Hot Electron Excitation of Luminescent Impurities in Ac Thin-Film Electroluminescent Devices

Abstract

AC thin-film electroluminescent (ACTFEL) devices are an essential technology for future flat-panel display applications. The basic structure of a typical ACTFEL device consists of a phosphor layer sandwiched between two insulating layers and a pair of electrodes. The phosphor consists of a wide bandgap semiconductor such as ZnS (Eg=3.7 eV) heavily doped (~ 1%) with a luminescent impurity such as Mn+2. Light emission is achieved by the application of large AC voltages to the layer structure such that electrons are injected into the high-field phosphor layer from interface states at the insulator-semiconductor interface either due to tunneling or due to field emission. For sufficiently high-fields (typically 1–2 MV/cm), electrons in the phosphor layer may gain sufficient energy to impact excite electrons in the luminescent impurities from the ground to excited states, which subsequently undergo radiative decay emitting photons1. The luminescent and power conversion efficiency of such devices critically depend upon the high-field distribution of electrons in the conduction bands of the phosphor, and the impact excitation rate for exciting luminescent impurities. Thus, an understanding of high-field carrier transport in the phosphor layer and of the physics of different threshold processes such as band-to-band impact ionization and impact excitation of luminescent impurities is essential for ACTFEL device design.