Temperature-dependent hysteresis of the emission spectrum of Eu-implanted, Mg-doped HVPE GaN

A red emission site (hereafter, Eu0), with its main ⁵D₀ to ⁷F₂ peak at 619 nm, is observed by photoluminescence (PL) spectroscopy of Eu-implanted, Mg-doped GaN, in samples annealed at high temperature and pressure (up to 1400 °C, 1 GPa) in order to remove lattice damage. The PL spectrum is strongly temperature-hysteretic between room temperature and ∼20 K: below 30 K, photochromic switching occurs between Eu0 and the usually dominant Eu1 center; upon warming the sample, the Eu0 signal does not recover until the temperature reaches ∼150 K. Photobleaching of Eu1 takes place at low temperatures after cooling, while photo-enhancement of Eu0 takes place at high temperatures after re-warming. These observations suggest a microscopic model of charge-driven defect interconversion in p-type GaN:Eu, Mg.