Phase-change Disk Research Articles

Numerical Analysis of Ridged-Circular Nanoaperture for Near-Field Optical Disk

A ridged-circular nanoaperture is investigated through three-dimensional (finite-difference time-domain) FDTD method. The motion equations of free electrons are inserted to analyze a metallic material. The electromagnetic field distributions of optical near-field around the aperture are investigated. The phase change disk illuminated by a near-field optical light through a ridged-circular nanoaperture is also analyzed. The far-field scattering patterns from the phase change disk and the crosstalk characteristics between plural marks are studied.

Synthesis of ZnS/SiO<sub>2</sub> Core-Shell by Sol-Gel Process and Covering then with Gold Nanoparticle and Study of its Photoluminescence Properties

Semiconductors have been suggested as ideal candidates for many electrical and optical applications and several groups have reported their successful synthesis in recent years. In particular, ZnS that is a direct wide band gap (3.91 eV) semiconductor and an important phosphor host lattice material, can be used in electroluminescent devices (ELD), due to its large band gap, which enables emission of visible light without absorption and efficient electron transport. In this work, we report synthesis and structural characteristics of gold covered core-shell nanometer size of ZnS/SiO2thin films as a patterning materials for use as a protective layer in optical phase change discs prepared through a simple sol-gel process. The results obtained by transmission electron microscopy (TEM) images showed that, as prepared core-shell materials were well-aligned nanoprticles grew in the same direction and through X-ray diffraction (XRD) analysis, we found that ZnS/SiO2core shell material was composed of crystalline ZnS core covered by an amorphous SiO2shell. Photoluminescence (PL) measurements performed on core-shell samples before and after gold coverig samples have provided information regarding stoichiometric vacancies or interstitial impurities and enhenced intensity of the green emission of ZnS nanowires at room temperature due to sheelding with SiO2.

Direct observations of Ge2Sb2Te5 recording marks in the phase-change disk

Atomistic structures of the Ge2Sb2Te5 thin film in the real phase-change disk have been investigated using transmission electron microscopy (TEM). As-deposited amorphous Ge2Sb2Te5 films were laser-irradiated for initialization (crystallization) and recording. Cross-sectional TEM observations revealed that the recording mark was fully amorphized by laser irradiation. A slight difference between the as-deposited and the laser irradiation-induced amorphous Ge2Sb2Te5 was observed in the intensity profile of nanobeam electron diffraction patterns and atomic pair distribution functions. This difference was attributed to structural relaxation of amorphous Ge2Sb2Te5, which gives rise to the alteration of chemical order.

Analysis of a Near-Field Optical Disk with an Acute-Edged Metallic Nano-Aperture

Readout characteristics of a near-field optical disk with an acute-edged metallic nano-aperture were investigated. The electromagnetic field distributions of the near field around the aperture were analyzed, and the far-field scattering of the waves transmitted through the phase change disk was calculated by finite-difference time-domain method into which motion equations of free electrons were incorporated. Adjusting the edge angle of the aperture increased the field intensity of near-field light. The influence of the edge angle on output through the recorded mark was also investigated. It was found that there is an optimum edge angle that differs depending on the aperture width.

Crystallization and thermal stability of Sn-doped Ge2Sb2Te5 phase change material

The influence of Sn doping on the crystallization and thermal stability of Ge2Sb2Te5 phase change material was studied. Thermal analysis shows that the phase change of Sn7.0Ge20.6Sb20.7Te51.7 occurs slightly higher than that of Ge2Sb2Te5 at 154 °C and has a lower melting point at 536 °C. The activation energy for crystallization for this material is also higher. It has a face-centred-cubic crystal structure. A fast crystallization speed of 60 ns is realized upon irradiation by a blue laser beam of 405 nm. The use of the Sn7.0Ge20.6Sb20.7Te51.7 phase change material as a mask layer in aperture-type super-resolution near-field phase change disk is realized to increase the carrier-to-noise ratio and thermal stability.

Ultrafast crystallization and thermal stability of In–Ge doped eutectic Sb70Te30 phase change material

Effect of In and Ge doping in the form of In2Ge8Sb85Te5 on optical and thermal properties of eutectic Sb70Te30 alloys was investigated. Crystalline structure of In2Ge8Sb85Te5 phase change material consists of a mixture of phases. Thermal analysis shows higher crystallization temperature and activation energy for crystallization. Isothermal reflectivity-time measurement shows a growth-dominated crystallization mechanism. Ultrafast crystallization speed of 30ns is realized upon irradiation by blue laser beam. The use of ultrafast and thermally stable In2Ge8Sb85Te5 phase change material as mask layer in aperture-type super-resolution near-field phase change disk is realized to increase the carrier-to-noise ratio and thermal stability.

