Abstract

A new type of the triode field-emission display constructed by the ceramic cold emitter was designed and performed. The display consisted of three electrodes, that is, ceramic cold emitter, extraction gate and accelerating electrode. The cold emitter used here possesses two major features: electrically conductive aluminum-doped zinc oxide (Al:ZnO) whisker with extremely sharp tip and amorphous carbon and related coating with relatively low-work function. The extraction metal grid applies the electric field to extract the electrons from the Al:ZnO ceramic whiskers. The electrons move toward the accelerating electrode and glow the phosphor film coated on the electrode. Intense and stable visible green and red emissions were obtained from the phosphors.
Keywords

Field Emission Display, Cold Emitter, Phosphor, Zinc Oxide, Carbon Nitride, Yttria, Whisker, CVD
Introduction

A new type of the flat displays, such as electroluminescence displays and field emission displays (FEDs), attracts many interests. Among them, FEDs have many similarities with conventional cathode ray tubes (CRTs). As for the FEDs and CRTs, electrons are accelerated in vacuum towards phosphors that then glow. The main difference is that the electrons are generated from the cold emitter rather than the thermal emitter so the device consumes much less power and can be turned on instantly. Instead of one single electron gun, each pixel comprises several thousands sub-micrometer tips from which electrons are emitted.

To achieve a low operating voltage of the cold emitter, the tip is manufactured into very sharp form so that the local field strengths become high enough for even a moderately low gate voltage. It is also made of a low-work function material such as molybdenum. To confirm the advantage of sharp tip working, Spindt et al. [1] measured that emission originated from the tip of molybdenum cones with an about 1.5 μm in tall and 50 nm in a tip radius was the maximum current of 50~150 μA per cone at the applied voltages in the range of 100~300 V. In addition, Saito et al. [2] investigated behavior of field emission from open multi wall carbon nanotubes (MWNTs). The current density in the rage of 10~100 μA/cm2 was obtained. It is well known that diamond and amorphous carbon and related films are low-work function materials. For example, hydrogenated diamond (111) surface has the negative electron affinity of –1.3 eV [3, 4]. Mueller et al. [5] prepared the diamond array, and evaluated behavior of field emission. The emission current of 3.5 mA was achieved at 0.8 V/μm in this configuration. Satyanarayana et al. [6] showed that the threshold fields of 3~5 V/μm was obtained at the current density of 1 μA/cm2 from the amorphous carbon (a-C) films. Furthermore, Huq et al. [7] prepared an array of Si tips with the a-C film coating. They reported that a maximum current of 50 μA was obtained with an applied field strength of 2.25 V/μm. The carbon and related materials with the sharp tips are the most important candidate for the field emission element.

Our research group develops the aggregation of the ceramic whiskers on the silicon and other solid-state materials [8-14]. The ceramic field emitter was formed by the electrically conductive whisker with the sharp tip and amorphous carbon and related coating with relatively low-work function. For example, a performance of the diode display operated by the cold emitter formed from aluminum-doped ZnO (Al:ZnO) whiskers coated with amorphous hydrogenated carbon nitride (a-CNx:H) films was reported [15]. The europium-doped yttria (Eu:Y2O3) red phosphor film was deposited on the anode electrode using an atmospheric chemical -vapor-deposition (CVD) apparatus and excited by the field emitted electrons at an operating voltage of ~600 V, implying the electric field of 2 V/μm. Very strong luminescence was observed entirely the area in which the red phosphor film was deposited. The work function of the amorphous phase of the carbon nitride films with various functional groups was also investigated using a Fowler-Nordheim (F-N) equation [16]. The F-N plots yielded work functions of 0.64 eV to 3.81 eV for the various a-CNx:H. The work function was dependent upon the functional group forming the termination structure of the a-CNx:H films.

In this study, we designed the new type of the triode display constructed using the ceramic cold emitter. The display consisted of three electrodes, that is, ceramic cold emitter, extraction gate and accelerating electrode with coating of the metal oxide phosphor. In this article, the performance of the triode display will be described and discussed.
Experimental
Emitter Fabrication

Al:ZnO whiskers were prepared using the atmospheric CVD apparatus that was previously employed in obtaining ZnO whiskers [8, 9]. The reactants, Zn(C5H7O2)2 (Soekawa Chemical Co., quoted purity of 99.9%) and Al(C5H7O2)3 (Soekawa Chemical Co., quoted purity of 99.9%), were loaded into a vaporizer and vaporized at 115°C. The inside temperature of the vaporizer measured using a K-type thermocouple is defined as the vaporizing temperature. The reactant vapor was first carried by nitrogen gas flowing at a rate of 1.2 dm3/min and then sprayed from the metallic nozzle directly onto the n-type silicon (Shin-etsu Semiconductor Co., resistivity of 2.00~50.0 Ωcm) substrate mounted on the electric heater. The substrate was heated to 550°C using the electric heater. The surface temperature measured using the K-type thermocouple is defined as the substrate temperature. The reactants, Zn(C5H7O2)2 and Al(C5H7O2)3, were immediately decomposed by the heat from the substrate heater to form whiskers. The distance between the nozzle and the substrate was maintained at 15 mm throughout the experiments. The a-CNx:H films were prepared by a CVD method assisted by electron-cyclotron-resonance (ECR) plasma [17-21]. The thickness of the films was maintained in the range of 20~50 nm. The surface morphologies of the whisker electrode were observed by field-emission scanning-electron-microscopy (FE- SEM, using JSM-6700F, JEOL).
Anode Fabrication

