Strain relaxation in semipolar $(202¯1)$ InGaN grown by plasma assisted molecular beam epitaxy

Strain relaxation in semipolar $(202¯1)$ InGaN layers grown by plasma assisted molecular beam epitaxy (PAMBE) was investigated with high-resolution X-ray diffraction (XRD) reciprocal space mapping, cathodoluminescence (CL), fluorescent light microscopy (FLM), and atomic force microscopy. We find that XRD detects lattice relaxation much later than its actual onset occurs. Other techniques used in this study allowed to detect local footprints of plastic relaxation before it was evidenced by XRD: at the initial stages of strain relaxation, we observed changes in layer morphology, i.e., formation of short trench line segments on the surface along the ⟨$112¯0$⟩ direction as well as dark lines in CL and FLM. The misfit dislocations formation and glide were observed in two slip systems: initially in basal slip system ⟨$112¯0⟩{0001}$ and for larger amount of strain in non-basal, prismatic slip system $⟨112¯0⟩{11¯00}$⁠. Experimentally determined critical thickness for InGaN layers grown by PAMBE on semipolar $(202¯1)$ bulk GaN substrates agrees well with literature data obtained with metalorganic vapor phase epitaxy and follows the Matthews-Blakeslee model prediction. We discuss the impact of substrate structural properties on the strain relaxation onset and mechanisms. We also describe the layer morphology and surface roughness evolution related to the increasing In content and strain relaxation of the semipolar $(202¯1)$ InGaN layers.