Ferroelectric Thin Films for Room Temperature Tunable Microwave Devices--The Current Status and Future

Ferroelectric have been considered as good candidates for room temperature tunable microwave elements for wireless communication devices. Much effort has been done in the past decade and many excellent results have achieved in highly epitaxial ferroelectric (Ba,Sr)TiO_3 thin films and related materials. However, high dielectric insertion loss and soft mode effects at high frequency have limited the practical applications of these ferroelectric materials. Various techniques have been adopted to improve the h...     

应变诱导外延Ba0.6Sr0.4TiO3薄膜铁电介电性能的频率依赖特性

基于修正的Landau-Devonshire唯象理论和Landau-Khalatnikov方程，研究了不同失配应变下外延Ba_0.6Sr_0.4TiO₃薄膜相稳定性和频率依赖的铁电介电性能。结果表明:室温下铁电c相、正交aa相和顺电相为稳态相且均为二级相变；在0.1 kHz的外加电场频率下，压应变增加了c相的面外剩余极化强度(P_r)和矫顽场(E_c)，而拉应变增加了aa相的面内剩余极化强度和矫顽场。此外，随着频率的增加，相界处P_r和E_c均增加，介电常数呈先快速后缓慢的下降趋势，最大调谐率逐渐减小且相应失配应变逐渐趋于零，这主要归因于频率的增加导致相变温度(ΔT_c)升高。进一步计算显示，面外的失配应变移动、调谐率降低和ΔT_c分别为0.017%、10.6%、7.3 ℃，其与面内结果(0.019%，9.7%，7.7 ℃)相近。     

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Materials in which magnetic and electric order coexist--termed 'multiferroics' or 'magnetoelectrics'--have recently become the focus of much research. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments but here is found to impede long-range order of the structural degrees of freedom that drive the formation of the ferroelectric state.     

Superconductivity in compressed lithium at 20 K

Superconductivity at high temperatures is expected in elements with low atomic numbers, based in part on conventional BCS (Bardeen-Cooper-Schrieffer) theory. For example, it has been predicted that when hydrogen is compressed to its dense metallic phase (at pressures exceeding 400 GPa), it will become superconducting with a transition temperature above room temperature. Such pressures are difficult to produce in a laboratory setting, so the predictions are not easily confirmed. Under normal conditions lithium is the lightest metal of all the elements, and may become superconducting at lower pressures; a tentative observation of a superconducting transition in Li has been previously reported. Here we show that Li becomes superconducting at pressures greater than 30 GPa, with a pressure-dependent transition temperature (T(c)) of 20 K at 48 GPa. This is the highest observed T(c) of any element; it confirms the expectation that elements with low atomic numbers will have high transition temperatures, and suggests that metallic hydrogen will have a very high T(c). Our results confirm that the earlier tentative claim of superconductivity in Li was correct.     

ESD对微波半导体器件损伤的物理机理分析

为了得到电磁脉冲对微波半导体器件的损伤规律,进而研究器件的静电放电损伤机理,首先对半导体器件静电放电的失效模式即明显失效和潜在性失效进行了介绍;其次分析了器件ESD损伤模型;最后通过对器件烧毁的物理机理进行分析,得到器件在静电放电应力下内在损伤原因。在ESD电磁脉冲作用下,器件会产生击穿效应,使内部电流密度、电场强度增大,导致温度升高,最终造成微波半导体器件的烧毁。     

钙钛矿La0.67Pb0.33MnO3薄膜的制备及表征

利用磁控溅射法在单晶LaAlO3(100)衬底上成功的外延生长了La0.67Pb0.33MnO3薄膜。用X射线衍射、原子力和超导量子干涉仪、振动样品磁强计对其进行了表征。结果表明,薄膜为赝立方钙钛矿结构,晶胞参数为a=3.861 nm,具有良好的单晶外延结构和光滑的表面。居里温度TC=345 K,在居里温度附近,发生铁磁-顺磁转变。此材料呈现出一种典型的自旋玻璃特性,是由于应力造成的。在室温条件下,当H=0.8T时,磁电阻效应非常明显,此现象是由于固有磁电阻效应引起的,并不是低场磁电阻效应引起的。室温下,其矫顽力只有50奥斯特。     

Properties of PbLa（Zr,Sn, Ti）O3 ceramics near ferroelectric-antiferroelectric phase boundary

Shirane found in the middle of the 20th century that the solid solution PZT had a lot of fine characteristics and carried out detailed researches[1]. The critical phase transition field of antiferroelectric decreased by adding titanate. Titanate expands t…     

Ultrasensitive solution-cast quantum dot photodetectors

Solution-processed electronic and optoelectronic devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 10(3) A W(-1). The best devices exhibited a normalized detectivity D* of 1.8 x 10(13) jones (1 jones = 1 cm Hz(1/2) W(-1)) at 1.3 microm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 10(12) jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 10(11) jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.     

