Spectroscopic analysis of Eu³⁺ in single-crystal hexagonal phase AlN

A detailed spectroscopic analysis of the crystal-field splitting of the energy levels of Eu³⁺(4f⁶) in single crystals of hexagonal phase aluminum nitride is reported based on assignments made to the high-resolution cathodoluminescence spectra observed between 500 nm and 750 nm obtained at 11 K and room temperature. Single crystals doped with trivalent europium were grown by high pressure, high temperature technology, and the crystal structure was confirmed by x ray diffraction methods to be the hexagonal phase. The Eu³⁺ ions substitute for Al³⁺ ions in sites of C_3v symmetry during crystal growth. More than 97% of the observed spectra are attributed to Eu³⁺ in the majority site. The spectra are identified as transitions from the excited ⁵D₀ and ⁵D₁multiplets of Eu³⁺ to the ground-state multiplets ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃,⁷F₄, ⁷F₅, and ⁷F₆ split by the crystal field into energy (Stark) levels. A parameterized Hamiltonian defined to operate within the 4f⁶ electronic configuration of Eu³⁺ was used to model the experimental Stark levels and their symmetry assignments or irreducible representations (irreps). The crystal-field parameters were determined through use of a Monte Carlo method in which the six $Bqk$ were given random starting values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting, which involved 20 Stark levels and their irreps from ⁵D₁, ⁵D₀, and ⁷F_0–4, resulted in a rms deviation of 6.7 cm⁻¹. The predicted splitting of the ⁷F₅ and ⁷F₆ multiplets was used to assign the experimental splitting for these manifolds since the spectra involved are weak and broad, precluding detailed Stark-level assignments.