Phase-change Optical Disk Research Articles

Structural investigation ofGeSb2Te4: A high-speed phase-change material

Pseudobinary ${\mathrm{G}\mathrm{e}\mathrm{T}\mathrm{e}\ensuremath{-}\mathrm{S}\mathrm{b}}_{2}{\mathrm{Te}}_{3}$ is in wide use today as a memory material in phase-change optical disks such as DVD-RAMs. ${\mathrm{GeSb}}_{2}{\mathrm{Te}}_{4},$ one of the intermetallic compounds used in this binary system, when in thermal equilibrium, shows a complex cubic close-packed structure with a 21-layer period. However, when an amorphous thin film of this compound is heated by laser irradiation and then suddenly cooled, it crystallizes into a simple NaCl-type structure as the metastable phase. These two structures are so different from each other that it is difficult to imagine that they have the same chemical composition. To reveal the relationship between the two, in this paper we examine the structures of both phases of ${\mathrm{GeSb}}_{2}{\mathrm{Te}}_{4}$ at various temperatures by x-ray powder diffraction using synchrotron radiation facilities. The results of our investigation are as follows: the metastable phase has a very open structure, which contains one vacancy per eight atoms. The structure transition from the metastable to the stable phase is due to the vacancy diffusion. Contrary to what was previously thought, the crystal structure of the stable phase is similar to that of ${\mathrm{PbBi}}_{2}{\mathrm{Se}}_{4}.$

The influence of oxygen and nitrogen doping on GeSbTe phase-change optical recording media properties

Nitrogen and oxygen doped and co-doped GeSbTe (GST) films for phase-change optical recording are investigated. It is found that the crystallization temperature increased as well as the crystalline microstructure refined by doping. The carrier-to-noise ratio (CNR) and erasability of phase-change optical disks are improved being up to 52 and 35 dB, respectively, by using an appropriate nitrogen doping or co-doping concentration in the recording layer. Optical disks with co-doped recording layer are found to be superior in the recording characteristics then the single doped recording layer disks.

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.

Near-Field Optical Storage Technology

Higher density optical disks with higher data rate and technological capability of super parallel optical heads using VCSEL (vertical cavity surface emitting laser) array are described. Optical heads using the VCSEL array and microlens array to get higher evanescent light from narrow apertures play a key role in supporting optical disks, which are coated with lubricant and protective film on their flat surfaces in order to keep the gap between the super parallel optical head and the disk surface within several 10 nano meter. Higher throughput efficiency in the near-field semiconductor optical probe array has been obtained in our optical head. However, the obtained evanescent light power is 10 μW from the 100 nm probe aperture, which is one tenth of the power needed to write a bit on the phase change optical disk surface. One solution to get ten times the power is to develop special nano fabricated optical probes of higher throughput efficiency. A metal fine grating fabrication method to get evanescent light wave resonant enhancement is planned after good 2D-FDTD simulation results.

書換型相変化光ディスクの多層化限界

We carried out a feasibility study on rewritable phase-change optical disks with more than two recording layers. Dynamic recording-playing evaluations of the test disks and Tx measurements revealed the thinning limit of the examined GeSbTe memory film to be 2 nm. The disk with a 2.5-nm thick recording film showed a high transmittance of 77 % at a wavelength of 405 nm, and a CNR of 45 dB. Based on our experimental results, we conclude that at minimum, a quadruple-layer disk for blue lasers will be possible pending some improvements in the maximum output of laser diodes, the sensitivity of optical detectors and the crystallization speed of memory films.

Effects of the Sb 2Te 3 crystallization-induced layer on crystallization behaviors and properties of phase change optical disk

The conventional phase-change optical disk is generally fabricated by the sputtering process, which has a drawback of requiring an initialization process to change the as-deposited recording layer in the disk from amorphous to crystalline phases before the disk can be used for reading or writing. In order to develop an initialization-free process, the Sb 2Te 3 alloy was used as an additional layer below or above the recording Ge 2Sb 2Te 5 layer to study its effect on crystallization behaviors of the recording layer. The layer structures were deposited on substrates of Si wafer, Cu-mesh to examine crystal structure (XRD), amorphous-to-crystal transformation (DSC) and microstructure (TEM). The complete disk specimens were prepared on PC board to measure their dynamic properties, such as reflectivity, jitter and modulation (dynamic tester); and to examine the effects of laser pulse duration time, position and thickness of Sb 2Te 3 layer on static reflectivity of the disk (static tester), where Avrami coefficient ‘ q’ in J-M-A rate equation can be derived. The results show that effect of Sb 2Te 3 layer is essentially to induce crystallization of Ge 2Sb 2Te 5 recording layer from (110) plane of Sb 2Te 3 crystals. This is due to the fact that the crystallization temperature of Sb 2Te 3 crystal is 85 °C below that of Ge 2Sb 2Te 5 crystal, in addition to a lower lattice mismatch between two crystals. The is in agreement with the J-M-A kinetic analyses that the rate controlling step for amorphous-crystal transformation in disk specimens with Sb 2Te 3 layer over 15-nm thickness is mainly governed by nucleation with q=2.53–2.79>2.5 in J-M-A equation. Regarding the effects of Sb 2Te 3 layer on disk properties, the results show that under the 10 nm Ge 2Sb 2Te 5 layer thickness, the Sb 2Te 3-assisted disks with lower Sb 2Te 3 layer thickness between 13 and 20 nm show the best combination of reflectivity and modulation. The most important advantage of this process is that the Sb 2Te 3-assisted disks require no initialization process, because the as-deposited disks can be directly written and erased.

