Mixed Functionality Semiconductor Surfaces: Formation, Characterization, Interfacial Dynamics, and Applications

• Main Author: O'Leary, Leslie Esther
• Format: Others
• Published: 2012
• Online Access: https://thesis.library.caltech.edu/7124/5/Leslie_OLeary_2012May9_Thesis.pdf; O'Leary, Leslie Esther (2012) Mixed Functionality Semiconductor Surfaces: Formation, Characterization, Interfacial Dynamics, and Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Y5DE-5W95. https://resolver.caltech.edu/CaltechTHESIS:06012012-205928241 <https://resolver.caltech.edu/CaltechTHESIS:06012012-205928241>

<p>The properties of any semiconductor device rely on the charge separation characteristics at interfaces within that device. The charge separation characteristics include relative energetics, interfacial electronic states, and the presence or absence of insulating layers. More importantly, the interfacial properties determine the maximum solar conversion efficiency for a photoelectrochemical or photovoltaic device. Solution-based halogenation/alkylation chemistry was used to functionalize Si surfaces. The chemistry was adapted to allow for the controlled formation of multicomponent molecular monolayers. Functional molecules were incorporated by the mixed monolayer approach, and lowered densities of surface electronic defect states and increased resistance toward the formation of deleterious Si oxides were observed. Heck coupoling reactions were developed at thiophene-containing monolayers. Thiophene terminated Si(111) surfaces had defect frequencies of &#62; 1 defect per 1,000 surface atoms, too large for solar energy conversion applications, while multicomponent CH₃/thiophene monolayers had defect densities of &#60; 1 per 500,000 surface atoms. Robust secondary chemistry at Si(111) with facile charge transfer to covalently linked molecules with preservation of surface electronic properties was shown for the first time. Molecular adsorbates with interesting electronic dipoles, such as bromothiophene, were incorporated into mixed monolayers. The electron distribution across the surface dipole caused a shift in the work function of Si by &#62; 600 mV. The fundamental mechanism of W_f shift was elucidated by a combined ab initio and experimental study, and the dependence of Si band-edge positions on pH was relieved using Si-C bonds. Designer surface chemistry was used to covalently link Si microwires within a exible PDMS matrix, and a direct correlation between the surface bonding mechanism and interfacial adhesion strength was unambiguously observed. The formation and electronic properties of Si/PEDOT junctions were studied, where PEDOT was covalently linked to the Si surface via electropolymerization initiation at a mixed molecular monolayer containing 2,2':5',2"-terthien-5"-yl- groups. Low resistance, ohmic contacts were made at p-Si. Aldehyde groups were also incorporated into mixed monolayers. Facile, low-temperature atomic layer deposition (ALD) of Al₂O₃, MnO₂,and TiO₂ on aldehyde-functionalized Si was achieved. Neither surface oxidation nor surface electronic defects formed during ALD, which has not been shown previously.</p>