Cathodoluminescence Properties of In₂O₃-coated Y₂O₃:Eu3+

Abstract

Europium-doped Y2O3 phosphor (Y2O3:Eu 3+) is a red phosphor that has been used in low-pressure fluorescence lamp, cathode ray tube (CRT) displays, and plasma display panels (PDPs) because of its characteristic sharp and hypersensitive luminescence.1-3 Y 2 O 3 :Eu 3+ exhibits a charge transfer excitation peaking at 242 nm and produces a strong 610-nm emission from Eu 3+ with a narrow bandwidth attributed to the 5 D0 → 7 F2 transition. [4][5][6] The quantum yield of this emission varies from 60 to 74% depending on the porosity of the Y2O3:Eu 3+ particle. 7Recently, this phosphor has attracted a great deal of attention for use in field emission displays (FEDs), since it is presumed to be inherently stable under electron beam excitation. [8][9][10][11][12] Most studies on the cathodoluminescence (CL) properties of Y2O3: Eu 3+ were conducted at accelerating voltages lower than 10 kV, although carbon nanotubes (CNTs), which are currently being investigated as electron field emitters for FEDs, require an applied voltage greater than 10 kV for reliable luminance. 13,14 Ntwacaborwa and co-workers 15 found that the CL intensity of Y2O3:Eu 3+ was significantly reduced by exposure to an electron beam. In contrast to the sulfide phosphors, Y2O3:Eu 3+ is an insulator with a bandgap energy of 6.0 eV. Electron irradiation of insulating materials builds a surface charge and creates a nonluminescent "dead layer" at the surface. Yamamoto and coworkers 16 found that the current must be increased to regain the desired CL intensity and that the emission appeared primarily at edges or rims of grooves or holes made on the phosphor screen. The surface of Y2O3:Eu 3+ can be coated with metal oxides, such as SiO2, MgO and Al2O3 to alleviate this problem. 17,18 However, the CL data of Y 2 O 3 :Eu 3+ collected up to 4 kV showed that the resulting energy loss reduced the CL intensity. For FED applications, this problem can be solved by using a mixture of conductive materials such as In2O3, ZnO and SnO2 with an insulating phosphor. For low-voltage FEDs, In2O3 was found most effective in this regard. 19,20 In the current study, a Y2O3: Eu 3+ surface was coated with In 2 O 3 using two different deposition methods and the effects of the coating on the CL intensity was investigated as a function of accelerating voltages up to 17 kV. The findings revealed that the morphology of the In2O3 coating played a key role in determining the CL intensity. ExperimentalThe In2O3 coating was fabricated using a precursor solution composed of InCl3 in ethanol or water. The phosphor was added to 50 mL of ethanol or 10 mL of water in which various concentrations of InCl3 had been dissolved. After adjusting to pH 10 with a NH4OH solution, the solutions were stirred for 1 h. The aqueous mixtures were heated at 70 o C to ensure the precipitation of In(OH)3 onto the surface of the phosphor. The mixtures were then dried at 40 o C and heated in a box furnace at 400 o C for 3 h to convert indium hydroxide to indium oxide. Phase information and surface morphology of the co...