L. P. Shi

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.

Universal HSPICE model for chalcogenide based phase change memory elements

We present a two terminal HSpice model for chalcogenide based phase change memory (CRAM) element. By including physical models of CRAM programming, this model can simulate not only the resistance change by different electrical pulses, but also temperature profile and crystalline fraction during the operation. Furthermore, it was successfully integrated with standard W/R circuit in memory technology. Output of sense amplifier vs writing current amplitude figure corresponded well with the typical R-I curve of CRAM elements.

Nonvolatile phase change memory nanocell fabrication by femtosecond laser writing assisted with near-field optical microscopy

The phase change memory cells were developed by using a combination system of a femtosecond laser with near-field scanning optical microscopy. The memory cells with feature size varying from 800nm down to 90nm were achieved. The cell functional performances were tested, and the scalability of the programming current as a function of the memory cell features was investigated. The optical near-field distance which is one of the critical factors to achieve high resolution nanostructures was studied experimentally with the consideration of the whole fabrication process for functional devices. The Bethe-Bouwkamp model was employed to study the effects of the optical near-field distance to the nanostructure geometry. The programming current of 0.8mA was observed for the memory nanocell at a feature size of 90nm.

Super-resolution near-field optical disk with an additional localized surface plasmon coupling layer

A structure of super-resolution near-field phase-change optical disk with localized surface plasmon coupling effect is proposed. A localized surface plasmon coupling layer (LSPCL) was introduced to form a coupled localized surface plasmons (CLSP) with the mask layer. Recording marks as small as 31 and 36 nm were observed in two structures, which were both much smaller than the smallest mark of 56 nm observed from the conventional one without LSPCL. CLSP not only can reduce mark size but it can also improve carrier-to-noise ratio of recording marks. The thermal stability of the disk was also studied.

Effect of metals and annealing on specific contact resistivity of GeTe/metal contacts

The circular transfer length method was employed to extract the specific contact resistivity, ρc of GeTe (amorphous and crystalline state) with metals (Ni, W, TiW) to quantify the series contact resistance. The ρc of amorphous-GeTe with metals was also determined for different annealing conditions. Higher metal work functions produce lower ρc for both GeTe states and the ρc was reduced further for annealing temperatures greater than the GeTe crystallization temperature. This is suggested to be a consequence of the higher temperature required to diffuse sufficient interstitial metallic atoms to transform the GeTe covalent bonding at the interface to metallic bonding.

Superlattice-like structure for phase change optical recording

In order to increase the crystallization speed and data transfer rate (DTR), a superlattice-like structure (SLL) was applied to the recording layer of phase change optical disks. Unlike the conventional phase change layer, the recording layer with the SLL structure consisted of alternating thin layers of two different phase change materials, i.e., GeTe and Sb2Te3. Although neither GeTe nor Sb2Te3 could be used as a phase change layer material for practical applications, present experimental results revealed that the phase change optical disk with the SLL structure demonstrated an excellent recording property that could meet practical recording requirements. X-ray photoelectron spectroscopy was employed to confirm that the SLL structure could be preserved after many times of melting and quenching. Dynamic properties of the optical recording disk with the SLL structure were investigated with a 1 T pulse duration of 8 ns and a constant linear velocity of 19 m/s. A clear eye pattern was observed. The carrier-...

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.

Blu-ray Type Super-Resolution Near-Field Phase Change Disk with In2Ge8Sb85Te5 Mask Layer

A new mask layer of In2Ge8Sb85Te5 was developed and used on super-resolution near-field phase change optical disks (super-RENS). The thermal and optical properties of the mask layer were investigated. Thermal analysis showed higher crystallization temperature and activation energy but lower melting temperature than Ge2Sb2Te5 phase change material. Ultra-fast crystallization within 30 ns was obtainable using a pulsed high-power laser beam of 405 nm wavelength. The recording performance of the new structure was evaluated on an optical recording system with a numerical aperture of 0.85 and a 405 nm blue-violet laser diode. Carrier-to-noise ratio (CNR) of 32, 25, 15, and 10 dB were obtained for 80, 50, 40, and 30 nm mark trains, respectively. Readout thermal stability of 20,000 cycles was realized for 80 nm mark trains. These performances were by far the highest obtainable for blu-ray aperture-type super-RENS 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.