Phase-change Media Research Articles

Nucleation of AgInSbTe films employed in phase-change media

In phase-change technology small volumes of a chalcogenide material are switched between amorphous and crystalline states by local heating with a short laser or current pulses. AgInSbTe is an alloy frequently used in optical data storage, which could also be applied in electronic data storage. For those applications it is crucial to understand the reliability and reproducibility of the switching process. In this work the first crystallization of an AgInSbTe alloy has been studied on a microsecond time scale using a focused laser beam. The experiments show that nucleation is a process governed by statistics. A correlation between the success of a nucleation event with the probability of nucleation is established. By measuring the nucleation probability as a function of laser pulse duration, the incubation time is determined to 11μs. The results are compared to measurements of the growth velocity of this material. The analysis of the temperature dependence of the growth velocity explains why AgInSbTe shows growth-dominated recrystallization. The implications of these findings to the application of such growth-dominated materials in electronic data storage are discussed.

An analytical model for nanoscale electrothermal probe recording on phase-change media

Scanning probe memories are now emerging as a means of achieving nanoscale resolution data storage. The use of microscopic conductive tips in contact with a phase-change material to record data as amorphous and crystalline marks is one such approach, making use of the large difference in electrical conductivity between the two phases to distinguish between two binary states on replay and hence provide a memory function. The writing process is complex and involves electronic, thermal, and phase-change processes that are difficult to model and study except using numerical techniques. A simplified analytical model of electrothermal writing by probe on a basic two-layer phase-change structure is developed here, and used to predict the required voltage levels for recording and the expected diameters of recorded crystalline and amorphous marks. A simplified model of cooling and solidification was also developed to study the cooling rates during amorphization. The predictions are shown to be in agreement with published experimental measurements and numerical simulations. The developed analytical models were extended to investigate the effects of introducing coating layers on recording voltage levels, to study the depth profiles of recorded marks, and to derive expressions for the capacitance and resistance of the phase-change layer that contribute to the transient behavior of the recording system.

Enhanced Readout Signal of Elliptic-Bubble Super Resolution Near Field Structure by Temperature-Dependent Complex Refractive Index of Phase-Change Medium

We propose a model elucidating the readout mechanism of the super resolution near-field structure (super-RENS) with an elliptic-bubble structure. The temperature dependence of the complex refractive index of Ge2Sb2Te5 is determined by in situ ellipsometry. The complex refractive index of Ge2Sb2Te5 shows quite an abrupt change at approximately 608°C. The temperature profile is calculated using a proposed first-order differential equation. Above the threshold readout power, Ge2Sb2Te5 is locally heated over the critical temperature when thickest region of Ge2Sb2Te5 is in commensurate with the center position of the Gaussian laser power distribution, which results in the abrupt amplification of signal-to-noise ratio (SNR) of the calculated reflectance. Thus, a model revealing the essential aspect of the elliptic-bubble super-RENS is proposed to elucidate its readout mechanism.

Terabit-per-square-inch data storage using phase-change media and scanning electrical nanoprobes

A theoretical study of the write, read, and erase processes in electrical scanning probe storage on phase-change media is presented. Electrical, thermal, and phase-transformation mechanisms are considered to produce a physically realistic description of this new approach to ultrahigh-density data storage. Models developed are applied to the design of a suitable storage layer stack with the necessary electrical, thermal, and tribological properties to support recorded bits of nanometric scale. The detailed structure of nanoscale crystalline and amorphous bits is also predicted. For an optimized trilayer stack comprising Ge/sub 2/Sb/sub 2/Te/sub 5/ sandwiched by amorphous or diamond-like carbon layers, crystalline bits were roughly trapezoidal in shape while amorphous bits were semi-ellipsoidal. In both cases, the energy required to write individual bits was very low (of the order of a few hundred picoJoules). Amorphous marks could be directly overwritten (erased), but crystalline bits could not. Readout performance was investigated by calculating the readout current as the tip scanned over isolated bits and bit patterns of increasing density. The highest readout contrast was generated by isolated crystalline bits in an amorphous matrix, but the narrowest readout pulses arose from isolated amorphous marks in a crystalline background. To assess the ultimate density capability of electrical probe recording the role of write-induced intersymbol interference and the thermodynamic stability of nanoscale marks were also studied.

Study of optical recording bits by scanning surface potential microscopy

We demonstrate a convenient method for the determination of recording bits in phase-change media based on scanning surface potential microscopy using a conductive tip. Commercially available digital versatile disks(DVD)±rewritable(RW) with initialization process are measured in experiments. The minimum size of the recording bits of DVD+RW, written by a commercial recorder, is around 425.22 nm. The mean work function (WF) value of the recording layer is found to be 4.27 eV from the contact potential difference images. After writing by a read∕write disk tester, the mean WF value of the amorphous bits has a drop of 15.46 meV. The results indicate that surface electronic energy of recorded regions is raised due to the writing process.

