Enhanced transport and transistor performance with oxide seeded high-κ gate dielectrics on wafer-scale epitaxial graphene

We explore the effect of high-κ dielectric seed layer and overlayer on carrier transport in epitaxial graphene. We introduce a novel seeding technique for depositing dielectrics by atomic layer deposition that utilizes direct deposition of high-κ seed layers and can lead to an increase in Hall mobility up to 70% from as-grown. Additionally, high-κ seeded dielectrics are shown to produce superior transistor performance relative to low-κ seeded dielectrics and the presence of heterogeneous seed/overlayer structures is found to be detrimental to transistor performance, reducing effective mobility by 30-40%. The direct deposition of high-purity oxide seed represents the first robust method for the deposition of uniform atomic layer deposited dielectrics on epitaxial graphene that improves carrier transport.     

Strategies for Achieving Strength–Ductility Synergy in Face-Centered Cubic High- and Medium-Entropy Alloys

High- and medium-entropy alloys (HEAs/MEAs), also called as multicomponent alloys, are a new class of materials that break through the traditional alloy design concept based on single principal element. However, they do not break away from the magic spell of strength–ductility trade-off. Therefore, designing HEAs/MEAs with both high strength and high ductility still remains a great challenge nowadays. This article provides a review on the recent progress in mechanical properties of face-centered cubic (FCC) HEAs/MEAs. First, several traditional strengthening strategies are briefly reviewed, focusing on the strengthening mechanisms and the optimized mechanical properties. Subsequently, various novel strategies for achieving strength–ductility synergy in HEAs/MEAs are summarized, which include lowering the stacking fault energy, regulating the short-range order, promoting transformation-induced plasticity, and constructing heterogeneous microstructures. The basic ideas and related underlying mechanisms from these strategies are discussed. Finally, the current challenges and the future outlooks are emphasized and addressed systematically. In brief, the present review is expected to provide a useful guide for the design of HEAs/MEAs with superior mechanical properties.     

Singularity parametersε andκ for interface cracks in transversely isotropic piezoelectric bimaterials

Two parameters, and (Suo et al., 1992), are of key importance in fracture mechanics of piezoelectric material interfaces. In this paper, it is shown, for any transversely isotropic piezoelectric (TIP) bimaterial, that one of the two parameters and always vanishes but the other one remains non-zero. Physically, it means that the non-oscillating crack-tip generalized stress field singularity exists for some TIP bimaterials (with vanishing ). Consequently, TIP bimaterials can be classified into two classes: one with vanishing performed crack tip generalized stress field oscillating singularity and the other one with vanishing is independent from the oscillating singularity. Some numerical results for and are given too.     

Preparation and characterization of TaAlOx high-κ dielectric for metal-insulator-metal capacitor applications

Metal-insulator-metal (MIM) capacitors with excellent electrical properties have been fabricated using high-κ TaAlO_x-based dielectrics. TaAlO_x films having thickness of 11.5-26.0 nm, with equivalent oxide thickness (EOT) of ~ 2.3-5.3 nm were deposited on top of Au/SiO₂ (180 nm)/Si (100) structures by radio frequency magnetron co-sputtering of Ta₂O₅ and Al₂O₃ targets. The surface chemical states of the as-deposited TaAlO_x films were characterized by high-resolution X-ray photoelectron spectroscopy. The crystallinity of the TaAlO_x films for various post-deposition annealing treatments was characterized by grazing incident X-ray diffraction, which reveals that an amorphous phase is still retained for rapid thermal annealing up to 500 °C. Besides a high capacitance density (~ 5.4 to 6.6 fF/μm² at 1 kHz), a low value of voltage coefficients of capacitance and a stable temperature coefficient of capacitance have also been obtained in MIM capacitors with TaAlO_x films. Degradation phenomenon of TaAlO_x-based MIM capacitors under constant current stressing at 20 nA is found to be strongly dependent on dielectric thickness. It is shown that Al-incorporated Ta₂O₅ (TaAlO_x) films with high band gap and good thermal stability, low leakage current and good voltage linearity make it one of the most promising candidates for metal-insulator-metal capacitor applications.     

