Spectra of optical absorption and energy levels diagram of Er$^{3+}$ ions in bulk crystals of aluminum nitride

The absorption spectra of the Er$^{3+}$ ions embedded in the AlN matrix have been investigated. The admixture of erbium was introduced in bulk AlN crystals by diffusion. The absorption lines, which are associated with the intraconfigurational electronic $f$–$f$-transitions from the ground $^{4}I_{15/2}$-state to the levels of ion Er$^{3+}$ excited states have been observed in the spectral range of 370–700 nm. The transitions to the state levels $^{4}F_{9/2}$, $^{2}H_{11/2}$, $^{4}F_{7/2}$, $^{4}F_{5/2}$, $^{2}H_{9/2}$, and $^{4}G_{11/2}$ have been investigated in detail at the temperature $T$ = 2 K. The number of the observed lines for these transitions coincides with the theoretically possible one for the electronic $f$–$f$-transitions in the ions Er$^{3+}$, which are in the crystal field with the symmetry below cubic. The narrowness of the observed lines and their number convincingly testify the replacement of preferably one regular crystalline position by erbium ions. The implementation of Er$^{3+}$ in the Al$^{3+}$ position with the local symmetry $C_{3v}$ appears the most probable. The energy positions of the levels of excited states for the investigated transitions have been determined. The diagram of the Er$^{3+}$ ion energy levels in the AlN crystals has been built.