Exciton spectra of AlN epitaxial films

Excitonic resonance energies in a C-plane AlN epilayer on the (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence (CL) measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral lineshape was fitted considering A (Γ_7v^u → Γ_7c) and BC (Γ_9v, Γ_7v^l → Γ_7c) exciton transitions. The energies them at 0 K were obtained to be 6.211 and 6.266 eV, giving the crystal field splitting (Δ_cr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature Θ_E was estimated to be 580 K.