High- and low-cycle fatigue influence of silicon,copper, strontium and iron on hypo-eutectic Al–Si–Cu and Al–Si–Mg cast alloys used in cylinder heads

In this publication, ambient condition fatigue investigations with different types of Al–Si–Cu and Al–Si–Mg cast alloys in rotating-bending high-cycle fatigue (HCF) and push–pull low-cycle fatigue (LCF) regimes have been performed with varying Si, Cu, Fe and Sr contents. The cast alloys investigated here are common used in cylinder heads for automotive application. Because the cylinder head is one of the most fatigued parts in combustion chamber engines, the microstructural knowledge of the damage process provides a tool of construction and its material selection. The investigations were also supported with an in-situ microstructural crack observation in high plasticity rotating-bending regimes. The specimens were directly processed out of serial produced T79 heat-treated cylinder heads to provide the equal microstructure for testing as under operational conditions.The observations clearly identified the effects of the individual alloying elements both under low- and high-cycle fatigue. The crack propagation speed and the crack paths were majorly influenced by the eutectic silicon. Additional, the precipitation hardening due to copper affected significantly the fatigue endurance, too. In high plasticities the silicon’s influence got almost lost and only the matrix strength was crucial. Thus, increased fatigue strength in high loaded LCF regimes was observed for alloys with less copper content, thus higher ductility. By contrast, improved HCF and low loaded LCF endurance was only achieved when the matrix strength was increased by copper’s precipitation hardening. Crack branching and deflections strongly influenced the microstructural damage of the ductile AlSi7Mg(Sr) and hence, gained its fatigue strength. Iron phases could not identified as harmful inclusions, since the phases were similar in size of other hard phase elements like the other primary intermetallic phases like ${\text{Al}}_2\text{Cu}$ and β-$\text{Si}$ phases under notch stress aspects, by the well defined solidification process in the test section. Because the crack nucleation mainly occurred on Si particles, strontium as a refinement agent influenced the early crack onset and accordingly the fatigue in total. Thus, the AlSi6Cu4(Sr) had increased lifetimes compared to AlSi6Cu4 both in HCF and LCF. Further, the presented results provide a modification of the Manson–Coffin approach to describe the relationship between plastic strain and lifetime, valid for all proposed alloys with only one set of parameters. Thus, it was possible to perform the fatigue calculation with a reduced range of scatter.     

Dynamic ageing and the mechanical response of Al–Mg–Si alloy through equal channel angular pressing

In this paper, dynamic ageing characteristics associated with the application of equal channel angular pressing (ECAP) to Al6061 alloy at elevated temperatures was investigated. Followed by ECAP, Vickers microhardness measurement on the cross-sectional planes and microstructural observations were undertaken using transmission electron microscopy. The combination of the ECAP process with dynamic ageing at both 100 °C and 150 °C resulted in a significant increase in hardness. The grain size was measured as ∼160 nm after four passes. A comparison with the published data on the same alloy processed by ECAP at room temperature and statically aged, suggests several advantages in incorporating dynamic ageing with ECAP. These advantages consist of the ability to attain better grain refinement, increased hardness and the potential for saving time and energy.     

Joint maximum-likelihood frequency offset and channel estimation for multiple-input multiple-output?orthogonal frequencydivision multiplexing systems

The problem of joint maximum-likelihood estimation of the carrier-frequency offset (CFO) and channel coefficients in multiple-input multiple-output orthogonal frequency-division multiplexing systems is investigated, assuming that a training sequence is available. The exact solution to this problem turns out to be too complex for practical purposes. To overcome this difficulty, the authors resort to the expectation?maximisation (EM) algorithm and propose an iterative scheme which iterates between estimating the channel parameters and the frequency offset. This results in an estimation algorithm of a reasonable complexity which is suitable for practical applications. Moreover, the Cramer?Rao bounds (CRB) for both CFO and channel estimators are developed to evaluate the performance of the proposed scheme. Simulation results show that the proposed algorithm achieves almost ideal performance compared with the CRBs in all ranges of signal-to-noise ratio for both channel and frequency offset estimates.     

