Hexagonal Boron Nitride (hBN) is a 2D, III-nitride wide bandgap semiconductor that has a structure very similar to graphene. Due to its extraordinary physical properties, such as high resistivity, high thermal conductivity, stability in the air up to 1000°C, large bandgap (Eg ∼ 5.9 eV), hBN appears to be a promising material for emerging applications, including deep UV (DUV) optoelectronics, electron emitters, and neutron detectors. There is also significant interest in monolayer and few-layer hBN as an encapsulating layer for 2D devices based on graphene and transition metal dichalcogenides. While high-quality hBN bulk crystals are available for exfoliation, the size of the crystals is limited, consequently, there is continued interest in the epitaxial growth of large-area hBN films using chemical vapor deposition (CVD). However, it should be kept in mind that film growth by CVD is accompanied by a sequence of chemical reactions (which is much more complex than the assumed stoichiometry) between precursors used in forming the final product, which in this case is hBN. In this work, we are studying the growth of crystalline sp2 BN in a vertical cold wall MOCVD reactor using a mixture of diborane (B2H6) and NH3 as precursors for B and N, respectively using H2 as a carrier gas. The growths are performed on c-plane sapphire substrates at temperatures ranging from 1100-1500oC. Diborane reacts with NH3 at temperatures as low as room temperature to form H3N-BH3 and other volatile B-N species, consequently, gas-phase chemistry plays an important role in this deposition process. The influence of temperature, N/B inlet gas ratio and precursor flow method (pulsed versus continuous) on the material quality, characteristics and deposition rate were studied. At 1350oC, stoichiometric sp2 BN films were obtained on sapphire using N/B ratio of ~7500 under continuous precursor flow giving a growth rate of ~12 nm/hr. Sequential pulsing of the B2H6 and NH3 precursors resulted in a significant increase in the growth rate (~300nm/hr) due to a reduction in the extent of gas-phase reaction. Additional studies are underway to investigate growth at higher temperatures and over a wider parameter space to ascertain the growth mechanism as a function of conditions to provide additional insights into the role of gas-phase chemistry on the properties of highly crystalline sp2 BN films on sapphire.

Influence of gas-phase reactions between B2H6 and NH3 towards the controlled synthesis of hexagonal boron nitride

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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