Phase-change Optical Disk Research Articles

Study of Cyclability Enhanced by Nitrogen Doping for Digital Versatile Disk-Random Access Memory Media

A nitrogen-doping process is commonly applied to improve the cyclability of phase change optical disks, but it is still difficult to attain the required number of overwrite cycles of 106 for digital versatile disk-random access memory (DVD-RAM) disks. In this study, we used a nitrogen doping process to prepare 2.6 GB DVD-RAM disks and successfully achieved more than 106 overwrite cycles of solely using by a four layer structure. The cyclability was found to increase with increasing nitrogen content in the N2/Ar sputtering gas flow then decrease with a further increase in the nitrogen content. Nucleation theory is used to explain the phenomena.

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks.

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.

Numerical study of near-field writing on a phase-change optical disk.

Absorption in the phase-change layer of an optical disk located in the near field of a Fabry-Perot laser diode is studied with a combination of finite-difference time domain (FDTD) analysis and a phenomenological laser model that predicts the operational characteristics of a laser diode. Some numerical simulations are performed and results are presented. In addition, the combined FDTD/laser-simulation model is described briefly.

Structural and thermal analysis of Ag-Sb-Te alloy and its films for phase change optical memories

An Ag-Sb-Te alloy and its films are prepared as a new optical recording amorphous crystalline (a?c) phase transformation material. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade-off problem between data (amorphous) stability and erasing sensitivity. phase change optical recording disks demonstrate long thermal stability of the amorphous recording marks. The crystallization process of Ag- Sb-Te material was studied using differential thermal analysis (DTA), and the nature of the material was studied by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The films were studied for both cases of before and after annealing. It was concluded that the alloy (Ag-Sb-Te) could be used as a phase change optical memory material.

Super-resolution near-field optical disk with an additional localized surface plasmon coupling layer

A structure of super-resolution near-field phase-change optical disk with localized surface plasmon coupling effect is proposed. A localized surface plasmon coupling layer (LSPCL) was introduced to form a coupled localized surface plasmons (CLSP) with the mask layer. Recording marks as small as 31 and 36 nm were observed in two structures, which were both much smaller than the smallest mark of 56 nm observed from the conventional one without LSPCL. CLSP not only can reduce mark size but it can also improve carrier-to-noise ratio of recording marks. The thermal stability of the disk was also studied.

Rewritable Dual-Layer Phase-Change Optical Disk with a Balanced Transmittance Structure

A rewritable dual-layer phase-change optical disk with a balanced transmittance structure for the layer located at the laser beam incident side (Layer 0) was developed. In this disk structure, transmittance of Layer 0 is almost constant, whether the layer is recorded or not. This structure was realized by adopting Ge–Sb–Te film that has appropriate optical constants, and optimizing the thickness of the dielectric layers. It is proved that this disk structure is effective in suppressing the influence of the recording state of Layer 0 on the layer beneath (Layer 1). Practical performances of the disk with this structure were confirmed for both layers. The feasibility of up to 55 GB capacity was examined for this rewritable dual-layer phase-change optical disk.

Superlattice-like structure for phase change optical recording

In order to increase the crystallization speed and data transfer rate (DTR), a superlattice-like structure (SLL) was applied to the recording layer of phase change optical disks. Unlike the conventional phase change layer, the recording layer with the SLL structure consisted of alternating thin layers of two different phase change materials, i.e., GeTe and Sb2Te3. Although neither GeTe nor Sb2Te3 could be used as a phase change layer material for practical applications, present experimental results revealed that the phase change optical disk with the SLL structure demonstrated an excellent recording property that could meet practical recording requirements. X-ray photoelectron spectroscopy was employed to confirm that the SLL structure could be preserved after many times of melting and quenching. Dynamic properties of the optical recording disk with the SLL structure were investigated with a 1 T pulse duration of 8 ns and a constant linear velocity of 19 m/s. A clear eye pattern was observed. The carrier-to-noise ratio was about 58 dB and a DTR of 47 Mbit/s was achieved. The DTR would be as high as 140 Mbit/s if the blue light is used. It has been proven that the SLL structure is a useful means to increase the DTR of phase change optical recording disks.

