Defect characterization and investigation of RST-silicon ribbon wafers

Abstract

The electronic and material properties of Ribbon on Sacrificial Template (RST) silicon wafers are investigated in this work. Like all multicrystalline wafers, RST wafers exhibit many grain boundaries, impurities and other crystal defects. The understanding of these defects and their influence on material quality is an important part of the characterization of solar cell material. The minority carrier lifetime of RST samples after POCl3 diffusion and SiNx:Hy hydrogenation is obtained by quasi-steady state photo conductance decay measurements. The distribution of lifetime on a wafer and the influence of grain boundaries and bulk resistivity are studied via photoluminescence imaging. Local defects, revealed with the help of a defect etch, are qualitatively correlated to lifetime values. The crystal orientation and the type of the grain boundaries between the different grains was measured with electron back scatter diffraction. The most common grain boundaries in the investigated area are of coincidence type 3. SiC particles on the silicon surface as residues of ribbon casting are studied by scanning electron microscopy. A focused ion beam cut through a SiC particle reveals the interface structure between these particles and the silicon wafer surface.