Several hurdles to further enhance the performance of semipolar III-Nitride laser diodes are addressed in this work. Particularly, we focused on improving their high operating voltage by thinning the p-GaN cladding layer and utilizing a transparent conductive oxide p-contact. On-wafer optical absorption measurements showed that a further reduction in voltage with thinner p-GaN was limited by increased optical loss due to increased mode overlap with the ITO/metal anode. In separate attempts to minimize bulk-related optical losses, we implemented a new design that consisted of an AlGaN electron blocking layer (EBL) placed remotely from the quantum wells (QWs) and a low p-waveguide Mg doping profile. A very low optical loss of about 2 cm-1 was extracted but the net improvement in differential efficiency was limited by lower internal injection efficiency due to carrier accumulation in the p-waveguide region. With an optimized design, that consisted of a lightly doped EBL close to the QWs and a UID p-waveguide, an improved light output power of 1.4 W at 1.5 A and a low threshold current density of 1.2 kA/cm2 were obtained.
Band potential fluctuations in InGaN/GaN quantum wells (QWs) induce carrier localization that affects emission linewidth and carrier recombination rate. Alloy composition and well width variations are considered as main sources of the potential fluctuations and are often treated indiscriminately. However, their impact on the emission linewidth and the carrier lifetimes may be different. Besides, the impact of the QW width fluctuations on the linewidth could possibly be reduced via optimization of growth, while random alloy composition fluctuations can hardly be avoided. In this work, we have studied these effects in green-emitting semipolar (20-21) plane InGaN/GaN single QW structures of different well widths (2, 4 and 6 nm) and in structures with different number of QWs (1, 5 and 10). Experiments have been performed by scanning near-field photoluminescence (PL) spectroscopy. It has been found that the well width fluctuations, compared to the InGaN alloy composition variations, play a negligible role in defining the PL linewidth. In multiple QW structures, the alloy composition fluctuations are spatially uncorrelated between the wells. Despite that the 10 QW structure exceeds the critical thickness, no PL linewidth changes related to a structural relaxation have been detected. On the other hand, the well width fluctuations have a large impact on the recombination times. In-plane electric fields, caused by the nonplanarity of QW interfaces, separate electrons and holes into different potential minima increasing the lifetimes in wide QWs.