平面電子發射光源之特性與發光機制探討

Abstract

本論文將介紹一種無汞及均勻發光的平面發光光源，此發光方式稱為平面電子發射光源(flat electron emission lamps, FEELs)。文中詳述元件的製程及製作方式；接著，藉由電壓及電流密度曲線與崩潰電壓特性分析氣體放電特性，了解FEEL操作於元件電場與氣體比值在4.3 kV/Torr-cm和35.7 kV/Torr-cm之間，此特性說明FEEL的兩電極間可能是操作於抑制型氣體放電區域。深入分析FEEL的放射光譜特性，確定FEEL藉由電子撞擊氣體激發氣體輝光的特性，在定電流密度下受氣體壓力影響，間接指出元件的發光特性強烈受到電子能量影響。因此，可推測電子數目與電子能量應是控制FEEL發光特性的兩大主因。 首先，以下說明FEEL的元件結構與製程方式部分。元件基本結構由5 cm x 5 cm面積玻璃電極的元件陰極及陽極電極與玻璃間隔三部分結合。陰陽極玻璃電極上皆鍍有一層氟錫氧化物(Fluorine-doppen Tin oxide, FTO)的透明導電薄膜。厚度10 mm的玻璃間隔距離為10 mm。元件製作過程需經玻璃表面清洗、陰極電漿鍍膜、陽極螢光粉沉積與燒結製程，最後再經過玻璃膠高溫爐膠合製程，將陰陽極、玻璃隔層與玻璃管膠合為元件內部放電面積3 cm x 3 cm的平板發光元件。 接著，為了瞭解元件氣體放電特性，我們將純度99.99%的氮氣或惰性氣體的工作氣體穩定通入於元件間兩平板內，氮氣或惰性氣體壓力皆為真空幫浦抽氣狀態下的動態壓力值。結果發現經有效控制的氮氣氣體操作氣壓及電壓為PN2=0.30~0.10 Torr與V≦5 kV，且不同氣體的發光元件需操作於不同操作壓力範圍。在放電特性部分，未崩潰前的元件具有暗區放電(dark discharge)特性；當操作電壓高於崩潰電壓發生氣體崩潰效應後，具有類似正常氣體放電(normal glow discharge)及異常氣體放電(abnormal glow discharge)特性，元件可能於輝光放電區(glow discharge)發光。FEEL崩潰特性與湯森表示式(Townsend expression) 畫出的帕邢曲線比較，元件崩潰電壓範圍操作在帕邢曲線左側區域，才能使FEEL內的螢光粉激發發光。藉由元件放電特性的了解，得知FEEL操作於帕邢曲線左半側，可獲得帶高動能電子；然而，隨著操作氣壓的降低，使電子自由路徑變長，接著因低氣壓元件需提高操作電壓，造成元件內電場值變大，結果元件內電場(electric field, E)對氣壓(pressure, p)的比值(E/p)由小變大，元件內電子帶有的電子動能平均值也隨操作氣壓下降而變大。 為了了解氮氣充填入元件的放電參數與元件發光特性的關係，探討可能的FEEL發光機制。我們藉著氮氣放射光譜儀、輝度計及積分球的量測與分析，發現FEEL發光特性，可以壓力範圍區分為PN2> 0.20 Torr、0.20 Torr >PN2> 0.11 Torr及PN2≦0.11 Torr三個範圍。 1. PN2> 0.20 Torr(以PN2 = 0.21 Torr為例): 元件內E/p < 10 kV/Torr-cm，操作電壓產生電流密度(current density, J)提高，元件螢光粉的發光放射光譜強度與氣體輝光光譜強度隨J近乎線性提升。表現出低E/p值下(近乎相同的元件電場)，氣壓高電子自由路徑短，隨著元件內電流密度值變大，電子數目增多，在電子由陰極飛行至陽極過程，電子與氣體的碰撞次數也隨J變大而變多，發光機制強烈受到受電子與氣體碰撞的影響，因此量測到的元件輝度值(luminance, L)，隨電流密度提升而微幅增加。 2. 0.20 Torr >PN2> 0.11 Torr: 隨操作壓力下降，元件內元件內E/p < 10 kV/Torr-cm提升至E/p > 30 kV/Torr-cm，氣體輝光強度隨降低而螢光粉強度提高，表示低氣壓下存在電子能量高於氣體輝光所需激發的能量，且足以激發螢光粉發光。 3. PN2≦0.11 Torr(以PN2 = 0.11 Torr為例): 元件內E/p < 10 kV/Torr-cm提升至E/p > 30 kV/Torr-cm，氮氣放射光譜強度下降至消失，螢光粉發光強度大幅提升，可能是隨著操作氣壓的下降電子自由路徑變大，E/p值提升電子動能提高，因此陰極飛行至陽極的高能量電子可能少數碰撞或不碰撞氣體，直接將電子能量轉移給螢光粉發光，因此元件量測得到高輝度值，元件發光效率於此範圍最高。 然而，元件操作於帕邢曲線左側會隨著操作氣壓的下降電子能量的提升，產生崩潰電壓值過大的缺點(如PN2= 0.11 Torr下，操作氣壓約為4 kV)，此缺點藉由高二次電子係數的陰極材料如氧化鋁與氧化鎂，可大幅降低崩潰電壓而獲得解決，(如氧化鎂的崩潰電壓可降至Vb < 1 kV)。推測MgO陰極材料的二次電子係數大於0.1，才可降低崩潰電壓且維持氣體放電，由於撞擊螢光粉的高能量分佈的電子數目變多，因此元件輝度值及元件效率值，也隨材料二次電子係數的提高而變大，目前最佳的發光效率為氧化鎂陰極材料，元件發光效率約為26.3 Lum/Watt。 最後，本論文介紹的平面電子發射光源(FEEL)，於帕邢曲線左側藉由氣體放電方式游離氣體及激發陰極獲得電子，FEEL的優點為可藉由降低元件操作氣壓的方式，產生高電子動能分佈的電子數目多，提高元件內的高電子動能數目撞擊陽極螢光粉層發光，此概念不同於螢光燈(fluorescent lamps, FLs)藉由紫外光激發螢光粉的發光方式，及電子束照射燈(cathodoluminescent lamps, CLs)藉由陰極產生電子撞擊螢光粉的發光方式，實驗結果發現FEEL的放電方式及發光特性屬於非線性行為，發光特性受電子數目和電子能量的強烈影響，有別於現今發光光源的特性與發光機制，由於結構簡單且兼具環保及節能特性，具有成為下一個綠色光源的潛力。最後，希望藉由本論文的基礎研究也提供了另一種發光方法的思維模式。

A planar lighting source featuring highly uniform light emission and mercury-free design, called flat electron emission lamps(FEELs), was studied. In this dissertation, the preparation and fabrication process of device are described. Furthermore, the gas discharge properties are analyzed by the voltage-current density curves and breakdown characteristics which the associated gas discharge of devices operating with the values of the ratio of electric field to gas pressure (E/p) between 4.3 kV/Torr-cm and 35.7 kV/Torr-cm indicated the width of cathode fall extends over the entire gap between the two electrodes and device is mostly in the obstructed discharge regime. The analysis of optical emission spectrums(OES) confirmed the electron collision-induced gas emission and strong effect of gas pressure on the phosphor emission