A slope-theory approach to electrical probe recording on phase-change media

A theoretical approach to predicting the spatial extent of the amorphous to crystalline transition region during the probe recording process on phase-change storage media is presented. The extent of this transition region determines the ultimate achievable linear density for data storage using phase-change materials. The approach has parallels with the slope theory used to find magnetic transition lengths in magnetic recording, and shows that the amorphous to crystalline transition length can be minimized by reducing the thickness of the phase-change layer, by minimizing lateral heat flow, and by maximizing the ratio of the activation energy for crystallization to the transition temperature Ec∕Tt.

Why Phase-Change Media Are Fast and Stable: A New Approach to an Old Problem

Present-day multimedia strongly relies on re-writable phase-change optical memories. We find that, different from current consensus Ge2Sb2Te5 (GST), the material of choice in digital versatile discs–random access memory (DVD-RAM), possesses a structure similar to ferroelectric GeTe, namely that Ge and Sb atoms are located off-center giving rise to a net dipole moment. Amorphisation of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in the short-range order. We demonstrate that the order-disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions cycles.

Enhanced Raman scattering for temperature measurement of a laser-heated atomic force microscope tip

Illuminating a silicon cantilever of an atomic force microscope with a focused laser beam creates heat that can be funneled into a nanoscale area at the apex of its tip. To characterize the heating dynamics and measure the temperature of the tip, a Raman scattering pump-and-probe method is used. It is found that at the apex of the tip the intensity of the Raman Stokes and anti-Stokes components are significantly enhanced relative to those obtained on a bulk silicon sample. Modeling the temperature rise at the tip of the cantilever by a closed-form analytical expression gives good agreement with the Raman measurements. This model can be used to design the structure of the cantilever so that the heat delivery to its tip is optimized. Such an optimized cantilever can potentially be used in high-density, heat-assisted magnetic recording, optical data storage using phase-change media and thermomechanical recording systems, for example, where nanoscale heated regions are of importance.

A Te-free phase change media with wider recording range

A new phase-change material with free tellurium element is introduced to the recording layer in the rewritable media. The recording speed of this phase-change media can extend from the DVD 1/spl times/ (3.5 m/s) to 8/spl times/ (28.0 m/s), which is promising to have the backward compatibility. The 2T writing strategy is conducted for this media indicating that the recording material is sensitive to the recording laser power. The media jitter values recorded after 1000 cyclic recordings and the simulated shelf and archived life in the media are reported.

Phase-change recording medium that enables ultrahigh-density electron-beam data storage

An ultrahigh-density electron-beam-based data storage medium is described that consists of a diode formed by growing an InSe/GaSe phase-change bilayer film epitaxially on silicon. Bits are recorded as amorphous regions in the InSe layer and are detected via the current induced in the diode by a scanned electron beam. This signal current is modulated by differences in the electrical properties of the amorphous and crystalline states. The success of this recording scheme results from the remarkable ability of layered III-VI materials, such as InSe, to maintain useful electrical properties at their surfaces after repeated cycles of amorphization and recrystallization.

Electrical probe storage using Joule heating in phase change media

We demonstrate the feasibility of ultrahigh-density probe recording in chalcogenide-based phase change media. The multilayer medium has been optimized with respect to electrical, thermal, and tribological requirements. Design of the multilayer takes into account the nonlinear dependence of the electrical properties of the phase change layer with respect to both electrical field and temperature. Memory dots as small as 15nm have been written and read repeatably. Data storage density of greater than Tbit∕in.2 density has been successfully achieved.

Numerical modeling of the subwavelength phase-change recording using an apertureless scanning near-field optical microscope

The electromagnetic field enhancement (FE) at the end of the probe of an Apertureless Scanning Near-field Optical Microscope (ASNOM) is used to write nanometric dots in a phase-change medium. The FE acts as a heat source that allows the transition from amorphous to crystalline phase in a Ge2Sb2Te5 layer. Through the 2D Finite Element Method (FEM) we predict the size of the dot as a function of both the illumination duration and the incoming power density. Numerical results are found to be in good agreement with preliminary experimental data.

Media Technologies of 20 GB Single-Layer Rewritable Phase-Change Disc for HD DVD System

Sufficient signal amplitude, erase ratio and low amount of cross-erase must be attained to achieve higher recording capacity of rewritable phase-change media. We have developed new media technologies, such as the GeTe-rich GeTe-Sb2Te3 pseudobinary alloy recording film partially substituted with bismuth and an additional layer to control thermal diffusion. The recording layer material enables a high erase ratio with a high signal amplitude. The additional layer also reduces the amount of cross-erase while maintaining a sufficient erase ratio. We have successfully demonstrated rewritable phase-change media having 20 GB capacity by using these media technologies for the HD DVD system with a blue-violet laser diode having the wavelength of 405 nm, a numerical aperture of the objective lens of 0.65 and light incident on a 0.6-mm-thick substrate. In this paper, we describe our new media technologies and the evaluation results, such as tilt margins, uniformity, overwriting cyclability and life durability.

