The chemical and physical control of semiconductor surfaces is crucial for various applications, including the performance enhancement of field-effect transistors, photocatalysts, and photocathodes. Despite significant advancements, there remains a need for in-depth research on various surface processes and characteristics of semiconductors. This thesis concentrates on the surface control of semiconductor photocatalysts and photocathodes, utilizing X-ray photoelectron spectroscopy and scanning tunneling microscopy.Investigation into the surface fluorination mechanism of rutile TiO2 (110) was performed. A mechanism akin to the Cabrera-Mott theory was proposed, where fluorination reduces surface charge density and induces an electric field. This field causes Ti cations to migrate to the surface, where they react with XeF2 and O2. Surface fluorination results in an atomically clean and non-stick surface, both before and after water rinsing. Additionally, this fluorination reaction is photo-switchable due to the photocatalyzed removal of the TiO2 surface carboxylate layer. Furthermore, the development of a method to protect photocathodes with atomically thin coatings, such as single-layer graphene and hexagonal boron nitride, was discussed. The feasibility of this method was proved by fabricating protected Mg photocathodes and detecting photoelectrons through the graphene layer. However, extending this approach to protect Cs3Sb photocathodes presented challenges, including the creation of clean substrates for photocathode growth and the nucleation of Cs3Sb on graphene and hexagonal boron nitride. These challenges require further investigation. Additionally, the surface chemistry of CsI-activated GaAs was investigated. Contrary to the conventional “yo-yo” activation method, the most stable oxide of Cs, Cs2O, was absent from the surface after annealing. Cs suboxides, such as Cs2O2 and CsO2, which possess lower work functions than Cs2O, were present in the activation layer. This hypothesis suggests a promising activation method for GaAs, potentially avoiding the formation of high work function Cs2O.