过渡层结构和生长工艺条件对Si基GaN的影响

为了提高MOCVD外延硅基GaN材料的质量，在硅（111）衬底上以HT-AlN为缓冲层，在缓冲层上再生长变组份过渡层后外延生长GaN。过渡层为多层复合结构，分为高温变组分AlGaN、GaN、低温AlN、高温变组分AlGaN。在高温生长AlGaN和GaN层中插入一层低温生长AlN以缓解降温过程中应力对厚GaN层的影响，为了缓慢释放热应力、采用合适的慢降温工艺。当外延层的厚度小于1．7微米时GaN外延层无龟裂，而厚度不断增加时，GaN外延层产生龟裂。本文研究了AlN缓冲层生长温度、高温变组分AlGaN生长过程中生长时间的变化对所生长GaN材料的影响。采用三维视频显微镜、高分辨率双晶X射线衍射（DCXRD）、扫描电子显微镜（SEM）、原子力显微镜（AFM）和室温光致荧光光谱（RT-PL）对样品进行了测试分析。测试结果表明所研制的硅基GaN表面光亮、平整，过渡层的引入有利于降低外延层中应力，提高GaN结晶质量。     

Fabrication of nanostructures by selective growth of C60 and Si on Si(1 1 1) substrate

Using two types of selective growth, selective C₆₀ growth and selective Si growth, on a common Si(1 1 1) substrate, an array of C₆₀ nanoribbons with controlled values of width and thickness is fabricated. On a surface that has Si(1 1 1)√3 × √3R30°-Ag (referred to as Si(1 1 1)√3-Ag hereafter) and bare Si(1 1 1) regions at the same time, the preferential growth of C₆₀ multilayered film is recognized on the Si(1 1 1)√3-Ag region. The growth of Si selectively occurs on a bare Si(1 1 1) region if the substrate surface has C₆₀-adsorbed and bare Si(1 1 1) regions at the same time. As a demonstration of the use of these selective growths, we fabricate an array of well-isolated C₆₀ nanoribbons, which show a well-ordered molecular arrangement and have sizes of about 40 nm in widths and 3-4 nm in thicknesses.     

Atomic and electronic structure of Sr/Si(0 0 1)-(2 × 2)

The adsorption of Sr on the Si(0 0 1) surface with the semiantiphase dimer (2 × 2) reconstruction is studied, based upon the ab initio pseudopotential calculations. It is calculated that the semiantiphase dimer (2 × 2) reconstruction (2 dimers per unit cell) is more favorable than the (2 × 1) phase (1 dimer per unit cell) by an energy of about 0.24 eV/dimer. Considering the energetically more stable reconstruction, we have assumed four possible locations for 1/4 monolayer (ML) Sr adsorption on this surface: (i) bridge, (ii) cave, (iii) pedestal, and (iv) valley-bridge. We find that Sr adsorption on the valley-bridge site is energetically more favorable than all other cases studied here. Interestingly, one of the dimers becomes symmetric, but the other one is still asymmetric with the buckling angle reduced from 18° to 14°, when compared with the clean Si(0 0 1)-(2 × 2) surface. The calculated bond length between Sr and Si in the case of valley-bridge adsorption site is 3.05 Å, and in good agreement with other theoretical calculations. We also present and compare the electronic band structures for the clean and covered surfaces as well as the corresponding charge density plots.     

Adsorption induced faceting and superstructural phase diagram of the Sb/Si(5 5 12) interface

The planar high index Si(5 5 12) surface consists of trenches formed by the several proximal surface planes, that can be employed as templates for the adsorption of low dimensional nanostructures. This paper reports the results of an extensive UHV study of the adsorption of Sb, in the sub-monolayer coverage regime, onto the Si(5 5 12) surface. The evolution of the surface phases, surface morphology and electronic structure is monitored by Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS). A careful control of substrate temperatures and Sb coverages formed at a low flux rate of 0.06 ML/min enable us to extract a complete adsorption phase diagram of the important interface, for the first time. The phase diagram clearly demonstrates the conversion of the large Si(5 5 12) unit cell into facets of planes of smaller (2 2 5), (3 3 7) and (1 1 3) base units. The study also reveals the formation of various superstructural phases formed by steering the kinetic parameters.     