Electrophoretic deposition from aqueous suspensions for near-shape manufacturing of advanced ceramics and glasses—applications

Due to high deposition rates and the avoidance of inflammable, often hazardous organic solvents EPD from aqueous suspensions is a fast and low-cost shaping technique for ceramics and glasses. Since the deposition rate is independent of particle size EPD has an outstanding ability for the shaping of nano-particles. In this paper the shaping of complex silica glass and zirconia components, like tubes or structured parts by means of the membrane method is shown. Three-dimensional shaped porous polymer moulds were used as ion-permeable deposition surface. To enable near-shape manufacturing, mixtures of nanosized and microsized particles were electrophoretically deposited. No size-dependent separation was observed. Due to the very high green density of these green bodies (up to 84% of the theoretical value) shrinkage could be reduced to 4.7%. Not only oxide ceramics but also silicon carbide was deposited from aqueous suspensions. Apart from bulk SiC, protective coatings with a thickness of app. 60 m were applied on top of CFC substrates by EPD. Good adhesion was observed and no cracking occurred. Furthermore, electrophoretic impregnation was used for the modification of porous green bodies. Thus silica glasses with graded density and pore size as well as functionally graded composites were prepared.     

Flexible carbon@graphene composite cloth for advanced lithium–sulfur batteries and supercapacitors with enhanced energy storage capability

Flexible carbon@graphene composite cloth was fabricated, and the resultant composite cloth consists of core–shell hollow structured carbon/graphene hybrid fibers with abundant micro- and mesoporosity and hydrophilic functionality. These unique features enable the composite cloth to be a promising material for energy storage application. As an efficient polysulfide adsorbent, the composite can be applied in lithium–sulfur batteries by being sandwiched between the sulfur cathode and polymeric separator. With this novel configuration, a high reversible capacity of ca. 900 mAh g^?1 and excellent cycle-life has been achieved, which is ascribed to the excellent polysulfides adsorption and confinement capability of the special core–shell and hollow structured porous hybrid fibers. Additionally, the composite cloth can be applied in supercapacitors as a flexible binder-free electrode, exhibiting a high specific capacitance of 271 F g^?1 (360 F cm^?3) at 0.1 A g^?1 in 6 M KOH, as well as excellent rate capability and cycling stability. The assembled symmetric supercapacitor supplies a high energy density of up to 9.4 Wh kg^?1 (12.5 Wh L^?1) with a power density of 25.0 W kg^?1 (33.3 W L^?1) and remains 5.7 Wh kg^?1 (7.6 Wh L^?1) with 4.5 kW kg^?1 (6.0 kW L^?1).     

Self-consistent solution of two-dimensional poisson and Schrödinger wave equations for nanoscale MOSFET approaching ballistic limit

A numerical solution of the potential distribution of two-dimensional Poisson equation and Schr?dinger wave equation under a set of boundary conditions has been obtained for a deep submicron and nanoscale MOSFET. The output characteristics can be found out by simply solving the two-dimensional Poisson equation under specific boundary conditions governed by the physics of the device. The channel potential profile has been presented. It is seen that the classical model underestimates the channel voltage and hence the longitudinal electric field in the channel as compared to that obtained through the quantum mechanical approach. For the purpose of validation, the results obtained on the basis of our model have been compared and contrasted with reported experimental result.     

An efficient joint channel estimation and decoding algorithm for turbo-coded space?time orthogonal frequency division multiplexing receivers

