Contact resistance and megahertz operation of aggressively scaled organic transistors

Bottom-gate, top-contact organic thin-film transistors (TFTs) with excellent static characteristics (on/off ratio: 10(7) ; intrinsic mobility: 3 cm(2) (V s)(-1) ) and fast unipolar ring oscillators (signal delay as short as 230 ns per stage) are fabricated. The significant contribution of the transfer length to the relation between channel length, contact length, contact resistance, effective mobility, and cutoff frequency of the TFTs is theoretically and experimentally analyzed.     

Improved Morphology and Performance of Solution‐Processed Metal‐Oxide Thin‐Film Transistors Due to a Polymer Based Interface Modifier

The influence of a polymer interface modifier on the performance of solution‐processed indium‐based metal‐oxide (MO) thin‐film transistors (TFTs) is investigated. We use the polymer ethoxylated polyethylenimine (PEIE). Compared to a reference sample this modification enhances the mobility by a factor of four, clearly reduces the contact and the sheet resistance, and decreases the charge carrier activation energy by about 20%. The improved electrical performance originates from both a reduced contact and a reduced sheet resistance of the TFTs. The molecular dipole of PEIE reduces the work function of the electrodes. Adversely the dipole enhances the off current and the trap density at the semiconductor/dielectric interface for bottom‐contact transistors with small channel length. The substrate becomes highly polar with a PEIE‐treatment. Accordingly, topographical studies of bottom‐contact TFTs show a very similar MO film morphology on the electrodes and in the channel for modified TFTs, whereas in the untreated samples the film has a higher roughness on the electrodes than in the channel. TFTs in top‐contact configuration with the polymer interface layer at the dielectric/semiconductor interface also show higher mobility compared to the reference MOTFTs which displays that the improved performance is due to the improved morphology of the MO film.     

探针尺寸对Si-DLC薄膜表面粘附和摩擦的影响

采用AFM尖头探针、球头探针和平头探针对Si-DLC膜进行摩擦实验,研究了薄膜的微观摩擦力学性能,探讨了不同接触尺度下薄膜表面粘附力及摩擦产生的机理,建立了不同探针与薄膜表面粗糙峰的接触模型,推导了表面粘附力与接触面积的关系表达式,表明在微观接触中,接触面积对粘附力起着主导作用。尖头探针与薄膜表面的微观摩擦系数取决于表面粗糙峰的斜率,与粗糙峰的高度相关不大;球头探针与薄膜表面的摩擦力主要取决于单位面积接触粗糙峰密度;平头探针与薄膜表面的摩擦力主要取决于外加载荷,表面形貌的微观尺寸效应可忽略。     

1-3型压电复合材料电极的制备

采用磁控溅射工艺以Cu、Ag为靶材在1-3型压电复合材料表面制备电极．研究了两种金属材料的溅射镀膜工艺，系统地分析了溅射功率对金属沉积速率、电极导电性能及附着力的影响．结果表明，两种金属的沉积速率随溅射功率的增加呈线性增加，电极方阻及电阻率均随功率增大而减小，溅射功率为100W时电极的附着力较好．     

TiOSO4水性溶胶制备TiO2薄膜及其亲水性研究

采用TiOSO4水性溶胶制备了TiO2薄膜，通过X射线衍射(XRD)、原子力显微镜(AFM)、紫外可见光谱(UV—Vis)和薄膜表面接触角对TiO2薄膜进行了表征并进行了亲水性研究．结果表明采用TiOSO4水性溶胶制备的TiO2薄膜具有优异的光照亲水性；在100-500℃之间热处理，温度的升高有利于亲水性的改善．     

脉冲电沉积法制备SnS薄膜及其光电性质研究

在ITO玻璃衬底上使用脉冲电沉积法制备了SnS薄膜,研究了不同的开启脉冲电位对薄膜表面,薄膜结构以及光学性质的影响。结果表明,在不同开启脉冲电位下制备出来的SnS薄膜的禁带宽度可以在1.3~1.62 eV范围内变化,并且随着开启脉冲电位的增大,薄膜的禁带宽度逐渐变大。在开启脉冲电位为-0.70V(vs.SCE)时,制备的薄膜禁带宽度为1.53 eV,在可见光范围内光吸收系数均达到104cm-1以上。扫描电子显微镜的测试结果表明所得薄膜的表面平整并且均匀。结合X射线衍射结果可证实制备出的薄膜是由正交结构晶粒组成的多晶体。对薄膜进行变温电学性质的测试,得到了薄膜电导率温度谱,发现室温下薄膜的电导率为10-6S.cm-1。同时,在实验上发现了薄膜与Al电极形成了肖特基接触,由电学测试推导出肖特基势垒高度为0.58 eV。     

