R. Zhao

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.

Enabling Universal Memory by Overcoming the Contradictory Speed and Stability Nature of Phase-Change Materials

The quest for universal memory is driving the rapid development of memories with superior all-round capabilities in non-volatility, high speed, high endurance and low power. Phase-change materials are highly promising in this respect. However, their contradictory speed and stability properties present a key challenge towards this ambition. We reveal that as the device size decreases, the phase-change mechanism changes from the material inherent crystallization mechanism (either nucleation- or growth-dominated), to the hetero-crystallization mechanism, which resulted in a significant increase in PCRAM speeds. Reducing the grain size can further increase the speed of phase-change. Such grain size effect on speed becomes increasingly significant at smaller device sizes. Together with the nano-thermal and electrical effects, fast phase-change, good stability and high endurance can be achieved. These findings lead to a feasible solution to achieve a universal memory.

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.

Role of Ge Switch in Phase Transition: Approach using Atomically Controlled GeTe/Sb2Te3 Superlattice

Germanium–antimony–tellurite (GST) is a very attractive material not only for rewritable optical media but also for realizing solid state devices. Recently, the study of the switching mechanism between the amorphous and crystal states has actively been carried out experimentally and theoretically. Now, the role of the flip-flop transition of a Ge atom in a distorted simple-cubic unit cell is the center of discussion. Turning our viewpoint towards a much wider region beyond a unit cell, we can understand that GeSbTe consists of two units: one is a Sb2Te3 layer and the other is a Ge2Te2 layer. On the based of this simple model, we fabricated the superlattice of GST alloys and estimated their thermal properties by differential scanning calorimetry (DSC). In this paper, we discuss the proof of the Ge switch on the basis of thermo-histories.

Band alignment between Ta2O5 and metals for resistive random access memory electrodes engineering

Band alignment of resistive random access memory (RRAM) switching material Ta2O5 and different metal electrode materials was examined using high-resolution X-ray photoelectron spectroscopy. Schottky and hole barrier heights at the interface between electrode and Ta2O5 were obtained, where the electrodes consist of materials with low to high work function (Φm,vac from 4.06 to 5.93 eV). Effective metal work functions were extracted to study the Fermi level pinning effect and to discuss the dominant conduction mechanism. An accurate band alignment between electrodes and Ta2O5 is obtained and can be used for RRAM electrode engineering and conduction mechanism study.

Ultrafast switching in nanoscale phase-change random access memory with superlattice-like structures

Phase-change random access memory cells with superlattice-like (SLL) GeTe/Sb(2)Te(3) were demonstrated to have excellent scaling performance in terms of switching speed and operating voltage. In this study, the correlations between the cell size, switching speed and operating voltage of the SLL cells were identified and investigated. We found that small SLL cells can achieve faster switching speed and lower operating voltage compared to the large SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved in the 40 nm SLL cells, respectively, both significantly faster than those observed in the Ge(2)Sb(2)Te(5) (GST) cells of the same cell size. 40 nm SLL cells were found to switch with low amorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which is much lower than the 1.6 V required by the GST cells of the same cell size. These effects can be attributed to the fast heterogeneous crystallization, low thermal conductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLL structure promises applications in high speed and low power memory devices.

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.

Superlatticelike dielectric as a thermal insulator for phase-change random access memory

Superlatticelike (SLL) dielectric comprising of Ge2Sb2Te5 and SiO2 was employed to reduce the power and increase the speed of phase-change random access memories (PCRAMs). In this study, we found that PCRAM cells with SLL dielectric require lower currents and shorter pulse-widths to switch compared to the cells with SiO2 dielectric. As the thickness of the SLL period reduces, the power and speed of the cells improved further due to the better thermal confinement of the SLL dielectric. Fast phase-change in 5 ns was observed in large cells of 1 μm, showing the effectiveness of SLL dielectric for advanced memory applications.

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.