The initial oxidation of molybdenum: IV. Epitaxial growth of oxide on molybdenum

The oxide which grows in low oxygen pressure and at temperatures between 700 and 1000 K on molybdenum is shown to be MoO₂. The epitaxial relationships between the oxide and the metal (100), (110) and (111) surfaces are given. The epitaxial relationships of oxide on the molybdenum (100) and (110) surfaces are geometrically equivalent. The oxide grows on the (111) molybdenum surface with no major oxide plane parallel to the substrate. It is suggested that the epitaxy of MoO₂ on the (111) surface is a consequence of growth on {211} molybdenum facets. The atomic positions in the pairs of interfacial planes found are given. There is little agreement between the positions of ions in the oxide and substrate lattice sites. Only in the postulated case of MoO₂ on {211} Mo facets is a small misfit found.     

The initial oxidation of molybdenum: III. Oxide nucleation rates on (100) and (110) molybdenum surfaces

The induction periods for the nucleation of epitaxial oxide on (100) and (110) surfaces of molybdenum have been determined. The role of impurities has been investigated by AES. It is found that only gross amounts of impurity are likely to be significant. Under isobaric conditions the activation energy associated with nucleation is 0.11 ± 0.2 MJ mole^?1 on both faces between 700 and 1050 K. At 850 K the induction rate is proportional to the pressure. Under isoposal conditions on the (110) surface an activation energy of 0.22 ± 0.03 MJ mole^?1 is found.     

Observation of surface composition of copper-nickel alloy by auger spectra in the lower energy region (at around 100 eV)

The interaction of oxygen with a clean (110) molybdenum surface has been studied by LEED and RHEED. At room temperature two simple chemisorbed structures are found, at higher exposures a multiple order pattern is seen. At temperatures above 750 K a complex LEED pattern is observed. Initially a corresponding RHEED pattern is seen which has a different structure. Epitaxial oxide nucleation may be followed by RHEED but the initial process is not seen by LEED. More extensive epitaxial films are needed before oxide reflexions appear in the LEED pattern. The formation of an epitaxial oxide film is accompanied by the growth of {100} molybdenum facets. These may be seen after the film has been carefully evaporated away. The nuclei of oxide which form are shown by both RHEED and transmission microscopy of replicas to be characteristically shaped.     

Oxidation and faceting of polycrystalline tungsten ribbons

We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10^?4 to 5 × 10^?3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and RHEED results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.     

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

The initial stages of the interaction of oxygen gas with a clean Ni (100) surface have been studied by a combination of LEED, AES, work function change and ion bombardment sectioning techniques. The reaction could be divided into three reaction regions: a fast dissociative chemisorption leading to surface structures based on the initial nickel interatomic spacing and resulting in an oxygen coverage of approximately 0.4 monolayers; a rapid oxidation leading to epitaxial NiO, two layers thick ; and a final slow thickening of bulk NiO. The first two regions were dependent only upon oxygen exposure. The third region was observed only at high gas-phase oxygen pressures or very low surface temperatures. Kinetics analyses are developed to explain the rate of oxygen chemisorption and the rate of oxide nucleation and growth.     

Two-scale modeling of adsorption processes at structured surfaces

We present an algorithm for the simulation of vicinal surface growth. It combines a lattice gas anisotropic Ising model with a phase-field model. The molecular behavior of individual adatoms is described by the lattice gas model. The microstructure dynamics on the vicinal surface are calculated using the phase-field method. In this way, adsorption processes on two different length scales can be described: nucleation processes on the terraces (lattice gas model) and step-flow growth (phase field model). The hybrid algorithm that is proposed here, is therefore able to describe an epitaxial layer-by-layer growth controlled by temperature and by deposition rate. This method is faster than kinetic Monte Carlo simulations and can take into account the stochastic processes in a comparable way.     

