Recent experimental studies on the electronic properties of thin films of tetrahedrally bonded amorphous carbon (ta-C) are reviewed. These films were grown either by vacuum arc deposition or by mass selected ion beam deposition. In several papers it was stated that ta-C is a p-type semiconductor. I will show that the given experimental evidence for p-type conductivity in ta-C is rather weak. Our studies show that doping of ta-C with B, N and P significantly influences the electrical conductivity. No evidence for p-type or n-type doping was found. In all cases, electrical conduction is due to hopping conduction and at high electric fields due to Frenkel-Poole conduction. This is shown with I-V measurements in metal/ta-C/metal structures, diode-like layered ta-C structures and ta-C/Si heterojunctions.
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