平面電子發射光源之電光轉換機制與二次電子係數探討

Abstract

平面電子發射光源(Flat Electron Emission Lamp, FEEL)為一新創之平面光源，擁有雙面均勻發光、透明、與隔熱等特性。其工作原理為電子在外加電場作用下加速激發陽極螢光粉致使發光，而電子主要有兩種來源：一為氣體游離產生自由電子；二為離子化氣體原子撞擊陰極材料後產生二次電子。 平面電子發射光源為一短電極間距之發光元件，元件操作於pd值(氣體壓力與電極間距離的乘積)小的操作區間，此帕邢曲線左側之氣體放電特性常被忽略。本論文以傳統氣體放電管的放射光譜與發光特性為基礎，探討氮氣為操作氣體的平面電子發射光源元件放電與發光機制。實驗結果顯示平面電子發射光源元件在操作壓力介於0.4torr至1torr之間時，元件內部的能量分別提供給氮氣分子光譜的第二正帶能階(337 nm and 357.5 nm)與氮氣離子光譜的第一負帶能階(391.5 nm and 427.5 nm)。當元件操作壓力低至0.3torr時，氮氣離子光譜的第一負帶能階(391.5nm)為主導FEEL元件發光的激發源，其光譜強度隨著氣壓下降而減弱並消失。此氮氣離子光譜訊號強度值皆遠大於第二正帶能階的氮氣分子光訊號，因此，在此操作區域大部分輸入元件的能量提供給氮氣第一負帶能階的離子光譜。此外，在接近於FEEL元件的陰極電極1mm處，氮氣操作氣壓範圍為0.12 torr ~ 1orr，元件內部幾乎由氮氣離子光譜中的第一負能帶所主導。FEEL放射光譜特性顯示，元件由氮氣離子光譜中的第一負能帶所主導，此離子在外加電場作用下將撞擊陰極產生二次電子，提供激發陽極螢光粉發光的電子來源。因此，由元件發光特性受到氮氣離子光譜強度、陰極材料的二次電子係數、電子動能與螢光粉電致發光效率等影響，可以進一步建立元件電光轉換機制的模型，並與FEEL元件實際量測之發光輝度進行比對驗證。由能量轉換模型與輝度值量測結果發現，平面電子發射光源的發光輝度隨著元件內部的操作氣壓下降而提高，隨著操作氣壓的下降，電子的自由路徑變大，因此高能量電子與少數氣體產生碰撞游離，將大部分的電子能量轉移給螢光粉發光，因此可得到較高輝度值。 由於FEEL元件操作於帕邢曲線左半部，此區域的氣體分子少、元件的崩潰電壓迅速提升, FEEL具有以下的缺點: (一)較高的元件操作電壓、(二)元件發光效率不佳、(三)較短的使用壽命等缺點。因此，在陰極導電薄膜上製鍍保護層能增加元件之使用壽命及提升元件發光效率。本論文比較FTO(fluorine-doped tin oxide)、氧化鋁(Al2O3/FTO)及氧化鎂(MgO/FTO)薄膜應用於平面電子發射光源元件之陰極材料，探討FEEL元件於崩潰電壓的物理特性，並分析三種不同陰極電極材料的崩潰電壓與二次電子係數。實驗結果發現發光元件內E/p~10000 V/torr-cm，陰極材料的二次電子係數，FTO為0.193，Al2O3/FTO為0.263，MgO/FTO為0.396。陰極材料二次電子發射係數越高，有效提升FEEL元件的發光效能，且降低元件的崩潰電壓，使得元件之使用壽命增長約四倍。

Flat Electron Emission Lamp (FEEL) is a newly developed type of uniform planar light source, featuring unique advantages of double-side lighting, transparency, and heat insulation over the conventional lighting sources. The working principle of FEEL utilizes electrons accelerated by external electric field to excite the phosphor powders coated on the anode to obtain desired luminescence. The abovementioned electrons can be generated from two different sources: the free electrons originated from gas ionization and the secondary electrons generated from the bombardment of ionized gases to the cathode material. However, since FEELs are devices with very short electrode-distance and low working pressure, (i.e. low pd values), they are inevitably operating on the left-hand side of the Paschen curve, and the detailed physical mechanisms are relatively unexplored. In this thesis, we shall focus our discussions on the discharge and lighting characteristics exhibited in typical FEEL devices based on the knowledge obtained from the conventional long discharge tubes by using optical emission spectroscopy. It is evident that, similar to that conventional long discharge tube, emissions from both of the first negative system (391.5 nm and 427.5 nm) of N2+ and the second positive system (337 nm and 357.5 nm) of N2 are present for pressures above 0.4 torr. As the pressure is further reduced to below 0.3 torr, the 391.5 nm emission from the first negative system becomes the dominant excitation, which diminishes gradually and disappears completely at 0.14 torr. Moreover, the relative emission intensity obtained at the position of 1 mm from the cathode, the emissions from the first negative system of N2+ are, in fact, already dominant over the entire pressure range (0.14-1.0 torr). The FEEL devices exhibit essentially the same pressure-dependent emission features as seen in the conventional long glow discharge tubes. In particular, similar to the long glow discharge tubes without positive columns, the FEEL devices are essentially working on the left hand side of the Paschen curve, as well. Under these circumstances, in addition to the collisional ionization processes necessary for maintaining a steady-state discharge, the primary energy transferring mechanism is utilizing electrons accelerated by external electric field to excite the phosphor powders coated on the anode to obtain desired luminescence. The results indicated that the lighting properties were dominated by first negative band B2∑u+→X2∑g+ of nitrogen ion and the secondary electrons were generated primarily from the bombardment of ionized gases to the cathode material, which, in turn, were accelerated by the applied voltage to excite the phosphor coating on the anode. Based on the proposed electro-optical transfer model, nitrogen ions emission, secondary electron coefficient of material, electron energy, and phosphor lighting efficiency were identified as the four most prominent parameters in determining the lighting of FEEL. We successfully prove the viability of using the proposed model to describe the luminance of FEEL by linking the four parameters obtained from independent experiments. The highest efficiency of FEEL is achieved presumably due to the reducing glow excitation of the nitrogen molecules as well as collisions encountered by the energetic electrons along the path across the space between cathode and anode. As a result, higher electron energy is preserved before landing on the phosphor coated on the anode to result in higher lighting efficacy. Previously, fluorine-doped tin oxide (FTO) had been used as the cathode electrode of the FEEL devices due to its transparent and conductive characteristics. Nevertheless, the requirement of relatively higher discharge voltage, presumably resulting from lower secondary electron emission coefficient ion-bombardment damage, have hindered the realization of the FEEL devices for practical use. Hence, developing protective layers capable of lowering the required discharge voltage and providing more robust endurance to ion-bombardment is necessary. Three different cathode materials, namely fluorine-doped tin oxide (FTO), aluminum oxide coated FTO (Al2O3/FTO) and magnesium oxide coated FTO (MgO/FTO) were prepared to investigate how the variations of γ and working gases influence the performance of FEEL devices, especially in lowering the breakdown voltage and pressure of the working gases. Our results show that, under the typical operation conditions of FEEL devices, the γ values for FTO, Al2O3/FTO, and MgO/FTO are 0.193, 0.263, and 0.396, respectively. The larger γ value obtained for MgO consistently accounts for both the significant reduction in breakdown voltage and marked enhancement in lighting efficacy and device lifetime.