碳化硅中氦离子辐照缺陷演化及SiCOI薄膜合成的研究

碳化硅是一种宽带隙半导体材料,具有禁带宽度大、击穿电压高、热导率高、电子饱和漂移速度大、介电常数小、抗辐射能力强、化学稳定性好等优良特性,使其在越来越多的领域如航空航天、太空探测、人造卫星、地热勘探、核能仪器、雷达通讯等, 所需要高温、高速、高频、大功率的微电子器件方面倍受青睐,并和氮化镓、金刚石一起被誉为发展前景十分广阔的第三代半导体材料。本论文采用He+离子注入,在SiC衬底一定深度引入纳米气泡/空腔的方法,来增强对氧原子的俘获以增加O原子在RP处局域浓度,使得更利于O与Si的反应,从而促进氧化埋层的形成,以达到降低注入O的剂量而形成优良的氧化物电绝缘层的目的。由于高剂量的O注入会引起表层SiC材料的损伤，该方法有望缓解目前SIMOX技术中O离子高剂量注入引起表层材料的损伤问题，以期获得低成本、低缺陷密度的SiCOI材料。论文主要开展了如下研究：（1）对He+离子高温（600 K）注入6H-SiC中产生的辐照缺陷,以及缺陷在阶梯温度退火的演化行为的特征进行了分析。实验采用100 keV的He+,辐照剂量范围为3.0×1015～3.0×1016 He+/cm2。利用拉曼光谱、室温光致发光谱、红外吸收光谱、沟道卢瑟福背散射谱的特征进行了分析。实验结果表明,离子注入所产生晶格损伤的程度与He+离子注入剂量有关；高温退火使得损伤得到恢复,不同注入剂量造成的晶格损伤需要不同的退火温度才可恢复。高剂量注入的样品在阶梯温度退火条件下呈现出了点缺陷的复合、氦-空位团的产生、氦泡的形核、长大等特性。与室温注入相比,高温注入引入的自退火作用使大部分简单缺陷发生复合,限制了损伤的积累,从而在材料中产生相对较小的损伤。在一定剂量范围内是避免注入层非晶化的一个重要方法,为后续利用氦离子注入空腔掩埋层吸杂或者制备低成本、低缺陷密度的绝缘层上碳化硅(SiCOI)材料提供了可能。 （2）对He的预注入引入的辐照缺陷与随后注入的氧原子的相互作用机理进行了初步分析。实验采用先He后O注入的方法，采用的离子能量为30 keV (He+)，100 keV (O+)；剂量分别为3.0×1016 (He+)、1.0×1017 (O+) ions/cm2。拉曼散射谱结果表明,空腔对氧的吸收主要是通过捕获简单缺陷释放出来的间隙氧原子实现的,进而促进了对氧的吸附,形成硅氧化合物,有利于氧化埋层的形成。紫外-可见吸收谱中的干涉带表明在材料表面下大概198 nm处是损伤层与晶体层的分界面，接近于SRIM2006估算得到的30 keV He+和100 keV O+辐照损伤的深度（He+辐照损伤的深度为195 nm；O+辐照损伤的深度为165 nm）。沟道卢瑟福背散射谱表明，在特定深度（约150 nm）处,样品中形成了接近非晶的埋层。He离子预注入的碳化硅基体由于含有较多的空位,注入的氧在退火过程中从简单缺陷中释放,向空腔层扩散并捕获在空腔层内,使得He离子预先注入形成的空腔层限制了随后O离子注入造成的损伤层的厚度

Adsorption and reaction of thiophene and H2S on Mo2C/Al2O3 catalyst studied by in situ FT-IR spectroscopy

The reactions of both thiophene and H2S onMo(2)C/Al2O3 catalyst have been studied by in situ FT-IR spectroscopy. CO adsorption was used to probe the surface sites of Mo2C/Al2O3 catalyst under the interaction and reaction of thiophene and H2S. When the fresh Mo2C/Al2O3 catalyst is treated with a thiophene/H-2 mixture above 473 K, hydrogenated species exhibiting IR bands in the regions 2800-3000 cm(-1) are produced on the surface, indicating that thiophene reacts with the fresh carbide catalyst at relatively low temperatures. IR spectra of adsorbed CO on fresh Mo2C/Al2O3 pretreated by thiophene/H-2 at different temperatures clearly reveal the gradual sulfidation of the carbide catalyst at temperatures higher than 473 K, while H2S/H-2 can sulfide the Mo2C/Al2O3 catalyst surface readily at room temperature (RT). The sulfidation of the carbide surface by the reaction with thiophene or H2S maybe the major cause of the deactivation of carbide catalysts in hydrotreating reactions. The surface of the sulfided carbide catalyst can be only partially regenerated by a recarburization using CH4/H-2 at 1033 K. When the catalyst is first oxidized and then recarburized, the carbide surface can be completely reproduced.

