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

A combination of junction capacitance, electron spin resonance and electric al conductivity measurements are used to investigate the electronic propert ies of two different types of a-C:H films grown in a dual ECR-RF glow disch arge system at substrate bias equal to -30 and -600 V, respectively. The an alysis of the steady state admittance (both capacitance C and conductance G ) as a function of frequency (omega = 5 Hz-l kHz) and temperature (20-350 K ) allows an estimate of the density of states at the Fermi level of approxi mately 7 x 10(15) and 9.7 x 10(16) eV(-1) cm(-3) for the -30 and -600 V dep osited samples, respectively, values well below those deduced for the densi ty of spins from the electron spin resonance experiments, of approximately 10(19)-10(20) cm(-3). Concerning the conductance results, two transport pro cesses operating, respectively, below and above 290 K are shown. The high t emperature 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 th e values obtained in the high temperature range (> 300 K) for the activatio n energy of the electrical conductivity. Regarding the effect of the freque ncy and temperature on the conductance, we show that fur temperatures below 290 Ii, a Variable Range Hopping mechanism is possible by facing our data to the Mott's model. Annealing at high temperature induces structural chang es accompanied by an increase in the spin density in both types of samples with however, a different behaviour from one type to another. (C) 2001 Else vier Science B.V, All rights reserved.