A.c. powder electroluminescence devices using ZnGa2O4:Mn phosphor

Abstract

Zinc gallate, ZnGa2O4 was recently investigated for the application to vacuum uorescent displays (VFDs) utilizing low-voltage cathodoluminescence. The ZnGa2O4 host lattice without dopant shows blue emission [5, 6]. The phosphor shows green [1±3] and red [4] emmision when it is doped with Mn2‡ and Cr3‡ activators. A high luminous ef®ciency had been obtained when the phosphor was operated at low voltage (10±300 V). The results of low accelerating voltage and high luminous ef®ciency led us to study the electroluminescence (EL) characteristics of the ZnGa2O4:Mn phosphor. There are a few papers reporting the electroluminescence (EL) of the ZnGa2O4:Mn phosphor. Thin ®lm electroluminescence devices (TFEL) had been developed using ZnGa2O4:Mn phophor thin ®lms which were deposited by RF magnetron sputtering on polished BaTiO3 ceramic sheets [7]. In this paper, we report the fabrication of the a.c. powder electroluminescence (ACPEL) devices. The ZnGa2O4:Mn ACPEL device was developed by the printing technique. First, ZnO (99.999%), Ga2O3 (99.999%) and MnO (99.9%) powders were mixed in 1 : 1 :0.001 mole ratio and ball-milled with alcohol for 24 h. The mixed powders were dried at 100 8C and sintered at 1375 8C for 5 h in air to form the doped zinc gallate (ZnGa2O4:Mn) phosphor powder. The phase identi®cation of the phosphor powder was carried out on a Siemens D5000 X-ray diffraction (XRD) equipment with CuKa radiation. Fig. 1 shows X-ray diffraction patterns of ZnGa2O4:Mn and ZnGa2O4 powders. There are no ZnO-phase or Ga2O3-phase peaks discernible in any of the samples. The subsolidus equilibria study of the ZnO-Ga2O3 system showed no appreciable solid solubilities of ZnO or Ga2O3 in the ZnGa2O4 compound [1]. All the sintered powders exhibit the spinel structure of the ZnGa2O4 powder. The lattice constant of the ZnGa2O4:Mn powder was calculated to be 0.8365 nm. The result is consistent with the value of 0.837 nm [8]. It is shown that the concentration of Mn dopant is much less so that it cannot in uence the crystal structure of host lattice. The a.c. powder electroluminescence (ACPEL) devices were prepared by the printing method. The cross-sectional structure of the ZnGa2O4:Mn ACPEL device is shown in Fig. 2. The device consisted of the indium tin oxide (ITO) coated glass, ZnGa2O4:Mn emitting layer, BaTiO3 insulating layer, and Ag paste electrode. Initially, an ITO coated glass substrate was etched in the requisite manner. Secondly, the ZnGa2O4:Mn phosphor powder was mixed with the dielectric organic binder (cyanoethyl cellulate) and deposited on the ITO substrate by printing. The high dielectric material (BaTiO3) was printed upon the phosphor ®lm to protect against breakdown. Finally, the Ag paste was printed as an electrode ®lm. Fig. 3 shows the scanning electron microscope (SEM) image and the corresponding cathodulminescence (CL) image of the ACPEL device in its cross-sectional view. It is interesting to note that the interface between the ZnGa2O4:Mn phosphor layer and the BaTiO3 insulator layer cannot be apparently distinguished in the SEM image (Fig. 3a). In contrast, we can easily characterize the thicknesses of each of the layers of the ACPEL device with the CL image (Fig. 3b). The thickness of the phosphor, the BaTiO3 and the Ag are evaluated to be 10, 13 and 6 im respectively. We can calculate the electric ®eld in the printed phosphor layer by evaluating the thickness of the phosphor layer. Among transition-metal ions, Mn2‡ ions are usually used as luminescent centres in the phosphor powder. Mn2‡ centres in the ZnGa2O4 host lattices show green luminescence with a peak at 508 m, which is due to 3d intra-shell transitions. The wave-