Analyses of Signals from Dual-Layer Phase Change Optical Disks

The signals obtained from dual-layer optical disks were analyzed experimentally and theoretically with emphasis on reducing layer cross talk (LCT) and spherical aberration (SA). Using the dual-layer phase change optical disk, the DC element of the LCT was measured and phase change marks were written on both layers. Diffraction calculations were used to explain the experimental results and to determine the amount of LCT and signal degradation caused by SA as functions of spacer layer thickness and optical detector size. Analytical methods to estimate LCT and SA were proposed, and their predictions turned out to agree well with the experimental and diffraction calculation results. These findings indicate that for a blue laser, the conventional techniques cannot simultaneously keep the LCT and SA to within practical values. Accordingly, designs for disks and drives for dual-layer optical disks are discussed.

Laser-induced changes on the complex refractive indices of phase-change thin film

The refractive indices of crystalline phase-change films are usually obtained by thermal-induced crystallization. However, this is not accurate, because the crystallization of phase-change film in rewritable optical disks is laser induced. In this study, we use the initializer to crystallize the phase-change films. The dependence of the refractive index n and the extinction coefficient k of the phase-change films on the initialization conditions are studied. Remarkable changes of the refractive indices (especially k) are found when the initialization laser power density is 6.63 mW/mum(2) and the initialization velocity is 4.0 m/s. At the same time, the structure changes of the phase-change films are also studied. This dependence is explained by the structure change of the films. These results are significant in improving the accuracy of optical design and the thermal simulation of phase-change optical disks, as well as in the study of phase-change optical disks at shorter wavelengths. (C) 2003 Society of Photo-Optical Instrumentation Engineers.

A Read/Write Experiment of Phase-Change Optical Disk for High-Data-Rate and High-Density

A Read/Write Experiment of Phase-Change Optical Disk for High-Data-Rate and High-Density

Optical properties and crystallization of AgInTeSbGe phase-change optical disk media

AgInTeSbGe thin films were deposited by the dc sputtering method. Due to germanium doping in the AgInTeSb film, new Ag2Te and Ge2Sb2Te5 crystalline phases were formed. The crystallization temperature and the activation energy of the AgInTeSbGe thin film increased manifestly with the addition of Ge. The refractive index n and the extinction coefficient k of the AgInTeSbGe film in the amorphous and crystalline states were measured. The reflectivity contrast of the AgInTeSbGe phase-change optical disk was greater than %30 in almost the whole visible region. A minimum recording mark of about 220 nm in length was recorded using the AgInTeSbGe thin film as the recording medium. The reflectivity contrast of the minimum recording mark was about %25.

Multispeed Superlattice-Like Phase Change Optical Disk

In order to extend the disk rotation speed margin of phase change optical disks, a superlattice-like structure was applied to the phase change layer. Results of the dynamic recording test show that superlattice-like (SLL) phase change disks are suitable candidates for multispeed recording and have better direct overwrite recording properties than conventional phase change optical disks.

Study of the Dynamic Crystallization Behavior of GeSbTe Phase Change Optical Disk

Dynamic erasing process is of great importance to increase the data transfer rate of rewritable phase change optical disks. Two disk structures with the GeSbTe phase change layer were employed. The first one is a disk with the conventional four-layered structure while the other one is a five-layered disk with an thermal buffer layer of Si. The dynamic erasing process of the GeSbTe disks was investigated through studying the dynamic crystallization behavior of these disks, which can be characterized by the changes of oscilloscopy signals such as mark length and amplitude. The correlations between the erasing power and the mark length and the amplitude were established. It has been shown that for the shorter marks the mark edge growth from the boundary between the amorphous mark and the crystalline space dominated the whole erasing process. However, for the longer marks, both mark edge growth and nucleation in the mark center and subsequent growth contributed to the dynamic erasing process.

Prospects of Doped Sb–Te Phase-Change Materials for High-Speed Recording

Recording parameters such as crystallization rate, archival life stability and media noise have been studied for phase-change optical discs based on doped Sb–Te materials. It has been found that the crystallization rate can be tuned easily by varying the Sb/Te ratio or the dopant atom; higher crystallization rates are obtained for higher Sb/Te ratios and for In- and Ga-doped Sb–Te. However, it has been found that measures that enhance the crystallization rate often also influence other material properties, such as archival life stability and media noise. For example, a higher Sb/Te ratio leads to a lower archival life and higher media noise. As a consequence, the demands on doped Sb–Te materials to combine sufficiently high crystallization rate, good archival life stability and low media noise become increasingly difficult to satisfy for high recording speeds.