The effects of temperature upon NiO formation and oxygen removal on Ni(110)

Oxygen chemisorption and NiO nucleation and growth on Ni(110) have been studied with low energy electron diffraction and Auger electron spectroscopy. Changes in the Auger peak energies and shapes were shown to occur only upon NiO formation. The effects of step-changes in temperature upon NiO nucleation and growth were studied and it was shown that temperature steps or annealing during the chemisorption regime did not significantly affect either chemisorption or NiO formation. During NiO growth, temperature steps to a higher temperature caused reduced growth rates, while steps to lower temperature caused increased growth rates. The reaction rate constant from the island growth model was calculated and shown to agree with literature data. The values obtained from temperature step measurements agreed within a factor of two with those obtained for reactions without temperature steps. Therefore, no systematic temperature effect upon the NiO nuclei density was observed for Ni(110). The activation energy for growth of NiO was found to be 5.5 kcal/mole. Dissolution of oxygen into bulk nickel was also studied and it was shown that bulk diffusion of oxygen in nickel was not rate controlling. Rather, surface phase transitions were observed which allowed incorporation of oxygen over the temperature range of 150°C to greater than 800°C, depending on the quantity of oxygen already incorporated.     

钙钛矿结构氧化物薄膜 的外延生长

在成功地外延生长超导、铁电、铁磁等多种性质的钙钛矿结构氧化物薄膜的基础上,讨论影响氧化物薄膜 外延生长的一些因素.考虑到相形成和薄膜长征动力学,在利用脉冲激光淀积法外延生长氧化物薄膜中衬底温度是十分重要的工艺参数.衬底温度对成相和生长薄膜的取向都有影响.考虑到薄膜是首先在衬底表面成核、成相并生长.因此衬底材料晶格的影响是不容忽视的.观察到衬底材料对薄膜外延生长温度的影响.在适当的工艺条件下,利用低温三步法工艺制备得到有很强织构的外延薄膜.这突出表明界面层的相互作用对钙钛矿结构薄膜的取向有着相当大的影响. 关键词：     

力和扩散机理下外延形貌的演化分析

本文提出了一个新的基于扩散界面的相场模型来描述外延生长中岛的形核、生长及熟化过程. 该模型同时考虑了弹性场、表面能、沉积、扩散、解吸和能量势垒等热力学及动力学过程对表面纳米形貌的影响. 采用经典的BCF模型来描述生长中的扩散形核过程, 而采用一个新的包含弹性应变能的自由能函数, 通过变分得到一个描述多层岛生长的相场方程, 该方法可以有效地描述外延生长中复杂的外延形貌. 采用有限差分格式对非线性耦合方程组进行求解. 数值结果显示, 该模型可以真实地再现外延生长中多层岛结构(即山丘状形貌)的演化过程, 模拟结果与已有实验结果一致. 同时模拟了生长过程中随外延形貌演化而形成的复杂生长应力, 研究表明, 在生长过程中, 岛中存在着复杂的应力分布, 且在岛边界处应力达到局部最大, 这与实验结果定性一致. 此外, 本文的重要发现是, 外延生长中的应力演化明显地影响原子的扩散过程, 当应力存在时, 外延结构变化较无弹性场时变快. 该项研究对理解外延生长中各物理机理的协同作用有重要的指导意义.     

APCVD生长碳化硅薄膜中汽相结晶过程的研究

对宽禁带半导体材料碳化硅薄膜的常压化学汽相沉积(APCVD)异质外延技术进行了讨论.在SiC薄膜的生长过程中对Si/C原子比例的控制一直是影响薄膜表面形貌和膜层掺杂浓度的重要因素.针对水平卧式反应室,分析了生长过程中气流流速分布、反应室内温度分布以及反应气体浓度分布,指出"汽相结晶"过程对薄膜生长区Si/C比例有很大影响,从而影响了碳化硅薄膜的生长速率,非常好的解释了薄膜生长过程中出现的现象.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

The initial interactions of beryllium with O2 and H2O vapor at elevated temperatures

