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) AlxGa1-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 Γ1 and Γ5. 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 Γ1-mixed Γ5-exciton-polaritons. The concentration of multiple vacancies consisting of an Al-vacancy (VAl) and N-vacancies (VNs), namely, VAlVN2−3, 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 VAlVN2−3 Shockley-Read-Hall nonradiative recombination centers (∼1 × 1016 cm−3) with hole capture coefficients of approximately 1×10−7 and 3×10−6 cm3 s−1, respectively.