自缓释镍源的横向诱导晶化多晶硅薄膜晶体管

以镍硅合金靶作为溅射源,采用磁控溅射方法制备了一种自缓释镍源.控制合适的自缓释镍源的准备条件,以单一方向横向晶化条件对非晶硅薄膜进行再晶化,可以获得低残余镍含量、大晶粒、高薄膜质量的多晶硅.以此多晶硅为有源层进行了薄膜晶体管研究.制备的p型TFT器件具有良好的特性,可有效地减小漏电流,同时具有很好的均匀性和稳定性.     

Toward multiple maximum power point estimation of photovoltaic systems based on semiconductor theory

Environmental conditions, such as temperature, non‐uniform irradiation, and solar shading, deeply affect the characteristics of photovoltaic (PV) modules in PV‐assisted generation systems. Several local maximum power points (MPPs) are found in the power–voltage curve of PV systems constructed by series/parallel‐connected PV modules under partially shaded conditions. The characteristics of PV systems change unpredictably when multiple MPPs occur, so the actual MPP tracking (MPPT) becomes a difficult task. Conventional MPPT methods for the PV systems under partially shaded conditions cannot quickly find the actual MPP such that the optimal utilization of PV systems cannot be achieved. Based on the p–n junction semiconductor theory, we develop a multipoint direct‐estimation (MPDE) method to directly estimate the multiple MPPs of the PV systems under partially shaded conditions and to cope with the mentioned difficulties. Using the proposed MPDE method, the multiple MPPs of the PV systems under partially shaded conditions can be directly determined from their irradiated current–voltage and power–voltage characteristic curves. The performances of the proposed MPDE method are evaluated by examining the characteristics of multiple MPPs of PV systems with respect to different shading strengths and numbers of the shaded PV modules and also tested using the field data. The experimental results demonstrate that the proposed MPDE method can simply and accurately estimate the multiple MPPs of the PV systems under partially shaded conditions. The optimization of MPP control models and the MPPT for PV systems could be achieved promisingly by applying the proposed method. Copyright © 2014 John Wiley & Sons, Ltd.     

Visualizing Thermally Activated Memristive Switching in Percolating Networks of Solution-Processed 2D Semiconductors

Memristive systems present a low-power alternative to silicon-based electronics for neuromorphic and in-memory computation. 2D materials have been increasingly explored for memristive applications due to their novel biomimetic functions, ultrathin geometry for ultimate scaling limits, and potential for fabricating large-area, flexible, and printed neuromorphic devices. While the switching mechanism in memristors based on single 2D nanosheets is similar to conventional oxide memristors, the switching mechanism in nanosheet composite films is complicated by the interplay of multiple physical processes and the inaccessibility of the active area in a two-terminal vertical geometry. Here, the authors report thermally activated memristors fabricated from percolating networks of diverse solution-processed 2D semiconductors including MoS₂, ReS₂, WS₂, and InSe. The mechanisms underlying threshold switching and negative differential resistance are elucidated by designing large-area lateral memristors that allow the direct observation of filament and dendrite formation using in situ spatially resolved optical, chemical, and thermal analyses. The high switching ratios (up to 10³) that are achieved at low fields (≈4 kV cm⁻¹) are explained by thermally assisted electrical discharge that preferentially occurs at the sharp edges of 2D nanosheets. Overall, this work establishes percolating networks of solution-processed 2D semiconductors as a platform for neuromorphic architectures.     

Cu2O Nanocrystal‐Templated Growth of Cu2S Nanocages with Encapsulated Au Nanoparticles and In‐Situ Transmission X‐ray Microscopy Study

Cubic and octahedral Cu₂O nanocrystals and Au–Cu₂O core–shell heterostructures are used as sacrificial templates for the growth of Cu₂S nanocages and Au–Cu₂S core–cage structures. A rapid sulfidation process involving a surface reaction of Cu₂O nanocrystals with Na₂S, followed by etching of the Cu₂O cores with HCl solution for ≈5 sec, results in the fabrication of Cu₂S cages with a wall thickness of 10–20 nm. Transmission electron microscopy characterization reveals the formation of crystalline walls and the presence of ultrasmall pores with sizes of 1 nm or less. Formation of Cu₂O–Cu₂S core–shell structures and their conversion into Cu₂S cages is verified by UV–vis absorption spectroscopy. X‐ray photoelectron spectra further confirm the composition of the cages as Cu₂S. The entire hollowing process via the Kirkendall effect is recorded using in‐situ transmission X‐ray microscopy. After shell formation, continuous ionic diffusion removes the interior Cu₂O. Intermediate structures with remaining central Cu₂O portions and bridging arms to the surrounding cages are observed. The nanocages are also shown to allow molecular transport: anthracene and pyrene penetration into the cages leads to enhanced fluorescence quenching immediately upon adsorption onto the surfaces of the encapsulated gold nanocrystals.     

Cross-layer QoS Analysis of Opportunistic OFDM-TDMA and OFDMA Networks

Performance analysis of multiuser orthogonal frequency division multiplexing (OFDM-TDMA) and orthogonal frequency division multiple access (OFDMA) networks in support of multimedia transmission is conducted in this work. We take a cross-layer approach and analyze several quality-of-service (QoS) measures that incilude the bit rate and the bit error rate (BER) in the physical layer, and packet average throughput/delay and packet maximum delay in the link layer. The authors adopt a cross-layer QoS framework similar to that in IEEE 802.16, where service classification, flow control and opportunistic scheduling with different subcarrier/bit allocation schemes are implemented. In the analysis, the Rayleigh fading channel in the link layer is modeled by a finite-state Markov chain, and the channel state information (CSI) is assumed to be available at the base station. With the M/G/1 queueing model and flow control results, the analysis provides important insights into the performance difference of these two multiaccess systems. The derived analytical results are verified by extensive computer simulation. It is demonstrated by analysis and simulation that OFDMA outperforms OFDM-TDMA in QoS metrics of interest. Thus, OFDMA has higher potential than OFDM-TDMA in supporting multimedia services.     

