The origin of the internal loss in ridge‐type laser diodes (LDs) fabricated using selective re‐growth is investigated through a systematic device characterization and additional optical measurements. We found that the internal loss of this LD is mainly caused by the absorptive layers at the re‐growth boundary and Mg‐doped GaN layer. The internal loss can be significantly reduced through a re‐design of the LD structure to avoid these absorptive regions by shifting the perpendicular optical field to the n‐cladding side. The re‐designed LDs had a very low threshold current of 10 mA and superior gain characteristics. These results indicate, that the InGaN‐quantum‐well (QW) active layer has a large differential gain and fewer non‐radiative defects. The fabrication method of this LD, i.e. epitaxial growth on low‐dislocation‐density GaN substrates combined with a process without dry‐etching, is responsible for the high quality of the QWs.

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Differential gain and carrier lifetime have been deduced experimentally for InGaN MQWs having different magnitudes of In compositional fluctuation and defect density in the active layer. It has been found that the compositional fluctuation and differential gain show a strong correlation in full accordance with the theoretical model for a band-tail modified by In compositional fluctuation as described in the companion paper (part I). Several laser characteristics, threshold current density, differential gain and response time, were found to be affected by the compositional fluctuation and defect density. The optimization of these growth parameters for producing high-performance blue-violet InGaN MQW LDs is also discussed.

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