In this thesis, the development of high surface quality, broad bandwidth, low cost antireflection coatings using the low optical index GaAs native oxide processed by a new oxygen-enhanced wet thermal oxidation method is demonstrated. The thicknesses and optical constants of the GaAs native oxide and the GaAs substrate used in the AR coating design have been determined by Variable Angle Spectroscopic Ellipsometry measurements within the visible light wavelength range at room temperature. The antireflection coatings have a simulated reflectance of <10% over bandwidths as large as 300 nm and a minimum reflectance of 2.3% for a design wavelength range of 400 to 800 nm. Good agreement between the simulated and measured reflectance data supports the validity of the modeling and indicates that a smooth interface is achieved between the semiconductor and the thermally grown native oxide.