Traditionally the critical value Gamma(crit) of the Gamma ratio of the pulling speed upsilon over the thermal gradient G at the melt-solid interface needed to grow defect free silicon is assumed to be a constant for a given crystal doping and resistivity. Recently it was however shown that Gamma(crit) depends on stress and thus alson on G. in particular on its derivative with respect to the distance from the melt-solid interface. As G also depends on v, a process window exists for the critcal pulling speed which depends both on the crystal radius and on Gamma(crit). The implications for the development of pulling processes for large diameter defect free silicon crystals are discussed. (C) 2013 Elsevier B.V. All rights reserved.