Activated carbon supported bimetallic CoMo carbides synthesized by carbothermal hydrogen reduction

A carbothermal hydrogen reduction method was employed for the preparation of activated carbon supported bimetallic carbide. The resultant samples were characterized by BET surface area measurement, X-ray diffraction, and temperature-programmed reduction-mass spectroscopy. The results showed that nanostructured beta-Mo2C can be formed on the activated carbon by carbothermal hydrogen reduction above 700 degreesC. The particle sizes of beta-Mo2C increase with increasing reaction temperatures and Mo loading. The bimetallic CoMo carbide can be synthesized by the carbothermal hydrogen reduction even around 600 degreesC. The bimetallic CoMo carbide is from carbothermal hydrogen reduction of CoMoO4 precursor and is easily formed when the Co/Mo molar ratio is 1.0. Separation of the bimetallic CoMo carbide phase into Mo carbide and Co metal occurs when the temperature of the reduction is above 700 degreesC. The addition of a second metal such as Co and Ni, decreases the formation temperature of carbide because the second metal promotes formation of CHx species from reactive carbon atoms or groups on carbon material and hydrogen, which further carburizes oxide precursors. (C) 2003 Elsevier Science Ltd. All rights reserved.

In situ FT-IR spectroscopic studies of CO adsorption on fresh Mo2C/Al2O3 catalyst

The surface sites of supported molybdenum carbide catalyst derived from different synthesis stages have been studied by in situ FT-IR spectroscopy using CO as the probe molecule. Adsorbed CO on the reduced passivated Mo2C/Al2O3 catalyst gives a main band at 2180 cm(-1), which can be assigned to linearly adsorbed CO on Mo4+ sites. The IR results show that the surface of reduced passivated sample is dominated by molybdenum oxycarbide. However, a characteristic IR band at 2054 cm-1 was observed for the adsorbed CO on MoO3/Al2O3 carburized with CH4/H-2 mixture at 1033 K (fresh Mo2C/Al2O3), which can be assigned to linearly adsorbed CO on Modelta+ (0 < delta < 2) sites Of Mo2C/Al2O3, Unlike adsorbed CO on reduced passivated Mo2C/Al2O3 catalyst, the IR spectra of adsorbed CO on fresh Mo2C/Al2O3 shows similarity to that on some of the group VIII metals (such as Pt and Pd), suggesting that fresh carbide resembles noble metals. To study the stability Of Mo2C catalyst during H-2 treatment and find proper conditions to remove the deposited carbon species, H-2 treatment of fresh Mo2C/Al2O3 catalyst at different temperatures was conducted. Partial amounts of carbon atoms in Mo2C along with some surface-deposited carbon species can be removed by the H, treatment even at 450 K. Both the surface-deposited carbon species and carbon atoms in carbide can be extensively removed at temperatures above 873 K.

On the induction period of methane aromatization over Mo-based catalysts

The behavior of different species during the temperature-programmed surface reaction (TPSR) of methane over various catalysts is traced by an online mass spectrometer, It is demonstrated that the transformation of MoO3 to molybdenum carbide hinders the activation of methane as well as the succeeding aromatization in the TPSR, If this transformation process is done before the reaction, the temperature needed for methane activation and benzene formation will be greatly lowered (760 and 847 K, respectively). On the basis of comparison of the catalytic behavior of molybdenum supported on different zeolites, it is suggested that the initial activation of methane is the rate-determining step of this reaction. For the cobalt catalysts supported on HMCM-22 or Mo catalysts supported on TiO2, no benzene formation could be observed during the TPSR, However, the prohibition of benzene formation is different in nature over these two catalysts: the former lacks the special properties exhibited by molybdenum carbide, which can continuously activate methane even when multiple layers of carbonaceous species are formed on its surface, while the latter cannot accomplish the aromatization reaction since there are no Bronsted acid sites to which the activated intermediates can migrate, although the activation of methane can be achieved on it. Only for the catalysts that possess both of these properties, together with the special channel structure of zeolite, can efficient methane aromatization be accomplished. (C) 2000 Academic Press.

