Thermodynamics-assisted solution growth of AlN single crystals with Fe-Cr based fluxes

In this study, Fe-Cr based fluxes were investigated for the solution growth of AlN single crystals due to the high nitrogen solubility. Thermodynamic calculations demonstrated that the addition of Cr contributed to achieving a higher nitrogen content in Fe-Cr flux, which was expected to improve the AlN growth rate. However, it was found that the growth rate of AlN and the Cr content in the flux were not simply positively correlated. To explain this phenomenon, a mechanism based on the effective nitrogen content in flux was proposed. By further utilizing this mechanism, the Fe-26 mass% Cr (Fe-26Cr) alloy was designed as a flux and the optimal growth temperature was estimated, and then experimentally validated. Using Fe-26Cr flux, the growth rate of AlN reached 48 μm/h at 1675 °C. The full-width at half-maximum (FWHM) of the X-ray rocking curves (XRCs) for the (0002) and (10−12) reflections in grown AlN single crystals were 72 and 90 arcsec, respectively. Furthermore, the introduction of 2 mass% Al into flux (Fe-26Cr-2Al flux) resulted in a reduction of the O impurity concentrations in the grown AlN crystals by approximately 70 % compared to the AlN synthesized without additional Al. • AlN single crystals were grown using the solution growth method with Fe-Cr fluxes. • Thermodynamic calculations revealed the solubility product of AlN in the flux. • A growth mechanism based on the [effective N] concept was proposed and validated. • With the designed Fe-26Cr flux, an AlN growth rate of 48 μm/h was achieved. • By introducing Al into flux, the O impurity concentrations were reduced by 70 %.