Measurements of metal-atom coverage, geometric structure, and electronic properties of ultrathin metal-semiconductor interfaces during all stages of growth can be used to relate microscopic structural properties to the behavior of the macroscopic interface. This includes the fundamental aspects of the transition from semiconducting to metallic behavior and interfacial optical response. Using a wide array of surface science techniques, we have studied the electronic and structural properties of alkali-metal - GaAs(110) interfaces to probe interfacial metallization as a density-dependent phenomenon with single atomic layer precision. The first alkali-metal layer on the semiconducting GaAs(110) surface was found to be non-metallic, even at metallic areal densities, and spectroscopic analysis of this interface suggests that this pre-metallized phase is a Mott insulator. Growth of a second alkali-metal layer produces metallization with a continuum of states in the band gap and the development of an interface plasmon. To relate geometric structure to electronic properties, we have demonstrated the occurrence of an alkali-induced unrelaxation of the underlying substrate surface structure for the K/GaAs(110) interface.