Rescently, there has been a renewed interest in the development of more efficient catholuminescent (CL) phosphors for low voltage display applications such as field emission displays (FED) or vacuum fluorescent displays (VFD). However, because of manufacturing and other practical constraints for low voltage display applications, the low voltage phosphors have serious concerns with respect to efficiency, spectral response time, long-term reliability and stability, screening and synthesis methods. Among the listed, the long-term reliability of CL efficiency of electron beam under the prolonged low voltage and high current density of electron beam is of great technological importance because it limits the lifetime of display devices. Especially, for low voltage display applications, the degradation behavior is expected to be a major selection criterion for a phosphor. A new red emitting low voltage phosphor , perovskite-structured $SrTiO_3$:Pr,Al,Ga has been investigated for its application to low voltage display devices because of high low voltage luminescent efficiency and excellent color purity that it offers. $SrTiO_3$:Pr,Al,Ga phosphor-based vacuum fluorescent displays (VFD), however, have exhibited short lifetimes. The characteristics of these drawbacks include rapid degradation of low voltage CL of $SrTiO_3$:Pr,Al,Ga phosphor, resulting in reduced luminescent efficiency. Furthermore, the reasons for the degradation are not clear.
So, in this study the degradation behavior during the electron irradiation was focused on, and also its CL measurements and various surface analysis et al. on the degraded samples by the electron irradiation were performed to study the mechanisms leading to the low voltage CL degradation of $SrTiO_3$:Pr,Al,Ga phosphors. In addition, several experiments were attempted to protect the degradation of $SrTiO_3$:Pr,Al,Ga phosphor. They were composed of (1) Calcinations under the reduced atmosphere, (2) $In_2O_3$-coating on the surface of phosphors, and (3) Injection of high-pressure hydrogen gas into the $SrTiO_3$:Pr,Al,Ga phosphor.
In the part to understand the degradation mechanism of $SrTiO_3$:Pr,Al,Ga phosphor it was found that both the formation of carbon overlayer and the surface damage on the phosphor surface impact CL output significantly and result in a serious degradation in the phosphor performance through the surface analysis such as AES and XPS combined with CL output measurements of $SrTiO_3$:Pr,Al,Ga phosphors.
Finally, it was found that in the case of injection of high-pressure hydrogen gas into the $SrTiO_3$:Pr,Al,Ga phosphor there was considerable effect to protect its degradation.
본 연구를 통하여 얻은 실험 결과를 정리해 보면,
1. $SrTiO_3$:Pr,Al,Ga 형광체는 다른 저전압용 형광체들에 비해 빠른 열화 특성을 보이며, 이 거동은 같은 power 기준에서 측정될 경우 저전압, 고전류밀도하에서 더 심하게 열화됨이 관측되었다.
2. 전자선이 조사되는 형광체 표면에는 browning 현상이 관찰되며, 이는 electron-stimulated에 의한 carbon의 흡착임이 확인되었다. 또한 이 carbon 층은 전자선 조사시간에 비례하는 경향을 보였으며, 저전압 열화 특성과 상관 관계가 있음을 알 수 있었다.
3. 전자선이 조사되는 동안 형광체 표면에서는 단순히 carbon 축적 현상외에 형광체 모체의 구성 성분인 oxygen의 desorption 현상이 일어남이 관찰되었다. 이는 $SrTiO_3$:Pr,Al,Ga 형광체의 표면에 결함 자리를 제공하여 생성되는 electron-hole pairs의 nonradiative surface recombination center로 작용하여 형광체의 표면 발광에 심각한 열화 특성을 보이게 하는 것으로 이해된다.
4. $SrTiO_3$:Pr,Al,Ga 형광체의 저전압 발광 열화 현상을 억제하기 위해 수소분위기에서의 소성, sol-gel법을 이용한 $In_2,O_3$ coating, 고압의 수소 주입실험이 행해졌다. 이 방법들 중 특히 고압의 수소 주입을 통해 $SrTiO_3$:Pr,Al,Ga 형광체의 열화 특성을 상당히 억제 시켰다.