A novel method for gas flow and impurity control in directional solidification of multi-crystalline silicon

doi:10.1016/j.jcrysgro.2014.04.019

Journal of Crystal Growth

Volume 399, 1 August 2014, Pages 33-38

https://doi.org/10.1016/j.jcrysgro.2014.04.019 Get rights and content

Highlights

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A novel gas injector system for gas flow and impurity control.
•
Effective evacuation of SiO and CO from the furnace interior.
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Inhibition of CO formation at graphite surfaces.
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A versatile tool to improve crystalline silicon properties.

Abstract

In this paper the potential of a specially designed argon gas injector for controlling the gas flow and transport of impurities in directional solidification of multi-crystalline silicon is evaluated. The gas injector which consists of a valve allows one to control the flow direction independently in the vertical and horizontal directions. Based on a gas flow model derived from a semi-industrial crystallization furnace the impact of different gas injection combinations on the gas flow pattern and impurity transport is studied. Special focus is given to the SiO evacuation from the melt-free surface, the CO formation at graphite surfaces and the CO evacuation from the furnace interior. It is found that for gas flow pattern formed through horizontal rather than vertical gas injection, SiO and CO are evacuated most effectively from the furnace interior and the formation of CO is inhibited. Such a type of gas injector presents a versatile tool for controlling the flow and impurity transport in the gas phase and possibly improving the material properties of crystalline silicon.

Introduction

In directionally solidified multi-crystalline silicon (mc-silicon), oxygen and carbon are the impurities that are present at the highest level. Typical pathways of these impurities [1], [2] are illustrated in Fig. 1. Dissolved oxygen from the silica crucible and silicon nitride coating is carried by the melt to the solid–liquid interface where it is incorporated into the solid or it evaporates as silicon monoxide from the melt-free surface. Argon gas, injected into the furnace chamber, carries the silicon monoxide (SiO) to the hot graphite fixtures where it reacts with carbon to form carbon monoxide (CO). At the melt-free surface CO dissociates into the melt and finally carbon and oxygen are incorporated into the solid. Oxygen related defects, like thermal and new donors, can reduce the minority carrier lifetime in solar cells [3]. Carbon precipitates can be responsible for the nucleation of new grains, the formation of locally induced stresses, wire-sawing defects [4] and can cause ohmic shunts in solar cells [5].

The final impurity distribution in the solidified ingot strongly depends on the melt [6] and the gas flow [1], [2] velocity fields. Controlling the melt flow by external force fields [7], [8] is a well acknowledged tool in crystal growth. Great effort has been drawn for investigating the effects of gas flow on heat and impurity transport in Czochralski [9], [10] single crystal growth and directional solidification of mc-silicon [1], [2], [11], [12]. In latter case, little attention has been drawn on tailoring the gas flow fields with the purpose on affecting chemical reactions and the distribution of impurities to the benefit of silicon crystal properties. Variations of the argon gas flow rate and the furnace pressure and their impact on the impurity distribution have been studied in [2]. Positive effects were reported on the implementation of a gas guidance device above the melt-free surface [11], [12].

In this paper we present for the first time a method for controlling primarily the gas flow pattern, and secondary, chemical reactions and the distribution of impurities in the gas phase. Based on a simplified local model derived from a semi-industrial crystallization furnace the impact of argon gas injected through a valve based on a dual nozzle on the flow pattern, chemical reactions and impurity distribution is investigated numerically. Such a specially designed valve allows one to control the flow direction independently in the vertical and horizontal directions or as a combination of the two. Details of the furnace and of the numerical model are described in Section 2. In Section 3, numerical results are presented and discussed, while concluding remarks are given in Section 4.

Section snippets

Numerical approach

The numerical study presented in this work was carried out for a semi-industrial vertical Bridgman furnace as shown in Fig. 2A. The induction heated furnace holds 120 kg of silicon, producing squared ingots of 55×55 cm and 16.5 cm in height. The furnace is equipped with two independently controlled susceptors which are placed above and underneath the silica crucible, each powered by a 100 kW generator. Heat is transferred to the ingot by radiation from the inductive heating of the upper and

Argon gas flow pattern

Fig. 4 shows the flow pattern at the diagonal plane obtained through variation of the mass flow rates of argon gas injected at the vertical and horizontal gas inlets ${\dot{m}}_{vert} | {\dot{m}}_{horiz}$ according to Table 1. Provided that the argon gas is injected vertically (case a, the horizontal inlet is closed), the gas is first transported to the melt-free surface where it bends laterally enforcing flow along the melt-free surface towards the crucible corner. Finally the gas streams upwards, leaves the furnace

Summary

The effect of argon gas injection on the flow pattern and impurity distribution in a semi-industrial silicon crystallization furnace has been studied numerically. It is demonstrated that the gas flow and the resulting impurity distribution can be tailored by applying a gas injector valve, which is based on a dual nozzle allowing for independent flow control in the vertical and horizontal directions. It has been shown that horizontal gas injection for this type of furnace is preferential in

Acknowledgment

This work has been performed within “The Norwegian Research Centre for Solar Cell Technology” (Project number 193829), a Centre for Environment-friendly Energy Research co-sponsored by the Norwegian Research Council and research and industry partners in Norway.

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A novel method for gas flow and impurity control in directional solidification of multi-crystalline silicon

Highlights

Abstract

Introduction

Section snippets

Numerical approach

Argon gas flow pattern

Summary

Acknowledgment

J. Cryst. Growth

J. Cryst. Growth

Mater. Sci. Eng. B

Sol. Energy Mater. Sol. Cells

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth