Phase-change Recording Materials Research Articles

GeBiTe相変化膜におけるBi L<sub>III</sub>吸収端のXAFS解析

The crystallization of GeBiTe (GBT) is faster than that of the well-known phase-change recording material GeSbTe (GST). Therefore, the investigation of GBT structure as well as its crystallization process is attractive. Accordingly, the high-speed crystallization of GBT is due to Bi. Thus, it was necessary to know the local structure around Bi within the amorphous GBT. Note that an interface layer, which is a very thin dielectric film adjacent to recording film, assists the crystallization of phase-change material in the optical recording media. The local structure around Bi in GBT within an actual media was analyzed using XAFS. Bi LIII-edge XAFS spectra of crystalline and amorphous GBT respectively, both with and without interface layer in the media, were obtained. As result of this analysis on GBT, nearest neighbor atom of Bi was found to be Ge. Moreover, within the amorphous GBT, the interatomic distance around Bi is larger than that around Ge. These are the differences between GST and GBT. We speculate that these factors contribute to the improvement of the GBT crystallization speed. On the other hand, the interface layer doesn’t influence the local structure of GBT; however, it does have an electric effect on the recording layer.

高速高密度相変化記録材料の原子配列と相変化過程の解明

高速高密度相変化記録材料の原子配列と相変化過程の解明

Structural transformation of Sb-based high-speed phase-change material

The crystal structure of a phase-change recording material (the compound Ag(3.4)In(3.7)Sb(76.4)Te(16.5)) enclosed in a vacuum capillary tube was investigated at various temperatures in a heating process using a large Debye-Scherrer camera installed in BL02B2 at SPring-8. The amorphous phase of this material turns into a crystalline phase at around 416 K; this crystalline phase has an A7-type structure with atoms of Ag, In, Sb or Te randomly occupying the 6c site in the space group. This structure was maintained up to around 545 K as a single phase, although thermal expansion of the crystal lattice was observed. However, above this temperature, phase separation into AgInTe(2) and Sb-Te transpired. The first fragment, AgInTe(2), reliably maintained its crystal structure up to the melting temperature. On the other hand, the atomic configuration of the Sb-Te gradually varied with increasing temperature. This gradual structural transformation can be described as a continuous growth of the modulation period γ.

SET/RESET properties dependence of phase-change memory cell on thickness of phase-change layer

The thermal properties of a phase-change random access memory (PCRAM) cell are dominated by the phase-change recording material. The SET/RESET resistances, the minimum width of the SET/RESET pulse, the threshold voltage and the maximum temperature of PCRAM cells are impacted by the thickness of the phase-change layer. In this paper, a PCRAM cell with different Ge–Sb–Te phase-change layer thickness are fabricated, and the electrical properties of the PCRAM cells are tested. The SET/RESET properties dependence of PCRAM cells on the thickness of the phase-change layer is investigated and compared for the low-power consumption of PCRAM.

Ultra-high-density phase-change storage using AlNiGd metallic glass thin film as recording layer

A nonchalcogenide inorganic blu-ray super-resolution near-field optical disk was successfully developed using an Al-based metallic glass as the recording layer. Data were recorded based on the large reflectivity change obtained during transformation from the amorphous state to the crystalline state upon laser irradiation. The optical and thermal properties of the Al 90Ni 3Gd 7 metallic glass thin film were studied. The crystallization and melting temperatures were higher and more thermally stable than that of conventionally used Ge 2Sb 2Te 5 chalcogenide phase-change recording material. Al 90Ni 3Gd 7 metallic glass also demonstrated fast phase-change response to blue laser. Dynamic recording performance of this new structure showed good carrier-to-noise ratio and readout stability.

Research on phase-change of disc recording media GaSbBi

A new phase-change recording materials, Bi-doped eutectic composition GaSb, was researched experimentally. The films were prepared by vacuum evaporation. The crystallization temperature decreases as the Bi concentration increases. The crystallization activation energy, calculated by Kissinger method, is more than 2.0eV and meets the requirement of the films stability. The crystallization rate enhances with the increase of the Bi concentration. The Ga12Sb79.2Bi8.8 satisfies the demand of optical property by means of spectrophotometer. The absorption of the Ga12Sb79.2Bi8.8 crystalline films is more than that of the Ga12Sb79.2Bi8.8 amorphous films, which can reduce the change when reads information. The least reflectivity of Ga12Sb79.2Bi8.8 is 28.78%, corresponding with the reflection demand.

