Review—Status and Challenges in Hetero-epitaxial Growth Approach for Large Diameter AlN Single Crystalline Substrates

Abstract

Aluminium nitride (AlN) crystalline substrate has emerged as a striking material and received tremendous attention for applications in high power electronics (HPE), deep-ultraviolet (DUV) light sources due to its exceptional properties. Single crystal growth of AlN by physical vapour transport (PVT) technique, and the necessity of large diameter AlN native substrates for the fabrication of HPE and DUV devices are described here. Two competing growth approaches in PVT are utilised to produce initial AlN single crystalline seeds namely, starting with self-nucleation followed by iterative homo-epitaxial growth for enlarging the crystal diameter in steps, and directly seeding on a closely lattice-matched foreign substrate of desired diameter by hetero-epitaxial growth. Both of these approaches are intended to grow bulk single crystals from which wafers might be prepared for further fabrication of devices. The hetero-epitaxial growth approach is specifically and comprehensively reviewed in this present work. A specific attention is given in using 6H- and 4H- polytype silicon carbide (SiC) substrates. The issues in hetero-epitaxially grown crystals such as presence of misfit dislocations, control of low-angle grain boundaries, incorporation of unintentional impurities, are highlighted together with the recent progress made in the achievement of about 2.5-inch dia. free-standing AlN wafer by this approach.