Catálogo de publicaciones - revistas

<jats:p>Understanding the evolution of irradiation-induced defects is of critical importance for the performance estimation of nuclear materials under irradiation. Hereby, we systematically investigate the influence of He on the evolution of Frenkel pairs and collision cascades in tungsten (W) via using the object kinetic Monte Carlo (OKMC) method. Our findings suggest that the presence of He has significant effect on the evolution of irradiation-induced defects. On the one hand, the presence of He can facilitate the recombination of vacancies and self-interstitial atoms (SIAs) in W. This can be attributed to the formation of immobile He-SIA complexes, which increases the annihilation probability of vacancies and SIAs. On the other hand, due to the high stability and low mobility of He-vacancy complexes, the growth of large vacancy clusters in W is kinetically suppressed by He addition. Specially, in comparison with the injection of collision cascades and He in sequential way at 1223 K, the average sizes of surviving vacancy clusters in W via simultaneous way are smaller, which is in good agreement with previous experimental observations. These results advocate that the impurity with low concentration has significant effect on the evolution of irradiation-induced defects in materials, and contributes to our understanding of W performance under irradiation.</jats:p>

<jats:p>The evolution of helium bubbles in purity Mo was investigated by <jats:italic>in-situ</jats:italic> transmission electron microscopy (TEM) during 30 keV He<jats:sup>+</jats:sup> irradiation (at 673 K and 1173 K) and post-irradiation annealing (after 30 keV He<jats:sup>+</jats:sup> irradiation with the fluence of 5.74 × 10<jats:sup>16</jats:sup> He<jats:sup>+</jats:sup>/cm<jats:sup>2</jats:sup> at 673 K). Both He<jats:sup>+</jats:sup> irradiation and subsequently annealing induced the initiation, aggregation, and growth of helium bubbles. Temperature had a significant effect on the initiation and evolution of helium bubbles. The higher the irradiation temperature was, the larger the bubble size at the same irradiation fluence would be. At 1173 K irradiation, helium bubbles nucleated and grew preferentially at grain boundaries and showed super large size, which would induce the formation of microcracks. At the same time, the geometry of helium bubbles changed from sphericity to polyhedron. The polyhedral bubbles preferred to grow in the shape bounded by {100} planes. After statistical analysis of the characteristic parameters of helium bubbles, the functions between the average size, number density of helium bubbles, swelling rate and irradiation damage were obtained. Meanwhile, an empirical formula for calculating the size of helium bubbles during the annealing was also provided.</jats:p>

<jats:p>The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics (MD) simulations. In this work, the short-range repulsive interaction of a reactive force field (ReaxFF), describing Fe–Ni–Al alloy system, is well modified by adding a tabulated function form based on Ziegler–Biersack–Littmark (ZBL) potential. The modified interaction covers three ranges, including short range, smooth range, and primordial range. The short range is totally predominated by ZBL potential. The primordial range means the interactions in this range is the as-is ReaxFF with no changes. The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function. Both energies and forces are guaranteed to be continuous, and qualified to the consistent requirement in LAMMPS. This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects' booming and recombination effectively.</jats:p>

<jats:p>Mo, as a dopant, is doped into SbTe to improve its thermal stability. It is shown in this paper that the Mo-doped Sb<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub> (Mo<jats:sub>0.26</jats:sub>Sb<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub>, MST) material possesses phase change memory (PCM) applications. MST has better thermal stability than Sb<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub>(ST) and will crystallize only when the annealing temperature is higher than 250 °C. With the good thermal stability, MST-based PCM cells have a fast crystallization time of 6 ns. Furthermore, endurance up to 4 × 10<jats:sup>5</jats:sup> cycles with a resistance ratio of more than one order of magnitude makes MST a promising candidate for PCM applications.</jats:p>