相変化記録材料ＧｅＴｅ‐Ｓｂ２Ｔｅ３擬二元系化合物の結晶構造

Crystal structures of the phase-change recording material, GeTe-Sb2Te3, pseudo-binary compounds have been investigated over a decade using synchrotron radiation and the large Debye-Scherrer camera installed at SPring-8’s BL02B2 beamline. These compounds can be crystallized into metastable single phase with NaCl-type structure, for instance, by instantaneous laser irradiation; however, they are transformed into stable phases with long period trigonal structures characterized by the chemical formula, (GeTe)n(Sb2Te3)m (n, m; integer) after sufficient heat treatments. These stable phases can be described as structure with cubic close-packing periodicity (…ABCABC…) where the stacking rules of the Ge/Sb and Te layers are different from each other.

Detection of Recording Marks on Digital Versatile Discs and Blu-ray Discs using Conductive Atomic Force Microscopy

Recording bits on recordable phase-change discs are studied using conductive atomic force microscopy (C-AFM) at high contrast. The grain size and boundary analysis of different phase-change recording materials can be performed at nanoscale spatial resolution. This study shows that C-AFM is not only suited to image the amorphous marks written on phase-change-based rewritable digital versatile discs (DVD) or Blu-ray discs (BD), but is also capable of revealing the appearance of bits in CuSi-based write-once BD discs.

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.

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.

Writing and erasing efficiency analysis on optical-storage media using scanning surface potential microscopy

The authors demonstrate a rapid and convenient method for the determination of writing and erasing laser powers in phase-change media based on scanning surface potential microscopy using a conductive tip. Commercially available digital versatile disks that are rewritable with initialization process are measured in experiments. The results of measurement show that when the disk is rotating at 3.5m∕s linear velocity, the adequate writing and erasing of laser power are 10–15 and 4–8mW, respectively. The critical laser power for crystallization conversion is 8mW. The method used for this research can be satisfyingly applied to the development of new phase-change recording materials.

Temperature Dependence of Phase-Change Random Access Memory Cell

The temperature dependences of phase-change random access memory (PCRAM) cells on different Ge–Sb–Te phase-change recording materials are studied and compared. A Ge2Sb2Te5 phase-change film has a larger resistance margin and a higher thermal stability than Ge1Sb2Te4 and Ge1Sb4Te7 films. The set resistance, reset resistance, resistance margin and threshold voltage of PCRAM cells decrease with increasing temperature. A Ge2Sb2Te5 PCRAM cell has a higher thermal stability of threshold voltage than Ge1Sb2Te4 and Ge1Sb4Te7 PCRAM cells.

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 crystallization rate or to adjust the optical constants.

Acceleration of Crystallization Speed by Sn Addition to Ge–Sb–Te Phase-Change Recording Material

We have demonstrated that a quaternary Ge–Sn–Sb–Te phase-change recording material obtained by adding Sn to Ge–Sb–Te has a higher crystallization speed than Ge–Sb–Te, and gives a larger erase ratio than Ge–Sb–Te when film thickness is decreased. Static evaluations have shown that a 6-nm-thick quaternary material was crystallized by laser irradiation of 50 ns. Measurements carried out under the conditions of a wavelength of 405 nm, a linear speed of 8.6 m/s and a mark length of 0.294 µm showed that the erase ratio of over 30 dB was obtained with the new composition for a 6-nm-thick layer. A carrier-to-noise ratio (CNR) exceeding 50 dB was also obtained. We think that these effects of Sn addition which give rise to complete crystallization are brought about by abundant nucleation in the amorphous phase even in thin layers. It was confirmed by X-ray diffraction analyses that the new Ge–Sn–Sb–Te material has a single-phase-NaCl-type structure, like the conventional compositions of Ge–Sb–Te.

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.

Compact Disc Erasable (CD-E) with Ag–In–Sb–Te Phase-Change Recording Material

The compact disc erasable (CD-E) disc has been developed with Ag–In–Sb–Te phase-change recording material. The disc has been designed for recording at a speed twice that of conventional CD's. The CD-E disc has high modulation, high sensitivity, wide power margin after 1000 overwrite cycles and high reliability. It is readable using CD-ROM players and recordable using CD-R recorders with minimum modification.