Preparation of Ni2Si contacts: effect on SiC diode operation

The excellent electrical and thermal properties of SiC make it a preferable semiconductor material for high-power and high temperature devices. A complete exploitation of the properties of SiC needs the manufacturing of low resistivity ohmic contacts stable at high temperatures. Ni/SiC contacts have been studied in order to achieve very low contact resistivity values on n-type SiC. From the first studies, it was observed that, a good ohmic contact can be made by annealing the nickel film at 900-950°C [1] in order to form nickel silicide (Ni2Si), which consumes a SiC layer by solid-state reaction, rising an interface at a larger depth inside the material. A 10m thick 6H-SiC p-type epilayer with 7.51015cm-3 nominal carrier concentration, underwent a selective phosphorous multiple implantation at 300°C, in order to achieve a box-shaped doping profile 200nm deep and 11020cm-3 high. The sample was annealed at 1300°C for 20 minutes in argon. A Ni layer 50nm thick was evaporated on the front of the wafer and defined by the lift-off technique in order to achieve contacts on the n+ regions. The back contact was made up of an Ti and Al alloy. A first electrical characterization (IV, CV, DLTS) was carried out. The samples subsequently underwent an alloy treatment at 900°C for 1 minute in vacuum in order to form nickel silicide. The diodes were electrically characterized by IV, CV, and TLM measurements. DLTS and EBIC analyses were carried out. Preliminary TLM measurements show the ohmic behaviour of the alloyed contact, yielding a contact resistivity value around 0.001  IV measurements evidenced some changes in the diode operation: an excess forward current arises in some diodes after the alloy treatment. A degradation of the reverse leakage current was observed as well. Two hypotheses were made on the cause of this change: 1) The further annealing induces the formation of electrically active defects, which act as generation-recombination centres in the current transport. 2) The reaction for the formation of nickel silicide consumes the n+ regions in one or more points, and a small Schottky diode in parallel with the n+/p junction is responsible of the increase of the current. The formation of defects was investigated by DLTS, the presence of a localised region with different conduction properties was investigated by EBIC analyses. The analyses show that no further defects are present in the device. A highly conductive layer (fig. 1) was observed by EBIC in the devices where the increase in forward current is most pronounced. (a) (b) (c) (d) Figure 1. SEM (a,c) and EBIC (b,d) images of the conductive area at 15 kV (a,b) and 7.5 kV (c,d) beam energy and 5200x magnification. [1] L. M. Porter, R. F. Davis, Mater. Sci. Eng. B 34 (1995) 83.