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

Abstract 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.