Chapter 5 The Reduction of Gate Leakage for the CNTs-Based
5.3 The Power Efficiency Improvements via Cutting Off the Leakage
The cross-section views of the conventional samples, the OPN samples, and the ONP samples were taken by SEM before the synthesis of the CNTs and the images were shown in Figs. 5.5(a), 5.5(b), and 5.5(c) respectively. The insets in Figs. 5.5 displayed a top view with 45° viewing angle for the samples that have undergone the process of CNTs synthesis. It could be observed that the horizontal distance from CNTs to the poly-Si gate was about 2.5 µm in all the samples. The emission current densities of anode in these samples were measured by altering the gate voltage from 0 to 100 V when the anode was applied at 1000 V, as plotted in Fig. 5.6. According to the measurement results, the emission current density of anode was 0.81 mA/cm2 in
conventional samples, 5.44 mA/cm2 in the OPN samples, and 2.01 mA/cm2 in the ONP samples when the gate was applied at 100 V. At the same time, the gate leakage current was 65.2 mA/cm2 in conventional samples, 58.14 mA/cm2 in the OPN samles, and 14.02 mA/cm2 in the ONP samples. If the total emission current density from cathode was defined as the summation of anode current density and gate current density and the current efficiency was defined as the ratio of anode current density to total emission current density, the current efficiency was 1.23% in conventional samples, 8.56% in OPN samples, and 12.54% in the ONP samples. Accordingly, the power efficiency was 1.23% in conventional samples, 48.34% in the OPN samples, and 58.6% in the ONP samples. The improvement of current efficiency may result from that the nitride film could block part of the emission current between gate and cathode. As shown in Fig. 5.7(a), there might be two main paths of gate leakage in conventional samples. The proposed OPN samples, as shown in Fig. 5.7(b), could clog the emission current from upside to suppress the gate leakage current and the proposed ONP samples, as shown in Fig. 5.7(c), could clog the emission current from downside. The higher current efficiency of the ONP samples than the OPN samples indicated that the leakage current from downside was the major path of the gate leakage current. Moreover, the total emission current of conventional samples and the OPN samples were both about 65 mA/cm2 but that of the ONP samples was only 16 mA/cm2. It resulted from that the nitride film in the ONP samples could greatly decrease the electric field induced from poly-Si gate, therefore, reduce the total emission current extracted from the cathode.
5.4 Summary
The current efficiency of the CNTs -based field-emission triodes has been greatly
reduced from 1.23% to 8.56% by capping a silicon nitride film above the poly-Si gate to prevent the leakage current from upside in the OPN samples and to 12.54% by inserting a silicon nitride film under the poly-Si gate to prevent the leakage current from downside in the ONP samples. The improvements of current efficiency showed that the added nitride film could effectively reduce the leakage current between gate and cathode by moderating the electric field around the poly-Si gate. For the proposed two device structures, the OPN device structure could both increase the current efficiency and high emission current density of anode and the ONP device structure could provide even better current efficiency, however, the total emission current density was notably suppressed resulting from that the nitride film could also moderate the electric field around cathode. As compared with the conventional device structure, the OPN and the ONP structures could not only improve the current efficiency remarkably but also increase the emission current density of anode. It showed a great potential in the application of field-emission displays.
Chapter 6
Summary and Conclusions
In this dissertation, several field-emission characteristics of the CNTs were improved by modifying the catalyst and the device structures. By modifying the catalyst structures, the emission current density, reliability, and uniformity were improved without causing any obvious structural destruction to the CNTs-based emitters or increasing the complexity of process. The power efficiency was also improved by modifying the device structures.
In Chapter 2, a thin Ti capping layer was deposited on the Fe nanoparticles to reduce the density of the CNTs. With 2-nm-thick Ti capping layer, an optimal density of the CNTs of about 2×107 cm-2 was obtained with the lowering of the turn-on field from 3.8 V/µm to 2.5 V/µm. It was due to the trade-off between the suppression of the screening effect and the reduction of the total emission area caused by the decrease of the density. Moreover, the abrupt decrease and the gradual degradation in the emission current density were also eased with the Ti capping layer. It might result from the improvements of the contact properties between the CNTs and the substrates. The Ti capping layer tended to merge with the Ti buffer layer under the Fe nanoparticles and held the Fe nanoparticles with large contact area. It could enhance the adhesion between the CNTs and the substrates and gained a strong contact structure to sustain the electrostatic force induced by the applied electric field from being pulled off from the substrates. Furthermore, the enlarged contact area also reduced the contact resistance between the CNTs and the substrates which could generate a great deal of Joule heat with high current density passing through. The high temperature induced
from the Joule heat would result in the attack from the oxygen remained in the vacuum chamber, cause an evaporation of the CNTs, or make the contact structure loose. It could cause a gradual degradation in the emission current density with the increase of operating time.
