Phase-change Optical Recording Media Research Articles

光記録媒体 (1) 相変化光記録媒体

光記録媒体 (1) 相変化光記録媒体

Amorphization induced by subnanosecond laser pulses in phase-change optical recording media.

We have investigated the dynamics of amorphization induced in phase-change optical recording media by focused laser pulses of subnanosecond duration. We initiated localized amorphism by using a focused laser beam to melt the phase-change material and completed the change by rapid cooling by means of thermal diffusion. These studies were conducted by use of real-time reflectivity measurements with a pump-and-probe technique in which both pump and probe pulses had a duration of approximately 510 ps. Our transient-reflectivity measurements indicate that the process that leads to amorphism has three distinct stages, namely, rapid melting, solidification, and slow relaxation.

Behavior of GeSbTeBi phase-change optical recording media under subnanosecond pulsed laser irradiation

We investigated the variations in reflectivity during the phase transition between amorphous and crystalline states of a Bi-doped GeTe-Sb2Te3 pseudobinary compound film with subnanosecond laser pulses, using a pump-and-probe technique. We also used a two-laser static tester to estimate the onset time of crystallization under 2.0-micros pulse excitation. Experimental results indicate that the formation of a melt-quenched amorphous mark is completed in approximately 1 ns, but that crystalline mark formation on an as-deposited amorphous region requires several hundred nanoseconds. Simple arguments based on heat diffusion are used to explain the time scale of amorphization and the threshold for creation of a burned-out hole in the phase-change film.

Analysis of writing and erasing behaviours in phase change materials

An understanding of the process involved in writing and erasing of phase-change optical recording media is vital to the development of new, and the improvement of existing, products. The present work investigates both experimental and theoretical laser-induced fast structural transformations of GeSbTe thin films. Optical and microstructural changes are correlated using both a static tester and transmission electron microscopy. In the second part of this paper we try to elucidate the physics underlying the amorphous-to-crystalline phase transformation under short-pulse laser excitation. Both thermal and thermodynamical behaviours must be taken into account to illustrate real processes.

Modeling thermal cross talk and overwrite jitter in growth dominant phase change optical recording media at high data rates

Two methods for the reduction of thermal cross talk (cross erase) and overwrite jitter in growth dominant phase change materials are addressed through simulations. An additional Au layer and a modified multipulse technique are considered. A model is described that simulates mark formation in growth dominant media. The disk with an additional Au layer is shown to reduce the temperature in adjacent tracks. The modified multipulse scheme, utilizes write power more efficiently and reduces the cross erase significantly. In growth dominant media, the main cause of overwrite jitter is shown to be the reduction in erasability with large marks at high velocities. Jitter is shown to be almost insensitive to parameters such as erase power, bias power at the cooling width region, and the length of the cooling width itself at high write powers. At lower write powers, clear minima in jitter are found for specific values of these parameters. At high write powers, jitter is substantially reduced using a modified multipulse scheme.

Measurement of the thermal coefficients of rewritable phase-change optical recording media.

We describe a method to estimate the heat capacity of the substrate, the dielectric layer, and the phase-change layer of phase-change optical recording media as well as the thermal conductivity of the phase-change layer in its crystalline state. Measurements were carried out on spinning disks with the beam of light focused and locked onto the groove track. The method relies on the identification of the solid-to-liquid phase transition that occurs in the phase-change layer and takes advantage of the dependence of thermal diffusion on track velocity and irradiation time.

Polarization dependence of thermal behavior in rewritable phase-change optical recording media.

Thermal behavior in land-groove phase-change optical recording has been examined for different polarizations of the incident beam. Three-dimensional temperature distribution in the grooved medium is evaluated by the numerical solution of Maxwell's equations and the heat transfer equation with the finite-difference method. Both experiments and calculations have shown that the thermal behavior in the medium is dependent on the state of polarization and the nature of the track. The calculated mark shapes in a quadrilayer stack are in good agreement with experimental observations.

Crystallization studies on phase-change optical recording media by use of a two-dimensional periodic mark array

We present the results of crystallization studies in thin-film samples of amorphous and crystalline Ge(x)Sb(y)Te(z). The experiments, conducted at moderately elevated temperatures, are based on measurements of the first-order diffraction efficiency from a two-dimensional periodic array of recorded marks. When the samples are slowly heated above room temperature, changes in the efficiencies of various diffracted orders give information about the on-going crystallization process within the sample. Two different compositions of the GeSbTe alloy are used in these experiments. Measurements on Ge(2)Sb(2.3)Te(5) films show crystallization dominated by nucleation. For the Sb-rich eutectic composition Ge-(SbTe), crystallization is found to be dominated by growth from crystalline boundaries. We also show that crystalline marks written by relatively high-power laser pulses are different in their optical properties from the regions crystallized by slow heating of the sample to moderate temperatures.

Effect of ZnS Adhesion Layer on Overwrite Cyclability of Phase Change Optical Recording Media

A high-performance phase change optical recording disk, which has a five-layered structure, was proposed. This disk has a ZnS adhesion layer between a GeSbTe recording layer and a ZnS–SiO2 second dielectric layer. A 2×105 overwrite cyclability was achieved with the original five-layered phase change optical recording disk. The cross-sectional transmission electron microscope observation and the in-plane X-ray diffraction measurement indicate that both the five-layered disk and the four-layered disk have almost the same crystalline structure. The contact angle measurements, however, indicate that the ZnS adhesion layer improves the adhesive force between the GeSbTe recording layer and the ZnS–SiO2 second dielectric layer. It is considered that the high overwrite cyclability results from this improvement of the adhesive force. Moreover, the electron probe microanalysis results indicate that the ZnS layer acts as a protective layer against the Ge diffusion from the GeSbTe recording layer.

