Metal nanocrystal memory with high-/spl kappa/ tunneling barrier for improved data retention

A nonvolatile memory (NVM) with metal nanocrystal (NC) embedded in high-/spl kappa/ dielectrics is proposed. With the larger work function of the metal NC compared to that of silicon NC, the metal NC memory exhibits the better data retention characteristic. The theoretical analysis showing the effect of the electron barrier height on tunneling current density is also presented to support the importance of work function engineering of the NC in NVM structure. The other electrical characteristics such as the programming transient and data endurance are also studied and described in this paper.     

纳米晶浮栅存储器的模拟、制备和电学特性

介绍了在纳米晶浮栅存储器数据保持特性方面的研究工作,重点介绍了纳米晶材料的选择与制备和遂穿介质层工程。研究证明,金属纳米晶浮栅存储器比半导体纳米晶浮栅存储器具有更好的电荷保持特性。并且金属纳米晶制备方法简单,通过电子束蒸发热退火的方法就能够得到质量较好的金属纳米晶,密度约4×1011cm-2,纳米晶尺寸约6～7nm。实验证明,高介电常数隧穿介质能够明显改善浮栅存储器的电荷保持特性,所以在引入金属纳米晶和高介电常数遂穿介质之后,纳米晶浮栅存储器可能成为下一代非挥发性存储器的候选者。     

金属纳米晶快闪存储器研究进展

金属纳米晶具有态密度高、费米能级选择范围广以及无多维载流子限制效应等优越性,预示着金属纳米晶快闪存储器在下一代闪存器件中具有很好的应用前景。从金属纳米晶存储器的工作原理、纳米晶的制备方法、以及新型介质材料和电荷俘获层结构等方面,对金属纳米晶存储器近年来的研究进展进行了总结。     

Asymmetric electric field enhancement in nanocrystal memories

The electrostatic model for nanocrystal memories is used to illustrate the fundamental difference of the metal nanocrystal memory in low-voltage program/erase (P/E) operations in comparison with semiconductor nanocrystal and trap-based memories. Due to repulsion of potential contours inside conductors, the metal nanocrystals will significantly enhance the electric field between the nanocrystal and the sensing channel set up by the control gate bias and, hence, can achieve much higher efficiency in low-voltage P/E. On the other hand, the electric field originated from the stored charge will only be slightly different for metal and semiconductor nanocrystal cases. We presented the electrostatic models by both approximate analytical formulation and three-dimensional numerical simulation in a nanocrystal array. Operations of P/E and read disturbance were analyzed for the cases of homogeneous charge distribution, silicon, and metal nanocrystals. In the P/E condition of +5/-5 V, the metal nanocrystal memory offers around 1.6 times higher peak fields than Si counterparts and almost three times higher than that from the one-dimensional model for homogeneous charge distribution. The field enhancement factor suggests the design criteria of oxide thickness, nanocrystal size, and spacing. The advantage of asymmetric field enhancement of metal nanocrystals will be even more prominent when high-K gate dielectrics are employed.     

纳米晶非挥发性存储器研究进展

介绍了纳米晶非挥发性存储器的发展状况和基本工作原理,比较了纳米晶非挥发性存储器所涉及到的各种不同的电荷输运机制,系统介绍了纳米晶非挥发性存储器在纳米晶材料设计、纳米晶晶体生长控制方法、隧穿/控制介质层工程和新型存储器器件结构等方面的一些最新研究进展,对纳米晶非挥发性存储器的研究趋势进行了展望。     

Self-assembly of metal nanocrystals on ultrathin oxide for nonvolatile memory applications

The self-assembly of metal nanocrystals including Au, Ag, and Pt on ultrathin oxide for nonvolatile memory applications are investigated. The self-assembly of nanocrystals consists of metal evaporation and selective rapid-thermal annealing (RTA). By controlling process parameters, such as the thickness of the deposited film, the post-deposition annealing temperatures, and the substrate doping concentration, metal nanocrystals with density of 2–4 × 10¹¹ cm⁻², diameter less than 8.1 nm, and diameter deviation less than 1.7 nm can be obtained. Observation by scanning-transmission electron microscopy (STEM) and convergent-beam electron diffraction (CBED) shows that nanocrystals embedded in the oxide are nearly spherical and crystalline. Metal contamination of the Si/SiO₂ interface is negligible, as monitored by STEM, energy dispersive x-ray spectroscopy (EDX), and capacitance-voltage (C-V) measurements. The electrical characteristics of metal, nanocrystal nonvolatile memories also show advantages over semiconductor counterparts. Large memory windows shown by metal nanocrystal devices in C-V measurements demonstrate that the work functions of metal nanocrystals are related to the charge-storage capacity and retention time because of the deeper potential well in comparison with Si nanocrystals.     

