Caracterización electrostática de sistemas altamente resistivos mediante microscopía de fuerzas

Laburpena

Electrostatic characterization of highly resistive systems through Scanning Force Microscopy Summary The objectives of the thesis are, on the one hand, to model and interpret the Kelvin Forces Microscopy (SKPM) images of insulating samples with presence of localized charges on its surface, and to study the dependence of the SKPM potential with the tip-sample distance, as well as the lateral resolution obtained when acquiring images. On the other hand, to study in the mesoscale the temporal evolution and the spatial dependence of the surface potential of Al granular thin films of high resistance, when changes in gate potential are applied. As well as studying at the nanoscale the behaviour of the system to obtain the substructure of surface potential. For the experimental measurements, Scanning Force Microscopy (SFM) has been used, while numerical simulations have been performed for the interpretation of the data. Firstly, a spherical tip approach was used based on the image method to analyze the data obtained in SKPM. Secondly, a conductivity-based hopping model based on a network of capacitors has been used to study the charging and discharging of thin films of highly resistive granular metals. As a result, lateral size and total charge domain has been obtained with the proposed model, and the predictions have been confirmed by experimental measurements. Finally, the effects of the tip geometry are discussed, and the most appropriate measurement parameters are determined. It has been shown that the SKPM technique is capable of monitoring the charging and discharging of granular Al films at mesoscopic scale and that the behaviour observed in the surface potential measurements can be reproduced with a hopping conductivity model with percolative parameters. Measurements with SKPM at the nanoscale have shown that granular Al shows a substructure with potential domains of size ?40nm, which do not correlate with the granular structure of the topography. It has been found that domains fluctuate, their dynamics is heterogeneous, and it is characterized by the presence of several characteristic times or scales.