Investigating the dynamics and enhancement mechanism of broadband optical nonlinearity in GaN HEMT

Background Gallium nitride high electron mobility transistors (GaN HEMT) demonstrate significant potential in high-efficiency power electronics and high-speed optoelectronics. However, the underlying mechanisms by which heterostructures and defect states influence their ultrafast carrier dynamics remain incompletely understood.

Purpose This study systematically investigates how dislocation defects and heterojunction interfaces modulate the ultrafast nonlinear optical and carrier dynamic properties of GaN HEMT materials, providing a physical foundation for the design of high-performance optoelectronic devices.

Methods Femtosecond transient absorption spectroscopy (TAS) was employed to comparatively analyze three GaN samples: an AlGaN/GaN heterostructure, high-resistivity Fe-doped GaN (HR-GaN), and unintentionally doped GaN (UID-GaN). A quantitative interference oscillation model was applied to eliminate thin-film artifacts, enabling the precise extraction of pure nonlinear absorption and broadband transient refraction spectra.

Results The AlGaN/GaN heterostructure exhibited a remarkably enhanced nonlinear optical response. Under 343 nm excitation, its maximum nonlinear refractive index change (Δn) reached 42.10×10⁻³, nearly three times that of the HR-GaN sample (16.70×10⁻³). Additionally, the carrier recombination lifetime of the AlGaN/GaN sample was halved compared to the HR-GaN sample. Drude model fitting, based on a wavelength-squared dependence, confirmed that these nonlinear responses originate primarily from free-carrier dynamics.

Conclusions The AlGaN/GaN heterojunction enhances broadband optical nonlinearity by introducing a high-density two-dimensional electron gas (2DEG) that amplifies the Drude effect. Simultaneously, dislocation defects introduce deep-level states that act as efficient recombination centers to accelerate carrier relaxation. The synergy between heterojunctions and dislocation defects enables GaN HEMT to achieve both strong optical nonlinearity and ultrafast response times, demonstrating clear application potential for advanced photoconductive switches and optical modulators.