OPTICAL-ABSORPTION AND ELECTRONIC TRANSPORT IN ION-IMPLANTATION-DOPEDPOLYCRYSTALLINE SIC FILMS

Fine-grained (d approximate to 0.1 mu m), polycrystalline SiC films we re prepared on top of insulating and optically transparent sapphire su bstrates by means of a thermal crystallization technique. Optical abso rption measurements indicate that the individual SiC grains consist of relatively defect-free beta-SiC surrounded by high-defect density gra in-boundary material. Nominally undoped material exhibits a low dc con ductivity (sigma approximate to 10(-8) Omega(-1) cm(-1)) in the dark a nd an efficient photoconductivity upon illumination with short-wavelen gth UV light. The temperature dependence of the de transport exhibits a quasi-Arrhenius-type behaviour with average activation energies of t he order to 0.6 eV. A characteristic feature of this kind of transport is a continuous increase in activation energy with increasing film te mperature. Upon doping with N, P and Al ions, the average activation e nergy is decreased and room temperature conductivities of the order of 0.1 Omega(-1) cm(-1) are reached. Doping with B ions, on the other ha nd, only leads to high-resistivity material. It is shown that the elec tronic transport in doped SiC-On-Sapphire (SiCOS) films can be success fully modelled in terms of a grain-boundary-dominated conduction proce ss. In this process thermal activation across potential barriers at th e grain-boundary surfaces competes with tunneling through these same b arriers.