Solvothermal Synthesis of Alloyed PtNi Colloidal Nanocrystal Clusters (CNCs) with Enhanced Catalytic Activity for Methanol Oxidation

Colloidal nanocrystal clusters (CNCs), which are composed of many nanocrystal subunits, have attracted intensive attention because they can possess not only the properties of each single subunit but also the collective properties and novel functionalities resulting from the ensembles. However, to date, the successful preparation of metal CNCs has rarely been reported. In this work, a simple one‐pot solvothermal method is developed to prepare PtNi‐alloyed CNCs with homogeneous distribution of Pt and Ni elements, porous feature, and interconnected steady framework. The PtNi CNCs are composed of many PtNi nanocrystals with size around 7–8 nm. Their growth mechanism is proposed through a systematic study. Thanks to the unique structure, the as‐prepared PtNi CNCs show better catalytic performance in methanol oxidation reaction than that of PtNi nanocrystals and Pt/C catalysts. This work is important because it not only provides a new method for the preparation of metal CNCs with desired morphology and properties, but also paves the way for practical applications of metal nanoparticles.     

金属纳米晶存储器件数据保持能力建模与验证

金属纳米晶存储器件具有低功耗、高速读写特性及较高的可靠性,因此近年来在非易失存储器研究领域备受关注.对比分析讨论了量子限制效应与库仑阻塞效应对金属纳米晶费密能级的影响后,发现库仑阻塞效应会严重削弱器件数据保持能力.在综合考虑金属纳米晶量子限制效应和库仑阻塞效应的基础上,提出了金属纳米晶存储器件数据保持能力分析模型,并通过与相关研究文献的实验数据对比分析,证实了本模型的合理性.     

Using different work function nanocrystal materials to improve the retention characteristics of nonvolatile memory devices

Recently, nanocrystal nonvolatile memory (NVM) devices have attracted great research interest. Taking into account the effect of work function to account for the better retention characteristics for nanocrystals with larger work function, utilizing different work functions Au, W and Si as floating gates is proposed and comparatively studied in this paper. It was found that Au nanocrystals have better retention characteristic than W and Si. The good retention characteristic of the Au nanocrystal device is due to the larger work function and it is difficult for electrons captured by Au nanocrystal to escape from them. So, the retention characteristic of the device can be improved by using larger work function nanocrystal materials.     

Microcavity light-emitting devices based on colloidal semiconductor nanocrystal quantum dots

This letter describes the design, fabrication, and characterization of a microcavity-electroluminescence (EL) device based on colloidal semiconductor nanocrystal quantum dots (NQDs). The device was fabricated by sandwiching a solution-cast film of light-emitting CdSe-CdS core-shell NQDs between two metal mirrors to form a resonant microcavity structure. We have observed a significant reduction in EL emission bandwidth from the fabricated device. Further improvement of the emission efficiency of the NQD-microcavity-EL devices can be achieved upon the minimization of the losses that are pertinent to metal mirrors.     

An integratable dual metal gate CMOS process using an ultrathin aluminum nitride buffer layer

We propose and demonstrate a novel approach for dual metal gate CMOS process integration through the use of a very thin aluminum nitride (AlN/sub x/) buffer layer between metal and gate oxide. This buffer layer prevents the gate oxide from being exposed to a metal etching process which potentially causes oxide thinning and damage. Subsequent annealing consumes the very thin AlN/sub x/ layer and converts it into a new metal alloy film by reacting with gate metals, resulting in no increase in EOT due to this buffer layer. The work function of the original gate metal is also modified as a result of its reaction with AlN/sub x/, making this approach extremely attractive for engineering the work function for dual metal gate CMOS applications.     

Metal nanocrystal memories-part II: electrical characteristics

This paper describes the electrical characteristics of the metal nanocrystal memory devices continued from the previous paper [see ibid., vol. 49, p. 1606-13, Sept. 2002]. Devices with Au, Ag, and Pt nanocrystals working in the F-N tunneling regime have been investigated and compared with Si nanocrystal memory devices. With hot-carrier injection such as the programming mechanism, retention time up to 10⁶ s has been observed and 2-bit-per-cell storage capability has been demonstrated and analyzed. The concern of the possible metal contamination is also addressed by current-voltage (I-V) and capacitance-voltage (C-V) characterizations. The extracted inversion layer mobility and minority carrier lifetime suggest that the substrate is free from metal contamination with continuous operations     

Polymorphism-Interface-Induced Work Function Regulating on Ru Nanocatalyst for Enhanced Alkaline Hydrogen Oxidation Reaction

Exploring high-performance Pt-free electrocatalysts for hydrogen oxidation reaction (HOR) in alkaline media is highly imperative for the development of alkaline polymer electrolyte fuel cells. Phase engineering is an effective strategy for boosting the catalytic performance of electrocatalysts; however, the fabrication of unconventional polymorphism-interfaced metal catalysts remains a significant challenge. In this study, a polymorphism-interfaced Ru nanocatalyst with a stable hexagonal close-packed (hcp) phase and a metastableface-centered-cubic (fcc) phase is successfully prepared. Owing to the built-in electric field and stacking fault on the unique polymorphic interface, the fcc-hcp-Ru catalyst exhibits outstanding alkaline HOR performance with a mass activity of 1016 A g_PGM^-1, which is six and three times higher than that of conventional hcp-Ru andcommercial Pt/C, respectively. The regulated electron distribution at the polymorphic interface is attributed to the discrepant work functions, which not only optimize the adsorption energy of hydrogen but also facilitate the water formation step to promote the alkaline HOR process. This study demonstrates that unconventional polymorphism-interfaced engineering is an efficient strategy to regulate the electronic structure of metal catalysts and identifies the prominent role of the work function in alkaline HORs, providing a new avenue for the rational design of highly efficient materials for electrocatalysis.     

