Polycrystalline Silicon Wafer Fabricated by Direct Growth from Liquid Silicon

Abstract

A polycrystalline silicon (Si) wafer was grown directly from liquid Si. There are three requirements to achieve a Si wafer applicable for solar cells. One is that the Si wafer must have a sharply edge-defined quadrangular shape with a good surface morphology. The others are that the Si wafer must have a good microstructure and low-end impurity concentrations. A Si wafer with size of 50 x 50 mm was successively grown using a new horizontal growth method. The thickness of the as-grown wafer was approximately 375 +/- 50 mu m and that of a wafer with both sides polished was about 300 mu m. Using Ar gas blowing, the grown wafer was quenched. In addition, the liquid Si residue On the surface of the wafer was effectively removed to enhance the surface morphology. The maximum growth velocity to continuously grow the Si wafer was 1280 cm/min. From investigations of the microstructures, randomly shaped grains with an average size of 50.22 mu m were grown vertically from the bottom to the top of the wafer. This columnar grain structure should be beneficial for solar-cell applications because the horizontal grain boundaries are major recombination sites for generated charge carriers. However, defects, such as dislocations, twins, pores and impurities, such as SiC and carbon particles, were detected. There were also contamination with metal impurities, such its Al, Ca, Ni, and at several ppmw, and a relatively high concentration of oxygen. The direct growth process for a Si wafer used in this study could be a good candidate for a new technology to fabricate Si wafers for use in solar cells due to the good shape properties and the high growth velocity. However, the microstructure must, be improved, and further impurity control is needed.