Silicon carbide crystals and processes and furnaces for making them

摘要

Crystals of hexagonal silicon carbide are prepared by heating Si and one or more carbon, preferably graphite, pieces spaced therefrom to 2300 DEG -2500 DEG C. in an inert gas atmosphere containing not more than 1 mol. per cent N2, one or more heat sinks existing at a distance from the carbon pieces not less than the maximum dimension of the largest crystal to be formed, and in directions substantially normal to the carbon surface at the junctions with each crystal. The crystals have "n" and/or "p" conductivity and have two faces substantially parallel to each other to within 1 degree, the maximum measurable length on either of these faces being not less than 1,6 mm., and the perpendicular distance between these faces is 0,013-2,5 mm. The content of components other than Si and C is not more than 1000 parts per million and the specific resistance is from 0,01-1000 ohm-cm. at 20-25 DEG C. Silicon 25 is placed in annular graphite crucible 21 supported on graphite bars. Graphite bars support cylindrical graphite sleeves 31-35 above crucible 21. The whole is contained in graphite container 13 having cover 14 with central hole 15 and graphite chimney 17. Heating is by induction coil 42 through asbestos sleeve 41 and zirconia thermal insulation 45. Heating may alternatively be by resistance. The crystals grow on the inside of sleeves 31-35 due to the heat gradient dropping from sleeve to sleeve towards the centre of the furnace produced by the heat sinks. The heat sinks are constituted by the base of the container 13, the chimney 17, and the evaporating silicon 25. The gas atmosphere may be CO, A, He, or H2 and may include as doping constituents PCl3, PH3, AsCl3, SbCl3, BCl3, or AlCl3. A "p"-"n" junction may be formed in the crystals by using different doping agents at different periods of the preparation. In an alternative apparatus (Fig. 3 not shown) the container is provided with a central pipe with axial and radial bores to introduce the gaseous atmosphere into the base of the container, and the graphite crucible is supported on graphite sleeves, and graphite plates rest on the crucible and against the container wall.