In-plane optical polarization and dynamic properties of the near-band-edge emission of an m -plane freestanding AlN substrate and a homoepitaxial film

For accelerating the development of deep-ultraviolet light-emitting diodes based on high AlN mole fraction (x) Al_xGa_1-xN for sterilization, disinfection, and skin therapy applications, in-plane optical polarization and dynamic properties of the near-band edge (NBE) cathodoluminescence (CL) peak of a low threading dislocation density (<10³ cm^-2) m-plane freestanding AlN substrate and a homoepitaxial film are assessed. Consistent with the polarization selection rules, the electric field (E) component of the NBE emission was essentially polarized parallel to the c-axis (E ∥ c). Low-temperature CL spectra of the homoepitaxial film exhibited exciton fine structures: CL peaks at 6.041₀ and 6.027₉ eV, which were polarized E ∥ c and E perpendicular to the c-axis (E ⊥ c), respectively, are assigned as being due to the recombination of free A-excitons of irreducible representations Γ 1 and Γ 5. The hydrogenic binding energy of the Γ₁ A-exciton being 51 meV is verified. Detectable CL peaks under E ∥ c polarization at 6.031₅ and 6.021₂ eV are tentatively assigned as Γ 1-mixed Γ 5-exciton-polaritons. The concentration of multiple vacancies consisting of an Al-vacancy (V_Al) and N-vacancies (V_Ns), namely, V Al V N 2 - 3, in the substrate was estimated by the positron annihilation measurement to be 2-3 × 10¹⁶ cm^-3, while that in the epilayer was lower than the detection limit (<10¹⁶ cm^-3). The NBE CL lifetime of 28 ps of the epilayer subsurface at 300 K is likely limited by the recombination at carbon deep-acceptors on nitrogen sites (3 × 10¹⁷ cm^-3) and/or V Al V N 2 - 3 Shockley-Read-Hall nonradiative recombination centers (∼1 × 10¹⁶ cm^-3) with hole capture coefficients of approximately 1 × 10 - 7 and 3 × 10 - 6 cm³s^-1, respectively.