Synergistic effect of bulk phase doping and layered-spinel structure formation on improving electrochemical performance of Li-Rich cathode material

In this study, La was doped into the lithium layer of Li-rich cathode material and formed a layered-spinel hetero-structure. The morphology, crystal structure, element valence and kinetics of lithium ion migration were studied by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The La doped lithium-rich cathode material exhibited similar initial discharge capacity of 262.8 mAh g^?1 at 0.1 C compared with the undoped material, but the discharge capacity retention rate can be obviously improved to 90% after 50 cycles at 1.0 C. Besides that, much better rate capability and Li⁺ diffusion coefficient were observed. The results revealed that La doping not only stabilized the material structure and reduced the Li/Ni mixing degree, but also induced the generation of spinel phase to provide three-dimensional diffusion channels for lithium ion migration. Moreover, the porous structure of the doped samples also contributed to the remarkable excellent electrochemical performance. All of these factors combined to significantly improve the electrochemical performance of the material.     

散射矩阵法研究弹性波在异质结声子晶体中的谐振

利用散射矩阵把弹性波在相邻介质层中的前向波和后向波联系起来。利用散射矩阵法计算一维完整和缺陷声子晶体的透射和反射特性,得到与传输矩阵法完全相同的结果。用该方法研究了弹性波在含两个子声子晶体:相同介质不同周期常数,不同介质相同周期常数的异质结声子晶体中的传输特性。结果表明:在异质结的声子晶体的带隙内出现强烈的局域谐振模,其数目由后一个声子晶体的周期数决定。     

Si/SiC-based DD hetero-structure IMPATTs as MM-wave power-source: a generalized large-signal analysis

A full-scale, self-consistent, non-linear, large-signal model of double-drift hetero-structure IMPATT diode with general doping profile is derived. This newly developed model, for the first time, has been used to analyze the large-signal characteristics of hexagonal SiC-based double-drift IMPATT diode. Considering the fabrication feasibility, the authors have studied the large-signal characteristics of Si/SiC-based hetero-structure devices. Under small-voltage modulation (～ 2%, i.e. small-signal conditions) results are in good agreement with calculations done using a linearised small-signal model. The large-signal values of the diode's negative conductance (5 × 10⁶S/m²), susceptance (10.4 × 10⁷ S/m^2}), average breakdown voltage (207.6 V), and power generating efficiency (15%, RF power: 25.0 W at 94 GHz) are obtained as a function of oscillation amplitude (50% of DC breakdown voltage) for a fixed average current density. The large-signal calculations exhibit power and efficiency saturation for large-signal (> 50%) voltage modulation and thereafter decrease gradually with further increasing voltage-modulation. This generalized large-signal formulation is applicable for all types of IMPATT structures with distributed and narrow avalanche zones. The simulator is made more realistic by incorporating the space-charge effects, realistic field and temperature dependent material parameters in Si and SiC. The electric field snap-shots and the large-signal impedance and admittance of the diode with current excitation are expressed in closed loop form. This study will act as a guide for researchers to fabricate a high-power Si/SiC-based IMPATT for possible application in high-power MM-wave communication systems.     

基于渐变组分体材料InGaAs的宽带长波长超辐射二极管

研制了一种新型的非选择性再生长掩埋异质结构长波长超辐射二极管(SLD).该器件采用渐变组分体材料InGaAs作为有源区,由金属有机物化学气相外延制备.150mA下,SLD发射谱宽的半高全宽为72nm,覆盖范围从1602到1674nm.发射谱光滑、平坦,光谱波纹在1550到1700nm的范围内小于0.3dB.室温连续工作,注入电流200mA下,器件获得了4.3mW的出光功率.器件适用于气体探测器和L-band光纤通信的光源.     

低维半导体异质结构光电探测材料及器件验证

面向国家"战略性先进电子材料"的发展目标,以国家重大战略需求,如全球气候观测、农林普查、国土资源探测、环境监测、深空探测和天文观测等领域的技术发展中面临的光探测器瓶颈问题为突破口,瞄准半导体材料及其光探测器正在朝全光谱覆盖、大面阵、高灵敏等方向快速发展的国际态势,开展低维半导体异质结构材料与光电器件研究,发展该体系材料人工设计精细调控的关键技术,推动多波段多种类型的光电探测器技术进步,促进我国经济、社会、国家安全及科学技术的发展.     

