Bulk AlN crystal growth: self-seeding and seeding on 6H-SiC substrates

Abstract

The properties of bulk AlN crystals grown by sublimation recondensation and either randomly nucleated (i.e. self-seeded) or seeded on 6H-SiC substrates or compared. Self-seeding produces crystals of the highest perfection, lowest stress, and low Si and C impurity content, but the crystals grow in random crystallographic orientations. Crystals grown in boron nitride crucibles typically form thin platelets with the fastest growth occurring in the c-axis direction. Growth striations run the length of the crystals in the c-axis direction. Anisotropic etching in aqueous 45 wt% KOH solutions shows that the growth (0 0 0 1) planes exposed to the AlN source predominately have an aluminum polarity. AlN crystals seeded on 6H-SiC(0 0 0 1) have a single crystallographic orientation and the largest dimensions are perpendicular to the c-axis, determined by the size of the substrate. Cracking and voids in the AlN layer produced by differences in thermal expansion coefficients of AlN and SiC and decomposition of the SiC were ameliorated by depositing an AlN–SiC alloy layer on the SiC before growing the AlN layer. Raman spectroscopy measurements suggest the AlN and AlN–SiC alloy layer are both under tensile stress. The defect density in AlN crystals grown on composite AlN–SiC/6H-SiC substrates was 3.7×10 5 cm −2, as determined by synchrotron white beam X-ray topography.