Prismatic Slip in AlN Crystals Grown By PVT

Abstract

Aluminum nitride (AlN) single crystals are desired as substrates for nitride based electronic and optoelectronic applications[1]. A low density of dislocations and other structural defects in bulk AlN substrates is in demand to grow high-quality heteroepitaxial epilayers. Physical vapor transport (PVT) grown AlN crystals possess low density dislocations and other structural defects[2]. In AlN crystals grown by PVT, basal plane slip is the most frequently observed deformation mechanism. However, prismatic slip takes place as well in such crystals [3, 4].When the diameter of AlN wafers expands to 50 mm and larger, managing the thermal gradients in the PVT growth chamber becomes critical for reducing thermal stresses that cause deformation. Recent synchrotron X-ray topography [5] studies of 50 mm diameter AlN wafers showed that while most wafers contained few to no basal plane dislocations (BPDs), some wafers possessed a 6-fold BPD dislocation pattern [2] which was aligned along the <11-20> directions. This configuration indicated that prismatic slip had likely occurred. Screw typed prismatic dislocations cross slipped from the prismatic plane onto the basal plane, and underwent basal plane slip, leading to dislocation multiplication. To investigate the origins of prismatic slip, a radial thermal gradient model was established showing[6] the resolved shear stress across the entire area of the crystal boule during growth. The results from the model were compared with experimental observations to investigate the role of radial thermal gradients in the nucleation of prismatic slip in AlN during bulk growth. This insight will help guide efforts to minimize structural defects in PVT AlN.