Reactivities of ultrathin alumina films exposed to intermediate pressures of H2O: Substrate-mediated mechanism for growth and loss of surface order

The response of ordered ultrathin Al₂O₃ films on NiAl(1 1 0) and Ni₃Al(1 1 0) substrates to sequential exposures at varying pressures of H₂O between 10⁻⁷ Torr and 10⁻³ Torr, ambient temperature, was characterized by LEED, AES and density functional theory (DFT) calculations. In all cases, an increase in average oxide thickness, as determined by AES, was observed, consistent with a field-induced oxide growth mechanism. Ordered oxide films of initial average thicknesses of 7 Å and 12 Å grown on NiAl(1 1 0) achieved a limiting thickness of 17(1) Å, while films of initial thickness of 7 Å and 11 Å grown on Ni₃Al(1 1 0) achieved a limiting thickness of 12(1) Å. The LEED patterns for the thinner (7 Å) films were not observed after exposure to 10⁻⁵ Torr (NiAl(1 1 0)), or 10⁻⁴ Torr (Ni₃Al(1 1 0)). In contrast, LEED patterns for the films of greater initial thickness persisted after exposures to 10⁻³ Torr UHV. DFT calculations indicate an Al vacancy formation energy that is significantly greater (by ∼0.5 eV) on the surface that has the thicker oxide film, directly opposite to what may be naively expected. A simple coordination argument supports these numerical results. Therefore, the greater limiting oxide thickness observed on NiAl(1 1 0) demonstrates that the rate determining step in the oxide growth process is not Al removal from the metal substrate and transport across the oxide/metal interface. Instead, the results indicate that the determining factor in the oxide growth mechanism is the kinetic barrier to Al diffusion from the substrate bulk to the oxide/metal interface. The persistence of the LEED patterns observed for the films of greater initial oxide thickness indicates that the surface disorder generally observed for alumina films grown on aluminide substrates and exposed to intermediate pressures of H₂O is due to the growth of a disordered alumina layer over an ordered substrate, rather than to direct H₂O interaction with terrace sites.     

Pressure effects on aluminium oxidation kinetics using X-ray photoelectron spectroscopy and parallel factor analysis

The effects of water vapour pressure on oxidation kinetics of aluminium have been studied using X-ray photoelectron spectroscopy (XPS) and three-way parallel factor analysis (PARAFAC). While the first technique is a powerful experimental tool for surface oxidation studies, the PARAFAC technique is a sophisticated analytical tool for analysing XPS data. The XPS Al(2p) and O(1s) core level have been used to follow the oxide film growth on clean surfaces at room temperature as a function of oxidation time (ranging from 1 to 60 min) and pressure of water vapour (ranging from 2.0×10⁻⁶ to 6.5×10⁻⁴ Pa). The growth of thin oxide films on aluminium surfaces has been found to follow the Cabrera–Mott inverse logarithmic law in all pressure ranges studied. The pressure effects have shown that the defect formation reaction at the oxide film/gas interface is the rate determining process in the aluminium oxidation. The pressure dependence of oxidation kinetics can be explained on the basis of metal vacancies in the defect structure of thin aluminium oxide films.     

Oxidation of titanium between 25°C and 400°C

The oxidation of Ti between 25°C and 400°C has been studied in an ultra high vacuum system with ellipsometry, Auger spectroscopy and surface potential difference. The surface potential difference is primarily sensitive to the adsorbed layer of oxygen, the Auger spectrometer is sensitive to the oxygen and Ti concentration at the oxide-gas interface and the ellipsometer is sensitive to the film thickness. Consequently, it was possible to follow the various processes separately during oxidation and oxide dissolution. Oxygen adsorbs in a percusor state as a molecule which dissociates and adsorbs in the atomic state. Reaction of atomic oxygen with Ti at the oxide-gas interface causes the stoichiometry to vary with the temperature, oxygen pressure and film thickness. At low temperatures and oxygen pressure the initial film growth follows a logarithmic time law. The predominant transport species is cationic. After initial oxidation, at higher temperatures and oxygen pressure, the predominant transport species is anionic. At constant temperature and oxygen pressure the oxide grows until the rate of growth is equal to the rate of dissolution into the metal. At this point the film thickness remains constant with time. Doping the Ti surface with Au results in a decrease in the oxide dissolution process due to blocking of interstitial diffusion paths by Au atoms.     