<jats:p>Needle-like single crystals of CeAu<jats:sub>2</jats:sub>In<jats:sub>4</jats:sub> have been grown from In flux and characterized as a new candidate of quasi-one-dimensional Kondo lattice compound by crystallographic, magnetic, transport, and specific-heat measurements down to very low temperatures. We observe an antiferromagnetic transition at <jats:italic>T</jats:italic> <jats:sub>N</jats:sub> ≈ 0.9 K, a highly non-mean-field profile of the corresponding peak in specific heat, and a large Sommerfeld coefficient <jats:italic>γ</jats:italic> = 369 mJ⋅mol<jats:sup>−1</jats:sup>⋅K<jats:sup>−2</jats:sup>. The Kondo temperature <jats:italic>T</jats:italic> <jats:sub>K</jats:sub> is estimated to be 1.1 K, being low and comparable to <jats:italic>T</jats:italic> <jats:sub>N</jats:sub>. While Fermi liquid behavior is observed deep into the magnetically ordered phase, the Kadowaki–Woods ratio is much reduced relative to the expected value for Ce compounds with Kramers doublet ground state. Markedly, this feature shares striking similarities to that of the prototypical quasi-one-dimensional compounds YbNi<jats:sub>4</jats:sub>P<jats:sub>2</jats:sub> and CeRh<jats:sub>6</jats:sub>Ge<jats:sub>4</jats:sub> with tunable ferromagnetic quantum critical point. Given the shortest Ce–Ce distance along the needle direction, CeAu<jats:sub>2</jats:sub>In<jats:sub>4</jats:sub> appears to be an interesting model system for exploring antiferromagnetic quantum critical behaviors in a quasi-one-dimensional Kondo lattice with enhanced quantum fluctuations.</jats:p>

<jats:p>Ultra-thin barrier (UTB) 4-nm-AlGaN/GaN normally-off high electron mobility transistors (HEMTs) having a high current gain cut-off frequency (<jats:italic>f</jats:italic> <jats:sub>T</jats:sub>) are demonstrated by the stress-engineered compressive SiN trench technology. The compressive <jats:italic>in-situ</jats:italic> SiN guarantees the UTB-AlGaN/GaN heterostructure can operate a high electron density of 1.27 × 10<jats:sup>13</jats:sup> cm<jats:sup>−2</jats:sup>, a high uniform sheet resistance of 312.8 Ω/□, but a negative threshold for the short-gate devices fabricated on it. With the lateral stress-engineering by full removing <jats:italic>in-situ</jats:italic> SiN in the 600-nm SiN trench, the short-gated (70 nm) devices obtain a threshold of 0.2 V, achieving the devices operating at enhancement-mode (E-mode). Meanwhile, the novel device also can operate a large current of 610 mA/mm and a high transconductance of 394 mS/mm for the E-mode devices. Most of all, a high <jats:italic>f</jats:italic> <jats:sub>T</jats:sub>/<jats:italic>f</jats:italic> <jats:sub>max</jats:sub> of 128 GHz/255 GHz is obtained, which is the highest value among the reported E-mode AlGaN/GaN HEMTs. Besides, being together with the 211 GHz/346 GHz of <jats:italic>f</jats:italic> <jats:sub>T</jats:sub>/<jats:italic>f</jats:italic> <jats:sub>max</jats:sub> for the D-mode HEMTs fabricated on the same materials, this design of E/D-mode with the realization of <jats:italic>f</jats:italic> <jats:sub>max</jats:sub> over 200 GHz in this work is the first one that can be used in Q-band mixed-signal application with further optimization. And the minimized processing difference between the E- and D-mode designs the addition of the SiN trench, will promise an enormous competitive advantage in the fabricating costs.</jats:p>

<jats:p>First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors. However, in addition to the high computational cost, it is still a challenge to obtain accurate mobility for carriers with a complex band structure, e.g., hole mobility in common semiconductors. Here, we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations. This treatment greatly reduces the computational cost in two ways: relieves the requirement of an extremely dense <jats:italic> <jats:bold>k</jats:bold> </jats:italic> mesh to obtain a smooth change in group velocity, and reduces the 5-dimensional integral to 3-dimensional integral. Taking Si and SiC as two examples, we find that this simplified approach reproduces the full first-principles calculation for mobility. If we use experimental effective masses to evaluate the group velocity, we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range. These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.</jats:p>

