Study of the bowing parameter and Urbach edge in AlGaN alloys grown by plasma-assisted molecular beam epitaxy

Abstract

Optoelectronic devices based on AlGaN alloys find a wide range of applications, such as water and surface purification, solar-blind photodetection, and skincare. The energy bandgap of the alloy is a critical input parameter for the design of such devices. However, widely different values for the bowing parameter exist in the literature. In addition, data for AlGaN alloys with high AlN mole fractions are relatively sparse. In this work, AlGaN alloys covering the entire range of compositions from GaN to AlN were grown by molecular beam epitaxy, and their compositions and optical absorption edges were estimated and compared to the existing literature. While for alloys with lower AlN mole fractions, we report a bowing parameter similar to established values, a large variation was observed for AlN mole fractions higher than 60%, where the bowing parameter increased significantly. Overall, three different bowing parameters were identified across the entire composition range. Alloy disorder of these AlGaN films was estimated by computing their Urbach energies, and their dependence on the alloy composition also exhibits a significant increase for these high Al containing films. While the exact reason for this phenomenon remains unclear, the growth mode employed, and the presence of alloy phenomena such as atomic ordering and compositional inhomogeneities in the material may play a major role. These results are expected to be useful in the design of optoelectronic devices for deep ultraviolet applications, including light emitting didoes for skin-safe germicidal action.