Motivation

As the field emission displays are going to be commercialized, it is very important to find a suitable cold cathode with low operating voltage and high emission current. According to the theory of field emission, Fowler-Nordheim tunneling, low work function (φ ), high field enhancement factor (β), and large emission area (α) are desirable for a high field emission current density.

For the techniques of silicon tips, several methods have been used to obtain a large enhancement factor β via sharpening the silicon tips, such as large oblique-angle thermal evaporation or sputtering to fabricate sharp metal cones, high temperature oxidation to sharpen silicon tips, or anisotropic etching of silicon using KOH to produce sharp tip molds.

However, these methods to fabricate sharp tips need extra fabrication process with complexity.

The novel material, CNTs, therefore, is proposed to be one of the most promising materials as the cathodes in field emission displays.

The high density of CNTs can provide a great deal of field emission sites which can raise the emission current density (due to the increase of the emission area, α) but the density of CNTs will affect the field enhancement factor (β) which is also strongly relative to the emission properties of CNTs. For CNTs with high density, the screening effects reduce the field enhancement factor (β), therefore, suppress the field emission current density [2.1-2.3], as shown in Figure 2-1. Obviously, it is important to obtain an optimized density of CNTs to improve the field emission properties, such as turn-on field, threshold field, and emission current density. Well control of density and surface morphology of CNTs is thus required for

applications in the near future. To effectively control the density of CNTs, three novel methods including two thin Ti capping layer processes and a co-deposition of Ti and Fe process are introduced to improve the emission properties via controlling the density of CNTs in this chapter.

2.2 Introduction

Carbon nanotubes (CNTs) have attracted great deal of attention because of its outstanding physical properties and potential applications since its first observation by Iijima in 1991 [2.4]. Due to several unique properties, such as high aspect ratio, low work function (5 ev), small tip radius of curvature, high chemical stability, high mechanical strength, high conductivity, and electron emission properties [2.5-2.6], it has been considered as one of the most potential materials for field emission displays. Besides, CNTs is also a very wonderful materials in many applications, such as flat-panel field-emission displays (FEDs), nanoprobes of atomic force microscopes (AFMs), microsensors, scanning tunneling microscopes (STMs) and microsized intense electron sources. Nowadays, several methods have been developed to synthesize CNTs, such as arc discharge [2.7-2.8], laser ablation [2.9], screen-printing [2.10-2.12], plasma-enhanced CVD [2.13], electron cyclotron resonance CVD [2.14], microwave plasma-enhanced CVD [2.15], and thermal CVD [2.16-2.17]. Generally speaking, arc-produced CNTs, laser-produced CNTs, and screen-printing techniques are used to fabricate low-cost CNT field-emission in diode structure for field-emission displays. However, the drawbacks are that the specific purification processes for arc-produced and laser-produced CNTs are required, and the uniformity of screen-printed CNT field-emitter arrays is not applicable in filed emission displays. The selective growth of CNTs by chemical vapor deposition (CVD) processes without extra purification or screen-printing process, therefore, sufficient for fabricating field-emission devices with better uniformity. Besides, via the CVD

process, the CNTs can be grown with great vertical align which can also increase the effective local electric field when a voltage is applied. For these reasons, the thermal CVD, a simple, low cost, and well-developed method, is preferred to synthesize the CNTs in this thesis.

However, some critical issues such as screening effects, reliability, high driving voltage, uniformity, and vacuum package have not been solved for the application of field emission displays. The effect of screening electric field by the dense arrangement of CNTs has been reported by several groups [2.1-2.3]. If the CNTs are too closely spaced, the electric field will be screened out. Groning et al. reported the field enhancement factor β of the tips decreases rapidly when the inter-tip spacing is smaller than twice the length of the tips. They also found that the maximum current density is obtained when the spacing between the tips is about two times their relative height by simulations, as shown in Figure 2-1. For larger spacing the current density decreases due to the decreasing the field emission sites, with a nearly constant emission current per tip as the field enhancement factor remains constant. For smaller spacing the current density decreases rapidly due to the decreasing β factor and this effect cannot be compensated for by increasing the field emission sites from tips. This shows that when the spacing between the emitting structures on a surface becomes comparable to its length, problems of shielding do occur and will limit the emission current density.

Therefore, the turn-on field of the high-density-CNTs is still high because of the screening effects, as shown in Figure 2-2. To obtain better field emission properties, the density of CNTs should be optimized. However, the density of CNTs synthesized by thermal CVD is too high and the screening effects are very serious. Many researches have reported different methods to suppress this phenomenon, such as plasma treatment [2.18-2.21], wet etching method [2.22], screen-printing method used pastes with different CNT contents [2.23-2.24], E-beam lithography [2.25-2.26], anodic aluminum oxide (AAO) nanoporous templates [2.27-2.28], excimer laser irradiation [2.29], and etc.. Most of the methods will increase the complexity in processes of fabrication or cause a defective affection on the CNTs.

In this chapter, three novel processes including two kinds of thin Ti capping layer and a co-deposition of Ti and Fe process are proposed to modify the density of CNTs grown by thermal CVD. With different thicknesses of the Ti capping layer on hydrogen pretreated catalytic nanoparticles or different weight percentages of Ti co-deposited with catalyst, Fe, the density of CNTs can be controlled simply without the need of extra high cost or high complex processes. By controlling the density of CNTs, these methods can reduce the turn-on field and threshold field, and improve the field emission current density due to the suppressing of the screening effects. Furthermore, reliability is also a very important issue for field emission displays if it is going to be commercialized. The main factors to affect the lifetime of a CNTs-based emitter are the adhesion and the contact resistance between the CNTs and the substrate. With the novel methods proposed in this chapter, the adhesion can be increased to obtain a higher electric breakdown field and the contact resistance can also be suppressed to avoid the Joule heating when the devices are operated in a high current density condition [2.30-2.31].