# Surface Charging on Insulating Films with Different Thicknesses in UPS
## 3 Results and Discussion
### Observations of Photoelectron Spectra  
Through the progressive sputtering removal of SiO\({}_{2}\) films with known thicknesses, samples 1 and 2 underwent valence band characterization using XPS under dual-beam charge neutralization conditions (Figure 2a,c) and UPS without charge neutralization (Figure 2b,d). For XPS analysis, the C 1s peak was deemed unsuitable for charge correction due to the sputter removal of adventitious carbon layers. The O 2s peak was selected for charge correction until significant oxygen depletion occurred [25], after which the literature-reported reference peak for Si-Si bonding was employed during the final stages of sputtering when the Si substrate became exposed. UPS spectra were acquired without charge neutralization and remained uncorrected. The emergence of the O 2s peak in XPS valence band spectra with an increasing film thickness corroborates our thickness determination. XPS results demonstrate stabilized valence band structures beyond critical thicknesses of 16 nm and 8.3 nm for respective samples, indicating uniform SiO\({}_{2}\) film distribution consistent with Si 2p and O 1s narrow spectra analyses. The difference in detection depth between XPS (1-5 nm) and UPS (0.5-2 nm) [1; 9; 26] leads to distinct manifestations of band structure features in their respective spectra, particularly pronounced in ultra-thin films. As shown in Figure 2c,d, when analyzing progressively thicker films, the characteristic band structure features (0-5 eV range) originating from the substrate persist longer in XPS valence band spectra compared to UPS measurements. At a critical thickness of 2.8 nm, these substrate-induced features remain clearly observable in XPS spectra while having completely vanished in UPS spectra. This phenomenon arises from the deeper information depth of XPS, which enables the simultaneous detection of signals from both the film and substrate, whereas the shallower probing depth of UPS restricts its detection to the film layer exclusively. Figure 1: Compilations of (**a**) O 1s and (**b**) Si 2p spectra collected during the sputtering removal process of the SiO\({}_{2}\) film with a thickness of 93 nm (Sample 1). (**c**) The atomic concentration of the constituent elements on Sample 1 as function of the sputtering time measured by XPS. (**d**) The thickness of the remaining film on Sample 1 with a certain sputtering time. Notably, as film thickness continues to increase beyond this critical value, both techniques eventually detect signals solely from the film, resulting in diminished spectral discrepancies. Furthermore, the implementation of effective charge neutralization in XPS significantly minimizes spectral distortions during valence band measurements. In contrast, UPS spectra exhibit more pronounced spectral changes due to charging effects arising from the absence of effective charge compensation (Figure 2b,d). The UPS spectral measurements across varying film thicknesses provide detailed insights into the evolution of electronic band structures and charge accumulation effects. Taking Sample 1 as a representative case (Figure 2b), the UPS spectra evolution can be systematically categorized into three continuous regimes:  
Regime I (0-16 nm, Figure 3a): Initial spectral intensity enhancement with thickness increment contrasts and with the absence of a discernible Fermi edge and the progressive attenuation of band features within 0-5 eV binding energy. This transitional behavior stems from two concurrent mechanisms: (1) electronic structure modification during the substrate transition from p-type Si to a SiO\({}_{2}\)-dominated surface, and (2) deteriorating electrical conductivity exacerbating charge accumulation effects. The characteristic intensity suppression in the 0-5 eV range becomes progressively pronounced with increasing thickness. Regime II (16-32.1 nm, Figure 3b): Spectral profiles exhibit remarkable consistency across this thickness range, indicating the stabilization of both interfacial electronic states and charge compensation dynamics.