In situ infrared and visible-light ellipsometric investigations of boron nitride thin films at elevated temperatures

In situ infrared (IR) spectroscopy and visible-light (VIS) spectroscopic ellipsometry over the spectral range from 700 to 2000 cm^-1 and 1.5-3.5 eV, respectively, were used to investigate the optical behavior of boron nitride (BN) thin films at temperatures from room temperature (RT) to 600°C The polycrystalline hexagonal (h) and mixed-phase h- and cubic (c)-BN thin films were deposited by magnetron sputtering on [001] silicon. We observe a reversible moisture incorporation process in as-grown h-BN samples. When stored in normal ambient, the h-BN thin films absorb water into thin-film micropores. When annealed in ultrahigh vacuum or a dry nitrogen atmosphere, the samples expel moisture but retain their microstructure. This is observable by reduction of the thin-film refractive indices in accordance with changes in the IR lattice resonance behavior. The optical properties of high c-BN content thin films remain unchanged during annealing. And both intrinsic h- and c-BN thin-film VIS refractive indices are nearly temperature independent, at least up to 600°C Therefore RT BN optical constants can be used for feedback loop control in in situ thin-film growth at temperatures up to 600°C.