found to occur along the rod axis. Single crystals are obtained more readily if a seed crystal is used on one side and if the initial floating liquid zone is formed at the joint with polycrystalline silicon. The floating zone is then made to travel along the polycrystalline mass while a single crystal forms from the seed.
Effective purification by successive zone melting has been observed in most samples. Single crystals with resistivities up to several hundred ohm-cm have been prepared, displaying without exception p-type conduction. We believe that the remaining impurity in these crystals is essentially Boron which cannot be effectively segregated by zone melting.
For the application of the floating zone method the silicon must be available in the shape of rods or at least as pieces which can be welded together to form a rod of irregular shape. In the case in which only powder is available as starting material, the powder can be pressed hydrostatically into a rod without the use of any binder 4). Such pressed rods have been successfully turned into silicon crystals by means of the floating zone equipment.
In conclusion three significant advantages of the floating zone method should be emphasized :
-
Avoidance of impurities originating from a container for the melt or from a heater element.
-
Possible application to materials with melting points even higher than that of silicon.
-
Preparation of single crystals from relatively small quantities of raw material.