Reduction of the threshold current of deep-ultraviolet laser diodes with embedded quantum dots in quantum wells

Abstract The relationship between electron state density and energy in deep ultraviolet laser diodes (LDs) with quantum well (QW) structures exhibits a step-like shape, which enhances differential gain and light confinement of the LD. However, the active layer in QW structures typically has limited ability to collect equilibrium carriers, leading to an increased threshold current. Quantum dot (QD) structures address this limitation by imposing quantum confinement on charge carriers in three dimensions, thereby reducing optical losses and improving carrier confinement. Despite these advantages, the presence of numerous QDs often introduces synthetic imperfections (e.g. size and shape variations) and surface/interface defects, which cause light to split into multiple wavelengths. This study investigated the embedding of QDs into AlGaN QWs and evaluated the impact of QD size on device performance. This configuration addresses the weak carrier collection in QW structures and leverages QDs to quantum-constrain carrier movement, while avoiding the problem of high threshold current densities due to the large number of QDs that are not uniform in size and indicative of defects. For a QW thickness of 4 nm, an optimal QD thickness of 1 nm was determined, enabling effective utilization of both QDs and QWs. This configuration led to a substantial increase in LD output power and a decrease in threshold current. Specifically, the output power of the LD with a 1 nm thick QD increased by 13.6 times, while the threshold current decreased by 63.6% compared to the QW structure alone. Besides, the findings also revealed that QD thickness significantly affects the photoelectric properties of the LD. Excessively thick QDs can form depressions in the middle of QWs, which lowers the restriction on charge carriers. Conversely, QDs that are too thin cannot effectively take advantage of the three-dimensional carrier constraints of QDs, thereby reducing the light confinement capability.