Microwave Plasma Chemical Vapor Deposition to Synthesize Multiphase Boron Nitride using External DC Bias. *

Boron nitride (BN) is a member of Group III-nitrides, and has aroused great interest among the scientific community during the past two decades for its outstanding properties including hardness, toughness, chemical inertness, thermally conductivity, and electrically insulation. Due to its low-cost and scalable production technology, chemical vapor deposition offers the most viable technology route for synthesizing BN. In this study, Microwave Plasma Chemical Vapor Deposition (MPCVD) was used to synthesize multiphase BN on silicon substrate. The produce BN coating has both sp² and sp³ bonding. The applied negative DC bias was found to yield a higher content of sp³ bonded BN in both cubic and metastable wurtzite structural forms. Another metastable phase known as Explosion BN (E-BN) is also present in the coating which has a mixture of sp² and sp³ bonding in the structure. To the best of our knowledge, we report the first synthesis of metastable phases of BN made by MPCVD. X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) were used to characterize sp² and sp³ bonded content in the BN coatings. Nano-indentation measurements reveal an average coating hardness of 25 GPa with some measurements as high as 31 GPa, consistent with a substantial fraction of sp³ bonding mixed with the hexagonal sp² bonded BN phase.