Transport Mechanism of SiGe Dot MOS Tunneling Diodes

The blockage of hole transport due to excess holes In SiGe dots was observed in the MOS tunneling diodes for the first time. The five layers of self-assembled SiGe dots are separated by 74-nm Si spacers and capped with a 130-nm Si. The hole tunneling current from Pt gate electrode to p-type Si dominates the inversion current at positive gate bias and is seven orders of magnitude higher than the Al gate/oxide/p-Si device. The large work function of Pt is responsible for the hole transport current from Pt to p-Si. The incorporation of SiGe dots confines the excess holes in the valence band and forms a repulsive barrier to reduce the hole transport current from Pt to SiGe dots by 2-3 orders of magnitude in comparison with the Pt/oxide/p-Si device. This repulsive barrier also reduces the hole tunneling current from SiGe dots to Pt at negative gate bias.     

Co‐tier downlink interference management in dense femtocell networks

With the adoption of long‐term evolution standard for 4G mobile communications, the deployment of femtocell base stations (FBSs) to cope with the surging traffic in mobile wireless communication is becoming increasingly popular. However, with the random installation of FBSs, the problem of interference among FBSs is still a challenge. In this paper, assuming the presence of a femtocell management system that can control and coordinate the densely deployed FBSs, a novel power backoff scheme is proposed that determines the appropriate transmit power of each FBS so that the interference is reduced. Simulation results for randomly deployed FBSs in an environment with shadowing using MATLAB are provided, showing that our proposed methods can effectively mitigate the co‐tier downlink interference while improving the system capacity in a densely deployed femtocell network with shared spectrum use. Quantitatively, the average interference is reduced by roughly 90% to 100% of dBm, and the average capacity is increased by more than 80%. These results attest to the effectiveness of the proposed scheme.     

Time and temperature-dependent mechanical behavior of underfill materials in electronic packaging application

The thermo-mechanical testing of HYSOL FP4549 polymer-filled underfill materials was conducted under different strain rate and temperature environment. A new specimen preparation procedure and further test methodology are developed to characterize the time–temperature mechanical behaviors of underfill materials. The stress–strain behavior of materials is simulated with constitutive framework, and the dependence of Young’s modulus on temperature and strain rate was evaluated. In addition, the specimens were tested with microforce testing system to evaluate the creep curve of underfill materials as a function of temperature and stress level. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with unloading–reloading technique to verify the test results and investigate its cyclic mechanical behaviors. On the other hand, the adhesion strength of underfill materials are tested between different adhesion surface by different deformation rate after some isothermal and hygro-thermal environments attack, which is to simulate the environment that the electronic components may be encountered. The results reveal that the rise of the temperature and moisture cause the apparent reduction of the surface adhesion strength, due to the microstructure transition of materials and the diffusion and concentration of moisture. For all conditions of the experiment after environmental preconditioning, the specimen fracture surfaces occur between solder mask and FR4 substrates, which means the measured strength is the adhesion strength between solder mask and FR4. Comparing different adhesion surface, the adhesion strength of underfill/FR4 is higher than solder mask/FR4. The interface of solder mask/FR4 is more sensitive to the temperature and moisture. In all of the cases, increasing the moisture level has a varying but significant effect on both fracture strength and absorption energy Ψ. The failure mode transfer and the strength degradation are attributed to the moisture uptake between the FR4/solder mask and solder mask/underfill interface.     

Cryptanalysis of Hsiang‐Shih's authentication scheme for multi‐server architecture

From user point of view, password‐based remote user authentication technique is one of the most convenient and easy‐to‐use mechanisms to provide necessary security on system access. As the number of computer crimes in modern cyberspace has increased dramatically, the robustness of password‐based authentication schemes has been investigated by industries and organizations in recent years. In this paper, a well‐designed password‐based authentication protocol for multi‐server communication environment, introduced by Hsiang and Shih, is evaluated. Our security analysis indicates that their scheme is insecure against session key disclosure, server spoofing attack, and replay attack and behavior denial. Copyright © 2010 John Wiley & Sons, Ltd.     

Hierarchical B-picture mode decision in H.264/SVC

To enable robust video transmission over heterogeneous networks, the hierarchical B-picture prediction structure is employed in the state-of-the-art video coding standard H.264/SVC, aiming to produce scalable bitstreams with various frame rates. However, the exhaustive mode decision process with the hierarchical B-picture structure increases the computational complexity of H.264/SVC encoding dramatically. In this paper, a fast mode decision algorithm is proposed to speed up H.264/SVC encoding with the hierarchical B-picture structure, which is achieved by utilizing macroblock (MB) features, correlation of temporal–spatial neighboring MBs, and the discrepant characteristics of hierarchical layers. Extensive experimental results demonstrate that the proposed algorithm is able to reduce the encoding time of H.264/SVC significantly for video sequences with a wide range of resolutions, and meanwhile the video quality and compression ratio are well preserved.