The challenge to embed a single monolayer of phosphorus during epitaxial gallium arsenide (GaAs) growth triggers numerous questions regarding practical preparation, effective analysis, and fundamental consideration of the resulting interlayers. Beyond better understanding of III-V heterointerface formation processes, precise interlayer incorporation may enable enhanced interface design, effective diffusion barriers, and advanced band structure engineering. We employ metalorganic vapor phase epitaxy (MOVPE) in various growth modes (continuous, with interruptions, pulsed, surface exchange) targeting the most abrupt incorporation of thinnest GaP films in the GaAs(100) matrix. The intensities of higher order interference fringes in high resolution X-ray diffraction (HR-XRD) serve as a measure of the effective GaPxAs1-x film thickness and P concentration, which is compared to compositional analysis based on scanning transmission electron microscopy (STEM). In situ reflection anisotropy spectroscopy (RAS) provided us with insights to the GaAs(100) surface configurations relevant during the P interlayer preparation.