Electronic properties of hydrogenated amorphous carbon films deposited using ECR-RF plasma method

摘要

A combination of junction capacitance, electron spin resonance and electrical conductivity measurements are used to investigate the electronic properties of two different types of a-C:H films grown in a dual ECR-RF glow discharge system at substrate bias equal to - 30 and - 600 V, respectively. The analysis of the steady state admittance (both capacitance C and conductance G) as a function of frequency (#omega# = 5 Hz-1kHz) and temperature (20-350 K) allows an estimate of the density of states at the Fermi level of approximately 7 X10~ (15) and 9.7X10~(16) eV~(-1) cm~(-3) for the -30 and -600 V deposited samples, respectively, values well below those deduced for the density of spins from the electron spin resonance experiments, of approximately 10~(19)-10~(20) cm~(-3). Concerning the conductance results, two transport processes operating, respectively, below and above 290 K are shown. The high temperature process is associated with an activation energy of 0.5 and 0.41 eV for the - 30 and-600 V samples, respectively, in good agreement with the values obtained in the high temperature range (> 300 K) for the activation energy of the electrical conductivity. Regarding the effect of the frequency and temperature on the conductance, we show that for temperatures below 290 K, a Variable Range Hopping mechanism is possible by facing our data to the Mott's model. Annealing at high temperature induces structural changes accompanied by an increase in the spin density in both types of samples with however, a different behaviour from one type to another.