Deposition of latex particles encapsulated in polyelectrolyte shells at heterogeneous metal surfaces modified by multilayer films

We used an oblique impinging jet (OIJ) cell to determine the initial deposition rate for the microcapsules deposited on the heterogeneous metal surfaces bare or modified by polyelectrolyte (PE) films. The dependence of reduced particle flux on the Reynolds number of the flow in the OIJ cell was determined by direct counting of particles deposited on the studied surfaces. We used fluorescently labelled latex particles and microcapsules built on these fluorescent cores and the use of fluorescent microscope allowed us to observe “in situ” the deposition processes of particles on rough, highly reflective surfaces. We demonstrated that modification of metallic surfaces of various materials and heterogeneity by the multilayer PE films result in the formation of uniformly charged film of nanometers thickness. The formation of such a film leads to the increase of deposition efficiency and its initial rate is governed by the charge of the film covered surface and the outermost layer of the capsule shell being in agreement with the prediction of the convective-diffusion theory.     

Nitric acid method for fabrication of gate oxides in TFT

We have developed low temperature formation methods of SiO₂ layers which are applicable to gate oxide layers in thin film transistors (TFT) by use of nitric acid (HNO₃). Thick (>10 nm) SiO₂ layers with good thickness uniformity (i.e., ±4%) can be formed on 32 cm × 40 cm substrates by the two-step nitric acid oxidation method in which initial and subsequent oxidation is performed using 40 and 68 wt% (azeotropic mixture) HNO₃ aqueous solutions, respectively. The nitric acid oxidation of polycrystalline Si (poly-Si) thin films greatly decreases the height of ridge structure present on the poly-Si surfaces. When poly-Si thin films on 32 cm × 40 cm glass substrates are oxidized at azeotropic point (i.e., 68 wt% HNO₃ aqueous solutions at 121 °C), ultrathin (i.e., 1.1 nm) SiO₂ layers with a good thickness uniformity (±0.05 nm) are formed on the poly-Si surfaces. When SiO₂/Si structure fabricated using plasma-enhanced chemical vapor deposition is immersed in 68 wt% HNO₃, oxide fixed charge density is greatly decreased, and interface states are eliminated. The fixed charge density is further decreased by heat treatments at 200 °C, and consequently, capacitance-voltage characteristics which are as good as those of thermal SiO₂/Si structure are achieved.     

Oxidation study of silicon nanoparticle thin films on HOPG

Thin films of silicon nanoparticles (diameter 5-10 nm) were deposited on highly oriented pyrolytic graphite (HOPG) by low-pressure DC magnetron sputtering. The effect of different room-temperature oxidation techniques was investigated using XPS sputter-depth profiling. Both oxygen treatment during deposition (using an argon-oxygen mixture in the sputter gas) as well as post-deposition oxidation techniques (exposure to oxygen plasma beam, ambient air conditions) were studied. In all cases oxidation was found to involve the whole film down to the film/substrate interface, indicating a network of open pores. Depending on the type of oxidation between 15 and 25 at% of oxygen, mostly associated with low oxidation states of silicon, were detected in the interior of the film and attributed to oxidized surfaces of the individual silicon nanoparticles. The highest oxygen concentrations were found at the very film surface, reaching levels of 25-30% for films exposed to air or prepared by reactive magnetron sputtering. For the oxygen plasma-treated films even oxygen surface concentrations around 45% and fully oxidized silicon (i.e., SiO₂) were achieved. At the Si/HOPG interface formation of silicon carbide was observed due to intermixing induced by Ar-ion beam used for sputter-depth profiling.     

Oxidation of nanocrystalline aluminum by variable charge molecular dynamics

We investigate the oxidation of nanocrystalline aluminum surfaces using molecular dynamics (MD) simulations with the variable charge model that allows charge dynamically transfer among atoms. The interaction potential between atoms is described by the electrostatic plus (Es+) potential model, which is composed of an embedded atom method potential and an electrostatic term. The simulations were performed from 300 to 750 K on polycrystalline samples with a mean grain size of 5 nanometers. We mainly focused on the effect of the temperature parameter on the oxidation kinetic. The results show that, beyond a first linear regime, the kinetics follows a direct logarithmic law (governed by diffusion process) and tends to a limiting value corresponding to a thickness of ∼3 nm. We also characterized at 600 K the effects of an external applied strain on the microstructure and the chemical composition of oxide films formed at the surface. In particular, we obtained a partially crystalline oxide films for all temperatures and we noticed a strong correlation between the degree of crystallinity of the oxide film and the oxidation temperature.     

Optical properties change of oxide film — metal system during the film growth: computer simulation

Results of computer simulation of reflective properties of the oxide film-metal system in the process of oxidation in the air environment are presented. The complex refractive indices for oxide film and metal were used as the initial data. Thin films (the thickness is comparable with the wavelength of incident radiation) and thick films (thickness is much larger than the wavelength of incident radiation) are considered. The parameter characterizing the cyclic character of system reflectivity during the growth of film thickness was derived for the thin film. It is shown that the cyclic parameter does not depend on optical properties of a metal substrate. In the air environment, this parameter is determined by a complex refractive index of the film, its thickness, and direction of incident radiation. Relationships for the estimate of system reflectivity in the process of oxide film growth are presented for the thick film.     

