Eutectic Sb70Te30 Research Articles

Local structural environments of Ge doped in eutectic Sb-Te film before and after crystallization

Electrical phase change device using the Ge-doped eutectic Sb-Te (e.g., Ge1Sb8Te2) film is known to exhibit improved energy efficiency thanks to lowered threshold voltage as well as decreased power consumption for the reset operation, as compared with Ge2Sb2Te5 film. Ge K-edge EXAFS analysis is employed in this study in an effort to elucidate such merits of Ge1Sb8Te2 film in connection with its local atomic arrangements. It is then verified that a Ge atom is four-fold coordinated in its nearest-neighboring shell both in the as-deposited and in the annealed films. It needs to be highlighted that approximately two Sb atoms constitute the Ge tetrahedral units in its amorphous state; however, after being crystallized, heteropolar Ge-Sb bonds hardly exist in this Ge1Sb8Te2 film. It has been known that crystallization temperature and activation energy for crystallization of this Ge1Sb8Te2 composition are greater than those of Ge2Sb2Te5 composition. In addition, these two phase change materials exhibit distinctly different crystallization mechanisms, i.e., nucleation-dominant for Ge2Sb2Te5 film but growth-dominant for Ge1Sb8Te2 film. These discrepancies in the crystallization-related properties are delineated in terms of the local structural changes verified from the present EXAFS analysis.

Crystallization and Melting Kinetics of Se-Doped Fast-Growth Sb70Te30 Phase-Change Recording Films

Doping Se into the fast-growth eutectic Sb70Te30 recording film can improve the archival stability and the recording sensitivity, but the initialization process will become more difficult. When the concentration of Se was increased, the nucleation rate of the Se-doped Sb70Te30 recording films will increase. To achieve smooth switch between the amorphous and crystalline states and simultaneously obtain the maximum optical contrast, the optimum doping concentration of Se into Sb70Te30 recording film should be slightly less than 2.1 at. %.

Thickness dependence of crystallization and melting kinetics of eutectic Sb70Te30 phase change recording film

As the thickness of eutectic Sb70Te30 phase change recording film was reduced, the crystallization temperature and activation energy for crystallization would increase, while the melting temperature and activation energy for melting would decrease. Accordingly, the archival stability and recording sensitivity can be improved. However, the recording speed for direct over write will be slowed down. Based upon the Johnson–Mehl–Avrami equation, it was found that the increase of film thickness would increase the nucleation rate of eutectic Sb70Te30 recording film and make the crystallization process become more nucleation-dominated.

Ultrafast crystallization and thermal stability of In–Ge doped eutectic Sb70Te30 phase change material

Effect of In and Ge doping in the form of In2Ge8Sb85Te5 on optical and thermal properties of eutectic Sb70Te30 alloys was investigated. Crystalline structure of In2Ge8Sb85Te5 phase change material consists of a mixture of phases. Thermal analysis shows higher crystallization temperature and activation energy for crystallization. Isothermal reflectivity-time measurement shows a growth-dominated crystallization mechanism. Ultrafast crystallization speed of 30ns is realized upon irradiation by blue laser beam. The use of ultrafast and thermally stable In2Ge8Sb85Te5 phase change material as mask layer in aperture-type super-resolution near-field phase change disk is realized to increase the carrier-to-noise ratio and thermal stability.

Crystallization behavior of Sb70Te30 and Ag3In5Sb60Te32 chalcogenide materials for optical media applications

AbstractAddition of In and Ag into eutectic Sb70Te30 fast‐growth material was studied. We explored the optical transitions and crystallization kinetics of Sb70Te30 and Ag3In5Sb60Te32 recording films. The results showed that Ag3In5Sb60Te32 has a higher crystallization temperature but a similar melting point compared to Sb70Te30. In both the crystalline and amorphous states, addition of Ag and In increased the refractive index and decreased the extinction coefficient of Sb70Te30. Ag3In5Sb60Te32 has better thermal stability and a lower crystallization speed than Sb70Te30 film. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Ge addition on the optical and electrical properties of eutectic Sb 70Te 30 films

The aim of this work is to study the influence of Ge addition on the optical and electrical properties in eutectic SbTe thin films (with the compositions: Sb 70Te 30, Ge 2Sb 70Te 28, Ge 5Sb 70Te 25 and Ge 10Sb 65Te 25) using visible optical reflectance, ellipsometry measurements, near infrared transmittance spectra, and four probe electrical resistivity measurements. From near infrared transmittance measurements the optical band gap was determined using Tauc’s expression for amorphous materials, a value of about 0.47 eV was obtained without any clear dependence on the Ge content. All amorphous films have approximately the same reflectance value, however the contrast ratio between the crystalline and amorphous phases decrease with increase of Ge. Using in situ four probe measurements versus temperature the dependence of the activation energy of conductance and the onset of the crystallization temperature have been determined for different materials. Four probe measurements have shown that the resistivity of amorphous films increases with increase of Ge. The results obtained have shown that optical and electrical properties of SbTe films with near eutectic composition change with the Ge content and depending on the application, in either optical or electrical memory devices, the most suitable composition needs to be determined.

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.

Effects of Ag and In addition on the optical properties and crystallization kinetics of eutectic Sb70Te30 phase-change recording film

Adding specific foreign elements into the eutectic Sb70Te30 fast-growth material is expected to be an effective way to increase its thermal stability for blue laser recording. We have studied the effects of Ag and In addition on the optical properties and crystallization kinetics of the eutectic Sb70Te30 recording film. The results showed that the addition of Ag and In increased the refractive index and decreased the extinction coefficient of amorphous Sb70Te30 film and decreased both the refractive index and extinction coefficient of crystalline Sb70Te30 film. The archival stability of the eutectic SbTe alloy could be effectively improved by adding Ag and In elements, however, the addition of Ag and In also made the initialization of the as-deposited eutectic SbTe film more difficult. During the isothermal crystallization process, the incubation time was extended, crystallization speed was reduced, and the crystallization process became more grain-growth dominated as Ag and In were added. All these effects were further enhanced by increasing the concentration of In element.

Optical Properties and Crystallization Characteristics of Ge-Doped Sb70Te30 Phase Change Recording Film

Eutectic Sb70Te30 fast-growth material is one of the promising candidates for high data transfer rate recording. However, it also shows the disadvantage of poor thermal stability. Germanium, with a high value of glass transition temperature, is expected to play the role of thermal stabilizer. It was found that the effect of Ge doping on reflectivity difference between as-deposited and annealed states is not significant. However, the addition of Ge element into Sb70Te30 can reduce the initialization or erasure power, and increase the stability of the phase change recording media at room temperature at the same time. Unfortunately, doping Ge element would also slow down the crystallization speed of eutectic Sb70Te30 recording film.

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks.

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.

Recording Characteristics of Ge Doped Eutectic SbTe Phase Change Discs with Various Compositions and Its Potential for High Density Recording

We describe the recording characteristics of discs having Ge doped eutectic SbTe alloy recording films with various compositions. The recording characteristics are fairly sensitive on the film composition. For our recording condition, the Sb/Te ratio of around 4 and the Ge content of about 5 at.% were preferable to obtain good carrier to noise ratio (CNR). A disc with eutectic recording film of that composition is compared with one with a GeTe–Sb2Te3 pseudo-binary alloy recording film, varying the mark length and the track pitch. The eutectic disc showed better CNR for the shorter mark lengths than the pseudo-binary disc. The cross-erase ratio of the eutectic disc is substantially higher than that of the pseudo-binary disc. The result suggests that the eutectic disc is suitable for higher linear bit density and the pseudo-binary disc is suitable for higher track density.

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.