J. M. Li

Phase change random access memory cell with superlattice-like structure

A superlattice-like structure (SLL) incorporating two nonpromising phase change materials was applied to phase change random access memory (PCRAM) cell. A properly designed SLL structure could balance both the phase change speed and stability of a PCRAM. Moreover, SLL PCRAM cells exhibited lower programming current and fast working time of 5ns. The main reason for the excellent performances is due to the much lower thermal conductivity of the SLL material compared to that of bulk materials. The thermal conductivity of eight SLL layers cycle was found to be smaller than 30% of that of single layer material.

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.

Properties and Reactive Sputtering Parameters of GeN Film for High-Density Phase-Change Optical Disk

The dependence of optical and thermal properties of phase-change optical disks on the diffusion of sulfur atoms from ZnS–SiO2 dielectric layers into a GeSbTe phase-change recording layer was simulated. The influence of reactive sputtering parameters on the properties of GeN films has also been studied. Significant improvement in the direct overwriting cycles was achieved in the 4.7 GB digital versatile disk-random access memory (DVD-RAM) disk with a GeN interface layer after optimizing the sputtering parameters of the GeN film.

A New Structure of Super-Resolution Near-Field Phase-Change Optical Disk with a Sb2Te3 Mask Layer

A super-resolution near-field phase-change optical disk with a new mask layer of Sb2Te3 was studied. The disks were fabricated and characterized. The problem of thermal stability for the conventional Super-RENS (Super-Resolution Near-field Structure) was studied for the first time. In order to solve the problem of poor thermal stability, two new structures with a thermal shield layer that can significantly improve thermal stability were proposed, fabricated, and analyzed. Recording marks as small as 56 nm were observed.

Investigation on mechanism of aperture-type super-resolution near-field optical disk

The mechanism of the aperture-type Blu-ray super-resolution near-field phase change optical disk with a mask layer of Sb2Te3 was studied theoretically and experimentally. Optical and thermal simulations were carried out to study the relationships of carrier-to-noise ratio (CNR) with reading speed and power. The theoretical prediction results were in good agreement with the experimental results. It was found that a thermal shield layer (TSL) with a high thermal conductivity, which is inserted into the dielectric layer sandwiched by a mask layer and a phase change layer, can significantly enhance readout thermal stability for the super-resolution near-field structure (super-RENS) optical disk.

Thermal Deformation Analysis of High-Density Optical Disks

The laser-beam spot size and track pitch of phase change optical disks are continuously decreasing. Hence any slight deformation in the disk will affect data storage performance efficiency. Thus said thermal deformation at high temperatures induced by laser irradiation becomes an important issue in optical disks. Thermal elastic deformation in blue-laser optical disks has been calculated by the finite element method (FEM). The associations of thermal deformation with disk structures and laser power have been investigated to supply methods to decrease thermal deformation. It was found that the peak temperature and peak deformation lies in different layers. Several methods of preventing deformation in blue-laser optical disks have been proposed. It was found that these methods can be used to optimize the structures of high density phase change optical disks.

Integrated Thermal and Optical Analyses of Phase-Change Optical Disk

An integrated analysis system based on thin film optics, thermal transfer and electromagnetics is developed. Thermal conductivity and generated heat are discussed for mark formation. The simulations of multilayer calorific sources and multibeam heating sources using the finite element method (FEM) are investigated. The readout of nanometer-scaled marks based on computational electromagnetics using the finite-difference time-domain (FDTD) analysis is discussed. The real marks captured with microscopes can be analyzed using this integrated analysis system combined with digital image technology. Material models in the electromagnetic vector method are discussed with reference to different layers of disk. It provides a powerful tool for structure design and failure analysis of phase-change optical disks.

Superlattice-Like Structure for High Recording Speed Phase Change Optical Discs

A recording performance comparison was conducted over a disc rotation speed range from 6.0 to 18.9 m/s using a dynamic tester between the disc with a superlattice-like structure (SLL) and conventional discs of Ge1Sb2Te4 and Ge1Sb4Te7. The recording layer of the SLL disc consisted of alternating thin layers of two different phase change materials, i.e., GeTe and Sb2Te3. The SLL disc demonstrated a better recording performance at high speed than the Ge1Sb2Te4 and Ge1Sb4Te7 discs in terms of jitter, erasability and durability. A distinct eye pattern was observed even after 100000 write/erase cycles. The carrier-to-noise ratio (CNR) above 50 dB and a data transfer rate of 47 Mbit/s were achieved at a rotation speed of 18.9 m/s. The excellent recording properties of the SLL disc were attributed to the reduced thermal conductivity and the favorable layout of the SLL structure for crystallization.

Local Thermal Expansion in Super-Resolution Near-Field Structure

Investigation focuses on the local strain–stress field in the laser-beam-spot region in super resolution near-field structure. The three-dimensional finite element method is used to analyze the elastic thermal expansion in the readout process. The temperature distribution around the laser beam in a large laser power margin has been studied, and it may be related to the second phase transition. The investigation has revealed that the center of the laser beam is always ahead of the centers of the peak-temperature, peak-stress, and peak-strain regions so that the stress and strain are very different around the center of the laser beam. This may lead to the change of the optical properties. The super-resolution mechanism caused by the local thermal expansion is discussed.

Integrated Analysis and Design of Phase-Change Random Access Memory (PCRAM) Cells

An integrated software for analysis and design of PCRAM cells has been developed. The research focuses on the discussion on electric-thermal -mechanical analyses. The software involves in the materials, geometrical and layer structure design and electric pulse strategy. It aims to provide a powerful tool for structure optimization and failure analysis of PCRAM cells.