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Pattern-dependent oxidation (PADOX) of silicon nanostructures fabricated on silicon-on-insulator (SOI) substrates is simulated. In order to reproduce the characteristic features of PADOX in the simulation, the volume expansion due to silicon oxidation is treated as a dilational strain, and the strain is applied to a transition region in which silicon is converted to oxide. In addition, the silicon oxide and transition layer are treated as viscoelastic solids, and the stress dependencies of the oxidation reaction, oxygen self-diffusion in the oxide, and oxide viscosity are taken into account. The simulated silicon and oxide shapes after oxidation satisfactorily reproduce the experimental results. The simulation results suggest that the rounded silicon shapes that appear after oxidation are mainly caused by the stress-induced reduction of oxide viscosity. Moreover, we obtain oxidation-induced strain and stress from the simulation. Based on the strain obtained, the electron potential profile responsible for the operation of single-electron transistor (SET) is investigated. The compressive strain in the silicon wire region of SETs reduces the bandgap, and this reduction is critical for the formation of the potential profile responsible for SET operation. [Copyright &y& Elsevier]

The atomistic mechanism of silicon oxidation is investigated by a combination of first-principles calculations and macroscopic approaches. The results of first-principles calculations show that oxidation-induced strain accumulates at the interface as oxidation proceeds, which indicates the emission of a large number of Si atoms to release the accumulated strain. Further calculations to investigate the favorable location of the emitted Si atoms indicate that most of them diffuse into the oxide layer and are oxidized there. Based on these results, we propose a model that the emitted Si atoms in the oxide govern the oxidation rate due to their high concentration. Next, based on the model, we construct the macroscopic diffusion equations, which include Si diffusion species in addition to oxidant species, and we successfully simulate the whole range of oxide thickness, including the thin film regime, in a wide range of oxidation conditions. In addition, combined with the elastic continuum theory, we show that the strain release by the Si emission can explain the recent observations of Si layer-by-layer oxidation together with the formation of many small oxide islands. [Copyright &y& Elsevier]

Abstract: Hydrogen-terminated diamond surface exhibits p-type conductivity during its exposure to air. To investigate this phenomenon, we examined the influence of different gases on the surface conductivity. Exposure to NO2 gas resulted in the biggest increase in conductivity, while H2O vapor decreased the surface conductivity. Moreover, even very low concentrations of NO2 molecules in air increased the hole sheet concentration, and with increasing NO2 concentration, the hole sheet concentration increased up to 2.3×1014 cm−2 (at 300ppm NO2). This increase of hole sheet concentration was observed during exposure to NO2 gas and simultaneous adsorption of NO2 molecules on the diamond surface, while it decreased when the exposure stopped and NO2 molecules desorbed from the surface. X-ray photoelectron spectroscopy investigation showed upward band bending and partial oxidation of the hydrogen-terminated surface after exposure to air and NO2. FETs exposed to NO2 gas exhibited lower source and drain resistances, which led to a 1.8-fold increase of maximum drain current, transconductance increased 1.5-fold and maximum frequency of oscillation increased 1.6-fold. [Copyright &y& Elsevier]

Graphene and related two-dimensional (2D) materials are attracting huge attention due to their wide-range potential applications. Because large-scale, high-quality 2D crystals are prerequisites for many of the applications, crystal growth of 2D materials has been intensively studied. We have also been conducting research to understand the growth mechanism of 2D materials and have been developing growth techniques of high-quality materials based on the understandings, in which detailed structural characterizations using low-energy electron microscopy (LEEM) have played essential roles. In this paper, we explain the principles of obtaining various structural features using LEEM, and then we review the status of our current understanding on the growth of graphene and hexagonal boron nitride. [ABSTRACT FROM AUTHOR]