Ultralow-power organic complementary circuits

The prospect of using low-temperature processable organic semiconductors to implement transistors, circuits, displays and sensors on arbitrary substrates, such as glass or plastics, offers enormous potential for a wide range of electronic products. Of particular interest are portable devices that can be powered by small batteries or by near-field radio-frequency coupling. The main problem with existing approaches is the large power consumption of conventional organic circuits, which makes battery-powered applications problematic, if not impossible. Here we demonstrate an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc). The monolayer dielectric is grown on patterned metal gates at room temperature and is optimized to provide a large gate capacitance and low gate leakage currents. By combining low-voltage p-channel and n-channel organic thin-film transistors in a complementary circuit design, the static currents are reduced to below 100 pA per logic gate. We have fabricated complementary inverters, NAND gates, and ring oscillators that operate with supply voltages between 1.5 and 3 V and have a static power consumption of less than 1 nW per logic gate. These organic circuits are thus well suited for battery-powered systems such as portable display devices and large-surface sensor networks as well as for radio-frequency identification tags with extended operating range.     

Critical thickness for ferroelectricity in perovskite ultrathin films

The integration of ferroelectric oxide films into microelectronic devices, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 A (ten unit cells), ref. 3. Recent atomistic simulations have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric-electrode interface. We show that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (approximately 24 A). A depolarizing electrostatic field, caused by dipoles at the ferroelectric-metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.     

Effects of SrTiO3 buffer layer on structural and electrical properties of Bi3.15Nd0.85Ti3O12 thin films prepared by a chemical method

Ferroelectric Bi3.15Nd0.85Ti3O12 (BNT) thin films have been grown on Pt/Ti/SiO2/Si substrates at 750 ℃ by a chemical solution deposition method using SrTiO3 (STO) as a buffer layer.The influence of STO...     

Strong polarization enhancement in asymmetric three-component ferroelectric superlattices

Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.     

微光夜视仪用无铅高压陶瓷电容的研发

本文采用BaTiO3作为基料,SrTiO3、CaCO3、Bi2O3·3TiO3等作为辅料,制备无铅高压陶瓷电容器介质材料,探索和研究了添加剂对介质材料性能的变化和影响,得到了介电常数可调、综合介电性能稳定的介质材料结果.从研究结果表明,在BaTiO3中加入摩尔数分数40%Sr-TiO3、15%CaCO3,3%Bi2O3...     

退火气氛对SrTiO3薄膜电学性能的影响

在不同的退火气氛下,采用金属有机物分解法在(111)Pt/Ti/SiO2/Si衬底上制备了SrTiO3薄膜材料,并对其结构和电学性能进行了研究。X射线衍射结果显示SrTiO3薄膜都呈现多晶立方钙钛矿结构。对比不同气氛下制备出的SrTiO3样品电学测量结果,发现在氧气氛中退火后,样品具有较低的漏电流和较好的介电性能,这可能是由于氧气氛中退火减少了SrTiO3薄膜内氧空位的浓度引起的。     

A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface

Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.     

采用Q—V系统研究掺稀土ZnS薄膜电致发光

Q-V特性测试系统及亮度测试系统对ZnS:RE及ZnS:REF_3 器件进行传输电荷密度、器件阈值电压、器件各层电容、输入电功率密度及发光亮度等参数的测量,通过实验公式对各项参数定量计算,作出特性曲线,并对结果作了讨论。     

高比容铝电解电容器关键技术进展

简述了铝电解电容器的国内外发展现状,评述了近年来铝电解电容器用高介电常数复合阳极氧化膜及工作电解液的研究进展,指出了相关的技术难点和存在问题,探讨了高比容铝电解电容器的未来技术发展趋势。     

耗散介观电容耦合电路的量子效应

从经典耗散介观电容耦合电路出发,研究了耗散介观电容耦合电路,在占有数表象中,计算了每个回路的电荷和电流的量子涨落。结果表明,每个回路中的电荷和电流的量子涨落不仅与自身回路的器件参数有关,同时还与另一回路的器件参数有关。     

CuO薄膜的阻变存储特性

利用等离子体辅助的分子束外延设备在SrTiO3掺Nb（Nb：STO）的衬底上制备了双层的Cu2O/CuO薄膜,经过测量其电流电压特性,发现Pt/Cu2O/CuO/Nb：STO器件的阻变存储特性非常显著,而且可重复程度高,适合多次数据写入与擦除,有望应用于信息存储领域.     

Two-dimensional electron gas with universal subbands at the surface of SrTiO(3)

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ～0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.     

Improper ferroelectricity in perovskite oxide artificial superlattices

Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds. Here we show that superlattices with very short periods possess a new form of interface coupling, based on rotational distortions, which gives rise to 'improper' ferroelectricity. These observations suggest an approach, based on interface engineering, to produce artificial materials with unique properties. By considering ferroelectric/paraelectric PbTiO3/SrTiO3 multilayers, we first show from first principles that the ground-state of the system is not purely ferroelectric but also primarily involves antiferrodistortive rotations of the oxygen atoms in a way compatible with improper ferroelectricity. We then demonstrate experimentally that, in contrast to pure PbTiO3 and SrTiO3 compounds, the multilayer system indeed behaves like a prototypical improper ferroelectric and exhibits a very large dielectric constant of epsilon(r) approximately 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications, is distinct from that of normal ferroelectrics, for which the dielectric constant is typically large but strongly evolves around the phase transition temperature and also differs from that of previously known improper ferroelectrics that exhibit a temperature-independent but small dielectric constant only.     

