Micro-Raman scattering from hexagonal GaN, AlN, and Al_xGa_1-xN grown on (111) oriented silicon: Stress mapping of cracks

We report post-growth micro-Raman stress mapping of cracks in GaN, AlN, and Al_xGa_1-xN grown on (111) oriented silicon. Cracks with an average spacing of ∼ 100 μm are observed. These cracks are categorized into two types. The first type of crack propagates through the epilayer, and several microns deep into the substrate and is observed in all the samples investigated. The second type cracks epilayer only and is observed only in GaN. The micro-Raman stress mapping of the first type of crack shows that the epilayers are under biaxial tensile (< 0) stress and the silicon substrate is under compressive (> 0) stress far away from the cracks. The stress in the epilayers as well the substrate is found to relax from the equilibrium (far away from the cracks) value of -0.5 GPa (AlN), -0.16 GPa (GaN), -0.6 GPa (Al_xGa_1-xN) and 0.36 GPa (Si) as the crack position is approached. Partial relaxation is observed to occur over a range of 10 μm. At the crack position, the epilayers and the substrate are relaxed to nearly zero stress values. The stress mapping of the second type of crack reveals that the substrate is completely relaxed (stress is close o zero) far away from the cracks. At the crack position the GaN epilayer is partially relaxed from -0.2 GPa to -0.08 GPa, while the silicon substrate is seen to be under tensile stress of -0.39 GPa. The stress map of epilayers is well described by the distributed force model for both types of cracks. Furthermore, the calculated stress profiles of cracked and uncracked substrate using the above mentioned model are in excellent agreement with the experimental data.