Improved Cathodoluminescence Output Coupling of ZnS:Tb Thin-Film Phosphors Deposited on 2D SiO[sub 2] Corrugated Glass Substrate

Abstract

A simple fabrication method of 2D SiO 2 corrugated layers and their suitability to enhance output coupling efficiency of a ZnS:Tb thin-film phosphor layer in an anode part of field emission display ͑FED͒ devices is discussed. For patterning of 2D dotted arrays of SiO 2 films, we used two-step irradiated hologram lithography and reactive ion etching. The formation of a wavelength-scale uniform 2D SiO 2 dotted array was confirmed by morphological analysis. We presented the finite-difference time-domain analysis for calculating the output light resulting from the insertion of a 2D SiO 2 corrugated layer into the interface between the glass substrate and the ZnS:Tb thin-film phosphor. A significant enhancement ͑a factor of 4.0-4.7͒ of the output coupling cathodoluminescence ͑CL͒ efficiency is reported in the low-voltage ͑0.5-4.5 kV͒ characteristics under electron-beam excitation for the ZnS:Tb thin-film phosphor deposited on a 2D SiO 2 corrugated substrate. The computational analysis, CL characterizations, and far-field measurement demonstrated that the 2D SiO 2 corrugated substrate served as diffraction scattering layers, increasing the output coupling efficiency, which is critically important for development of commercial FEDs employing thin-film phosphors.Recent interest in a phosphor for field-emission-displays ͑FEDs͒ has triggered the search for thin-film phosphors. Thin-film phosphors have several advantages and disadvantages over powder phosphors. 1 The merits of thin-film phosphors are better thermal stability, reduced outgassing, improved uniformity, smaller grains, and better adhesion to substrates. Due to these properties, thin-film phosphors are replacing candidates for powders to improve the lifetime and resolution of FED devices. However, the low brightness of thin-film phosphors is the biggest drawback in application to FEDs. The low level of brightness of thin-film phosphors is associated with an internal reflection effect and a smaller interaction volume between incident beam and film due to the reduced scattering of the excitation beam. To realize an FED device employing thin-film phosphors on the market, the critical problem of low efficiency must be overcome.A typical low-voltage FED ͑LVFED͒ device employing thin-film phosphor consists of two major components, the cathode and anode layer. An anode layer consists of a thin-film phosphor layer, a transparent indium tin oxide ͑ITO͒ layer, and the glass substrate; they generate the light. 2 In general, the refractive index of the ZnS:Tb phosphor layer (n ϭ ϳ2.5) and the ITO (n ϭ ϳ1.9) is larger than that of the glass (n ϭ ϳ1.5), which is in turn significantly larger than that of the air (n ϭ ϳ1.0). The emitting light from the anode layer is divided into five components: two extraction modes, the substrate waveguide modes, the ITO modes, and the phosphor modes. Figure 1 shows the light paths corresponding to the five modes. For LVFED devices employing thin-film phosphor, the three confined modes and one extraction mode toward the anode do not contribut...

Enhanced Light Extraction from SrGa2S4:Eu2+ Film Phosphors Coated with Various Sizes of Polystyrene Nanosphere Monolayers

“…The waveguide modes of light emitted from the TFPs arise from the total internal reflection (TIR) at their interfaces, at which there are significant differences in refractive index (n). 8 In the case of SrGa 2 S 4 :Eu 2+ films (n ) 2.3), 9 the fraction of light externally propagating from the phosphor side is approximately 4.7% (the fraction of escaping light is approximately 1/4n 2 ). 5 The remaining light generated inside the TFP is internally reflected at the interface and is eventually absorbed or emitted from the edge of the SrGa 2 S 4 : Eu 2+ film-coated substrate.…”

Structural effect of a two-dimensional SiO2 photonic crystal layer on extraction efficiency in sputter-deposited Y2O3:Eu3+ thin-film phosphors

“…The percentage of photons trapped a͒ Author to whom correspondence should be addressed; electronic mail: yrdo@kookmin.ac.kr inside the Y 2 O 3 :Eu 3+ thin film phosphor is about ϳ87% of the total emitted light, 20 so the experimentally obtained enhancement of the extraction efficiency is lower than expected. We suggest several reasons for this mismatch between the expected and obtained extraction efficiencies, based on the results of our recent reports 18,21,26 and this study. These reasons fall into four main categories: ͑1͒ difficulties in controlling the structural parameters of 2D photonic crystals ͑lattice constant ⌳, nanorod height h, and filling ratio f͒, 18 ͑2͒ the intrinsic optical parameters of thin-film phosphors ͑refractive index n and extinction coefficient k͒, 26 ͑3͒ the difference between the crystallinities of thin-film phosphors on flat substrates and of those on 2D PC assisted substrates, 21 and ͑4͒ the difference between the thermal expansion coefficients of thin film phosphors and substrates after the annealing process.…”

“…We suggest several reasons for this mismatch between the expected and obtained extraction efficiencies, based on the results of our recent reports 18,21,26 and this study. These reasons fall into four main categories: ͑1͒ difficulties in controlling the structural parameters of 2D photonic crystals ͑lattice constant ⌳, nanorod height h, and filling ratio f͒, 18 ͑2͒ the intrinsic optical parameters of thin-film phosphors ͑refractive index n and extinction coefficient k͒, 26 ͑3͒ the difference between the crystallinities of thin-film phosphors on flat substrates and of those on 2D PC assisted substrates, 21 and ͑4͒ the difference between the thermal expansion coefficients of thin film phosphors and substrates after the annealing process. In the present study, we examined in detail the structural effects of 2D SiO 2 PCLs on the photoluminescence properties of as-grown and post-annealed Y 2 O 3 :Eu 3+ thin film phosphors fabricated by sputtering deposition, after trying to minimize the structural and optical problems in ͑1͒ and ͑2͒.…”

“…18 Briefly, a 400-nm-thick SiO 2 film is coated onto an amorphous quartz glass substrate ͑JMC GLASS͒ by using plasma enhanced chemical vapor deposition ͑PECVD͒, and then a 40-nm-thick Cr film is deposited as a hard mask using thermal evaporation. A photoresist ͑PR͒ film is spin coated on top of the Cr-coated SiO 2 layer.…”

“…[18][19][20][21] To reduce the waveguide effect and, in turn, improve the efficiency of light extraction from the glass side of the thin-film phosphor layer, we have previously fabricated ZnS: Tb 3+ , 18 ZnS: Mn 2+ , 19 20 and Gd 2 O 3 :Eu 3+ thin-film phosphors 21 on 2D SiO 2 corrugated PCLs in an attempt to enhance the roughness of the interface between the quartz ͑or glass͒ substrate and the phosphor, and succeeded in improving the extraction efficiencies ͑out-coupling efficiencies͒ by factors in the range 4.2− 9.3. In agreement with the results for PC light emitting diodes, 22,23 and PC organic light emitting diodes, 24,25 these improvements in the extraction efficiency are due to the diffraction scattering produced by the 2D periodic array.…”