Abstract
The field emission behavior of carbon nanotubes (CNTs) in terms of their potential applications in vacuum microelectronics has been studied extensively in recent years. CNTs have extraordinary strength, impressive thermal and chemical stabilities and excellent electronic properties beneficial for making field emission cathodes. A vertically aligned CNT cathode is a beneficial geometrical structure for achieving high performance vacuum micro- and nano-electronic devices such as field emission diodes, triodes and flat panel displays. Although the vertical alignment of CNTs has been achieved in many ways [1-3] , less experimentation has been conducted at ambient pressure using a simple tube furnace which would be a most promising method to mass produce CNTs by catalytic chemical vapor deposition (CVD). In this work, Ni catalytic film was deposited on SiO 2 substrates and CNTs synthesis was performed in a quartz tube furnace. NH 3 was applied with acetylene (C 2 H 2 ) to produce vertically aligned CNTs. The catalyst deposition parameters and C 2 H 2 /NH 3 flow rates are critical to the size and density of Ni catalytic particles, which strongly affect the morphology of as-grown CNTs. The temperature and pressure at which CNTs were grown were at 750°C and 1 atm., respectively. During the experiments, a mixture of Ar (80 vol.%) and H 2 (20 vol.%) was introduced into the furnace at room temperature, the furnace was then heated to 750°C at the rate of 15°C/min. When the temperature was stabilized, NH 3 was introduced as a processing reagent, followed by C 2 H 2 , the carbon source. The effects of NH 3 on the size and density of Ni particles were investigated, and the mechanism of CNT alignment was analyzed. Specifically, ammonia breaks larger catalyst particles into smaller ones, thereby increasing the particle density. The size of catalyst particles is critical, because CNTs cannot grow if the diameter of catalyst particles is far beyond the diffusion length of carbon atoms [4] . Moreover, NH 3 hampers amorphous carbon generation [4-5] , therefore keeping the catalyst sites active, which leads to higher density of as-grown CNTs. The morphologies of CNTs produced under different conditions were examined by scanning electron microscope (SEM). It was found that the vertical alignment of CNTs (Figure 1) could be achieved only at certain C 2 H 2 /NH 3 flow ratios; otherwise randomly oriented CNTs are produced. The Raman spectroscopic characteristics of asgrown CNTs were obtained for further understanding of the CNT structures. As shown in Figure 2, the peak at about 1590 cm -1 is attributed to the G-band of tangential mode of graphene sheet while the D-band peak at around 1330 cm -1 corresponds to the defects or limited dimensions of CNT crystal structure [6] . The D-band signal intensity is stronger than that of the G-band suggesting that the CNTs synthesized in this experiment have significant lattice defects. In addition, the absence of any apparent second-order peaks indicates the low crystallinity of the CNT structure [6] . The Raman system used for CNT characterization operates with backscattering geometry and a notch filter installed to block the scattering light. As a result, all peaks with the Raman shift below 50cm -1 were completely cut off. According to Bandow [7] et al., the radial breathing mode (RBM) frequency can be estimated as ω τ = 223.75(cm -1 nm) / d (nm), where d is the diameter of the outmost graphene shell of the CNT. Therefore the RBM mode cannot be seen if all the CNTs have diameter more than 4-5nm. Since the smallest MWCNTs obtained in this work were about 15-20nm in diameter, no RBM peaks were observed. Vacuum diode structures were fabricated from the as-grown CNTs and tested in vacuum (∼10 -6 torr) for field emission. The turn-on field is about 3V/um, which is comparable to data reported elsewhere [8-13] . Field emission characteristics of the aligned CNTS will be reported. © 2005 IEEE.