Tailoring of Contacts on Silicon Carbide - Procedures and Mechanisms

Doktorsavhandling, 2007

Silicon carbide (SiC) exhibits very good electrical, thermal, chemical and mechanical properties which make it suitable for the next generation of wide band gap electronic devices, where silicon (Si) cannot be used due to its limitations with respect to the mentioned properties. Nickel (Ni) and Tantalum (Ta) are among the metals used for the contact formation. Both these metals are suitable as precursors for creating Schottky and ohmic contacts onto n-SiC. Ni is widely used because of its ability to form silicides while Ta can form both silicides and carbides. The initial stage of the reaction between a given metal and SiC is important for the silicide formation on SiC. In order to address this problem experimentally, SiC was coated with an ultra thin Ni layer and shown that the silicide formed grows as islands on the SiC surface upon heat treatment. By using the same method, a comparison between different initial Ni thicknesses is presented. This gives an idea of the Ni critical thickness that is needed, before diffusion becomes the only growing parameter for the silicide formation. During silicide formation, C depletion from the SiC takes place. Lately, the influence of C to the contact behaviour has been a controversial issue. Thus, the capability to control the C formation is of importance. Control of C formation and its distribution was achieved following a simple and practical method by means of co-sputtering of Ni and Si. Presence of C can be also avoided by using Ta or Ni/Ta for the metallization. Moreover, Ta products have the advantage of an increased thermostability compared to the ones of Ni. The results summarised here were obtained by means of X-ray photoelectron spectroscopy (XPS) and angle-resolved X-ray photoelectron spectroscopy technique (ARXPS). Depth profiles were carried out by means of XPS and Auger electron spectroscopy (AES). Grazing-angle X-ray diffraction (XRD) was used to identify the phases formed. Characteristic I-V curves of the materials were recorded and a comparison between the traditional metallization and co-sputtering technique was done.