Abstract
A defect band is formed in hydrogenated amorphous silicon nitride (a-SiNx:H) due to current stressing of the material. This gives rise to an increase in conductivity, referred to as current induced conductivity. This thesis investigates the current transport mechanisms that occur in the induced defect band, by comparing the temperature dependence of the conductivity of several sets of a-SiNx:H thin film diodes. These sets were systematically current stressed to different levels with one set remaining unstressed. Samples with energy gaps of 2.06 eV and 2.28 eV were considered. We show that around room temperature a modified Poole-Frenkel description of conduction (i.e. field enhanced hopping of carriers via charged defect states) provides a good fit to the data. Using this model the activation energy of current transport was calculated and shown to depend on the material band gap. Data fitting to the Poole-Frenkel model provided further support for the field-assisted hopping mechanism. Previous investigations had suggested that the defect band resides in the lower half of the band gap, so that current transport through the defect band was then expected to be due to the movement of holes, in a manner consistent with Poole-Frenkel conduction. By considering samples grown on p-type and n-type substrates, we demonstrated that transport was indeed the result of the movement of holes through the defect states within the induced defect band. At lower temperatures the experimental data is poorly described by a modified Poole-Frenkel type process, so further mechanisms were considered, including variable-range hopping and nearest-neighbour hopping. Due to the similar nature and slight temperature dependence of each process, differentiating between the two mechanisms proved difficult. However, other factors such as the temperature range and defect density favoured variable-range hopping transport. By assuming this form of low temperature hopping transport, conduction through the defect-band of the a-SiNx:H, could then be convincingly explained over the entire temperature range from 320 K to 20 K in terms of two dominant transport mechanisms, Poole-Frenkel conduction and variable-range hopping.