准自由支撑铝薄膜中有序表面结构的自组织生长

利用真空热蒸发方法在液体基底表面成功制备出具有自由支撑边界条件的金属铝薄膜系统，研究了薄膜中自发形成的自边界向内部区域逐渐生长而呈带状分布的有序表面结构.该有序结构的形成与薄膜厚度、沉积速率和真空环境中的生长时间等实验参数密切相关，其形成过程可用一个三阶段生长模型来描述.实验证明此类有序结构是在薄膜内应力作用下，铝原子及原子团簇在液体表面自由扩散凝聚所致.进一步的理论研究表明：基于特征的边界条件和固液相互作用，该自由支撑铝薄膜系统中包含了丰富的正弦形内应力分布，各种具有不同振幅和频率的正弦形内应力的合成可形成矩形状畴块和带状有序结构. 关键词： 液体基底 铝薄膜 自组织生长 有序结构     

Formation mechanism of ordered stress-relief patterns in a free sustained Cu film system

A nearly free sustained copper （Cu） film system has been successfully fabricated by thermal evaporation deposition of Cu atoms on silicone oil surfaces, and a characteristic ordered pattern has been systematically studied. The ordered pattern, namely, band, is composed of a large number of parallel key-formed domains with different width w but nearly uniform length L; its characteristic values of w and L are very susceptible to the growth period, deposition rate and nominal film thickness. The formation mechanism of the ordered patterns is well explained in terms of the relaxation of the internal stress in the films, which is related to the nearly zero adhesion of the solid-liquid interface. By using a two-time deposition method, it is confirmed that the ordered patterns really form in the vacuum chamber.     

n型有序多孔硅基氧化钨室温气敏性能研究

利用电化学腐蚀方法制备了n型有序多孔硅, 并以此为基底用直流磁控溅射法在其表面溅射不同厚度的氧化钨薄膜. 利用X射线和扫描电子显微镜表征了材料的成分和结构, 结果表明, 多孔硅的孔呈柱形有序分布, 溅射10 min的WO₃薄膜是多晶结构, 比较松散地覆盖在整个多孔硅的表面. 分别测试了多孔硅和多孔硅基氧化钨在室温条件下对二氧化氮的气敏性能, 结果表明, 相对于多孔硅, 多孔硅基氧化钨薄膜对二氧化氮的气敏性能显著提高. 对多孔硅基氧化钨复合结构的气敏机理分析认为, 多孔硅和氧化钨薄膜复合形成的异质结对良好的气敏性能起到主要作用, 氧化钨薄膜表面出现了反型层引起了气敏响应时电阻的异常变化. 关键词： 有序多孔硅 氧化钨薄膜 二氧化氮 室温气敏性能     

Studies of electrochemically grafted thin organic layers on inorganic surfaces with infrared spectroscopic ellipsometry

We present IR spectroscopic ellipsometry (IRSE) measurements of electrochemically grafted organic ultrathin films down to monolayer thickness. The formation of organic layers of 4-methoxybenzene (anisole) on TiO₂, Au, and Si(111) surfaces was confirmed from observation of the respective absorption bands in the ellipsometric spectra. We discuss the orientation of molecules in a thin film on an Au substrate and in a thick film grafted on a TiO₂ substrate. The appearance of silicon oxide related bands in the IRSE spectra shows that oxidation is a side reaction of the electrochemical grafting on Si substrates. These results demonstrate the potential of IRSE application in studies of interfacial structures and thin film engineering. PACS 68.35.Ja; 82.45.Wx; 82.45.Jn     

Crystallization of Poly(Ethylene Oxide) in Thin Films

Crystallization of poly(ethylene oxide) (PEO) in thin films was studied using hot-stage polarized optical microscopy. Isothermal linear crystal growth rates were measured for various film thicknesses at various degrees of undercooling. At a given crystallization temperature, the linear crystal growth rate decreased exponentially with decreasing film thickness below a film thickness of 80 nm. Films showed similar spherulitic morphology down to a film thickness of 30 nm. Control experiments on hydrophilic and hydrophobic surfaces showed that surface chemistry affects stability of the polymer films and causes a competition between crystallization and dewetting.     

Low temperature surface chemistry and nanostructures

The new scientific field of low temperature surface chemistry, which combines the low temperature chemistry (cryochemistry) and surface chemistry approaches, is reviewed in this paper. One of the most exciting achievements in this field of science is the development of methods to create highly ordered hybrid nanosized structures on different organic and inorganic surfaces and to encapsulate nanosized metal particles in organic and polymer matrices. We consider physical and chemical behaviour for the systems obtained by co-condensation of the components vapours on the surfaces cooled down to 4–10 and 70–100 K. In particular the size effect of both types, the number of atoms in the reactive species structure and the thickness of growing co-condensate film, on the chemical activity of the system is analysed in detail. The effect of the internal mechanical stresses on the growing interfacial co-condensate film formation and on the generation of fast (explosive) spontaneous reactions at low temperatures is discussed. The examples of unusual chemical interactions of metal atoms, clusters and nanosized particles, obtained in co-condensate films on the cooled surfaces under different conditions, are presented. The examples of highly ordered surface and volume hybrid nanostructures formation are analysed.     