<jats:p>Two-dimensional (2D) van der Waals material is a focus of research for its widespread application in optoelectronics, memories, and spintronics. The ternary compound Nb<jats:sub>2</jats:sub>SiTe<jats:sub>4</jats:sub> is a van der Waals semiconductor with excellent air stability and small cleavage energy, which is suitable for preparing a few layers counterpart to explore novel properties. Here, properties of bulk Nb<jats:sub>2</jats:sub>SiTe<jats:sub>4</jats:sub> with large in-plane electrical anisotropy are demonstrated. It is found that hole carriers dominate at a temperature above 45 K with a carrier active energy of 31.3 meV. The carrier mobility measured at 100 K is about 213 cm<jats:sup>2</jats:sup>⋅V<jats:sup>−1</jats:sup>⋅s<jats:sup>−1</jats:sup> in bulk Nb<jats:sub>2</jats:sub>SiTe<jats:sub>4</jats:sub>, higher than the reported results. In a thin flake Nb<jats:sub>2</jats:sub>SiTe<jats:sub>4</jats:sub>, the resistivity ratio between the crystalline axes of <jats:italic>a</jats:italic> and <jats:italic>b</jats:italic> is reaching about 47.3 at 2.5 K, indicating that there exists a large anisotropic transport behavior in their basal plane. These novel transport properties provide accurate information for modulating or utilizing Nb<jats:sub>2</jats:sub>SiTe<jats:sub>4</jats:sub> for electronic device applications.</jats:p>

<jats:p>To deal with the invalidation of commonly employed series model and parallel model in capacitance–voltage (<jats:italic>C</jats:italic>–<jats:italic>V</jats:italic>) characterization of organic thin films when current injection is significant, a three-element equivalent circuit model is proposed. On this basis, the expression of real capacitance in consideration of current injection is theoretically derived by small-signal analysis method. The validity of the proposed equivalent circuit and theoretical expression are verified by a simulating circuit consisting of a capacitor, a diode, and a resistor. Moreover, the accurate <jats:italic>C</jats:italic>–<jats:italic>V</jats:italic> characteristic of an organic thin film device is obtained <jats:italic>via</jats:italic> theoretical correction of the experimental measuring result, and the real capacitance is 35.7% higher than the directly measured capacitance at 5-V bias in the parallel mode. This work strongly demonstrates the necessity to consider current injection in <jats:italic>C</jats:italic>–<jats:italic>V</jats:italic> measurement and provides a strategy for accurate <jats:italic>C</jats:italic>–<jats:italic>V</jats:italic> characterization experimentally.</jats:p>

<jats:p>The slower response speed is the main problem in the application of ZnO quantum dots (QDs) photodetector, which has been commonly attributed to the presence of excess oxygen vacancy defects and oxygen adsorption/desorption processes. However, the detailed mechanism is still not very clear. Herein, the properties of ZnO QDs and their photodetectors with different amounts of oxygen vacancy (V<jats:sub>O</jats:sub>) defects controlled by hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>) solution treatment have been investigated. After H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> solution treatment, V<jats:sub>O</jats:sub> concentration of ZnO QDs decreased. The H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> solution-treated device has a higher photocurrent and a lower dark current. Meanwhile, with the increase in V<jats:sub>O</jats:sub> concentration of ZnO QDs, the response speed of the device has been improved due to the increase of oxygen adsorption/desorption rate. More interestingly, the response speed of the device became less sensitive to temperature and oxygen concentration with the increase of V<jats:sub>O</jats:sub> defects. The findings in this work clarify that the surface V<jats:sub>O</jats:sub> defects of ZnO QDs could enhance the photoresponse speed, which is helpful for sensor designing.</jats:p>