Sb Te Phase-change Materials Research Articles

Structure of the Ge–Sb–Te phase-change materials studied by theory and experiment

We have studied the structure of GeSb 2Te 4, GeSb 4Te 7, and Ge 3Sb 2Te 6 compounds using theoretical and experimental means. Based on ab initio calculations, we propose the stacking sequence in the [111] direction of GeSb 2Te 4, GeSb 4Te 7, and Ge 3Sb 2Te 6 phases to be Te–Ge–Te–Sb–Te–v–Te–Sb–, Te–Ge–Te–Sb–Te–v–Te–Sb–Te–Sb–Te–v–Te–Sb–, and Te–Ge–Te–Ge–Te–Sb–Te–v–Te–Sb–Te–Ge–, respectively, where v is an ordered vacancy layer. This structural model agrees with the X-ray diffraction data of sputter-deposited Ge 2Sb 2Te 5, GeSb 2Te 4, GeSb 4Te 7, and Ge 3Sb 2Te 6 thin films.

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.

A method to enhance the data transfer rate of eutectic Sb-Te phase-change recording media

This work describes the effect of nitrogen doping to eutectic Sb-Te phase-change materials in order to enhance the speed of the amorphous-to-crystalline phase transformation. When nitrogen at a sputtering gas flow ratio of N2/Ar=3% was doped in the eutectic Ge-In-Sb-Te recording layer, the data transfer rate was increased up to 1.6 times. When thin GeNx nucleation promotion layers were further added in below and above the recording layer, an overall enhancement up to 3.3 times in data transfer rate was achieved. The nitrogen contents corresponding to the N2/Ar flow ratios (N2∕Ar=0%–10%) were calibrated by electron spectroscopy for chemical analysis. Transmission electron microscopy revealed that nitrogen doping was able to promote the phase transformation by generating numerous nucleation sites uniformly distributed in the recording layer and hence increased the recrystallization speed.

In situ transmission electron microscopy analysis of electron beam induced crystallization of amorphous marks in phase-change materials

Crystallization of amorphous data marks in crystalline Ga15Sb85 and Ge,In doped SbTe phase-change material was studied in situ in a Transmission Electron Microscope (TEM). Electron irradiation induced crystallization was obtained at room temperature using a 120kV beam. In general, electron beam (e−-beam) induced crystallization started from the amorphous-crystalline interface and was growth dominated for both materials. A dependence of growth velocity on electron beam intensity and crystal direction was observed. A comparison with laser-crystallized amorphous marks was made. For laser-induced crystallization also crystal growth from the amorphous-crystalline interface was seen. However, differences in morphology between the e−-beam and laser-recrystallized data marks of the GaSb phase-change material were observed. The electron beam erased data marks contained crystals with (extremely) large periodicities found in three dimensions. For the Ge,In doped SbTe phase-change material identical morphologies were observed for the e−-beam and laser-recrystallized data marks. Both methods that induce crystallization displayed a rhombohedral Sb structure, the same structure as the laser-crystallized surroundings.

Te-free, Sb-based phase-change materials for high-speed rewritable optical recording

High-speed rewritable optical disks based on conventional (eutectic) Sb–Te phase-change materials have low archival life stability and high media noise. We propose Te-free, Sb-based phase-change materials for recording at linear velocities over 28 m/s. These materials combine good optical contrast, rapid crystallization, and high amorphous phase stability.

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.

Structural Analysis of Ag–In–Sb–Te Phase-Change Material

The Ag–In–Sb–Te system is widely used for phase-change discs such as compact disc rewritable (CD-RW) and rewritable digital versatile disc (DVD+RW). To clarify the effect of Ag and In in the Ag–In–Sb–Te system, we studied the local structure of Ag–In–Sb–Te phase-change material by extended X-ray absorption fine structure (EXAFS). The results suggest that the existence of Ag contributes to the thermal stability of amorphous marks and existence of In contributes to high-speed phase-change.

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.

Feasibility of High-Data-Rate Media with Ge–Sb–Te Phase-Change Material

We experimentally studied an optical disk with a large capacity and high data-transfer rate that can record high-definition television signals. A high rotation speed was used to achieve the high data-transfer rate. Using Ge–Sb–Te phase change material, a carrier-to-noise ratio of about 50 dB was achieved at a linear velocity of 15–30 m/s with a 0.4-µm-long mark. High-speed crystallization was necessary to erase recording marks. Therefore, by appropriately designing layers that assist nucleation, we obtained a DC erase ratio of 29 dB with a 0.4-µm mark and 1-mW power margin for over-25-dB erase ratio at a linear velocity of 18 m/s. Furthermore, the 0.4-µm mark-space repetition jitter was less than 7% even after 1000 cycles. A data transfer rate of 60 Mbps was obtained by optimizing the write strategy using a multi-pulse with an optical system equivalent to that of current digital versatile disk (DVD).

High-Density and High-Data-Transfer-Rate Optical Disk with Blue Laser Diode and Ag–In–Sb–Te Phase-Change Material

We studied phase-change recording using a 405 nm blue laser diode, an objective lens of numerical aperture (NA)=0.6 and Ag–In–Sb–Te phase-change material. We report land and groove recording on a conventional disk structure with a 0.6-mm-thick and 120-mm-diameter PC substrate. We confirmed the possibility of achieving a high density, 12 GB capacity and a high data-transfer rate of 30 Mbps, corresponding to that of a read-only digital versatile disk (DVD-ROM) under the conditions of 6.5 m/s recording speed, 0.18 µm/bit linear density (minimum mark length 0.27 µm) and 0.42 µm trackpitch.

Potentiality of the Ag–In–Sb–Te Phase Change Recording Material for High Density Erasable Optical Discs

The feasibility of using Ag–In–Sb–Te phase change material for high density of directly overwritable optical discs has been studied. A linear density of 0.34 µm/bit with a track pitch of 0.74 µm is confirmed to be achievable with a light source wavelength of 680 nm, with an objective lens NA of 0.60 and the criteria of data-to-data jitter of less than 14% of the channel bit length. The recording characteristics are also studied using an optical system with a 635 nm red laser diode in order to investigate the attainment of a linear density of 0.267 µm/bit which is equivalent to the DVD-ROM (read only memory) specification.