**2. Experiment methods**

Fig. 1 shows the schematic illustration of the silicon solar cell used in this work. The substrate of polycrystalline silicon thin film is Borosilicate glass, which is 10×10×0.07cm3 in size. A pure tungsten layer of 1.2µm was sputtered on the glass substrate at DC of 500W in

Application of Electron Beam Treatment in Polycrystalline Silicon Films Manufacture for Solar Cell 79

The P-doped polycrystalline silicon absorber of 10cm² was melted and recrystallized by a controlled line shaped electron beam (size in 1×100mm2) as described in Fig.2. The appearance of the sample after recrystallization was shown in Fig.3. The samples are preheated from the backside to 500°C within 2 min by halogen lamps. The electron beam energy density applies to the films is a function of the emission current density, the accelerating voltage and the scan speed. The scan speed is chosen to 8mm/s and the applied energy density changes between 0.34J/mm2 and 0.4J/mm2. To obtain the required grain size, the silicon should be melted and re-crystallized. Therefore, temperature in the electron beam radiation region should be was over the melting point of silicon of 1414°C. The surface morphology of the film, as well as distribution of WSi2 phase under different energy

densities has been investigated by means of a LEO-32 Scanning Electron Microscopy.

 **Without recrystallization**

**Recrystallized area**

Fig. 3. Appearance of polycrystalline silicon absorber after recrystallization

The applied recrystallization energy density strongly influences the surface morphology and microstructure of the recrystallized silicon film. With the energy increasing, the capping layer becomes smooth and continuous and less and small pinholes form in the silicon film. Excess of recrystallization energy density leads to larger voids in the capping layer, more WSi2/Si eutectic crystallites, a thinner tungsten layer and a thicker tungstendisilicide layer. Fig.4 gives the top view of the polycrystalline silicon film after the recrystallization. The EB surface treatment leads to recrystallization to obtain poly-Si films with grain sizes in the order of several 10µm in width and 100µm in the scanning direction as shown in Fig.5. The polycrystalline silicon films in Fig.4 are EB remelting with four different EB energy densities. Area A was treated with an energy density of 0.34J/mm2 (the lowest of the four areas) while area D was treated with an energy density of 0.4J/mm2 (highest of the four

**A B CD**

Fig. 4. Top view of the recrystallized silicon film, with increase of applied energy density

**3. Results and discussion** 

from the left to the right

**3.1 Microstructure of the capping layer** 

areas) on the same nanocrystalline silicon layer.

an argon atmosphere, which has almost the same thermal expansion coefficient of 4.5×10-6K-1 as that of the silicon film (Linke et al., 2004; Goesmann et al., 1995). This tungsten interlayer was used as a thermal and mechanical supporting layer for deposition of the silicon film. Nanocrystalline silicon films were then deposited on the tungsten interlayer by the plasma enhanced chemical vapour deposition process (PECVD) within SiHCl3 and H2 atmosphere. Details of the process were described in References (Rostalsky et al., 2001; Gromball et al., 2004, 2005). The power density used was 2.5W/cm2. The gap in the PECVD parallel plate reactor was 10mm and the substrate temperature was 550℃. The flow rate H2/SiHCl3 is 0.25 to reduce the hydrogen and chlorine content in the film. Boron trichloride (BCl3) was added in the gas for an in-situ p-doping. The process pressure was chosen to 350 Pa for the minimized stress. At the above conditions, the deposition rate up to 200nm/min was obtained. After a silicon film of 15-20μm thickness was deposited, a SiO2 layer of 400nm thickness was deposited on the top of the silicon from SiHCl3 and N2O within 5 min to prevent balling up.

Fig. 1. Structure of thin film silicon solar cell

Fig. 2. Schematic of the linear electron beam recrystallization system (Gromball et al., 2005)

The P-doped polycrystalline silicon absorber of 10cm² was melted and recrystallized by a controlled line shaped electron beam (size in 1×100mm2) as described in Fig.2. The appearance of the sample after recrystallization was shown in Fig.3. The samples are preheated from the backside to 500°C within 2 min by halogen lamps. The electron beam energy density applies to the films is a function of the emission current density, the accelerating voltage and the scan speed. The scan speed is chosen to 8mm/s and the applied energy density changes between 0.34J/mm2 and 0.4J/mm2. To obtain the required grain size, the silicon should be melted and re-crystallized. Therefore, temperature in the electron beam radiation region should be was over the melting point of silicon of 1414°C. The surface morphology of the film, as well as distribution of WSi2 phase under different energy densities has been investigated by means of a LEO-32 Scanning Electron Microscopy.

Fig. 3. Appearance of polycrystalline silicon absorber after recrystallization
