VGF growth of 4in. Ga-doped germanium crystals under magnetic and ultrasonic fields

To achieve high-quality VGF-grown semiconductor single crystals of germanium a favorable nearly flat slightly convex solid/liquid interface shape is required. This has been obtained by applying a traveling magnetic field (TMF). The process was optimized by numerical simulation. For the first time a “double-frequency” TMF was used experimentally to control the interface shape very effectively. Due to the superposition of two independent frequencies a precise tailoring of the Lorentz force density distribution within the melt has been obtained. Besides flow pattern control and damping of convective fluctuations, the diffusion boundary layer could be reduced, that increases the morphological stability and purification effect. Additionally, to remove nearly totally the diffusion boundary layer a more effective Schlichting stream immediately at the interface was generated by ultrasonic (US) vibration. Lateral photo-voltage scanning (LPS) on longitudinal crystal cuts was used to analyze the interface curvature and micro-homogeneity. Further, the electrical properties vs. TMF and US vibration were investigated. Ga-doped crystals showed a high doping efficiency. The axial distribution of the carrier concentrations follows the regime of nearly complete melt mixing.