In the 310-790 K temperature range, the mechanism of initial oxidation by O₂ is oxide island nucleation and growth. At the lower temperature range, oxygen is first chemisorbed and the oxide nucleates at coverage of ∼0.2. Increasing the temperature causes the oxide islands to nucleate at lower coverage and at 700 K and above, the oxide nucleates without any significant stage of chemisorbed oxygen. The temperature dependence shows that while the dissociation stage is not activated, the oxide nucleation and growth are thermally activated. Also, opposite to O₂ adsorption, the initial H₂O adsorption and oxidation rate was found to decrease with temperature. Opposite to the oxygen case, upon exposure to water vapor there is no noticeable stage of chemisorbed oxygen (or OH) and oxide is directly nucleated. Only after oxide coalescence, this tendency changes and the oxidation rate is increased with temperature.     

Oxygen chemisorption,surface oxidation,and the oxidation of carbon monoxide on cobalt (0001)

Oxygen chemisorbs on clean Co(0001) at 300 K with an initial sticking probability of ~0.3. The chemisorbed overlayer (which is very reactive towards CO) readily undergoes conversion to cobalt oxide, even at room temperature. This transformation is accelerated at higher temperatures, and the oxygen uptake rate falls as CoO growth proceeds. At a certain point, however, the uptake rate rises sharply, and this behaviour is ascribed to nucleation and growth ofCo₃O₄. This interpretation is consistent with the available Δφ, Auger, LEED, and reactivity data. Thus Δφ changes sign as lattice penetration by the ad sorbed oxygen takes place, and this is accompanied by a shift and broadening of the O(KLL) Auger signal. LEED indicates the epitaxial growth firstly of CoO(111) and then, at higher oxygen exposures, of Co₃O₄(111). At 300 K CO rapidly reduces the Co₃O₄ surface back to CoO, and the oxidation/reduction behaviour by O₂/CO appears to be completely reversible. Steady-state measurements yield a value of 19 ± 7 kJ mol^?1 for the activation energy to CO₂ production from CO + O₂. Earlier photoelectron spectroscopic studies by other authors are considered in the light of these results.     

A phase-field/Monte-Carlo model describing organic crystal growth from solution

In this paper work we present a phase-field/Monte-Carlo hybrid algorithm for the simulation of solutal growth of organic crystals. The algorithm is subsequently used for an investigation of diffusion effects on the growth mechanisms. This method combines a two-scale phase-field model of the liquid phase epitaxial growth and a Monte-Carlo algorithm of the 2D nucleation and thus is faster than previous purely Monte Carlo simulations of crystal growth. The inclusion of supersaturation and diffusion in the method allows the study of crystal growth under various growth conditions. Parameters used in the hybrid algorithm are bound to the energetic parameters of crystal faces, which can be estimated from a detailed study of the actual crystal structure based on a connected nets analysis, which allows the prediction of the shape and morphology of real crystals. The study of the diffusion effect is carried out based on an example of a hydroquinone crystal, which grows from the water solution at various supersaturations. The dependencies of the growth rate and the nucleation rate on the supersaturation indicate the change of the growth mechanism from spiral growth to 2D nucleation. The difference in the growth rate for various faces is in agreement with the crystal morphologies derived from the attachment energy method and observed experimentally. The main result of the simulation is the evaluation of engineering limits for choosing appropriate external process conditions.     

Influence of silicon cluster on epitaxial growth of silicon carbide

Precursor concentration dependences of growth rate, doping concentration and surface morphology have been investigated in the epitaxial growth of 4H-SiC(0001) epilayers with horizontal hot-wall CVD system using various precursor concentrations under constant C/Si ratio. Form the experimental data it is found that silicon cluster which is formed through gas phase nucleation plays an important role in controlling the doping concentration and epitaxial growth rate of the silicon carbide. It was observed that t...     

Continuous monitoring of Mg oxidation by internal exoemission

Thin-film Mg/Si(111) Schottky diodes are exposed to oxygen to detect chemicurrents in the devices. The detected charge is created by nonadiabatic energy dissipation and due to either internal exoemission currents or surface chemiluminescence induced photocurrents. Both contributions can be distinguished by changing the metal film thickness of the device. Auger electron spectroscopy to study the oxygen uptake demonstrates that the chemicurrent transients represent truly the time dependent reaction rate at the surface. Model calculations indicate that the current monitors Mg oxide island nucleation and growth.     

