Phase-change Optical Disk Research Articles

Impact of Se and Te addition on optical characteristics of ternary GeInSb chalcogenide films as promising materials for optoelectronic applications

The present study investigates the linear and non-linear optical characteristics of non-crystalline quaternary Ge15In5Sb5Se75 (GIS-Se) and Ge15In5Sb5Te75 (GIS-Te) chalcogenide thin films. The amorphous nature of the studied compositions was confirmed by x-ray diffraction (XRD). The values of transformation temperatures determined by differential thermal analysis (DTA), such as glass transition and crystallization temperatures, are 524.3 and 639.6 K for GIS-Se and 503.6 and 542.0 K for GIS-Te. The calculated values of glass forming ability () and thermal stability () for GIS-Se are higher than those for GIS-Te. The transmittance and reflectance spectrophotometric measurements in the UV–vis. wavelength range (400–2500 nm) are used to estimate the index of refraction and extinction coefficient The obtained values of and are found to be thickness independent in the investigated range. Tauc’s extrapolation model was used to determine both Urbach tail () and optical band gap () energies. The values of () are 0.416, 0.526 eV and () are 1.83, 1.31 eV with indirect allowed transitions for GIS-Se and GIS-Te, respectively. The optical conductivity increased as photon energy () increased, and it was enhanced in the presence of Te than Se addition. The analysis of the index of refraction () is used to evaluate the dispersion energy the high frequency dielectric constant the oscillator strength the lattice dielectric constant the ratio and the optical electronegativity which were found to be higher in GIS-Te than those in GIS-Se composition. The volume () and surface () energy loss functions were found to be increased with increasing () and the presence of Te. Furthermore, the calculated values of nonlinear optical susceptibility and nonlinear index of refraction for GIS-Se and GIS-Te are 0.195 × 10−12, 2.44 × 10−11 esu and 0.868 × 10−12, 10.19 × 10−11 esu, respectively. The detailed outcomes show that the studied film samples are suitable for various optical applications such as phase-change optical rewritable disks, optical sensors, optoelectronic devices and high-speed optical fibers.

Toxicity of tellurium and its compounds

Abstract Tellurium (Te) is widely used in industry because of its unique physicochemical properties. In the general population, foodstuff like meat, dairy products, and cereals is the major source of tellurium exposure. In the occupational environment, inhalational exposure predominates. Due to its exceptional properties as a metalloid, Te is broadly used in the industry. For example, Te is used as an alloy for solar panels, phase change optical magnetic disks, and Peltier devices. Recently, alloys of Te with cadmium, zinc, and other metals are used for nanomaterials, such as quantum dots. Thus, it is suggested that there is an existence of risk of exposure to Te in everyday life. Commercial Te is mostly obtained from slimes of electrolytic copper refineries. Te concentration in the slimes can extend up to 10% or more. Slight levels of its organic compounds may also be absorbed via skin. Not much information is available to prove Te as carcinogenic but its toxicity is well established. The present paper will review the toxicity of Te and its compounds.

Design of a four-sided magnetic circuit for an optical pickup actuator for multilayer high-density optical disks

Large-capacity optical storage materials have reached a stage of relative maturity, but progress in the design of optical pickup actuators is currently slow. To improve the reading and writing capabilities of optical recording systems, we propose a design for a four-sided magnetic circuit for an optical pickup actuator for multilayer high-density optical disks that make full use of the space around the actuator. The basic magnetic circuit model in this design, which is derived by the equivalent magnetic charge method, can be applied to the design of all such actuators. The static and dynamic magnetic circuit evaluation parameters established in this paper can be used conveniently and directly to improve the magnetic circuit performance of the actuator. The design was verified by finite element simulation, and linear working ranges of ±2.6 mm, ±0.6 mm, and ±16° were found in the focusing, tracking, and tilting degrees of freedom. This is sufficient to compensate for errors in phase-change optical disks with more than 50 layers in high-speed rotation. The theoretical servo accuracy of the actuator can reach tens of nanometers. This can be used with a dual-objective-lens layout that can support layer-skip reading and writing of optical disks with an effective working bandwidth greater than 10 000 Hz.

Simplified Transfer Function of Write Compensation for Phase-Change Optical Disks

A simplified transfer function for write compensation is given by the inverse function of the cooling transfer function of a phase-change optical disk. The areas at a phase-change recording layer irradiated with an optical beam melt and change to marks. The marks are formed when the area is rapidly cooled; however, the marks disappear in a slowly cooled situation. Thus, the cooling process was considered essential for the mark formation on a phase-change optical disk. Therefore, the transfer function of the mark formation combined a heating component before mark formation with that after mark formation using a lag element. It was also found that the transfer function for write compensation could be achieved using a two-stage pulse train method. Experimental results showed that long marks were recorded by the pulse train method with the same frequency characteristic as our theoretical analysis.

