Influence of Ni Catalyst Layer and TiN Diffusion Barrier on Carbon Nanotube Growth Rate

Dense, vertically aligned multiwall carbon nanotubes were synthesized on TiN electrode layers for infrared sensing applications. Microwave plasma-enhanced chemical vapor deposition and Ni catalyst were used for the nanotubes synthesis. The resultant nanotubes were characterized by SEM, AFM, and TEM. Since the length of the nanotubes influences sensor characteristics, we study in details the effects of changing Ni and TiN thickness on the physical properties of the nanotubes. In this paper, we report the observation of a threshold Ni thickness of about 4 nm, when the average CNT growth rate switches from an increasing to a decreasing function of increasing Ni thickness, for a process temperature of 700°C. This behavior is likely related to a transition in the growth mode from a predominantly “base growth” to that of a “tip growth.” For Ni layer greater than 9 nm the growth rate, as well as the CNT diameter, variations become insignificant. We have also observed that a TiN barrier layer appears to favor the growth of thinner CNTs compared to a SiO2 layer

Field emission devices on silicon

Three different types of field emission diodes (FEDs)---Si field emitters, metal wedge emitters and Si emitters with an air bridge---have been fabricated. These FE diodes were studied to determine the emission uniformity and stability.Effects of diamond-like carbon (DLC) film on the emission characteristics were specifically studied. The coating of DLC films was carried out by laser ablation and rf sputtering. Significant reduction in turn-on voltage was observed for the first time in the Si microtips with a DLC film coated by laser ablation. Effects of substrate temperature on the quality of the DLC films were also observed. For the Si microtips with DLC films prepared by rf sputtering, the onset voltages of emission were higher. This was found to be due to the high resistivity and the crystalline-like structure of the films.Using the processes developed, arrays of Si microtips were fabricated and tested for emission stability and uniformity. A structure with a built-in pn junction for each tip was adopted to fabricate FE diodes. The measurement results from these FE diodes showed more stable emission than those without the built-in junctions. In the FEDs with pn junctions, the emission current was limited by the reverse junction characteristics. In order to minimize the effect of junction breakdown on the current regulating mechanism, a novel field emission device with a p-i-n junction for current regulation was proposed and implemented. An extremely stable and uniform emission was obtained from these FEDs with p-i-n junctions due to the large breakdown voltages.In addition to the above experimental research, an electric field computation program based on charge density method was developed. The electron emission of FEDs with different tip size, half angle and gate planar levels was simulated. The results showed that the electric field at a given voltage increases when the tip size and half angle are decreased. Moreover, a slightly recessed emitter yields a larger field than the one with a coplanar configuration.In the final part of the thesis, a novel FE triode structure and the required fabrication procedure were proposed. This was done in order to manufacture an operational FE triode without the need of advanced lithography tools, and to overcome the obstacles encountered during the first triode study. FE triodes with submicrometer emitter/gate spacing were successfully fabricated with the procedure developed. In this novel triode, high quality thermal oxide was used as an insulator between the gate electrode and the substrate even without the need of advanced tools for submicrometer lithography. Currents collected by the anode were measured at different gate voltages