Mn掺杂PBST薄膜的溶胶-凝胶工艺制备及介电调谐性能

采用溶胶-凝胶工艺在Pt/Ti/SiO2/Si衬底上制备了Mn掺杂的钛酸锶铅钡PBST铁电薄膜.探讨了掺杂后PBST薄膜的微观结构及其铁介电性能.实验结果表明:随着Mn的掺杂量的增加,PBST薄膜的晶化质量变好,介电常数、介电损耗和调谐量均有减小的趋势,其优值因子有显著的提高.在测定频率为1 MHz下,掺杂后的PBST薄膜介电常数和介电损耗呈下降趋势,薄膜的介电常数从未掺杂的1 250降低至掺杂后的610,同时介电损耗由0.095减小到0.033,当Mn为J4 mol％时,有最小的介电损耗0.033,虽然调谐量不是最高的,但有最大的优值因子(FOM),其微波介电综合性能有所改善.     

基于线性可调全差分OTA的电容倍增器

为了解决在集成电路中制作大容量电容器困难的问题,设计了一个线性可调跨导运算放大器(LO-TA),利用3个所设计的LOTA和1个接地电容实现了1个浮地电容倍增器.提出的电容倍增器能够通过调节相应LOTA的外部控制电流的比值实现电容容量的线性调节,调节范围为1～103倍时误差小于0.3%.电路的性能通过一个二阶电流模式滤波器和PSPICE仿真得到了验证.     

CaCu3Ti4O12陶瓷的介电性能及其内耗

采用固相反应法制备CaCu₃Ti₄O₁₂陶瓷材料,用阻抗分析仪测定其介电常数和内耗.结果表明：在频率一定的条件下,CaCu₃Ti₄O₁₂陶瓷的内耗在某一温度时出现了一个最大值,偏离该温度时内耗迅速下降;随着频率增加,内耗峰向高温区域移动.在低温条件下,CaCu₃Ti₄O₁₂陶瓷的介电常数较小;当达到某一温度后,介电常数发生突变;当大于该温度后,介电常数基本维持在较高值,且随频率增加,所对应的温度逐渐升高.CaCu₃Ti₄O₁₂陶瓷的介电常数与其内禀结构、阻挡层电容有关,内耗和频率与温度的关系满足极化子模型;而在一定温度下介电常数发生的突变亦与极化子本身的特性有关.     

Soft-mode hardening in SrTiO3 thin films

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM). But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices. Extensive studies have shown that this phenomenon could be caused by a 'dead layer' of very low dielectric constant between the ferroeletric film and the electrode. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane-Sachs-Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.     

The interface between silicon and a high-k oxide

The ability of the semiconductor industry to continue scaling microelectronic devices to ever smaller dimensions (a trend known as Moore's Law) is limited by quantum mechanical effects: as the thickness of conventional silicon dioxide (SiO(2)) gate insulators is reduced to just a few atomic layers, electrons can tunnel directly through the films. Continued device scaling will therefore probably require the replacement of the insulator with high-dielectric-constant (high-k) oxides, to increase its thickness, thus preventing tunnelling currents while retaining the electronic properties of an ultrathin SiO(2) film. Ultimately, such insulators will require an atomically defined interface with silicon without an interfacial SiO(2) layer for optimal performance. Following the first reports of epitaxial growth of AO and ABO(3) compounds on silicon, the formation of an atomically abrupt crystalline interface between strontium titanate and silicon was demonstrated. However, the atomic structure proposed for this interface is questionable because it requires silicon atoms that have coordinations rarely found elsewhere in nature. Here we describe first-principles calculations of the formation of the interface between silicon and strontium titanate and its atomic structure. Our study shows that atomic control of the interfacial structure by altering the chemical environment can dramatically improve the electronic properties of the interface to meet technological requirements. The interface structure and its chemistry may provide guidance for the selection process of other high-k gate oxides and for controlling their growth.     

影响聚吡咯铝电解电容器漏电流的因素及改进措施

利用电化学沉积的方法在腐蚀并赋能的铝箔试样表面合成了聚吡咯，并制备了聚吡咯铝电解电容器。在聚吡咯被覆的过程中，由于聚合溶液的毛细现象使电容器的阴极与阳极之间形成了微小的电气通路，另外聚合溶液对铝氧化膜有一定的腐蚀作用，导致氧化膜的缺陷增加，这两种情况是聚吡咯电解电容漏电流增加的根本原因。研究发现，采用合适的阻断材料和缩短聚合时间均可以有效地降低它们的影响。通过热重－差热分析，说明了在贮存过程中环境对这种电容器漏电流的影响，并提出了保持该电容器长期性能稳定的措施。     

Fabrication and Electrochemical Properties of Carbon Nanotube-based Composite Electrodes for Electrochemical Capacitor Applications

1 Results Electrochemical capacitors (ECs) are expected to be used in hybrid electric vehicles in combination with batteries or fuel cells because of their higher power density than batteries. ECs using electrical double layer capacitance of carbon based materials and pseudocapacitance of transition metal oxides are called electrochemical double layer capacitors (EDLC) and supercapacitors (or pseudocapacitor), respectively. Transition metal oxides are considered the best candidates for high energy dens...