Crystal Growth Behaviors of Silicon during Melt Growth Processes

Kozo Fujiwara

doi:10.1155/2012/169829

Abstract

It is imperative to improve the crystal quality of Si multicrystal ingots grown by casting because they are widely used for solar cells in the present and will probably expand their use in the future. Fine control of macro- and microstructures, grain size, grain orientation, grain boundaries, dislocation/subgrain boundaries, and impurities, in a Si multicrystal ingot, is therefore necessary. Understanding crystal growth mechanisms in melt growth processes is thus crucial for developing a good technology for producing high-quality Si multicrystal ingots for solar cells. In this review, crystal growth mechanisms involving the morphological transformation of the crystal-melt interface, grain boundary formation, parallel-twin formation, and faceted dendrite growth are discussed on the basis of the experimental results of in situ observations.

Highlights

The expectations for solar cells have been increasing yearly toward solving energy and environmental problems worldwide
The crystal structure of an mc-Si ingot obtained by casting based on a unidirectional growth technique is markedly different from that of sc-Si, as illustrated by the formation of grain boundaries and the distribution of crystallographic orientations, which prevent the realization of high-efficiency solar cells
Dendritic casting is a method in which seed crystals are created from the melt during crystallization by inducing dendrite growth at the bottom of the crucible [1,2,3]

Summary

Introduction

The expectations for solar cells have been increasing yearly toward solving energy and environmental problems worldwide. The crystal structure of an mc-Si ingot obtained by casting based on a unidirectional growth technique is markedly different from that of sc-Si, as illustrated by the formation of grain boundaries and the distribution of crystallographic orientations, which prevent the realization of high-efficiency solar cells. Various new methods for controlling the macro- and microstructures have been presented, which focus on controlling the nucleation or crystal growth mechanism in the earlier stage of casting. Many studies to establish such technologies for obtaining high-quality mc-Si ingots have been continuously performed on all these methods In this review, such technologies for mc-Si ingot growth will not be discussed; only the melt growth mechanisms of Si will be discussed because fundamental understanding of crystal growth mechanisms is crucial to developing all such technologies. The crystal growth phenomena during melt growth processes including the morphological transformation of crystal-melt interfaces, grain boundary formation, parallel-twin formation, and faceted dendrite growth will be discussed by providing a review of our recent studies

Morphological Transformation of Crystal-Melt Interface

Parallel-Twin Formation

Faceted Dendrite Growth

Summary