|M.Sc Student||Keren Zohar-Hauber|
|Subject||Pressure Sensitive Thin Composite Films|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Berger Shlomo|
|Full Thesis text|
Our research work focuses on piezoresistive nanocomposite thin films that consist of nickel nano-particles embedded inside hydrogenated amorphous carbon matrix (Ni/a-C:H). The research studies the influence of the nickel content on the microstructure, electrical resistivity and piezoresistivity of the films. The films are deposited in a vacuum chamber using two parallel processes: plasma decomposition of ethylene gas and RF sputtering of a nickel target. The C, H and Ni atoms are deposited on heated (about 300°C) substrates (Si and Al2O3) to a thickness of about 300 nanometers. The content of Ni in the films was varied by changing the ethylene gas flow rate.
Microstructure studies using HRTEM show a non-uniform microstructure along the films thickness which consists of three main layered regions: amorphous carbon close to the substrate followed by equiaxed nanometer size nickel particles and ended with columnar nickel particles. In all regions the nickel particles are surrounded by amorphous carbon phase.
X-ray diffraction patterns show that the nickel particles in all compositions don’t have the FCC crystallographic structure, which is the stable phase of nickel at room temperature, but rather the HCP structure or the rombohedral structure of Ni3C phase. The presence of either of these two phases or both indicates a non-thermodynamically stable deposition process controlled by the process kinetics.
The electrical conductivity of the films was found to be thermally activated and fits the Arrhenius expression. The activation energy for the electrical conduction decreases and the electrical conduction increases with increasing the Ni concentration in the film. The electrical conductivity dependence on the applied voltage-frequency fits the variable range hooping mechanism. The ac electrical measurements as a function of applied voltage frequency in the range of 1kHz to 1MHz, reveal a major dielectric resonance peak at nickel compositions of 0-45at% Ni. The resonance peak position is shifted linearly with increasing the mechanical force on the film. High Gauge Factors (GF), between 200 and 400, were measured using the shift of the resonance peak. These values are higher by one order of magnitude as compared to a GF maximum value of 20 measured under dc electrical measurements, taking into account the dc electrical resistance change under no resonance conditions. It is therefore concluded that using the dielectric resonance is by far a more sensitive tool to evaluate applied force on the Ni/a-C:H films, which can be applied for measuring ultra-small or high-frequency applied forces.