The Phase Change Optical Disc with the Data Recording Rate of 140 Mbps

We have been developing a new phase change optical disc on the base of the recording system (DVR) with a blue laser (405 nm) and dual objective lens with a numerical aperture (NA) of 0.85. We have achieved the data recording rate of 140 Mbps. The significant techniques to achieve the recording rate as follows: rearrangement of the AgInSbTeGe composition for the rate of 140 Mbps and controlling the re-crystallization by SRC structure (the super-rapid cooling structure) and development of Write Strategy for the high data recording. As a result of the techniques, less than 10% jitter value and more than 1,000 times overwriting possibility at 140 Mbps have been achieved.

Thermal Analysis of Multi Track Phase Change Optical Recording Using a Special Parallel Recording Array Head

Thermal analyses are performed on landed multi-track recording on phase change optical disk using a parallel recording head. A new arrangement of parallel recording head is proposed. Temperature uniformity for adjacent tracks was studied. The minimum angle between the slanting line array and the cross tracking line array to achieve temperature uniformity of adjacent track was investigated. This angle was found to be a function of the power and so on. The simulation results can be used to help the design of parallel recording array head as well as the structure of media.

New Prml for Asymmetrical Signals in High-Density Optical Disks

The partial response maximum-likelihood (PRML) signal processing technique is effective for high-density recording media. However, asymmetry in the playback signal, which is caused not only by inter-symbol interference, but also by mismatched recording power in phase-change optical disks, markedly degrades the quality of PRML signal processing. In this paper, we propose an adaptive equalizer with a LMS algorithm for asymmetrical signals. We show the improved results in recording and playback experiments.

New Approach to Initialization-Free Phase-Change Optical Disk

The mechanism of the initialization-free phase-change optical disk was discussed. The computer simulation about the optical and thermal properties of the initialization-free disk was carried out. The results showed that Sb2Te3 film was a suitable additional layer of initialization-free phase-change optical disk for Ge2Sb2+xTe5 phase-change media.

Development of Recording Stacks for a Rewritable Dual-Layer Optical Disc

In this paper results are presented on the development of recording layers for a rewritable dual-layer phase-change optical disc. The disc is based on the digital video recording (DVR) system utilising a blue laser and a high numerical-aperture objective lens. With such a disc the capacity of the DVR system could be increased to about 45 GB. Various stack designs, write strategies and recording characteristics of the media are discussed. For each recording layer a design with a fairly good performance is found.

Oxygen and Nitrogen Co-Doped GeSbTe Thin Films for Phase-Change Optical Recording

Nitrogen and oxygen co-doped GeSbTe films for phase-change optical recording are investigated. It is found that optical properties of amorphous and crystalline state are similar to that of GeSbTe, crystallization temperature increases and crystalline microstructure is refined which indicate a nucleation rate increases by co-doping. The carrier-to-noise ratio (CNR) and erasability of phase-change optical disks are improved being up to 52 dB and 35 dB, respectively, by using an appropriate co-doping concentration in the recording layer.

New Structure of Dual-Layer Rewritable Phase-Change Optical Disc

In this paper, we proposed a new simple structure of dual-layer rewritable phase-change optical discs. The disc consists of only three conventional materials for rewritable disc, a phase-change layer, dielectric layers and a reflective layer. Recording, erasing and overwriting in both recording mediums of the new disc structure are demonstrated and characterized. Both recording mediums can be rewritten using the conventional recording power defined by digital versatile disc (DVD) standard.

Crystallization of Ge 2 Sb 2 Te 5 phase-change optical disk media

In this paper, the crystallization behaviour of amorphous Ge2Sb2Te5 thin films is investigated using differential scanning calorimetry), x-ray diffraction and optical transmissivity measurements. It is indicated that only the amorphous phase to face-centred-cubic phase transformation occurs during laser annealing of the normal phase-change structure, which is a benefit for raising the phase-change optical disk's carrier-to-noise ratio (CNR). For amorphous Ge2Sb2Te5 thin films, the crystallization temperature is about 200°C and the melting temperature is 546.87°C. The activation energy for the crystallization, Ea, is 2.25eV. The crystallization dynamics for Ge2Sb2Te5 thin films obeys the law of nucleation and growth reaction. The sputtered Ge2Sb2Te5 films were initialized by an initializer unit. The initialization conditions have a great effect on the reflectivity contrast of the Ge2Sb2Te5 phase-change optical disk.

