## 3 Results and Discussion
The results showed that as the laser fluence increased, the doping level increased and sheet resistance decreased. A heavily doped emitter with a sheet resistance of approximately 11-48 \(\Omega\)/sq was obtained. Under a laser fluence of 0.3 J/cm\({}^{2}\), the laser process failed. The laser energy was used to melt Si, and the diffusion of the dopant atoms occurred in the liquid phase [13]. However, the energy was insufficient to allow for diffusion under a laser fluence of 0.3 J/cm\({}^{2}\) or less. At a laser fluence of 0.8 J/cm\({}^{2}\), the sheet resistance was reduced by 32.92 \(\Omega\)/sq. In addition, it was found that a heavily doped region with a sheet resistance of 11.38 \(\Omega\)/sq was formed under a laser fluence of 2.25 J/cm\({}^{2}\) because more dopants entered the emitter layer and sheet resistance decreased. In the case of laser doping, damage to the surface of a crystalline wafer is inevitable. Here, the damage caused by laser irradiation on the sample surface was evidenced by cracks, debris, and a melted surface; dislocations and cracks were also observed at the bottom of the sample. In this study, the surface damage caused by laser irradiation was analyzed with SEM. Figure 4: Sheet resistance of the emitter layer and its decreasing magnitude versus laser fluence. Figure 5 shows a cross-sectional image of the c-Si wafer with respect to laser fluence. The relationship between surface characteristics and laser fluence after laser doping was analyzed. The results confirmed that when the laser fluence was excessive, the c-Si wafer was destroyed, and the surface characteristics deteriorated due to melting. In addition, a heavily doped region was obtained without breaking the shape of the Si surface pyramid at laser fluencies of 0.8 and 1.29 J/cm\({}^{2}\), so the laser fluence was fixed at those values while the laser speed was varied from 10 to 70 mm/s in order to make an accurate comparison between the two fluence conditions. The next important parameter was laser speed, which was determined based on the X-Y stage motion speed. Reducing the laser speed increases the percentage of overlap among laser pulses, which can be expressed as \(\left(1-\frac{V}{\gamma\times D}\right)\times 100\), where \(V\) is the laser scan speed, which was 10, 40, and 70 mm/s; \(f\) (Hz) is the pulse frequency; and \(D\) (mm) is the laser spot size [14]. A green laser beam with a spot size and frequency of approximately 0.025 mm and 500 kHz, respectively, was used at various laser speeds ranging from 10 (99.92% overlap) to 70 mm/s (99.44% overlap). As the process progressed, a circular wave-shaped process area, called ripple, was formed from the overlapping wave pulses. As the laser speed decreased, a higher degree of overlap occurred during the laser pulse, causing the ripples to decrease in size and smoothen. In addition, increasing the overlap among the pulses increased the laser time on the wafer, which meant that the doped region had a large amount of cumulative energy from the laser. Figure 6 shows the graph of the sheet resistances and their decreasing magnitudes with respect to laser speed after fixing the laser fluence at 0.8 and 1.29 J/cm\({}^{2}\). The change in sheet resistance as a function of laser speed was obtained under a laser fluence of 0.8 J/cm\({}^{2}\); however, this change was small under the laser fluence of 1.29 J/cm\({}^{2}\), which indicated that the dopant diffused into Si could possibly increase with decreasing laser speed under a laser fluence of 0.8 J/cm\({}^{2}\). As shown in the SEM image in Figure 6, under a laser speed of 10 mm/s, for the two laser fluence conditions, the surface structure deteriorated because of the surface melting caused by the application of the laser for an excessive period in the wafer. The sheet resistances were 43.7 and 59.1 \(\Omega\)/sq under laser fluencies of 0.8 and 1.29 J/cm\({}^{2}\), respectively. When the laser speed changed under the laser fluence of 0.8 J/cm\({}^{2}\), there was a change in resistance.