Field-enhanced electrical transport mechanisms in amorphous carbon films

In order to investigate the localized electronic states in hydrogenated amorphous carbon (a- C : H) films, the temperature and electric field dependences of the current density have been measured in low-field coplanar (Al/ a- C : H/Al) and in high-field transverse (TiW/a- C : H/TiW) geometries. The very-low-field conductivity sigma = sigma(00) exp[-(T-0/T)(1/4)] reveals a band-tail hopping transport mechanism in an Ohmic regime (at least for films above 20 nm thickness) while, for electric field values F> 4 x 10(3) V cm(-1), a different behaviour is evidenced of the form sigma = sigma(n)(0) exp [(F/F-n)(n)] with a transition in the exponent value from n = 2 at low fields (F< 3 x 10(4) V cm(-1)) towards n≥1/2 at high fields (F> 3 x 10(5) V cm(-1)). This field enhancement of electrical transport can be interpreted as arising from either a three-dimensional Poole-Frenkel effect for charged empty defects, or field-assisted hopping out of neutral empty defects (the Apsley - Hughes hopping model). Although a clear discrimination between both models would require very high electric fields (F> 8gammakT/e, where gamma(-1) is the localized wavefunction radius), the Apsley - Hughes model describes accurately the experimental temperature-dependent and field-enhanced transport within localized states (scaling as eF/2gammakT with gamma(-1) = 2.8 +/- 0.4 nm) and is also consistent with the variable-range hopping mechanism observed in the Ohmic regime.