The red and green phosphors Eu:Y2O3 was also prepared using the atmospheric CVD apparatus. The reactant, yttrium dipivaloyl methanato (Y(DPM)3, C33H57O6Y, Showa Denko Co., quoted purity of 99.9%) and europium dipivaloyl methanato (Eu(DPM)3, C33H57O6Eu, Showa Denko Co., quoted purity of 99.7%) were loaded into a vaporizer and vaporized using an electric heater. The vaporizing temperature was kept constant at 205°C. The reactant vapor was first carried by nitrogen gas flowing at a rate of 1.2 dm3/min and then sprayed from the metallic nozzle directly onto the metal mesh with a relative area occupied by holes to be ~53% used for the accelerating electrode. The deposition duration of Eu:Y2O3 crystallites was varied for 20~30 minutes for each experiment to maintain a film thickness of 5 µm. The substrate was heated to 600~620°C using the electric heater. The distance between the nozzle and the substrate was maintained at 15 mm throughout the experiments.
Assembly of FED Device

Field emission properties of the element were studied using a triode display device. The accelerating electrode or anode consists of the phosphor-coated metal-mesh whereas the field emission element acts as the cathode. In addition, the metal-mesh extraction gate was fixed between anode and cathode. Three electrodes have an area of 20×30 mm2 and are parallel to each other and separated by spacers, 50 μm and 7 mm thick-insulators, for insulating of cathode-gate electrodes and gate-anode electrodes, respectively. Figure 1 (a) and (b) show a photograph of FED device and its schematic illustration, respectively. The property measurements were carried out under a pressure of 8×10-5 Pa.

AZojomo - AZoM journal of Materials Online - Photograph of the FED device assembled in this study

Schematic diagram of the FED device assembled in this study

Figure 1. (a) Photograph of the FED device assembled in this study
(b) Schematic diagram of the FED device assembled in this study.
Results and Discussion

Figure 2 shows the SEM images of the Al:ZnO whiskers with a-CNx:H coating grown on the single crystalline silicon (100) wafer. The radius of curvature of the whisker tip was evaluated using the cross-sectional image. The top view (a) shows that the crystallites grew to sizes of about 0.7~1.0 mm with a density of 1×105 mm-2. The size of the crystallites was heterogeneous and the edge direction of each crystallite did not indicate any particular alignment. A cross-sectional SEM observation (b) reveals that the crystallites were whisker-shaped with an aspect ratio of 10. The deviation of the length of the whiskers was within 10~30%. The typical radius of curvature of the whisker tip was 85 nm. Although direct measurement of the resistivity of the Al:ZnO whisker is impossible, the Al:ZnO film exhibited a resistivity as low as 1 Ωcm, with an n-type characteristic.

SEM images of the Al:ZnO whiskers with a-CNx:H coating

Figure 2. SEM images of the Al:ZnO whiskers with a-CNx:H coating.

The fundamental field-emission property of the whisker emitter was examined under conditions of cathode-gate electrode distances of 50 µm and 300 µm. Figure 3 indicates gate electrode current (Ig) obtained at each distance. The threshold emission voltage Vth at 1 µA/cm2 for the distance of 50 µm was 120 V, corresponding to a value of 2.4 V/µm. It increased to 2.7 kV at 300 µm, that corresponds to a value of 9 V/µm. These threshold electric fields are higher than that (1.5~7.8 V/μm) reported previously [16]. This is explained as that the gate electrode captured only a part of current due to its mesh structure.

AZoJomo - AZoM Journal of Materials Online - Cathode-gate electrode current obtained at each distance

Figure 3. Cathode-gate electrode current obtained at each distance.

Figure 4 shows the voltage and current profiles of the FED during operation. On the initial stage of the experiment, an accelerating voltage (Va) applied between the cathode and the anode was increased with an increment rate of approximately 12.5 V/s. In this procedure, phosphor showed no luminescence. As a next step, a gate voltage (Vg) applied between the cathode and the gate electrode was increased with an increment rate of approximately 4 V/s. When Vg reached to 800 V, Ig and anode current (Ia) increased suddenly followed by dropping Vg to 500 V. In this time, emitted electrons started to glow phosphor at the accelerating electrode. The Ia value was in the range of 0.3~1 mA.

AZojomo - AZoM Journal of Materials Online - Voltage and current profiles of the FED during operation

Figure 4. Voltage and current profiles of the FED during operation.

A series of photographs of red display device during operation

A series of photographs of red display device during operation

A series of photographs of red display device during operation

A series of photographs of red display device during operation

Figure 5. A series of photographs of red display device during operation.

Although Ig was saturated at 1 mA that is limit current of the Ig source, it dropped to zero when the Ia value increased to 1 mA. This induced the instability of the anode current or the phosphor glow intensity. After 900 s, Va was decreased slightly. The Ia value was also decreased to the range of 0.02~0.3 mA. After 1750 s, the Va value was increased to 7 kV again. The Ia value was also increased. The intensity of red phosphor was also changed with the variation of Va as indicated in Figure 5. A series of photographs of red display device shows that the intensity of the phosphor increased with the Va increment. Figure 6 shows the luminance of red display device as a function of Va. As evident from Figure 6 the luminance up to 88.9 cd/m2 can be achieved at 7 kV.

Luminance of red display device measured as the function of the anode voltage

Figure 6. Luminance of red display device measured as the function of the anode voltage.
Summary

The voltage and current profiles of the FED with the Al:ZnO cold emitter were measured during operation. Under conditions of Va of 5 kV and Vg of 500 V, the Ia value of 0.3~1 mA was obtained. This current range was enough to glow the ceramic phosphor for obtaining the intense luminescence.