Fabrication and characterization of CdTe nanoparticles attached to poly(4-vinylpyridine) nanofibers

The major objective of this work was to characterize the status of CdTe nanoparticles attached to the surface of poly(4-vinylpyridine) (P4VP) nanofibers. Scanning electron microscopy and transmission electron microscopy images indicated that the attachment of CdTe nanoparticles enlarged the diameter of P4VP nanofibers. Moreover, the results of the energy-dispersive X-ray spectrum and the electron diffraction pattern revealed that the deposition on the surface of P4VP nanofibers was CdTe in a cubic lattice

Controllable synthesis of VSB-5 micirospheres and microrods: Growth mechanism and selective hydrogenation catalysis

Nanoporous VSB-5 nickel phosphate molecular sieves with relatively well controllable sizes and morphology of microspheres assembled from nanorods were synthesized at 140 degrees C over a short time in the presence of hexamethylenetetramine (HMT) by a facile hydrothermal method. The pH value, reaction time, and ratio of HMT to NaHPO2-H2O crucially influence the morphology and quality of the final products.

Preparation and characterization of Gd2O3:Eu3+ luminescence nanotubes

The size controllable Gd2O3:Eu3+ luminescence nanotubes were successfully prepared using a simple method by coating gadolinium compounds on the carbon nanotubes and then firing the carbon nanotubes. The morphology of the obtained Gd2O3:Eu3+ nanotubes was determined by transmission electron microscopy (TEM). It was found that the obtained nanotubes have the outer diameters of similar to 100 nm, the inner diameters of similar to 50 nm, and the lengths of several tens of microns. The sizes of Gd2O3:Eu3+ nanotubes can be easily controlled by changing the reaction times and the concentration of reactants.

Synthesis of Cu-In-S ternary nanocrystals with tunable structure and composition

Nearly monodisperse Cu-In-S ternary nanocrystals with tunable composition, crystalline structure, and size were synthesized by a hot-injection method using mixed generic precursors. Such ternary nanocrystals with zincblende and wurtzite structure were reported for the first time.

Synthesis and characterization of 1D Co/CoFe2O4 composites with tunable morphologies

1D Co/CoFe2O4 composites with tunable morphologies were fabricated by a facile solvothermal route in the presence of a surfactant poly(vinylpyrrolidone) (PVP); they may be very attractive for potential applications because of their outstanding soft magnetism.

Hydrothermal synthesis and thermoelectric transport properties of impurity-free antimony telluride hexagonal nanoplates

Impurity-free single-crystalline antimony telluride hexagonal nanoplates (see figure) are synthesized by a facile and quick hydrothermal treatment without any organic additives or templates. The inherent crystal structure is the driving force for the growth of these Sb2Te3 hexagonal nanoplates. Films of these nanoplates shows p-type behavior, and exhibit a promisingly high Seebeck coefficient of 425 mu V K-1 at room temperature.

A new method for studying the interaction between chlorpromazine and phospholipid bilayer

The spectroscopic and transmission electron microscopy (TEM) studies of interaction between chlorpromazine (CPZ) and dimyristoyl phosphatidylglycerol (DMPG) bilayer by using gold nanoparticles (AuN-Ps) as probes are reported. The DMPG bilayer-protected AuNPs were prepared by a simple one-step method. The DMPG bilayer tethered on the AuNPs was considered as a biomembrane model. The addition of CPZ affected the surface plasmon resonance (SPR) and morphology of the prepared AuNPs, and this effect was monitored by UV-vis spectroscopy and TEM.

A novel and efficient route to Se nano/microstructures with controllable phase and shape

In this paper, we demonstrate a novel and efficient route by which the shape-controlled synthesis of t-Se nano/microstructures including nanowires, nanorods, nanobelts, microtubes, and flowers, as well as uniform spheres of a-Se, can be readily realized based on solution-mediated heat treatment with commercially available Se powders. X-ray diffraction (XRD), energy-dispersive X-ray spectra (EDS), Raman spectra, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to characterize the samples.