Traps centers and deep defects contribution in current instabilities for AlGaN/GaN HEMT's on silicon and sapphire substrates

AlGaN/GaN high electron mobility transistors (HEMTs) with Si and Al₂O₃ substrates reveals anomalies on I_ds-V_ds-T and I_gs-V_gs-T characteristics (degradation in drain current, kink effect, barrier height fluctuations, etc.). Stress and random telegraph signal (RTS) measurements prove the presence of trap centers responsible for drain current degradation. An explanation of the trapping mechanism responsible for current instabilities is proposed. Deep defects analysis performed by capacitance transient spectroscopy (C-DLTS), frequency dispersion of the output conductance (G_ds(f)), respectively, on gate/source and drain/source contacts and RTS prove the presence of deep defects localized, respectively, in the gate and in the channel regions. Defects detected by C-DLTS and G_ds(f) are strongly correlated, respectively, to barrier height inhomogeneities and kink anomalies. Gate current analysis confirms the presence of (G-R) centers acting like traps at the interface GaN/AlGaN. Finally, the localization of these traps defects is proposed.     

Prediction of curved oil–water interface in horizontal pipes using modified model with dynamic contact angle

In this study, interface shapes of horizontal oil–water two-phase flow are predicted by using Young-Laplace equation model and minimum energy model. Meanwhile, the interface shapes of horizontal oil–water twophase flow in a 20 mm inner diameter pipe are measured by a novel conductance parallel-wire array probe(CPAP). It is found that, for flow conditions with low water holdup, there is a large deviation between the model-predicted interface shape and the experimentally measured one. Since the variation of pipe wetting characteristics in the process of fluid flow can lead to the changes of the contact angle between the fluid and the pipe wall, the models mentioned above are modified by considering dynamic contact angle. The results indicate that the interface shapes predicted by the modified models present a good consistence with the ones measured by CPAP.     

结合小波变换和双边滤波的SICM图像降噪算法

扫描离子电导显微镜(Scanning Ion Conductance Microscope, SICM)能够实现微纳米级的形貌测量,引起广大学者的关注研究。针对SICM形貌图像易受噪声污染,影响后续应用的问题,提出了一种基于小波分层阈值处理的双边滤波算法。针对SICM形貌图像的多特征融合噪声,采用伪中值滤波局部处理图像中的强斑点噪声,有机结合小波阈值去噪和双边滤波去除图像高频和低频噪声,最终得到去噪效果较好的形貌图像。通过仿真实验和实测实验进行多次验证,本文算法对比中值滤波、双边滤波和小波去噪三种去噪算法,其峰值信噪比提升幅度均大于9.8%。实验表明本文算法在SICM形貌图像去噪方面具有更大优势。     

Micellar Properties of Surface Active Ionic Liquid Lauryl Isoquinolinium Bromide and Anionic Polyelectrolyte Poly(Acrylic Acid Sodium Salt) in Aqueous Solution

The complex formation between anionic polyelectrolyte poly(acrylic acid sodium salt) [NaPAA] and surface active ionic liquid (SAIL) lauryl isoquinolinium bromide [C₁₂iQuin][Br] in aqueous media has been investigated by surface tension, isothermal titration calorimetry (ITC), and conductance. The self‐assembled structures have been characterized using dynamic light scattering (DLS) and turbidity measurements. A range of surface parameters have been calculated from tensiometric measurements including critical micelle concentration (CMC), surface excess concentration (Γ_cmc), surface pressure at the interface (Π_cmc), minimum area occupied at air–solvent interface (A_min), adsorption efficiency (pC₂₀), and surface tension at the CMC (γ_cmc). The thermodynamic parameters, i.e., standard enthalpy of micellization , standard free energy of micellization (), and standard entropy of micellization () have also been evaluated. Four different stages of transitions, corresponding to the progressive formation of NaPAA–[C₁₂iQuin][Br] complex (C₁)_, critical aggregation concentration (CAC), critical saturation concentration (C₃) and CMC have been observed owing to strong electrostatic and hydrophobic interactions. The results obtained from DLS and turbidity measurements show that size of the aggregates first decreases and then increases in the presence of polyelectrolyte. The binding isotherms obtained using isothermal titration calorimetry (ITC) show the concentration dependence as well as the highly cooperative nature of interactions corresponding to formation of polyelectrolyte–SAIL complexes.     

Electrical, optical and morphological properties of nanoparticle indium-tin-oxide layers

Porous layers were prepared from DEGUSSA's ITO (In₂O₃:Sn) nanoparticle dispersion by doctor blading followed by annealing in air. We investigated the influence of various annealing parameters on electrical, optical and morphological thin film properties.Conductance rises with increasing annealing temperature and time by more than three orders of magnitude up to 44 Ω^− 1cm^− 1. Besides this we found an abrupt decrease in free charge carrier concentration above a critical annealing temperature of 250 °C, which leads to a step in conductance curve. In spite of particle growing during annealing no decrease in porosity was observed and in opposite to compact material, nanoparticle layers do not exhibit an appreciable shrinkage below recrystallisation temperature. These both indicate a densification hindering particle pinning effect, which is believed to be currently the main obstruction to achieve higher electrical conductivities.     

