Aluminum nitride (AlN) offers exceptional properties necessary to explore the development of large area substrates for nitride based electronics and photonics. Recent studies on AlN bulk growth using the sublimation-recondensation method developed at Crystal IS demonstrated high-quality crystals with low dislocation density and crystallographic uniformity. The diameter enlargement of these AlN boules is often associated with extensive defect generation. The goal of this study is to evaluate the origin and distribution of growth defects in AlN bulk crystals.

AlN crystals were grown using the sublimation-recondensation technique and then they were sliced into wafers. The defect evaluation in this study was performed using x-ray topography, differential image contrast and polarized-light optical microscopy, atomic force microscopy (AFM) and etch pit pattern delineation. Special attention was paid to crack development and propagation, grain boundary distribution, micro-scale inhomogenities as well as to the origin and density of dislocations. The major cause of growth defect appears to be non-linearity of both axial and radial temperature gradients. Growth optimization results in lower defect density and improved crystallinity of the AlN crystals.