Abstract: Process and apparatus for heating substrates to form semiconductor regions. A gaseous reactant is introduced into a reaction chamber formed from a material, such as quartz, which is transparent and non-obstructive to radiant heat energy transmitted at a predetermined short wave length. A graphite susceptor, which is opaque to and absorbs the radiant heat energy, is positioned within the reaction chamber and supports the substrates to be processed. The susceptor and substrates are heated directly while the walls of the reaction chamber remain cool. The substrates are heated uniformly, and single crystal semiconductor wafers processed by this technique have little or no crystallographic slip. To further insure uniform heating, the susceptor may be moved relative to the radiant heat source which, in the preferred embodiment, comprises a bank of tungsten filament quartz-iodine high intensity lamps.