Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.     

Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices

We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films on graphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.     

Observation of antiferromagnetic domains in epitaxial thin films

Antiferromagnetic domains in an epitaxial thin film, LaFeO(3) on SrTiO(3)(100), were observed using a high-spatial-resolution photoelectron emission microscope with contrast generated by the large x-ray magnetic linear dichroism effect at the multiplet-split L edge of Fe. The antiferromagnetic domains are linked to 90 degrees twinned crystallographic regions in the film. The Neel temperature of the thin film is reduced by 70 kelvin relative to the bulk material, and this reduction is attributed to epitaxial strain. These studies open the door for a microscopic understanding of the magnetic coupling across antiferromagnetic-ferromagnetic interfaces.     

Heterogeneous three-dimensional electronics by use of printed semiconductor nanomaterials

We developed a simple approach to combine broad classes of dissimilar materials into heterogeneously integrated electronic systems with two- or three-dimensional layouts. The process begins with the synthesis of different semiconductor nanomaterials, such as single-walled carbon nanotubes and single-crystal micro- and nanoscale wires and ribbons of gallium nitride, silicon, and gallium arsenide on separate substrates. Repeated application of an additive, transfer printing process that uses soft stamps with these substrates as donors, followed by device and interconnect formation, yields high-performance heterogeneously integrated electronics that incorporate any combination of semiconductor nanomaterials on rigid or flexible device substrates. This versatile methodology can produce a wide range of unusual electronic systems that would be impossible to achieve with other techniques.     

氮化镓(GaN)纳米材料的制备

通过直流电弧等离子体方法制备了氮化镓纳米晶,并研究了制备的样品的基本特性。使用N2和NH3的混合气体进行反应,并对样品进行了电子显微镜扫描(SEM),X射线衍射(XRD)和Raman散射分析。试验发现,样品为纤锌矿结构,平均大小为50nm左右,晶格常数为a=3.186,c=5.174。     

GaN photonic-crystal surface-emitting laser at blue-violet wavelengths

Shorter-wavelength surface-emitting laser sources are important for a variety of fields, including photonics, information processing, and biology. We report on the creation of a current-driven blue-violet photonic-crystal surface-emitting laser. We have developed a fabrication method, named "air holes retained over growth," in order to construct a two-dimensional gallium nitride (GaN)/air photonic-crystal structure. The resulting periodic structure has a photonic-crystal band-edge effect sufficient for the successful operation of a current-injection surface-emitting laser. This represents an important step in the development of laser sources that could be focused to a size much less than the wavelength and be integrated two-dimensionally at such short wavelengths.     

Imaging coherent electron flow from a quantum point contact

Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.     

Van der Waals Epitaxial Growth of agr-Alumina Nanocrystals on Mica

Lattice mismatch stresses, which severely restrict heteroepitaxial growth, are greatly minimized when thin alumina films are grown by means of van der Waals forces on inert mica substrates. A 10-nanometer-thick epitaxial film exhibits crystallographic sixfold symmetry, a lattice constant close to that of the basal plane [0001] of alpha-alumina (sapphire), and an aluminum: oxygen atomic ratio of 1:1.51 +/- 0.02 (measured by x-ray photoelectron spectroscopy), again the same as for bulk sapphire. The film is free of steps and grain boundaries over large areas and appears to be an ideal model system for studying adhesion, tribology, and other surface phenomena at atomic scales.     

A Benzene-Thermal Synthetic Route to Nanocrystalline GaN

A thermal reaction of Li3N and GaCl3 in which benzene was used as the solvent under pressure has been carried out for the preparation of 30-nanometer particles of gallium nitride (GaN) at 280°C. This temperature is much lower than that of traditional methods, and the yield of GaN reached 80%. The x-ray powder diffraction pattern indicated that sample was mainly hexagonal-phase GaN with a small fraction of rocksalt-phase GaN, which has a lattice constant a = 4.100 angstroms. This rocksalt structure, which had been observed previously only under high pressure (at least 37 gigapascals) was observed directly with high-resolution electron microscopy.     

