Local electrostatic potential and strain characterization of semiconductor nanostructures
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Materials characterization techniques that determine the local charge transport properties of electronic devices are of increasing importance. Dopant profiles, electrostatic potential distributions near interfaces, strains, and defects within nanoscale devices should be characterized very locally, two-dimensionally and quantitatively for a complete understanding of device characteristics. Transmission electron microscopy (TEM) has the ability to resolve the atomic structure of materials, but conventional TEM methods do not give all the information that would be desirable for complete device characterization. This dissertation examines two advanced phase reconstruction techniques that can be used to characterize electrostatic potentials and strains in semiconductor nanostructures. Electron holography was utilized to measure the electrostatic potentials associated with charges and their distribution within a core/shell nanowire. Electron holography was optimized for the nanowire geometry using a dual-lens imaging configuration. Using this method, the mean inner potential of intrinsic germanium and its oxide were determined to be 15.50 ± 0.44 V and 9.10 ± 0.42 V, respectively. The B concentration within the B-doped shell of the core/shell nanowire was determined through a comparison of measured and simulated phase profiles. Fermi level pinning at interface states between the doped shell and the inner germanium oxide was also observed by electron holography. The screening length and the potential in the interface charge region were quantitatively measured. These characteristics compared favorable with the values obtained from numerical solution of Poisson’s equation. The local strain in a strained silicon (sSi) wafer was characterized using geometric phase analysis of high-resolution TEM (HRTEM) images. The method enables the reconstruction of strain maps from HRTEM images by digital image processing alone, when the HRTEM images were taken under careful controlled imaging conditions. Using specimens with known strain values, this method was confirmed to give a reliable, quantitative measure of strains in a sSi structure. Geometric phase analysis was also applied to real sSi layers grown on relaxed SiGe alloys containing either 43.9 or 17.7 atomic percent Ge. The defects and stress relaxation of these wafers were also analyzed.