  The efficacy of oxygen (O) surface terminations on diamond is an important
factor for the performance and stability for diamond-based quantum sensors and
electronics. Given the wide breadth of O-termination techniques, it can be
difficult to discern which method would yield the highest and most consistent O
coverage. Furthermore, the interpretation of surface characterization
techniques is complicated by surface morphology and purity, which if not
accounted for will yield inconsistent determination of the oxygen coverage. We
present a comprehensive approach to consistently prepare and analyze oxygen
termination of surfaces on (100) single-crystalline diamond. We report on X-ray
Photoelectron Spectroscopy (XPS) characterization of diamond surfaces treated
with six oxidation methods that include various wet chemical oxidation
techniques, photochemical oxidation with UV illumination, and steam oxidation
using atomic layer deposition (ALD). Our analysis entails a rigorous XPS
peak-fitting procedure for measuring the functionalization of O-terminated
diamond. The findings herein have provided molecular-level insights on oxidized
surfaces in (100) diamond, including the demonstration of clear correlation
between the measured oxygen atomic percentage and the presence of molecular
contaminants containing nitrogen, silicon, and sulfur. We also provide a
comparison of the sp2 carbon content with the O1s atomic percentage and discern
a correlation with the diamond samples treated with dry oxidation which
eventually tapers off at a max O1s atomic percentage value of 7.09 +/- 0.40%.
Given these results, we conclude that the dry oxidation methods yield some of
the highest oxygen amounts, with the ALD water vapor technique proving to be
the cleanest technique out of all the oxidation methods explored in this work.
