Recombination in plasma-deposited amorphous Si:H. Luminescence decay

Abstract

An investigation of the photoluminescence decay in plasma-deposited amorphous Si: H is presented. The relative contribution of radiative and nonradiative transitions are shown in undoped samples with efficiencies varying over two orders of magnitude. In high-efficiency samples the 10-K radiative rate spans nearly six orders of magnitude, and is interpreted as radiative tunneling of band-tail electrons and holes. A systematic shift of time-resolved spectra by 0.15 eV to low energy is related to the width of the band tails. The decay is found to be excitation-intensity-dependent when the electron-hole pair density exceeds about 1.5\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. This result is explained by overlap between neighboring pairs, and we deduce that the radius of the larger carrier is 11 \AA{}\ifmmode\pm\else\textpm\fi{}20%. A decrease of the radiative transition rate as the temperature increases is seen as evidence of carrier diffusion and is believed to explain a corresponding increase in the luminescence intensity. From this and other evidence it is argued that the electron-phonon coupling observed in the luminescence is associated with hole states, while electrons are not strongly coupled. We suggest that self-trapped hole states are split off from the top of the valence band, and account for the ${E}_{y}$ band in the field-effect density of states.