Surface Modification and Charge Transfer Studies at Silicon and Gallium Arsenide Interfaces

Citation

Bansal, Ashish (1997) Surface Modification and Charge Transfer Studies at Silicon and Gallium Arsenide Interfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5k6w-kt74. https://resolver.caltech.edu/CaltechTHESIS:07162025-164213008

Abstract

The thesis describes some chemical modifications of Si and GaAs surfaces, as a means of gaining control over the physical, chemical and electrical properties of these surface and of the interfaces formed from these surfaces.

The Current-voltage properties of n-GaAs photoanodes were evaluated in KOH-Se^/2-(aq), CH₃CN-ferrocene (Fc)^+/0, and CH₃CN-methyl viologen (MV)^2+/+ solutions. Chemisorption of transition-metal ions (Rh^III, Rh^III, Co^III, Os^III) onto GaAs has been shown previously to effect improved photoanode behavior for n-GaAs/KOH-Se^/2- (aq) contacts, but it is not clear whether the chemisorbed metal forms a buried semiconductor/metal (Schottky) junction or results in a "hybrid" semiconductor/metal/liquid contact. Metal ion treated n-GaAs photoanodes displayed different open circuit voltages in contact with each electrolyte solution investigated. The role of the chemisorbed metal in the n-GaAs/KOH-Se^/2-(aq) system is, therefore, best described as catalyzing interfacial charge transfer at the semiconductor/liquid interface, as opposed to forming a semiconductor/metal or semiconductor/insulator/metal contact.

The ability to modify Si surf ace without partial oxidation or formation of electrical defects is potentially important. However, little is known about the chemistry of these surfaces under ambient temperature and pressure. A two-step halogenation/alkylation route to chemical functionalization of Si(111) surf ace is described, that allows covalent attachment of alkyl functionalities without concomitant oxidation of the silicon surface. In the first step, a hydrogen terminated silicon surface is chlorinated to obtain a chlorine terminated silicon surface. In the second step, the chlorinated surface is reacted with alkyl lithium or alkyl Grignard to obtain an alkyl terminated surface. The surface of silicon is extensively analyzed using a number of techniques such as XPS, HREELS, IRS, AES, TPD etc. The alkyl terminated surfaces are more resistant to oxidation in air and in contact with wet chemical environments than the H-terminated surface.

Current-voltage and capacitance-voltage measurements of the alkyl terminated surfaces in CH₃OH-Me₂Fc^+/0 indicate that the electrical properties of these surfaces are very similar to those of a H-terminated surface. The alkyl overlayers provide a small resistance to charge transfer across the Si/liquid interface but do not shift the band edges or induce additional surface recombination. I-V characteristics of n-Si/alkyl/Au MIS devices indicate that these junctions behave largely liken-Si/Au Schottky junctions. The efficacy of alkyl overlayers in preventing photooxidation and photocorrosion of n-silicon surfaces was measured in contact with Fe(CN)₆^3-/4-(aq) and with CH₃OH-Me₂Fc^+/0 containing known amounts of water. The alkyl terminated surfaces consistently show better I-V characteristics and lower oxidation than the H-terminated surface, indicating that stability to oxidation had been achieved without any significant compromise in the electrical quality of the silicon surface.

Thesis Files