 phase-change memory thin films

In this Letter, new concepts of fluorescence phase-change materials and fluorescence phase-change multilevel recording are proposed. High-contrast fluorescence between the amorphous and crystalline states is achieved in nickel- or bismuth-doped Ge2Sb2Te5 phase-change memory thin films. Opposite phase-selective fluorescence effects are observed when different doping ions are used. The fluorescence intensity is sensitive to the crystallization degree of the films. This characteristic can be applied in reconfigurable multi-state memory and other logic devices. It also has likely applications in display and data visualization.

Polarization readout analysis for multilevel phase change recording by crystallization degree modulation

Four different states of Si15Sb85 and Ge2Sb2Te5 phase change memory thin films are obtained by crystallization degree modulation through laser initialization at different powers or annealing at different temperatures. The polarization characteristics of these two four-level phase change recording media are analyzed systematically. A simple and effective readout scheme is then proposed, and the readout signal is numerically simulated. The results show that a high-contrast polarization readout can be obtained in an extensive wavelength range for the four-level phase change recording media using common phase change materials. This study will help in-depth understanding of the physical mechanisms and provide technical approaches to multilevel phase change recording.

Simplified Transfer Function of Write Compensation for Phase-Change Optical Disks

A simplified transfer function for write compensation is given by the inverse function of the cooling transfer function of a phase-change optical disk. The areas at a phase-change recording layer irradiated with an optical beam melt and change to marks. The marks are formed when the area is rapidly cooled; however, the marks disappear in a slowly cooled situation. Thus, the cooling process was considered essential for the mark formation on a phase-change optical disk. Therefore, the transfer function of the mark formation combined a heating component before mark formation with that after mark formation using a lag element. It was also found that the transfer function for write compensation could be achieved using a two-stage pulse train method. Experimental results showed that long marks were recorded by the pulse train method with the same frequency characteristic as our theoretical analysis.

Laser Pulse Compensation Applied to Phase-Change Recording Film to Form Nano-Sized Marks

We have examined the application of the laser pulse compensation to the phase-change recording film of the triple-layer optical disc by using an original writing laser pulse controller, which compensates dynamically the pulse edges in order to form the marks at the correct position. We confirmed 112 nm sized marks smaller than the optical spot formed on the thin recording film. The results of the compensation had some characteristics depending on the data pattern and/or the layer. We discussed the causes by considering about the structure of the recording films and the phase-change process to form the marks.

高速高密度相変化記録材料の原子配列と相変化過程の解明

高速高密度相変化記録材料の原子配列と相変化過程の解明

Structural transformation of Sb-based high-speed phase-change material

The crystal structure of a phase-change recording material (the compound Ag(3.4)In(3.7)Sb(76.4)Te(16.5)) enclosed in a vacuum capillary tube was investigated at various temperatures in a heating process using a large Debye-Scherrer camera installed in BL02B2 at SPring-8. The amorphous phase of this material turns into a crystalline phase at around 416 K; this crystalline phase has an A7-type structure with atoms of Ag, In, Sb or Te randomly occupying the 6c site in the space group. This structure was maintained up to around 545 K as a single phase, although thermal expansion of the crystal lattice was observed. However, above this temperature, phase separation into AgInTe(2) and Sb-Te transpired. The first fragment, AgInTe(2), reliably maintained its crystal structure up to the melting temperature. On the other hand, the atomic configuration of the Sb-Te gradually varied with increasing temperature. This gradual structural transformation can be described as a continuous growth of the modulation period γ.

SET/RESET properties dependence of phase-change memory cell on thickness of phase-change layer

The thermal properties of a phase-change random access memory (PCRAM) cell are dominated by the phase-change recording material. The SET/RESET resistances, the minimum width of the SET/RESET pulse, the threshold voltage and the maximum temperature of PCRAM cells are impacted by the thickness of the phase-change layer. In this paper, a PCRAM cell with different Ge–Sb–Te phase-change layer thickness are fabricated, and the electrical properties of the PCRAM cells are tested. The SET/RESET properties dependence of PCRAM cells on the thickness of the phase-change layer is investigated and compared for the low-power consumption of PCRAM.

Ultra-high-density phase-change storage using AlNiGd metallic glass thin film as recording layer

A nonchalcogenide inorganic blu-ray super-resolution near-field optical disk was successfully developed using an Al-based metallic glass as the recording layer. Data were recorded based on the large reflectivity change obtained during transformation from the amorphous state to the crystalline state upon laser irradiation. The optical and thermal properties of the Al 90Ni 3Gd 7 metallic glass thin film were studied. The crystallization and melting temperatures were higher and more thermally stable than that of conventionally used Ge 2Sb 2Te 5 chalcogenide phase-change recording material. Al 90Ni 3Gd 7 metallic glass also demonstrated fast phase-change response to blue laser. Dynamic recording performance of this new structure showed good carrier-to-noise ratio and readout stability.

Research on phase-change of disc recording media GaSbBi

A new phase-change recording materials, Bi-doped eutectic composition GaSb, was researched experimentally. The films were prepared by vacuum evaporation. The crystallization temperature decreases as the Bi concentration increases. The crystallization activation energy, calculated by Kissinger method, is more than 2.0eV and meets the requirement of the films stability. The crystallization rate enhances with the increase of the Bi concentration. The Ga12Sb79.2Bi8.8 satisfies the demand of optical property by means of spectrophotometer. The absorption of the Ga12Sb79.2Bi8.8 crystalline films is more than that of the Ga12Sb79.2Bi8.8 amorphous films, which can reduce the change when reads information. The least reflectivity of Ga12Sb79.2Bi8.8 is 28.78%, corresponding with the reflection demand.

