Electronic processes in electroluminescent device structures
Electronic processes in two different electroluminescent device structures, the forward biassed metal/thick insulator/semiconductor (MIS) diode and the high field metal/insulator/metal (MIM) panel, are investigated. Models are produced to explain the behaviour of two particular MIS systems which have been studied experimentally. One of these systems is the Au/cadmium stearate/n-GaP structure, where the insulator is deposited using Langmuir-Blodgett (LB) technology. The other is the Au/i-ZnS/n-ZnS structure. In the MIS devices electroluminescence occurs as a result of the recombination of electrons and holes in the semiconductor and so it is necessary to have an efficient minority carrier (hole) injection mechanism. Attention is paid to the impact excitation of the electron gas in the metal by the electrons injected from the semiconductor because this has been proposed by other workers as a process for producing holes in the metal that are energetically capable of entering the semiconductor valence band, provided they can traverse the insulator. The characteristics of the LB film devices are found to be best described by assuming the minority carrier injection to be limited by the hole transport through the insulator. Hopping between interface states on the successive LB layers is proposed as the transport mechanism. However, the device incorporating a II-VI semi-insulator is shown to be more characteristic of hole transport in the insulator valence band and a minority carrier injection which is limited by the supply of holes from the metal. In high field MIM panels the mechanism of electroluminescence is quite different with impurity centres being impact excited or impact ionised by injected electrons and subsequently luminescing. Such devices driven by a dc signal are susceptible to the formation of high current filaments which burn out and result in device failure. A model is developed which predicts that there is a voltage range over which the device can exist in either a low current state or two higher current states and the resultant instability is expected to be destructive. Current-voltage characteristics are produced using this model and their general features are found to be relatively insensitive to material and device parameters. In order to understand the evolution of the electrical state of the MIM device after switch-on, a time dependent theory of system behaviour is also developed. This is particularly important as the devices are usually driven by a pulsed signal. For an homogeneous system the current is found to converge to the lower current state of the steady state characteristic.