PURPOSE: To solve a problem that, when Al or Ga is diffused into a silicon substrate inside a sealed quartz tube, (1) the vapor pressure of the Al or the Ga is not increased as the Al or the Ga reacts with quartz, and (2) the Al or the Ga cannot be diffused stably as evaporated Si adheres to a source, and a problem that a heavy-metal substance is diffused and passes through, the sealed quartz tube from the outside, reducing the lifetime of the silicon substrate.

CONSTITUTION: Before a heat treatment process in which Al or Ga is diffused into a silicon substrate and which exceeds 1000°C, the inner wall of a sealed tube is covered with alumina or a Ga oxide by a heat treatment at a temperature near 950°C. An oxide film prevents the Al or the Ga from being reacted with quartz, the vapor pressure of the Al or the Ga is not lowered, the vapor pressure of Si becomes releatively low, and a source is not covered with an Si film. In addition, the oxide film prevents a heavy metal from creeping.