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

Abstract

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 C3v 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 7F0–4, 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.