The challenging problem in the design of digital receivers of today's and future high-speed, high data-rate wireless communication systems is to implement the optimal decoding and channel estimation processes jointly in a computationally feasible way. Without realising such a critical function perfectly at receiver, the whole system will not work properly within the desired performance limits. Unfortunately, direct implementation of such optimal algorithms is not possible mainly due to their mathematically intractable and computationally prohibitive nature. A novel algorithm that reaches the performance of the optimal maximum a posteriori (MAP) algorithm with a feasible computational complexity is proposed. The algorithm makes use of a powerful statistical signal processing tool called the expectation maximisation (EM) technique. It iteratively executes the MAP joint channel estimation and decoding for space'time block-coded orthogonal frequency division multiplexing systems with turbo channel coding in the presence of unknown wireless dispersive channels. The main novelty of the work comes from the facts that the proposed algorithm estimates the channel in a non-data-aided fashion and therefore except a small number of pilot symbols required for initialisation, no training sequence is necessary. Also the approach employs a convenient representation of the discrete multipath fading channel based on the Karhunen'Loeve (KL) orthogonal expansion and finds MAP estimates of the uncorrelated KL series expansion coefficients. Based on such an expansion, no matrix inversion is required in the proposed MAP estimator. Moreover, optimal rank reduction is achieved by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the iterative estimation approach.     

Study of electrical and micro-structural properties of high-κ gate dielectric stacks deposited using pulse laser deposition for MOS capacitor applications

The electrical properties of hafnium oxide (HfO₂) gate dielectric as a metal–oxide–semiconductor (MOS) capacitor structure deposited using pulse laser deposition (PLD) technique at optimum substrate temperatures in an oxygen ambient gas are investigated. The film thickness and microstructure are examined using ellipsometer and atomic force microscope (AFM), respectively to see the effect of substrate temperatures on the device properties. The electrical J–V, C–V characteristics of the dielectric films are investigated employing Al–HfO₂–Si MOS capacitor structure. The important parameters like leakage current density, flat-band voltage (V_fb) and oxide-charge density (Q_ox) for MOS capacitors are extracted and investigated for optimum substrate temperature. Further, electrical studies of these MOS capacitors have been carried out by incorporating La₂O₃ into HfO₂ to fabricate HfO₂/La₂O₃ dielectric stacks at an optimized substrate temperature of 800 °C using a PLD deposition technique under oxygen ambient. These Al–HfO₂–La₂O₃–Si dielectric stacks MOS capacitor structure are found to possess better electrical properties than that of HfO₂ based MOS capacitors using the PLD deposition technique.     

Guidelines for establishment of correlations between mechanical properties and microstructure in Al–Si alloys

Abstract

This paper reports an analysis of the accuracy and sensitivity of ultimate tensile strength and strain to failure to changes in microstructure constituents in Al–Si alloys ranging from 7 to 18%Si. The influence of amount of constituents, namely dendrites, eutectic, silicon particles and intermetallics as well as their geometrical features, namely secondary dendrite arm spacing, silicon eutectic thickness and intermetallics thickness on both ultimate tensile strength and strain to failure is evaluated. This study provides information that will be useful in the establishment of robust correlations between mechanical properties and metallurgical features, since they are highly dependent on their sensitivity and accuracy.     

X-Ray Micro-Calorimeter Based on Si Thermistors for X-Ray Astronomy: Design and First Measurements

X-ray Astronomy provides a unique window on a wide variety of astrophysical phenomena. The currently operating X-ray space observatories perform X-ray spectral imaging with the use of CCDs. When available, cryogenic X-ray microcalorimeter arrays will far outperform CCDs in terms of spectral resolution, energy bandwidth and count rate. Experience has been gained with Infra-Red bolometer arrays at CEA-LETI (Grenoble) in collaboration with the CEA-SAp (Saclay); taking advantage of this background, we are now developing an X-ray spectro-imaging camera for the next generation space astronomy missions, using silicon technology (implanted and high temperature diffused thermistors). Each pixel of this array detector is made of a tantalum absorber bound, by indium bump hybridization, to a silicon thermistor. The absorber array is bound to the thermistor array in a single automatic step. The thermo-mechanical link, provided by hybridization, is being improved in terms of thermal capacitance. Finally, our main effort is in developing arrays of silicon thermistors with negligible excess 1/f noise. The thermistor has been simulated with the 2D simulator ATHENA (SILVACO International). We studied the effects of the implants and their thermal treatment on both vertical and lateral dopant distributions at the edges of the thermistor. Prototypes have been created following the procedure optimized by the ATHENA simulation. We present the status of the development and results of measurements performed on these four main building blocks required to create a detector array up to 32×32 pixels in size.      