Thermal simulation for a two-dimensional near-field optical recording system using a vertical-cavity surface-emitting laser

An optical recording method with high throughput is required to create a two-dimensional near-field optical memory system using a vertical-cavity surface-emitting laser (VCSEL). Optical recording is possible with the combination of a near-field probe and patterned medium. A patterned medium consisting of 40 nm periodic dots with phase change medium and silver nanoparticles were used for plasmon resonance induced by an electromagnetic wave. The electric field power density at the silver nanoparticles was increased about 87 000 times over that with a structure without patterned medium. Heat from this structure is effectively used by thermal conduction out of a silver nanoparticle and by concentrating the heat on a particle. When the 1 mW optical intensity from a VCSEL is used, near-field optical recording is achieved. The recorded feature size was estimated at 40 nm. In this letter, a recording method is described using plasmon resonance and thermal conduction effects.

Local characterization and transformation of phase-change media by scanning thermal probes

Scanning thermal microscopy is applied to investigate the local thermal properties of thin phase-change (Ge2Sb2Te5) layers. The thermal contrast allows crystalline and amorphous areas to be detected unambiguously, and can be quantitatively linked to the crystalline ratio of the phase change layer. It is demonstrated that the heated tip can be used to locally crystallize parts of an amorphous Ge2Sb2Te5 layer, thereby suggesting thermal probes may be used for data storage applications on phase-change media (write once).

Surface Plasmon Enhancement Effect and Its Application to Near-Field Optical Recordin

We studied on enhanced photon-tunneling and the near-field enhancement effect for a nano-scale aperture with a concentric surface plasmon resonance structure. About two-orders larger light transmission and an enhanced near-field were achieved by optimizing the grating structures. This enhanced near -field was applied to optical recording, and about 100 -nm recorded patterns were successfully made on phase change media. This surface plasmon optics would open up the ultra-high-density recording technology.

Multilevel optical data recording methods on phase-change media

Multilevel data storage (ML) is a new method in the optical storage field, which is also a trend for improving the capability of future optical discs. This article introduces several ML methods based on phase-change media including pit depth modulation (PDM) and mark radial width modulation (MRWM). In addition, some disadvantages and advantages concerning the principle of these methods will be discussed. Finally, a new ML method will be advanced, through which the levels in one recording pit will be increased evidently.

Advanced Phase Change Media for Blue Laser Recording of 18 GB Capacity for 0.65 Numerical Aperture and 30 GB Capacity for 0.85 Numerical Aperture

We attempted to improve phase change media to achieve larger capacity for systems with blue lasers using numerical apertures (NA) of both 0.85 and 0.65. It was found that the Bi-added GeTe-rich pseudo-binary composition promised high erase ratio even when we did not use any interface layers on a recording layer. The use of an absorption control layer (ACL) was also attempted to suppress the cross erase (XE). It was found that the ACL was very effective in suppressing the XE. The advanced phase change media that used the above novel technologies showed good analog performance. The advanced media also showed bER of less than 1.0×10-4, promising 18 GB capacity for an NA of 0.65 and 30 GB capacity for an NA of 0.85.

Optimization of Write Strategy in a Partial-Response-Maximum-Likelihood System for High-Density Recording

It is difficult to optimize the write strategy by conventional methods in high-density recording systems with very severe intersymbol interference. We have proposed a new method using the index accounting for the linearity of the channel, which is very important in the partial-response-maximum-likelihood detection scheme. It has been confirmed that the minimum bit error rate can be obtained when the linearity of the channel becomes maximum. We have shown the feasibility of a recording capacity of over 20 GB with 120-mm-diameter phase-change media using the blue-violet laser diode with an objective lens whose numerical aperture is 0.65.

Development of Organic Recording Media for Blue High Numerical Aperture Optical Disc System

Organic write once media is developed for the Blu-ray disc format, namely, a system comprising a blue wavelength laser diode of 405 nm and a high numerical aperture (NA) lens of 0.85. The first generation Blu-ray disc format employs phase change media. Thus, the organic write once media was designed with a reflectivity modulation scheme in order to retain compatibility. A very simple layer structure is proposed, SiO2/Dye/SiO2 without any metal reflective layer, and the media showed sufficient modulation amplitude and signal quality. The optical constants before and after the recording were measured to clarify the recording mechanism, and the result matched well with the readout waveform in the optical disc system. The measured jitter level after random pattern recording at a density of 23.3 GB per disc was 6.7% using a limit equalizer, which satisfies the Blu-ray disc format.