Interpretation of X-Ray photoelectron spectra of Ge(111), GeO₂/Ge(111), C₆₀F₁₈/Ge(111) samples using quantum chemical calculations

The valence band of photoelectron spectra of complex samples (multicomponent samples, samples with oxide films, molecules adsorbed on the surface) has a complex structure, which complicates the interpretation of the contributions of various sample components to the spectral structure. A method is considered for interpretation of valence band spectra as a result of calculating the density of electronic states for a physical volume using quantum chemistry — an atomic model that most fully characterizes the sample under study. The optimal position of atoms in a given physical volume of the computational model was found using the iterative Broyden–Fletcher–Goldfarb–Shanno numerical optimization method taking into account the spatial distribution of the potential obtained from quantum chemical calculation. The calculation was performed using code written in Python using the ASE and GPAW libraries (atomic simulation environment and grid-based projector-augmented wave) on the equipment of a supercomputer computing cluster. The data obtained by calculation were compared with the measured photoelectron spectra of various systems, such as Ge(111), GeO₂/Ge(111), C₆₀F₁₈/Ge(111), and C₆₀F₁₈/GeO₂/Ge(111). The analysis made it possible to determine the contributions of various atoms and bonds to the final photoelectron spectrum, estimate the thicknesses of individual layers, and determine the types of bonds between molecules and the substrate.