The formation of barriers to charge transfer at semiconductor interfaces has been a focus of considerable research, much of it led by AVS scientists, for over fifty years. While early work centered on the intrinsic physical properties of the semiconductor, ultrahigh vacuum surface studies revealed the importance of extrinsic, interface-specific effects in understanding the systematic behavior of these Schottky barriers. Without intervening adsorbate layers, chemical reactions and interdiffusion can occur, even near room temperature, which alter the interface region, introducing new phases, crystal defects, and localized electronic states. Surface science techniques display local atomic bonding that depends systematically on thermodynamics and a qualitative transition between reactive and unreactive interfaces. Indeed, atomic-scale interlayers that change such chemistry introduce macro-scale electronic effects. These effects increase in importance as electronic structures shrink to nanoscale dimensions and interfaces constitute much of the entire structure. Detecting these states at or near interfaces requires techniques orders of magnitude more sensitive than conventional surface science provides. Low energy, nanoscale depth-resolved cathodoluminescence spectroscopy (NDRCLS) yields this capability, revealing states at the surface as well as tens to hundreds of nanometers below. DRCLS studies show the importance not only of interface chemistry but also crystal defects. Defects both resident in the semiconductor as well as created by interface reaction and diffusion can impact Schottky barrier formation. GaN, AlGaN, SiC and ZnO - metal junctions provide representative examples of chemically-induced defects near interfaces. Indeed, such defects appear to play a role in limiting Schottky barrier heights for semiconductors in general. These systematics suggest new ways to predict and control Schottky barriers in the nanoscale regime.