when operated at constant current density, both are indicative of the primary roles played by the electron energy. Thus, the result of OES data shows that the two parameters of electron numbers and electron energy may play the key roles for the lighting properties of FEELs. First, we describe the fabrication and structure of FEEL device. The basic structure of device consists of a cathode glass plate, a glass spacer, and an anode glass plate. Both electrodes are coated with a transparent and conductive films of fluorine-doped tin-oxide (FTO) on each inner surfaces. The inner surface area of FTO-electrode is 5 cm x 5 cm, and the thickness of the glass spacer is 10 mm which the gap between the two conductive electrodes is d = 10 cm. After the surface cleaning process, plasma coating on cathode electrode surface, phosphor depositing on anode electrode surface following the furnace process, and the glass glue gluing process, the FEEL is integrated to be a planar gas discharge area with 3 cm x 3cm. The following part is to understand the gas discharge characteristics of FEEL. We use the nitrogen gas and noble gas(Ne and Ar) with purity of 99.99% as the working gas filling inside the space between the two electrodes. The pressure of the nitrogen and noble gas were monitored by a vacuum gauge attached to pumping system and was kept at a certain constant value during the whole course of measurement. The results are found that the two parameters of the nitrogen pressure (PN2) and the discharge voltage (V) should be controlled at the PN2 = 0.30~0.10 Torr and operated at V≦ 5 kV. The operated gas pressure ranges are different for the different gases. The discharge characteristics of FEELs are divided from the breakdown voltage (Vb). Before V< Vb, it is found the dark discharge characteristics; and V≧Vb, the discharge characteristics show almost the same as the normal glow discharge and abnormal glow discharge. As the results, the lighting region of FEEL may be operated on the region of glow discharge. If the breakdown characteristics of FEEL are compared with Paschen curve shown as Vb-pd curve drawing from the Townsend expression . The Vb of FEEL is located on the left-hand side of minimum-Paschen curve and excited the electrons with high kinetic energy(Ee). However, the free path of electron (λe) increased as we decreased the maintaining pressure, and the following electric field(E) between the planar electrode increased as the operated voltage further increased. To summarized, the values of the ratio of electric field to gas pressure (E/p) are also increased. It means that the Ee further increased as the maintain pressures of FEEL decreased. Furthermore, we try to realize relationship between the nitrogen gas discharge parameters and the FEEL lighting properties; and then, the lighting mechanism of FEEL was investigated. Thus, the measurement the data by the optical emission spectroscopy, the luminance meter and the integrating sphere were analyzed. There are three lighting modes of FEEL which were PN2> 0.20 Torr, 0.20 Torr >PN2> 0.11 Torr, and PN2≦0.11 Torr. 1. PN2> 0.20 Torr (PN2= 0.21 Torr for an example): At E/p < 10 kV/Torr-cm, the phosphor emissions or photo-luminance as a function of current density (L-J) for the corresponding devices. In general, the photo-luminance was observed and measured only after the breakdown of gas. In this state, the OES data show the measured emission intensity as a function of current density of light emission at different current density. The linear dependence between the emission intensity and device current density for the gas peaks (391.5 and 427.5 nm) indicates that the predominant emission mechanism is indeed due to electron collisions with the gas environment. Thus, the lighting properties are strongly affected by the electron numbers and the electron collisions with the gas environment. 