Visible and infrared photoluminescence of Er-doped AlN films: Excitation of Er ions via efficient nonradiative energy transfer from the host level

Aluminum nitride (AlN) is a multifunctional semiconductor. AlN doped with rare earth ions has broad application prospects in monochromatic lighting, color display, laser medium, medical treatment, etc. In this paper, an Er3+-doped AlN film has been prepared by radio frequency magnetron sputtering and characterized by surface morphology, chemical composition and photoluminescence (PL), focusing on the emission mechanism of Er3+ in the AlN host. The films show PL in a wide range of the visible (540, 560 and 668 nm) and near-infrared (816, 869, 985 and 1534 nm) ranges. It has been found a certain energy transfer bridge between Er3+ and AlN defect-related optical transitions. At 532 nm excitation, V-Al-O-N vacancy-oxygen complex in the AlN host efficiently absorbs photons, then, nonradiative energy transfer (NRET) occurs from the defect levels to the levels of Er3+ since they are in resonance. Since there is a resonance of a donor emission level and an acceptor absorption level, Forster resonance energy transfer (FRET) occurs with the transfer efficiency about 51%. This results in a very strong enhancement of excitation of the 4f related PL. On the contrary, optical excitation at other energies results in a low intensity of the Er3+ PL. As a matter of facts, a strong absorption of the AlN host and the FRET-enabling resonance of donor-acceptor levels allow for a great enhancement of the PL efficiency of Er3+ ions. The results show that the impurity defect state of AlN has an important influence on the Er3+ luminescence efficiency.