A device for heating a substrate according to a predetermined temperature profile for crystallizing a material on the substrate includes: a housing, at least a process chamber situated inside the housing and provided with a first and second opening for passing through a substrate, an inlet for introducing a process gas which includes the material in vapor phase into the chamber, at least two transport rollers attached to the housing for transporting the substrate into the chamber. The device further includes passage spaces for preventing the escape of process gas from the chamber to a space between the chamber and housing, which are situated near respective ends of the transport rollers in the chamber, the respective passage spaces having a first passage opening on an inner wall of the chamber, a second passage opening on an outer wall of the chamber and a first flange fixed around the transport roller.