Effects of oxygen impurity on the energy distribution of gap states in hydrogenated amorphous silicon studied by post-transit photocurrent spectroscopy

Abstract

Oxygen is normally present in the highest concentration (${10}^{18}--{10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ range) among the impurities in hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ and affects the electrical properties and light-induced changes in this material. However, little is known about how the energy distribution of trap states, $g(E)$, in undoped $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ changes with the presence of oxygen. In this study, we prepare undoped $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$ samples in an ultrahigh-vacuum plasma-enhanced chemical-vapor-deposition chamber and intentionally introduce oxygen atoms at a concentration of $3\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ by adding $\mathrm{C}{\mathrm{O}}_{2}$ to the source gases. By comparing the results obtained for high-purity and oxygen-doped samples by post-transit photocurrent spectroscopy, we discuss the effects of oxygen on the energy distribution of electron and hole trap states before and after long exposure to light. It is found that the presence of oxygen atoms enhances the defect reactions during prolonged illumination, while the shapes of $g(E)$ in the as-deposited state are similar between the high-purity and oxygen-doped samples in the energy range studied (from ${E}_{c}\ensuremath{-}0.35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{E}_{c}\ensuremath{-}0.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for electron traps and from ${E}_{v}+0.38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{E}_{c}+0.78\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for hole traps). A light-induced decrease of the hole trap density in the energy range from ${E}_{v}+0.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{E}_{v}+0.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is observed and discussed.