Rational design of graphene @ nitrogen and phosphorous dual-doped porous carbon sandwich-type layer for advanced lithium–sulfur batteries

Lithium sulfur (Li–S) batteries show great prospect as a next generation high energy density rechargeable battery systems. However, the practical utilization of Li–S batteries is still obstructed by the shuttle effects which inducing the fast capacity fading and the loss of active sulfur. Herein, a special graphene @ nitrogen and phosphorous dual-doped porous carbon (N–P–PC/G) is presented to modify a commercial separator for an advanced Li–S battery. The N–P–PC/G nanosheet employs graphene layer as an excellent conductive framework covered with uniform layers of N, P dual-doped porous carbon on both sides which possessing massive interconnected meso-/micropores. It is demonstrated that the N–P–PC/G-modified separator can suppress the shuttle effects by coupling interactions including physical absorption, chemical adsorption and interfacial interaction. With the aid of the N–P–PC/G-modified separator, the pure sulfur cathode with high-sulfur loading of 3 mg cm^?2 offers a high initial discharge capacity of 1207 mA h g^?1 at 0.5 C (1 C = 1675 mA h g^?1), and a maintained capacity of 635 mA h g^?1 (fading rate of only 0.095% per cycle), after 500 cycles. This work suggests that combining hybrid nanocarbon with multi-heteroatom doping to modify the commercial separator is an effective approach to obtain high electrochemical performance Li–S batteries.     

Microstructure and thermal expansion behavior of spray-formed Al–27Si alloy used for electronic packaging

The hypereutectic Al–27Si (mass fraction) alloys are prepared by spray deposition and extrusion. The effect of thermal aging process on the coefficient of thermal expansion (CTE) and microstructure of the Al–27Si alloys are investigated. The results show that the distribution of Si particles in α-Al matrix is uniform, and the primary Si phase grows gradually during the process of thermal aging. The CTE between room temperature to 100 °C increases gradually with the ascending of aging temperature, attributed to the relaxation of residual thermal stress and the coarsening of primary Si phases in the alloys. On the other hand, the CTE increases linearly as the cycling temperature increases up to 500 °C, and the measured values are in good agreement with the Kerner model.     

An advanced model for initiation and propagation of damage under fatigue loading – part II: Matrix cracking validation cases

A series of tests were conducted to act as validation cases for the numerical model developed in part I of this paper to predict the initiation and propagation of damage in composite materials. The onset of matrix cracking in [0₂/θ₄]_s specimens under tension–tension fatigue loading was studied using acoustic emission (AE) and dye-penetrant enhanced X-rays. The number of cracks identified by significant AE hits correlated well with the number of cracks identified by X-rays. Finite Element simulations of the [0₂/θ₄]_s specimens using the model from part I for cohesive interface elements fatigue loading showed a good correlation with the experimental results.     

Mo–Si–B alloys for ultra-high-temperature space and ground applications: liquid-assisted fabrication under various temperature and time conditions

Journal of Materials Science - Boron-doped molybdenum silicides have been already recognized as attractive candidates for space and ground ultra-high-temperature applications far beyond limits of...     

Thermodynamic analysis and characterization of Bi–Cu–Sn alloys as advanced lead-free solder materials for high temperature application

Due to the frequent use of Cu as a substrate material in electronics, it is important to understand the interactions between the solders and the substrates, based on the knowledge of the phase equilibria, thermodynamics and other characteristics of the Bi–Cu–Sn system. The results of thermodynamic analysis and characterization of Bi–Cu–Sn alloys as advanced lead-free solder materials for high temperature application are presented in this paper. The research includes analytical investigations done by thermodynamic predicting according to general solution model in temperature range 1,073–1,273 K and experimental investigations performed by differential thermal analysis, differential scanning calorimetry, optical microscopy, hardness and electroconductivity measurements for the sections with molar ratio Bi:Cu = 1:1, 1:3 and 3:1.