Electrical and Structural Properties of Twin Planes in Dendritic Web Silicon

We have extensively analyzed dendritic web silicon using EBIC, DLTS, minority-carrier diffusion length measurements, highresolution TEM, and X-ray topography to investigate its electrical and structural properties and compare the results with solar cell efficiencies. The twin planes in the web and their effect on minority-carrier diffusion length were of particular interest to us. Measurements were made on as-grown material and on solar cells. The starting material and the cells were always from the same web strips. For high-resolution electrical measurements we developed cross-sectional EBIC with Schottky contacts formed on the web cross-sectional area. For high-efficiency cells we find flat EBIC linescans across the web cross sections, high minority-carrier diffusion lengths, few dislocations and no defect clusters. For low-efficiency cells we find EBIC linescans of reduced amplitude near the twin planes, low diffusion lengths, many dislocations, and electrically active defect clusters at the twin planes. Excessive recombination at the twin planes seems to limit the efficiency of these cells. In both high- and low-efficiency material, DLTS peaks present in the as-grown material disappear upon cell processing.