This thesis deals with a new method to engineer stoichiometric gallium arsenide (GaAs) [100] surfaces by deposition of organic monomolecular films, which can be used as stable and functional platforms for the design of novel bio-inspired semiconductor devices. Here, instead of commonly used inorganic insulators, a new class of self-assembled monolayers (SAMs) composed of 4-mercaptobiphenyl derivatives (X-MBPs) is used to stabilize the GaAs/electrolyte interface as well as to functionalize the GaAs surface. These functional organic domino-like molecules with rigid and bulky biphenyl backbones are chosen to accomplish high surface stabilities against the degradation in air and in water, and to provide various chemical functions to the surface via flexible 4-substitutions. In Chapter 4, two dominant parameters, solvent polarity and temperature, which determine the film qualities were systematically changed to optimize the grafting conditions of the X-MBPs. Qualities of the SAMs were evaluated in terms of the thickness using ellipsometry and the electrochemical stability checked by impedance spectroscopy. In Chapter 5, the optimized SAMs and the bare GaAs (prepared by wet chemical etching) were characterized using various surface sensitive techniques in dry states, i.e. either in ambient or in vacuum. Atomic force microscopy (AFM) implied that the surfaces of SAMs have the comparable smoothness to bare GaAs, while ellipsometry measurements verified the monolayer formation. Near edge x-ray adsorption fine structure (NEXAFS) spectroscopy yielded the tilt angle of highly ordered MBP backbones to the surface normal. Furthermore, high resolution x-ray photoelectron spectroscopy (HRXPS) confirmed the covalent binding of sulfur and arsenide, and demonstrated the high chemical stability of the engineered surface against the oxidation in ambient. In Chapter 6, the functionalized GaAs surfaces were characterized in contact with water. Firstly, the surface free energies were calculated from the contact angles of different liquid droplets, which provide quantitative measures for the surface compatibilities to lipid membranes and biopolymers. In the next step, the electrochemical characterizations of the functionalized GaAs in physiological electrolytes were carried out by cyclic voltammetry and AC impedance spectroscopy. Cyclic voltammetry demonstrated that the grafting of SAMs resulted in a significant reduction of charge transfer across the GaAs/electrolyte interface. Furthermore, impedance spectroscopy experiments denoted excellent electrochemical stabilities of the monolayer-coated GaAs in physiological electrolytes for more than 20 h. Chapter 7 introduces the direct application of the established engineering protocol on the planar FET with a Hall-bar configuration, where the two-dimensional electron gas (2DEG) is confined in the vicinity of the GaAs surface. Here, the molecular dipoles from 4-substituents and the solvent polarities were found to influence the sheet resistance of the 2DEG significantly, suggesting that the functionalized FETs are highly sensitive to the surface dipole moments. The surface engineering method developed in the present study demonstrated a reliable device stability for the operations of various GaAs-based semiconductors with well defined surface characteristics, and suggests its large potentials to fabricate new biofunctional devices by deposition of biopolymers and model cell membranes.
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This thesis deals with a new method to engineer stoichiometric gallium arsenide (GaAs) [100] surfaces by deposition of organic monomolecular films, which can be used as stable and functional platforms for the design of novel bio-inspired semiconductor devices. Here, instead of commonly used inorganic insulators, a new class of self-assembled monolayers (SAMs) composed of 4-mercaptobiphenyl derivatives (X-MBPs) is used to stabilize the GaAs/electrolyte interface as well as to functionalize the Ga...
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