Phase-change Recording Films Research Articles

Laser Pulse Compensation Applied to Phase-Change Recording Film to Form Nano-Sized Marks

We have examined the application of the laser pulse compensation to the phase-change recording film of the triple-layer optical disc by using an original writing laser pulse controller, which compensates dynamically the pulse edges in order to form the marks at the correct position. We confirmed 112 nm sized marks smaller than the optical spot formed on the thin recording film. The results of the compensation had some characteristics depending on the data pattern and/or the layer. We discussed the causes by considering about the structure of the recording films and the phase-change process to form the marks.

Thickness dependence of crystallization and melting kinetics of eutectic Sb70Te30 phase change recording film

As the thickness of eutectic Sb70Te30 phase change recording film was reduced, the crystallization temperature and activation energy for crystallization would increase, while the melting temperature and activation energy for melting would decrease. Accordingly, the archival stability and recording sensitivity can be improved. However, the recording speed for direct over write will be slowed down. Based upon the Johnson–Mehl–Avrami equation, it was found that the increase of film thickness would increase the nucleation rate of eutectic Sb70Te30 recording film and make the crystallization process become more nucleation-dominated.

Effect of Interface Layers on Phase-Change Recording Material Analyzed by Hard X-ray Photoelectron Spectroscopy Method

The influence of interface layers on the chemical and electronic states of a phase-change recording material, GeBiTe (GBT) alloy, used in high-speed rewritable HD DVD media was investigated for the first time by hard-X-ray photoelectron spectroscopy (HX-PES). The binding state of elements for the amorphous state of the phase-change recording film with interface layers is closer to that of the crystalline state than the amorphous film without interface layers. The density of states (DOS) for the valence band of the amorphous state without an interface layer was smaller than that of the crystalline state. The band-edge energy of the amorphous state without an interface layer was lower than that of the crystalline state by about 0.5 eV. On the other hand, the DOS and the band-edge energy of the amorphous state of GBT with interface layers were almost the same as those of the crystalline state. This result may lead to almost the same carrier density for electrical conduction for the crystalline state as the amorphous state, which is totally unexpected, thus very interesting, because the atomic arrangements should differ from each other. We speculate that these effects are a factor allowing high-speed crystallization.

Thermal- and Laser-Induced Order–Disorder Switching of Ag-Doped Fast-Growth Sb70Te30 Phase-Change Recording Films

Doping Ag into the fast-growth Sb70Te30 recording film can increase the crystallization temperature and activation energy for crystallization so that the archival stability can be improved, however, the initialization will become more difficult. The activation energy for melting of the Ag-doped Sb70Te30 recording films will decrease with Ag concentration so that the recording sensitivity can also be improved. New compounds such as Ag5Te3 and AgSbTe2 will form due to the addition of Ag. When the concentration of Ag was increased, the crystallization mechanisms of the Ag-doped Sb70Te30 recording films will gradually transfer from polyhedral growth to linear growth. Under pulsed laser irradiation, the melt-quenched amorphous zone for the annealed Ag-doped Sb70Te30 recording films will enlarge as the Ag content was increased, representing that the recording sensitivity will increase. However, once the concentration of Ag was increased to 10.8 at. %, the as-deposited Ag-doped Sb70Te30 recording film will become difficult to be initialized.

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.

Bi–Ge–Sb–Sn–Te films for reversible phase-change optical recording

Bi–Ge–Sb–Sn–Te phase-change recording films (Films A–D) were prepared by the co-sputtering of Bi 30Ge 10Te 60, Ge 20Sb 80, Sb and Sn targets. The composition of the recording films was controlled by the dc power. The film composition, surface roughness and reflectivity were also closely related. Dynamic tests revealed that disks with Film C exhibited a low jitter value and a high modulation value after direct overwriting. Therefore, the composition of the recording film dominated the reversible recording properties of the phase-change optical disk.

Doped Ge–Sb–Te phase-change materials for reversible phase-change optical recording

Differently doped Ge–Sb–Te phase-change recording films were prepared by the individual dc magnetron sputtering of Sn 5Ge 3Sb 72Te 20, In 5Ge 3Sb 72Te 20, Ga 5Ge 3Sb 72Te 20 and Bi 5Ge 3Sb 72Te 20 targets. A close relationship was observed between surface roughness and reflectivity. The Ge–Sb–Te films doped with Bi showed the highest surface roughness and reflectivity. The phase-change optical disks with Bi-doped Ge–Sb–Te recording films showed the best overwriting characteristics. Therefore, the reversible recording properties of the phase-change optical disk were dominated by the composition of the recording film.

The effect of thickness on the Bi–Ge–Sb–Te films for reversible phase-change optical recording

The Bi–Ge–Sb–Te phase-change recording films were prepared by the dc magnetron sputtering of a Bi 5Ge 9Sb 68Te 18 target in an atmosphere of Ar. The surface of the Bi–Ge–Sb–Te films grew in the form of needles as the film thickness increased. There was the close relation among the film thickness, surface roughness, and optical properties. The Bi–Ge–Sb–Te film with a thickness of 16 nm had the lowest jitter value and the highest modulation value. As the results, the film thickness affected the Bi–Ge–Sb–Te film for phase-change optical recording significantly.

Theoretical analysis of nanosecond crystallization kinetics of phase-change optical recording films

A theoretical investigation of nanosecond crystallization kinetics of the phase-change optical recording films is presented. An extended Kissinger equation for the square-root heating is derived, which properly describes the temperature evolution of the films by nanosecond laser heating. The extended Kissinger equation was used to explain our previous experimental results.

