Hydrodynamic and drift-diffusion modelling of charge carrier transport in ZnS:Mn thin-film electroluminescent structures

A set of model equations in the framework of the hydrodynamic approach is set up to describe the longitudinal charge carrier transport in ac-driven ZnS:Mn thin-film electroluminescent structures. Band-to-band impact ionization and a single type of hole recombination centres are taken into account to model charge carrier generation and recombination in the ZnS:Mn layer. The influence of the electric-field dependence of the impact ionization coefficients is analysed by comparing different models for this coefficient which have been used in the literature. Extensive numerical simulations within the framework of the drift-diffusion approach are presented. The main focus is on the average current-voltage characteristic and its dependence on several parameters such as the frequency of the driving voltage or geometry parameters of the ZnS:Mn layer. By comparing the results with those obtained within a hydrodynamic model it is found that, although very high carrier temperatures are reached, the longitudinal transport behaviour is similar in both models. The reason for this is the dominance of the longitudinal drift currents compared to diffusion and temperature-induced currents. The results are in good agreement with experiments. In contrast to simplified transport models which have been used previously the model equations formulated in this paper can be applied to describe the behaviour in thin-film structures with spatially extended contact areas exhibiting lateral pattern formation in certain parameter ranges.