Jpn. J. Appl. Phys. Vol. 40 (2001) pp. 6088–6091 Part 1, No. 10, October 2001 c 2001 The Japan Society of Applied Physics
Undergate-type Triode Carbon Nanotube Field Emission Display with a Microchannel Plate SeGi Y U∗ , Sunghwan J IN, Whikun Y I, Jeongho K ANG, Taewon J EONG, Yongsoo C HOI, Jeonghee L EE, Jungna H EO, Nae Sung L EE1 , Ji-Beom YOO2 and Jong Min K IM† NCRI, Center for Electron Emission Source, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea 1 Department of Advanced Materials Engineering, Sejong University, Seoul 143-747, Korea 2 Department of Material Engineering, Sungkyunkwan University, Suwon 440-746, Korea (Received May 22, 2001; accepted for publication June 25, 2001)
The characteristics of a field emission display (FED), which is based on an undergate-type triode carbon nanotube (CNT), have been examined by incorporating an electron-multiplying microchannel plate (MCP) between the anode and cathode plates of a FED. The MCP was fabricated by electroless plating and the sol–gel process on punched alumina. By applying appropriate voltages between the two faces of an MCP within a FED, the current at the anode plate of a FED was found to be enhanced more than three to five times, leading to higher brightness. The focusing of field emitted electrons was also improved by adjusting the bottom voltage of the MCP, which resulted in a clear image. Therefore, the incorporation of the MCP improved the performance of an undergate-type CNT FED, which can now be considered as one of the key candidates for flat panel displays. KEYWORDS: secondary electron, microchannel plate, carbon nanotube, field emission, field emission display, triode, gate
1. Introduction 1)
Since the discovery of carbon nanotubes (CNTs) a number of potential applications for them have been proposed. Among them, field emission2, 3) is one of the most promising due to the rapid progress in this area as evidenced, for example, by the successful demonstration of CNT-based field emission displays (FEDs),4) field emission based lamps,5, 6) and the possibility of realizing vertically aligned growth of CNTs.7, 8) The extremely high geometric aspect ratio may be thought of as being one of the key reasons for easy electron emission from CNTs. A CNT-based FED with the diode driving type4, 9) has been fabricated and the size of FED panels is being increased in order for them to be considered seriously for possible medium-, or even large-, sized flat panel displays.10, 11) However, in order to display high-quality pictures, the FED should be operated in the triode mode, which leads to smooth gray scale imaging and fast response for moving pictures. Although several triode-type CNT FEDs have been proposed,12, 13) an undergate-type carbon nanotube field emission display (UCFED),14, 15) where gate electrodes are located beneath the cathode electrodes, unlike in the conventional triode structure, is chosen for further study with an electronmultiplying microchannel plate (MCP) in this report due to its simplicity in terms of the structure and fabrication process. Here, we have applied an electron-multiplying microchannel plate (MCP) to a UCFED in order to improve its display characteristics; the performance of this MCP-incorporated UCFED was investigated with respect to current amplification, brightness enhancement, and focusing improvement. 2. Experimental Unlike conventional triode FEDs, the gate electrodes, which modulate field emitted electrons according to the video image signals, in the undergate-type triode FEDs are located under the cathode electrodes as the name implies. One of the main reasons for investigating the undergate triode type is that the process of attaching nanotubes on the cathode elec∗ E-mail † E-mail
address:
[email protected] address:
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trodes can be performed as the last step in the fabrication of a cathode plate. This leads to the small chance for CNT contamination, which is inevitable in the conventional CNT triode structure due to the fact that gate electrodes are fabricated after CNT printing. Thus, the simplicity in the structure and fabrication process as well as the small chance of CNT contamination cause this undergate triode structure to possess high potential for practical application. As a first step towards UCFED fabrication, a 150-nm-thick Cr film for gate electrodes was formed on the soda-lime glass substrate by sputtering. After patterning of the gate electrodes and the successive etching process, a 1500-nm-thick SiO2 insulating layer was grown by plasma enhanced chemical vapor deposition. Cr cathode electrodes of 300 nm