Effect of vacuum annealing on charge transport and trapping in a-Si _1-xC_x:H/c-Si heterostructures

The processes of charge transport and trapping in amorphous Si _{1 - x}C_x:H films deposited on crystalline p-type Si wafers and annealed in vacuum in the temperature range 300-650 °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 °C. C-V characteristics indicate a slight increase of the dielectric constant k and a large hysteresis after annealing at 450 °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 °C anneal. From I-V data the maximum rectification ratio is observed after annealing at 450 °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 °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 °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-Si_{1 - x}C_x:H/p-Si structure annealed at 450 °C is proposed.