On the solubility of boron nitride in supercritical ammonia-sodium solutions

Abstract

Cubic boron nitride (cBN) is considered to be one of the leading candidates for next generation electronic devices yet has no current synthesis route for single crystals greater than a few mm in size leading to a lack of suitable substrates. New synthesis methods need to be explored, with the ammonothermal method being a potential candidate for synthesis of large area, thick cBN and hexagonal BN (hBN) single crystals. This paper investigates the solubility of BN in supercritical ammonia using sodium amide (NaNH2) as a mineralizer and cBN crystals as feedstock. Solubilities were measured based on mass loss of the feedstock at temperatures from 450—600 deg. C, with pressures of 150–190 MPa, using durations from 24—96 h and with varying amounts of Na. A positive solubility trend is demonstrated with respect to temperature with solubility varying from 0.017—0.050% molBN/molNH3. Solubilities did not vary as a function of time or when>0.07 molNa/L mineralizer content was used suggesting both that equilibrium was reached and the solutions were saturated. These values are high enough to motivate further investigation for solution-based growth of BN.