Thermal stability of food additives of glutamate and benzoate type

Summary An unsaturated polyester/sisal flame retardant composite was formulated using decabromine diphenyl oxide associated with antimony trioxide as additives. The development and use of natural or vegetable fiber reinforced composites is increasing worldwide, since natural fibers come from renewable sources and their use contributes to the so-called 'green technology'. In the present study, the synergic effect of a bromine/antimony (3:1 molar ratio) based flame retardant system with 7.5% Br (mass/mass) added to composites investigated by TG, UL-94V and pyrolysis on a coupled Pyr-GC/MS device. The efficiency of the flame retardant system is confirmed by TG and UL-94V test where the composite containig flame retardant system obtained the highest classification (V-0).     

Resonant photoemission at the oxygen K edge as a tool to study the electronic properties of defects at SiO2 /Si and SiO2 /SiC interfaces

Silicon is by far the most important material used in microelectronics, partly due to the excellent electronic properties of its native oxide (SiO₂), but substitute semiconductors are constantly the matter of research. SiC is one of the most promising candidates, also because of the formation of SiO₂ as native oxide. However, the SiO₂/SiC interface has very poor electrical properties due to a very high density of interface states which reduce its functionality in MIS devices. We have studied the electronic properties of defects in the SiO₂/Si and SiO₂/SiC interfaces by means of XAS, XPS and resonant photoemission at the O 1s and the Si 2p edges, using silicon dioxide thermally grown with thicknesses below 10 nm. Our XAS data are in perfect agreement with literature; in addition, resonant photoemission reveals the resonant contributions of the individual valence states. For the main peaks in the valence band we find accordance between the resonant behaviour and the absorption spectra, except for the peaks at −15 eV binding energy, whose resonant photoemission spectra have extra features. One of them is present in both interfaces and is due to similar defects, while another one at lower photon energy is present only for the SiO₂/SiC interface. This is related to a defect state which is not present at the SiO₂/Si interface.     

Substrate temperature dependence of the photoluminescence efficiency of co-sputtered Si/SiO2 layers

Si particles embedded in an SiO₂ matrix were obtained by co-sputtering of Si and SiO₂ at various deposition temperatures T_d (200–700°C) and annealing at different temperatures T_a (900–1100°C). The systems were characterized by X-ray photoelectron, Raman scattering, infrared absorption and photoluminescence spectroscopy techniques. The results show that the photoluminescence efficiency is strongly dependent on the degree of phase separation between the Si nanocrystals and the SiO₂ matrix. This is likely connected with the Si/SiO₂ interface characteristics, together with the features indicating the involvement of quantum confinement.     

Highly stereoselective synthesis of C-(alkynyl)-pseudoglycals from δ-hydroxy-α,β-unsaturated aldehydes

An efficient and novel methodology for the synthesis of C-(alkynyl)-pseudoglycals from δ-hydroxy-α,β-unsaturated aldehydes has been developed.     

ZnO／AlN／Si（111）薄膜的外延生长和性能研究

用常压金属化学气相沉积法(MOCVD)在Si(111)衬底上制备了马赛克结构ZnO单晶薄膜。引入低温Al N缓冲层以阻止衬底氧化、缓解热失配和晶格失配。薄膜双晶X射线衍射2θ/ω联动扫描只出现了Si(111)、ZnO(000l)及Al N(000l)的衍射峰。ZnO/Al N/Si(111)薄膜C方向晶格常量为0.5195nm,表明在面方向处于张应力状态;其对称(0002)面和斜对称(1012)面的双晶X射线衍ω摇摆曲线半峰全宽分别为460″和1105″;干涉显微镜观察其表面有微裂纹,裂纹密度为20cm-1;3μm×3μm范围的原子力显微镜均方根粗糙度为1.5nm;激光实时监测曲线表明薄膜为准二维生长,生长速率4.3μm/h。低温10K光致发光光谱观察到了薄膜的自由激子、束缚激子发射及它们的声子伴线。所有结果表明,采用金属化学气相沉积法并引入Al N为缓冲层能有效提高Si(111)衬底上ZnO薄膜的质量。     

Caesium/xenon dual beam depth profiling: Velocity of the sputtered atom and ionization probability

In this work, a caesium/xenon co-sputtering gun was used to perform depth profiles of a RhSi layer with varying caesium beam concentration. The positive ion yields were monitored with respect to the varying work function of the solid and the intensities of the ions were plotted with respect to the caesium surface concentration. As expected by the tunneling model, all the M⁺ signals decrease exponentially with the increasing caesium beam concentration. Moreover, the heaviest ion yields decrease faster than the lighter ion ones. This phenomenon can be explained by the different velocities of the departing atoms, which has an important impact on the ionization processes. We then studied the variations of the MCs⁺ yields with respect to the caesium surface concentration and with respect to the nature of the departing atom. Finally, we applied models based on the tunneling model in order to fit our results.