The development of GaAs vertical gradient freeze (VGF) growth process is focused on the increase of process efficiency by a reduction of the production costs with simultaneous improvement of the crystal quality. To meet this technological and scientific challenge, different strategies were proposed, e.g. an increase of crystal size, simultaneous crystallization in multi crucible furnace or an increase of growth rate. For process enhancement, an exact and permanent control of the melt flow is of crucial importance that is easily provided by traveling magnetic fields (TMF). The beneficial influence of Lorentz forces of crystal quality, which were generated by a KRISTMAG^® internal heater-magnet module (HMM), was already reported for VGF Ge growth [1,2]. In analogous way to germanium growth, in the present investigation  the HMM was positioned sidewise around the crucible and was supplied by a combination of direct current (DC) and alternating current (AC) for a coupled generation of thermal and magnetic fields. Thereby a wide range of electromagnetic parameters (frequency, current amplitude, phase shift) were available for process optimization. The influence of these magnetic parameters on the crystals growth in a HMM will be presented. Figure 1 shows an image of a 4 inch VGF GaAs single crystal grown in the HMM.

Fig. 1: Si doped 4 inch VGF-GaAs crystal grown in HMM

The key aspect of the presented investigation was the well-defined control of the solid/liquid interface bending in the grown VGF crystals. Therefore, a numerous preliminary 3D CFD global simulations of the whole furnace are inevitable. The both results, numerical and experimental, showed that the downward directed Lorentz forces generated near the crucible wall, if properly adjusted, may lead to a significant reduction of the concavity of the solid-liquid interface in comparison to the deflection characterizing crystals grown without TMF and to the data reported in the literature [3].

Study of charge carrier concentrations by Hall measurements also pointed out the differences in the crystals that were grown with and without TMF. The crystals exposed to TMF showed higher axial charge carrier densities. The radial charge carrier distribution was flattened due to the reduction of the interface deflection.

Moreover, simulation results revealed a potential for a further process improvement by an increase in a crystallization rate using TMF [4].

[1] Ch. Frank-Rotsch, P. Rudolph, Journal of Crystal Growth, 311 (2009) 2294-2299.

[2] Ch. Frank-Rotsch, U. Juda, B. Ubbenjans, P. Rudolph, Journal of Crystal Growth352 (2012) 16-20.

[3] R. Lantzsch,PhD Thesis, TU Freiberg (2009)

[4] N. Dropka, Ch. Frank-Rotsch, Journal of Crystal Growth, 367 (2013) 1-7.

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