Fabrication of MgB_2 superconducting films by different methods

MgB2 superconducting films have been successfully fabricated on single crystal MgO(111) and c-AL2O3 substrates by different methods. The film deposited by pulsed laser deposition is c-axis oriented with zero resistance transition temperature of 38.4 K, while the other two films fabricated by chemical vapor deposition and electrophoresis are c-axis textured with the zero resistance transition temperature of 38 K and 39 K, respectively. Magnetization hysteresis measurements yield critical current density Jc of 107 A/cm2 at 15 K in zero field for the thin film and of 105 A/cm2 for the thick film. For the thin film deposited by chemical vapor deposition, the microwave surface resistance at 10 K is found to be as low as 100μΩ, which is comparable with that of a high-quality high-temperature superconducting thin film of YBCO.     

微波等离子体电子回旋共振放电粒子模拟分析

电子回旋共振放电产生的等离子体在微电子工业中材料加工、空间电推进方面有着广泛的应用。为了研究微波等离子体电子回旋共振的放电特性,使电子回旋共振放电产生的等离子体密度和能量转换效率更高,建立了微波等离子体电子回旋共振放电的1D3V模型,描述了带电粒子在外加静磁场、微波场共同作用下的微观运动。结果表明:微波频率为2.45 GHz时,随着静磁场磁感应强度的增加,平均电子能量先持续增大达到峰值,随后又不断地减小,且在0.087 5 T时电子加速效果最明显,结果符合电子的回旋频率公式,验证了该模型的正确性;共振区域内,发现在0.087 5 T磁感应强度下,微波频率为2.45 GHz下拟合的电子速度分布才与微波电场分布趋势相似,说明微波电场推动了电子运动。这为进一步研究微波等离子体放电的粒子模拟-蒙特卡罗碰撞模拟奠定了基础,也为进一步研究微波等离子体源中粒子产生效率及微波等离子体源的物理性质提供了重要参考。     

铁电相变的简谐子软模图像

在自由边界条件下计算了立方钛酸钡有限尺寸晶体中原子的简谐振动模 ,发现许多简谐子软模。用这些软模花样说明了晶体冷却时发生具有a畴和c畴结构的铁电相变。理论表明铁电相变过程涉及屏蔽电荷的激发及其在界面的缓慢扩散 ,以最后得出各个电畴内部的均匀自发极化。铁电相变过程的这些细节 ,都得到了实验的有力证明     

Ni-Al底电极对偏氟乙烯-三氟乙烯共聚物铁电薄膜性能的影响

利用磁控溅射法,在Si(100)衬底上制备了不同厚度的非晶导电薄膜Ni-Al底电极,并采用直滴法、退火工艺和掩膜技术,首先制备了偏氟乙烯-三氟乙烯P(VDF-TrFE)共聚物铁电薄膜,并构架了Al/P(VDF-TrFE)/Ni-Al/Si铁电电容器异质结.采用X线衍射仪(XRD)、铁电测试仪(Precision LC unit)等测试手段对薄膜的性能进行了表征.结果表明:Ni-Al薄膜厚度对偏氟乙烯-三氟乙烯共聚物薄膜的漏电流产生较大影响,当厚度为36 nm时,其漏电流密度达到2.09×10-5A/cm2;所构架的Al/P(VDF-TrFE)/Ni-Al/Si电容器呈现2种漏电机理,在较低的电场范围内,电容器的导电机理为欧姆导电机理,在高电场下为界面肖特基导电机理.     

Cu_2O薄膜的电化学制备及EPIR效应

以FTO玻片为基底,在不同pH值下采用恒电位电化学沉积法制备了Cu2O薄膜样品和Cu/Cu2O/Cu/FTO器件.通过XRD、SEM、EDS对样品的相组成、晶体结构、微观形貌和化学成分进行了表征和分析,并对Cu/Cu2O/Cu/FTO器件的电脉冲诱导电阻转变(EPIR)效应进行了测量.结果表明室温下Cu/Cu2O/Cu/FTO中存在明显的EPIR效应和忆阻器行为且与溶液酸碱性有关.在酸性和中性条件下,即pH=5、6、7时,Cu/Cu2O/Cu/FTO存在显著的EPIR效应,随着pH值的增加效应趋于减弱,当脉冲电压为6 V,脉冲宽度为0.001 s时,样品具有最大EPIR值.随pH进一步增加,在pH=8、9、10的碱性条件下,Cu/Cu2O/Cu/FTO的EPIR效应消失.     