Nanosize fabrication using etching of phase-change recording films

An etching technique called phase-change etching was developed. In this technique, only crystalline regions in a phase-change recording film are selectively etched by an alkaline solution, and amorphous regions remain on the sample surface, which means that a phase-change recording film can be used as a resist for pattern formation. By combination of this technique and phase-change recording, fabrication of the dot pattern with a size of about 1∕10 of the fabricating spot was demonstrated. This result indicates the possibility of nanosize fabrication using the phase-change etching technique.

Effects of Ag and In addition on the optical properties and crystallization kinetics of eutectic Sb70Te30 phase-change recording film

Adding specific foreign elements into the eutectic Sb70Te30 fast-growth material is expected to be an effective way to increase its thermal stability for blue laser recording. We have studied the effects of Ag and In addition on the optical properties and crystallization kinetics of the eutectic Sb70Te30 recording film. The results showed that the addition of Ag and In increased the refractive index and decreased the extinction coefficient of amorphous Sb70Te30 film and decreased both the refractive index and extinction coefficient of crystalline Sb70Te30 film. The archival stability of the eutectic SbTe alloy could be effectively improved by adding Ag and In elements, however, the addition of Ag and In also made the initialization of the as-deposited eutectic SbTe film more difficult. During the isothermal crystallization process, the incubation time was extended, crystallization speed was reduced, and the crystallization process became more grain-growth dominated as Ag and In were added. All these effects were further enhanced by increasing the concentration of In element.

Optical Properties and Crystallization Characteristics of Ge-Doped Sb70Te30 Phase Change Recording Film

Eutectic Sb70Te30 fast-growth material is one of the promising candidates for high data transfer rate recording. However, it also shows the disadvantage of poor thermal stability. Germanium, with a high value of glass transition temperature, is expected to play the role of thermal stabilizer. It was found that the effect of Ge doping on reflectivity difference between as-deposited and annealed states is not significant. However, the addition of Ge element into Sb70Te30 can reduce the initialization or erasure power, and increase the stability of the phase change recording media at room temperature at the same time. Unfortunately, doping Ge element would also slow down the crystallization speed of eutectic Sb70Te30 recording film.

Crystallization of Ag–In–Sb–Te Phase-Change Optical Recording Films

Crystalline phases formed on thermally annealed and laser-annealed Ag12.4In3.8Sb55.2Te28.6 four-element alloy films were observed to be different. After 1 h isothermal annealing at temperatures between 190°C and 450°C, hexagonal Sb and chalcopyrite AgInTe2 phases were observed, whereas laser annealing by initialization at laser power higher than 2.86 mW/µm2 yielded cubic crystalline Sb and AgSbTe2 phases. There was only one exothermic peak at 170°C determined by differential scanning calorimetry (DSC) measurement. Only the hexagonal Sb phase was observed by X-ray diffraction of samples subjected to DSC measurement. These experimental results suggest that the activation energy for crystallization derived from Kissinger's equation using DSC data may not be the same as that for crystallization during erasing of phase-change optical recording disks.

Measurement of the thermal coefficients of nonreversible phase-change optical recording films

We have developed a procedure to obtain the critical temperature for the amorphous-to-crystalline phase transition as well as the thermal conductivity and the specific heat of the phase-change media of optical recording. The procedure involves estimating the thermal conductivity from the data obtained by measuring the threshold cw laser power required for inducing phase transition. Then, from the data obtained in short-pulse measurements, we estimate the specific heat. In principle this method can yield the thermal parameters of any number of layers, so long as one of the layers is made of a phase-change material having a well-defined transition temperature.

高融点成分添加相変化光ディスク記録膜の開発

高融点成分添加相変化光ディスク記録膜の開発

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 2Sb 2Te 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.

Phase-Change Optical Recording Films with AgInTe 2–Sb–Te System

Reversible optical change due to laser irradiation or thermal annealing in amorphous films of the composition triangle AgInTe2-Sb-Te is described. Distinctive features are that there are a composition H (Ag9In14Te37Sb40) representing the best cyclability (≈106) for static repetition tests and a composition L (Ag8In8Te46Sb38) showing the largest reflectivity difference (≈26%) between the as-deposited and the annealed states at λ=830 nm.

The Effect of Doping on the Erasure Speed and Stability of Reversible Phase-Change Optical Recording Films

The crystallization kinetics of chalcogenide films, to which small concentrations of foreign elements such as Cu, Co, Pb, Ag and Sb are added, were investigated theoretically. It is found that the critical cooling rate can be taken as the criterion for evaluating the roles of various elements on the erasure speed. Since the elements having lower Tg/Tm possess higher critical cooling rates, they are favorable for enhancing the erasure speed of the above films. On the other hand, the Tg of the elements having lower Tg/Tm is found to be the main factor in determining the stability of corresponding films at room temperature because these elements possess higher crystallization speed in the temperature range between Tg and Tm. Therefore, the elements having lower Tg/Tm and higher Tg can be used to improve both the erasure speed and the stability of the reversible phase-change optical recording films.

Single-beam overwrite experiment using In-Se based phase-change optical media

Single-beam overwrite in an optical disk is reported using In-Se-Tl amorphous-crystalline phase-change recording films having very short erasing (crystallization) time of 0.2 μs. The lifetime of the amorphous state in an In-Se-Tl film is estimated to be longer than 10 years at 60 °C. This is due to its high crystallization temperature (135 °C) and high activation energy (2.6 eV). The phase-change cycles can continue over 106 cycles in stationary state experiments using test samples. The possibility of single beam overwrite (rewriting without prior erasing as with a magnetic disk) is verified by using a 1.6-μm-diam round laser beam spot irradiated on a 5-in.-diam disk rotating at 2400 rpm.