Computational study of intersubband absorption in strain-free δ-doped GaNAsBi/GaAs quantum wells for infrared photodetectors

In this work, we investigate the intersubband optical absorption properties of δ-doped GaN0.04As0.89Bi0.07/GaAs single quantum wells using a Self-Consistent Schrödinger-Poisson approach within the effective mass approximation numerically solved using finite difference method combined with a density-mixing procedure. The selected composition exploits the compensation between the compressive strain induced by bismuth and the tensile strain induced by nitrogen at y/x≈0.58, yielding an alloy lattice-matched to the GaAs substrate. This strain-free configuration suppresses misfit dislocations and interfacial defects, preserves crystalline quality, and removes the need for strain-compensation layers, offering a direct manufacturing advantage over strained III-V heterostructures. We analyzed the electronic structure and infrared absorption response by varying the quantum well width LQW, donor density Nd, δ-doping thickness δ, and δ-doping position α. Our results show that increasing LQW induces a pronounced redshift of the fundamental T(1→2) transition from the mid-wave infrared (MWIR) toward the long-wave infrared (LWIR) region while the wider well providing supports multiple allowed intersubband transitions over 2.8-11.8 μm range. We demonstrate that increasing Nd significantly enhances the absorption amplitude through carrier accumulation and Hartree-induced confinement, while varying δ mainly affects the carrier confinement and redistribution, with a strong influence on the T(2→3) transition. Most importantly, we demonstrate that shifting the δ-doping layer from the QW center breaks the inversion symmetry of the confinement potential and activates the symmetry-forbidden T(1→3) transition through parity mixing of the envelope wavefunctions. The T(1→3) transition remains resolved under moderate Lorentzian linewidth broadening. Within the present theoretical model, a favorable design window around LQW=6nm, Nd=5x1019cm-3, δ=2-3 nm, and ∣α∣=1- 2 nm provides strong and tunable infrared absorption. These findings highlight the potential of this strain-free GaNAsBi/GaAs quantum-well system for tunable infrared photodetector applications.