Electrosynthesis and comparative studies on carboxyl-functionalized polythiophene derivatives

Electrochemical synthesis of a novel carboxylic acid functionalized polythiophene – poly(3-thiophene-butyric-acid), PTBA – has been realized. Its morphology, electrochemical, spectral and conducting properties have been compared to those of poly(3-thiophene-acetic-acid), PTAA, which is widely used to immobilise both bioactive molecules and inorganic nanoparticles. According to scanning electron microscopic (SEM) images, the difference in the real and geometric surface area of the modified electrodes is much more expressed in the case of PTBA. Both the symmetry of the cyclic voltammograms and the concurrent, sustained optical changes proved that this polymer possesses an improved and more stable redox activity. According to simultaneously performed in situ ac. impedance and UV–Vis measurements, both films could be uniformly transformed between the insulating and conducting forms, but PTAA exhibited some degradation. The development of the conducting state during the redox switching of both thiophene derivative polymers proved to be primarily connected to the formation of di-cationic species. The electrochemical quartz crystal microbalance (EQCM) results evidenced also differences between the two polymers, which difference can be interpreted by assuming the more expressed effect of the deprotonation-connected (self-) doping process in PTAA. The results confirm that the new conducting polymer, PTBA is much more convenient for being considered as the polymer matrix of practically applicable composites.     

Switching and memory devices based on a polythiophene derivative for data-storage applications

In this article, we report electrical characteristics of devices based on oriented and unoriented films of a polymer, namely poly[3-(6-methoxyhexyl)thiophene]. The current–voltage characteristics of sandwiched devices, based on unoriented polymer, showed hysteresis behavior, while oriented versions exhibited switching characteristics, i.e. presence of two conducting states depending on sweep direction of voltage scans. The ratio between the device current of two conducting states has been as high as 10⁵. This is comparable, if not better, than the results reported so far with complicated device architecture or doped polymeric materials. We have also demonstrated that the switching devices have an associated memory effect for data-storage applications.     

Numerical investigation of conductance of porous media in high vacuum systems

In high vacuum systems or materials that have fine capillaries, the molecular transport can be characterized as being free-molecular flow regime. In this flow regime intermolecular interactions can be ignored and flow is determined entirely by molecule-surface collisions. The transport of gases and volatile compounds through porous media and filters with variety of geometries is of great interest in various industrial applications. Although the effect of porosity on gas flow in the most of the flow regimes has been explored, but there are a few investigations on gas transport in porous media and filers at free-molecular regime. In this investigation gas transport in porous media with various porosities and geometries is explored. Test Particle Monte-Carlo (TPMC) method is employed. The walls are assumed to be diffusive. The skeletal portion of the porous media (frame) is modelled by solid spheres. The developed numerical scheme is validated with non porous cases. The effect of porosity, sphere sizes of frame, porous geometry, gas type and temperature on the conductance is examined. The simulations are performed for a porous pipe and porous nozzle. Results demonstrate that porosity and filtration highly affects the conductance of pipe and nozzle and causes great pressure drop in high vacuum systems. The increase of sphere sizes at constant porosity causes conductance to grow. The gas type and temperature of gas affects the conductance of pipe and nozzle too.     

Investigation of 4H-SiC MOS capacitors annealed in diluted N2O at different temperatures

Thermally grown oxide on 4H-SiC has been post-annealed in diluted N₂O (10% N₂O in N₂) at different temperatures from 900 to 1100 °C. The quality of the nitrided oxide and the SiO₂/4H-SiC interface was investigated by AC conductance and high frequency C-V measurements based on Al/SiO₂/4H-SiC metal-insulator-semiconductor (MOS) structure. It is found that N₂O annealing at 1000 °C produces the lowest interface state density, though the difference is not so significant when compared to the other samples annealed at 900 and 1100 °C. These results can be explained by the high temperature dynamic decomposition process of N₂O. By fitting the AC conductance data, it is found that higher temperature nitridation increases the capture cross-section of the interface traps.     

The electronic configuration and the conductance of silicon nanograins: An application of the scattering approach

The purpose of this study is the assessment of the properties of the electronic structure and of the transport characteristics of silicon nanostructures of a size comparable with the experimental ones. Accordingly, crystalline columnar grains, sandwiched between two adsorbing aluminum contacts, with a size of several hundreds atoms and linear dimensions up to a few nanometers, have been considered. The calculation method elaborates on the scattering approach, as reported in the recent chemical literature, using the extended Hückel theory for the evaluation of either the electronic charge or the transmission function. The calculations show that the binding energy has bulk-like features, i.e. its size dependence has a nearly flat asymptote with a value close to the cohesive energy of the solid. However the effect of the loosely coordinated boundary atoms is perceptible even at the sizes few hundreds atoms and produces an oscillatory behaviour of the binding energy. The comparison between this energy and the conductance indicates that both quantities increase at the same sizes and therefore the orbitals of the loosely bounded atoms are the important conductive channels.