Electrical properties of annealed MPCVD grown vertically aligned carbon nanotube films

Vertically aligned multiwalled carbon nanotubes (MWNTs) were grown on silicon substrate at a low temperature (<520 °C) using microwave plasma-enhanced chemical vapor deposition (MPCVD). From the Raman spectra, it was found that the I_D/I_G ratio of MWNTs decreased after annealing, indicating that more graphenes were formed by the annealing process. Nevertheless, a strong Si signal was found in Raman spectra after annealing at a high-temperature (600 °C). From X-ray photoelectron spectroscopy (XPS) analysis it was observed that the ratio of the oxygen to carbon (O/C) signal intensity was from 0.15 to 1.88 for the increasing annealed temperatures of MWNTs, and a Si signal was found nearby the surface of MWNTs after annealing at 600 °C. Moreover, from the I–V measurement, the less symmetric I–V characteristic was found for the metal/MWNTs/metal (MIM) sandwich structure of unannealed MWNTs. After 300 °C annealing process, the positive current was increase and the negative current was decrease. It was conjectured that the MWNTs could obtain more graphenes structure by the 300 °C annealing process. Moreover, the I–V trace of the sample annealed by 600 °C exhibited rapid current descent, indicating the oxygenated and partly silicided phenomena might cover outer graphite layer of MWNTs. The equivalent circuit for the MIM sandwich structure could be represented as two Schottky barrier diodes in a back-to-back configuration. From the data fitting, it was found that the Schottky barrier height (_B0) decreased and the current density (J) increased from unannealing to 300 °C annealing temperature. However, the Schottky barrier height (_B0) was increased from 300 to 600 °C annealing temperature. Comparison with the XPS, this may due to the oxygenated and partly silicided phenomenon on the surface of the MWNTs.