Phase Change Optical Recording Research Articles

Metal-induced Effect of Antimony Addition on Some Physico-chemical Properties of Se80Te20 Inorganic Glass

Chalcogenide glasses are well known advanced functional materials for widely use in phase change optical recording technology. The present papers reports our novel research work regarding the metal-induced effects of Sb on some significant physico-chemical properties (coordination number, constraints, density, molar volume, cohesive energy, heat of atomization etc) of binary Se80Te20 glass. Our results indicate that there is a strong correlation between the structural modifications and significant variation in various physico-chemical parameters of parent glassy alloy after inclusion of Sb.

Thermal characterization of Se78 − x Te20Sn2Pb x (0 ≤ x ≤ 6) glassies for phase change optical recording technique

New multi-component glasses of Se78 − x Te20Sn2Pb x (0 ≤ x ≤ 6) system have been synthesized using melt-quench technique. Differential Scanning Calorimetric measurements are performed at different heating rates under non-isothermal conditions to study the glass transition kinetics of prepared chalcogenide glass. Different kinetic parameters such as the activation energy of glass transition, thermal stability, glass forming ability etc have been determined. The composition dependence of kinetic parameters of glass transition is also discussed.

Electric field driven optical recording

A physical mechanism of transformations between the amorphous and crystalline phases induced by the electric field of a laser beam is proposed. It creates needle-shaped crystal particles aligned to the beam polarization. The polarization driven orientation of particles can significantly increase the information storage capacity achievable with phase change optical recording. The nucleation and postnucleation stages of the laser field induced crystallization are discussed establishing the conditions under which the polarization aligned particles can be created.

Plasma enhanced germanium nitride dielectric thin films for phase-change recording systems

The more efficient dielectric material and structure have been developed for fast phase-change optical recording systems. The germanium nitride thin film was reactively sputtered using a d.c. chamber, operated at 3 kW with a mixed gas plasma of argon and nitrogen. The dielectric thin film of 11–13 nm has been designed as the upper dielectric layer between the Ge–In–Sb–Te active layer and the silver reflective layer. The optical storage disks were initialized by a Hitachi POP120-8E device to provide crystalline active layers before the dynamic tests. The initialization was conducted using 810 nm laser at 2300 mW while the disks were rotated at a constant linear velocity of 10 m/s. The optical properties were analyzed using a Steag ETA-RT optical measuring system at 410–1010 nm wavelength. The power margin window was shown to be effectively improved by 60% to 18–26 mW when compared with the traditional ZnS–SiO 2 dielectric system. TEM (transmission electron microscope) micrograph showed clear profile for the standard 3T laser marks on the active layer. The plasma enhanced germanium nitride dielectric thin film could improve digital versatile disk's (DVD) production yield and efficiency.

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Structure and properties of amorphous thin film for optical data storage

In this paper the magnetic and magneto-optical properties of amorphous rare earth-transition metal (RE-TM) alloys as well as the magnetic coupling in the multi-layer thin films for high density optical data storage are presented. Using magnetic effect in scanning tunneling microscopy the clusters structure of amorphous RE-TM thin films has been observed and the perpendicular magnetic anisotropy in amorphous RE-TM thin films has been interpreted. Experimental results of quick phase transformation under short pulse laser irradiation of amorphous semiconductor and metallic alloy thin films for phase change optical recording are reported. A step-by-step phase transformation process through metastable states has been observed. The waveform of crystallization propagation in micro-size spot during laser recording in amorphous semiconductor thin films is characterized and quick recording and erasing mechanism for optical data storage with high performance are discussed. The nonlinear optical effects in amorphous alloy thin films have been studied. By photo-thermal effect or third order optical nonlinearity, the optical self-focusing is observed in amorphous mask thin films. The application of amorphous thin films with super-resolution near field structure for high-density optical data storage is performed.

Pressure-induced amorphization of quasibinary GeTe–Sb2Te3: The role of vacancies

The authors demonstrate that the cubic phase of quasibinary GeTe–Sb2Te3 alloys (GST), the material of choice in phase-change optical recording (such as digital versatile disk-random access memory) can be rendered amorphous by the application of hydrostatic pressure. The amorphization pressure depends on the GST composition. The pressure-induced amorphous phase possesses a local structure around Ge atoms similar to that of laser-amorphized GST. They argue that vacancies are crucial for the pressure-induced amorphization.

Confocal spectromicroscopy of amorphous and nanocrystalline tungsten oxide films

A Raman confocal spectromicroscopic system was used to study in situ phase composition and surface morphology in amorphous and nanocrystalline tungsten oxide and tungstate thin films, prepared on silicon and glass substrates by dc magnetron co-sputtering technique. The possible use of these films for the phase-change optical recording was demonstrated using 442 nm He–Cd laser with a variable power of up to 50 mW. The formation of nanocrystalline tungsten trioxide or tungstate phases was observed under the laser irradiation. These nanocrystalline phases show relatively strong Raman activity, which can be used for information reading purposes. A multilayer structure composed of several tungstate films with different chemical composition is proposed as potential write-once optical recording media.

Rapid and accurate measurement for phase‐change optical recording bits

Conducting atomic force microscopy (CAFM) and scanning surface potential microscopy (SSPM) have been used to image the phase-change optical recording bits. Commercially available digital versatile discs (DVD) + rewritable (RW) with initialization process were measured in experiments. Comparing the measurement results of both, the measurement resolution of CAFM is far superior to that of SSPM. With the DVD + RW disc rotating at a linear speed of 3.5 m/s, appropriate writing laser power range, may be precisely identified by CAFM as 10-15 mW. This is sufficient to verify the high-resolution recording bits research method. This new method may also be applied to the development of new types of phase-change recording materials.

