MONTE-CARLO SIMULATION OF ELECTRON-TRANSPORT IN ALTERNATING-CURRENT THIN-FILM ELECTROLUMINESCENT DEVICES

An ensemble Monte Carlo simulation of electron transport in bulk ZnS at different electric fields is presented. Scattering mechanisms associated with polar optical phonons, acoustic phonons (through deformation potential coupling), intervalley scattering, and impurities (neutral and ionized), are included in a nonparabolic multivalley model. Simulation indicates that the polar optical phonon and intervalley scattering mechanisms are dominant, whereas neutral and ionized impurity scattering are of no significance in determining the high-field electron transport in bulk ZnS. The simulated results show that approximately 26% of the electrons possess total energies exceeding 2.1 eV, the threshold energy for Mn impact excitation, at an electric field of 1 MV/cm. This fraction of electrons with energies exceeding 2.1 eV is estimated to be 50% and 65% at electric fields of 1.5 and 2.0 MV/cm, respectively. Transient overshoot effects are found to be of negligible importance in the operation of alternating-current thin-film electroluminescent (ACTFEL) devices. The steady-state electron distribution at high fields is sufficiently energetic to explain the observed efficiency of ACTFEL devices. No evidence for a significant electron population with energies in excess of 5 eV is found, even during the brief nonstationary regime, and thus very few carriers possess sufficient energy to induce band-to-band impact ionization.