Influence of buffer layer and 6H-SiC substrate polarity on the nucleation of AlN grown by the sublimation sandwich technique

The initial nucleation stage of AIN crystal grown by the sublimation method on on-axis (0 0 0 1)(Si) and (0 0 0 1)(C) 6H-SiC as well as 3.5 degrees off-axis (0 0 0 1)si 6H-SiC substrates was investigated. Sublimation growth from a pure sintered AIN source was carried out in a resistively heated furnace with a source temperature of about 1800 degreesC at a nitrogen pressure of 500 Torr. Direct growth on Si-terminated as-received SiC substrates was discontinuous, marked by sparse nucleation and slow lateral growth of nuclei. Several hexagonal sub-grains were usually obtained on the substrates after a long growth time (more than 3 h) due to the incomplete coalescence of the nuclei. In contrast, no sublimation growth occurred on the as-received C-terminated substrates. To enhance two-dimensional (2D) growth, an AIN epitaxial layer was first deposited on the substrates by MOCVD before sublimation growth. Continuous films could then be grown on all the substrates with AIN MOCVD buffer layers. The tensile stress of the AIN layer due to thermal expansion coefficient mismatch between SiC and AIN caused cracking across the AlN/SiC interface into the SiC substrates during the cooling process limiting the maximum of the thickness and lateral size of the AIN crystals. (C) 2001 Elsevier Science B.V. All rights reserved.