Outline of Dissertation

In the first chapter, the relative backgrounds of this dissertation are introduced briefly including the vacuum microelectronics, the field emission theory, and the technologies of field-emission devices. Here the motivation and main ideas for the design of our experiments are also described simply.

In Chapter 2, the mechanism for the destructions of CNTs in field-emission devices during the field-emission measurements are described according to the measurement data and the material analysis results. A method of thin Ti capping layer on the hydrogen pretreated nanoparticles is also proposed for better reliability.

Moreover, this method is also applied to control the CNTs density for increasing the emission current density.

In Chapter 3, a Fe-Ti codeposited metal layer is utilized as the catalyst of the CNTs. For reliability issue, a partially immersed structure is achieved and therefore

suppresses the abrupt decrease and gradual degradation of the emission current density. Besides, for the uniformity issue, the coalescence between Fe nanoparticles is suppressed by the Ti in the codeposited layer and, therefore, uniform CNTs are obtained for homogeneous light emission.

In Chapter 4, the pillar-like CNTs are synthesized from the Fe-Ti codeposited catalyst. The growth rate and morphologies of the pillar-like CNTs are discussed here.

Moreover, the optimal inter-pillar distance is also obtained via controlling the distance between pillars with lithography. The man-made structure provides uniform distributed electron emitters with small variation in morphologies and high emission current density.

In Chapter 5, a silicon nitride layer is added under the poly-Si gate or above the poly-Si gate in the triode-type field-emission devices to reduce the leakage current between the gates and the cathodes. By cutting off the current paths of the leakage current, the power efficiency of the triode-type field-emission devices was remarkably improved. It also shows a trade-off between the resisting of leakage current and the shielding of extracting electric field.

Finally, all experimental results in the previous chapters and all the suggested works for further researches are summarized in Chapter 6 and Chapter 7, respectively.

Chapter 2

The Improvements of Emission Current Density and Reliability for the CNTs Synthesized from the Ti-Capped Fe Nanoparticles

In this chapter, a novel method of preparing the catalytic metal layer was proposed for the synthesis of the CNTs in the thermal-CVD. By depositing a very thin Ti layer on the hydrogen pretreated Fe nanoparticles, part of the nanoparticles were covered and the density of CNTs was greatly reduced. With a suitable thickness of the thin Ti capping layer, the density of CNTs could be well controlled and suppress the screening effect which could reduce the emission current density seriously. Moreover, the Ti capped on the Fe nanoparticles also played a rule of adhesive between the CNTs and the substrates after being heated to improve the contact properties. It suppressed the abrupt decrease and gradual degradation of the emission current.

2.1 Introduction

The emission current density between the cathodes and the anodes is one of the important features that can affect the performance of field-emission devices. With higher emission current density, the driving ability of vacuum microelectronics or the brightness of field-emission displays and back-light units can be greatly improved.

According to the description of the field emission theory in chapter 1, there are

several factors that can seriously affect the emission current density, such as the strength of the applied electric field and the work-function Φ, field enhancement factor β, and emission area α of the electron emitters. Among them, the strength of the applied electric field and the work function of emitters have been defined by the architecture of field-emission devices and the chosen emitter material, respectively.

The most effective way to increase the emission current density is to enlarge the local enhancement factor and emission areas of the electron emitters. It has been reported that the screening effect caused from the high density of electron emitters can greatly reduce the strength of local electric fields around the tips of emitters and, therefore, decrease the emission current density from the cathodes to the anodes[2.1-2.2].

According to the simulation results, the optimal distance between two CNTs is about two times of its length[2.1]. However, the densities of CNTs synthesized from chemical vapor deposition systems are generally much higher than the optimal density calculated from the simulation. It causes a very serious screening effect around the tips of the CNTs and decreases the local enhancement factor of emitters. The emission current density of CNTs-based emitters is therefore reduced seriously. Several methods have been reported to suppress the screening effect by reducing the density of CNTs in field-emission devices to a suitable value. Some of them separate or passivate the catalytic nanoparticles by adding some processes before synthesizing the CNTs to reduce the density of CNTs[2.2-2.11]. Some of them try to control the density of CNTs by changing the growth conditions during the synthesis processes[2.12-2.15]. The others make some post-treatments after the synthesis process to eliminate part of the grown CNTs[2.16-2.17]. However, some of them increase the complexity or cost of processes or cause defects in the crystal structure of CNTs.

Another critical issue of the CNTs-based field-emission devices is the reliability.

Although CNTs exhibit great mechanical strength and inert chemical properties, the degradation of the emission current density as being applied at high electric field or operated with high emission current density still occur and cause a fetal fail to the field-emission devices. According to several researches, the fails of CNTs during operation can generally be classified into two types: (i) a break of CNTs or a detachment between the CNTs and the substrate in high electric field[2.18] and (ii) a gradual degradation of emission current density for a long operating time or with high emission current density[2.19-2.22]. For the first type, mechanical damages cause an abrupt decrease in the field emission current when we apply a constant or an increasing electric field on the CNTs. It was also found that the CNTs are broken or pulled off from the substrate after being applied at high electric field and exhibit an abrupt drop in the emission current density. For the later one, a gradual degradation of the emission current density was found which might result from the Joule heat generated from high current density. At high temperature, the oxygen remained in the vacuum chamber tends to attack the defective regions in the CNTs. Additionally, the interface structure between the CNTs and the substrates is also getting loose at high temperature that makes the CNTs more easily to be pulled off from the substrate. It causes a gradual degradation of the field-emission current density.

Several methods have been reported to improve the reliability of the CNTs-based electron emitters via modifying the interface between the CNTs and the substrate such as utilizing different interfacial layers[2.23-2.24], making some post-treatments of the CNTs[2.25], and coating a binder or spin-on-glass (SOG) on the CNTs[2.26-2.27].

Via improving the adhesion between the CNTs and the substrate, the breakdown electric field could be increased to prevent from the abrupt decrease in emission current. On the other hand, the improvements of the contact resistance between the CNTs and the substrate could suppress the Joule heating generated with high current

density passing through the defective regions. However, some of the methods cause structural damages to the crystallinity of the CNTs and the others increase the complexity and cost of processes.

In this chapter, a thin Ti layer was capped on the hydrogen pretreated catalytic Fe nanoparticles before the growth of the CNTs. The Ti capping layer can effectively reduce the diffusion of the carbon radicals generated in the thermal-CVD to suppress the growth of CNTs. With different thicknesses of the Ti capping layer, the density of the CNTs was well controlled to a suitable density of about 2×10⁷ cm^-2. Furthermore, no obvious damage in the crystallinity of the CNTs was found according to the results of material analysis. By this way, the density of CNTs was greatly reduced and the turn-on field was greatly decreased from 3.8 V/µm to 2.1 V/µm with 2-nm-thick Ti capping layer.

Moreover, the abrupt decrease and gradual degradation in the emission current density was also suppressed with a 5-nm-thick Ti capping layer. Two different mechanisms were proposed and discussed here to describe the improvements of the reliability. One was the adhesion enhancement and the substrates and the other was the reduction of the contact resistance between the CNTs. According to the measurement results and the SEM images, the breakdown and degradation of the emission current density were both suppressed by depositing a thin Ti capping layer on the hydrogen pretreated Fe nanoparticles.

2.2 Experimental Procedures