Kelvin probe force gradient microscopy of charge dissipation in nano thin dielectric layers

Abstract

Application of Kelvin probe force gradient microscopy (KPFGM) to visualize the local charge dissipation in thin dielectric layers is considered. By this method, the local charge behavior in nano thin SiO2, Si3N4, and LaScO3 dielectric layers has been studied. Local charging of the layers has been performed at the point contact with a conductive probe. KPFGM potential images reveal variations of the surface potential in the locally charged areas, which makes it possible to detect the injected charge and to study its behavior. Special experiments on the SiO2 layers with embedded Si-nanocrystals, when lateral spreading of injected charge had been suppressed, permitted to demonstrate high (better than 20 nm) lateral resolution of KPFGM observations. A simple electrostatic model has been developed to estimate the total amount of injected charge. The obtained estimations made it possible to control charge retention in the dielectric layer and possible leaks into the substrate. The studied dielectric layers demonstrate a broadening of the charged area with time t, proportionally to t1/2, what indicates the domination of the diffusion mechanism in charge lateral spreading on the large time scale. These observations permitted to determine the diffusion coefficients, mobilities, and diffusion activation energies for charges in the studied dielectric layers. To obtain the correct information on the injected charge behavior, the parasitic charge dissipation through the surface film of adsorbed water should be reduced to a negligible level. It was achieved by working in moderate vacuum conditions with an additional sample heating.