Rewritable Dual-Layer Initialization-Free Phase-Change Optical Disk

The initialization-free dual-layer phase-change disk was proposed for the skipping of the initialization process and the reduction of production time and product cost in the production of dual-layer phase-change disk. The computer simulation and disk design of the initialization-free dual-layer disk were carried out. The initialization-free dual-layer phase-change optical disks with Sb2Te3 additional layers were successfully fabricated. The experimental results were in good agreement with the simulation and design results. No significant deterioration was observed in the initialization-free dual-layer phase-change optical disk compared with the conventional dual-layer phase-change disk.

Multilevel Signal Processing Technique for a 30 Gbit/inch2 Phase-Change Optical Disk System

We developed a multilevel recording technique and a Viterbi decoding technique for increasing the recording density of a phase-change optical disk. In this multilevel recording technique, each mark recorded on the disk changes its edge position in four steps according to four-level data. Since two consecutive mark edges are reproduced simultaneously, their reproduced signal takes seven levels. We devised a Viterbi decoder with reference matrices in order to improve data detection performance by compensating the nonlinearity of this multilevel signal. We evaluated the error rate performance of this optical disk system by computer simulation. The Viterbi decoder with the reference matrices showed superior performance, and had an improved noise margin which is 3.5 dB higher than that of a conventional Viterbi decoder. By employing the multilevel recording technique and the Viterbi decoding technique, we confirmed the feasibility of the phase-change optical disk system with a recording density of 30 Gbit/inch2.

Characterization of Ultra-Fast Phase Change Optical Disk with GaSb Material

ABSTRACTWe studied the crystallization mechanism of ultra-fast phase change optical disks with recording layers made of GaSb material for digital versatile disk (DVD) systems. The results of a static recording test and an amorphous mark formation simulation suggest that GaSb maintains a high crystal growth rate even at temperatures 150 degrees lower than the material's melting point. Disks with recording layers made of this material have a write speed margin ranging from DVD 3× to 8× or more.

Optical Properties of Multi-layer Chalcogenide Thin Film for Optical Recording Media

ABSTRACTWe present the results of optical properties of multi layer thin films as in the media of phase change optical disk data storage. Reflectance and optical contrast of multi layer thin films increased rapidly between 100 °C and 150 °C. Moreover, optical contrasts at different wavelength were also studied. The refractive index and the optical band gap decreased, while the extinction coefficient increases as the crystallization occurs. The Egopt of crystalline thin film was ∼0.6 eV lower than that of amorphous thin film. Egopt decreased as the number of stacked layer increased.

Effect of Ionic Bond in Recording Materials on Characteristics of Phase-Change Optical Disk

ABSTRACTThis paper discusses a crystallization model that takes difference of covalent bond and ionic bond into consideration. Ionic bond makes crystallization easier than covalent bond because ionic bond is more flexible than covalent bond. The diameter and spaces around large minus-ions should also be taken into consideration. Elements••having electronic negativity far from that of Te or Sb has a tendency to form ionic bonds. For example, Tb and In are considered to make chemical bonds having more ionic characteristics than Ge. In the case of already reported Sn or Bi addition, large ion diameter seems to increase crystallization speed.

Surface roughening of recording media and readout performance of phase-change optical disk

Scaling characteristics of the surface roughening of the phase-change optical media have been investigated. Studies have shown that the variation-correlation function of the surface profile of recording media follows an asymptotic power law. The influence of the film surfaces with different roughness on the readout performance of the optical media has been investigated based on a scalar diffraction model. The results show that only one roughness parameter cannot reflect the responding readout properties of a phase-change optical disk such as signal noise and jitter due to the surface roughening. Fractal Brownian motion is applied to simulate the scaling characteristics of the surface roughening. Influence of the surface roughening with different Hurst exponents on the readout performance of the optical media has been studied. Results of the numerical calculation from both simulated and physical surfaces of the phase-change media show that the surface roughening with a lower Hurst exponent will cause a higher deterioration of the detected signals. Therefore, the fractal method provides an effective approach to the failure analysis of the optical phase-change disk.

Diamond-like carbon film overcoats for phase-change optical recording discs

Diamond-like carbon (DLC) films are applied as a protective layer in phase-change optical discs due to their high hardness, low friction and high optical transmission. This article aims to present the effect of surface root-mean-square ( R rms) roughness, optical transmission, electrical signals, adhesion and alkali resistance of DLC films grown by the plasma enhanced chemical vapor deposition (PECVD) technique. The studies were conducted by atomic force microscopy (AFM), UV-visible spectrometry, disc-testers, three adhesion-tests and alkali solution, respectively. The results reveal that it is novel and useful to protecting the storage data by making DLC film overcoats on discs.