Performance enhancement of GaN-based VCSELs by composition-graded structures

The carrier-injection efficiency and confinement within the active region play a crucial role in determining the threshold and slope efficiency of GaN-based vertical-cavity surface-emitting lasers (VCSELs). In this study, two types of composition-graded epitaxial structures were proposed: a composition-graded last quantum barrier (LQB) and a composition-graded electron blocking layer (EBL). GaN-based optically pumped VCSELs incorporating these structures exhibit significant improvements over conventional composition-uniform designs, with threshold reductions of 71.1% and 53.2%, respectively. Band structure analysis reveals that these performance improvements are attributed to enhanced electron injection efficiency and improved hole confinement within the active region as well. Furthermore, pulsed lasing operation in electrically injected VCSELs was demonstrated by employing the composition-graded LQB structure. This study systematically validated the effectiveness of the composition-graded LQB and EBL structures in improving GaN-based VCSEL performance from experimental perspectives. These findings provide valuable insights for the development of GaN-based VCSELs.