Optical characterization of nanocrystalline boron nitride thin films grown by atomic layer deposition

• Atomic layer deposition of BN with< 1 nm per cycle deposition rates
• A narrow self-limiting temperature window was found.
• Nanocrystalline h-BN films with a wide transparence window with Eg up to 5.85 eV

Boron nitride thin films were grown on sapphire and Si substrates by atomic layer deposition from triethylborane (TEB) and NH3 precursors in the temperature range of 500 to 900 °C. By varying the TEB exposure the film thickness can be controlled with< 1 nm precision. At 600 °C the process is self-limiting, but films are found to be amorphous. Films grown at higher temperatures were identified as sp2 BN, but the process is no longer self-limiting. From Raman and IR absorption spectroscopy films deposited at 900 °C were identified as nanocrystalline sp2 BN with crystallite sizes in the range of 3 to 8 nm depending on the NH3 dosage. Films deposited at lower temperatures had broad red shifted IR absorption peaks indicating the lack of long range ordering. The visible and UV optical properties of these films were characterized by UV–Vis transmission measurements over the range of 800 to 190 nm. Nanocrystalline films are highly transparent over this range up to the band gap, which was measured to be in the range of 5.83 to 5.65 eV depending on the NH3 dosage.