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.

Acknowledgements

The authors wish to thank Professor B. Equer for his encouragement and helpful comments during this work and Dr G. Adamopoulos for the deposition of the carbon films and the ellipsometry measurements. One of the authors (S.K.) is grateful to the Department of Science and Technology, Government of India (New Delhi), for providing a BOYSCAST fellowship during the present work.

Notes

‡Present address: Thin Film Technology Group, National Physical Laboratory, New Delhi-110 012, India.

Additional information

Notes on contributors

Sushil Kumar‡

‡Present address: Thin Film Technology Group, National Physical Laboratory, New Delhi-110 012, India.