Electronic properties and phonon-assisted optical absorption of 4H-SiC/GaN heterojunctions under strain

To elucidate the interfacial physics and optical performance limits in 4H-SiC/GaN ultraviolet (UV) photodetectors, first-principles density functional theory and many-body quasi-degenerate perturbation theory calculations are performed to quantitatively characterize the electronic properties and phonon-assisted optical absorption of 1.5–2.5-nm-thick 4H-SiC/GaN heterojunctions under in-plane strain ranging from −8% to 8%. The stable type-II heterojunctions exhibit optical absorption edges of 1.75 eV (SiN1) and 1.40 eV (SiN2). Remarkably, at the critical UV wavelength of 375 nm, the absorption coefficient of SiN1 reaches 2.72 ×105 cm−1, substantially outperforming those of bulk GaN (2.21 ×105 cm−1 ) and 4H-SiC (5.44 ×103 cm−1 ). Furthermore, biaxial strain induces pronounced asymmetric spectral tunability: 8% tensile strain drives a large 1.14 eV redshift (extending the absorption edge to 0.63 eV) and boosts the 375-nm absorption coefficient to 6.27 ×105 cm−1 , whereas −8% compressive strain causes a 0.41 eV blueshift up to 2.18 eV. In contrast, uniaxial strain yields a noticeably weaker modulation effect. These quantitative findings elucidate the microscopic optoelectronic mechanisms governed by interfacial polarization, spatial charge separation, and the quantum-confined Stark effect, establishing fundamental design principles for strain-tunable UV photodetectors operating in extreme environments.