Plasma enhanced germanium nitride dielectric thin films for phase-change recording systems

The more efficient dielectric material and structure have been developed for fast phase-change optical recording systems. The germanium nitride thin film was reactively sputtered using a d.c. chamber, operated at 3 kW with a mixed gas plasma of argon and nitrogen. The dielectric thin film of 11–13 nm has been designed as the upper dielectric layer between the Ge–In–Sb–Te active layer and the silver reflective layer. The optical storage disks were initialized by a Hitachi POP120-8E device to provide crystalline active layers before the dynamic tests. The initialization was conducted using 810 nm laser at 2300 mW while the disks were rotated at a constant linear velocity of 10 m/s. The optical properties were analyzed using a Steag ETA-RT optical measuring system at 410–1010 nm wavelength. The power margin window was shown to be effectively improved by 60% to 18–26 mW when compared with the traditional ZnS–SiO 2 dielectric system. TEM (transmission electron microscope) micrograph showed clear profile for the standard 3T laser marks on the active layer. The plasma enhanced germanium nitride dielectric thin film could improve digital versatile disk's (DVD) production yield and efficiency.

光記録媒体 (1) 相変化光記録媒体

光記録媒体 (1) 相変化光記録媒体

相変化記録材料ＧｅＴｅ‐Ｓｂ２Ｔｅ３擬二元系化合物の結晶構造

Crystal structures of the phase-change recording material, GeTe-Sb2Te3, pseudo-binary compounds have been investigated over a decade using synchrotron radiation and the large Debye-Scherrer camera installed at SPring-8’s BL02B2 beamline. These compounds can be crystallized into metastable single phase with NaCl-type structure, for instance, by instantaneous laser irradiation; however, they are transformed into stable phases with long period trigonal structures characterized by the chemical formula, (GeTe)n(Sb2Te3)m (n, m; integer) after sufficient heat treatments. These stable phases can be described as structure with cubic close-packing periodicity (…ABCABC…) where the stacking rules of the Ge/Sb and Te layers are different from each other.

The effect of thermal anisotropies during crystallization in phase-change recording media

The problem discussed is the significance of anisotropies in the thermal parameters of different phases of phase-change materials as used for data storage purposes during recording. The particular phase change in interest is from the amorphous-to-crystalline state. Applying the method of correlation moment analysis produced upper estimators for the time dependence of the width of the crystalline mark and the time at which phase change ceases based on the heat flow process alone. These upper estimators are closed-form analytical expressions that can be used to estimate the recording resolution for any general spatial profile of initial temperature in the medium. This analysis showed that, up to a first order, the specific heat anisotropies have considerably less influence on the heat flow than the thermal conductivity differences. In general, for the material parameters used in phase-change data storage applications, the theory showed that the anisotropy in thermal parameters can be neglected.

High-Temperature Complex Refractive Index of Phase Change Recording Medium of Ge2Sb2Te5 Determined Using Phase Change Static Tester and Spectroscopic Ellipsometer

We investigated the high-temperature optical properties of the optical phase change material Ge2Sb2Te5 (GST) using an in situ ellipsometer equipped with a convetional heating chamber and using a spectroscopic ellipsometer combined with a phase change static tester. In addition, we compared the results for the high-temperature complex refractive indices of GST thin films determined using a spectroscopic ellipsometer after the samples were crystallized by the in situ and a phase-change random access memory (PRAM) methods. The ellipsometric constants and the RMS value of the surface roughness of the GST samples were measured by in situ ellipsometry and atomic force microscopy (AFM), respectively. Results showed wide variations at high-temperatures when the GST samples were crystallized by the in situ method. The complex refractive indices of GST thin films were consistent at mid-range temperatures up to 370 °C for the two methods. In contrast, they showed a large variation at a higher temperature of ∼450 °C for the in situ method and remained stable in the entire temperature range for the PRAM method. This indicates that the optical properties of GST thin film are affected by the film structure and protective layer for the in situ method but not for the PRAM method at high-temperatures. It is concluded that the combined use of the spectroscopic ellipsometer with the phase change static tester gives us more precise and consistent results not affected by either the film structure nor the protective material.

Thickness dependence of crystallization and melting kinetics of eutectic Sb70Te30 phase change recording film

As the thickness of eutectic Sb70Te30 phase change recording film was reduced, the crystallization temperature and activation energy for crystallization would increase, while the melting temperature and activation energy for melting would decrease. Accordingly, the archival stability and recording sensitivity can be improved. However, the recording speed for direct over write will be slowed down. Based upon the Johnson–Mehl–Avrami equation, it was found that the increase of film thickness would increase the nucleation rate of eutectic Sb70Te30 recording film and make the crystallization process become more nucleation-dominated.

Study of the optical response of phase‐change recording layer with zinc oxide nanostructured thin film

Recently, use of nanostructured materials as a near-field optical active layer has attracted a lot of interest. The non-linear optical properties and strong enhancements of metallic oxide nanostructured thin films are key functions in applications of promising nanophotonics. For the importance of ultra-high density optical data storage, we continue investigating the ultra-high density recording property of near-field optical disk consisting of zinc oxide (ZnO(x)) nanostructured thin film. A carrier-to-noise ratio above 38 dB at a recording mark size of 100 nm can be obtained in the ZnO(x) near-field optical disk by a DVD driver tester directly. In this article, we use an optical pump-probe system (static media tester) to measure the optical response of a phase-change recording layer (Ge(2)Sb(2)Te(5)) and demonstrate the high contrast of optical recording with a ZnO(x) nanostructured thin film in short pulse durations. Also, we investigate the dependence of writing power and the optical response in conventional re-writable recording layers and the phase-change material with ZnO(x) nanostructured thin film.