Electrical conduction in undoped ultrananocrystalline diamond thin films and its dependence on chemical composition and crystalline structure

Abstract

The electrical conduction behavior of undoped ultrananocrystalline diamond (UNCD) and its dependence on deposition temperature and chemical structure are presented. UNCD films were grown using a microwave plasma-enhanced chemical vapor deposition technique at deposition temperatures of 400 °C and 800 °C. The chemical structure of the UNCD films is characterized with several tools including: Elastic recoil detection analysis, Fourier transform infrared spectroscopy, electron energy loss spectroscopy, Raman spectroscopy, and environmental scanning electron microscope. The results show a higher content of sp2-bonded carbon for the 800 °C deposition samples (∼65%) in comparison with the 400 °C samples (∼38%). In both kinds of films, the hydrocarbon bonds have the saturated sp3 structures, while there is lower hydrogen content in the 800 °C samples (∼8%) than in the 400 °C samples (∼10%). For conduction properties, experiments are conducted using a probe station and conductive-atomic force microscopy. Experimental data show that the samples deposited at 800 °C are several orders of magnitude more conductive than the 400 °C samples. The conduction occurs primarily along the grain boundary for both types of samples. The conductivity of both types of films also shows field dependent nonlinear behavior. Both the Poole–Frenkel models and single and overlapping Coulombic potential models show that the conduction is directly correlated with the sp2 bond carbon density, and the role of the hydrocarbon bonds in the conduction path is formed by the network of the sp2 bonded carbon.