Measurements of Nonlinear Mark Edge Shift for Phase Change Optical Disk Systems

A new measurement method using the autocorrelation of readout pseudorandom sequence signals is proposed for evaluating the nonlinear mark edge shift of the phase change optical disk. The measurement abilities of the method are evaluated using 255-bit maximal length sequence signals. The nonlinear mark edge shifts are determined experimentally by using pulse train recordings and these results also agree with theoretical analysis.

Measurements of Nonlinear Mark Edge Shift for Phase Change Optical Disk Systems

A method for evaluating the nonlinear mark edge shift of the phase change optical disk using auto-correlations of readout signals was presented and verified.

Development of Ultrafast Laser Flash Methods for Measuring Thermophysical Properties of Thin Films and Boundary Thermal Resistances

Reliable thermophysical property values of thin films are important to develop advanced industrial technologies such as highly integrated electronic devices, phase-change memories, magneto-optical disks, light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), semiconductor lasers (LDs), flat-panel displays, and power electronic devices. In order to meet these requirements, the National Metrology Institute of Japan of the National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) has developed ultrafast laser flash methods heated by picosecond pulse or nanosecond pulse with the same geometrical configuration as the laser flash method, which is the standard method to measure the thermal diffusivity of bulk materials. Since these pulsed light heating methods induce one-dimensional heat diffusion across a well-defined length of the specimen thickness, the absolute value of thermal diffusivity across thin films can be measured reliably. Using these ultrafast laser flash methods, the thermal diffusivity of each layer of multilayered thin films and the boundary thermal resistance between the layers can be determined from the observed transient temperature curves based on the response function method. The thermophysical properties of various thin films important for modern industries such as the transparent conductive films used for flat-panel displays, hard coating films, and multilayered films of next-generation phase-change optical disks have been measured by these methods.

Rewritable Triple-Layer Phase-Change Optical Disk Providing 100 Gbyte Capacity

A rewritable phase-change optical disk providing a large capacity of 100 Gbyte on a 120 mm disk was first demonstrated using the multilayer Blu-ray DiscTM (BD-XL) format. The doubled capacity of this optical disk compared with that of a conventional dual-layer disk was achieved firstly by stacking triple recording layers and secondly by increasing the recording capacity per layer from 25 to 33.4 Gbyte at 33.6%. The high transmittances of 50% (middle layer) and 60% (front layer) were achieved by thinning a Ge–Sb–Te phase-change film to 7.5 and 6 nm and also by thinning a Ag-alloy film to 9 and 7 nm, respectively. An additional TiO2-based film formed on the Ag-alloy film was effective in improving the transmittance at 3%, compared with the structure using a conventional TiO2 film. Furthermore, a transmittance-balanced structure was adopted for these layers in order to stabilize the recording-reading properties. To improve cyclability, ZrO2–Cr2O3-based interface films were provided on both sides of the phase-change film for the middle and front layers. The increase in recording capacity per layer was achieved by reducing the minimum mark length from 0.149 to 0.112 µm. Since the optical changes degrade with the reductions in the mark lengths and thicknesses of the Ge–Sb–Te and Ag-alloy films, a phase-change material with a GeTe-rich composition on a GeTe–Sb2Te3 pseudo-binary line was adopted for every layer to compensate it. It was confirmed that the sample disk successfully satisfies all the requirements of the BD-XL format.

Rewritable Triple-Layer Phase-Change Optical Disk Providing 100 Gbyte Capacity

A rewritable phase-change optical disk providing a large capacity of 100 Gbyte on a 120 mm disk was first demonstrated using the multilayer Blu-ray DiscTM (BD-XL) format. The doubled capacity of this optical disk compared with that of a conventional dual-layer disk was achieved firstly by stacking triple recording layers and secondly by increasing the recording capacity per layer from 25 to 33.4 Gbyte at 33.6%. The high transmittances of 50% (middle layer) and 60% (front layer) were achieved by thinning a Ge–Sb–Te phase-change film to 7.5 and 6 nm and also by thinning a Ag-alloy film to 9 and 7 nm, respectively. An additional TiO2-based film formed on the Ag-alloy film was effective in improving the transmittance at 3%, compared with the structure using a conventional TiO2 film. Furthermore, a transmittance-balanced structure was adopted for these layers in order to stabilize the recording-reading properties. To improve cyclability, ZrO2–Cr2O3-based interface films were provided on both sides of the phase-change film for the middle and front layers. The increase in recording capacity per layer was achieved by reducing the minimum mark length from 0.149 to 0.112 µm. Since the optical changes degrade with the reductions in the mark lengths and thicknesses of the Ge–Sb–Te and Ag-alloy films, a phase-change material with a GeTe-rich composition on a GeTe–Sb2Te3 pseudo-binary line was adopted for every layer to compensate it. It was confirmed that the sample disk successfully satisfies all the requirements of the BD-XL format.

