# Surface Charging on Insulating Films with Different Thicknesses in UPS
## 1 Introduction  
Ultraviolet photoelectron spectroscopy (UPS) is one of the most commonly used techniques for measuring the valence band electron structure of materials. This is particularly important for understanding the interactions between chemical adsorbents and surfaces, as well as for measuring the band structure of inorganic or organic semiconductors [1]. UPS also aids in determining the work function (\(\Phi\)) of metal or semiconductor surfaces [2, 3]. The work function is a crucial physical parameter of materials, directly related to their photoelectric, tribological, oxidation, corrosion, and catalytic properties. The accurate measurement of the work function is also essential in experiments. However, insulating materials are often deemed unsuitable for UPS measurements due to the impact of the charging effect [1, 4]. Based on the photoelectric effect, electrons (i.e., photoelectrons) are continuously excited from the surface area irradiated by ultraviolet light during the measurement process. To maintain the electrical neutrality of the surface, the rate of electron compensation on the surface needs to be sufficiently rapid. However, for samples with poor electrical conductivity, such as insulating materials, the lost electronscannot be promptly replenished, leading to the accumulation of positive charges on the surface, a phenomenon commonly referred to as the charging effect [5; 6; 7]. The impacts of charging effects have been studied deeply with an X-ray photoelectron spectrometer (XPS). The presence of the charging effect reduces the initial kinetic energy of the emitted photoelectrons, causing the peak in the spectrum to shift towards a higher binding energy [8; 9]. Moreover, the spatial non-uniformity of charge distribution results in phenomena such as peak shape distortion, broadening, and intensity attenuation, thereby significantly reducing the reliability of the information obtained from the spectrum [10; 11; 12; 13]. In XPS, a suitable charge compensation method in combination with binding energy (BE) referencing is typically used to avoid the misleading of charging effects on spectral analysis [6; 8; 14; 15; 16; 17]. However, this method is not applicable to UPS measurements due to the limitations of the measurement range of BE [1; 18]. Therefore, researchers investigating valence band structures typically circumvent direct measurements on bulk insulating materials, opting instead for ultra-thin films [1; 19; 20]. However, the film fabrication requires precise thickness control to balance competing requirements: sufficient thickness to suppress substrate photoelectron interference while maintaining thinness to prevent charging effects [1]. However, significant discrepancies persist in valence band configurations across different film thicknesses, with the critical thickness selection criteria remaining inadequately characterized [18]. Furthermore, while reduced resistivity in thin films mitigates charging artifacts [21], the conductivity of the films actually depends on the film composition and on the low thickness limit on the composition of the substrate. Structural/energetic defects inevitably introduced during deposition (e.g., point defects, dislocations, and interfacial strain) further distort electronic structures [22; 23]. These combined factors raise fundamental questions regarding the representativeness of thin-film electronic structures compared to bulk insulators. Consequently, establishing reliable UPS measurement protocols for insulators necessitates comprehensive thickness-dependent studies correlating charge dynamics with spectral evolution. This study investigates the evolution of valence band spectra and charging-induced surface potential in SiO\({}_{2}\) thin films with varying thicknesses obtained by the sputtering method. The observed UPS spectra exhibit a stage-dependent evolution with increasing film thickness.