X. S. Miao

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.

Subwavelength and super-resolution nondiffraction beam

A light spot that is smaller than a half wavelength will subsequently diverge in all directions. In this letter, the authors model a subwavelength (0.42λ) super-resolution light beam which propagates over a long distance without any divergence. This can be achieved by placing a multibelt pure-phase-type binary optical element on the lens pupil. The authors also report a useful approach for designing the optical element, based on vector diffraction theory, which can be used in paraxial and nonparaxial focusing and imaging systems.

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.

Phase change behaviors of Sn-doped Ge–Sb–Te material

Sn-doped Ge–Sb–Te material was prepared by laser synthesis. It has a rocksalt crystal structure for Sn doping content less than 30at.%. A phase change temperature tester was developed to in situ measure crystallization temperature and melting point of Sn-doped Ge–Sb–Te. The crystalliza-tion temperature of Sn-doped Ge–Sb–Te is close to that of Ge2Sb2Te5 while its melting point is much lower than that of Ge2Sb2Te5. The melting points of Sn9.8Ge20.3Sb28.4Te41.5 and Sn18.8Ge19.5Sb25.3Te36.4 are 475 and 450°C, respectively. The crystallization speed was tested by an ultraviolet light at pulse duration of 30ns. It exhibits a high crystallization speed.

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.

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.

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.

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.

Dynamics of Ultrafast Crystallization in As-Deposited Ge2Sb2Te5 Films

Femtosecond laser-induced ultrafast crystallization in 80 nm as-deposited Ge2Sb2Te5 films has been investigated by time-resolved microscopy. With an average fluence of approximately 10 mJ/cm2, a transient nonequilibrium state of the excited material was formed within 2 ps. The results can be interpreted as an electronically induced nonthermal phase transition.