Current–temperature analysis of DC glow discharge CVD diamond films

We have investigated the electrical transport properties in CVD diamond films prepared with a DC plasma glow discharge method. The analyzed samples have been grown with different methane concentrations and substrate temperatures; the resulting resistivities at room temperature are in the range from 7.10(8) Ohm.cm to 2.10(13) Ohm.cm. The current has been measured as a Function of the temperature, in the region from liquid nitrogen to 700 K. The electrical conduction process has been found to depend strongly on the temperature range considered. For T>600 K, the conductivity can be described as an exponential function of the reciprocal temperature: in this region, two samples with the highest resistivities, show an activation energy of 1.7 eV. This value is related to the donor impurity of isolated nitrogen which has been detected by electron spin resonance analysis. A variable range hopping process is proposed to describe the experimental conductivity in the low temperature region. Material characterization performed with micro-Raman spectroscopy and electrical measurements depict the transport process as a conduction due to low resistivity graphite-like phases mainly present along grain boundaries.