# Surface Charging on Insulating Films with Different Thicknesses in UPS
## 4 Conclusions  
This study demonstrates that the charging dynamics in SiO\({}_{2}\) insulating films during UPS measurements exhibit a three-stage evolution strongly correlated with film thickness. In the transitional phase, the electronic state recombination dominated by interfacial remodeling together with conductivity degradation triggers the characteristic spectral attenuation. As film thickness increases, stabilization occurs through established charge transport pathways, followed by absolute intensity reduction due to the decrease in the quantum yield, while the valence band top (VBM) undergoes a weak shift.  
A direct correlation exists between surface potential evolution and UPS spectral shifts. Benefiting from tunneling-assisted charge compensation, minimal surface potential forms on the film with the thickness range of 0-8 nm. Beyond the critical thickness threshold, there is a sudden enhancement of the surface potential value (3-5 V) due to the growth of film resistivity. Concurrently, the formation of an internal electric field between the charged surface and conductive substrate establishes competing migration pathways for UV-generated holes and compensating electrons under field-driven conditions, thereby suppressing further surface potential elevation through a dynamic charge compensation equilibrium.  
The validity of conventional work function determination methods is constrained by film thickness and substrate properties. While bias voltage application partially mitigates charging effects within a certain thickness, its efficacy diminishes progressively in thicker films and the reliability of the results of the analysis of work function is lost due to the overlap of the spectrometer with the sample cutoff edges. This work further highlights the  
Figure 6: Overlaid UPS spectra obtained during depth profiling of (**a**) Sample 1 and (**b**) Sample 2. Without any charge neutralization method, UPS spectra were performed with bias of \(-5\) V. (**c**) Spectral shifts induced by negative bias and (**d**) work function analysis during depth profiling of Sample 1 and Sample 2.  
necessity of substrate resistivity and strategies in UPS measurements of insulating films to bridge the gap between ultra-thin-film measurements and bulk insulator characterization in valence band spectroscopy.  
Conceptualization, L.Z. and XX; investigation, L.Z. and X.X.; writing--original draft preparation, L.Z.; writing--review and editing, L.Z.; supervision, X.X.; project administration, L.Z.; funding acquisition, X.X. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the National Natural Science Foundation of China (52375166).  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries may be directed to the corresponding author.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} UPS & Ultraviolet photoelectron spectroscopy \\ XPS & X-ray photoelectron spectroscopy \\ BE & Binding energy \\ VBM & Valence band maximum \\ WF & Work function \\ \end{tabular}