# Surface Passivation of Crystalline Silicon Wafer Using H\({}_{2}\)[PERSON]
## 3 Results and Discussion  
Passivation quality was measured with a Sinton WCT-120 instrument. It is widely known that silicon wafers show huge fluctuations in minority carrier lifetime. Due to wafer quality fluctuations, the gain in minority carrier lifetime is used to represent passivation quality. After the wafer cleaning procedure, H\({}_{2}\)S annealing experiments were carried out to investigate the relationship between temperature and passivation quality. Figure 1 and Table 1 show the minority carrier lifetime gain vs. temperature. In this minority carrier lifetime gain-temperature plot, 2 peaks were observed; one is 1610% at 525 \({}^{\circ}\)C, and the other one is 2030% at 600 \({}^{\circ}\)C. [PERSON] found that H\({}_{2}\) is the desorption product of thermal decomposition of H\({}_{2}\)S on silicon [9]. Later, [PERSON] experimentally proved that passivation quality on silicon is related to the combined effect of hydrogen and sulfur atoms [14]. Hence, the 2 peaks in the table probably stem from the same passivation mechanism as reported by [PERSON]. Compared with the highest minority carrier lifetime gain (2750%) obtained by [PERSON], the H\({}_{2}\)S passivation quality seems inferior. However, it should be noted that minority carrier lifetime gain is strongly related to the silicon wafer properties itself. Specifically, the differences in material properties between the monocrystalline silicon wafer and the multicrystalline silicon wafer used in the previous case most likely resulted in the different upper limits of minority carrier lifetime gain. A large number of bulk defects are known to exist along the grain boundaries in multicrystalline silicon [18,19]. However, in monocrystalline silicon, the absence of grain boundaries means that these kinds of bulk defects are practically negligible. In other words, monocrystalline silicon is superior to multicrystalline silicon in terms of bulk defects. This is also proven by the low initial minority carrier lifetime of multicrystalline vs. monocrystalline silicon. Hence, a more profound gain in minority carrier lifetime can be obtained on multicrystalline silicon by the same method. Moreover, a strange phenomenon was observed: the minority carrier lifetime gain increased again when the annealing temperature was higher than 650 \({}^{\circ}\)C. To elucidate this phenomenon, XPS characterization was used to analyze silicon surface composition and further investigate the passivation mechanism. Figure 2 shows the S 2p XPS spectra for the H\({}_{2}\)S-passivated Si surface. Most conditions exhibit peaks at 162.3 eV, corresponding to Si-S bonds [20]. At 500, 525, 550, and 600 \({}^{\circ}\)C, the peaks' intensity exhibits the same tendency as the minority carrier lifetime gain when the annealing temperature increases from 500 \({}^{\circ}\)C to 600 \({}^{\circ}\)C. For example, the highest and second-highest peak perfectly corresponds to the largest and second-largest minority carrier lifetime gain. This means that sulfur plays a dominant role in passivation performance. However, it seems that the 550 \({}^{\circ}\)C condition does not follow this trend. It exhibits a lower Si-S peak intensity than that seen at 500 \({}^{\circ}\)C, but a larger minority lifetime gain. Both hydrogen and sulfur are believed to play important roles in passivation performance. Thus, it is possible that the more positive passivation effect of hydrogen compensates for the loss of sulfur and enhances the total passivation performance to a higher level than that seen under 500 \({}^{\circ}\)C conditions. However, there is no peak at 162.3 eV at 700 \({}^{\circ}\)C, meaning that no Si-S bonds form on the Si surface after 700 \({}^{\circ}\)C H\({}_{2}\)S annealing treatment. However, a high minority carrier lifetime gain was observed in reality, which contradicted the conclusion that sulfur plays a dominant role in passivation performance. The answer to this conundrum can be found in Figure 3. The peaks at 103.8 and 99.1 eV correspond to SiO\({}_{2}\) and Si, respectively [21].