Abstract:
Temperature-dependent electron paramagnetic resonance (EPR) and conductivity measurements were performed on unhydrogenated amorphous carbon nitride (a-CNx) 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.percent 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 × 1020 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 T0, 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 T0 values become much smaller than the ones obtained from conductivity measurements. These results are explained in terms of the different arrangement of the carbon sp2 bonded domains in the films. © 2003 Elsevier B.V. All rights reserved.