Electronic behavior of rare-earth dopants in AlN: A density-functional study

Local density functional calculations are carried out on Er, Eu, and Tm rare-earth (RE) dopants in hexagonal AlN. We find that the isolated impurities prefer to substitute for Al and, in contrast with isolated RE dopants in GaAs and GaN, REAl defects are electrically active and introduce deep donor levels around E-v+0.5 eV. RE complexes with oxygen and vacancies are discussed; some of these have deep levels in the upper third of the gap and could account for a threshold excitation energy around 4 eV observed for intra-f transitions at 465 and 478 nm in AlN:Tm.