Numerical study and experimental investigation of zone refining in ultra-high purification of gallium and its use in the growth of GaAs epitaxial layers

A numerical study has been carried out to optimize the zone refining process parameters such as zone length, zone travel rate, number of passes and the effect of molten zone stirring in ultra-high purification of gallium. Theoretical interpretation of the impurity concentration profile along the refined gallium ingot is also ascertained. Based upon the optimized parameters obtained through the study, 5N2 (99.9992% pure) gallium is subjected to zone refining for 50 passes and mechanically stirred at 20–35 rpm by a rotational mechanism in the directional freezing system. The refined gallium has a purity level of 7N2 (99.999992%) and the total impurity concentration is reduced to 77.7 ppb with respect to 24 impurities, as analyzed by glow discharge mass spectrometry (GDMS). The purified gallium is used as the starting material to grow GaAs epitaxial layers by liquid phase epitaxy technique. The quality of the grown layer is tested by secondary ion mass spectroscopy (SIMS), Hall measurements and low-temperature photocapacitance techniques.