Electronic and optical properties of biaxially strained Al1-xGaxN and In1-xGaxN

This study presents a comprehensive first-principles investigation of the structural, electronic, and optical properties of Al_1-xGa_xN and In_1-xGa_xN ternary alloys (x = 0, 0.25, 0.5, 0.75, 1) under biaxial strain conditions ranging from −6 % to +6 %. Density functional theory (DFT) with the PBE and TB-mBJ functionals was employed to obtain accurate predictions of band structures and dielectric responses. The results reveal that biaxial strain induces significant band-gap modulation—up to ∼1.5 eV in Al-rich alloys and ∼1.1 eV in In-rich alloys—providing tunability across a broad spectral range from the ultraviolet to the near-infrared. The imaginary part of the dielectric function, ε₂ (ω), exhibits clear strain-induced shifts: tensile strain leads to a pronounced redshift and intensity enhancement, particularly in Ga- and In-rich alloys, while compressive strain induces a blueshift. The refractive indices n_x and n_z exhibit systematic composition- and strain-dependent trends, increasing monotonically with the incorporation of Ga or In and displaying notable anisotropy under strain. Specifically, tensile strain enhances both n_x and n_z by up to ∼0.3–0.4 across the alloy series, whereas compressive strain reduces them. Comparative analysis between Al_1-xGa_xN and In_1-xGa_xN highlights distinct strain sensitivities, reflecting differences in bond character and electronic polarizability. These findings demonstrate that a strategic combination of composition and strain engineering can effectively tailor the optoelectronic response of III-nitride alloys for applications in UV–visible light emitters, detectors, and high-efficiency photonic devices.