Nobuo Akahira

Rapid‐phase transitions of GeTe‐Sb2Te3 pseudobinary amorphous thin films for an optical disk memory

Amorphous films having a component of the stoichiometric GeTe‐Sb2Te3 pseudobinary alloy system, GeSb2Te4 or Ge2Sb2Te5 representatively, were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high‐speed one‐beam data rewriting. The material films being sandwiched by heat‐conductive ZnS layers can be crystallized (low power) or reamorphized (high power) by laser irradiation of very short duration, less than 50 ns. The cooling speed of the sandwiched film is extremely high: more than 1010 deg/s, which permits the molten material to convert to the amorphous state spontaneously; whereas, a low‐power pulse irradiation of the same duration changed the exposed portion into the crystalline state. The optical constant changes between the amorphous state and the crystalline state of them were measured to be large: from 4.7+i1.3 to 6.9+i2.6 and from 5.0+i1.3 to 6.5+i3.5, respectively. The crystallized portion was known to have a GeTe‐like fcc structure ...

High Speed Overwritable Phase Change Optical Disk Material

It was found that GeTe-Sb2Te3 pseud-binary amorphous alloy films showed remarkably fast switching properties to laser irradiation. By the static laser irradiation test, the film whose composition corresponded to stoichiometric compound of GeSb2Te4 were crystallized within 50ns of pulse duration at power of 8mW, whilst they could be amorphized with the same pulse duration at power of 20mW. Direct overwriting cycle test was performed on the revolving disk system for 105 times using single laser beam. CNR of more than 50dB and erasability of -22dB were obtained for linear velocity of 22m/s and overwriting frequencies of 5 and 7 MHz. The laser powers were 22 mW for recording and 10 mW for erasing. These materials will be applicable to high data rate direct overwritable disk media.

TeOx thin films for an optical disc memory

Tellurium suboxide thin films TeOx were found to change in refractive index and extinction coefficient on thermal or optical heating, with accompanying changes in the reflectivity and transmission. The preparation method and thermal or optical properties of the TeOx thin films were investigated to obtain a stable and highly sensitive optical disc memory. A two‐source evaporation method using Te and TeO2 provided uniform and any desired composition. The properties of the film depended on the x value; increasing x to as large as x=1.2, which represents a Te‐poor composition, produced an excellent humidity and heat stability, and decreasing x to 0.8 made the film more sensitive to a laser diode, but susceptible to humidity. The TeO1.1 thin film was found to sufficiently satisfy all requirements for practical disc applications. A reflective optical disc was prepared using the TeO1.1 thin film deposited on a polymethylmethacrylate substrate with grooves for optical tracking. This disc is capable of recording v...

Land and Groove Recording for High Track Density on Phase-Change Optical Disks

We have proposed a high-track-density optical recording method, which records signals on both the land area and groove area. From a computer simulation based on the scalar diffraction theory, we found that crosstalk between recording tracks depends on the groove depth, and is most reduced at the groove depth of 80 nm~90 nm for a laser wavelength of 780 nm, a numerical aperture of 0.55 and a track pitch of 0.8 µm. Applying this method to a phase-change optical disk, we experimentally confirmed that crosstalk depends on the groove depth and obtained a crosstalk level of less than -32 dB for a groove depth of 64 nm to 78 nm and 0.9 µm length marks. In order to prove that this recording method can be used for rewritable media, we also measured change of carrier-to-noise ratio (CNR) when signals were repeatedly recorded on an adjacent track. The reduction of CNR after 1000 repetitions of writing was less than 2 dB for a linear velocity of 5 m/s.

Thermal changes of optical properties observed in some suboxide thin films

Suboxide thin films of SbOx, TeOx, MoOx, and GeOx (x is smaller than the stoichiometric value for each component) were found to have the property of showing a critical change in their absorption coefficients and refractive indices at elevated temperatures. The thin‐film samples were prepared by evaporating a mixture of the stoichiometric oxide powder and a deoxidization metal powder such as tungsten. The critical temperatures of these thin films are 150, 120, 150, and 280 °C, respectively. The absorption coefficients before and after the heat treatment are 2.5×104 (before) and 6.1×104 (after), 8×104 and 1.0×105, 5.6×103 and 1.1×104, and 4.9×104 and 1.8×105 cm−1, respectively. Their refractive indices are 1.8 (before) and 1.9 (after), 3.1 and 3.5, 1.8 and 2.1, and 2.5 and 2.8, respectively. As determined by x‐ray diffraction analysis, these thin films are composed of very small metal grains and stoichiometric oxide grains. The thermal changes accompanied by the optical constant changes are mainly due to st...

