Rate-Limiting Reactions of Growth and Decomposition Kinetics of Very Thin Oxides on Si(001) Surfaces Studied by Reflection High-Energy Electron Diffraction Combined with Auger Electron Spectroscopy

摘要

The growth and decomposition kinetics of very thin oxides on Si(001) surfaces were investigated by reflection high-energy electron diffraction combined with Auger electron spectroscopy (RHEED-AES) to monitor the reaction rates of oxide growth and decomposition in real time, and changes in surface structure and interface morphology simultaneously. The oxides prepared by stopping the second oxide layer growth at various coverages following the first oxide layer growth by Langmuir-type adsorption at 500℃ under an O_2 pressure of 2.6 x 10~(-4) Pa were isothermally annealed by increasing temperature from 500 to 666℃ at the same time that O_2 gas was quickly evacuated. It was observed that (1) oxide thickness continued to decrease significantly as measured on the basis of changes in O KLL Auger electron intensity ΔI_(O-KLL) before voids appeared at a nucleation time t_N as recognized by the appearance of half-order spots in RHEED patterns, (2) the SiO_2/Si interface morphology was considerably roughened corresponding to ΔI_(O-KLL) during void nucleation as observed by the evolution of the RHEED intensity of bulk diffraction spots ΔI_(bulk), (3) the ratio of ΔI_(bulk) to ΔI_(O-KLL) was almost constant, and (4) oxide decomposition rate during void nucleation, which was approximately given by ΔI_(O-KLL)/t_N, showed a good linear correlation with the second oxide layer growth rate α immediately before starting the decomposition reaction. The good correlation between ΔI_(O-KLL)/t_N and α clearly indicates that the rate-limiting reaction of the second oxide layer growth is closely related to that of the oxide decomposition during void nucleation. All the above-mentioned observations can be comprehensively interpreted using a reaction model proposed for the rate-limiting reactions of oxide growth and decomposition, in which the point defect generation (emitted Si atoms + vacancies) caused by the strain due to the volume expansion of oxidation plays a crucial role because of the high reactivity of the emitted Si atoms and vacancies with dangling bonds. Under an O_2 atmosphere, both emitted Si atoms and vacancies are the preferential adsorption sites of O_2 molecules in the oxide and at the SiO_2/Si interface, respectively. The oxide can be decomposed by the emitted Si atoms to produce SiO molecules, which desorb from the surface, leading to oxide removal in vacuum with SiO_2/Si interface morphology roughening, but may be oxidized within the oxide under an O_2 atmosphere.