Green luminescence from Cu-diffused LiGaO₂ crystals

An intense green luminescence is observed from single crystals of LiGaO₂ doped with copper. Czochralski-grown undoped crystals are wrapped in thin copper foil and then held at 900 °C for 1 h in a flowing nitrogen atmosphere. Large concentrations of Cu⁺ ions enter the crystals during this process and occupy Li⁺ sites. These copper-diffused crystals are characterized with optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), thermoluminescence (TL), and electron paramagnetic resonance (EPR). An optical absorption band peaking near 350 nm is assigned to the Cu⁺ ions at Li⁺ sites and represents an excitation from a 3d¹⁰ ground state to a 3d⁹4s¹ excited state. A broad PL emission from these excited Cu⁺ ions has a peak near 523 nm and the related PLE band has a peak near 356 nm (this PLE band links the emission to the optical absorption band). Illuminating a Cu-diffused crystal at room temperature with 325 nm laser light converts a portion of the Cu⁺ ions to Cu²⁺ ions. EPR spectra from these 3d⁹ ions are easily seen at low temperatures and their angular dependence is used to determine the g matrix and the ⁶³Cu hyperfine matrix. Subsequent heating produces a TL peak near 122 °C with a maximum in its spectral dependence near 535 nm. Correlated EPR measurements show that this TL peak occurs when trapped electrons are thermally released from unintentionally present transition-metal ions (most likely Fe) and recombine with holes at the Cu²⁺ ions.