Abstract:
The electrical properties of hydrogenated amorphous silicon sa-Si:Hd metal–semiconductor–metal
(MSM) devices are investigated as a function of Si bombardment dose prior to and after annealing.
We observe that conduction in unbombarded devices is surface-barrier controlled whereas it is bulk
controlled in bombarded devices. The resistance decreases with bombardment dose in a manner
consistent with increased hopping conductivity in highly damaged structures. A relative permittivity
of between 8 and 12, depending on dose, was calculated from experimental Poole–Frenkel plots for
bombarded devices. These values compare closely with the theoretical relative permittivity for
amorphous silicon of 11.7 and confirm that conduction is by Poole–Frenkel mechanism. For
bulk-controlled conduction, we observe an increase in the zero-field Coulombic trap barrier height
with decreasing dose, ranging from 0.53 for a Si dose of 531013 cm−2 to 0.89 for a dose of 2
31012 cm−2. We attribute this to a decrease in the concentration of charged defects with decreasing
dose and find that the change in concentration of charged centers needs to be about 4
31019 cm−3 to account for the change of 0.35 eV from the lower to the upper dose. Activation
energies obtained from Arrhenius plots of current density against temperature varied with dose and
temperature in a similar way as Coulombic barrier height. We explain these results in terms of the
variation in the number of charged defect centers with dose and annealing temperature and a shift
in the Fermi level.
