Electrical transport properties of a carbon nanostructure obtained by plasma-enhanced chemical vapor deposition during thermal cycling

Abstract

We have investigated the structure and electrical conductivity of carbon nanographite layers grown by chemical vapor deposition, enhanced by microwave plasma (PECVD) on an setup by IPLAS Innovative Plasma Systems GmbH (Germany). The samples were grown on fused silica substrates with deposition times of 20 and 40 min, respectively. The study of the formed layers of nanographite by the method of Raman light scattering and scanning electron microscopy showed that the surface of the nanographite sample deposited for 20 min is covered with a large number of unconnected vertical graphene nuclei with an average size of less than 10 nm. An increase in the growth time to 40 min led to an increase in the size of the nuclei to 20 –30 nm; however, their overlap does not occur. This confirmed that the samples corresponded to the initial stages of the formation of vertical graphene in the grown nanographite layers and there is no percolative structure in them. The obtained samples were used to study the temperature dependences of the sheet electrical resistance at direct current in the range of 4 –300 K and the effect on them of the number of cycles N cooling – heating (300 K – 2 K – 300 K) in an atmosphere of gaseous helium, as well as the change in the atmosphere storage of samples (by placing them in the air after warming up to room temperature). It was found that the electrical resistance of the sample deposited for 20 min is very sensitive to two technological parameters of measurement – the number of cycles N and the change in the storage atmosphere after heating. This manifested itself in the fact that after four cooling – heating cycles and one change of the atmosphere (helium – air – helium) after warming up, the resistance increased by more than 20 %, reaching saturation. The resistance of the sample, deposited for 40 min, showed less sensitivity during thermal cycling, increasing by no more than 10 %. The effect of thermal cycling we attribute to the rearrangement of defects formed at the boundaries of grains in the nanographite layer, and in the case of a change in the atmosphere, with the passivation of dangling bonds with atmospheric gases.