Akemi Hirotsune

Nanometer-sized Phase-Change Recording using a Scanning Near-Field Optical Microscope with a Laser Diode.

We present for the first time a nanometer-sized phase-change recording using a scanning near-field optical microscope (PC-SNOM recording). The recording experiments were performed with a SNOM using a 785-nm-wavelength semiconductor laser diode, shear force detection for gap control and reflected light detection for observing the domains (reading). The recording media of ZnSSiO2(20 nm)/GeSbTe(30 nm)/ZnSSiO2(150 nm)/polycarbonate substrate were used. The writings were done at laser powers of 8.4–7.3 mW in the probe for pulse widths of 5 or 0.5 ms. As a result, we obtained a minimum recorded domain size of 60 nm in diameter. This size shows a potential to achieve an ultrahigh density PC-SNOM recording with about 170 Gb/in2. A possibility of achieving high speed readout for the future data storage is also discussed.

Phase change recording using a scanning near‐field optical microscope

The formation and observation, with reflected light, of 60‐nm‐diam phase‐changed domains in a thin GeSbTe film using a scanning near‐field optical microscope with a 785 nm wavelength laser diode is demonstrated. The dependence of the domain size on incident laser power was obtained, and the size changed from 150 to 60 nm in diameter with incident power of 8.4–7.3 mW in the probe. At the threshold power of 7.3 mW, the film temperature rose to around 180 °C to partially phase change the local area of the film from amorphous to crystalline. A detected reflectivity increase due to phase change in the formed domain was 8%–2%. The observing (reading) was performed with an incident laser power of 0.2 mW, which corresponds to 10−2–10−3 times less than in a magneto‐optical recording. The incident laser power shows that the phase change reading using the reflection scanning near‐field optical microscope has the potential to read the recorded bit at a speed over 10 MHz.

Scanning near-field optical microscope with a laser diode and nanometer-sized bit recording

We demonstrate 80 nm diameter bit recording for the first time using a phase change recording film and a reflection scanning near-field optical microscope with a 785 nm wavelength laser diode. The sample structure was a 20 nm thick ZnS-SiO 2 protection layer/30 nm thick Ge 2 Sb 2 Te 5 recording film/150 nm thick ZnS-SiO 2 protection layer/polycarbonate substrate. Writing was performed with pulsed laser light of 8.4 mW for 5 ms and 0.5 ms, and 8.0 mW for 5 ms. Written bits were observed in reflection by illuminating a small light of 0.2 mW. In this form of recording, a formation of phase change domains of about 50 nm in diameter is expected if the surface deformation is suppressed. Our results indicate the possibility to achieve an ultra-high recording density of more than 100 Gb in -2 .

Sub-Terabyte-Data-Capacity Optical Discs Realized by Three-Dimensional Pit Selection

To realize optical discs with the sub-terabyte data capacity, we propose the three-dimensional pit selection (3DPS) method where a single data pit to be read out in a multi-layer disc is selected three-dimensionally to obtain super-resolution in the disc plane and to reduce layer cross-talk. To examine the feasibility of this method, the phase-change pit capsule method was tested where the data pits consist of a phase-change material which melts during readout. The super-resolution effect was observed for both layers of a dual-layer disc. It was shown that a quadric-layer disc can be designed because of the high transmittance of each layer. Thus, 3DPS is considered to have the potential for a data capacity of hundreds of gigabytes with a conventional optical system.

Improved Grain Isolation in [Co/Pd]n Multilayer Media for Thermally Assisted Magnetic Recording

The effect of the magnetic and structural properties on recording performance was investigated. Well grain-isolated media were obtained by adding boron and oxygen into the [Co/Pd]n multilayer and cooling the substrate. We found that the recorded bit size could be decreased by reducing the magnetic cluster size. Twenty-flve-nm bits were successfully recorded on the improved media with a magnetic cluster size of about 30 nm.

High-Density Recording on a Phase-Change Optical Disk with Suppression of Material Flow and Recording-Mark Shape-Deformation.

Double-reflective-layer structure disks with recording film containing a high-melting-point component that does not melt during recording are studied. Cr–Te is chosen for the high-melting-point component to be added to Ge–Sb–Te recording film to suppress a recording film material flow. A Si–Al double-reflective-layer structure has the advantage of increasing the material flow stopping power margin. The structure is also effective in preventing deformation of the recording marks due to the difference in light absorption between the crystallized region and the amorphized recording mark. A Si–Al double-reflective-layer-structure disk with Cr–Ge–Sb–Te recording film shows very little distortion in the reproduced signal waveform even after 104 overwrites of pulse-width modulation (PWM). This makes it suitable for high density recording as a rewritable digital video disk (DVD).

Optimization of Crystallization Characteristics for Phase-Change Optical Disk with Ag–Ge–Sb–Te Recording Film

A Ag–Ge–Sb–Te recording film was developed, and its chemical composition was optimized to minumize overwrite jitter. The cause of the jitter was investigated by measuring the crystallization ratios and by transmission electron microscopy. It was found that the overwrite jitter increased with a shortage of Ge or Ag because the crystallization speed increased when these elements were added, while it increased with an excess of Ge or Ag because the crystallization speed decreased when these elements were added. A model based on the atomic bond distance and the strength of interatomic interactions in the crystal was devised. The model can be used to explain the mechanism by which a slight change in the chemical composition of recording films changes the crystallization speed. It was confirmed that the proposed disk with recording film having a small amount of Ag had the minimum overwrite jitter (under 10%).

Layer-Selection-Type Recordable Optical Disk with Inorganic Electrochromic Film

We have introduced a tungsten oxide film to a layer-selection-type recordable optical disk (LS-R) as an inorganic electrochromic (EC) film. The EC film showed a large transmittance change in a wide wavelength range. Layer selection was demonstrated with a dual-layer disk whose spacing was 0.3 µm, and the capacity of a 20-layer LS-R was estimated to be 1 TB.

GeSbTe Phase Change Material for Blue-Violet Laser at High Linear Speed

High-speed recording and read-out were carried out under the condition of numerical aperture (NA) 0.65 and substrate thickness 0.6 mm using a blue-violet laser of 405 nm. Using a GeSbTe phase change material of the crystal nucleus generation dominant type, we obtained a carrier-to-noise ratio (CNR) of 50 dB with 0.24 µm mark and a DC erase ratio of 30 dB at a velocity of 18 m/s. It was proven that the realization of the 100 Mbps data transfer rate was possible by optimizing record compensation.

New phase‐change rewritable optical recording film having well suppressed material flow for repeated rewriting

Characteristics of a phase‐change optical disk using a (Ge–Sb–Te)‐(Cr–Te) recording film containing a high‐melting‐point component Cr–Te are studied. The high‐melting‐point component Cr–Te precipitates in the recording film, and prevents a material flow of the recording film during repeated rewrites in which the recording film is melted. There is no drawback such as noise increase by the addition of Cr–Te. Thus the phase‐change optical disk using this recording film shows very small distortion in the reproduced signal wave form even after 2×104 rewrites of high‐density recording signals that are liable to cause the material flow by forming long and short recording marks on the disk. With a phase‐change optical disk, it is easy to overwrite (rewrite without prior erasure) information, which makes it highly suitable for digital recording of motion pictures and sound. They are expected to be used as rewritable compact disks, rewritable CD‐ROM disks, rewritable video disks, and small recording media for perso...