Phase-Change Optical Disc with the Data Recording Rate of 100 Mbps

We have developed a new phase-change optical disc having a transparent cover layer on the basis of the new optical recording system (DVR) with a blue laser (405 nm) and a dual objective lens with a numerical aperture (NA) of 0.85. We have developed the new technologies as follows. We used a transparent cover layer made of spin-coated UV resin, realized the rearrangement of the AgInSbTeGe composition to achieve the rate of 100 Mbps and controlled the recrystallization using a super-rapid cooling structure. As a result of using the above new technologies, a jitter value of less than 10% and more than 1,000 times overwriting ability at 100 Mbps have been achieved.

記録補償用マルチパルスを用いない相変化光ディスクヘの高速記録

In broadcasting stations, the data transfer rate of phase change optical disk systems for HDTV must be more than 100Mb/s. A conventional phase change disk system must drive a laser diode with multipulse in order to record data. However, using this method seems to make laser diode driving difficult if the data transfer rate is more than 100Mb/s to achieve. High data transfer rates requires high linear velocity. We theorized that the same cooling effect as occurs in multipulse appears at high linear velocity. A bit error rate of less than 10-5 was obtained in 100Mb/s read/write experiments in which random data was recorded without multipulse onto a phase change disk by using RLL (1, 7) code and Partial Response Maximum Likelihood (PRML) at a scanning velocity of 25 m/s. This result proved that our theory was correct.

Optical recording; phase-change and magneto-optical recording

Phase-change optical disk promoted digital versatile disk and magneto-optical disk promoted audio and digital still camera disk applications. Now these optical disk technologies start the high density racing again and have the capability of more than 100 Gbit/in 2 density. Ultra-short pulse 120 fs laser response on phase-change media shows capability of breakthrough the laser recording limitation and has the candidate of 1 Tbps recording data rate.

Determination of optical constants of thin films and multilayer stacks by use of concurrent reflectance, transmittance, and ellipsometric measurements

Using measurements of reflectance, transmittance, and the ellipsometric parameter D, we have determined the thickness, refractive index, and the absorption coefficient of various thin films and thin-film stacks. (D, the relative phase between the p- and s-polarized components, is measured for both reflected and transmitted light.) These optical measurements are performed with a specially designed system at the fixed wavelength of lambda = 633 nm over the 10 degrees -75 degrees range of angles of incidence. The examined samples, prepared by means of sputtering on fused-silica substrates, consist of monolayers and trilayers of various materials of differing thickness and optical constants. These samples, which are representative of the media of rewritable phase-change optical disks, include a dielectric mixture of ZnS and SiO(2), an amorphous film of the Ge(2)Sb(2.3)Te(5) alloy, and an aluminum chromium alloy film. To avoid complications arising from reflection and transmission losses at the air-substrate interface, the samples are immersed in an index-matching fluid that eliminates the contributions of the substrate to reflected and transmitted light. A computer program estimates the unknown parameters of the film(s) by matching the experimental data to theoretically calculated values. Although our system can be used for measurements over a broad range of wavelengths, we describe only the results obtained at lambda = 633 nm.

Performance of Ge–Sb–Bi–Te–B Recording Media for Phase-Change Optical Disks

We investigated the physical properties of GeSbBiTeB materials and examined the feasibility for phase change recording. The studied compositions were Ge4Sb0.5Bi0.5Te5 and Ge2Sb1.5Bi0.5Te5 with B doping. The coexistence of Bi and B atoms into both Ge4SbTe5 and Ge2Sb2Te5 lattice maintains single fcc structure without phase separation. The Bi substitution shows benefits in decreasing crystallization temperature and activation energy, however the reflectivity is slightly reduced. [rf3] With small amount addition of boron about 1 at.%, the reflectivity can be increased. [rf2] Conventional 4-layer structure of digital versatile disk-random access memory (DVD-RAM) 2.6 GB format was used to prepare the disks for dynamic characterization and overwrite cyclability evaluations. The disk with Ge4Sb0.5Bi0.5Te5(B) recording layer shows large noise fluctuation and low overwrite erase ratio, suggesting that the crystallization speed is still insufficient. While the disk with Ge2Sb1.5Bi0.5Te5(B) recording layer shows lower writing and erasing powers, stable noise level and high overwrite erase ratio, indicating the capability for DVD-RAM applications. The effect of B doping was verified to enhance the signal amplitude and modulation.