Sub-bandgap optical absorption of MOVPE-GaN grown under controlled nucleation.

Photothermal deflection spectroscopy (PDS) was used to measure the absorbance of GaN films grown from a 3D nucleation layer. A modeling of absorbance was compared with the experimental data. The results indicate the presence of a highly defective region (∼50 nm) near the substrate in the 3D nucleation template. During the subsequent growth, the films form by lateral overgrowth while the defects remain localized at the interface, leading to a two-layer system. The upper layer exhibits a much lower defect density compared to GaN grown on conventional buffer layers.