Absorption spectra of bulk aluminum nitride crystals doped with Er$^{3+}$ ions

This Letter presents results of analysis of the absorption spectra of AlN : Er$^{3+}$ bulk crystals. In the spectral of 370–700 nm, absorption lines responsible for intraconfiguration electron transition from the ground state $^{4}$I$_{15/2}$ to the excited states of Er$^{3+}$ ions are found. Transitions to the levels of the $^{4}$F$_{9/2}$, $^{2}$H$_{11/2}$, and $^{4}$G$_{11/2}$ states at 2 K are studied in detail. The number of observed lines for these transitions fully agrees with that theoretically possible for f–f electron transitions in Er$^{3+}$ ions found in a noncubic crystal field. The small width of the observed lines and their number indicate that erbium ions displace mostly one regular crystal position. Most probably, Er$^{3+}$ occupies the position of Al. Energy positions of excited states for the considered transitions are determined.