Modeling and laboratory scale proof of concept of the horizontal ribbon growth process: Application to silicon wafer manufacturing
In this work we focus on the development of the Horizontal Ribbon Growth technique to produce highly pure silicon wafers for use in solar cells. We divide this preliminary work in two parts: a small scale experiment and the development of mathematical models to describe the crystallization phenomena. We begin by constructing a laboratory scale experiment to demonstrate the HRG concept using water as the working fluid. We made use of the fact that ice floats on water just like silicon would float on top of its melt. Therefore it is possible to conveniently test and analyze the operability of this technique. We find that the initial seeding process needs to be carefully controlled in order to extract a uniform wafer. Appropriate surface cooling and wall heating are necessary to guarantee the continuous formation of an ice film. We then develop simple mathematical models to predict the crystallization rate and thermal profiles of the system. We state that the driving force for crystallization is convective cooling and the film grows in one direction (downwards). Finally we propose how to validate the model using experimental data and how to extend the work to grow silicon ribbons.
A new method for growing crystal ribbons
Abstract A new method has been devised to pull crystal ribbons from the free surface of a melt without using a shaping aperture. In this method the crystal ribbon is pulled in a direction parallel to the melt surface while the heat of crystallization is removed in a direction perpendicular to the melt surface with a heat sink in contact with the upper surface of the ribbon. Special features of the growth process are discussed and variations of the technique are presented. Crystal ribbons of ice and germanium have been grown.
Growth of monocrystals of germanium from an undercooled melt
Dissipation of latent heat being the main rate-controlling factor in crystal growth from the melt, a novel technique has been developed for the rapid growth of monocrystals of germanium and other materials with the diamond or zincblende structure. By dipping a properly orientated seed crystal into an under-cooled melt and withdrawing it at the same rate as it grows, monocrystals of indefinite length can be obtained with various shapes which are under reasonable control; in particular, crystals can be grown in the form of thin lamellae, demonstrating in a striking fashion that even in an isotropic medium of cubic symmetry, crystal growth can take place by the deposition of individual layers orientated along the close-packed {111} planes. Further evidence for growth by layer formation is deduced from etch experiments. The surfaces of some of these crystals have a high degree of perfection. Crystal surfaces as grown, and also after having been etched, have been examined in detail under the optical, phase-contrast and electron microscopes, as well as with a fine-focus X-ray beam; certain growth features are discussed and a possible mechanism is suggested for dendritic growth.