In recent years, Nickel Oxide (NiO) thin films have drawn considerable attention due to their wide range of applications, e.g. as gas sensors, in fuel cells, as electrochromic devices, in magnetoresistance heads etc. NiO properties are responsible for its versatility: for example, a) stoichiometric NiO is an insulator at room temperature b) non-stoichiometric NiO is a p-type semiconductor c) its conductivity can be modified through doping d) its has good chemical stability e) it belongs to the category of Transparent Conductive Oxides (TCO), which are used today in the most developed fields of industry such as microelectronics, optoelectronics, photovoltaic devices etc.
The subject of this MSc thesis is the growth of NiO thin films and the correlation of their structural, electrical and nanomechanical properties as well as their dependence on the deposition parameters. NiO thin films were grown on Si substrate, which had a thin oxide layer on the surface, by Pulsed laser Deposition (PLD). The deposition took place in a vacuum chamber with oxygen flow. Oxygen played the role of reactive gas and its pressure was kept at 10 Pa. To find a correlation between the structural, electrical and nanomechanical properties film properties, a common link to a deposition parameter is necessary. Such a common parameter revealed the substrate temperature (Ts). Therefore, NiO thin films were grown at room temperature (RT) and at Ts= 100, 200, 300, 400οC. Hence, the purpose the present work was to elucidate the influence of substrate temperature during deposition on the structural, electrical and nanomechanical properties of the film.
It proved that the structural properties are affected by the substrate temperature. Its increase results into the improvement of the crystalline structure which means bigger crystallite size, which is in the order of tens of nm. Bigger grain size results into fewer defects which make NiO thin films appropriate for microelectronics applications. Furthermore, a higher substrate temperature results into a thicker NiO film that is due to increasing grain size. The film thickness was estimated to around 400 nm for the higher temperatures. Calculation of resistivity showed that there is a minimum while the substrate temperature changes.
For the first time, the nanomechanical properties were studied and it was found that the improvement of crystalline structure affects the nanomechanical properties. Experiments of nanoindentation were carried out and showed that hardness (H), elastic modulus (E) increase with increasing substrate temperature, though, surface roughness decreases.
Finally, for the purpose of this work, an improved apparatus of measuring the electrical film properties by a four point method was developed. This was done in order to achieve greater precision of the values of resistivity.
The films have been grown in the LATA lab and the nanomechanical measurements in the School of Chemical Engineering Department/National Technical University of Athens.
For details, see:
"Structural, electrical and mechanical properties of NiO thin films grown by Pulsed Laser Deposition". I. Fasaki, A. Koutoulaki, M. Kompitsas, C. Charitidis. Applied Surface Science 257 (2010) 429.