Critical review of band offset determination by X-ray photoelectron spectroscopy for the heterojunctions on diamond substrates

Valence band offsets (VBO) of heterojunctions can be determined by X-ray photoelectron spectroscopy (XPS) directly. XPS VBO determinations for heterojunctions on diamond substrates have been reported by many workers for the last decades. We have reviewed these reports critically and found that none of the results of these reports are rigorous enough, and that most of the results are incorrect. The reasons for this are the following three. First, most of the researchers working on diamond devices are not aware of the fact that the top of valence band (VB) does not coincide with an extrapolation point of leading edge in VB XPS spectrum. It is important to note the special nature of diamond valence band. Second, most of the researchers working on diamond devices are not aware of the fact that the ex-situ samples normally used in XPS measurements are covered with adventitious carbon-containing molecules. This prevents the identification of C 1s XPS peak from the substrate diamond. Third, morphology of sample surfaces is rarely examined. It is notable that the probing depth of XPS is so small (∼ 2 nm) that the sample surface must be uniform in an atomistic scale of ∼0.2 nm.