Wu Liangcai

Memory characteristics of an MOS capacitor structure with double-layer semiconductor and metal heterogeneous nanocrystals

An MOS (metal oxide semiconductor) capacitor structure with double-layer heterogeneous nanocrystals consisting of semiconductor and metal embedded in a gate oxide for nonvolatile memory applications has been fabricated and characterized. By combining vacuum electron-beam co-evaporated Si nanocrystals and self-assembled Ni nanocrystals in a SiO2 matrix, an MOS capacitor with double-layer heterogeneous nanocrystals can have larger charge storage capacity and improved retention characteristics compared to one with single-layer nanocrystals. The upper metal nanocrystals as an additional charge trap layer enable the direct tunneling mechanism to enhance the flat voltage shift and prolong the retention time.

Germanium Nitride as a Buffer Layer for Phase Change Memory

The results of adhesion improvement and SET-RESET operation voltage reduction for the GeN buffer layer are presented. It is found that the adhesive strength between the Ge2Sb2Te5(GST) layer and the layer below could be increased at least 20 times, which is beneficial for solving the phase change material peeling issue in the fabrication process of phase change memory (PCM). Meanwhile, the RESET voltage of the PCM cell with a 3-nm-thick GeN buffer layer can be reduced from 3.5 V to 2.2 V. The GeN buffer layer will play an important role in high density and low power consumption PCM applications.

Si3.5Sb2Te3 Phase Change Material for Low-Power Phase Change Memory Application

Novel Si3.5Sb2Te3 phase change material for phase change memory is prepared by sputtering of Si and Sb2Te3 alloy targets. Crystalline Si3.5Sb2Te3 is a stable composite material consisting of amorphous Si and crystalline Sb2Te3, without separated Te phase. The thermally stable Si3.5Sb2Te3 material has data retention ability (10 years at 412K) better than that of the Ge2Sb2Te5 material (10 years at 383 K). Phase change memory device based on Si3.5Sb2Te3 is successfully fabricated, showing low power consumption. Up to 2.2 × 107 cycles of endurance have been achieved with a resistance ratio lager than 300.

Ge1Sb2Te4 Based Chalcogenide Random Access Memory Array Fabricated by 0.18-μm CMOS Technology

Ge1Sb2Te4-based chalcogenide random access memory array, with a tungsten heating electrode of 260 nm in diameter, is fabricated by 0.18-μm CMOS technology. Electrical performance of the device, as well as physical and electrical properties of Ge1Sb2Te4 thin film, is characterized. SET and RESET programming currents are 1.6 and 4.1 mA, respectively, when pulse width is 100 ns. Both the values are larger than those of the Ge2Sb2Te5-based ones with the same structure and contact size. Endurance up to 106 cycles with a resistance ratio of about 100 has been achieved.

Total dose radiation tolerance of phase change memory cell with GeSbTe alloy

Phase change memory (PCM) cell array is fabricated by a standard complementary metal–oxide–semiconductor process and the subsequent special fabrication technique. A chalcogenide Ge2Sb2Te5 film in thickness 50 nm deposited by rf magnetron sputtering is used as storage medium for the PCM cell. Large snap-back effect is observed in current–voltage characteristics, indicating the phase transition from an amorphous state (higher resistance state) to the crystalline state (lower resistance state). The resistance of amorphous state is two orders of magnitude larger than that of the crystalline state from the resistance measurement, and the threshold current needed for phase transition of our fabricated PCM cell array is very low (only several μA). An x-ray total dose radiation test is carried out on the PCM cell array and the results show that this kind of PCM cell has excellent total dose radiation tolerance with total dose up to 2×106 rad(Si), which makes it attractive for space-based applications.

Direct Tunneling and Storage of Electrons in Ni Nanocrystals Embedded within MOS Structure

Recently, nanocrystal nonvolatile memory (NVM) devices using nanocrystals (NCs) have attracted great research interest. In this work, we investigated the feasibility of Ni nanocrystals embedded in metal oxide semiconductor (MOS) capacitor structure for NV

Remarkable Resistance Change in Plasma Oxidized TiOx/TiNx Film for Memory Application

We report the experimental phenomenon of large resistance change in plasma oxidized TiOx/TiNx film fabricated on W bottom-electrode-contact (W-BEC) array. The W-BEC in diameter 260?nm is fabricated by a 0.18??m CMOS technology, and the TiOx/TiNx cell array is formed by rf magnetron sputtering and reactive ion etching. In current?voltage (I?V) measurement for current-sweeping mode, large snap-back of voltage is observed, which indicates that the sample changes from high-resistance state (HRS) to low-resistance state (LRS). In the I?V measurement for voltage-sweeping mode, large current collapse is observed, which indicates that the sample changes from LRS to HRS. The current difference between HRS and LRS is about two orders. The threshold current and voltage for the resistance change is about 5.0?10?5?A and 2.5?V, respectively. The pulse voltage can also change the resistance and the pulse time is as shorter as 30?ns for the resistance change. These properties of TiOx/TiNx film are comparable to that of conventional phase-change material, which makes it possible for RRAM application.

Chalcogenide Random Access Memory Cell with Structure of W Sub-Microtube Heater Electrode

In order to reduce the reset current of chalcogenide random access memory, a W sub-microtube heater electrode with outer/inner diameter of 260/100 nm, which was fabricated with standard 0.18-μm technology, is proposed for the first time to achieve a reset current of about 0.5 mA. The reasons may be that sub-microtube increases the number of electrode edge and thermal efficiency is improved greatly because the thermal density on the edge of sub-microtube electrode is generally the highest.

From octahedral structure motif to sub-nanosecond phase transitions in phase change materials for data storage

Phase change random access memory (PCRAM) has been successfully applied in the computer storage architecture, as storage class memory, to bridge the performance gap between DRAM and Flashbased solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatible with CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfully developed 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems. 3D Xpoint (3D PCRAM), invented by Intel and Micron, has been regarded as a new breakthrough in the last 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology. This technology has some remarkable features, such as the confined device structure with 20 nm size, the metal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic threshold switching selector that can provide a high drive current and a low leakage current. A good understanding of phase change mechanism is of great help to design new phase change materials with fast operation speed, low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAM and different understandings on phase change mechanisms in recent years, and then propose a new view on the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structure motifs are generally found in both amorphous and crystalline phase change materials. They are considered to be the basic units during phase transition, which are severely defective in the amorphous phase. These configurations turn into more ordered ones after minor local rearrangements, the growth of which results in the crystallization of rocksalt (RS) phase with a large amount of vacancies in the cation sites. Further driven by thermodynamic driving force, these vacancies move and layer along certain directions; consequently, the metastable RS structure transforms into the stable hexagonal (HEX) structure. Based on our results, we find that reversible phase transition between amorphous phase and RS phase, without further changing into HEX phase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacancies in both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessary space, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.

Study of phase change materials for phasechange random access memory

Phase change memory technology, which is regarded as one of the most promising candidates for the next generation non-volatile memory technology, has achieved rapid development in the past ten years. Meanwhile, related products based on this technology have been put into the market and mass production has come true. With the development of phase change memory technology, the fundamental research has become a hot topic in the fields of information, materials, and so on. Phase change storage medium based on chalcogenide is the basis and core of phase change memory. The performance of phase change memory is determined by phase change material’s performance. In this paper, the industrialization status of phase change memory are briefly introduced firstly, then the research progress of the commonly used GeSbTe phase change materials and the main phase transition mechanism is summarized. Finally, the C doping modification of the traditional GeSbTe and the phase change mechanism of C-doped GeSbTe materials is analyzed.