Baowei Qiao

Si–Sb–Te films for phase-change random access memory

Electrical properties and crystallization behaviour of Si–Sb–Te films were studied and compared with those of a Ge2Sb2Te5 film. The resistivity ratio of Si–Sb–Te films can reach 106 during the amorphous–crystalline transition, accompanied with a small thickness change (less than 3%). The melting temperature of Si–Sb–Te films is lower than that of a Ge2Sb2Te5 film. Furthermore, the switching characteristics of the device using a Si10.7Sb39.5Te49.8 film were also studied. The device can be successfully switched with a 4 V, 100 ns pulse for SET operation and a 5 V, 40 ns pulse for RESET operation. A small RESET current and low power consumption can be obtained, which may be attributed to the low melting temperature and the high crystalline resistivity of Si–Sb–Te films. These results indicate the feasibility of Si–Sb–Te films in the application of phase-change random access memory.

Design of Multi-States Storage Medium for Phase Change Memory

A method has been introduced to realize multi-states storage in phase change memory (PCM) by using stacked phase change films with different amorphous resistivity. The amorphous resistivity of each phase change layer was selected to make the stacked films crystallize successively with the increase of current, and so obtain four resistance states. The thickness of each phase change layer was also optimized to ensure enough resistance difference between neighboring storage state. The thermal conduction simulation was performed, and the results indicate that the stacked Si–Sb–Te films with suitable composition, resistivity and thickness in each layer can crystallize successively with the increase of current and realize multi-states storage in PCM. A memory cell with stacked film structure (Si16Sb33Te51/Si4Sb46Te50/Si11Sb39Te50) was prepared. The film thicknesses of Si16Sb33Te51, Si4Sb46Te50 and Si11Sb39Te50 were optimized to be 40, 80, and 20 nm, respectively. The current–voltage (I–V) characteristics and the resistance–current (R–I) characteristics were measured. The results confirmed the four-state storage capability of the memory cell with this stacked film structure.

Effects of Si Doping on Phase Transition of Ge2Sb2Te5 Films by in situ Resistance Measurements

The amorphous-to-crystal transition has been studied by in-situ resistance measurements for Ge2Sb2Te5 (GST) thin films doped with silicon (Si) or nitrogen (N). It was found that the electrical properties and thermal stability of GST films can be improved by doping small amount of Si in the GST film. In comparison with Si doping, the N doping effects have also been studied and it was revealed that both the Si- and N-doping can increase the electrical conduction activation energy and enhance thermal stability of amorphous GST.

A nano-scale-sized 3D element for phase change memories

A nano-scale-sized 3D phase change memory element has been successfully fabricated using novel metal sidewall pattern technology, based entirely on a CMOS process which is independent of lithographic size limitations. With Ge2Sb2Te5 (GST) as the phase change material, a 280 nm × 35 nm contact area cell (equal to 100 nm contact diameter) has been successfully fabricated by 5 µm university lithographic techniques. The effects of annealing temperature on the GST resistivity and crystal structure have been studied. Static mode switching (dc tests) and pulse mode tests were performed, and an on/off ratio greater than 100 has been achieved.

Nano-crystalline phase change memory with composite Si-Sb-Te film for better data retention and lower operation current

Si-Sb-Te (SST) system materials were investigated for non-volatile phase change memory application. By contrast to the conventional GST films, the SST films exhibit stronger thermal stability and smaller RESET current, which were resulted from the self-confine and self-heat mechanisms by the amorphous Si-rich regions surrounding the phase change crystalline in SST. It shows a good promise for non-volatile memory applications.

Characterization of Ge2Sb2Te5 thin film transistor and its application in non-volatile memory

Abstract With the increasing requirement of high density memory technology, a new cell structure—1TR has received much attention. It consists of a single thin film transistor (TFT) with chalcogenide Ge 2 Sb 2 Te 5 as the channel material. In order to evaluate the feasibility of its application in the field of non-volatile memory, we take a further step in researching on the characteristics of GST-TFT. We fabricated a back-gate GST-TFT and investigated the output and transfer characteristics of its two states. The experimental results show that gate voltage can modulate the GST channel currents in both the amorphous and the crystalline states. Based on the experiments, we can expect that this novel device can ultimately lead to a new nonvolatile memory technology with even higher storage density.

THE PERFORMANCE OF Ge 2 Sb 2 Te 5 MATERIAL FOR PCRAM DEVICE

ABSTRACT The performance of Nonvolatile phase-change-memory material Ge2Sb2Te5 and nitrogen doped Ge2Sb2Te5 as well as its device cell was investigated. The effects of the annealing temperature on the resistivity and crystal structure of the Ge2Sb2Te5 were studied. The Hall measurement shows the hole mobility decreased with the changing of film thickness from 10 nm. to 100 nm The GST resistivity can be increased effectively by doping nitrogen and the decrease of the film thickness. The high resistive GST is indispensable to minimize threshold voltage of PCRAM.

Some solutions for writing current reduction and high density application of phase change memory

High writing current is the bottle-neck of PCM application and efforts focus on merely materials doping to improve crystal resistivity, novel structure to decrease active area etc. Here some solutions of our group are shown, including Si doped GST, novel 2D and 3D memory cell structure and multistate storage