Float Zone Silicon Cells (FZ)

Introduction

Float zone silicon cell is a high-purity option to crystals grown by the conventional Czochralski (CZ) process. The concentrations of light impurities, such as carbon and oxygen, are extremely low in FZ cells. Another light impurity, nitrogen, helps to improve the mechanical strength of the wafers. These days nitrogen is intentionally added during the growth stages.

What is the float zone method?

The float Zone (FZ) method is based on the zone-melting principle and was invented by Theuerer in 1962.  The production takes place under vacuum or in an inert gaseous atmosphere. In this set up, a high-purity polycrystalline rod and a monocrystalline seed crystal are held face to face in a vertical position and are rotated.A radio frequency field is used to partially melt both. The seed from below is brought in contact with the drop of melt and a necking process establishes a dislocation free crystal before the neck is allowed to reach the desired diameter for steady-state growth. As the molten zone is moved along the polysilicon rod, the molten silicon solidifies into a single Crystal.Typical oxygen and carbon concentrations in FZ silicon are below 5 10¹⁵ cm^-3. FZ crystals are doped by adding the doping gas phosphine (PH³) or diborane (B₂H₆) to the inert gas for n- and p-type, respectively. Unlike CZ growth, the silicon molten Zone only contains doping gas and hence is high in purity and has higher resistivity. 

CZ process versus Float Zone Solar

Even though the CZ process is popular method of choice for commercial substrates, it has several disadvantages. For example the CZ wafers contain a large amount of oxygen impurities that reduce the minority carrier lifetime in the solar cell, thus impacting the voltage, current and efficiency. In addition, the oxygen and complexes of the oxygen can make the wafers sensitive to high temperature processing. Float Zone (FZ) wafers overcome all these issues. Since growing of large diameter ingots and higher costs are a constraint, FZ wafers might only be used for laboratory cells and are less common in commercial production.

Application of FZ cells

They are used for:

• Power devices and detectors
• Being highly transparent to terahertz radiation, they are used to fabricate optical components, such as lenses and windows, for terahertz applications.
• They are also used in solar arrays of satellites because of their higher conversion efficiency.