Temperature dependence of the electron paramagnetic resonance of unhydrogenated carbon nitride

Abstract

Temperature-dependent electron paramagnetic resonance (EPR) and conductivity measurements were performed on unhydrogenated amorphous carbon nitride (a-CN x ) thin films in an attempt to investigate electronic transport mechanisms in the layers. The films were synthesized by pulsed laser ablation of graphite in a nitrogen remote plasma. As nitrogen incorporation decreased from 25 to 16 at.% with increasing laser intensity, the electrical conductivity at room temperature, σ, increased from 0.33 to 17 (Ω·cm) −1. This behavior correlates well with increasing spin densities (from 4 to 9×10 20 cm −3) and a broadening of the Lorentzian EPR line ( g=2.0028), from 2.6 to 3.8 G. The EPR linewidth can be decomposed into temperature independent and temperature dependent parts. At 4 K, the EPR signal is 1.9 G wide, independent of nitrogen content. However, the temperature-dependent part of the EPR linewidth broadens with temperature, with a T −1/4 dependence, thus indicating that spin relaxation also occurs through a hopping mechanism. Between 100 and 270 K, a hopping conduction mechanism is evidenced by a linear dependence of ln( σ) vs. T −1/4. Values of T 0, as deduced from the EPR measurements, are close to those obtained from conductivity measurements for films with the lowest nitrogen, thus suggesting a correspondence between spins and charge carriers. However, such a correspondence is not observed at higher nitrogen content, as the EPR-deduced T 0 values become much smaller than the ones obtained from conductivity measurements. These results are explained in terms of the different arrangement of the carbon sp 2 bonded domains in the films.