In Chapter 3, an Fe-Ti codeposited metal layer was utilized as the catalyst of the CNTs. By utilizing this novel catalyst, the reliability of the CNTs-based field-emission devices could be improved by enhancing the contact properties between the CNTs. No obvious abrupt decrease in the emission current density was observed in five measurements from 0 to 7.7 V/µm and no serious degradation of the emission current density occurred during being stressed at 7.7 V/µm for 2,500 sec. Additionally, the Ti codeposited with the Fe in the metal layer could suppress the coalescence of the Fe nanoparticles during being heated in the hydrogen pretreatment or the CNTs growth processes. Uniform CNTs with small variation in length were obtained. It could greatly reduce the local field enhancement due to the length variation of the CNTs and obtained a uniform emission current. As compared to the CNTs synthesized from the pure Fe catalyst, the percentages of emission area for the CNTs synthesized from the Fe-Ti codeposited catalyst was improved from 12% to 29% at 500 V, from 41% to 75% at 600 V, and from 86% to 100% at 700 V.
In Chapter 4, a man-made structure, the pillar-like CNTs, was synthesized from the Fe-Ti codeposited catalyst to obtain a reliable emission current from straight CNTs with small variation in its length. As compared with the pillar-like CNTs synthesized from the pure Fe catalyst, those synthesized from the Fe-Ti catalyst remained a constant growth rate in 2 hours. It was due to that the smaller catalytic nanoparticles synthesized from the Fe-Ti codeposited layer could keep the activity better than those synthesized from the pure Fe metal layer. To design the field-emission characteristics of the pillar-like CNTs, the diameter and the inter-pillar spacing of the pillar-like
CNTs were defined by the pattern. An optimal R/H ratio, 2.5, for the pillar-like CNTs of 12 µm high was obtained with low turn-on field of 1.01 V/µm. It was also due to the trade-off between the suppression of the screening effect and the reduction of the total emission current density as described in Chapter 2.
In Chapter 5, a silicon nitride layer was added into the triode-type CNTs -based field-emission devices to block the leakage current between the cathodes and the gates.
The current efficiency of the CNTs-based field-emission triodes has been greatly reduced from 1.23% to 8.56% by capping a silicon nitride film above the poly-Si gate to prevent the leakage current from upside in the OPN samples and to 12.54% by inserting a silicon nitride film under the poly-silicon gate. For these two proposed device structures, the OPN device structure could both increase the current efficiency and high emission current density of anode and the ONP device structure could provide even better current efficiency. However, the total emission current density was notably suppressed resulting from that the nitride film could also moderate the electric field around cathode. For application purpose, the ONP device structure showed more potential in the triode-type field-emission devices.
Chapter 7
Future Prospects
For further research, some important topics about the CNTs-based field-emission devices are suggested.
(1) Low Temperature Processes
Although some of the field-emission characteristics, the emission current density, reliability, and uniformity, were improved by the modification of the catalyst and device structures, the process temperature is still too high for practical application purposes. The high temperature during the processes of hydrogen pretreatment and CNTs synthesis makes it difficult to utilize a glass or even a plastic substrate. Glasses are the most common used substrates in flat-panel displays because of its flat and large area and also one of the most promising substrates for field-emission displays.
However, the melting point of glasses is generally below 600 °C which is lower than the process temperature. Therefore, a low temperature process for the fabrication of CNTs is essential for the commercialization of the CNTs-based field-emission displays or back-light units.
For a catalytic nanoparticle with small diameter, the activity can be kept during the synthesis process of CNTs. In this dissertation, the Fe nanoparticles synthesized from the Fe-Ti codeposited layer also have the feature of small diameter. It shows a very great chance to synthesize the CNTs from the codeposited metal layer at relative low temperature. Moreover, it would be interesting to utilize other catalytic metal and codeposited metal as the catalyst for the low temperature purpose.
(2) Pillar-Like CNTs with Large Emission Area
In our work, the pillar-like CNTs with uniform length were achieved by using the Fe-Ti codeposited catalyst. It shows a very great potential in the application of back-light units or flat light source. Due to the limitation of the thermal-CVD in chamber size and the compliance of the measurement system, the pillar-like CNTs with large area are not fabricated. However, the reliability and uniformity for the pillar-like CNTs with large emission area still need to be studied.
For this purpose, we should build a thermal-CVD with large reaction chamber and a large vacuum measurement system which could sustain the high voltage and high emission current density.
(3) The Reliability and Uniformity of the Triode-Type Field-Emission Devices
In Chapter 5, the issues of the reliability and uniformity are not discussed for the triode-type field-emission devices. It would be helpful to utilize the Fe-Ti codeposited catalyst in the triode-type field-emission devices for a reliable and uniform emission current.
In addition to the topics mentioned above, there are still many interesting issues remained for the CNTs-based field-emission devices.
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