The objective of this work was to select and process an electrically insulating thin film suitable for the photoelectrodes used in solar to chemical energy conversion where the current flow would occur by quantum tunneling. A number of aluminum oxide coatings with various processing parameters were deposited onto silicon to explore the relationships between the deposition variables and the performance of the electrodes in 1 N H\sb2SO\sb4 aqueous solution subjected to visible light. The oxide thin films deposited by ultra high vacuum (UHV) reactive evaporation technique possessed 70-90% transmittance in UV-visible light range. Their optical transmittance (τ) and the open-circuit photopotentials (V\sboc) were found to depend on the deposition variables. The film growth on single crystal Si occurred from initially oriented islands and clusters, to a coalesced amorphous film as the atomic coverage increased. The surface compositions and depth profiles showed clean and uniform films. It was found that the oxide films can improve the open-circuit photovoltage of silicon and also act as anti-reflection (AR) coatings on Si. (Abstract shortened by UMI.)