The present work studies the disorder in ion-implanted and ion-etched GaAs semiconductors. The primary targets in this study consist of two types of systems:45-keV Be⁺-implanted GaAs and low-energy Ar⁺-etched GaAs. Electronic and lattice structural disorder in these systems are investigated by means of optical reflectivity measurements and Raman-scattering techniques. Visible-ultraviolet reflectivity measurements have identified finite-size effects on the interband electronic excitations in microcrystalline GaAs (μ-GaAs), which is known from previous work to exist in Be⁺-implanted disordered GaAs. The optical properties of μ-GaAs differ appreciably from those of the bulk crystal, the difference increasing with L⁻¹, the inverse of the characteristic size of the microcrystals. The linewidths of the prominent interband features E₁, E₁+∆₁, and E₂ increase linearly and rapidly with inverse microcrystal size: Γ_μ = Γ₀ + AL⁻¹, where Γ₀ (Γ_μ) is the linewidth in the bulk crystal (μ-GaAs), and A is a constant. A simple theory is proposed which semi-quantitatively accounts for the observed size effects. Small microcrystal size implies a short time for an excited carrier to reach, and to be scattered by, the microcrystal boundary, thus limiting the excited-state lifetime and broadening the excited-state energy. An alternative uncertainty-principle argument is also given in terms of the confinement-induced k-space broadening of electron states. The near-surface structural disorder in Ar⁺-etched GaAs has been investigated using a combination of Raman scattering and optical reflectivity measurements. The longitudinal optical (LO) Raman mode in the ion-damaged medium preserves its crystalline lineshape, indicating that the crystalline long-range order is retained in the disordered structure. The structural damage is depth-profiled with LO Raman intensity measurements together with wet chemical etching. A graded damage model proposed in the work well explains the observed LO intensity in the ion-damaged, chemical-etched GaAs. The reflectivity measurements qualitatively support the Raman scattering findings. In addition, the reflectivity spectrum exhibits a red-shift of the peaks associated with the interband electronic transitions. Such a peak shift is likely to arise from the electron-defect interaction in the disordered surface medium. / Ph. D.