Raman Spectroscopy for Analysis of Silicon Precursor Layers for Liquid Phase Crystallized Solar Cells

Abstract

Amorphous silicon thin films based solar cells have a low efficiency comparing to the conventional poly-crystalline silicon wafer based solar cells. To overcome the efficiency limitation, the attempt to crystallize the amorphous silicon thin layers deposited by Plasma enhanced chemical vapor deposition (PECVD) on glass is investigated. the crystallization process is based on zone-melting technology and called liquid phase crystallization (LPC). Raman spectroscopy is used to analyze the properties of the silicon thin layers that is deposited by PECVD process and crystallized by LPC process; for better understanding of these processes. The present thesis provides the possibility of engaging Raman spectroscopy as a characterization method with MATLAB software as the analysis tool to investigate the properties of thin silicon precursor layers for liquid phase crystallized solar cells. The exported data after analysis has been plotted into two different categories; depth profiling as well as two-dimensional mapping. The first category includes three samples deposited on glass by plasma enhanced chemical vapour deposition with different deposition parameters (e.g.: silane concentration, rf power, and pressure) to investigate the influence of these parameters on having amorphous or micro-crystalline structure, as well as hydrogen content and micro-structure parameters. The first sample has a µc-Si structure with crystallinity changing with depth. The other two samples have a-Si structures. All samples have nearly a constant hydrogen content and micro-structure parameters with depth. Then, the amorphous samples have been annealed to investigate the influence of annealing on the structural order and hydrogen diffusion. The first one is thermally unstable, only 0.50 µm remains on glass after annealing. While the other is stable and the whole thickness remains unchanged during annealing. All hydrogen has been diffused in both samples. Raman spectroscopy is a useful technique to create data that can be plotted as depth profiles for the crystallinity, structural order, hydrogen content, and micro-structure parameters for PECVD deposited precursors; hence gives a better understanding of the changes in these properties with depth. PECVD parameters of the last sample (SiH4 flow rate of 6 sccm, H2 flow rate of 12 sccm, pressure of 1 mbar, rf power of 25W, and heater temperature of 450 oC) have been taken as the standard, since they deposit an a-Si structure, that is thermally stable after annealing among all samples. The second section includes one sample, which has been deposited by electron beam deposition, cut into five small samples, and then crystallized by liquid phase crystallization with different crystallization parameters (e.g.: laser scan speed and power) to investigate the influence of these parameters on stress inside the crystallized precursors. Three 2D maps have been taken in grain boundaries, where c-Si peak is shifting to lower wavenumbers; hence a tensile stress behavior along grain boundaries is mapped. Two 2D maps have been taken in the middle of a crack, where the stress behavior is varied between a tensile stress on one side of the crack and a compressive stress on the other side. The last 2D maps have been taken at a crack tip, where the crack starts at the surface but does not continue with depth. higher tensile stress has been mapped from the glass side, which does not reach its threshold value to form the crack yet. Raman spectroscopy is a helpful tool in investigate the stress resulted in LPC-Si precursors for better understanding of LPC process. LPC parameters of the first sample (red laser beam of 808 nm wavelength, substrate temperature of 510 oC, laser speed of 1 mm/s, and power of 45W) have been taken as the standard, since they produce a non-cracked precursor with minimum number of grain boundaries among all samples.