Phase-Change Optical Disk Having a Nitride Interface Layer

A thin nitride layer formed at the interface of a Ge–Sb–Te recording layer and a ZnS–SiO2 protective layer successfully suppresses the phenomenon that reflectivity or signal amplitude becomes markedly small due to repeated overwrites. Based on secondary ion mass spectrometry (SIMS) observations, the 5-nm-thick interface layer was found to restrain sulfur atoms in the ZnS–SiO2 layer from diffusing into the Ge–Sb–Te layer and from changing the optical characteristics of the layer. Among several nitride materials, germanium nitride (Ge–N) sputtered film is found to have the most suitable properties as an interface layer: high barrier effect and good adhesiveness with Ge–Sb–Te and ZnS–SiO2 layers. The optical disk having the Ge–N interface layer achieves more than 5×105 cycles of overwrites with almost no changes in signal amplitude, reflectivity and jitter based on DVD-RAM specifications. The disk shows no degradation such as cracking, peeling, and corrosion after exposure to accelerated environmental conditions of 90°C and 80% RH for 200 h.

Rewritable Dual-Layer Phase-Change Optical Disk

This paper describes the development of a rewritable phase-change optical disk with a dual-layer structure. To obtain equivalent signal amplitude and recording sensitivity from the medium located on the beam incident side (the first medium) and the medium beneath (the second medium), we developed a high transmittance first medium featuring a very thin recording film (7 nm) and no reflection layer, and a high recording sensitivity and high reflectivity second medium with a semi-transparent front layer and a reflection layer. We confirmed a jitter of approximately 10% for both media at a recording density corresponding to 8.5 GB on a 120 mm disk. The laser power required to record on the first medium was 12 mW, and the second medium, 13 mW. It was also confirmed that the new rewritable dual-layer phase-change optical disk possesses the potential for operation with a blue laser.

Dual-Layer Optical Disk with Te–O–Pd Phase-Change Film

A recordable dual-layer optical disk that uses a Te–O–Pd phase-change film was studied for high-density recording. In order to reduce thermal influence to adjacent tracks, we selected the optimum composition of Te42O46Pd12 that gives low thermal conductivity. To get equivalent signal amplitude and absorbance of layers, the thicknesses of the laser incident side recording layer (Layer 1) and the other layer (Layer 2) are designed to be 20 nm and 55 nm, respectively. We confirmed that there was less than 10% bottom jitter for both layers under the following recording densities: recording bit length of 0.41 µm and track pitch of 0.74 µm. The laser powers required for recording on Layer 1 and Layer 2 were typically 10 mW and 12 mW, respectively. The jitter was less than 13% for both layers with large power tolerance of 30%p-p.

High-density recording on phase-change optical disks

Current status of high density recording on phase change optical disks is summarized. More than 4.1 Mbits/mm2 density of direct overwriting on phase change disks was achieved using red (680 nm) high power (35mW) LDs, large NA (0.6) optics, thin (t0.6 mm) substrates, mark-edge recording with write compensation, and land-groove recording technologies.

Sub-Oxide Thin Films For An Optical Recording Disk

Thin films of SbOx, GeOx, TeOx, MoOx, InOx and PbOx ( x is smaller than the stoichiometric value for each component ) were found to have a feasibility to be applied to an optical recording disc. Upon heating, these thin films are critically changed in their optical constants at 150°, 280°, 120°, 150°, 120° and 100°C, respectively. The contrast ratios before to after heat treatment of the GeOx, TeOx and MoOx films of 1000 - 2000 Å thick are very high, they are 50, 20 and 15, respectively. Among them, the TeOx (x ≈1.1) film was excellent in both the sensitivity and the contrast ratio, and an optical recording disc was prepared by depositing the TeOx on a PMMA disc, substrate. Video-signals can be recorded and played back using a laser diode (λ =8300 Å, N.A.= 0.5) at the laser power of 6 - 8 mW incident. The carrier to noise ratio at 5 MHz was in excess of 55 dB.