Optical Phase Change Materials Research Articles

Design principle of super resolution near-field structure using thermally responsive optical phase change materials for nanolithography applications

The super resolution near-field structure (Super-RENS) which is composed of a thin layer of a thermally responsive optical phase change material (PCM) between two dielectric layers, can be a means of resolving the limited resolution of the laser beam for direct laser lithography. In Super-RENS, incident laser irradiation induces the direct, reversible opening and closing of a nanoaperture in the PCM layer, and a nanoscale pattern is realized in the lithography system. Here, we first introduce the complete modeling procedures and optimization methodology for Super-RENS in nanolithography based on a rigorous analysis of near-field structure, thermal analysis in the finite-element method, and analysis of the corresponding feature size on the photoresist (PR) layer. Multiple combinations of the PCM layer and the two dielectric layers with varying dimensions are considered as design parameters to achieve the required resolution in the nanolithography system. The feasible line profiles are investigated at the general operating conditions of the pulsed laser beam, based on varying dimensions of the PCM layer (5–30nm) and the two dielectric layers (10–200nm). This work will provide a detailed methodology for the design and optimization of the Super-RENS for applications in the nanolithography system.

On the atomic structure of thin amorphous Ge-Sb-Te films

The structure of thin films of Ge2Sb2Te5 was studied by high-resolution transmission electron microscopy (HRTEM) and by X-ray diffraction (XRD). The as-evaporated films prepared by flash evaporation were amorphous. Nearly, all the atoms were randomly distributed but even in as-evaporated films small areas with some ordering were noticeable. The number and size of the ordered areas increased with thermal treatment of films to temperatures ∼80 °C, i.e., ∼70 °C below the crystallization temperature of amorphous Ge2Sb2Te5. After thermal treatment, small embryos of nucleation centers (size <8 nm) were formed. According to XRD measurements the films as a whole were still fully amorphous. The crystallite model of the structure of amorphous solids is then partly valid, at least for Ge2Sb2Te5. We expect that it can be applied also for other Ge–Sb–Te-based films, as well as for the other amorphous chalcogenides. The possible impact of the formation of nucleation “embryos” on storage time of data in optical and electrical phase-change memory materials and devices is discussed.

High-Temperature Complex Refractive Index of Phase Change Recording Medium of Ge2Sb2Te5 Determined Using Phase Change Static Tester and Spectroscopic Ellipsometer

We investigated the high-temperature optical properties of the optical phase change material Ge2Sb2Te5 (GST) using an in situ ellipsometer equipped with a convetional heating chamber and using a spectroscopic ellipsometer combined with a phase change static tester. In addition, we compared the results for the high-temperature complex refractive indices of GST thin films determined using a spectroscopic ellipsometer after the samples were crystallized by the in situ and a phase-change random access memory (PRAM) methods. The ellipsometric constants and the RMS value of the surface roughness of the GST samples were measured by in situ ellipsometry and atomic force microscopy (AFM), respectively. Results showed wide variations at high-temperatures when the GST samples were crystallized by the in situ method. The complex refractive indices of GST thin films were consistent at mid-range temperatures up to 370 °C for the two methods. In contrast, they showed a large variation at a higher temperature of ∼450 °C for the in situ method and remained stable in the entire temperature range for the PRAM method. This indicates that the optical properties of GST thin film are affected by the film structure and protective layer for the in situ method but not for the PRAM method at high-temperatures. It is concluded that the combined use of the spectroscopic ellipsometer with the phase change static tester gives us more precise and consistent results not affected by either the film structure nor the protective material.

Re-Writability and its Mechanism in DVD: Phase-Change Mechanism Chalcogenide Alloy between Amorphous and Crystal

In optical phase-change materials, especially, Ge2Sb2Te5 has widely been used as a heat of rewritable optical storage media. The chalcogenide material shows a good performance in write-erase cyclability and optical contrast: refractive index change between amorphous and crystal state. However, the high-speed switching mechanism has long been not well understood because of the difficulty of the amorphous analysis. In 2004, we gave one solution for the issue. Since then, a number of discussions and critics have raised up on the new model because the amorphous state in the model is not amorphous like. In this review, we want to give a clear solution to understand what is amorphous in chalcogenides and how different is the amorphous from normal glass amorphous.

Atomic Ordering and Gap Formation in Ag-Sb-Based Ternary Chalcogenides

Novel semiconductors with tailored properties can be designed theoretically based on our understanding of the interplay of atomic and electronic structures and the nature of the electronic states near the band-gap region. We discuss here the realization of this idea in Ag-Sb-based ternary chalcogenides, which are important optical phase change and thermoelectric materials. Based on our studies we propose new systems for high-performance thermoelectrics.

Thermal writing using a heated atomic force microscope tip

Resistive heating of an atomic force microscope tip was used to perform thermally induced surface modifications. Heating was achieved by dissipating power in the legs of an electrically conducting silicon cantilever. Temperatures of up to 170 °C were obtained using 40 mW of input power. Electrical measurements used to monitor the temperature showed thermal time constants of 0.35–0.45 ms, depending on whether the tip was in contact with a substrate. The heated tip was used to demonstrate thermomechanical writing on a polycarbonate substrate, as well as thermal writing of an optical phase change material.