Optical Investigation of Nanoscale Electronic Inhomogeneities in Thin Films

Abstract

In this dissertation, nanoscale electronic inhomogeneities in thin film layers (oxide, metal, and semiconductor) are explored using nano-infrared (nano-IR) spectroscopy and optical pump terahertz probe spectroscopy (OPTP) techniques. First, We investigated structural and electronic inhomogeneities in a VO2 thin film grown on a (001)-oriented TiO2 substrate by exploiting nano-scale and macroscopic probing techniques. A compressive strain along the out-of-plane direction becomes ad- ditionally relaxed via microcracks which form a micron-sized rectangular pattern. A large inhomogeneity in the dielectric response is observed near the crack, and this signi- fies a strong coupling between electronic and lattice degrees of freedom. Interestingly, the strong inhomogeneity is observed also inside of the rectangular pattern, and it shows a gradient along one crystalline axis. We attribute such peculiar inhomogeneity observed in a relatively large length scale possibly to a combined effect of the strain relaxation and an oxygen vacancy distribution. As the nano-scale inhomogeneities in structural and electronic properties will eventually determine macroscopic responsiv- ities, this work can be a good guide in designing VO2 thin films with appropriate controls of the strain and the chemical composition to realize better functionalities. Second, We visualized surface plasmon in poly- and single-crystalline Copper (Cu) films by exploiting nano-infrared imaging. We clearly observed oscillating patterns in both films which are attributed to the surface plasmon launched from the film edge and the atomic force microscope tip. The surface plasmons observed for poly- and single-crystalline Cu films have different oscillating periods for the given wavelength of incident beam, and different slopes of the surface plasmon dispersion. These behaviors could be understood by a corresponding difference in dielectric constants of the dielec- tric layer on top of the Cu films; a relatively smaller dielectric constant is required to fit the surface plasmon’s dispersion relation of the single-crystalline Cu film implying that the oxidized layer formed on the Cu film surface is thinner than for the poly- crystalline film. This result is in good agreement with the previous observation about the robustness of the single-crystalline Cu film against the surface oxidation. Third, We monitored the distribution of boron ions in a silicon (Si) wafer near the surface by utilizing pump-energy dependent OPTP techniques. 800 nm fundamental laser pulse and its second- and third-harmonic waves are used to adjust the probing depth corresponding to 8000, 80, and 8 nm, respectively. We obtained photoconductiv- ity spectra fitted by Drude-Smith model describing the backscattering or localization of free carriers in the terahertz range. In addition, We observed a significant increase in the scattering rate and backscattering parameter at Boron-implanted regions, and this tendency follows the depth profile of Boson. Therefore, We have shown that infor- mation about the heterogeneous distribution of impurities can be obtained based on photoelectron dynamics, which we expect to provide a new direction for non-destructive measurement of impurity distribution in a semiconductor.