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(sigma/sigma degrees degrees) = -(T-0/T)(1/4), corresponding to 3D hopping transport. As the electric field F increases from 10(4) to 10(5) V cm(-1), the decrease of both apparent slope T-0(1/4)(F) from 230 to 100 K-1/4 and prefactor sigma degrees degrees(F) from 10(16) to 10(4) S cm(-1) is quantitatively similar for forward and reverse bias, with however a shift Delta V of 0.28 eV attributed to the heterojunction built-in potential. The observed strong decrease in the apparent prefactor sigma degrees degrees(F) is consistent with recent modeling of field-enhanced hopping transport in exponential band tails. (c) 2006 Elsevier B.V. All rights reserved.