The MOCVD overgrowth studies of III-Nitride on Bragg grating for distributed feedback lasers

The AlGaN film was overgrowth on the GaN grating substrates by a metal organic chemical vapor deposition method. The gratings were fabricated with fine step shape structure by nanoimprint and inductively coupled plasma (ICP) dry etching. The overgrowth epi-layer acts as the up-cladding layer within the distributed feed-back (DFB) laser diodes, which is the kernel process for the embedded grating DFB. The high quality AlGaN could obtain within the limited space in the DFB structure. It is found that the grating shape changes from rectangle to zigzag with reduced threading dislocation density (TDD), leading to better quality crystalline. The optimized overgrowth contributes to bending the TDs originating from the GaN/sapphire interface by image force during the film coalescence over the zigzag grating. The COMSOL model was built to simulate the effect for shape changing of the grating caused by overgrowth on the grating. The shape changing also affects the resonant wavelength of the DFB via effective refractivity index distribution. And the zigzag grating could achieve a higher Q factor than that of rectangle grating with optimized grating depth, which would contribute to improving the ability of mode selection for DFB laser diodes.