Calibration of the Thermal Conductivity of Thin Films in Phase-Change Optical-Recording Stacks

We describe two different methods of calibrating the thermal conductivity of thin films in phase change optical recording stacks, the melt threshold method and the transmission electron microscope (TEM) picture based method. Both methods use numerical modeling to approximate the thermal conductivities of some commonly used materials in phase change recording. The aim of our study is to find a consistent set of values for the thermal conductivities, such that temperatures in different recording stacks at different velocities are correctly predicted. Both methods were found to give consistent results. The relationships between thermal conductivity, the type of interface, and the thickness of the thin film were also investigated. A good match between simulated results and experimental results was demonstrated.

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.

Pressure-Induced Site-Selective Disordering ofGe2Sb2Te5: A New Insight into Phase-Change Optical Recording

We demonstrate that , the material of choice in phase-change optical recording (such as DVD-RAM), can be rendered amorphous by the application of hydrostatic pressure. It is argued that this structural change is due to a very strong second-nearest-neighbor Te-Te interaction that determines the long-range order in the metastable cubic phase of and also to the presence of vacancies. This newly discovered phenomenon suggests that pressure is an important factor for the formation of the amorphous phase which opens new insight into the mechanism of phase-change optical recording.

Thermal characterization of dielectric and phase change materials for the optical recording applications

Advances in the phase change optical recording technology strongly depend on the optical and thermal optimizations of the metal/ZnS–SiO2/phase change multilayer structure, which requires accurate modeling and thermal characterization of the phase change media structure. In the present work, the thermal conductivities of the amorphous and crystalline Ge4Sb1Te5 phase change and ZnS–SiO2 dielectric layers of thicknesses in the range of 50–300nm have been measured using the transient thermoreflectance technique. The data are between factors of 2–4 different from the previously measured values for thin film and bulk samples. The thermal boundary resistance at a metal/ZnS–SiO2 interface is found to be around 7×10−8m2W−1. This might have serious implications for the future phase change recording application which attempts to achieve the high writing speeds by decreasing the thickness of a ZnS–SiO2 dielectric layer.

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.

Nanometer-scale mechanism of phase-change optical recording as revealed by XAFS

We demonstrate that the Ge(Sb)–Te bond lengths in crystallized cubic Ge2Sb2Te5 (GST) are significantly lower than the values expected from the previous X-ray diffraction (XRD) analysis. At the same time, the second nearest-neighbour Te–Te distances are in perfect agreement with XRD. We conclude that the structure of GST is a distorted rocksalt structure. Upon amorphization, Ge–Te and Sb–Te bonds get shorter and stronger. This unusual behaviour is due to a switch of Ge atom from an octahedral symmetry position in the crystalline state into a tetrahedral symmetry position in the amorphous state. It is this switching of the Ge atoms that is responsible for the fast and stable media performance.

Investigation of phase changes in Ge1Sb4Te7 films by single ultra-fast laser pulses

The phase transformations induced in a Ge1Sb4Te7 system by a femtosecond (fs) laser exposure were investigated. The system has a multilayer structure of 15 nm ZnS–SiO2/80 nm Ge1Sb4Te7/100 nm ZnS–SiO2/0.6 mm polycarbonate substrate. The morphology and contrast of marks formed in both amorphous and crystalline backgrounds by single fs pulses were characterized using an optical microscope. X-ray diffraction was applied to identify the crystal structures formed by single fs shots. Phase-reversible transformations in the system have been achieved through careful adjustment of the laser fluence. The mechanism of reversibility triggered by fs laser pulses is discussed. The feasibility of phase-change reversible optical recording with the active Ge1Sb4Te7 layer using single fs pulses with a duration of 400 fs within well-defined fluence and pulse energy windows is therefore demonstrated. Our work also demonstrates that it is possible to record and retrieve data rapidly in the Ge1Sb4Te7 film within a conventional optical disk structure using non-amplified laser systems as laser sources.

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.

Identification of soft phonon modes in Ge–Sb–Te using electron diffraction

The atomistic structure of crystalline Ge–Sb–Te thin film for phase-change optical recording was investigated using transmission electron microscopy and nanobeam electron diffraction. Nonradial diffuse streaks were observed in electron diffraction patterns obtained from laser-induced crystalline phases of Ge–Sb–Te thin films. The intensities of the diffuse streaks were pronounced in particular directions in this alloy. The diffuse streaks were due to low-frequency transverse lattice waves that occur along directions perpendicular to the near neighbor zigzag atomic chains.

Identification of Ag37Sn33Te30 Phase-Change Filmsfor Optical Data Storage

The Ag37Sn33Te30 film has been investigated todetermine its suitability as phase change optical recording alloy basedon static tester and atomic force microscopy measurements. Switchingproperties and the recording bit topography of the film are studied.With a writing pulse of power 2.5 mW, width 10 μs, and an erasure pulseof power 0.25 mW, width 10 μs, optical contrast of 31.8% is obtained.Re-crystallization experiments identify the Ag37Sn33Te30film as a suitable phase change material for optical data storage with acomplete erasure time of 1.1 μs at low erasure power.