Hopping current density in amorphous carbon/crystalline silicon heterojunctions

High density amorphous carbon (a-C) films deposited by femtosecond pulsed laser ablation were incorporated in p-type c-Si/a-C/Al devices. Rectifying I–V characteristics were measured versus temperature T (200–300 K). At low fields, the conductivity behaves as Ln (σ/σ∘∘) = −(T0/T)1/4, corresponding to 3D hopping transport. As the electric field F increases from 104 to 105 V cm−1, the decrease of both apparent slope T 0 1 / 4 ( F ) from 230 to 100 K1/4 and prefactor σ∘∘(F) from 1016 to 104 S cm−1 is quantitatively similar for forward and reverse bias, with however a shift ΔV of 0.28 eV attributed to the heterojunction built-in potential. The observed strong decrease in the apparent prefactor σ∘∘(F) is consistent with recent modeling of field-enhanced hopping transport in exponential band tails.