Recently developing new types of display has been needed. One of those new types of display is transparent display. Transparent displays have attracted great interest because of their great need in car navigation systems and decoration lighting devices. Nontransparent displays hinder one from seeing some objectives through them, and reduce sightlines. On the other hand, transparent displays can overcome these problems. In order to develop transparent displays, it is necessary to obtain transparent thin film phosphors. However, thin film phosphors have a critical problem, low brightness, which is originated from low light extraction efficiency due to total internal reflection (TIR) and wave guide effects. Because of the TIR and wave guide effects, only a small fraction of total number of photons can be extracted from thin film phosphors.
In recent years, many research groups have tried to solve those problems by introduction of patterned micron-lenses, surface roughening, and photonic crystal. Of these schemes, we focused on the introduction of 2-dimensional photonic crystal layers.
Red emitting $YVO_4:Eu^{3+}$, green emitting $Zn_2SiO_4:Mn^{2+}$, and blue emitting $BaMgAl_{10}O_{17}:Eu^{2+}$ thin film phosphors were deposited by RF magnetron sputtering system. We optimized the deposition condition of those thin film phosphors by changing some parameters and found the optimized condition. And then 2-dimensional hexagonally close packed polystyrene monolayer was coated on the thin film phosphors as a 2-dimensional photonic crystal layers to find the appropriate lattice constant. Polystyrene monolayer was transferred on the thin film phosphors by lift-off method. Well ordered polystyrene monolayer was transferred on the thin film phosphors by this method and served as photonic crystal layers. Polystyrene nanospheres having diameter of 330nm, 400nm, 460nm, 600nm, and 800nm were used and light extraction efficiency was increased by introduction of 2-dimensional polystyrene photonic crystal layers. Highest enhancement light extraction ratio was observed at 460nm, 460nm, and 600nm for the $BaMgAl_{10}O_{17}:Eu^{2+}$, $Zn_2SiO_4:Mn^{2+}$ thin film phosphors, and $YVO_4:Eu^{3+}$ thin film phosphors
Light extraction efficiency of thin film phosphors coated with 2-dimensional polystyrene photonic crystal layers was increased, but adhesion between polystyrene monolayer and thin film phosphor was not good. So we changed polystyrene photonic crystal layers to $SiO_2$ photonic crystal layers. And height of photonic crystal layers could not be changed when polystyrene monolayer served as photonic crystal layers. To investigate the effect of height of 2-dimensional photonic crystal layers deposited $SiO_2$ layers were used and height of photonic crystal layers was changed by deposition of $SiO_2$ having various height. To fabricate $SiO_2$ photonic crystal layers $SiO_2$ thin films were deposited on fabricated thin film phosphors by magnetron sputtering system. And then NSL (Nanosphere Lithography) and ICP - RIE (Induced Coupled Plasma - Reactive Ion Etching) process were used to pattern $SiO_2$ photonic crystal layers. $SiO_2$ thin films deposited on $BaMgAl_{10}O_{17}:Eu^{2+}$ thin film phosphors were etched by ICP-RIE process using $CF_4$ gas and polystyrene monolayer was served as a etch mask. Metal thin film layers serve as a etch mask in the usual NSL process and polystyrene monolayer is used as a mask to pattern metal thin film layers. In this study, however, $SiO_2$ thin film layers were etched by using polystyrene monolayer as a etch mask. By which process, highly ordered polystyrene monolayer was well transferred to the $SiO_2$ thin film layers and we could reduce the step of process.
By introduction of $SiO_2$ photonic crystal layers, we also could observe the enhancement of light extraction efficiency of $BaMgAl_{10}O_{17}:Eu^{2+}$ thin film phosphors. It can be ascribe to the Bragg diffraction and leaky mode of light generated in the thin film phosphors. Light emitted at an angle more than critical angle is waveguided, and is eventually either absorbed or emitted from the edge of the substrate. Photonic crystal layers act as a phase grating and leaky mode is induced by changing phase of wave. In the case of blue emitting $BaMgAl_{10}O_{17}:Eu^{2+}$ thin film phosphors, highest enhancement light extraction ratio was observed when $SiO_2$ photonic crystal layers, lattice constant at 460nm, was introduced. This result was well matched with that of polystyrene photonic crystal layers. So we fixed lattice constant of $SiO_2$ photonic crystal layers as 460nm and changed the height of photonic crystal layers to investigate the effect of height of photonic crystal layers. Enhancement of light extraction efficiency of $BaMgAl_{10}O_{17}:Eu^{2+}$ thin film phosphors coated with $SiO_2$ photonic crystal layers was increased with increasing height of photonic crystal layers. To confirm possibility to apply thin film phosphors to transparent display devices transmittance of thin film phosphors coated with $SiO_2$ photonic crystal layers was recorded. Transmittance of thin film phosphors was decreased with increasing height of photonic crystal layers and transmittance was above 70% in the range of visible light.
In conclusion, light extraction efficiency of transparent $BaMgAl_{10}O_{17}:Eu^{2+}$ thin film phosphors was enhanced about 3time by introduction of 2-dimensional $SiO_2$ photonic crystal layers.
최근 들어 새로운 형태의 디스플레이 개발에 대한 요구가 증가하고 있다. 이러한 새로운 형태의 디스플레이 중 투명디스플레이는 자동차의 네비게이션 시스템이나 쇼윈도의 장식용등으로 활용하고자 그 관심이 증가하고 있다. 불투명한 디스플레이의 경우 관찰자의 시야를 가리지만 투명디스플레이의 경우 이러한 문제를 극복하여 관찰자로 하여금 디스플레이 너머의 관찰을 가능하게 한다. 투명디스플레이를 개발하기 위해서는 투명한 박막형광체를 합성하는 것이 필요하다. 그러나 박막형광체의 경우 내부전반사와 waveguide효과로 인해 광추출 효율이 낮으며, 이로 인하여 휘도가 낮다는 문제점을 갖고 있다. 따라서 본 연구에서는 스퍼터링을 이용하여 높은 투과도를 갖는 박막형광체를 합성하였으며, 동시에 광결정층을 도입하여 박막형광체가 갖는 낮은 휘도 문제를 해결하고자 하였다. 적색 발광 박막형광체로는 $YVO_4:Eu^{3+}$녹색 발광 박막형광체로는 $Zn_2SiO_4:Mn^{2+}$, 청색 발광 박막형광체로는 $BaMgAl_{10}O_{17}:Eu^{2+}$ 형광체를 RF magnetron sputtering을 이용하여 증착 하였으며, 휘도와 투과도를 고려하여 증착 조건을 최적화 하였다. 또한 박막형광체의 낮은 휘도 문제를 해결하고자 앞서 증착한 박막형광체 위에 나노스피어 리소그래피 방법을 이용하여 $SiO_2$ 2차원 광결정층을 도입하였다.