Comparative analysis of the thickness and electrical conductivity of thin chalcogenide semiconductor films

The structure and thickness of zinc and cadmium chalcogenide semiconductor films are studied by X-ray radiography. The film thickness is shown to be comparable with the half-value layer depth. The electrical conductivity of the films increases upon heating in the hydrogen atmosphere and decreases upon heating in carbon oxide. The opposite trend is observed in the ratio between the electrical conductivity and band gap of the initial and oxidized film surfaces.

     

Oxidation of nickel studied by UV and X-ray photoelectron spectroscopy

The nature of argon-ion bombarded nickel surfaces (polycrystalline, and (111), (110) and (100) single crystals) and their subsequent interaction with oxygen at ordinary temperatures have been studied using X-ray and UV photoelectron spectroscopy, including angular variation measurements and the determination of work function changes, in combination in the same apparatus. Variations between the HeI spectra of the four clean substrates were taken to confirm the presence of substantial order within the depth sampled by UPS. The four surfaces exhibited similar but not identical behaviour during oxidation, resembling that reported by other workers from studies of both annealed single crystals and evaporated polycrystalline films. The initial process was deduced to be essentially dissociative chemisorption: no evidence supporting a previous suggestion of associative adsorption at low coverages was found. Oxygen commenced to penetrate below the surface of all samples before oxygen equivalent to a monolayer had been taken up (~10 L exposure) and further substantial uptake followed resulting in the formation of a stable film (~18 Å) of nickel oxide by ~100 L exposure. This oxide layer was not stoichiometric nickel(II) oxide: it was characterized by the presence of two distinct O 1s signals, the relative intensities of which depended on the crystallographic nature of the surface. It is tentatively suggested that the oxygen signal with the higher BE be associated with Ni^III. Comparison of the X-ray and UV spectra suggests that the oxide film is very non-uniform in thickness, some Ni metal remaining very close to the surface.     

The determination of the refractive index and the thickness of thin films of aluminium oxide on aluminium by the polarimetric method

Formulas for the reflection of light from glass (i. e. a dielectric) coated with a thin non-metallic film are generalized for the case of the reflection of light from a metal coated with a thin non-metallic film, e. g. a film of aluminium oxide on aluminium. It is shown how the refractive index and the thickness of the aluminium oxide film on an aluminium mirror can be determined by measurements in polarized light. In conclusion the results of Drude's classical theory of thin non-metallic films on metallic mirrors are compared with the results obtained by the author on the basis of the interference of light in thin films.     

Tuning surface reactivity via electron quantum confinement

The effect of electron quantum confinement on the surface reactivity of ultrathin metal films is explored by comparing the initial oxidation rate of atomically flat magnesium films of different thickness, using complementary microscopy techniques. Pronounced thickness-dependent variations in the oxidation rate are observed for well ordered films of up to 15 atomic layers. Quantitative comparison reveals direct correlation between the surface reactivity and the periodic changes in the density of electronic states induced by quantum-well states crossing the Fermi level.     

Low-temperature oxidation of CoCu films with long-term irradiation by oxygen ion beams

The surface and surface layers of Co_xCu_100?x inhomogeneous thin films irradiated by an oxygen ion beam for a long time (to 100 min) are studied. The films are obtained by electrolytic deposition. With X-ray photoelectron spectroscopy and conversion electron Mössbauer spectroscopy, it is shown that the irradiation leads to the formation of an oxidized surface layer. The continuity and thickness of the layer depend on the roughness of the initial film. For a cobalt content of 8≤x≤20 at. %, the oxide layer is continuous and nonuniform in thickness, the mean thickness being estimated at several tens of nanometers. The interface between the layer and the underlying film is sharp. The films irradiated are smoother than the asdeposited ones. The formation of the oxide layer is treated in terms of a qualitative model.     

多孔氧化铝薄膜的制备和光学特性研究

采用阳极氧化法制备了二维有序纳米孔氧化铝膜.研究了工艺参数对多孔薄膜有序性、孔径、膜厚度等的影响，测量了多孔氧化铝有序膜的光透过、光吸收和光发射等光学特性.结果表明，在波长360 nm附近多孔氧化铝有序膜的光透过谱线和光吸收谱线发生突变，波长大于360 nm时，光透过增强；波长小于360 nm时，光吸收增强.多孔氧化铝有序膜的光致发光强度和峰位与激发光波长有关，光致发光谱范围在340~600 nm.