P. K. Tan

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.

Study of the Partial Crystallization Properties of Phase-Change Optical Recording Disks

The partial crystallization properties of Ge1Sb2Te4 phase-change optical disks are studied using two methods. The first one involves annealing of the samples in a vacuum oven while controlling annealing time and temperature, while the second one involves the use of a static tester. A difference in reflectivity was observed, indicating that different crystallization fractions give rise to different reflection levels. The optical constants of amorphous, partial and full crystaline states were measured by spectroscopic ellipsometry. The optical constant of the partial crystalline state was calculated under the assumption that the partial crystalline state is a combination of full crystalline and amorphous states. The crystallization fraction was determined by simulating the refractive index of the partial crystalline state. The stability of the partially crystalized disk was measured for more than 200 days demonstrating that the partially crystallied disk is very stable. A possible recording strategy using the multilevel reflection to realize multi-level reflection modulation recording in write-once media is discussed.

Dependence of Optical Constants on Film Thickness of Phase-Change Media

The optical constants of phase-change films are assumed to be constant regardless of film thickness in conventional optical design and thermal simulation of multilayer structure optical disks. However this assumption is not valid when the phase-change film thickness in the optical disks becomes very small. In this study, the dependence of the optical constants on the thickness of Ge1Sb2Te4 phase-change films is investigated. The changes of the extinction coefficient k and the refractive index n (especially k) become significant when film thickness is in the range of 10 nm–30 nm, and become larger at shorter wavelengths such as the blue region. This dependence of optical constants (n and k) on the film thickness can be explained and confirmed based on the discontinuous film model of very thin film. These results are significant in improving the accuracy of optical design and thermal simulation of phase-change optical disks, as well as in the study of phase-change optical disks at shorter wavelengths.

Investigation on Super-Resolution Near-Field Blu-Ray-Type Phase-Change Optical Disk with Sb2Te3 Mask Layer

Super-resolution near-field phase-change Blu-ray-type optical disks with a mask layer of Sb2Te3 were studied theoretically and experimentally with a laser wavelength of 405 nm and a numerical aperture (NA) of 0.85. The modulation transfer function (MTF) was calculated and the thermal performance was simulated by the finite element method. Super-resolution near-field phase-change Blu-ray-type optical disks with different structures were fabricated and measured. It was shown that the thermal shield layer was helpful in reducing recorded mark size and improving readout stability. Recorded marks as small as 16 nm were observed. The relevant mechanism was discussed.

Optical Transition of Chalcogenide Phase-Change Thin Films

The optical transitions of as-deposited and thermally annealed chalcogenide phase-change thin films were studied. Allowed indirect transition occurs in Ge, GeTe, and Ge2Sb2.3Te5 and allowed direct transition in Sb, Sb2Te3, Ge1Sb1Te2, Ge1Sb2Te4, Ge1Sb4Te7, and Ge2Sb2Te5. For both amorphous and crystalline states, optical energy gap Egopt decreases with increasing film thickness due to the blue shift effect. Egopt of crystalline is ~0.5 eV lower than their amorphous counterparts. Egopt has a linear proportional relationship with the percentage of partial crystalline phases Rpc. The different kinds of optical transition and the decrease of Egopt between Ge2Sb2Te5 and Ge2Sb2.3Te5 may be attributed to the structural change, caused by the presence of the additional Sb as a structure modifier, which changes the interaction among the components of the Ge2Sb2Te5 system.

Study of the Multi-Level Reflection Modulation Recording for Phase Change Optical Disks

Multi-level reflection was obtained on rewritable phase change optical disks with a static tester using different laser amplitudes. Different levels of input power amplitude give rise to different reflection levels. In order to study the mechanism for forming the multi-level reflection, partial crystallization effect and size effect were studied. The partial crystallization effect was studied using differential scanning calorimeter (DSC) measurement and X-ray diffractometer (XRD) measurement. The size effect was studied by simulation and measurement. Based on the simulation and experimental results the combination of size effect and partial crystallization effect is proposed as the main reason to cause the multi-level reflection. This model can be used to analyse the writing, erasing, and overwriting processes of multi-level reflection modulation recording.

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.

New Additional Layer to Realize Initialization-Free Function for Digital Versatile Disk-Random Access Memory Disk

Digital versatile disk-random access memory (DVD-RAM) with the Sb2Te3 additional layer demonstrated the initialization-free function. The results of the reflectivity, X-ray diffraction (XRD) and differential scanning calorimeter (DSC) confirmed the initialization-free function. No significant decrease of the modulation amplitude after 1000 overwriting cycles was observed. A reduced time was required for complete erasure.

Study of the Superlattice-Like Phase Change Optical Recording Disks

Superlattice-like phase change optical disks, where the recording layer consists of alternating thin layers with two different phase change materials of GeTe and Sb2Te3, have been fabricated. The optical and thermal properties were simulated and measured. Samples were measured by X-ray diffraction (XRD) after annealing. The peaks of Ge2Sb2Te5 were observed, which indicates that Ge2Sb2Te5 was formed at the interface between GeTe and Sb2Te3. The recording, erasing and overwriting properties were investigated. The signal differentiation in writing, reading and erasing was observed at the pulse width of 7 ns, which indicates that the superlattice-like structure can effectively shorten the crystallization time. The overwriting cycle was measured using a static tester. Within 10000 times no significant change in the modulation amplitude was observed.