In situ stress measurements during MOCVD growth of AlGaN on SiC

Deposition of Al_xGa_1-xN (x>0.3) for UV devices is challenging due to defect formation and film cracking that are commonly observed in thick layers. Cracking is generally attributed to lattice constant and thermal expansion mismatch between film and substrate, and also between AlGaN layers of different compositions. However, growth-related stresses due to developing film morphology also play a role. In situ stress measurements were used to investigate effects of growth conditions on stress evolution during MOCVD growth of AlGaN on SiC. An initial compressive stress was measured during AlGaN growth on thin AlN buffer layers, which evolved with film thickness into a tensile growth stress. The thickness at which the transition occurred was found to depend on the buffer layer growth conditions and on the AlGaN Al fraction. Increasing the AlN buffer layer V/III ratio from 750 to 10,600 increased the AlGaN initial compressive stress from 1.9 to 8.7 GPa, which reduced the film's final tensile stress. GaN was not observed to transition into tensile growth stress within the 3.8 μm thickness grown, while increasing Al fraction corresponded to a more rapid change into tensile growth. The observed stress evolution is explained in terms of an initial compressive growth stress that changes to a final tensile stress due to morphological evolution during growth.