Electronic properties of pulsed laser deposited amorphous carbon and carbon nitride thin films

This thesis is concerned with the electrical properties of the disordered amorphous material, amorphous carbon and carbon nitride films. At first, hydrogenated amorphous carbon was deposited using rf plasma enhanced chemical vapour deposition with methane as a precursor. Changing the deposition parameter, such as the input power results in the negative self bias modification and thus changes in the properties of the deposited film. The hydrogen condition in the deposition chamber is important for the growth of these films. With the demonstration of an achieve electronic device, a resonance tunnel diode, was reported using pulsed laser deposition; pulsed laser deposited amorphous carbon and carbon nitride thin films were studied. To understand the electrical properties of these films, a wide range of measurements were performed. The surface morphology was examined using atomic force microscopy and scanning tunnelling microscopy. The microstructure was investigated using electron energy loss spectroscopy giving data on the sp2 fraction, density, nitrogen content and Raman spectroscopy showed the degree of sp2 clustering. The band structure was investigated using electron energy loss spectroscopy, scarnning tunnelling spectroscopy and ultraviolet photoelectron spectroscopy, giving information on the density of empty conduction band states, close to the Fermi level and the occupied valence band states. The joint density of states was also measured by ultraviolet-visible-infrared optical transmittance, spectroscopic ellipsometry and Photothermal deflection spectroscopy. Electrical characterisations were carried out using both sandwich and coplanar structures. Pulsed laser annealing of amorphous carbon films was also studied, and the change on the surface morphology, microstructural and electrical properties studied. The conduction mechanism in amorphous carbon films at high electric fields was found to be based on classical Poole-Frenkel conduction, and the dielectric constants estimated from the model were found to be consistent with optical measurements. The neutral trapping centres were postulated to be localised sp2 sites below the conduction band according the analysis of the total band structure. Low field conduction in amorphous carbon films were thought to be controlled by band tail hopping through localised sp2 sites. Laser annealing shows the increase of the number of the sp2 sites which increase the conductivity of the film. However, the sp2 clustering does not necessarily increase the conductivity of the film. The optical band gap in high stress amorphous carbon films can be smaller than the other reports, as a bandtail exists in the bandgap which contributes to the hopping and Poole- Frenkel conduction process. The influence to the nitrogen atoms incorporated to laser deposited amorphous carbon nitride films was also studied. It was found that the nitrogen gas background pressure in the deposition chamber strongly affects the properties of the films. It was demonstrated that a higher nitrogen pressure does not always give rise to higher nitrogen content in the films. Higher nitrogen pressure reduces the velocity of the incident carbon species ablated by the laser, and less dense (less stress) films were deposited. Consequently, the conductivity of the film was reduced. However, the conduction mechanism appears still to be similar to that of amorphous carbon. The analysis of the change in the band structure due to the incorporation of the nitrogen atoms supports the analysis. Thus, the entire band structure of amorphous carbon was linked to the electrical conduction mechanism at both high and low electric fields, including the effect of nitrogen atom incorporation, and pulsed laser annealing. In this thesis we report the highest field effect mobility of a-C and a-CNx films ever reported in the literature of 0.01-0.02 cm2/Vs. This mobility is obtained due to the very high electric field that can be applied to our devices.