High-field transport and noise properties of sputter-deposited amorphous carbon-silicon heterojunctions

The electrical conductivity of heterojunctions of amorphous carbon (a-C) films (25 and 75 nm thick) grown on silicon by magnetron sputtering has been studied as a function of the applied electric field and temperature. At low electric fields and high temperatures, the conductivity exhibits thermally activated ohmic behaviour with activation energy 0.14 eV. At high electric fields, photoconductance measurements indicate that the conductivity is primarily due to a field-activated mobility with its activation energy decreasing as the electric field increases. At very high electric fields, band-to-band tunnelling is the dominant conduction mechanism. The mobility field-activated conduction model indicates an energy distribution of trapping states consisting of two exponential distributions. The exponential distributions correspond to tail states arising from clustering of sp(2) sites and to deep states caused by isolated sp(2) sites. Low-frequency noise measurements show that thicker a-C films contain a higher concentration of the trapping states. This result was explained by an increase of the sp(2)/sp(3) bonding ratio found from the analysis of Raman spectroscopic measurements