Reactive Molecular Dynamics Investigation of the Thermal Stability of Organic Monolayers Grafted on Si(111).

F. A. SORIA; P. PAREDES OLIVERA; E. M. PATRITO

Boston, MA

Congreso; Materials Research Society Fall Meeting & Exhibit.; 2015

Materials Research Soc

Organic monolayers covalently anchored to silicon surfaces represent a topic of fundamental and applied interest because such layers may be used as a thin dielectric, as a protection barrier or as a primer layers for use in microelectronics. The functionalization of silicon surface is also of interest in the development of chemical or biochemical sensors.A key issue for such applications is the chemical and thermal stability of the monolayers. In previous works we investigated the chemical stability of hydrogenated and alkylated Si(111) surfaces (1, 2) towards O2 and H2O oxidizing species using Density Functional Theory.In this work have we have employed reactive molecular dynamics simulations using the ReaxFF force field in order to understand the mechanisms of thermal decomposition of −CH3, −CH2CH3, −CH2CH2CH3, −CH2(CH2)2CH3, −CH2(CH2)3CH3 and −CH2(CH2)8CH3 grafted to the Si(111) surface. A maximum theoretical coverage of around 69% is predicted (3) for alkane monolayers on Si(111), implying that hydrogenated silicon atoms still remain on the surface. The only exception is the −CH3 monolayer which has every atop Si atom is bound to a methyl group. Surface SiH groups play a key role in the stability of the monoalyers.The thermal decomposition of the monolayers was investigated in the temperature range from 500 to 1500 K. The first step in the decomposition mechanism involves the breakage of surface SiH bonds and the diffusion of hydrogen atoms into the bulk, leaving surface silyl radicals on the surface. These radicals first abstract hydrogen atoms from the CH2 group of beta carbon atoms which produces the desorption of alkene molecules. As the reaction proceeds, the surface coverage of organic molecules decreases and this allows the molecules to lie down on the surface as a consequence of the interaction with surface sylil radicals. This initiates the dehydrogenation of all the methylene groups. At the highest temperatures a full dehydrogenation occurs giving rise to the formation of silicon carbide on the surface.