Effect of the Surface Structure of Thin Layers on Their Reactivity
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1074-I03-22
Effect of the Surface Structure of Thin Layers on Their Reactivity Fedor Dultsev SB RAS, Institute of Semiconductor Physics, Lavrentiev ave., 13, Novosibisrk, 630090, Russian Federation Effect of the surface structure of thin layers on their reactivity F.N. Dultsev ABSTRACT Changes of the electron properties of surface atoms of silicon dioxide film caused by the adsorption of donor-type gas-phase molecules (H2O, O2, NH3, HF) were modelled with the help of semiempirical methods and molecular mechanics. It was shown that the adsorption capacity and reactivity of the layers depend on the Si-O-Si bond angle. The results of calculations are in good agreement with the experimental data and confirm our hypothesis concerning the rearrangement of the surface layer stimulated by the adsorbed molecule. INTRODUCTION Surface processes are important in catalysis, in the development of gas sensors, in the synthesis or dry etching of thin silicon dioxide-based layers. The reactivity of a solid is known to depend on its structure. Speaking of thin films, we may state that their porosity is one of the essential characteristics. A decrease in pore radius below 1 nm causes changes in the electron structure of surface atoms, which causes changes in chemical properties. In this situation, the electron structure of surface atoms should be taken into account. Using porous materials as low-k dielectrics one should keep in mind the high chemical activity of their surface. Understanding of the dependence of chemical properties on film structure will allow us to modify the film in the necessary direction. A key to such an understanding may be investigation of changes in the electron structure of surface atoms. The contact of a surface with the gas phase is accompanied by the adsorption of gas molecules on the surface. This causes changes in the surface itself. Speaking of silicon dioxide, one may state that the diversity of its chemical properties is due to the lability of Si-O-Si bonds: angle changes determine the reactivity of SiO2 films. This is
explained by the large nuclear charge of Si, which causes lowering of the energy of unoccupied 3d orbitals, so that their energy becomes close to the energy of 2p orbitals of O atoms. So, the possibility of additional overlapping of 3d orbitals of Si atoms and 2p orbitals of O atoms appears; this is the reason of the variety of chemical properties of silicon dioxide. For example, it was shown in [1] that 200 oC is a critical temperature point for the low-pressure chemical vapor deposition of SiO2 layers (LPCVD). One of the reasons is a change in the electron structure of surface atoms causing a decrease in the adsorption capacity in the higher-temperature region. A structural rearrangement occurs at a temperature of about 200 oC in silicon dioxide films characterized by the cristobalite-type structure [2], which may actually be a reason of the observed competition between the two routes (gas-phase and heterogeneous ones) of SiO2 deposition process. The experimentally determined activa
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