Interface-abruptness-limited physics in AlGaN UV-B laser diodes

Abstract

AlGaN-based ultraviolet-B (UV-B) laser diodes have recently shown improved performance through low-temperature metal-organic vapor phase epitaxy growth; however, the dominant physical mechanism remains unclear. In this study, we investigate the mechanism governing the performance improvement by comparing the heterointerface structures actually formed and the resulting device characteristics in samples in which the growth temperature was varied for the layers from the n-side waveguide to the p-side region, while using the same target epitaxial structure and device design. Reducing the growth temperature from 1000 to 775 °C suppresses Al/Ga interdiffusion, resulting in a significant reduction in the thickness of the composition-graded region at the p-side waveguide/electron-blocking layer interface. As a result, the carrier injection efficiency ηi increases from 18% to 50%, accompanied by a fourfold reduction in threshold current density and a twofold enhancement in slope efficiency. Despite increased impurity incorporation and degraded surface morphology under low-temperature growth conditions, the internal optical loss remains nearly unchanged. These results indicate that the observed improvement in device performance is primarily attributed to improved carrier injection resulting from enhanced guide/EBL heterointerface abruptness. Furthermore, these findings suggest that heterointerface engineering provides a useful design strategy for AlGaN-based UV-B laser diodes and may also offer design guidelines for other wide-bandgap nitride semiconductor devices containing heterointerfaces with large composition differences.