Built-In Electric Field for Efficient Charge Separation and Prolonged Carrier Lifetime at the Doped GaN Surface

Abstract

GaN-based materials have recently attracted continuous interest in optoelectronic and photocatalytic applications; however, the role of surface built-in electric fields, a critical determinant of carrier dynamics and device performance, remains poorly understood. Herein, we take GaN films as an example to explore the built-in field and charge separation dynamics at various GaN surfaces using transient reflection (TR) spectroscopy. Unlike undoped GaN films without a built-in field, both the TR spectra of n-type and p-type doped GaN films exhibit remarkable Franz–Keldysh oscillation above the bandgap, confirming the presence of an intrinsic built-in field. Driven by the built-in field, photogenerated carriers at the doped GaN surfaces undergo ultrafast charge separation within ∼4.0 ps, achieving a remarkably prolonged carrier lifetime of up to 13.9 μs, which is 4 orders of magnitude longer than that in undoped GaN. Furthermore, the charge separation dynamics in the n-GaN film is quantitatively analyzed using a one-dimensional drift-diffusion model, yielding an intrinsic built-in field of ∼37 kV/cm. Our findings might offer new insights into the rational design of efficient GaN-based photocatalytic systems and optoelectronic devices.