Crack face contact in X‐FEM using a segment‐to‐segment approach

In this study, we propose a segment‐to‐segment contact formulation (mortar‐based) that uses Lagrange's multipliers to establish the contact between crack faces when modeled with the extended finite element method (X‐FEM) in 2D problems. It is shown that, in general, inaccuracies arise when the contact is formulated following a point‐to‐point approach. This is due to the non‐linear character of the X‐FEM interpolation along the crack faces that leads to crack face interpenetration. However, the segment‐to‐segment approach optimizes the fulfilment of the contact constraints along the whole crack segment, and in practice the contact is modeled precisely. Convergence studies for mesh sequences have been performed, showing the advantages of the proposed methodology. Copyright © 2009 John Wiley & Sons, Ltd.     

Cu_xTe薄膜的结构及其对CdTe太阳电池性能的影响

用真空共蒸发法制备了CuxTe薄膜并将其运用于CdTe太阳电池中.对薄膜进行了X射线衍射(XRD)分析,比较了有、无CuxTe插层的CdTe太阳电池的暗态,I-V特性和C-V特性.结果表明,刚沉积的薄膜非晶结构占主导地位,只有部分Cu/Te配比较低的薄膜出现多晶结构.CuxTe插层的引入有利于消除roll over(暗态I-V曲线饱和)现象,使电池的二极管理想因子和暗饱和电流密度降低,CdTe掺杂浓度增加,有效地改善了CdTe太阳电池的性能.用CuxTe薄膜作为背接触层,获得了效率为12.5%的CDS/CdTe小面积(0.0707cm2)太阳电池.     

Vertex‐to‐face contact searching algorithm for three‐dimensional frictionless contact problems

This article presents a new vertex‐to‐face contact searching algorithm for the three‐dimensional (3‐D) discontinuous deformation analysis (DDA). In this algorithm, topology is applied to the contact rule when any two polyhedrons are close to each other. Attempt is made to expand the original contact searching algorithm from two‐dimensional (2‐D) to 3‐D DDA. Examples are provided to demonstrate the new contact rule for vertex‐to‐face contacts between two polyhedrons with planar boundaries. Copyright © 2005 John Wiley & Sons, Ltd.     

A node‐to‐node scheme with the aid of variable‐node elements for elasto‐plastic contact analysis

A strategy for a two‐dimensional contact analysis involving finite strain plasticity is developed with the aid of variable‐node elements. The variable‐node elements, in which nodes are added freely where they are needed, make it possible to transform the non‐matching meshes into matching meshes directly. They thereby facilitate an efficient analysis, maintaining node‐to‐node contact during the contact deformation. The contact patch test, wherein the contact patch is constructed out of variable‐node elements, is thus passed, and iterations for equilibrium solutions reach convergence faster in this scheme than in the conventional approach based on the node‐to‐surface contact. The effectiveness and accuracy of the proposed scheme are demonstrated through several numerical examples of elasto‐plastic contact analyses. Copyright © 2015 John Wiley & Sons, Ltd.     

Micro‐indentation fracture from flat‐ended cylindrical indenter

Unlike a Hertzian ring crack induced by a spherical indenter in absence of a singular stress field, a ring crack generated by a rigid flat cylindrical indenter can be explicitly linked to a K‐dominant singular stress field at the perimeter of the flat indenter. This means microcrack initiation induced by a flat indenter and relevant properties such as the critical indentation load and fracture toughness can be formulated explicitly using the fracture mechanics approach. It is shown in this paper that the indentation stress intensity factor, , for such a stress field is similar to that of a mode I crack. Based on the energy‐releasing rate and the Griffith's theorem, a flat indentation cracking model has been proposed; the critical load and critical cracking angle for crack initiation are derived. A new concept of fracture toughness for negative mode I singular stress field,, has been defined and a relationship between and the traditional K_IC has been derived. The experimental investigation validates the existence of such , from which the K_IC value of the glass had been determined to be 0.772 ± 0.003 MPa m^1/2, agreeing well with the literature data. This analysis for indentation fracture or crack initiation due to surface contact of a flat indenter is particularly useful in determining K_IC of brittle materials with dimensions in micro/nanoscales, e.g. thin films and other microstructures as flat micro/nano‐indenters are available and can be used on various nano‐indentation machines.     

