Martijn Henri Richard Lankhorst

Phase-change recording materials with a growth-dominated crystallization mechanism: A materials overview

The influence of phase-change material composition on amorphous phase stability, crystallization rate, nucleation probability, optical constants and media noise is reported for materials with a growth dominated crystallization mechanism. Two material classes have been studied, doped Sb–Te and doped Sb-based compositions. The material properties of both are greatly influenced by their composition, and in a similar way. For both materials systems hold that the antimony content especially influences the crystallization rate, amorphous phase stability and media noise of the phase-change material. Compositions rich in antimony generally show high crystallization rates, low archival life stability and high media noise. The material properties are further influenced by the presence of dopants like tellurium, germanium, gallium, indium or tin. Germanium and tellurium reduce the crystallization rate, but are essential to increase the amorphous phase stability. Dopants like tin or indium are added to increase the c...

Prospects of Doped Sb-Te Phase-Change Materials for High-Speed Recording

Recording parameters such as crystallization rate, archival life stability and media noise have been studied for phase-change optical discs based on doped Sb–Te materials. It has been found that the crystallization rate can be tuned easily by varying the Sb/Te ratio or the dopant atom; higher crystallization rates are obtained for higher Sb/Te ratios and for In- and Ga-doped Sb–Te. However, it has been found that measures that enhance the crystallization rate often also influence other material properties, such as archival life stability and media noise. For example, a higher Sb/Te ratio leads to a lower archival life and higher media noise. As a consequence, the demands on doped Sb–Te materials to combine sufficiently high crystallization rate, good archival life stability and low media noise become increasingly difficult to satisfy for high recording speeds.

Phase-Change Media for High-Numerical-Aperture and Blue-Wavelength Recording

We have studied the feasibility of doped eutectic SbTe alloys for practical application in digital video recording (DVR) rewritable phase-change media for high-numerical-aperture (NA=0.85) and blue-wavelength (λ=405 nm) recording. Remaining issues such as thermal cross-erase in land/groove recording and the thermal stability of recorded amorphous marks have been investigated and resolved. This work has resulted in the realisation of 22.5 GB phase-change media complying with the DVR blue format. The future of doped eutectic SbTe alloys in the phase-change recording-speed race is also promising, as the crystallisation speed and archival life stability can be optimised more or less independently by tuning the Sb/Te atomic ratio and adding Ge, respectively. We have demonstrated that a user data transfer rate of 70 Mbit/s (DVR double speed) is within reach using our current phase-change composition, and are expecting to realise data rates of over 100 Mbit/s in the near future.

Crystallization behavior of phase change materials: comparison between nucleation- and growth-dominated crystallization

A comparative study of the crystallization behavior of nucleation- and growth-determined materials is presented. The influence of phase change layer thickness, additions and amorphous mark size on the crystallization rate is addressed. The long nucleation time of doped SbTe-type materials allows one to measure the nucleation probability separately, which may provide important information for initialization of optical storage disks.

Remote phosphor LED modules for general illumination: toward 200 lm/W general lighting LED light sources

Phosphor converted white LEDs are becoming more and more attractive for general lighting applications because of the steadily increasing luminous efficacy numbers reported by LED-suppliers. Despite these high numbers, a further significant improvement step can be made when a low-to-medium brightness (<500 kCd/m2) source is acceptable. The wall plug efficiency of a blue LED is generally better than that of a conventional white LED made from the same die. To take full advantage of this, we have developed medium-brightness LED-modules (~150 kCd/m2) for general lighting in which the phosphor is applied remote from the blue LEDs. By direct comparison with modules in which conventional high power white LEDs with almost identical dies are applied, we have shown that on system level the remote phosphor modules can have up to 50% better efficacy. Using a downlight module as a carrier, we have shown that in the relevant color temperature range of 2700 to 4000K a high CRI (>80) can be obtained in combination with a high luminous efficacy, while the optical efficiency of the module can be over 85%. A module efficacy of over 100 lm/W at 4000K with CRI 80 seems to be within reach, with a long-term expectation of over 180 lm/W. The remote phosphor LED modules deliver well homogenized white light with a Lambertian radiation profile. They are ideal for general illumination, as they combine glare reduction with high system efficacy and enable high optical efficiencies of the luminaries. The RP modules enable forward compatibility by well defined interfaces and optical properties that are decoupled from the actual performance, form factor and number of LEDs in the module. The Philips Fortimo downlight system is based on this remote phosphor concept, featuring forward compatibility and a total system efficacy (including driver) of over 60 lm/W under operating conditions using currently available Luxeon Rebel emitters.