Distinct uptake of tellurate from selenate in a selenium accumulator, Indian mustard (Brassica juncea)

Tellurium (Te) is widely used in industry because of its unique chemical and physical properties, and has recently become a part of everyday life as a component of phase-change optical magnetic disks. However, the recovery of Te from the environment has not been discussed yet. In this regard, we evaluated the potential use of Indian mustard (Brassica juncea), a selenium (Se) accumulator, for the phytoremediation of Te. The Indian mustard plant was exposed to selenate and tellurate and the concentrations of Se and Te and the chemical species in the plant were determined. The Indian mustard plant accumulated less Te than Se, and the amount of Te accumulated in the plant was approximately 1/69 of that of Se. Although the incorporation of selenate was reduced by increasing sulfate concentration in the medium, the incorporation of Te was not affected by it, suggesting that this plant was able to discriminate tellurate from selenate in the roots. Three Te species were detected in the plant. The major species was tellurate. The other two species were not identical to available Te standards and thus could not be identified. Consequently, the Indian mustard plant is inappropriate for the phytoremediation of Te because it can strictly distinguish tellurate from selenate.

Super-Resolution Near-Field Disk with Phase-Change Sn-Doped GeSbTe Mask Layer

A new mask layer of Sn-doped (7 at. %) Ge2Sb2Te5 was developed and used on super-resolution near-field phase-change optical disks (super-RENS). Temperature-dependent reflectivity result showed a reflectivity change at 169 °C with Sn doping into Ge2Sb2Te5. The mask material also showed high thermal stability. Optical study indicated the suitability of the film for use in blue-laser recording and as a mask layer. Fast crystallization within 90 ns was achieved using a pulsed high-power laser beam of 405 nm wavelength. Dynamic recording performance of the new structure showed carrier-to-noise ratios (CNR) of 37 and 18 dB obtained for 80 and 50 nm mark sizes, respectively. Readout thermal stability of 12,000 cycles was realized for 80 nm mark sizes. The incorporation of Sn-doped GeSbTe (GST) as mask layer in the super-RENS structure significantly improved the CNR and thermal stability of the disk.

Metabolism of tellurium, antimony and germanium simultaneously administered to rats

Recently, tellurium (Te), antimony (Sb) and germanium (Ge) have been used as an alloy in phase-change optical magnetic disks, such as digital versatile disk-random access memory (DVD-RAM) and DVD-recordable disk (DVD-RW). Although these metalloids, the so-called "exotic" elements, are known to be non-essential and harmful, little is known about their toxic effects and metabolism. Metalloid compounds, tellurite, antimonite and germanium dioxide, were simultaneously administered to rats. Their distributions metabolites were determined and identified by speciation. Te and Sb accumulated in red blood cells (RBCs): Te accumulated in RBCs in the dimethylated form, while Sb accumulated in the inorganic/non-methylated form. In addition, trimethyltelluronium (TMTe) was the urinary metabolite of Te, whereas Sb in urine was not methylated but oxidized. Ge was also not methylated in rats. These results suggest that each metalloid is metabolized via a unique pathway.

Blu-ray Type Super-Resolution Near-Field Phase Change Disk with In2Ge8Sb85Te5 Mask Layer

A new mask layer of In2Ge8Sb85Te5 was developed and used on super-resolution near-field phase change optical disks (super-RENS). The thermal and optical properties of the mask layer were investigated. Thermal analysis showed higher crystallization temperature and activation energy but lower melting temperature than Ge2Sb2Te5 phase change material. Ultra-fast crystallization within 30 ns was obtainable using a pulsed high-power laser beam of 405 nm wavelength. The recording performance of the new structure was evaluated on an optical recording system with a numerical aperture of 0.85 and a 405 nm blue-violet laser diode. Carrier-to-noise ratio (CNR) of 32, 25, 15, and 10 dB were obtained for 80, 50, 40, and 30 nm mark trains, respectively. Readout thermal stability of 20,000 cycles was realized for 80 nm mark trains. These performances were by far the highest obtainable for blu-ray aperture-type super-RENS disk.

Effects of Bi on crystallisation in Ge–Sb–Te–Bi

Blue light optical storage products will become the next generation devices for recording media in the near future. To increase the crystallisation speed of recording by 405 nm wavelength blue laser diode, bismuth (Bi) is applied to substitute small quantities of antimony (Sb) in a pseudobinary GeTe–Sb2Te3 (GeSb2Te4) alloy material. It is shown that the activation energies for crystallisation of Ge–Sb–Te–Bi are reduced by addition of Bi indicating the improvement of recording speed. The Ge–Sb–Te–Bi material is incorporated into actual blue light phase change optical disks and is found to provide good land and groove recording characteristics, cross-talk characteristics, and cross-erase characteristics. It is demonstrated a promising candidate for HD DVD-RAM applications.