2. 0.20 Torr >PN2> 0.11 Torr: The photo-luminance appears to be enhanced at lower gas pressure, and the emission exhibits very different behaviors from PN2> 0.20 Torr. In this regime, the gas excitation glow and phosphor emission are coexistent, indicating that the predominant mechanisms operating in the system are strongly dependent on the gas pressure. The OES spectrum evidently shows the weakened peak intensity from the N2 excitations, with an apparent centering at the characteristic wavelength (530 nm) of the phosphor. We believe that in the lower pressure range, there exist some population of electrons with residual kinetic energy higher than the threshold energy for activating phosphor emission. 3. PN2≦0.11 Torr(PN2= 0.11 Torr for an example): The E/p values (lower gas pressures) become higher, and the emission exhibits very different behaviors from those described above. By assuming a uniform electric field E across the gap between the two electrodes, the diminishing of the emission intensity of the 391.5 nm line may merely reflect the electron energy has become too large to give optimum excitation coefficient for relevant energy levels. It may be the advantage for FEEL to directly transfer the electron energy from the cathode to the phosphor coated on the anode. Thus, the luminance value and the luminance efficiency of FEEL is the best in the regime. However, this kind of discharge devices, operated on the left hand-side of minimum Paschen curve, are disadvantage with the higher breakdown voltage following the reduced operated-pressure (for FTO, the Vb~4 kV at 0.11 Torr). The disadvantage is to solve by the exchanging with the cathode materials with high secondary electron emission coefficient(γ). In the results, we predict the γ of magnesium oxide(MgO) may be higher than 0.1 and the device voltage of 0.11 Torr pressure maintaining at about 1 kV. We also found that the MgO-cathode material can achieve the luminance efficiency achieve 26.3 Lum/Watt now. Finally, we develop a lighting device for generating uniform planar light. The device integrates electron beams induced by gas discharge with CL-phosphor on the anode. The advantage of FEEL can generate the electron beams with electron kinetic energy distribution at high value to excite the phosphor coating on the anode. Consequently, UV-light is not required and the usage of mercury can be avoided in FEEL. This design is different from the fluorescent lamp (FLs), the phosphors excited by the UV-light generating from the gas, and the cathodo-luminescent lamps (CLs), the phosphors excited by the electron generating from the cathode emitter. Despite many obstacles still needed to be overcome, the current FEEL devices have been demonstrated the highly friendly environment and application potential for the candidate of the next generation mercury free green lighting sources.