Strain relaxation in AlGaN under tensile plane stress

Abstract

Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cracks but also by the extension of cracks into the GaN layer and a thermal activated change in the defect structure.

Internal strains and crystal structure of the layers in AlGaN/GaN heterostructures grown on a sapphire substrate

“…32 As can be deduced from the data, a deformation jump for the Al 2 O 3 TKs is one order of magnitude lower than that of TNs. For the GaN film, the jump is practically the same in both cases.…”

Intersubband transitions in nonpolar GaN/Al(Ga)N heterostructures in the short- and mid-wavelength infrared regions

“…In this case, cracks are isotropically distributed, and present vertical {11 ̅ 00} facets. 25,26 Due to the anisotropy of the nonpolar lattices, relaxation along the c and a/m directions must be analyzed independently.…”

Relaxation in crack‐free AlN/GaN superlattices

“…A first investigation of the SL film was performed by light microscopy (not presented here) and showed some surface roughening but no macroscopic cracks despite the fact that the film contained more than 60% Al in average and was about 400 nm thick, which is far above the critical thickness for a homogenous Al 0.6 Ga 0.4 N film grown on GaN [4,5]. However, the bright field (BF) TEM image taken along the [0 110] zone axis -shown in Fig.…”