Optical gain evolution and quantum-to-bulk transition in wide AlGaN quantum wells

Abstract

We present a theoretical study of the optical gain characteristics in wide Al0.8Ga0.2N quantum wells (QWs) designed for ultraviolet C emission strained to AlN substrates. The electronic band structure was computed using a self-consistent solution of the Schrödinger and Poisson equations coupled with a 6-band k⋅p model, accounting for strain and polarization effects. Our simulations reveal a transition from quantum-confined to bulk-like behavior with increasing well width and carrier concentration. In narrow QWs, optical gain is dominated by ground-state transitions, while in wider structures, excited states and continuum-like carriers contribute significantly. A “dead-width” region is identified, where strong internal electric fields suppress gain unless high carrier densities are applied. We introduce a gain contribution coefficient to quantify the role of individual transitions and distinguish active from inactive states across the quantum-to-bulk regime. These findings provide physical insight into the optical behavior of wide AlGaN QWs and offer practical guidelines for the design of high-efficiency deep-UV laser structures with relaxed confinement requirements.