Large-scale modeling of ultra-thin SiO2 films on Si(001) surfaces has been performed by means of molecular dynamics utilizing our original inter-atomic potential energy function for Si, O mixed systems. The SiO2 film is formed by layer-by-layer insertion of O atoms into Si-Si bonds in a Si wafer from the surface. The obtained models reproduce quantitatively the structural transition layers near the interface. Through a modeling of vicinal SiO2/Si(001) model including atomic steps, it has been found that oxide film near the step-edge is preferentially amorphized. For a more advanced modeling method, we propose a new simulation procedure where O atoms are introduced into the substrate in one-by-one manner. In the calculation, the oxidation is started from the surface and abrupt change in composition at the SiO2/Si interface is reproduced. Thus, the classical molecular dynamics is a powerful method together with a simplified inter-atomic potential function applicable to mixed systems.
Adsorption and diffusion of Ge atoms on a H/Si(001)-(2×1) surface is investigated by using first-principles total energy calculations and kinetic Monte Carlo simulations. The site exchange between the Ge adatom and substrate Si atoms has a significant effect on the energetics and the kinetics. We found that there are a number of adsorption geometries available for Ge atoms and that the dominant adsorption geometry temporally varies depending on the substrate temperatures. Ge atoms diffuse not on the surface but in the subsurface region. The diffusion species changes from Ge atoms to Si atoms at higher temperatures, resulting in formation of Ge-Si dimers or Si-Si dimers in the initial growth stage. These findings imply that transient states may have an important effect on the Ge growth.
Step dynamics is crucial for the growth of a crystal at temperatures lower than the roughening transition. Recent advancement of microscopy has made direct comparison of statistical theory and quantitative atomic scale experiment possible. A short review of basic ideas on the motion of steps and its application to the shape relaxation in micro-crystals is presented. Problems such as realization of the equilibrium shape of a micron size crystal and decay of a 3 D island on a substrate are discussed.
Autocatalytic-reaction model, a chemical-kinetics model that provides an integrated description of various dynamical processes in the very initial oxidation of Si(100) surface, is illustrated. The nonlinear rate equation of this model has an analytical solution. With only two fitting parameters as it is, the solution quantitatively reproduces the experimental time-evolution of the surface oxide coverage in various oxidation modes, which range from the Langmuir-type in the low-temperature/high-pressure regime to the two-dimensional-island-growth-type in the high-temperature/low-pressure regime, or even to the one in the etching regime containing simultaneous decomposition of grown oxides in addition to their growth. Microscopic background behind the model is discussed in comparison with existing oxidation models and thin-film-growth models.
We have simulated dry and wet oxidation of silicon taking into account the emission of a large number of silicon species from the interface that governs the silicon-oxidation rate. The silicon emission model enables the simulation to be done by use of the oxidant self-diffusivity in the oxide with a single activation energy. Using a unified set of physically reasonable parameters, the whole range of oxide thickness for both dry and wet oxidation is fitted in a wide range of oxidation temperatures (800−1200oC) and oxygen pressures (1−20 atm) for both (100) and (111) silicon substrates without any empirical modifications.
Effects of the tip induced nonconservative atomic processes on the cantilever dynamics of dynamic-mode atomic force microscopy are theoretically analysed on the basis of the time averaging perturbation theory. Atomic adhesion is considered as an example of nonconservative process. Typical magnitude of the Q value due to adhesion is estimated to be of the order of 104, which is comparable to the intrinsic Q value of the cantilever. The additional frequency shift due to adhesion is estimated to be of the order of 10 Hz, which sets in suddenly when the tip turning point becomes less than a certain threshold height. This feature explains the experimental observation of the discontinuous frequency shift at chemical reactive sites on the semiconducting surface.
A single wall carbon nanotube (SWNT) has an extremely small diameter of 1∼2 nm and a high aspect ratio. For the use of the SWNT as an atomic force microscopy (AFM) cantilever, the SWNT was grown directly onto the top of the conventional silicon (Si) AFM cantilever using chemical vapor deposition (CVD) at 900oC in flowing CH4 gas. The length of the SWNT was hardly controlled in the growing process. Therefore, we cut the SWNT and adjusted its length at the top of the cantilever by bending and by applying the bias between the cantilever and the conductive substrate with monitoring the force curve and the vibration amplitude of the cantilever. The resolution of AFM using the optimized SWNT cantilever was compared with the one using conventional Si cantilever by observing the Au surface and TiOx lines fabricated utilizing the AFM nano-oxidation process. The radius of the SWNT cantilever was estimated by observing the width of the DNA in air.