An Approach to Eliminate Initializing Process from Manufacturing of Phase Change Optical Recording Media.

A new method of eliminating the initializing process from phase change recording media production has been proposed. The process is based on the use of krypton instead of argon for the deposition of GeSbTe recording film. By this method the kinetic energy of sputtered atoms can be reduced by collisions with gas atoms. Fairly good recording performance was demonstrated on a non-initialized disc based on the DVD-RAM ver.2.0 format.

Experimental and theoretical investigations of laser-induced crystallization and amorphization in phase-change optical recording media

We describe the numerical procedure for calculating three-dimensional profiles of temperature in a multilayer stack illuminated by a laser beam, and model the crystallization and amorphization kinetics for phase-change rewritable media. Experimental methods have been used to determine indirectly the probabilities of nucleation and growth for Ge2Sb2Te5 alloy. Some of the fundamental behaviors of phase-change erasable media, such as the crystallization of as-deposited amorphous phase, amorphization of supercooled liquid, and recrystallization of quenched amorphous phase, have been illustrated based on our three-dimensional temperature calculations and the model kinetics. The calculated transient reflectance behavior of as-deposited Ge2Sb2Te5 amorphous films in a single layer and in a quadrilayer stack, as well as the erasure behavior of Ge2Sb2Te5 alloy in a quadrilayer disk are in good agreement with experimental observations.

Laser-induced crystallization in Ge-Sb-Te optical recording materials

Laser-induced crystallization behaviour in Ge-Sb-Te optical recording thin film materials, that are sandwiched between dielectric layers, was studied by a multipuise laser technique. The reflection changes at 780 nm laser wavelength caused by local heating by a pulsed high-power laser beam are monitored. Both the nucleation time and the complete crystallization time are found to decrease significantly with increasing phase change layer thickness up to about 25 nm, above which both tend to become constant. Furthermore, the maximum crystallization speed depends on the type of dielectric material. The strong dependence of crystallization speed on layer thickness is explained in terms of competing interface and bulk processes. This finding is of practical use because it allows the design of phase change optical recording media which can be used at high linear recording speeds.

GaGeTe films as phase-change optical recording media

Ga 20Ge 30Te 50 thin films deposited by vacuum evaporation on various substrates have been studied for their structural and optical properties. The as-deposited amorphous films were crystallized by thermally annealing them. The optical constants of the amorphous films indicate semiconducting behaviour ( n > k). The optical bandgap ( E g) determined from Tauc's plot is 0.7 eV. The change in reflectance on crystallization has been utilized to obtain maximum optical contrast by optimising the thickness of the film.

Effect of multilayer structure and laser pulse width on the reversible cycling of phase change optical storage media

The static cycling behavior of chalcogenide phase change optical recording media was investigated. Thin-film multilayer structure and laser pulse conditions were both varied to determine optimum conditions for good cycling behavior. For all structures, cyclability was always best for short pulses. An aluminum overcoat layer was found to substantially increase the performance. The effect of the temperature excursion experienced during writing on cycling was determined. At the shortest pulses it was demonstrated that the thermal excursions did not depend upon the presence of an aluminum overcoat, suggesting that mechanical strength was important for enhancing cyclability. For longer pulses the aluminum changed the thermal characteristics of the stack. These differences between various samples are compared to try to elucidate modes of failure in optical data storage structures.

Repeatability dependence of phase-change optical disks on morphology of protective layers

The surface or the Si3N4-sputtered protective layer of phase change optical recording media was observed with a scanning tunneling microscope. We found the morphology of the protective layer can be controlled by changing the sputtering gas conditions and that overwrite cycle repeatability strongly depends on it. As the Ar gas pressure in the sputtering chamber is decreased, the protective layers become denser and have a flatter surface, and the repeatability is improved from 3×103 to 5×105. The media sensitivity also depends on the morphology of the protective layers. The causes of media deterioration after a large number of overwrite cycles are discussed

Mechanism of transformations in phase-change optical recording media

Multielement alloys composed of two binary compounds such as SbTe, SbSe, InTe, InSe and MTeSe compounds, useful for optical recording media, have been prepared. The kinetics of phase change in these media are discussed. The activation energy of crystallization, Δ E, and the transformation index n are calculated. The data show that the film with stoichiometric composition not only has the best stability, but also has the largest transformation index. In addition, the times required for the media to change from the glassy state to the crystalline state by optical crystallization is much less than that of the pure thermal crystallization. A new model for the transformations induced by laser pulses based on the electronic structural changes of the alloys is proposed.

Compound materials for reversible, phase-change optical data storage

Results of rapid (laser induced) and slow (heating stage induced) crystallization studies on Te1−xGex, 0≤x≤0.6, are reported. The time it takes to laser crystallize varies with x by more than four orders of magnitude. Films with stoichiometric compositions, Te and GeTe, can be crystallized using laser pulses of less than 100 ns duration. Unlike Te, which spontaneously crystallizes at room temperature, GeTe has a crystallization temperature of >150 °C. From these results we argue that, in general, compound materials allow realization of fast-switching, reversible, phase-change optical recording media. Furthermore, this fast-switching capability, from the amorphous to the crystalline state and back, can be attained simultaneously with long term data (amorphous phase) stability, provided the melting temperature of the compound is sufficiently high.