Impact of high-κ dielectric and metal nanoparticles in simultaneous enhancement of programming speed and retention time of nano-flash memory

A methodology to simulate memory structures with metal nanocrystal islands embedded as floating gate in a high-κ dielectric material for simultaneous enhancement of programming speed and retention time is presented. The computational concept is based on a model for charge transport in nano-scaled structures presented earlier, where quantum mechanical tunneling is defined through the wave impedance that is analogous to the transmission line theory. The effects of substrate-tunnel dielectric conduction band offset and metal work function on the tunneling current that determines the programming speed and retention time is demonstrated. Simulation results confirm that a high-κ dielectric material can increase programming current due to its lower conduction band offset with the substrate and also can be effectively integrated with suitable embedded metal nanocrystals having high work function for efficient data retention. A nano-memory cell designed with silver (Ag) nanocrystals embedded in Al₂O₃ has been compared with similar structure consisting of Si nanocrystals in SiO₂ to validate the concept.     

声频工程适用标准规范解析(一)——音响扩声、公共广播等系统工程设计中技术标准规范适用性探讨

简要介绍了当前现行的一些国家和行业主管部委,针对声频工程制定的技术标准规范,并撰文说明在声频工程设计过程中,如何正确选择引用对口并适用的国家和行业,针对声频工程制定的技术标准规范,避免滥用和误用.真正将声频工程的规划、设计、施工、检测验收工作搞好.努力构建一套比较完美的声频工程系统,为使用场所和用户提供一个满意的,并符合实际应用需要的声频工程项目.     

Inside Front Cover: Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly (Adv. Funct. Mater. 7/2006)

The synthesis of three‐dimensionally ordered, transparent gold‐nanocrystal (NC)/silica superlattice thin films using the self‐assembly (by spin‐coating) of water‐soluble gold nanocrystal micelles and soluble silica is reported by Fan and co‐workers on p. 891. The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

"电子产品设计"课程教学的重要内容

从工程设计实际出发,指出了现行"电子产品设计"课程教学中容易被忽视的重要内容——工程设计控制程序,结合标准分析了设计控制程序的各项内容,包括工作程序、设计输入控制、设计过程控制、设计输出控制、设计更改控制、技术服务和记录,并明确了具体的内在要求,对设计课程的适用性和实用性,以及提高学生电子产品设计能力具有较重要作用和意义。     

低散射目标支撑金属支架的外形参数优化

低散射金属支架用于电磁散射测量中的目标支撑,是隐身飞行器设计制造过程中的重要装备。合适的支架外形可以降低自身电磁散射及变形,对于减小测量误差十分重要。通过理论计算、软件仿真、紧缩场测量的方法,从尖劈边缘绕射和行波散射的角度分析了金属支架外形参数对支架散射的影响机理,并考虑了支架变形带来的背景对消误差。在理论分析基础上,通过建立三维参数模型利用FEKO软件优化的方法,获得了工作在C频段和X频段下金属支架的最优散射倾角和截面形状,并比较了其力学变形的优劣,综合考虑散射性能和力学性能得到优化结果。提出了60°的最优倾角和70°的重载目标用倾角,以及一个后部长度占全长26.9%的非对称卵形优化截面,该截面形状可均衡降低支架垂直极化和水平极化方向的电磁散射。给出了倾角变化下尖劈绕射和行波散射变化趋势不同决定了存在优化倾角的结论,和支架前劈夹角和照明区截面长度尽量同时减小的低散射截面设计原则。研究成果对低散射金属支架设计工程化和降低电磁散射测量误差具有重要意义。     

Engineering Perovskite Nanocrystal Surface Termination for Light‐Emitting Diodes with External Quantum Efficiency Exceeding 15%

Hybrid organic–inorganic metal halide perovskites are particularly promising for light‐emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide‐based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5–10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole‐free films. As a result of time‐resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.     

太赫兹Si/SiGe量子级联激光器波导模拟

波导层结构设计是制备太赫兹(THz)量子级联激光器的关键问题之一.本文基于德鲁得(Drude)模型,利用时域有限差分(FDTD)法,对Si/SiGe量子级联激光器的波导层进行优化设计,从理论上对传统的递变折射率波导、单面金属波导、双面金属波导以及金属/金属硅化物波导横磁模(TM模)的模式损耗和光场限制因子进行了对比分析.结果表明,金属/金属硅化物波导不但可以减小波导损耗,而且有很高的光学限制因子,同时其工艺也比双面金属波导容易实现,为Si/SiGe太赫兹量子级联激光器波导层的设计提供了一定的理论指导.