Floating Zone Crystal Growth

This chapter outlines one of the two practically important bulk crystal growth methods for silicon, the crucible-less floating zone (FZ) technique, which cannot be evaluated without comparing it to the other one, the Czochralski (CZ) method. The main advantage of FZ silicon is the high purity and the resulting high electrical and structural material quality. Although, till now, FZ silicon for solar cells is mainly a matter of R&D and not of cell production for terrestrial utilization, it has a big potential for future applications, because efficiency and long-term stability of FZ silicon cells are considerably higher than that of CZ silicon cells.

Presently, the main problem with FZ solar cells is not only the high price of the special feed material for FZ, boosted by the currently unbalanced situation of the feed stock market, but also the limited crystal cross section of FZ crystals, which does not fit the standard cell formats. New concepts to overcome these difficulties are to grow crystals directly with the desired square cross section of the solar wafer, the use of cheaper feed material like granular silicon, or pulling feed rods after a downgraded CZ technique from low-price raw silicon like upgraded metallurgical grade (UMG) silicon with the benefit of further purification by segregation.