Two-Dimensional FDTD Analysis of the Readout Characteristics of an Optical Near Field Disk

Various kinds of optical near-field apertures have been proposed for higher throughput and smaller spot size. However, few studies have mentioned the readout characteristics of the recorded marks on an optical disk illuminated by a near field optical light. In this paper, we have investigated the scattering light by a two-dimensional recorded mark on a phase change disk with Finite-Difference Time-Domain (FDTD) simulations. Instead of using Recursive Convolution (RC) and Piecewise Linear Recursive Convolution (PLRC) scheme, we integrated the motion equation of free electron into conventional FDTD method to solve the electromagnetic field in the metallic materials. The validity of the proposed method is illustrated by comparing its results with those from the analytic exact solution. We analyzed the distributions of optical near-field around a two-dimensional metallic nano-aperture, and then calculated the far-field scattering pattern from a two-dimensional recorded mark on a phase change disk while it was illuminated by an optical near-field though a nanoaperture. The sum signal by a condenser lens was calculated from far-field pattern, and its relationships with the width of recorded mark and the thickness of each layer in the phase change disk were illustrated. The cross-talk between the recorded marks was also discussed.

Detection of Recording Marks on Digital Versatile Discs and Blu-ray Discs using Conductive Atomic Force Microscopy

Recording bits on recordable phase-change discs are studied using conductive atomic force microscopy (C-AFM) at high contrast. The grain size and boundary analysis of different phase-change recording materials can be performed at nanoscale spatial resolution. This study shows that C-AFM is not only suited to image the amorphous marks written on phase-change-based rewritable digital versatile discs (DVD) or Blu-ray discs (BD), but is also capable of revealing the appearance of bits in CuSi-based write-once BD discs.

Micromachined Polarization Beam Splitter With Adjustable Leak Ratio for Optical Pickup

In this letter, we propose the optomechanical design and fabrication of a novel batched-processed polarization beam splitter (PBS) with an adjustable leak ratio for optical recording applications. The micro-PBS was fabricated by a 410-nm low stress silicon nitride Si3N4 membrane integrated with a bimorph actuator. A dc drive voltage was applied to allow an adjustable leak ratio (0-0.24) with incident angle from 65deg to 75deg for operation in the different optical requirements (phase-change and magneto-optical disks) at lambda = 633 nm. The characteristics of the fabricated PBS are in close agreement with the theoretical calculations and confirm its feasibility.

Analysis of the Degradation Mechanism during Repeated Overwrite of Phase-Change Discs

Rewritable optical digital versatile discs (DVDs) were analyzed to identify alterations in layer composition after repeated overwrite cycles. Elemental depth profiling revealed the presence of S in the phase-change layer in areas with high jitter values. This we relate to a decomposition of the ZnS:SiO2 dielectric layer during repeated overwrite. The S concentration in the phase-change layer at the location of the repeatedly overwritten tracks is estimated to range between 0.1 and 1 at. %, depending on whether or not a caplayer is present between the ZnS:SiO2 layer and the phase-change layer. From electron microscope images we established that repeated overwrite cycles induce voids in the phase-change layer. By analyzing the repeatedly overwritten areas of various recording stacks we collected evidence that a caplayer may suppress void formation. Both S diffusion and void formation will limit the overwrite cyclability of optical discs, which explains the frequently reported beneficial effect of a caplayer on overwrite performance.

High-Speed Write/Read Techniques for Blu-ray Write-Once Discs

We have been developing high-speed write/read techniques for Blu-ray discs. We carried out 1–12× write/read tests on a phase-change Blu-ray write-once disc using an improved tester, an adaptive partial-response maximum-likelihood (PRML) technique, and a new write adjustment method. Sufficiently good limit equalizer jitter values of less than 6.5% were obtained under all write-speed conditions. Good bit error rates of less than 10-6 and fair power margins of more than ±10% were obtained under all write/read speed conditions.

High Density Near-Field Optical Disc System

Key technologies for near-field recording/readout systems using a solid immersion lens (SIL) are summarized in this paper. Our system employing a SIL of 1.84 numerical aperture (NA) and a laser diode (LD) of 405 nm wavelength has realized the capacity of a 112 GB in a 12 cm diameter phase-change disc along with a write-power margin of ±11.2% and a data transfer rate of 36 Mbps. Such a larger-capacity data storage system should provide a high data transfer rate. A preliminary result of 1.84 NA dual-channel near-field recording/readout using a monolithic dual-beam LD with 412 nm wavelength is presented.

Analysis of Near-Field Readout Characteristics Using Approximate Vector Diffraction Calculation

In a near-field optical readout system using a solid immersion lens (SIL), comparisons of the readout characteristics of different types of disc structure were made. We measured the frequency responses of the readout signals of a Si-etched disc, an injection molded polycarbonate disc and a phase-change disc. The calculation results obtained by an approximate vector diffraction method agreed well with the measured differences between the readout characteristics of the three discs.

Optimized Write Strategy for 4× Speed BD-RE

We studied an optimized write strategy for a 4× speed phase-change Blu-ray disc system for further practical implementation. This was performed by simulation using three-dimensional alternating directional implicit thermal analysis and crystallization modeling. We evaluated and confirmed this approach by implementing it in 2× recording with commercially available 23 GB discs.

Read/Write Technology for 50 GB/layer at 100 Mbps

We developed read/write technology for high-density optical data storage of 50 GB/layer at a data transfer rate of 108 Mbps. We demonstrated the writing and reading capabilities of a 50 GB/layer optical disk recording system at 36 Mbps with a sufficient power margin, using a three-beam crosstalk canceller with optical response equalizers, using a phase-change write-once disk. Bit error rates were below 1×10-4 at 108 Mbps. Experimental results showed that the technology realizes writing and reading densities two times higher and data transfer rates three times higher than those of Blu-ray with a simple optical component such as Blu-ray.

Performances of phase-change recording disks based on GaSbTe media

Phase-change optical disks with novel GaSbTe recording media were prepared in the DVD+RW format and evaluated by a dynamic tester at 1X speed. The studied compositions locate along the Sb/sub 7/Te/sub 3/-GaSb and Sb/sub 2/Te/sub 3/-GaSb pseudo-binary tielines. The disks were prepared by an RF co-sputtering method. Enhanced recrystallization has been demonstrated in the disks with Sb/sub 7/Te/sub 3/-rich recording layer. An approximated composition Ga/sub 2/Sb/sub 5/Te/sub 3/ even exhibited a nearly complete erasure (CNR/spl sim/45 dB, dcerasability/spl sim/-40 dB). Relationship between the dynamic characteristics and crystal structure of the GaSbTe materials has been proposed.

In situ X-ray diffraction study of crystallization process of GeSbTe thin films during heat treatment

The crystallization processes of the Ge 2Sb 2Te 5 thin film used for PD and DVD-RAM were studied in its realistic optical disk film configurations for the first time by X-ray diffraction using an intense X-ray beam of a synchrotron orbital radiation facility (SPring-8) and in situ quick detection with a Position-Sensitive-Proportional-Counter. The dependence of the amorphous-to-fcc phase-change temperature T 1 on the rate of temperature elevation R et gave an activation energy E a: 0.93 eV much less than previously reported 2.2 eV obtained from a model sample 25–45 times thicker than in the real optical disks. The similar measurement on the Ge 4Sb 1Te 5 film whose large reflectance change attains the readability by CD-ROM drives gave E a: 1.13 eV with larger T 1 than Ge 2Sb 2Te 5 thin films at any R et implying a lower sensitivity in erasing as well as a better data stability of the phase-change disk.

Recording of Multi-Level Run-Length-Limited Modulation Signals on Compact Disc/Digital Versatile Disc Rewritable Discs

Multi-level (ML) recording technology increases the capacity of standard compact disc (CD) and digital versatile disc (DVD) rewritable systems with no change to the optical/mechanical unit. The feasibility of applying ternary run-length-limited (3L-RLL) modulation to recording/playback on standard CD/DVD phase change discs is discussed. The use of 3L-RLL modulation increases the recording density 50% relative to conventional binary RLL (2L-RLL) modulation currently used in CD and DVD systems.

New Improvement Methods for Multispeed Initialization-Free Phase-Change Optical Disk

Disk structure optimization, thermal balance design and a new writing strategy are proposed to further widen the speed range of multispeed recording for an initialization-free phase-change disk. Simulated results show a high cooling rate. Experimental results show that the initialization-free phase-change disk with three improvement methods has a broad range of recording speeds from 3.49 to 20.94 m/s, which correspond to a rotation speed of 1 to 6 times that of digital versatile disk-read only memory (DVD-ROM), respectively.

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.