Current-voltage plots reveal details of the energy level alignment during photoelectron spectroscopy experiments

The photoelectron current $I_s$ emitted from samples exposed to soft X-rays, such as is the case during X-ray photoelectron spectroscopy (XPS) analyses, is dominated by secondary electrons (SE) with energies not exceeding several eVs. Because of that, both the magnitude and the direction of $I_s$ is highly sensitive to the applied sample bias $V_s$ with the magnitude of just a few volts. By measuring current-voltage characteristics for a series of samples with different work functions ${\varphi }_{SA}$ a clear correlation between ${\varphi }_{SA}$ and the shape of the $I_s-V_s$ curves is demonstrated. While all $I_s-V_s$ plots have a characteristic “reversed S” shape, a clear and consistent shift towards positive $V_s$ values is observed with increasing ${\varphi }_{SA}$. The effect is explained by variations in the contact potential $V_c$ established between the sample and the spectrometer. For all samples there is an excellent agreement between the $V_c$ values (derived by ultraviolet photoelectron spectroscopy from the SE cut-offs) and the critical $V_s$ value at which the photocurrent begins to drop. Hence, simple to perform current-voltage measurements, provide a unique insight into the details of the energy level alignment between sample and the spectrometer. In particular, the sign and the magnitude of the contact potential as well as relative changes in the sample work function can be determined. The knowledge of these parameters is often essential for correct interpretation of XPS spectra.