基于氧化锌的紫外探测器研究进展北大核心

氧化锌(ZnO)是一种天然的宽禁带半导体材料,其理论上的禁带宽度为3.37 eV,近年来已经成为制备紫外探测器件的热门材料之一。然而,由于ZnO材料的本征缺陷,直接制备的紫外探测器件总是存在响应率低、暗电流大、响应速度慢等问题。为了获得更好的紫外探测性能,各种可行的器件改善和修饰方法被提出。文章从元素掺杂、表面修饰和异质结构造等三个方面评述了提升ZnO紫外探测器件性能的典型方法,分析了这些方法存在的问题,并展望了未来高性能紫外探测器的发展方向。     

三种纵向结构的AlGaN/GaN HFET性能比较

对AlGaN/GaN HFET纵向的常规结构、倒置结构和双异质结进行了研究，结果表明：常规结构的材料生长简单、容易控制，倒置结构的直流性能低于常规结构，而双异质结虽然在材料生长方面较为复杂，但它可以获得较常规结构更为优良的直流特性.     

High-quality ZnO growth, doping, and polarization effect

The authors have reported their recent progress in the research field of ZnO materials as well as the corresponding global advance. Recent results regarding(1) the development of high-quality epitaxy techniques,(2) the defect physics and the Te/N co-doping mechanism for p-type conduction, and(3) the design, realization, and properties of the ZnMgO/ZnO hetero-structures have been shown and discussed. A complete technology of the growth of high-quality ZnO epi-films and nano-crystals has been developed. The co-doping of N plus an iso-valent element to oxygen has been found to be the most hopeful path to overcome the notorious p-type hurdle. High mobility electrons have been observed in low-dimensional structures utilizing the polarization of ZnMgO and ZnO. Very different properties as well as new physics of the electrons in 2DEG and 3DES have been found as compared to the electrons in the bulk.     

Effect of Lattice Mismatch on Luminescence of ZnO/Si Hetero-Structure

研究了ZnO薄膜中应力对发光的影响.实验样品为ZnO体单晶、在Si基片上直接生长的ZnO薄膜以及通过SiC过渡层在Si基片上生长的ZnO薄膜.测量了这三种样品的X射线衍射图形、喇曼光谱和光致发光光谱.由X射线衍射图形可以看出,由于SiC过渡层缓解了ZnO与Si之间的晶格失配,使得通过SiC过渡层在Si上生长的ZnO薄膜的结晶质量好于直接在Si上生长的ZnO薄膜的质量.进一步通过喇曼谱测量发现,与ZnO体单晶相比,直接在Si上生长的ZnO薄膜的E2(high)峰红移1.9cm-1,根据喇曼谱峰位移与应力的关系可以推出薄膜中存在0.4GPa的张应力;而通过SiC过渡层在Si上生长的ZnO薄膜的E2(high)峰红移0.9cm-1,对应着0.2GPa的张应力.对照X射线衍射图形的结果可以看出,薄膜中张应力的大小与薄膜的结晶质量密切相关,表明张应力来源于外延层和基片间的晶格失配,晶格失配越大,外延层中产生的张应力越大.有无SiC过渡层的两种薄膜样品的PL光谱中都存在紫外和绿光两种谱带,随样品热处理时氧气分压增加,两种样品都出现绿光增强的相似的变化规律,但有SiC过渡层的样品的变化幅度较小.这一结果说明,绿色发光中心与薄膜的质量,也就是与薄膜中存在的张应力大小有关.在以往研究中得出的非故意掺杂ZnO薄膜的绿色发光中心来源于氧反位缺陷(Ozn),文中研究的结果正好可以解释氧反位缺陷形成的原因.由于薄膜中存在张应力,使得样品的能量升高,其结果必然会产生缺陷来释放张应力,以便降低系统能量.而氧离子半径大于锌离子半径,氧替位锌有利于释放张应力,也就是说,在存在张应力的情况下,Ozn的形成能降低.这一结果进一步证明Si上生长的ZnO薄膜中的绿色发光中心与氧反位缺陷有关.     

Ag_3PO_4体系光催化材料的研究进展:基础与改性(英文)

Ag3PO4是一种高效的光催化剂,因而受到了极大的关注。其高光催化活性归功于其内禀电子结构。采用形貌控制和纳米复合体构筑的方法提高其性能和实用性。综述Ag3PO4单晶的晶体结构、性能及其基础理论,并用一些典型例子来说明提高其光催化活性的主要策略,即形貌控制和纳米复合体的构筑。