Ferroelectric Bi3.25La0.75Ti3O12 films of uniform a-axis orientation on silicon substrates

The use of bismuth-layered perovskite films for planar-type nonvolatile ferroelectric random-access memories requires films with spontaneous polarization normal to the plane of growth. Epitaxially twinned a axis-oriented La-substituted Bi4Ti3O12 (BLT) thin films whose spontaneous polarization is entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates using SrRuO3 as bottom electrodes. Even though the (118) orientation competes with the (100) orientation, epitaxial films with almost pure (100) orientation were grown using very thin, strained SrRuO3 electrode layers and kinetic growth conditions, including high growth rates and high oxygen background pressures to facilitate oxygen incorporation into the growing film. Films with the a-axis orientation and having their polarization entirely along the direction normal to the film plane can achieve a remanent polarization of 32 microcoulombs per square centimeter.     

Ferroelectric Field Effect in Epitaxial Thin Film Oxide SrCuO2/Pb(Zr0.52Ti0.48)O3 Heterostructures

A ferroelectric field effect in epitaxial thin film SrCuO(2)/Pb(Zr(0.52)Ti(0.48))O(3) heterostructures was observed. A 3.5 percent change in the resistance of a 40 angstrom SrCuO(2) layer (a parent high-temperature superconducting compound) was measured when the polarization field of the Pb(Zr(0.52)Ti(0.48))O(3) layer was reversed by the application of a pulse of small voltage (<5 volts). This effect, both reversible and nonvolatile, is attributed to the electric field-induced charge at the interface of SrCuO(2) and Pb(Zr(0.52)Ti(0.48))O(3). This completely epitaxial thin film approach shows the possibility of making nonvolatile, low-voltage ferroelectric field effect devices for both applications and fundamental studies of field-induced doping in novel compounds like SrCuO(2).     

等离子增强型化学气相沉积氮化硅的淀积

等离子增强型化学气相淀积（ＰＥＣＶＤ）氮化硅是目前器件唯一能在合金化之后低温生长的氮化硅．本文研究了各种波积参数对薄膜性能的影响，提出了淀积优质氮化硅钝化膜的条件，对折射率随生长速率而变化给出了新的解释．     

Imaging the condensation and evaporation of molecularly thin films of water with nanometer resolution

The polarization force between an electrically charged atomic force microscope tip and a substrate has been used to follow the processes of condensation and evaporation of a monolayer of water on mica at room temperature. Condensation proceeds in two distinct structural phases. Up to about 25 percent humidity, the water film grows by forming two-dimensional clusters of less than a few 1000 angstroms in diameter. Above about 25 percent humidity, a second phase grows, forming large two-dimensional islands with geometrical shapes in epitaxial relation with the underlaying mica lattice. The growth of this second water phase is completed when the humidity reaches about 45 percent. The reverse process of evaporation has also been imaged.     

Enhanced enantioselectivity in excitation of chiral molecules by superchiral light

A molecule or larger body is chiral if it cannot be superimposed on its mirror image (enantiomer). Electromagnetic fields may be chiral, too, with circularly polarized light (CPL) as the paradigmatic example. A recently introduced measure of the local degree of chiral dissymmetry in electromagnetic fields suggested the existence of optical modes more selective than circularly polarized plane waves in preferentially exciting single enantiomers in certain regions of space. By probing induced fluorescence intensity, we demonstrated experimentally an 11-fold enhancement over CPL in discrimination of the enantiomers of a biperylene derivative by precisely sculpted electromagnetic fields. This result, which agrees to within 15% with theoretical predictions, establishes that optical chirality is a fundamental and tunable property of light, with possible applications ranging from plasmonic sensors to absolute asymmetric synthesis.     

基于CIGS薄膜光伏电池的三相并网研究与仿真

在金融时间序列中,一组金融序列可被视为由不同时间段的分段函数拟合连接而成.利用3σ准则确定分段函数的临界点,并根据AIC准则及调整后R2对分段点进行验证,从而分段点把数据分割成两部分.对两序列分别用合适的函数进行拟合,并用ARMA-GARCH模型对残差序列进行修正.由上证综合指数数据的实证分析结果表明：3σ准则能很好地检索出临界点,同时建立的分段函数模型预测效果要优于ARMA与EGARCH模型,以及ARMA-GARCH模型的引入对模型的精确度有所提高.所介绍的方法简单易懂、便于操作、精度高,为金融投资者和学者提供参考价值.     

