Oxidation of the Surface of Polycrystalline Boron

The growth of an oxide film on the surface of polycrystalline β-rhombohedral boron during thermal oxidation in air at temperatures of 400, 500, 600, and 700°C was studied using in situ single-wave ellipsometry. At temperatures above the melting point of B₂O₃, the oxidation was significantly activated. Having reached its maximum at 500, 600, and 700°C, the thickness of the oxide film decreased. This is attributed to the dynamic equilibrium between the processes of evaporation of liquid oxide and film growth caused by the diffusion of boron and oxygen ions. The refractive index of the substrate (pure boron) decreased from 3.1 to 2.95 during the annealing due to a change in its porosity. It was established by Raman spectroscopy that after annealing at 700°C, the surface contained not only the oxide B₂O₃, but also traces of suboxide B₆O, previously observed during the oxidation of boron at higher temperatures.