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

Abstract

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°C have been evaluated. Current–voltage (I–V), capacitance–voltage (C–V) and admittance–temperature (G–T) characteristics were measured in the temperature range 100–350K. 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 0.12eV. 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 (EV+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°C is proposed.