High-performance n-channel organic thin-film transistors based on the dual effects of heterojunction and surface modification

This paper presents two n-channel organic heterojunction transistors with modified insulator by using hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/copper phthalocyanine (CuPc) and F₁₆CuPc/pentacene as the active layers. Compared with a single-layer device, it reports that an improved field-effect mobility and a 6-fold higher drain current are observed. The highest mobility of 0.081~cm²/(V.s) was obtained from F₁₆CuPc/CuPc heterojunction devices. This result is attributed to the dual effects of the organic heterojunction and interface modification. Furthermore, for two heterojunction devices, the performance of the F₁₆CuPc/CuPc-based transistor is better than that of F₁₆CuPc/pentacene. This is attributed to the morphologic match of two organic components.     

Substrate bias effects on collector resistance in SiGe heterojunction bipolar transistors on thin film silicon-on-insulator

An analytical expression for the collector resistance of a novel vertical SiGe heterojunction bipolar transistor(HBT) on thin film silicon-on-insulator(SOI) is obtained with the substrate bias effects being considered.The resistance is found to decrease slowly and then quickly and to have kinks with the increase of the substrate-collector bias,which is quite different from that of a conventional bulk HBT.The model is consistent with the simulation result and the reported data and is useful to the frequency characteristic design of 0.13 μm millimeter-wave SiGe SOI BiCMOS devices.     

Analytical base–collector depletion capacitance in vertical SiGe heterojunction bipolar transistors fabricated on CMOS-compatible silicon on insulator

The base--collector depletion capacitance for vertical SiGe npn heterojunction bipolar transistors (HBTs) on silicon on insulator (SOI) is split into vertical and lateral parts. This paper proposes a novel analytical depletion capacitance model of this structure for the first time. A large discrepancy is predicted when the present model is compared with the conventional depletion model, and it is shown that the capacitance decreases with the increase of the reverse collector--base bias--and shows a kink as the reverse collector--base bias reaches the effective vertical punch-through voltage while the voltage differs with the collector doping concentrations, which is consistent with measurement results. The model can be employed for a fast evaluation of the depletion capacitance of an SOI SiGe HBT and has useful applications on the design and simulation of high performance SiGe circuits and devices.     

Photoelectric property of LaAlO3-δ/Si heterojunctions with different oxygen contents

Three oxide heterojunctions made of LaAlO_3-δ/Si are fabricated under various oxygen pressures by laser molecular-beam epitaxy. They all show nonlinear and rectifying current--voltage characteristics, and the distinct difference in rectification behaviour among them. Their photoelectric properties are examined by a visible HeNe laser and an ultraviolet Hg lamp. We find that their photovoltaic responses are closely related to the oxygen contents in the LaAlO_3-δ films. The junction fabricated under the lower oxygen pressure has a higher photovoltaic sensitivity. The possible mechanism is suggested based on the band structure of the p--n heterojunction.     

Designing power heterojunction bipolar transistors with non-uniform emitter finger lengths to achieve high thermal stability

With the aid of a thermal-electrical model,a practical method for designing multi-finger power heterojunction bipolar transistors with finger lengths divided in groups is proposed.The method can effectively enhance the thermal stability of the devices without sacrificing the design time.Taking a 40-finger heterojunction bipolar transistor for example,the device with non-uniform emitter finger lengths is optimized and fabricated.Both the theoretical and the experimental results show that,for the optimum device,the peak temperature is lowered by 26.19 K and the maximum temperature difference is reduced by 56.67% when compared with the conventional heterojunction bipolar transistor with uniform emitter finger length.Furthermore,the ability to improve the uniformity of the temperature profile and to expand the thermal stable operation range is strengthened as the power level increases,which is ascribed to the improvement of the thermal resistance in the optimum device.A detailed design procedure is also summarized to provide a general guide for designing power heterojunction bipolar transistors with non-uniform finger lengths.     

薄膜异质结中磁性斯格明子的相关研究

磁性斯格明子由于具有拓扑保护、尺寸小、驱动电流密度低等优异的属性,有望作为未来超高密度磁存储和逻辑功能器件的信息载体.为了满足器件中信息写入和读取的基本要求,需要在室温下实现斯格明子的精确产生、操控和探测.该综述简要介绍最近我们针对上述问题取得的一系列研究进展,包括:1)证明可以通过控制磁性薄膜材料的垂直磁各向异性在室温下产生斯格明子,并进一步在基于反铁磁的薄膜异质结中发现了室温、零磁场下稳定存在的斯格明子;2)证明能够利用电流产生的自旋轨道力矩操控斯格明子,并进一步制备出一种基于斯格明子的原理型器件,实现了利用电学方式产生和操控数量可控的斯格明子.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

Effect of fatigue fracture on the resistive switching of TiO2-CuO film/ITO flexible memory device

We fabricated the GaIn/TiO₂-CuO/ITO resistive memory and studied the effect of fatigue fracture on the switching performance. The device shows the stable bipolar resistive switching over 10⁸ s under ambient condition. The ON/OFF ratio decreases seriously with increase of bending cycles. The main fatigue fracture caused by dynamic strain includes micro defect between nanoparticles, vertical crack along the film thickness and interfacial delamination between layers. Finite element analysis indicates that channel crack plays a key role to cause the interfacial delamination between function layer and ITO electrode. The channel crack and interfacial delamination can hinder the formation of tree−like conduction filaments. Moreover, oxygen via the cracks can be easily transformed to ions and reduce the density of oxygen vacancies under the catalytic assistance of CuO. Our studies may provide some useful information for inorganic materials applied in flexible nonvolatile memory.     

Revisiting the Optimal Nano‐Morphology: Towards Amorphous Organic Photovoltaics

Organic photovoltaic cells commonly use an active layer with a polycrystalline bulk heterojunction. However, for simplifying the fabrication process, it may be worthwhile to use an amorphous active layer to lessen the burden on processing to achieve optimal performance. While polymers can adopt amorphous phases, molecular glasses, small molecules that can readily form glassy phases and do not crystallize over time, offer an appealing alternative, being monodisperse species. Our group has developed a series of reactive molecular glasses that can be covalently bonded to chromophores to form glass‐forming adducts, and this strategy has been used to synthesize glass‐forming donor and acceptor materials. Herein, the results of devices incorporating these materials in either partially or fully amorphous active layers are summarized. Additionally, these molecular glasses can be used as ternary components in crystalline systems to enhance efficiency without perturbing the morphology.     

金属氧化物异质结气体传感器气敏增强机理

金属氧化物异质结由于费米能级效应、不同组分之间的协同作用,常被用来提高电阻型金属氧化物半导体气体传感器的气敏特性。本文简述了近年来国内外金属氧化物异质结材料的类别,主要分为混合氧化物结构、层状结构、第二相粒子修饰结构、一维纳米结构和核-壳结构;重点综述了金属氧化物异质结的气敏增强机理,包括异质结效应、协同效应、催化溢流效应、响应反型、载流子分离及微结构调控六大机理;分析了当前异质结气体传感器面临的瓶颈。最后对纳米异质结气体传感器的发展进行了展望,今后金属氧化物异质结气体传感器可以从明确异质结界面机理展开,这将为自下而上地设计出符合实际需要的气体传感器提供一定参考。