Phase Change Optical Recording Research Articles

Suitability of Ge-As-Te system for optical data storage

We report the study of crystallization kinetics and time-temperature-transformation in alloys of Ge7.5AsxTe92.5-x (x=20, 40, 45, 47.5, 50, 52.5, 55) and Ge10AsxTe90-x (x=15, 20, 22.5, 35, 40, 45, 50), in order to examine the suitability of these alloys for reversible phase change optical recording (RPCOR) films, which is one of the leading optical storage devices. Using the values of viscosity and kinetics of crystal nucleation and growth process, the time-temperature-transformation curves were obtained. The variation of critical parameters such as minimum erasure time and thermal stability has been calculated as a function of composition. It has been observed that for our samples, the critical cooling rate is high (2.1*105-4.3*104 K/min) for Ge10AsxTe90-x (x=15, 20, 22.5, 35, 40, 45, 50) and 1.51*105-2.91*104 K/min for Ge7.5AsxTe92.5-x (x=15, 20, 22.5, 35, 40, 45, 50) ensuring fast erasure speed ( approximately=1 mu s) which is an essential requirement for high quality RPCOR films. It is suggested that the material with lower concentration of Germanium (<10 at.%), Arsenide and Tellurium concentration between (35-55 at.%) are better contenders for the above application.

Existence of electronic excitation enhanced crystallization in GeSb amorphous thin films upon ultrashort laser pulse irradiation.

The energy density crystallization threshold of amorphous GeSb films has been studied for the first time as a function of the laser pulse duration in the range from 170 fs to 8 ns. The results obtained provide evidence of the existence of enhanced crystallization upon irradiation with pulses shorter than 800 fs, which is most likely related to electronic excitation effects. The study of the interaction of ultrashort laser pulses with solids has raised some important questions concerning the nature of the transformations occurring at the material surface in the presence of very strong levels of electronic excitation. The existence of electronic excitation induced phase transitions has been already demonstrated in several materials (Si [1], GaAs [2‐ 4], graphite [5]). In the case of crystalline semiconductors, excitation with pulses shorter than the longitudinal optical phonon emission time [6] may lead to a structural instability of the lattice, leading to the formation of a metastable transient phase with (semi)metallic character. It is not clear, however, whether the presence of such transient strong electronic excitation and/or transient phases may influence the final state or structure induced since, once the energy of the excited carriers has been transferred to the lattice, the structural transformation path is thermal in nature. Different nonthermal mechanisms which could lead to enhanced crystallization and to crystal damage recovery upon pulsed laser irradiation were proposed during the early 1980s [7,8]. More recently, both photonic effects and electron-hole plasma induced network softening have been invoked to explain structural changes induced in amorphous Si and Ge upon ns and ps laser pulse irradiation [9 ‐ 11]. High Sb-content GeSb amorphous and crystalline thin films have been shown to both crystallize and amorphize upon irradiation with subnanosecond laser pulses. Both processes show optical contrast [12] making this material a potential candidate for applications in ultrashort laser pulse driven phase change optical recording [13,14]. These characteristics also make this material interesting for the study of possible high electronic excitation induced and enhanced crystallization processes. The aim of this work is to elucidate whether the high electronic excitation, induced by irradiation with subnanosecond laser pulses, can induce or enhance the crystallization process of the amorphous phase. Such an enhancement process should lead to appreciable changes in the minimum energy density required for crystallization as the laser pulse duration changes, according to the differences in the excited carrier populations induced. We have performed what we believe to be the first experiment to determine the minimum energy density required for crystallization of an amorphous phase upon irradiation with laser pulses whose durations extend over 3 orders of magnitude, covering the range 170 fs to 8 ns. The samples used in the experiment were 50 nm thick, Ge 0.07Sb 0.93 films grown at room temperature in a multitarget magnetron dc sputtering system from Ge and Sb (99.999% purity) targets onto glass, carbon-coated mica, and Si(100) substrates. In order to improve the sensitivity of the material and to enhance the crystallization rate upon pulsed laser irradiation, two different kinds of samples were grown: (a) fully amorphous layers with the composition Ge 0.07Sb 0.93 and (2) bilayered films formed by an amorphous surface layer of similar composition and an underlayer slightly richer in Sb and therefore polycrystalline. Since the bilayered films showed smaller transformation times and improved sensitivity upon pulsed laser irradiation with 10 ps laser pulses at 583 nm, they were chosen for the present work. The samples on mica and Si were used, respectively, to prepare planar and cross-section transmission electron microscopy (TEM) specimens in order to characterize film structure and composition. The latter was determined by energy dispersive x-ray spectroscopy on planar TEM specimens. An ultrashort CPA laser pulse source (described in Ref. [15]) was used to irradiate the samples on glass. The experimental configuration is shown in Fig. 1. A commercial fs Ti:Al 2O 3 oscillator providing 150 fs duration pulses at a repetition rate of 82 MHz at 830 nm was used to seed an Ar 1 ion pumped Cr-LiSAF solid state regenerative amplifier. Before seeding the amplifier, the oscillator pulses were stretched to approximately 300 ps with a diffraction grating pulse stretcher. After amplification the pulses were recompressed with a grating pair compressor. The typical output of the laser system consisted of a 10 kHz repetition rate pulse train with energies per pulse in the order of 5 mJ. A second Pockels cell, synchronized to the one in the regenerative amplifier, was used

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.

Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb2Te3 and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.

New phase-change rewritable optical recording film having well suppressed material flow for repeated rewriting

Characteristics of a phase-change optical disk using a (Ge–Sb–Te)-(Cr–Te) recording film containing a high-melting-point component Cr–Te are studied. The high-melting-point component Cr–Te precipitates in the recording film, and prevents a material flow of the recording film during repeated rewrites in which the recording film is melted. There is no drawback such as noise increase by the addition of Cr–Te. Thus the phase-change optical disk using this recording film shows very small distortion in the reproduced signal wave form even after 2×104 rewrites of high-density recording signals that are liable to cause the material flow by forming long and short recording marks on the disk. With a phase-change optical disk, it is easy to overwrite (rewrite without prior erasure) information, which makes it highly suitable for digital recording of motion pictures and sound. They are expected to be used as rewritable compact disks, rewritable CD-ROM disks, rewritable video disks, and small recording media for personal computers.

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.

相変化を利用した新規な４元系Ａｇ‐Ｉｎ‐Ｓｂ‐Ｔｅ光記録材料

Reversible phase change optical recording material with a new quarternary system of Ag-In-Sb-Te has been studied. The starting material for the Ag-In-Sb-Te system is a combination of AgInTe2 with the chalcopyrite structure and Sb. In this work, we report the basic properties of the Ag-In-Sb-Te films investigated by means of DSC, XRD, ED, and HR-TEM. We also discuss the phase change process of the Ag-In-Sb-Te system.

Study of Se–Te–Sb System for Application to Reversible Optical Data Storage

In the present work variation of the two main parameters–viscosity and critical cooling rate, required for the recent upcoming optical data storage devices, have been analysed for different compositions of Se80-xSbxTe20 (0≤x≤9) system. Temperature dependence of viscosity has been determined using the activation energy for glass transition and for crystallization obtained from heating rate dependence of glass transition temperature and peak crystallization temperature from DSC measurements. Vicsocity has been estimated with the help of Vogel-Tamman-Fulcher equation. It has been found that for samples containing Sb≤4 at.%, the viscosity increases with the increase in Sb content. Further increase in Sb concentration causes a decrease in the viscosity. These results are consistent with the structural changes taking place in the Se–Te system with increase in Sb content. Using these values of viscosity and kinetics of crystal nucleation and growth process, the time-temperature-transformation curves were obtained. The critical cooling rate Rc is found to increase for Sb>4 at.%. For good reversible phase change optical recording, the critical cooling rate of the material should be high enough so as to get an erasure time of less than 1 µs. From the analysis of the data the suitability of the SeTeSb system for optical recording has been discussed.

Structural and optical properties of Bi x Se1?x films

Bi x Se1−x thin films have been studied because of their structural and optical properties with a view to judging their suitability as the recording medium in phase-change type optical recording. Amorphous films deposited at room temperature were crystallized by thermal annealing. X-ray diffraction analysis and surface morphological studies are reported. A maximum reflectivity difference of 25% at λ=830 nm was obtained upon amorphous-to-crystalline transition. The optical constants calculated by the Newton-Raphson method using the experimental transmittance, reflectance and thickness data are reported.

Thin binary In 1−xSb x films prepared by flash evaporation

Thin binary In 1− x Sb x films of 1000 Å thickness have been prepared by flash evaporation. The antimony concentration x has been controlled to vary from 0.23 to 0.70. Grain sizes and homogeneity of concentration both before and after annealing at 250 °C, 350 °C, and 450 °C were examined by both X-ray diffraction and transmission electron microscopy. Application of differential scanning calorimetry allows discussion of an atomic migration mechanism and phase transformation kinetics as a function of concentration. Also, the relationship between the microstructures and the optical properties, such as reflectances, transmittances and absorption coefficients at 830 nm, are presented. Both the grain sizes and the optical reflectances observed show that the In 1− x Sb x film with an antimony concentration x ≈ 0.54 is a candidate for phase change optical recording.

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.

Discussion on the Mechanism of Reversible Phase Change Optical Recording

In stoichiometric GeTe-Sb2Te3 films, reversible phase change optical recording materials, Sb addition is used widely in order to improve the crystallization speed, the data retention time and the stability of cyclic operation, but its mechanism is not clear. V-VI compounds (GeTe, SnTe) are well known as compounds in which the ferroelectric structural phase transition occurs. The dielectric constant ε0 changes largely due to a change in the short-range order of weakly bound nearest neighbors in the crystal to more strongly bound nearest neighbors in the amorphous phase. Therefore, it is necessary to discuss the effect of Sb addition in the stoichiometric GeTe-Sb2Te3 composition upon the thermal stabilities, the dynamic properties of crystallization and the dielectric constant change Δε0 based on the amorphous-to-crystalline phase transition.

