Intersubband absorption properties of high Al content AlxGa1?xN/GaN multiple quantum wells grown with different interlayers by metal organic chemical vapor deposition

High Al content Al_xGa_1−xN/GaN multiple quantum well (MQW) films with different interlayers were grown by metal organic chemical vapor deposition. These MQWs were designed to achieve intersubband (ISB) absorption in the mid-infrared spectral range. We have considered two growth conditions, with AlGaN interlayer and GaN/AlN superlattice (SL) interlayer, both deposited on GaN-on-sapphire templates. Atomic force microscopy images show a relatively rough surface with atomic-step terraces and surface depression, mainly dominated by dislocations. High-resolution X-ray diffraction and transmission electron microscopy analyses indicate that good crystalline quality of the AlGaN/GaN MQW layer could be achieved when the AlGaN interlayer is inserted. The ISB absorption with a peak at 3.7 μm was demonstrated in MQW films with AlGaN interlayer. However, we have not observed the infrared absorption in MQW films with GaN/AlN SL interlayer. It is believed that the high dislocation density and weaker polarization that resulted from the rough interface are determinant factors of vanished ISB absorption for MQW films with the GaN/AlN SL interlayer.     

Nearly full-dense and fine-grained AZO:Y ceramics sintered from the corresponding nanoparticles

ABSTRACT: Aluminum-doped zinc oxide ceramics with yttria doping (AZO:Y) ranging from 0 to 0.2 wt.% were fabricated by pressureless sintering yttria-modified nanoparticles in air at 1,300 [DEGREE SIGN]C. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, a physical property measurement system, and a densimeter were employed to characterize the precursor nanoparticles and the sintered AZO ceramics. It was shown that a small amount of yttria doping can remarkably retard the growth of the as-received precursor nanoparticles, further improve the microstructure, refine the grain size, and enhance the density for the sintered ceramic. Increasing the yttria doping to 0.2 wt.%, the AZO:Y nanoparticles synthetized by a coprecipitation process have a nearly sphere-shaped morphology and a mean particle diameter of 15.1 nm. Using the same amount of yttria, a fully dense AZO ceramic (99.98% of theoretical density) with a grain size of 2.2 mum and a bulk resistivity of 4.6 [MULTIPLICATION SIGN] 10[MINUS SIGN]3 [OHM SIGN][MIDDLE DOT]cm can be achieved. This kind of AZO:Y ceramic has a potential to be used as a high-quality sputtering target to deposit ZnO-based transparent conductive films with better optical and electrical properties.     

Gallium nitride nanowires doped with magnesium

GaN nanowires doped with Mg have been synthesized on Si (111) substrate through ammoniating Ga₂O₃ films doped with Mg under flowing ammonia atmosphere. The Mg-doped GaN nanowires were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL). The results demonstrate that the nanowires were single crystalline with hexagonal wurzite structure. The diameters of the nanowires ranged 20-30 nm and the lengths were about hundreds of micrometers. The intense PL peak at 359 nm showed a blueshift from the bulk band gap emission, attributed to Burstein-Moss effect. The growth mechanism of the crystalline GaN nanowires is discussed briefly.     

FTIR Spectra of the M(EDTA)n? Complexes in the Process of Sol-Gel Technique

Sol-gel is one of the effective techniques for preparing YBa₂Cu₃O_{7 – uj727811q2870584/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">} (YBCO) coatings at relatively low cost. Its process from gel to oxide coatings, however, was not yet fully understood. This is one of the obstacles for achieving YBCO coatings with good superconducting properties. In this paper, we chose EDTA as the complexant to prepare gels with different kinds of M(EDTA)^n– complexes, where M is any combination of Y, Ba, and Cu cations, in order to investigate the decomposition process of the gels with the help of Fourier transform infrared (FTIR) spectra. Comparing the FTIR spectra of the gel powders with those of the calcined powders, we found that the decomposed temperatures were different for different M(EDTA)^n– complexes. We considered this was the cause of the Ba segregation (as BaCO₃) in the coatings at low heating temperature, which in turn blocked the formation of high quality YBa₂Cu₃O_{7–uj727811q2870584/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">}.     

Scientific aspects of deformability of aluminum alloys during extrusion

The value of the limiting flow rate (V _max) indicated by formation of tearing on the surface of extruded products has been selected as an indicator of deformability during extrusion of aluminum alloys. Formation of tearing is due to embrittlement of grain boundaries occurring when the alloy temperature rises and reaches uj8jpl6/xxlarge8220.gif" alt="ldquo" align="MIDDLE" BORDER="0">subsolidusuj8jpl6/xxlarge8221.gif" alt="rdquo" align="MIDDLE" BORDER="0"> temperatures. The temperature rise to uj8jpl6/xxlarge8220.gif" alt="ldquo" align="MIDDLE" BORDER="0">subsolidusuj8jpl6/xxlarge8221.gif" alt="rdquo" align="MIDDLE" BORDER="0"> temperatures occurs because of deformation heat (adiabatic heating) which depends on the deformation resistance of the alloy for the given temperature/strain rate conditions present during the extrusion. The deformation resistance is a fundamental feature of the alloy determining theV _max. The amount of alloying elements in solid solution have a major influence on the limiting flow rate. Second phase particles and grain and dendrite structures are not as important as the amount of alloying elements in solid solution. Scientific principles for the optimum alloying and heat-treatment of aluminum alloys intended for extrusion are presented. The second part of the article discusses the potential for using a uj8jpl6/xxlarge8220.gif" alt="ldquo" align="MIDDLE" BORDER="0">heterogenizationuj8jpl6/xxlarge8221.gif" alt="rdquo" align="MIDDLE" BORDER="0"> anneal for aluminum alloy ingots before extrusion, allowing significant increases in theV _max.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_{7–uj0n652r65/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">} and YBa₂Cu₃O_{7–uj0n652r65/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">}-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_{7–uj0n652r65/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">} on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_{7–uj0n652r65/xxlarge948.gif" alt="delta" align="BASELINE" BORDER="0">} thick films with thickness as low as 3 uj0n652r65/xxlarge956.gif" alt="mgr" align="MIDDLE" BORDER="0">m even on polycrystalline substrates.     

Formation and properties of defects and small vacancy clusters in SiC: Ab initio calculations

Large-scale ab initio simulation methods have been employed to investigate the configurations and properties of defects in SiC. Atomic structures, formation energies and binding energies of small vacancy clusters have also been studied as a function of cluster size, and their relative stabilities are determined. The calculated formation energies of point defects are in good agreement with previously theoretical calculations. The results show that the di-vacancy cluster consists of two C vacancies located at the second nearest neighbor sites is stable up to 1300 K, while a di-vacancy with two Si vacancies is not stable and may dissociate at room temperature. In general, the formation energies of small vacancy clusters increase with size, but the formation energies for clusters with a Si vacancy and nC vacancies (V_Si-nV_C) are much smaller than those with a C vacancy and nSi vacancies (V_C-nV_Si). These results demonstrate that the V_Si-nV_C clusters are more stable than the V_C-nV_Si clusters in SiC, and provide possible nucleation sites for larger vacancy clusters or voids to grow. For these small vacancy clusters, the binding energy decreases with increasing cluster size, and ranges from 2.5 to 4.6 eV. These results indicate that the small vacancy clusters in SiC are stable at temperatures up to 1900 K, which is consistent with experimental observations.