Oxide phosphor and dielectric thin films for electroluminescent devices

Since its inception in the early 1990s, the field of high brightness, high stability oxide phosphor electroluminescence (OPEL) continues to develop with efforts now underway in several groups around the world. Green-emitting phosphors Zn2SiO4:Mn, Zn2Si1−xGexO4:Mn and ZnGa2O4:Mn achieve efficiencies of 0.5–2.5 lm/W. Lifetimes of 50000 h in air are achieved, even without control of humidity. Red emission from Ga2O3:Eu and MgGa2O4:Eu has also been realized, with efficiencies approaching 1 lm/W. Recently, higher efficiencies of up to 10 lm/W have been reported in yellow emitting Y2O3:Mn and (Y2O3)x(GeO2)y:Mn. Commercialization of OPEL for flat panel displays and lamps on glass substrates requires the development of phosphors that may be deposited at temperatures of 700 °C or lower. Several approaches are described here. A vacancy mechanism is shown to control the enhancement of crystallization in ZnGa2O4 by the introduction of CdO to form compounds of Zn1−xCdxGa2O4:Mn. Here, sputtering targets and as-deposited thin films contain Cd atoms, but the annealed thin films undergo complete loss of Cd as they crystallize. It is also shown that a flux such as LiF maybe added to Zn2SiO4:Mn to lower crystallization temperature. Here it is likely that point defects LiZn and Fo permit charge compensation while lowering the crystallization temperature. High breakdown strength, high dielectric constant and transparent dielectric layers have been developed to permit long life OPEL on Corning 1737 glass substrates. Whereas it is commonly reported that SrTiO3 and other perovskite titanate and zirconate dielectrics are not self-healing, it has been shown that a SrTiO3 dielectric of thickness 2 μm prepared using three sub-layers does achieve self-healing. In conjunction with a Zn2Si0.5Ge0.5O4:Mn phosphor, excellent steep-threshold electroluminescence (EL) behaviour is observed with L50, the brightness 50 V over threshold at 60 Hz of over 300 cd/m2. Finally, results for a new phosphor system Ga2−xInxO3:Eu on glass substrates are reported, in which the electric field for EL is reduced.