thickness were patterned on the insulating layer; these electrodes formed a crisscross pattern with the gate electrodes. Finally, singlewalled CNTs synthesized by a conventional arc-discharge method16) were screenprinted only on the edge areas of the cathode electrodes as field emitters in order to utilize the strong electric field at the edge.14, 15) The cathode plate was separated from a green phosphor-coated anode plate by 200 or 1100 µm high spacers. The detailed fabrication process for a UCFED can be found in refs. 14 and 15. MCPs are two-dimensional arrays of micrometer-sized channel multipliers in a plate where electron amplification is performed by avalanche multiplication of secondary electrons in the cylindrical pore channels.17) When incident particles, electrons in this paper, strike the inner walls of the channels of an MCP, secondary electrons are emitted from the wall surface. When a suitable bias voltage is applied to an MCP, electrons strike the wall releasing more secondary electrons. Repetition of this action to the end of the channels creates an avalanche of electrons. Since the current density for a FED may reach up to the range of µA/cm2 ,18) the input current density for our specially designed MCP for a FED is several orders of magnitude higher than that for the conventional MCPs.19) Thus, we have developed a new type of MCP with new materials and methods which can sustain and also amplify such a high input current.20) Within the channels in an MCP, there exist two thin layers, i.e., a conductive layer and a
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Va Va
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Fig. 1. (a) A FED image with the schematic driving circuit where Va = 1000 and Vg = 180 V, and (b) an MCP incorporated FED image with the schematic driving circuit where Va = 1000, Vt = 800, Vb = 200, and Vg = 180 V. From the comparison between the two photographs, the FED was found to be brighter by inserting an MCP between the anode and cathode plates. Since the voltage supplies were connected in cascade formation, the final voltages at the anode are Va and (Vb + Vt + Va ) from the ground potential for (a) and (b), respectively.
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secondary electron emission layer, where secondary electrons are released from the emissive layer by bombardment of electrons and electrons are supplied through the conductive layer from the reservoir in order to replenish the emitted secondary electrons. A 2-mm-thick MCP with 170-µm-diameter holes on 220 µm centers was built from mechanically punched alumina. The CuAl2 O4 was used as a conductive layer formed by electroless copper plating and subsequent thermal oxidation, and the SiO2 emissive layer was deposited on the conductive layer by a sol–gel process and heat treatment. The detailed fabrication process can be found in refs. 20 and 21.
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3. Results and Discussion The characteristics of an MCP-incorporated UCFED were tested in a vacuum chamber with low pressure of the order of 10−7 Torr, which is the typical pressure range for FED operation. Figure 1 shows that UCFED images with and without MCP incorporation, which were operated with the driving circuits and the corresponding applied voltage (Va : anode voltage, Vg : gate voltage, Vb : MCP bottom voltage, and Vt : MCP top voltage) setups. In order to simulate the actual operation of a FED, a square wave pulse was used for the gate voltage (100 µs duration and the duty ratio of 100) while other voltages were DC. From the photographs for the two cases taken from the phosphor screen images at the anode, it was found that the MCP enhances the brightness and the overall uniformity of a UCFED. Figure 2 indicates that incorporation of the MCP enhances the current at the anode by the multiplication of electrons through the MCP, which was also observed in the images of Fig. 1. The four to seven (three to four) times current amplification for Vb = 300 V marked by solid squares in Fig. 2 (200 V marked by solid triangles) is quite a satisfactory result for UCFED performance improvement, considering the fact
Fig. 2. A current detected at the anode as a function of the anode voltage where Vg = 180, Vt = 800 V. The lowest curve marked with empty circles corresponds to a UCFED without an MCP. The upper two curves corresponds to a UCFED with an MCP, where solid squares (solid triangle) are correspond to Vb = 300 V (200 V). The rapid increase of current for a bare UCFED around Va = 1100 V is expected by the action due to transition from triode to diode emission (see the text).
