Toshio Fukaya

Interfacial phase-change memory

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

Ferroelectric catastrophe: beyond nanometre-scale optical resolution

The optical diffraction limit is rigidly determined as a simple equation of wavelength ? and lens numerical aperture NA (): ?/2/NA. In this paper, we report that Ag5.8In4.4Sb61.0Te28.8 and Ge2Sb2Te5 chalcogenide thin films, which are typical of optical recording materials used in digital versatile discs (DVDs), enable a resolution of under ?/10 due to their ferroelectric properties. In the Ag5.8In4.4Sb61.0Te28.8 film it was found that this optical super-resolution can be observed between?350 and 400??C, resulting in a second phase transition from a hexagonal (A7 belonging to ) to a rhombohedral structure of R32 or R3m. In Ge2Sb2Te5, on the other hand, the temperature range is much wider, between?250 and 450??C, which is also due to a second phase transition from a NaCl-type fcc to a hexagonal structure.

Optical switching property of a light-induced pinhole in antimony thin film

Optical near-field recording, called a super-resolution near-field structure, records and retrieves small marks beyond the diffraction limit. A thin layer of an antimony (Sb) film, added to the usual phase-change optical disk, is the key material of this technique. Nonlinear optical properties of an Sb film, especially optical switching, were studied in the stationary state using a nanosecond pulse laser. Clear switching was observed under microscopic measurement.

The Near-Field Super-Resolution Properties of an Antimony Thin Film

A multilayered super-resolution near-field structure (Super-RENS) disk with the order of SiN/Sb/SiN/GeSbTe/SiN was produced on a polycarbonate substrate. The readout resolution properties of small phase change marks recorded through an antimony thin film were investigated. The linear and radial densities have been carefully examined, and it was found that the recorded phase change marks with a size of 0.15 µm were mostly circle. Two different signal frequencies of 7.0 and 7.2 MHz were recorded on one land area between grooves for tracking by offsetting the tracking to the right and left from the center. The width of the land area was 0.6 µm. The recording linear velocity was 3.0 m/s. In the reading, both signals were detected at the center of the tracking. However, when the tracking was offset at the positions of ±0.2 µm from the center, each signal was clearly separated without crosstalk owing to an Sb Super-RENS.

Sputtered silver oxide layers for surface-enhanced Raman spectroscopy

We present results of reactively sputtered silver oxide thin films as a substrate material for surface-enhanced Raman spectroscopy (SERS). Herein, we show that deposited layers develop an increasingly strong SERS activity upon photoactivation at 488 nm. A benzoic acid/2-propanol solution was used to demonstrate that the bonding of molecules to SERS active sites at the surface can be followed by investigating temporal changes of the corresponding Raman intensities. Furthermore, the laser-induced structural changes in the silver oxide layers lead to a fluctuating SERS activity at high laser intensities which also affects the spectral features of amorphous carbon impurities.

On a thermally induced readout mechanism in super-resolution optical disks

We have simultaneously measured the carrier-to-noise ratio (CNR) as well as the transmitted and reflected light intensities from platinum oxide based super-resolution near-field structure (PtOx super-RENS) disks. The the reflected and transmitted light intensities were found to decrease and increase, respectively, as the CNR value increased. The phase-change material AgInSbTe (AIST) used in PtOx super-RENS disks was found to exhibit a strong optical nonlinearity with respect to readout laser power. AIST becomes transparent at higher laser powers. To ascertain whether the presence of Pt nanoparticles is important to the readout mechanism, a super-RENS disk was fabricated in which the PtOx layer was replaced with a metal-free phthalocyanine (H2Pc) layer and the CNR of the H2Pc disk was measured. From the observation that the CNR value was equivalent to that of a disk made using PtOx, we conclude that the presence of nanoparticles does not play an important role in the super-RENS readout mechanism. Finally, ...

Measurements of temperature dependence of optical and thermal properties of optical disk materials

We have experimentally determined the temperature dependences of the refractive index and thermal conductivity for several optical disk materials from room temperature to 200–300°C. A novel system for obtaining thermal conductivity in the films is also introduced. As a demonstration, temperature simulation inside a simple-structured disk with and without considerating the temperature dependence of the refractive index at a 405 nm wavelength was carried out, and the difference was approximately 15°C at maximum temperature.

Micro-optical nonlinearity of a silver oxide layer

We studied the near-field formation mechanism of a silver oxide thin film sandwiched between protective layers using a pump probe with nanosecond laser pulse irradiation and tapping-mode tuning-fork near-field scanning optical microscopy. A multilayer composed of ZnS–SiO2/Ag2O/ZnS–SiO2 showed abnormal optical nonlinear properties compared to ordinary third-order nonlinear materials. The Ag2O layer played a role in generating a strong light scattering center in a reversible chemical reaction in only a small area of the multilayer, and the optical near field produced about the scattering center was 40 times stronger than that produced by an antimony layer instead of Ag2O.

Super-Resolution Near-Field Structure and Signal Enhancement by Surface Plasmons

We describe surface plasmon enhancement by near-field interaction with Ag-scattering centers. By inserting mark trains between two Ag-scattering centers, the near-field signals were further enhanced in a super-resolution near-field structure. Observing the enhancement in more detail, it was found that surface plasmons are generated along the mark trains and are dispersed in a range of 1 µm along the marks and 40–50 nm normal to the multilayer stack.

Non-melting super-resolution near-field apertures in Sb–Te alloys

The super-resolution near-field structure permits the formation of short-lived subdiffraction limit apertures that channel electromagnetic energy at the aperture boundary. This effect is commonly observed when a laser is focused onto a thin film of Sb based material. The aperture formation within Sb, Sb2Te3, Sb2Te, and SbTe is investigated by time resolved optical pump-probe techniques and found to occur without melting. Ab initio modeling has shown a threshold-like change in the optical properties below the melting temperature which leads to the formation of a near-field aperture. This threshold is shown to be a consequence of thermally induced misalignment of p-type bonding.