CHEMICAL AND ELECTRICAL PROPERTIES OF GRAIN BOUNDARIES IN POLYCRYSTALLINE SILICON

Experimental techniques for the investigation of the electrical and chemical properties of grain boundaries are developed and utilized for the study of cast polycrystalline silicon. Totally automated electron beam induced current (EBIC) techniques are developed, including hardware and software development, and are used for the study of recombination properties of silicon grain boundaries. Automation was used to maximize the precision and accuracy of the EBIC data and to allow large numbers of measurements to be performed to determine the effects of experimental parameters on reproducibility and beam damage. EBIC, secondary electron and optical microscopy techniques are used to characterize the grain size and structure in Wacker Silso cast polycrystalline silicon. The grain size is found to be 1.63 (+OR-) 0.43 millimeters. Grain structure was found to be controlled by the casting process used to form the material. Twin boundaries are found to be electrically inactive. Preferential grain boundary diffusion of phosphorus in silicon grain boundaries is measured using both bevel and stain, and bevel/EBIC techniques. The effects of heat treatments at 600, 750 and 900(DEGREES)C are investigated extensively. An MIS device structure which does not require high temperature processing (> 200(DEGREES)C) for fabrication is developed and utilized for these studies. EBIC analysis results show that grain boundary recombination velocity is substantially increased by heating cast polysilicon to temperatures as low as 600(DEGREES)C, whereas no effects were seen at 400(DEGREES)C. Heat treatments up to 900(DEGREES)C are investigated. Use of complementary surface analysis techniques are developed which show that these heat treatments result in oxygen segregation to grain boundaries. In-situ fracturing techniques for Auger electron spectroscopy and secondary ion mass spectroscopy analysis of grain boundaries are developed. Ion microscopy techniques are developed and used to complement the fracture studies. Local oxygen concentrations of 0.1 atomic % were measured. Based on the time and temperature dependence of the activation of grain boundary recombination, it is concluded that oxygen segregation is responsible for the activation.