Determination of the crystallisation kinetics of fast-growth phase-change materials for mark-formation prediction

A mark simulation tool for fast-growth phase-change materials was developed to predict the formation and erasure of amorphous marks in phase-change discs that are based on doped Sb2Te compositions. One of the main challenges of such a simulation tool is the accurate determination of the input parameters. The thermal conductivity of the thin films in the recording stack was determined from melt-threshold experiments in combination with numerical modelling. The crystallisation process, modelled as crystal growth from a crystalline-amorphous interface, is governed by the temperature-dependent velocity of crystal growth. In this paper, we discuss a procedure for measuring the low-temperature velocity of crystal growth from the size of an amorphous mark and the time required for complete erasure under isothermal conditions. Furthermore, to determine the velocity of crystal growth in the temperature range encountered during recording conditions, we performed time-resolved erasure experiments in combination with numerical modelling. The derived time-dependent growth velocity was used in a numerical model to simulate the formation of amorphous marks. Transmission electron microscopy (TEM) measurements of recorded marks were compared to predicted mark shapes to validate the numerical model.

Numerical simulation of mark formation in dual-stack phase-change recording

Dual-stack phase-change recording is an option to further increase the data capacity of rewritable optical disks. Such disks consist of two recording stacks that are both recorded and read from the same side of the disk. Consequently, the first recording stack needs therefore to be semitransparent to allow recording in the second recording stack. Thick nontransparent metal layers can therefore not be used in the first recording stack, which makes the first recording stack the most challenging stack from a thermal point of view. A numerical model based on crystal growth was developed to study formation and erasure of amorphous marks in phase-change stacks that are based on fast-growth doped SbTe phase-change materials. The validity of the model was demonstrated from transmission electron microscopy analyses of recorded marks that showed a good correspondence with the calculated mark shapes in a conventional single-stack recording stack. The model was subsequently applied to analyze formation and erasure of marks in slow-cooling phase-change stacks for digital versatile disk, (DVD) and digital video recording (DVR) recording conditions. The effect of the recording velocity, the erase power, and the crystal growth velocity on the erasability of amorphous marks was simulated. The calculated phenomena are in good agreement with the phenomena observed from DVD and DVR erasability measurements. Mark formation in slow-cooling recording stacks is characterized by severe recrystallization during writing. Two possible solutions are indicated in this article, aiming at reducing the heat accumulation and the resulting recrystallization during writing of amorphous marks. Additional semitransparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is the application of modified write strategies. Modeling and recorder results are discussed for both approaches.

Thermal Cross-Erase Issues in High-Data-Density Phase-Change Recording

We report on the numerical simulations of high-data-density phase-change recording (DVR-blue and DVR-red) in order to analyse the phenomenon of thermal cross-erase in land/groove recording. In particular, if a maximum data capacity is pursued, which involves a small radial track pitch, thermal cross interference may become an important issue. The question of whether or not the temperature distributions and the related thermal cross-erase probability scale with the optical spot size is addressed. Two different thermal models (a planar model and a land/groove model) were used in which laser heating was simulated. Reliable temperature predictions and related trend analyses require a proper model and accurate input parameters. The thermal conductivity of the different layers in the stack was obtained from measurements performed with a time-resolved static tester setup in combination with a novel melt-threshold method. Since thermal cross-erase is dependent on the temporal temperature distribution in the stack, we analysed the most important parameters that possibly contribute to a reduction of the temperature, and thus the cross-erase probability, in the adjacent track.

Phase-Change Media for High-Density Optical Recording

Abstract : Rewritable optical-storage systems are quickly gaining market share in audio, video and data-storage applications. The development of new rewritable optical-storage formats with higher capacity and data rate critically depends on innovations made to the recording media incorporating so-called phase-change materials. These materials allow reversible switching between a low and high reflective state induced by laser heating. In this paper, we highlight phase-change media aspects as optical and thermal design, sputter-deposition, materials optimization, and the development of new recording strategies. Focus is on the speed race in optical recording.

Advances in thermal modeling of dual-layer DVR-blue fast-growth media

Dual-layer phase-change recording is an option to further increase the data-capacity of re-writeable optical discs. A numerical model based on crystal growth was used to study formation and erasure of amorphous marks in dual-layer stacks based on fast-growth doped eutectic SbTe phase-change materials. The effect of the linear velocity, erase power, and crystal growth velocity on the erasability of amorphous marks was simulated. The calculated effects are in good agreement with the phenomena observed from DVD and DVR measurements. Mark formation in slow-cooling dual-layer stacks is characterized by severe re-crystallization during writing. Two possible solutions are indicated in the paper aimed at reducing the heat accumulation in the recording stack, and thus re-crystallization, during writing of amorphous marks. Additional transparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is based on an appropriate write strategy. Recording results obtained with this such a write strategy are shown.