Development of nanostructured carbon thin films
Date of Issue2011
School of Electrical and Electronic Engineering
In this thesis, experimental investigation of the growth mechanism of unique nanostructured carbon films and their properties as well as different fabrication techniques of these films is reported. Using experimental results, the presented mechanisms have been evaluated and developed. This work has enabled a larger range of deposition parameters to create user-specific nanostructured carbon films. The parameters examined include heat, depositing ion energy, plasma density and post treatments such as laser and thermal annealing. Electrical, thermal and field emission properties of nanostructured carbon films have also been studied. First, the effects of deposition temperature and ion energy on the microstructure of the carbon films have been investigated. To do this, the microstructure of carbon films deposited at temperature range of 25 to 6000C and substrate bias range of 25 to 600 V have been studied by plan view and cross section transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and Raman spectroscopy. It is found that at low deposition temperatures ( 1500C) the microstructure of the film depends on the substrate bias. at low substrate biases (lower than 400V) the films are amorphous in the microstructure. Increasing the bias to 600V results in formation of preferred oriented nanocrystals in the microstructure. This is attributed to the formation of high temperature thermal spikes due to impinging of high energy ions to the growing film. Increasing the substrate temperature to 400 and 6000C leads to formation of preferred oriented nanocrystals even at floating substrate bias. The nature of the nanocrystals however depends on the applying bias. Low substrate biases (lower than 600V) results in the formation of graphitic like nanocrystals while at 600V tubular nanostructures are formed. This is due to higher formation enthalpy of tubular carbon structures compare to graphene sheets. In order to study the effect of plasma parameters, carbon films were prepared under two different plasma densities (2.5 and 12.5 mA/cm2) and different substrate biases (25 to 500 V). It is found that by applying high ion density plasma, nanocrystals are formed at room temperature even at low substrate biases (300 V). Meanwhile, decreasing the ion density increases the threshold ion energy for graphitization. More importantly it is experimentally shown that the nature of the nanocrystals strongly depends on the depositing ion energies. High ion energy (higher than 500 eV) results in formation of tubular nanostructures while lower ion energies (300 to 500 eV) results in formation of graphitic nanostructures. Stability of different nanostructures have been discussed in terms of the thermal spike temperature. The experimental results of the formation of different nanostructures have been proven by molecular dynamics simulations. Separately, the properties of textured nanostructure carbon films were also studied. The first property investigated was the electrical conductivity of the films. It is found that formation of preferred oriented nanocrystals results in significant increase in the conductivity. The conduction in the amorphous films is limited through Poole-Frenkel mechanism. Electron will hoop between the conductive sp2 sites. Therefore, the conductivity of the amorphous films is controlled by the amount, size and distribution of sp2 bonded nanocrystals embedded in the amorphous sp3 matrix. Formation of preferred oriented nanocrystals results in the formation of continuous sp2 bonded channels which enhances the conductivity by three orders of magnitude. In order to induce the nanostructures locally a local post deposition treatment is needed. Hence, the application of the laser annealing has been studied. To do this, a wide range of initial a-C structures (from ta-C to high sp2 content a-C films) have been irradiated by a KrF Excimer laser with pulse width of 23 ns. The structural changes have been studied by Raman spectroscopy, TEM and EELS. It has been shown that the behavior of carbon films upon laser irradiation strongly depends on the initial bonding structure of the films. Using high sp2 content a-C film as the initial structure, results in the formation of graphitic nanocrystals at moderate laser energies (higher than 360 mJ/cm2). However, ta-C films are stable even at higher laser energies. Field emission and thermal conductivity of textured carbon films have also been investigated. It is found that formation of conductive sp2 channels throughout the thickness of the films which is achieved by the formation of texture in the microstructure, affects the emission threshold field significantly. This is mainly due to simultaneous activation of two field enhancement mechanism namely the presence of highly conductive phase (sp2 bonded filaments) embedded in an insulative (amorphous sp3) matrix and the formation of high aspect ratio graphitic filaments. As such, the threshold emission filed on an a-C film decreased from 12 to 3.5 V/μm by a single nsec laser irradiation at 462.5 mJ/cm2. It is also shown that thermal conductivity of carbon films depend on the microstructure of the films. Pulsed photothermal reflectance (PPR) has been used to study the thermal conductivity of carbon films deposited at different temperatures (amorphous and nanocrystalline carbon films). It is found that formation of highly conductive graphitic nanostructures perpendicular to the substrate, increases the thermal conductivity. Compare to thermal conductivity of a-C films (~ 1 W/m.K) textured carbon films show an order of magnitude ( upto 17 W/m.K) increase in thermal conductivity.