that the MCP’s aspect ratio is not high, i.e., 12. (Generally, a higher aspect ratio yields higher gain. See ref. 22.) The rapid increase in the current for the non-incorporation case around Va = 1100 V is the typical phenomenon for an undergate-type FED, i.e., transition from triode-type emission to diode-type emission. This cannot be avoided for the undergate-type triode structure due to its deficiency in shielding electric field generated by the anode voltage, whereas the gate electrodes for the conventional triode type are located above the cathode electrodes protecting the diode-type extraction of electrons. This anode voltage range for diode emission should be avoided for normal FED operation. The anode current as a function of the top voltage (Vt ) of an MCP is plotted in Fig. 3. Increasing the top voltage of an
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Fig. 3. Anode current as a function of Vt for an MCP incorporated UCFED where Vb = 200 (empty triangle symbols) and 300 V (solid square symbols), Vg = 180 V, and Va = 1000 V. The current increases as Vt increases, which is the typical phenomenon for an MCP.
Vb = 250 V
Fig. 4. Focus enhancement by an increase of Vb . The left (right) photo corresponds to Vb = 150 V (250 V). From the brightness comparison, it is easily found that Vb = 250 V yields a higher current than Vb = 150 V does.
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MCP yields a high current at the anode plate. This is direct evidence of electron multiplication within an MCP, since a monotonic increase in current with the MCP bias voltage is the typical phenomenon for common MCPs before the current saturation.22, 23) Arcing at the high voltage, however, due to narrow spacing in a FED together with the relatively low vacuum condition prevented us from observing the MCP gain saturation for the higher bias voltage. From Fig. 3, the higher Vb (300 V, marked by solid squares in Fig. 3) exhibits a higher overall current amplification than that exhibited by the lower Vb (200 V, marked by empty triangles), which is confirmed in Fig. 4 by the higher brightness for the higher Vb case. Since the high brightness is the result of the high current, it is concluded that high Vb produces high current amplification. Moreover, it was found that better focusing is obtained for the higher Vb , which was verified by Monte Carlo simulation in ref. 21. Figure 5 illustrates dynamic behavior in a UCFED with/without MCP incorporation, where the anode current was monitored with respect to the voltage drop across the resistor leading to negative values in Fig. 4. The anode current for Vb = 300 V (marked by a dashed line in Fig. 5) is higher than that for Vb = 200 V (marked by a dotted line), i.e., higher Vb yields higher current, which was observed in Figs. 2, 3, and 4. This Vb dependence was not found in the conventional triode FED where the gate electrodes were located above the cathode electrode. The main reason for the current enhancement at high Vb is ascribed to the unprotected undergate-type field emitters from the electric field, i.e., the field from the bottom face of an MCP for the MCP incorporation case and the field from the anode plate for MCP nonincorporation case. It is found that the integrated area under the anode current signal is proportional to the applied Vb voltage. In addition, the decay behaviors of the anode current for all three cases are almost the same, which indicates that MCP insertion does not hinder the dynamic behavior of an FED operation. Consequently, the MCP does not deteriorate the FED’s response time.
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Fig. 5. Dynamic characteristics of a UCFED under the pulse mode operation. The pulse width was 100 µs with the duty ratio of 100 and the voltage height of 180 V. The bottom curve represents the input gate voltage, while the upper three curves correspond to the anode voltages which are shown in the negative scale due to the measurements of voltage drops at the resistor. The anode voltage (Va ) was maintained to be 1000 V, while the top face voltage of the MCP (Vt ) was 800 V. The solid line corresponds to non-MCP incorporation, and dotted (dashed) line corresponds to MCP incorporation with Vb = 200 V (300 V).
4. Conclusions We have tested the undergate-type triode carbon nanotube field emission display by incorporating a microchannel plate within a FED. From the comparison of the anode current and FED brightness, we found that MCP incorporation improves UCFED performance without any serious deterioration. In addition, the focusing in the UCFED was also improved by varying the bottom voltage of an MCP. Therefore, MCP incorporation is one of the methods that can be used for improving the characteristics of a UCFED. Acknowledgements This work was supported by The Korean Ministry of Science and Technology through the National Creative Research Initiatives program.
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