Germanium supersaturation during the crystallization of amorphous Te–Ge–Sn thin films

The structure and phase relations of Te–Ge–Sn thin films are examined with application to erasable optical storage media. Free energy data from the literature predict that the region of the Te–Ge–Sn phase diagram between Ge, Sn, and the TeGe–TeSn pseudobinary consists of one two-phase field [α–Ge and Te50 (GexSn1−x)50] and one three-phase field (α–Ge, β–Sn, and TeSn). Electron diffraction from five different Te–Ge–Sn films annealed at 623 K experimentally confirms this prediction. One composition from the two-phase field is deposited as a tri-layer film with the structure 150 nm SiO2/75 nm Te36.3Ge47.4Sn16.3/150 nm SiO2 on a grooved disk substrate, and the microstructure resulting from low-power (12 mW) CW and higher-power (∽50 mW) pulsed laser exposure is studied by transmission electron microscopy and electron diffraction. Of particular significance is that laser-induced crystallization produces a single-phase structure consisting of the Te–Ge–Sn compound phase which is supersaturated with respect to the excess Ge. This supersaturation leads to a disordering of the equilibrium NaCl-type structure of this phase. Crystallization of a micron-sized amorphous spot on a ∼200 ns time scale occurs by a diffusionless process. The fast erase times required by a phase-change optical recording application can thus be achieved in off-stoichiometric compound compositions by way of a nonequilibrium crystallization process.

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.

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

Crystallization process of InTe alloy films for optical recording

The crystallization process of sputtered InTe alloy films was studied in detail using a differential scanning calorimeter (DSC) in order to determine if the alloy can be used as a phase-change type optical recording material. The crystallization process is well described by the Johnson-Mehl-Avrami (JMA) equation. In a 50–60 at.% TeIn alloy system, good material characteristics were obtained, that is, a high crystallization temperature above 250°C, a high activation energy of more than 3.0 eV and a frequency factor of more than 10 30 s −1. The exponent n varied from 1.5 to 3 in these compositions. An incubation time was also observed in the present experiments. In addition to studying these material characteristics, the crystallization speed was estimated with a static write-erase tester using semiconductor laser irradiation. It was found that the incubation time greatly influences high speed crystallization.

Multilayered Thin-Film Materials for Phase-Change Erasable Storage

Phase-change erasable optical recording uses a focused laser beam as a heat source to reversibly switch a micron-sized area in a thin film between the amorphous and crystalline states. A bit of information is stored as an amorphous spot in a crystalline background, and the state of the bit is determined by the differing optical properties of the amorphous and crystalline phases. This concept was first demonstrated in 1971 and then, after about a decade of exploratory work, the field accelerated throughout the 1980s at several research laboratories. Currently the subject of number of reviews, the field of phase-change materials promises to broaden and intensify in the 1990s.The active layer, where the storage occurs, is typically a tellurium-based alloy with a variety of solute species. Early work studied the recording properties of single-layered films, but it has been clearly shown that multilayered films, where the active layer is sandwiched between two or more dielectric layers, have superior recording properties and resistance to irreversible damage caused by laser heating. The dielectric layers (typically SiO2, Si3N4, or ZnS) provide barriers to active-layer oxidation and contamination, help prevent the hole formation associated with ablative write-once storage methods, and act as crucibles and heat sinks which contain the molten spot and influence its cooling properties, respectively. A typical multilayer structure is shown in the cross-sectional transmission electron micrograph of Figure 1.

Progress and issues of phase-change erasable optical recording media

Recent progress in realizing practical media for phase-change reversible optical recording is described. Key attributes required of the media and the material science which underlie them are discussed. These include: (1) the kinetics of crystallization and amorphization, (2) chemical and data stability, (3) reversibility, and (4) readout signal-to-noise ratio. The crystallization kinetics of recording media is discussed using two examples: (GeTe) 0.85Sn 0.15 and the pseudo-binary InSbGaSb stoichiometric compound system. A discussion of the recorded signal strength, sources of media noise and resulting signal-to-noise ratio is presented. Non-uniform optical index of refraction was shown to increase media noise. Methods to reduce the effect of media noise are presented.

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.

Kinetic studies of the crystallization of Te 92Pb 8 thin films

A method to obtain the crystallized volume fraction X and the time taken for crystallization t is proposed. The Avrami equation X = 1 − exp(− kt n ) is used to calculate the time exponent n and rate constant of crystallization k. The nucleation mechanism, nucleation rate and the growth rate of nuclei of Te 92Pb 8 are discussed in relation to different n and k values. It is found that the nucleation at the start of the transformation, plus continuing nucleation at grain boundaries or grain edges, determine the kinetics at lower temperatures. Both the nucleation and the growth rate of nuclei are low at 32° C. They increase slowly with temperature below 46° C. From 46° C, the nucleation rate increase rapidly. Its maximum may be in the temperature region between 65° C and 80° C. The maximum growth rate of nuclei is at a temperature higher than 80° C. A nonlinearity in the plots of lnln 1/(1 − X) vs ln t caused by the variation of n is also found and discussed. The amorphous state of Te 92Pb 8 is unstable. The possibility of using it as erasable phase change optical recording material is evaluated.