Role of SiC substrate polarity on the growth and properties of bulk AlN single crystals

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum a parts per thousand 380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (< 2 wt%).