Effect of annealing on the structural and electrical properties of d.c. multipolar plasma deposited a-C:H films

Abstract

The changes upon annealing in CH and CC bonding in relation to the structural and electronic properties have been investigated in two different series of a-C:H samples prepared in a direct current (d.c.) multipolar plasma system from pure methane at quite different substrate bias (−40 and −600 V). Using a combination of infrared absorption, elastic recoil detection analysis, high resolution transmission electron microscopy and electrical resistance measurements, we fully characterize the samples in their as-deposited state as well as after successive annealing cycles at increasing temperatures up to 700°C. The results show clearly that the two types of series exhibit quite different microstructures and hydrogen incorporation in their as-deposited state. The low bias (−40 V) series exhibits a highly disordered structure, while the high bias (−600 V) one already contains well ordered regions. They also have a completely different behavior upon annealing up to high temperature. A microstructure conversion such as from hydrogenated as well as non-hydrogenated sp 3 C sites to sp 2 C ones occurs in the temperature range 400–500°C in all cases. However, a more efficient graphitization is observed in the high bias series (−600 V) for annealing temperatures as high as 700°C. Quite surprising results are obtained for the low bias series (−40 V): contrarily to what is usually observed for this type of sample, this series is found to be more thermally stable for high annealing temperature (>400°C) than the high bias one (−600 V). These results are discussed and explained in terms of the relaxation process in the local microstructure.