Effect of vacuum annealing on charge transport and trapping in a-Si1-xCx:H/c-Si heterostructures

The processes of charge transport and trapping in amorphous Si1 - xCx:H films deposited on crystalline p-type Si wafers and annealed in vacuum in the temperature range 300-650 degrees C have been evaluated. Current-voltage (I-V), capacitance-voltage (C-V) and admittance-temperature (G-T) characteristics were measured in the temperature range 100-350 K. The spectrum of thermal effusion of hydrogen was measured from room temperature up to 1000 degrees C. C-V characteristics indicate a slight increase of the dielectric constant k and a large hysteresis after annealing at 450 degrees C. The hysteresis is believed to be associated with mobile hydrogen effusion from the a-SiC:H film, and it is not seen after a 650 degrees C anneal. From I-V data the maximum rectification ratio is observed after annealing at 450 degrees C. Variable-range hopping (VRH) conduction at the Fermi level is found to dominate the forward current of the as-deposited structure. After annealing at 450 degrees C the forward current can be described by space-charge limited (SCL) mechanisms with trapping at shallow levels with energy of about 0.12 eV. After annealing at 650 degrees C the process of VRH conduction appears again, but the density of hopping sites is much higher than in the as-grown sample. From admittance spectra, the energy position of respective traps in a-SiC:H is at (E-v + 0.45) eV for as-deposited material and it decreases slightly after vacuum annealing. On the basis of these results, an energy band diagram of the a-Si1 - xCx:H/p-Si structure annealed at 450 degrees C is proposed. (C) 2011 Elsevier B.V. All rights reserved.