高次模碳纤维微波加热装置的设计与实验

为了使T300碳纤维达到微波石墨化的反应温度,需要对其进行微波加热实验。设计了一种基于TM_(110)高次横磁波模式的碳纤维微波加热装置,该装置是一种圆柱形谐振腔,腔体两侧分别存在一个轴向电场最大值区域,理论上可以同时加热多束碳纤维。为了保证碳纤维的加热效率,在设计时需要同时兼顾腔体的谐振频率和阻抗匹配特性。使用仿真软件优化了耦合点的具体位置,并对优化后的圆柱形谐振腔进行了多物理场的耦合仿真。仿真结果表明:出口处碳纤维的表面温度最高,且靠近耦合点的左侧碳纤维温度高于右侧。实验结果与仿真结果较为吻合,验证了碳纤维微波加热的可行性和装置的有效性。     

非理想表面层对一级相变铁电薄膜性质的影响

应用Ginzburg-Landau-Devonshire(GLD) 理论研究了 非理想表面对夹持在两个金属电极间的一级铁电薄膜性质的影响. 结果表明, 非理想表面层的存在降低了一级铁电薄膜的铁电相变转变温度, 当两个表面层不对称时, 铁电薄膜的电滞回线失去了中心对称性.     

B替位Ca离子对BaTiO_3铁电相变的影响

用X和射线衍射、透射电镜、扫描电镜、介电性能与电导平衡等方法研究了B位Ca离子对BaTiO_3;陶瓷铁电相变的影响.X射线衍射数据表明,随着B位Ca离子含量的增加(直至5%)c/a比下降至接近1.从介电系数的温度关系可见:含5%B位与6%A位Ca离子的组份其居里峰展宽且下降至室温.发现了当含A位Ca离子(约5%)时,B位Ca离子含量的变化对降低居里点有较显著的作用.据此结果,本文发展了一套热力学模型以解释Ca掺杂BaTiO_3中B位Ca离子对居里峰移动与展宽的作用.     

Ferroelectricity of weak-polar organic molecules in alternate Langmuir-Blodgett multilayer films

Pyroelectric material is a polar solid and has been interested since the 1960s. Today it is one of important materials used in infrared detectors[1]. With the development of the devices, the focus point on the materials applied has been transferred from bulk materials to film ones. Ferroelectric material is a kind of pyroelectric materials which spontaneous polarization can be reversed with the direction change of external electric field. Observation of ferroelectric hysteresis loop or not cou…     

Thermally fluctuating superconductors in two dimensions

In many two-dimensional superconducting systems, such as Josephson-junction arrays, granular superconducting films, and the high-temperature superconductors, it appears that the electrons bind into Cooper pairs below a pairing temperature (T(P)) that is well above the Kosterlitz-Thouless temperature (T(KT)) the temperature below which there is long-range superconducting order). The electron dynamics at temperatures between T(KT) and T(P) involve a complex interplay of thermal and quantum fluctuations, for which no quantitative theory exists. Here we report numerical results for this region, by exploiting its proximity to a T = 0 superconductor-insulator quantum phase transition. This quantum critical point need not be experimentally accessible for our results to apply. We characterize the static, thermodynamic properties by a single dimensionless parameter, gamma(T). Quantitative and universal results are obtained for the frequency dependence of the conductivity, which are dependent only upon gamma(T) and fundamental constants of nature.     

梯度铁电薄膜热释电性质的理论研究

为了探讨提高铁电薄膜热释电性质的理论途径,建立组分梯度铁电薄膜的理论模型,在热力学唯像理论的框架下,引入一个分布函数描述组分梯度的特性,通过改变参数a、外电场的强度及方向等因素,重点研究了组分梯度铁电薄膜的热释电性质.研究表明:薄膜内各组分的梯度分布导致了极化强度的梯度分布;参数a和所施加外电场的情况是影响铁电薄膜性质的两个重要因素,对改变热释电材料的工作温区和探测灵敏度都有显著影响,该论文的研究为铁电薄膜热释电器件的制备及实际应用提供了理论参考依据.     

Phenomenon of multiple field-induced phase transition in PLZST antiferroelectric ceramics

The multiple field-induced phase transition in 4 at.% La modified Pb(Zr,Sn,Ti)O 3 family with temperature from -40℃ to 45℃ in reported. Two electric field-induced transitions from a metastable antiferroelectric phase to two ferroelectric phases are observed is polarization at the applied field of 4 MV/m. The critical field of phase transition between two ferroelectric phases is not larger than 2.5 MV/m, about ten to twenty percent of that ever found in PZT based ceramics. Lattice structure is shown to be orthorhombic by X-ray diffraction. Dielectric investigation reveals a relaxor-like ferroelectric behavior. Temperature-electric field phase diagram is also presented. An appreciate kind of materials is provided to investigate multiple field-induced phase transition with PZT-based ceramics.