Field-enhanced electrical transport mechanisms in amorphous carbon films

Abstract

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 σ = σ00 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 × 103 V cm−1, a different behaviour is evidenced of the form with a transition in the exponent value from n = 2 at low fields (F<3 × 104 V cm−1) towards n ⩾̸ 1/2 at high fields (F > 3 × 105 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 > 8γkT/e, where γ −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/2γkT with γ −1 = 2.8 ± 0.4 nm) and is also consistent with the variable-range hopping mechanism observed in the Ohmic regime.