Effect of Dielectric Material Films on Crystallization Characteristics of Ge2Sb2Te5 Phase-Change Memory Film

Reduction of the film thickness of phase-change film and the adoption of GeN- or ZrO2-based dielectric films are both effective in achieving good thermal stability in phase-change optical disks. It was experimentally confirmed that, at a heating rate of 10 °C/min, the crystallization temperature Tx of the Ge2Sb2Te5 amorphous film when sandwiched by ZnS–SiO2 films markedly increases from 162 to 197 °C, while the thickness of the Ge2Sb2Te5 film decreases from 10 to 3 nm. Tx also slightly increases when ZnS–SiO2 films are substituted for GeN-based films (from 162 to 165 °C) and ZrO2-based films (from 162 to 167 °C). At the same time, the activation energy of crystallization is 2.4 eV for both GeN- and ZrO2-based films, and is higher than 2.2 eV for ZnS–SiO2 films.

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%).

Evaluation of Thermal Tolerance in Initializing Thin Optical Discs

We simulated a temperature change in thin phase-change optical discs that have various substrate thicknesses in order to evaluate the thermal tolerance of a polycarbonate substrate during DC sheet beam initialization. We simulated the temperature change by applying a finite difference method to the partial differential equation for heat conduction. In discs with substrates on both sides, the difference between the sums of the given heat while the temperature of each substrate is more than or equal to glass transition temperature for a disc with a 100-µm-thick substrate on both sides and a disc with a 5-µm-thick substrate on one or both sides, was within 5%. The sum of the given heat to the substrate for a disc with a substrate on one side, was about 1.5 times larger than that for a disc with substrates on both sides. These results showed that the initialization margin needed to obtain a good-quality initialization state with no thermal damage to the substrate is smaller for a disc with a substrate on one side than for that with substrates on both sides.

Using High Speed Phase Change Material for ${\rm DVD}+{\rm RW}$ 2.4x Dual Layer

The authors succeeded in developing an innovative high-speed phase change material applied for DVD+RW dual layer recording media. The high transmittance of the front-side recording layer combining the stacking material and thin film structure was found. This front-side recording layer is a key functional layer structure for the dual layer phase change optical disk. The same writing powers can be used for both recording layers and the same reflection of 7% is obtained. The transmittance for the front-side layer is 58.1% and 59.4% in amorphous and crystalline state, respectively. The jitter after 10 direct-overwriting cycles is 6.5% and 7% for the front-side and back-side recording layer, respectively

ZrO2-Based Interface Films Realizing a Dual-Layer Phase-Change Optical Disk Utilizing a Blue-Violet Laser

A ZrO2-based dielectric material shows superior properties for interface films of phase-change optical disks, in particular, when utilizing a blue-violet laser. i) The ZrO2-based film possesses a high transparency at a wavelength of 405 nm, ii) it has a superior adhesiveness with phase-change recording films, and iii) it shows no crystallization or decomposition below 1000 °C, as shown by temperature-programmed desorption (TPD) measurement. The film was adopted for interface films of a semitransparent recording layer (L1) that was located on the laser-incident side of a dual-layer optical disk, and a high optical transmittance of more than 50% and a large reflectivity contrast of more than 0.8 were realized. By dynamic evaluation based on the Blu-ray Disc Format version 1.0, the obtained jitter measured with limit equalizer (LEQ jitter) was 6.7% and the cycle number was more than 10,000, whereas those of the reference disk using conventional GeN-based interface films were 7.3% and 2000, respectively. From the optical and thermal simulation, the maximum temperature in the interface film located on the laser-incident side of L1 was around 700 °C.

Bi-Ge-Sb-Te Films for Reversible Phase-Change Optical Recording

The Bi-Ge-Sb-Te phase-change recording films were prepared by DC magnetron sputtering of Bi5Ge9Sb68Te18 and Bi5Ge3Sb74Te18 targets, individually. The surface roughness of Bi5Ge9Sb68Te18 film was relatively low. At the recording speed of 27.92 m/s, the jitter value of Bi5Ge9Sb68Te18 disk was 21.33%, but Bi5Ge3Sb74Te18 disk could not be recorded. After the aging test, the reflectivity change (Rbef.−Raft.) of Bi5Ge9Sb68Te18 film was obviously less. It could be found that the Bi5Ge9Sb68Te18 film was suitable for high-speed recording and showed the better archival life stability. As the results, the recording performance and archival life stability of phase-change optical disks were dominated by the composition of recording film.