Nanoscale observations of the epitaxial growth of hashemite on barite (0 0 1)

The heteroepitaxial growth of hashemite BaCrO₄ on barite BaSO₄(0 0 1) from supersaturated aqueous solutions was observed in situ using an atomic force microscope (AFM). It was shown that the first hashemite layer grows via two-dimensional nucleation easily forming a complete epitaxial layer, which is likely to have a low level of intrinsic stress. Two-dimensional nucleation of the second and subsequent layers proceeds with significantly lower rates, and growth occurs with lower step velocities. These layers seem to have significant level of intrinsic stress and tend to reduce it via the formation of free surface normal to the growth layer (holes in the layer, dendrite-like shape of nuclei and steps, preferable formation of nuclei at the step edges). As a result, the initially flat surface becomes rough. The process described corresponds to the Stranski-Krastanov epitaxial growth mode, which is well known for growth of semiconductor and metal films but not previously recognised for crystals grown from aqueous solutions.     

A molecular dynamics simulation study on the cavitation inception of water with dissolved gases

The promotion/prevention mechanism of dissolved gases on cavitation inception is essential for many high-tech industries and research. In the present study, large-scale molecular dynamics simulations are performed to investigate the effects of water cavitation caused by different gas types by using nitrogen and oxygen gases with TIP4P/2005 water. The cavitation inception behaviour is analyzed via Mean First Passage Time method. Water with dissolved gases has a higher nucleation rate and is easier to cavitate than pure water. At the same gas concentration, the cavitation of water with nitrogen is promoted to a greater extent than that with oxygen. The number and energy of hydrogen bond (HB) are further calculated by the Acceptor-Hydrogen-Donor method to explain this promotion mechanism. The number and energy of HB in water with gases decrease compared with those in pure water. The introduction of gases weakens the HB network and promotes cavitation inception because of weaker interactions between gas and water molecules. A model is developed to describe the relationship between nucleation rate and HB energy. Gas molecules assemble on the surface of bubbles during water cavitation, which may decrease the free energy of bubble surface, maintain the existing bubble, and contribute to the growth process.     

Surface nucleation and growth in the system of interacting particles

Recent experiments on epitaxial growth of metals on graphene have shown a strong dependence of island densities on coverage. These investigations cannot be explained by the standard mean-field nucleation theories. To understand them, we extend to higher coverage the former theory of rate equations developed for the initial state of nucleation, in a system where adsorbate interaction is included. We account for that, in the case of high coverage, the repulsive interaction influences both the attachment of monomers to clusters and the mobility of atoms. In our work we analyze the modification of the dependence of the island density on coverage, temperature and F/D ratio. In some regimes our theory results in the experimentally observed substantial growth of island density with coverage for a high deposited amount and a weak dependence on deposition rate F. We also find out the local maxima in temperature dependence of island density, as a consequence of long-range repulsive interactions.     

晶体相场法模拟异质外延过程中界面形态演化与晶向倾侧

采用晶体相场模型研究了异质外延过程中失配应变与应力弛豫对外延层界面形态演化的影响, 并对由衬底倾角引起的外延层晶向倾侧进行了分析.研究结果表明: 在有一定倾角的衬底晶体上进行外延生长时,若衬底和外延层之间失配度较大 (ε>0.08),外延层中弹性畸变能会以失配位错的形式释放, 最终薄膜以稳定的流动台阶形式生长且外延层的晶向倾角与衬底倾角呈近似线性关系. 而当衬底和外延层之间失配度较小(ε<0.04)不足以形成失配位错时, 外延层中弹性畸变能会以表面能的形式释放,最终使薄膜以岛状形态生长. 在高过冷度条件下,衬底倾角和失配度较大时,衬底和外延层之间会形成由大量位错规则排列而成的小角度晶界从而显著改变外延层的生长位向.