Herein, the optical and electrical property improvements of the InGaN MQW laser diode with an emission wavelength around 410 n1m are theoretically investigated by tuning the composition of AlGaN/GaN/AlGaN EBL, which is achieved with the simulation software Crosslight. The simulation results show that, compared with the reference structure (p-Al0.2Ga0.8N EBL), composition-graded AlGaN/GaN/AlGaN EBL structure A can increase the electron concentration in the quantum well, and ultimately increase the laser power, in which the Al content of EBL decreased from 0.2 to 0.05 along the growth direction. On the one hand, the height of EBL itself does not decrease compared to reference structure. On the other hand, gradually decreasing the Al content can reduce the polarization effect of the overall EBL, further increase the barrier height of EBL to electrons, resulting in reduced leakage current from 14.5% to 4.8% and increased output power by 0.28 times. For structure B, the EBL Al content increases from 0.05 to 0.2 in the growth direction, the barrier height of the EBL is significantly reduced, and a large amount of electrons leak into the EBL and p-GaN regions, resulting in an increase in leakage current. Therefore, the efficiency is significantly reduced compared to the structure A. The AlGaN/GaN/AlGaN composite EBL with gradually decreasing Al composition is an effective method to increase output power and suppress electron leakage in InGaN 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.
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