Hopping Conduction and Chemical Structure : a study on Silicon Suboxides

Abstract

Using an rf magnetron reactive sputtering technique thin films (<~ 1 mu-m) of amorphous silicon suboxides (a-SiOx, 0 < x < 2) have been deposited. The substrate temperature during deposition remained below 100oC. The films appear rather homogenous in composition (O/Si ratio), with an incorporated fraction of hydrogen and argon atoms of about 2-5 at.%. Taking into account the incorporated argon atoms no large (empty) void fraction has been detected. The films appear composed of Si-Si4-nOn (n= 0,...,4) building blocks, with an occurrence of the five individual building blocks in the different compounds generally in agreement with the statistical distribution of randomly dispersed O atoms in a bridging con- figuration between Si atoms. Furthermore, a large density of neutral paramagnetic defects (~ 1021 cm) has been observed. These defects are predominantly associated with silicon dangling bonds (DBs) in a silicon rich environment ( Si-Si3), although in the as deposited compounds with higher O/Si ratio x also Si DBs in more oxygenated environments ( Si-Si2O, Si-SiO2) are observed. Simultaneously, the electrical conductivity of the films indicates a dominant variable range hopping (vrh) mechanism in the compounds, at least up to room temperature. This conduction mechanism is described by a charge transport via thermally assisted tunneling events through localized (defect) states. Since the host network of the defect states in our SiOx films appears rather homogeneous on both macroscopic (mu-m) and microscopic (nm) scale the localized hopping states are expected to be more or less randomly distributed throughout the material. The energy distribution of the corresponding hopping sites is expected to extend over several tenths of an eV, as indicated by measurements on the conductivity around room temperature and supported by a theoretical Density Of States (DOS) model based on a defect-poole model. As a result the vrh conduction process in these SiOx films is adequately described by a system of localized states, randomly distributed in both space and energy. Analytically, it is possible to express the conductivity in such a system qualitatively in terms of the density and localization of the states around the position of the chemical potential. The conductivity appears to depend on the external parameters temperature (T) and electric field (F) and is described differently in different regimes of T and F. In this thesis a more quantitative description of the vrh process is derived numerically using percolation theory. To this purpose calculations on the percolation threshold of the site-to-site impedance in a system of randomly distributed hopping sites were performed. The results appear in qualitative agreement with the analytically derived results. Moreover, the calculations result in a clear quantified description of 1. the temperature dependence of the vrh conduction at low field strengths and 2. the field dependence of the conduction at low field strengths and/or low temperatures. Furthermore, a numerical study on the vrh conduction in a system of non-uniform localized states shows that the conductivity is effectively dominated by the charge transport through a subset of more weakly localized states. Measurements on the temperature and field dependence of the electrical conductivity in the deposited SiOx films appear in good agreement with the theoretical vrh models. Furthermore, quantitative information on the density and localization of the electronic states dominating the macroscopic charge transport is obtained by fitting the measured data with the numerically derived relations. The observed differences in resistivity of the investigated SiOx films appear well explained by a stronger localization of the hopping sites in the more oxygenated environments. Furthermore, it appears that only a small fraction of the neutral paramagnetic states, viz. the more weakly localized states, contributes significantly to the macroscopic charge transport in the vrh conduction process. Both observations are supported by measurements on annealed samples, showing a preferential annihilation of paramagnetic spins in the more oxygenated environments