Abstract
Single crystalline silicon spheres for use in a solar electrochemical convertor array have been fabricated by the melt drop technique in a quartz shot tower. Molten silicon flows through a quartz capillary tube as a liquid jet which breaks up into droplets. The molten droplets later freeze during free fall within the shot tower. Based on Lord Rayleigh's theory of jet breakup, a sinusoidal perturbation of a specific frequency applied to the molten jet is effective in controlling the diameter of resulting droplets to predetermined values for various liquid jets studied. With molten silicon jets, the size of the frozen silicon is not very narrow. This is due to the continuous erosion of quartz capillary by molten silicon, which results in increasing jet diameter during the course of the experiment. The molten silicon droplets falling down in the shot tower can be frozen into single crystals by properly choosing the heat transfer medium in the shot tower. Nucleation theory and computer modelling of the cooling process for a typical silicon sphere of 325 micro diameter have been developed from thermodynamic principles. Based on the theories developed, if such a droplet is allowed to supercool to about 1260(DEGREES)C and be kept at that temperature for about a second, after which it freezes, single crystalline structure will result due to the very few homogeneous nucleation sites that grow in the droplet. Polycrystalline spheres result on further supercooling below 1260(DEGREES)C. Single crystalline spheres have a tear drop shape with a smooth, shiny surface. Polycrystalline spheres have an "orange peely" spherical surface with a pronounced tail. X-ray diffraction of the single crystal sphere indicates growth along the <111> axis. In order to achieve a complete yield of single cyrstals, a 370 cm tall shot tower with a 120 cm long heat zone at 1250(DEGREES)C is required with an ultra pure Argon ambient at 760 Torr. In this study, the shot tower column is limited to 200 cm and a large number of single crystalline and bicrystalline spheres and some coarse grained polycrystalline spheres are obtained on freezing.
Sridhar, Uppil (1983). Single crystal silicon sphere growth for solar cell applications. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -537943.