In this work, we investigated the chemical bonding states in highly P-doped Si thin films epitaxially grown on Si (001) substrates using high-resolution X-ray photoelectron spectroscopy (HR-XPS). HR-XPS P 2p core-level spectra clearly show spin-orbital splitting between P 2p(1/2) and P 2p(3/2) peaks in Si films doped with a high concentration of P. Moreover, the intensities of P 2 p(1/2) and P 2p(3/2) peaks for P-doped Si films increase with P concentrations, while their binding energies remained almost identical. These results indicate that more P atoms are incorporated into the substitutional sites of the Si lattice with the increase of P concentrations. In order to identify the chemical states of P-doped Si films shown in XPS Si 2p spectra, the spectra of bulk Si were subtracted from those of Si: P samples, which enables us to clearly identify the new chemical state related to Si-P bonds. We observed that the presence of the two well-resolved new peaks only for the Si: P samples at the binding energy higher than those of a Si-Si bond, which is due to the strong electronegativity of P than that of Si. Experimental findings in this study using XPS open up new doors for evaluating the chemical states of P-doped Si materials in fundamental researches as well as in industrial applications. (C) 2018 Elsevier B.V. All rights reserved.