# Surface Passivation of Crystalline Silicon Wafer Using H\({}_{2}\)[PERSON]
## 3 Results and Discussion
From Figure 3, it is clear that the SiO\({}_{2}\) peak is much higher than the Si bulk peak. This means that the silicon wafer was thoroughly covered by SiO\({}_{2}\). It explains why the passivation performance was still good when there was no sulfur bonded with silicon dangling bonds. It also coincides with previous research showing that sulfur atoms will desorb at high temperatures [22]. Thermal oxidation for silicon oxide growth is widely used in laboratory or industrial production. Here, the 2-zone furnace provided a similar environment. Even though the experiment was conducted in a vacuum environment, a tiny amount of O\({}_{2}\) cannot be evacuated from the furnace. Because surface dangling bonds were not terminated by sulfur, the bare silicon surface is more positive and sensitive to O\({}_{2}\), especially under high-temperature conditions. In the next section, the effects of various annealing temperatures on the wafers' morphology will be discussed. Figure 4 shows SEM images of silicon wafers after H\({}_{2}\)S annealing treatment. In this study, the wafers (100) were processed with saw damage etching by KOH solution. Figure 4a shows an SEM image of a Si wafer without H\({}_{2}\)S annealing treatment. Many square shapes were formed on the wafer surface, which is caused by isotropic etching [23; 24]. Previous research showed that H\({}_{2}\)S treatment would result in the formation of a thin sulfur layer on Si or Ge substrates [20]. The morphology shown in Figure 4b,c is obviously different from that shown in Figure 4a. The silicon surface was covered with a thin layer after 525 \({}^{\circ}\)C H\({}_{2}\)S annealing treatment, as shown in Figure 4b. The identity of this layer was confirmed to be sulfur by the above-mentioned XPS spectra and EDX mapping results. In Figure 4b, the wafer surface is flat, and the boundaries between the square shapes are difficult to distinguish, which can be attributed to the increased thickness of the sulfur film. It can be further concluded that the thin sulfur layer becomes thicker under the higher annealing temperature of 600 \({}^{\circ}\)C, which is consistent with the intensities of the XPS peaks. Based on the above discussion  
Figure 3: XPS spectra of Si 2p for an H\({}_{2}\)S-passivated Si surface (700 \({}^{\circ}\)C). Figure 2: XPS spectra of S 2p for an H\({}_{2}\)S-passivated Si surface. of the XPS spectra, we concluded that the passivation performance reflects the amount of sulfur. Here, the SEM findings show the same result, that passivation performance is related to the amount of surface sulfur. The sulfur thin layer becomes rough again under 700 \({}^{\circ}\)C annealing treatment, as shown in Figure 4d. This finding is following XPS results showing that there are no sulfur atoms on the silicon surface when the annealing temperature is raised to 700 \({}^{\circ}\)C or higher. Silicon-based solar cell passivation performance mainly stems from two factors: termination of surface dangling bonds and rejection of the minority carrier due to the field effect. Herein, the potential second passivation theory was investigated. Electrostatic force microscopy (EFM) is a very useful technique to investigate surface charge-related phenomena, such as the surface field so, EFM was used to measure the silicon wafer surface, as shown in Figure 5. The existence of a fixed charge in SiNx and BSF (Al contact) is known to bring pretty good passivation performance to commercial solar cells [25; 26; 27]. Hence, we suspect that the cause must be related to the surface charge if the possibility of dangling bond termination was denied. The detection and distribution of a surface charge on silicon were first investigated by [PERSON] [28]. The response of surface charge to bias voltage is believed to result in differences in the topography or EFM images. Before measurements were taken, aluminum contact was deposited on half of the samples' area for comparison purposes. Then, forward bias voltage (5 V) was applied to the samples first, and reverse bias voltage (\(-\)5 V) was applied second.