Development of contact algorithm for three‐dimensional numerical manifold method

This paper customizes a contact detection and enforcing scheme to fit the three‐dimensional (3‐D) numerical manifold method (NMM). A hierarchical contact system is established for efficient contact detection. The mathematical mesh, a unique component in the NMM, is utilized for global searching of possible contact blocks and elements, followed by the local searching to identify primitive hierarchies. All the potential contact pairs are then transformed into one of the two essential entrance modes: point‐to‐plane and crossing‐lines modes, among which real contact pairs are detected through a unified formula. The penalty method is selected to enforce the contact constraints, and a general contact solution procedure in the 3‐D NMM is established. Because of the implicit framework, an open‐close iteration is performed within each time step to determine the correct number of contact pairs among multi‐bodies and to achieve complete convergence of imposed contact force at corresponding position. The proposed contact algorithm extensively utilizes most of the original components of the NMM, namely, the mathematical mesh/cells and the manifold elements, as well as the external components associated with contacts, such as the contact body, the contact facet and the contact vertex. In particular, the utilization of two mutually approaching mathematical cells is efficient in detecting contacting territory, which makes this method particularly effective for both convex and non‐convex bodies. The validity and accuracy of the proposed contact algorithm are verified and demonstrated through three benchmark problems. Copyright © 2013 John Wiley & Sons, Ltd.     

Analysis of edge crack behaviour influenced by non‐symmetrical contact tractions and bulk tension

This paper analyses a crack growth behaviour, which is initiated from the contact edge between a square punch with rounded edges and a half plane. Investigated are the influences of the contact profile, magnitude of the bulk tension and, crack obliquity, in particular, misalignment between the punch and half plane on the variation of the stress intensity factors K_I and K_II during the crack growth. The misalignment is simulated by a tilting of the punch. A partial slip regime is considered for the contact shear force to accommodate a general fretting fatigue condition. It was found that a crack closure occurs if only the contact forces are applied. The crack grows longer before it is closed if the punch is tilted (clockwise, in this paper) such that it initiates at the opposite site with respect to the direction of tilting. The closure phenomenon disappears when the bulk tension is added and exceeds a certain magnitude, which significantly depends on not only the contact profile but also the degree and direction of tilting. Provided are the lowest values of the bulk tensile stress due to a fatigue load necessary to extend the crack without a closure for each condition of the contact profile and misalignment. This may be used as a design guideline to restrain the contact‐induced failure.     

磁控溅射MoO_3半导体功能薄膜的制备与结构性能研究

采用射频磁控溅射技术分别在Si(100)和玻璃衬底上通过调整不同的溅射功率和退火温度成功制备了MoO3薄膜。利用X射线衍射、扫描电镜、紫外可见光分光光度计、接触角测量等进行了表征和分析。X射线衍射表明400℃以上沉积的MoO3结晶薄膜属正交晶系,沿(0k0)(k=2n)取向择优生长,衍射峰强度和薄膜的结晶度随溅射功率的提高逐渐增强;利用扫描电镜观察表面形貌,发现结晶的薄膜表面发生了不同程度的变化,由初期均匀分布的纳米细长状颗粒长大成二维片状结构;紫外可见光分光光度计测试表明,薄膜在可见光区具有良好的光学透过性,平均透过率达60%以上;接触角测量发现薄膜呈明显亲水性,通过后续的表面氟化改性热处理,实现了薄膜亲水-疏水的润湿性能转换。     

Thermophysical Characterization of a CuO Thin Deposit

CuO thin deposits on a tungsten carbide (consisting of 9% cobalt) substrate are obtained by physical vapor deposition (PVD) at ambient temperature. The longitudinal thermal conductivity as well as the thermal contact resistance at the deposit–substrate interface are investigated. A periodic photothermal experiment based on infrared radiometry is implemented. The amplitude between the periodic heat flux applied on the sample and the average temperature on the heated area are measured over a low frequency range. The method does not require the absolute measurement of these two quantities given that the thermal properties of the substrate are known. Scanning electron microscopy observations show strong anisotropy and columnar structure of the deposits. Moreover, the chemical composition of the films is obtained using the Auger technique. Cobalt diffuses from the substrate toward the deposit during the deposition process. It is demonstrated that the measured thermal conductivity of the CuO layer mainly rests on the microstructure of the layer instead of the roughness of the sample. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5-8, 2005, Bratislava, Slovak Republik.