This paper reviewed the recent study on the growth kinetics of very thin oxide layers on the Si(001) surface performed by a real-time monitoring method of Auger electron spectroscopy combined with reflection high energy electron diffraction (RHEED-AES). The RHEED-AES method enabled us to measure simultaneously the oxide coverage and etching rate during Si thermal oxidation. The time evolution of O KLL Auger electron intensity is applicable for discriminating definitely three kinds of oxidation schemes appearing at the initial stage: Langmuir-type adsorption, two-dimensional (2D) oxide island growth and active oxidation. In the Langmuir-type adsorption, the time evolution of RHEED intensity ratio between the half-order spots of (1/2, 0) and (0, 1/2) I(1/2, 0)/I(0, 1/2) suggested emission of Si atoms from the oxidized area, which was interpreted in terms of the interfacial strain due to volume expansion resulting from oxidation. In the 2D oxide island growth and active oxidation, the I(1/2, 0)/I(0, 1/2) showed a periodic oscillatory behavior, the period of which was independent of temperature, oxide coverage and oxidation scheme, but changed in proportion to O2 pressure. This means that all of the adsorbed oxygen atoms are associated with etching of the surface in the 2D oxide island growth as well as in the active oxidation. Based on the experimental results, a surface reaction model of 2D oxide island growth was proposed, in which (1) repeated collisions between desorption precursors SiO* migrating on the surface lead to nucleation and 2D growth of oxide islands, (2) etching of the surface originates from oxide growth as well as SiO desorption, (3) the resultant oxide layers are enriched with Si atoms, and (4) the interfacial strain of oxide islands is rather small in comparison to that for Langmuir-type adsorption. Using the Si atom emission due to the interfacial strain as a key concept, the progress of oxidation at the interface following the Langmuir-type oxidation and 2D oxide island growth and furthermore the decomposition kinetics of very thin oxide layers can be interpreted comprehensively.
Potential energy barriers for dissociative chemisorption of O2 molecules on clean and H2O-preadsorbed Si(001) surfaces were verified using supersonic O2 molecular beams and synchrotron radiation photoemission spectroscopy. The saturated oxygen amount on both kinds of Si(001) surfaces were measured as a function of incident energy of O2 molecules. The saturated oxygen amount was dependent in both cases on the incident energy. Especially, two energy thresholds appeared in the H2O-preadsorbed Si(001) surface oxidation. An Si-2p photoemission spectrum for the oxygen-saturated Si(001) surface formed by O2 gas possessing incident energy below the first threshold on the clean surface revealed the oxygen insertion into backbond sites of Si dimers. The dimer backbonds, however, were not oxidized by O2 irradiation without incident energy larger than 1.0 eV in the H2O-preadsorbed surface. These facts indicate that a chemisorption reaction path of the oxygen insertion into dimer backbonds through bridge and dangling bond sites is open for the clean surface oxidation, and the path is cut by termination of dangling bonds by H and OH radicals.
This article describes an application of reflectance-difference (RD) spectroscopy, that is a surface-sensitive optical technique, to monitoring of the oxidation process on Si. The RD spectrum of a single-domain Si(001) surface shows repeated inversion of its polarity as oxidation proceeds in the layer-by-layer mode. Taking advantage of this oscillation, we have demonstrated in situ measurements of the oxide thickness with atomic-layer precision, which allows the determination of oxidation rate for each monolayer of Si. Such layer-resolved kinetic data will expectedly advance our understanding of the atomistic mechanisms of the oxidation processes.
By decreasing probing depth down to 0.41 nm, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV, that was equivalent with excitation energy required for direct interband transition at Γ point in energy band structure of Si, was observed even through a 1.12-nm-thick oxide film. It was found, by considering the penetration of electronic states from Si substrate into SiO2 in the analysis of thickness dependence of the energy loss of O 1s photoelectrons caused by the direct interband transition, that the top of valence band of compositional transition layer is almost equivalent to that of bulk Si. In other words, the SiO2/Si interface defined by electronic structure exists in the oxide located effectively 0.61 nm away from the nominal interface defined by chemical structure.
The oxidation properties of Si nanostructures on SOI substrate were quantitatively evaluated by novel microscopic methods. Si structures embedded in thermal oxide were non-destructively observed through the oxide layer using scanning electron microscopy and atomic force microscopy. A pattern-dependent oxidation (PADOX) of the Si nanostructures on SOI substrate was quantitatively evaluated by use of these techniques. PADOX is combined phenomena, suppression of oxidation by mechanical stress and oxidation from under Si layer caused by oxygen diffusion in the buried oxide layer. As a result of co-occurrence of these two phenomena, the local oxidation rate strongly depends on the shape and size of the Si nanostructures.
The Si surface oxidation by O2 molecules was studied by the first principles calculations. It was revealed that an O2 molecule is adiabatically chemisorbed from a spin-triplet to spin-singlet state conversion through a spin-orbit interaction when the O2 molecule arrives at the Si surface with a low incident energy. The sticking probability of the O2 molecule is strongly dependent on its incident energy at the early stage of oxidation. The energy barrier for O2 dissociative chemisorption was found to increase according to the depth from the Si surface. The layer-by-layer oxidation was predicted for the topmost several layers because of this energy barrier increase with the subsurface layers, but it is not well understood for deeper subsurface oxidation by this mechanism. The mechanism for the deeper layer-by-layer oxidation is proposed.
SiGe/Si heterostructure with abrupt interface was formed by molecular-beam epitaxy combined with solid-phase epitaxy. We find that nucleation during the solid-phase epitaxy is a thermally activation-type phenomenon. The activation energy of the nucleation decreases from 4.3 eV to 2.2 eV with increasing Ge concentration (x = 0 → 0.3), while the activation energy of crystal growth decreases from 3.1 eV to 2.1 eV. A phase diagram of crystallinity of SiGe layer was obtained. We found that the crystallinity depends on trade-off between the nucleation- and the growth-rates. Transmission electron microscopy confirmed that the solid-phase epitaxial growth completely suppressed Ge segregation between the Si channel and the SiGe layer. Electron mobility of 1500 cm2/Vs at room temperature was obtained.