GB1154934A - Improvements in or relating to the Surface Treatment of Silicon Carbide Crystals - Google Patents

Abstract

1,154,934. Electrolytic polishing and etching of silicon carbide crystals; gold alloys. GENERAL ELECTRIC & ENGLISH ELECTRIC COMPANIES Ltd. June 6, 1967 [June 17, 1966], No.27300/66. Headings C7A and C7B. A surface of a crystal of P-type SiC is anodically etched by placing the surface in a horizontal position in contact with an electrolyte consisting of a 0À1 N to 5N aqueous solution of HF wherein part, e.g. up to 50% by volume, of the water is optionally replaced by a weak acid such as acetic acid, and passing a current such that the current density at the crystal surface is 5 to 500 amps. /sq. dm. Electro-polishing may be effected with 1À5N to 4N aqueous HF with a current density of 20 to 350 amps. /sq. dm., the crystal potential relative to an adjacent measuring electrode being 4 to 10 volts. 10% by volume of glacial acetic acid may also be present. N-type SiC crystals may be electro-polished by introducing positive holes into the surface layer e.g. by injection from a forward-biased PN junction or by exposure to U. V. radiation. Delineation of a PN junction in a single crystal of SiC can be effected in an electrolyte suitable for polishing at a current density less than that at which polishing occurs e.g. less than 30 amps/sq. dm. resulting in a thin white or brown on whichever region is connected as anode. Leakage current across a PN junction may be reduced by removing weak points or surface damage adjacent the junction by connecting the P-region as anode, masking the contact to this region and any metal contacts on the N-region from the electrolyte and employing a current density above that which effects polishing. As shown a polythene vessel 1 having a screw-threaded neck 2 supports a threaded recessed polythene holder 3 for a SiC crystal 5 with the crystal surface horizontal and parallel to a noble metal e.g. gold cathode 7. A reference electrode 11 is provided by a gold wire sheathed except at its tip by a polythene tube. By this arrangement irregularities due to convection of the electrolyte are reduced. The viscosity of the electrolyte may be increased by the addition of a water-miscible inert viscous liquid such as glycerine, such addition lowering both current density limits defining the polishing range. The contact to the SiC crystal which contact is insulated from the etching electrolyte may be a pressure contact, an evaporated metal layer, silver paste or an electrolyte contact, or may be formed by a molybdenum disc bonded to the crystal by a gold alloy containing 5% by weight of tantalum or of each of tantalum and aluminium.