The disclosure is directed to a double crucible Czochralski crystal semiconductor growing apparatus (10). An inner crucible (14) floats in a melt within an outer crucible (13) and a single crystal semiconductor billet (23) is pulled from the melt (16) in the inner crucible. An elongated tubular member (26), having at least one small aperture (33) in the wall thereof, provides a channel between the outer and inner crucibles. The tubular member (26) permits flow of the melt in the outer crucible (14) to the inner crucible (13) while inhibiting the diffusion of dopant material from the inner to outer crucible while any gas in the member will pass through the aperture (33).