Domain dynamics during ferroelectric switching

The utility of ferroelectric materials stems from the ability to nucleate and move polarized domains using an electric field. To understand the mechanisms of polarization switching, structural characterization at the nanoscale is required. We used aberration-corrected transmission electron microscopy to follow the kinetics and dynamics of ferroelectric switching at millisecond temporal and subangstrom spatial resolution in an epitaxial bilayer of an antiferromagnetic ferroelectric (BiFeO(3)) on a ferromagnetic electrode (La(0.7)Sr(0.3)MnO(3)). We observed localized nucleation events at the electrode interface, domain wall pinning on point defects, and the formation of ferroelectric domains localized to the ferroelectric and ferromagnetic interface. These results show how defects and interfaces impede full ferroelectric switching of a thin film.     

株行距配置对机采棉生长发育、产量及品质的影响

【目的】研究不同株行距配置对棉花生长发育、产量及品质的影响,分析机采棉适宜的种植方式。【方法】在同一密度(18×10⁴株/hm²)下设置3种株行距配置方式:一膜三行(76 cm+76 cm+76 cm等行距,株距7 cm);一膜四行(76 cm +66 cm +10 cm +76 cm,平均行距57 cm,株距10 cm);一膜六行(66 cm +10 cm,平均行距38 cm,株距14.6 cm),分析不同株行距配置对棉花农艺性状、叶面积指数、棉铃时空分布、干物质积累及产量的影响。【结果】一膜三行模式下的棉花株高、果枝始节高度均优于其他模式;叶面积指数在盛铃期达到峰值,其中一膜三行处理叶面积指数较一膜四行、一膜六行处理分别高出11.57%、4.50%。产量以一膜三行处理最高,为6 269.46 kg/hm²,较一膜四行、一膜六行分别高出4.06%、4.85%,各处理间棉花纤维品质基本无差异。【结论】一膜三行等行距种植模式更适合作为机采棉种植方式。     

对等密度条件下机采棉不同种植模式的综合评价

【目的】研究等密度条件下北疆机采棉最优的种植模式。【方法】以棉花品种新陆早57号(株型松散)、新陆早48号(株型紧凑)为材料,进行一膜三行(R3)、一膜四行(R4)、一膜六行(R6)三种种植模式等密度种植,测定棉花生长发育、脱叶采收效果以及产量品质指标,分析种植模式对不同株型结构棉花的影响,综合评价不同种植模式的生产适用性。【结果】一膜三行模式棉花较一膜四行、一膜六行模式生育期提前2~6 d且棉花株高、果枝始节高表现较优,更利于后期的机械采收;增大种植行距会提高棉花的脱叶率、吐絮率,棉花叶片的挂枝率也会因株距减小而增加;种植模式对棉花产量影响较大,而对品质无明显影响;株型松散型棉花较株型紧凑型棉花更容易受到种植模式的影响。【结论】等密度条件下,棉花品种在一膜三行模式下的整体表现优于一膜四行、一膜六行模式,适合在生产中推广应用。     

Phonon dispersions of fcc delta-plutonium-gallium by inelastic x-ray scattering

We report an experimental determination of the phonon dispersion curves in a face-centered cubic (fcc) delta-plutonium-0.6 weight % gallium alloy. Several unusual features, including a large elastic anisotropy, a small-shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes, are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory calculations for delta-plutonium.     

品种和栽培方式对春大白菜抽薹性的影响

综合比较了6个品种和4种栽培方式对春大白菜抽薹性的影响,试验结果表明:不同品种之间的抽薹性存在着较大的差异,强春、良庆的耐抽薹性最强,未熟抽薹率都为0;对照品种丰抗80的耐抽薹性最差,未熟抽薹率最高,达83.33%。“春大将”综合性状表现良好,适宜温州春季栽培。在4种栽培方式中,露地直播+不盖地膜的未熟抽薹率最高,达87.46%;育苗移栽+覆盖地膜、育苗移栽+不盖地膜的无发生未熟抽薹。