Nanoscale Structural Characterization of Oxide and Semiconductor Heterostructures
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According to a recent report from International Technology Roadmap for Semiconductors (ITRS), semiconductor industry based on silicon Complementary metal–oxide–semiconductor (CMOS) technology is facing challenges in terms of making the device faster with higher density and lower power consumption. To overcome the challenges, various methodologies are attempted using different state variables instead of electric charges, for example, polarization, phase states, and electron spin information. Different materials can also be chosen instead of silicon, for example, carbon, complex metal oxides in 1D or 2D nanostructure formations. A different concept of operating devices is also another option, for example, single electron transistors, spintronics, and quantum electronics. A tremendous number of stages during microfabrication manufacturing for integrated circuits consist of a series of deposition and etching processes. During these processes, unknown problems can arise from the design of their structural geometry. For example, unwanted strain distribution from the electrode patterns can change the electric properties of underlying materials regarding the decrease in charge carrier mobility or increase in leakage current in dielectrics, which all occur in nanoscale. So, it is important to understand the effects of structural phenomena on the electronic properties of materials using nanoscale characterization. The first work shows the changes in electronic property in Si quantum dot devices fabricated on Si/SiGe heterostructure is discussed. The electrode deposition process on the heterostructure surface is necessary for the device operation, but the electrodes also induce external nanoscale strain fields. These strain fields are transferred to the substrate materials via electrode edges and change electronic band structure. The magnitudes of the strain and their impact on changing the band structure are studied. In the second project, the alignment of ferroelectric polarization nanodomains in PbTiO3/SrTiO3 (PTO/STO) superlattice heterostructures is discussed. The PTO/STO nanostructure was created using a focused-ion beam technique. The domain alignment was observed using the x-ray nanodiffraction. A thermodynamic theoretical approach calculates the free energy density of the system to understand the origin of domain alignment. In the final project, the origin of photoinduced domain transformation in PTO/STO superlattices is discussed. Charged carriers are excited by the above-bandgap optical illumination, and transported by the internal electric fields arising from depolarization fields. These photoexcited charge carriers eventually screen the depolarization fields, and the initial striped nanodomain patterns transform to a uniform polarization state. After the end of illumination, the striped nanodomains patterns recover for a period of seconds at room temperature. The transformation time depends on the optical intensity, and the recovery time depends on the temperature. A charge trapping model with a theoretical calculation reveals that the charge trapping is a dominant process for the domain transformation, and the de-trapping process is for the recovery. Simulated domain intensity changes are in good agreements with the X-ray diffraction data.
oxide heterostructures, x-ray diffraction, nanoscale characterization